Description

Neurotrophin family. Biological effects of all the members of this family are mediated through activation of 1 or more of the 3 members of the tropomyosin-related kinase (Trk) family of receptor tyrosine kinases, which leads to activation of Ras, phosphatidylinositol 3-kinase, phospholipase C-γ1, and signaling pathways controlled through these proteins, including the mitogen-activated protein kinases (33). Neurotrophins also activate the p75 neurotrophin receptor (p75NTR), a member of the tumor necrosis factor receptor superfamily. Neurotrophins are required during growth into adulthood, where they control synaptic function and plasticity and sustain neuronal cell survival, morphology, and differentiation. Two members of this family, nerve growth factor and brain-derived neurotrophic factor, have been used in clinical trials in neurologic disorders.

Nerve growth factor. Nerve growth factor is the prototype of a target-driven neurotrophin. It is essential for the development and differentiation of peripheral sympathetic and neural crest-derived sensory nerve cells. Nerve growth factor also plays a role in the central nervous system as a trophic agent for basal forebrain cholinergic neurons. Nerve growth factor is internalized by binding to the trkA receptor and travels by retrograde axonal transport of sensory and sympathetic neurons to reach the cell body. There, it activates the second messenger system to influence transcription of several genes. Altered amounts of specific proteins are transported by anterograde transport in the axons. Various agents that regulate nerve growth factor are listed below:

Nerve growth factor exerts a modulatory role on sensory nociceptive nerve physiology in the adult and appears to correlate with hyperalgesic phenomena occurring in tissue inflammation. In addition to its classical actions in the nervous system, nerve growth factor maintains a balanced interplay between the nervous, immune, and endocrine systems.

Examples of nerve growth factor interaction with the neuroendocrine system are listed below:

The potential of nerve growth factor as a treatment for degenerative neurologic disorders and peripheral neuropathies is being explored.

Brain-derived neurotrophic factor. Brain-derived neurotrophic factor and its receptor are widely expressed in the developing as well as the adult nervous system and plays an important role in the development of brain circuits and neuronal network plasticity. Brain-derived neurotrophic factor signaling in the brain can increase peripheral insulin sensitivity, suggesting a mechanism whereby the brain can control lifespan. The expression of brain-derived neurotrophic factor is modulated by activity of GABAergic and cholinergic systems, and brain-derived neurotrophic factor, in return, can influence neuronal synaptic activity. In neuronal cells, brain-derived neurotrophic factor-immunoreactivity has been found in several regions of the central nervous system, as well as in the peripheral and enteric nervous system. Brain-derived neurotrophic factor can also be released from many non-neuronal cell types using similar molecular mechanisms (05). One form of brain-derived neurotrophic factor enters the nucleus and may directly influence transcription, whereas another fraction travels by retrograde axonal transport out of the synthesizing cell and can be detected in the basal forebrain cholinergic neurons. Synthesis and release of brain-derived neurotrophic factor is targeted by activation of glutamate receptors, and disturbance of this process probably underlies neurodegenerative and psychiatric disorders. Brain-derived neurotrophic factor has a neuroprotective effect because of its ability to reverse the N-methyl-D-aspartate-induced inactivation of protein kinase C in cortical neurons. Upregulating brain-derived neurotrophic factor-activated pathways may be therapeutically relevant for neurologic disorders.

Exposure to stress decreases levels of brain-derived neurotrophic factor in brain regions associated with depression, whereas antidepressant treatment produces the opposite effect and blocks the effects of stress on brain-derived neurotrophic factor. Patients with major depressive disorder and posttraumatic stress disorder who express lower allopregnanolone (a GABAergic neuroactive steroid) levels also show reduction of brain-derived neurotrophic factor in the peripheral blood and in the brain that could function as a biomarker axis for the diagnosis of both disorders (02).

Persistence of long-term memory requires a late protein synthesis- and brain-derived neurotrophic factor-dependent phase in the hippocampus. Intrahippocampal delivery of brain-derived neurotrophic factor reverses the deficit in memory persistence caused by inhibition of hippocampal protein synthesis.

CDNF/MANF. These neurotrophic factors protect midbrain dopaminergic neurons and restore motor function in a 6-hydroxydopamine rat model of Parkinson disease. The receptors and mode of action of these novel factors, which are potential therapeutic treatments for Parkinson disease, are being investigated.

Neurotrophin-3. Neurotrophin-3 is expressed in noradrenergic neurons of the locus coeruleus and acts as a trophic factor for these neurons. This raises the possibility that some of the effects of stress and antidepressants on locus coeruleus function and plasticity could be mediated through neurotrophin-3. Neurotrophin-3 prevents degenerative changes in striatal projection neurons after excitotoxicity in vivo and has the potential to treat Huntington disease. NT-3 has potential applications in the treatment of spinal cord injury and cerebral ischemia.

Neurotrophin-4/5. Although neurotrophin-4 and neurotrophin-5 were originally described separately, they are similar in several respects and are referred to as neurotrophin-4/5. The biological actions of neurotrophin-4/5 are believed to be mediated by trkB receptor tyrosine kinase. The mRNA for trkB is expressed in hippocampal cells in vivo and may play a neuroprotective role against excitotoxic damage in the hippocampus and the cerebral cortex by enhancement of the neuronal calcium homeostatic system.

Neurotrophin-6. Recombinant purified neurotrophin-6 has a spectrum of actions like those of nerve growth factor on experimental animal models, eg, in chick sympathetic and sensory neurons, although it is less potent. Neurotrophin-6 is expressed in the embryo, but expression persists in some adult tissues. Interaction of neurotrophin-6 with heparin-binding molecules may modulate its action in the nervous system.

Fibroblast growth factors. The fibroblast growth factor family encompasses at least 23 members and, of these, 10 are expressed in the developing central nervous system, along with 4 fibroblast growth factor receptors. Acid fibroblast growth factor (aFGF) is present in a select set of neurons including some that are at risk for developing neurodegenerative disorders. Basic fibroblast growth factor (bFGF) is widely distributed in tissues. Its gene is located on chromosome 4 of the human genome. bFGF is present in most astrocytes and a specific hippocampal neuronal population. Immunoreactivity of bFGF is expressed weakly in the neurons and strongly in the choroid plexus, whereas glial cells express little of these proteins.

Normally, both aFGF and bFGF are stored in the intracellular compartments and released in the extracellular space following injury to the cell membrane. bFGF is involved in wound healing; the levels are markedly increased in brain wounds. Fibroblast growth factors are major mitogens and can stimulate proliferation of blood vessels, fibroblasts, glial cells, and Schwann cells in vitro. Basic fibroblast growth factor and its receptors are constitutively expressed in dorsal root ganglia and the peripheral nerve. These molecules display an upregulation in dorsal root ganglia and in the proximal and distal nerve stumps following peripheral nerve injury; thus, bFGF could be a promising candidate for the development of new therapeutic strategies to treat peripheral nerve injuries. FGFs can induce nerve fiber extension and support survival of central nervous system sensory and sympathetic neurons. They also protect neurons from death after experimental contusive spinal cord injury and have potential application in stroke.

Neuropoietic cytokines. Neuropoietic cytokines have been recognized to play an important role in normal development of the brain, as well as following injury during the healing process, when they act as neurotrophic factors. Elevated levels are associated with many neurologic disorders. Interleukin-1 beta, interleukin-3, interleukin-6, interferon-gamma, transforming growth factor-beta, and tumor necrosis factor-alpha all play a role in human disorders such as stroke, multiple sclerosis, and Alzheimer disease.

Ciliary neurotrophic factor. Ciliary neurotrophic factor is released from Schwann cells and astrocytes in response to injury and is considered to play a key role in nerve cell maintenance and repair. The human ciliary neurotrophic factor gene has been mapped to chromosome 11 and contains a single 1 kb intron within the coding domain. Axokine-1 is rCNTF with substitution of alanine for cysteine at position 63, and deletion of 13-C terminal amino acids. The synthesis of ciliary neurotrophic factor occurs mainly in the Schwann cells and astrocytes and appears to be regulated by either direct or indirect signals from neurons. The exact mechanism of release of ciliary neurotrophic factor is not known. Polymerase chain reaction has been used to show that ciliary neurotrophic factor mRNA is widely expressed in the brain, heart, lung, liver, kidney, and testis of the rat, in addition to preferential expression in the sciatic nerve. The highest concentration in the central nervous system is found in the optic nerve and the olfactory bulb. These concentrations are several times higher than those required for the biological activity of ciliary neurotrophic factor and may indicate that ciliary neurotrophic factor is trapped in the optic nerve and olfactory bulb's cells of synthesis and is not readily released into the extracellular space to reach its neuronal receptors. The following biological actions have been attributed to ciliary neurotrophic factor:

Hepatocyte growth factor. Hepatocyte growth factor is a multifunctional cytokine originally identified and purified as a potent mitogen for hepatocyte. Along with its receptor, c-Met, it can promote formation of neurites and enhance elaboration of dendrites in mature neurons. It is expressed in various brain regions under normal conditions and can enhance the survival of hippocampal and cortical neurons during the aging of cells in culture. Hepatocyte growth factor has a neuroprotective effect. It is 1 of the most potent survival-promoting factors for motor neurons, and intrathecal hepatocyte growth factor has shown therapeutic efficacy in transgenic rat models of amyotrophic lateral sclerosis.

Granulocyte colony-stimulating factor. G-CSF is a growth factor approved for the treatment of neutropenia following chemotherapy. G-CSF has known neuroprotective effects in a variety of experimental brain and spinal cord injury models and protects the CNS from immune-mediated damage in microglia-mediated reactive T cell activation with the potential to be a neuroprotective agent in multiple sclerosis (28).

Bone morphogenetic proteins. Bone morphogenetic proteins, with 20 members, are a rapidly expanding subclass of transforming growth factor-beta superfamily. Recombinant human bone morphogenetic protein-2 is in clinical trials as a bone graft substitute in spinal fusion surgery. Bone morphogenetic protein ligands and receptors exert a broad range of effects during multiple stages of neural development. In addition, bone morphogenetic proteins act on more lineage-restricted embryonic central nervous system progenitor cells to promote regional neuronal survival and cellular differentiation.

Growth differentiation factor-5 is a novel member of this subclass. It is expressed in the developing central nervous system, including the mesencephalon, and acts as a neurotrophic, survival-promoting molecule for rat dopaminergic midbrain neurons. Recombinant human growth differentiation factor-5 supports dopaminergic neurons in culture to the same extent as both transforming growth factor-3 and glial cell line-derived neurotrophic factor. However, in contrast to transforming growth factor-beta 3 and glial cell line-derived neurotrophic factor, growth differentiation factor-5 augments members of astroglial cells in cultures, suggesting that it may act indirectly. Also, it may act through pathways different from those triggered by transforming growth factor-beta 3 and glial cell line-derived neurotrophic factor. Growth differentiation factor-5 also protects dopaminergic neurons against N-methylpyridinium ion-induced toxicity that selectively damages dopaminergic neurons.

Osteogenic protein-1. Osteogenic protein-1 is a member of transforming growth factor-beta superfamily and plays an important role in the formation of new bone. It is better known for its use in orthopedic surgery; clinical trials have been conducted for treatment of nonunion fractures by osteogenic protein-1. Osteogenic protein-1 is a powerful regulator of genes for neural cell adhesion molecules and promotes the development of adrenergic phenotypes in neural cell cultures. It induces dendritic growth in rat sympathetic neurons in the presence of nerve growth factor as a cofactor. In the presence of optimal concentration of nerve growth factor, osteogenic protein-1-induced dendritic growth from cultured perineural neurons is comparable to that observed in situ. Osteogenic protein-1 enhances recovery of motor function following stroke in an animal model.

Neurturin. This neurotrophic factor has a 45% homology with the coding region of glial cell line-derived neurotrophic factor. It promotes cell growth and is neuroprotective. It is being developed for the treatment of neurodegenerative disorders. Recombinant human neurturin was shown to prevent Parkinson disease symptoms as well as protect dopaminergic neurons and preserve dopamine content in midbrain substantia nigra in rhesus monkeys exposed to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. A preparation of neurturin plus adeno-associated virus serotype 2, delivery by stereotaxic intraputaminal injection, has completed phase 2 trials in Parkinson disease.

Persephin. This is a novel neurotrophic factor that is 40% identical to glial cell line-derived neurotrophic factor and neurturin; it promotes the survival of ventral midbrain dopaminergic neurons and prevents their degeneration after exposure to 6-hydroxydopamine in vivo. Persephin has potential applications for degenerative neurologic disorders but is more selective and has less adverse effects because it acts via different receptor complexes, unlike glial cell line-derived neurotrophic factor and neurturin.

Glial cell line-derived neurotrophic factor. Glial cell line-derived neurotrophic factor is a neurotrophic polypeptide and is distantly related to transforming growth factor-beta. It lies on the short arm of the human chromosome. Purified recombinant glial cell line-derived neurotrophic factor promotes neurite outgrowth and is a neurotrophic factor for developing peripheral neurons. It has also been implicated in the survival and morphological and functional differentiation of midbrain dopaminergic neurons in vitro. Glial cell line-derived neurotrophic factor travels by retrograde transport via mesencephalic dopamine neurons of the nigrostriatal pathway. The pattern of retrograde transport following intrastriatal injections indicates that there may be subpopulations of neurons that are glial cell line-derived neurotrophic factor responsive.

In addition to its role in the dopaminergic system, glial cell line-derived neurotrophic factor may act on several types of nondopaminergic neurons such as noradrenaline neurons, cerebral Purkinje cells, and neurons in peripheral ganglia. Glial cell line-derived neurotrophic factor may function as a target-derived trophic factor for neuronal populations innervating skeletal muscle, including sensory neurons and spinal cord motor neurons. It is a more potent survival factor for dopaminergic neurons and the noradrenergic neurons of the locus coeruleus than other neurotrophic factors. Glial cell line-derived neurotrophic factor has a neuroprotective effect and protects astrocytes from ischemia-induced apoptosis by inhibiting the activation of caspase-3. Based on a comprehensive review of the pharmacology of glial cell line-derived neurotrophic factor, it appears that this molecule may be useful in the treatment of neurodegenerative diseases such as Parkinson disease, amyotrophic lateral sclerosis, and cholinergic deficit-related dementia.

Insulin-like growth factors. The insulin-like growth factor system consists of 2 biologically active polypeptides (insulin-like growth factor-1 and insulin-like growth factor-2) with their receptors and 6 binding proteins (insulin-like growth factor binding protein 1 to 6). Insulin-like growth factor-1 has endocrine, autocrine, and paracrine properties. It increases glucose and amino acid uptake and stimulates synthesis of protein and RNA in a variety of cells, including neurons and glial cells. It is involved in tissue regeneration and repair in several organs, including peripheral nerves following transection. Insulin-like growth factor participates in multiple cell processes besides promoting nerve regeneration. As a myogenic factor, insulin-like growth factor-1 promotes muscle hypertrophy, and as an angiogenic factor, it promotes angiogenesis in regenerating skeletal muscle by activating vascular endothelial growth factor receptors. In animal models, insulin-like growth factor-1 protects against cerebral ischemic injury and facilitates recovery from it. Alterations in the blood insulin-like growth factor-1 system may constitute a marker of some degenerative disorders of the cerebellum.

Insulin-like growth factors have neurotrophic action on sensory, sympathetic, and motor neurons. Insulin-like growth factor-1 has a potential in the treatment of neurodegenerative disorders and has shown beneficial effect in amyotrophic lateral sclerosis. An interaction takes place between estrogen receptors and insulin-like growth factor-1 receptor in the promotion of neuronal survival and in the response of neural tissue to injury. Insulin-like growth factor activity is reduced in diabetic neuropathy; this is implicated in the pathogenesis of the condition. This also forms the basis of treatment of diabetic neuropathy by insulin-like growth factor-1.

Neuregulins. Neuregulins constitute a family of structurally related signaling proteins comprising at least 12 members. The glial growth factors and 3 neuregulin isoforms stimulate the proliferation of cultured Schwann cells. Neuregulins may play an important role in neuroglia and neuron-target interaction, peripheral nerve development, and neuromuscular synaptogenesis.

Neuregulin mRNA is widely expressed in early development, particularly in the central nervous system and the peripheral nervous system. The epidermal growth factor-like growth factors bind to and activate members of a subfamily of receptor tyrosine kinases that include the epidermal growth factor receptors, ErbB2-Neu, ErbB3, and ErbB4. Neuregulin-ErbB signaling is essential for the development of the heart and the nervous system. Sustained expression of these genes after birth and in adult life is required for tissue maintenance and repair. Some disorders, such as cardiomyopathies, neuropathies, and demyelinating diseases may be alleviated by augmenting neuregulin-ErbB signaling.

Glial growth factors. Glial growth factors are proteins produced in peripheral nerves, and they probably play a significant part in regulating the function of Schwann cells; they were even called "Schwann cell mitogens" at 1 time. Schwann cells have an important function in the development and regeneration of peripheral nerves following injury. The development of peripheral axons depends on Schwann cells as a source of trophic factors. Axon-Schwann cell trophic interactions play a role in the development of the neuromuscular system. During later stages of development, Schwann cells produce the myelin sheath.

Three distinct but structurally related glial growth factors have been identified: (1) glial growth factor-1, (2) glial growth factor-2, and (3) glial growth factor-3, and are ligands of members of the epidermal growth factor receptor family. These play a complex role in the development, regeneration, and tumor biology of the peripheral nervous system. The 2 cell types that express glial growth factor mRNA at the highest levels are motor neurons and primary sensory neurons.

Glial growth factors may have an important role to play in the treatment of nerve injuries, neuropathies, and tumors of glial cell origin. Glial growth factor-2 concentrations greater than necessary to saturate the mitotic response induce the secretion by Schwann cells of activities that promote sympathetic neuron survival and outgrowth. Glial growth factor-2 is being developed for several indications, including peripheral neuropathies, multiple sclerosis, and muscle disorders.

Sonic hedgehog protein. This is a member of a family of molecules that are active during development of dopaminergic neurons. Signaling by Sonic hedgehog controls important developmental processes in the nervous system, including dorsoventral neural tube patterning, neural stem cell proliferation, and neuronal and glial cell survival. The neurotrophic and neuroprotective activities make them potentially valuable in the treatment of neurodegenerative diseases, central nervous system trauma, and stroke.

Neuritin. This is an activity-induced axonal protein that is mainly expressed in the brain by a gene of the same name. Expression is induced by neuronal activity and neurotrophic factors. Neuritin promotes neurite outgrowth, regulates synaptic plasticity, and has a neuroprotective effect (36). Neuritin is implicated in several neurologic disorders and is a potential therapeutic target.

Gangliosides. Gangliosides are complex lipids contained in all tissues of the body, but the highest concentrations are found in neurons where they constitute one tenth of the total membrane-bound lipid. Although present on the whole neuron surface, they are more concentrated in the synaptic region. Gangliosides are expressed in a differential manner during growth, development, and maturation of the nervous system. Gangliosides affect the activity of several functional proteins. Some examples of this are listed below:

Ganglioside GM1 (monosialoganglioside). GM1 ganglioside has been reported to enhance the effect of nerve growth factor on sprouting and neurite extension, but they are not considered to be a substitute for appropriate neurotrophic factors. Exogenously administered gangliosides were shown to enhance peripheral nerve sprouting and repair. GM1 ganglioside has been suggested to act as a possible neurotrophic factor in noradrenergic, serotonergic, cholinergic, and dopaminergic systems. Some evidence exists for a synergistic effect of gangliosides on nerve growth factor-induced neuronal cell recovery and synaptogenesis. It is likely that interactions between nerve growth factor and gangliosides occur due to their incorporation into cell membranes.

GM1 provides trophic support for damaged dopamine neurons and, in combination with fibroblast growth factor-2 (and epidermal growth factor).

Cerebrolysin. Cerebrolysin, which contains active fragments of neurotrophic factors obtained from purified porcine brain proteins, has long been used for the treatment of dementia and stroke in Europe. Experimental studies have shown that the neuroprotective effect of cerebrolysin is by inhibition of oxidative stress-induced apoptosis and suggest potential use for neurodegenerative disorders (08).

Peptide-6. This peptide was designed based on a biologically active region of the CNTF and was shown to induce proliferation, and it increased survival and maturation of neural progenitor cells into neurons (06). It can permeate blood-brain barrier, and subcutaneous implants improve reference memory in mice. It has potential for prevention and treatment of learning as well as memory disorders.

Davunetide. This is an 8 amino acid snippet derived from activity-dependent neuroprotective protein (ADNP). Possible mechanism actions of davunetide include antiinflammatory effect, antioxidant activity, inhibition of protein aggregation, and interaction with microtubules, which addresses the pathology in several neurologic disorders (23).

Neurotrophic factor receptors. Cell-cell communication is facilitated by the production of protein ligands in 1 cell that are recognized by cell-surface receptors on other cells. Two families of protein ligands that use receptors limited largely to neuronal cells can be classified as either tyrosine kinases or cytokine receptors. The action of these protein ligands is also specific to neuronal cells. The receptor system used by ciliary neurotrophic factor is different from the one used by the neurotrophins, but parallels are numerous; both have neurotrophic effect and both activate either an intrinsic or an extrinsic associated tyrosine kinase activity by inducing dimerization of signal transducing components. Multiple classes of neurotrophic factors may signal interactively or collaboratively, explaining the synergistic effect of brain-derived neurotrophic factor and ciliary neurotrophic factor.

Each neurotrophic factor family (ie, nerve growth factor, glial cell-derived neurotrophic factor, and ciliary neurotrophic factor) has a set of receptors with specificity for individual members of the family and a common receptor without member specificity that, in some families, generates the cellular signal and retrograde transport.

Axonal transport of nerve growth factors. Although some effects of the nerve growth factor are elicited directly at the nerve terminals, the long-term responses to nerve growth factor require transport of a signaling molecule to the cell body. Due to the extreme length of the axon, special mechanisms have developed for the delivery of substances from the cell body to the synapse (anterograde transport) and from the synapse to the cell body (retrograde transport). For transport in both directions, proteins are packaged into vesicles that are propelled along the microtubules by molecular motors. Obstruction of retrograde transport by ligation of the axon or disruption of microtubules by drugs results in the death of the neuron, but this can be prevented by application of nerve growth factor directly to the cell bodies. Traditional views of neurotrophic factor biology held that trophic factors are released from target cells, retrogradely transported along their axons, and rapidly degraded on arrival in cell bodies. Neurotrophic factors, such as brain-derived neurotrophic factor, fibroblast growth factor, glial cell-line derived neurotrophic factor, insulin-like growth factor-1, and neurotrophin-3, can move anterogradely along axons; thus, they can escape the degradative pathway on internalization and are recycled for future uses. Anterograde transfer of neurotrophic factors is involved in various physiological processes, including the regulation of developmental neuronal death, the modulation of synaptic transmission, and the control of axonal and dendritic architecture.

Role of neurotrophic factors in neuronal development, maturation, and survival. The role of neurotrophic factors in neuronal development, maturation, and survival is shown in Table 2.

Table 2. Potential Role of Neurotrophic Factors in Neuronal Development and Maturation

Intrinsic neurotrophic factors, including BDNF, VEGF, and IGF-1, participate in hippocampal neurogenesis and are implicated in a broad spectrum of pathophysiological changes in the human brain. Further investigation of roles of such neurotrophic factors may provide an insight into how adult hippocampal neurogenesis is related to psychiatric disease and synaptic plasticity.

Neural stem cells are a promising cellular source of neurotrophic factors for the treatment of diseases of the nervous system. Oligodendrocyte progenitor cells derived from human embryonic stem cells express neurotrophic factors.

Neurotrophic factors and apoptosis. Virtually all cell populations in the vertebrate nervous system undergo massive, "programmed" cell death, or apoptosis, early in development. Initially, neurons and glia are overproduced, followed by the demise of approximately one half of the original cell population. Survival of cells during apoptosis depends critically on their ability to access "trophic" molecular signals derived primarily from interactions with other cells. Inability of such signals to suppress apoptosis may contribute to cell death. Various cell signaling pathways and biochemical activities involved in apoptotic death are currently under intense study. Nerve growth factor signals through tropomyosin receptor kinase A to promote neuronal survival, whereas brain-derived neurotrophic factor signals through receptor p75NTR to promote neuronal apoptosis on the same neuron. This functional antagonism plays an important role in matching neurons with their targets throughout the nervous system. The connection of multiple stimuli that induce neuronal cell death to an apoptotic mechanism suggests that apoptosis could play a central role in neurodegeneration in the brain. Apoptosis is implicated in the following degenerative neurologic disorders:

The longevity of postmitotic cells is regulated by neurotrophic factors and expression of antiapoptotic genes. Loss of these factors leads to inappropriate cell death. Further understanding of this mechanism could reveal effective strategies for arresting neuronal and glial cell death induced by injury, disease, or aging in humans. The role of individual neurotrophic factors in apoptosis is as follows.

Nerve growth factor. Apoptosis is triggered following withdrawal of nerve growth factor. The low-affinity neurotrophin receptor (called p75NTR) appears to be involved in inducing apoptosis in the absence of nerve growth factor, and its suppression by antisense oligonucleotides increases the survival of sensory neurons. The mechanism by which p75NTR expression induces apoptosis in the absence of nerve growth factor binding is not known, but it has been shown that it enhances the sensitivity of neural cells to oxidative damage. Such an expression has been shown following axotomy, following exposure to beta-amyloid peptide, and in the cortical neurons of some patients with Alzheimer disease or advanced age.

Ciliary neurotrophic factor. This promotes the survival of spinal sensory neurons following axotomy but not during the period of programmed cell death.

Glial cell line-derived neurotrophic factor. This has been shown to rescue developing avian motor neurons from natural programmed cell death in vivo, and it promotes the survival of enriched populations of cultured motor neurons. Furthermore, treatment with this agent in vivo prevents the induced death and atrophy of spinal motor neurons following peripheral axotomy.

Basic fibroblast growth factor. Basic fibroblast growth factor can override the axotomy-induced cascade of events that leads to death of the vulnerable neurons.

Insulin-like growth factor-1. This growth factor suppresses apoptosis effectively in many cell types treated with cycloheximide.

Epidermal growth factor. This factor suppresses apoptosis effectively in cells treated with actinomycin D. This is mediated by posttranslational modification of preexisting factors rather than by synthesis of new genes or proteins.

Neuregulins. Schwann cell apoptosis may play an important role in peripheral nerve development. Schwann cell survival may be regulated by access to neuregulins derived from axons.

Role of neurocytokines and neurotrophic factors in cell signaling. Soluble and membrane-embedded neurotrophic factors bind to specific receptors on responsive neurons, thereby initiating dramatic changes in the proliferation, differentiation, and survival of their target cells. Many of the intracellular pathways that neurotrophins and ciliary neurotrophic factors use to regulate gene expression and, thereby, achieve diverse biological responses have been elucidated. Characterization of the enzymes, linker proteins, and transcription factors that are sequentially activated in response to neurotrophic factors has provided significant insight into the mechanisms by which these agents elicit specific biological responses during normal development. This characterization has also provided insight into the adaptive responses of mature neurons. Synaptic activity regulates the synthesis, secretion, and action of neurotrophins, and these can in turn induce immediate changes in synaptic efficacy and morphology. Neurotrophins may lead to cell survival, growth, and differentiation or cell death, depending on the signaling pathway that is activated. In addition, neurotrophins exert various effects through interactions with other receptors and ion channels.

Signal transduction pathways between beta-amyloid precursor protein, transforming growth factor-beta, and tumor necrosis factor-alpha can orchestrate intricate injury-related cellular and molecular interactions. The links between these 3 factors include the following:

Astrocytes and microglia each produce and respond to transforming growth factor-beta and tumor necrosis factor-alpha in characteristic ways when the brain is injured. Transforming growth factor-beta, tumor necrosis factor-alpha, and secreted forms of beta-amyloid precursor protein can protect neurons against excitotoxic, metabolic, and oxidative insults and may thereby serve neuroprotective roles. On the other hand, under certain conditions, tumor necrosis factor-alpha and the fibrillogenic derivative of beta-amyloid precursor protein can promote damage of neuronal and glial cells and may play roles in neurodegenerative disorders. Studies of genetically manipulated mice that had altered transforming growth factor-beta and tumor necrosis factor-alpha levels or altered ligand or beta-amyloid precursor protein receptor levels suggest important roles for each factor in cellular responses to brain injury. These studies indicate that mediators of neural injury responses also have the potential to enhance amyloidogenesis, or to interfere with neuroregeneration, or both, if expressed at abnormal levels or modified by strategic point mutations. Knowledge of these signaling pathways is revealing novel molecular targets to focus on for neuroprotective therapeutic strategies in disorders ranging from stroke to Alzheimer disease.

Nerve growth factor and neuronal gene expression. Nerve growth factor has been shown to upregulate the expression of a subset of genes that are important for the growth and differentiation of neonatal sympathetic neurons, including those encoding the p75 receptor. These nerve growth factor-induced alterations in gene expression can be elicited in mature neurons in vivo as well. Increased terminally derived nerve growth factor specifically upregulates the ratio of p75 to trkA nerve growth factor receptors, a phenomenon that is postulated to provide a molecular mechanism for a modulatory feedback loop that regulates neuronal responses to nerve growth factor itself.

Serine protease inhibitors. Serine protease inhibitors play an important role in the regulation of several physiological processes, including coagulation, fibrinolysis, fertilization, development, malignancy, neuromuscular patterning, and inflammation. Serine protease inhibitors have also been shown to promote neurite growth in vitro from different neuronal cell types. These include neuroblastoma cells, hippocampal neurons, and sympathetic neurons. Protease nexin-1 is a prominent member of this family. It was first discovered in skin fibroblasts as a molecule that links serine proteases to the cell surface; since then, it has been found to be identical to glia-derived nexin on basis of activity as well as nucleotide sequence. Protease nexin-1, a 110-kd protein, is also identical to the secreted form of amyloid protein precursor. Protease nexin-1/amyloid precursor protein is decreased in various anatomical regions, including the hippocampus in patients with Alzheimer disease. The gene for protease nexin-1 has been mapped and is in the same locus as the gene for the autosomal recessive Tunisian form of familial amyotrophic lateral sclerosis.

Protease nexin-1 can rescue spinal motor neurons in chickens during the period of naturally occurring programmed neuron death and has been found to prevent axotomy-induced spinal motor neuron death in the neonatal mouse. These results are consistent with the finding that protease nexin-1 is a neurotrophic agent. It is possible that a balance between serine proteases and their inhibitors is involved in regulating the fate of neuronal cells during development.

Hedgehog family of inducing proteins. Vertebrate hedgehog genes comprise a small family with potent development-inducing abilities. Three members of human family hedgehog proteins are known: (1) Sonic, (2) Indian, and (3) Desert. Each of these has a distinct expression pattern in the embryo and the adult. Hedgehog signaling proteins induce cells to develop from a precursor state to a more functional state. They have been involved in the patterning and growth of vertebrate neural tube. Sonic hedgehog has neurotrophic and neuroprotective effects and is involved in the early development of the motor neurons, whereas the survival of motor neurons at later stages of embryonic development requires other neurotrophic factors. Amino-terminal product of Sonic hedgehog autoproteolysis (also called Shh-N) is an inductive signal expressed by floor plate cells that can induce dopaminergic neurons in vitro. Shh-N can function in a dose-dependent manner to induce different cell types within the neural tube. These results provide the basis for potential cell transplantation therapy for Parkinson disease. The expression and known embryologic activities suggest the possibility of clinical application of Sonic hedgehog proteins in neurologic disease.

Proteins involved in neuromuscular development. These proteins include agrin, laminin, and acetylcholine receptor-inducing activity (also called ACh-inducing activity), and they play an important role in neuromuscular development.

Agrin. Agrin was first isolated from the marine ray Torpedo californica electric organ. Agrin is released from the nerve terminals and deposited in the basal lamina. Its functions include the following:

Laminin. Laminin is composed of 3 independent chains named alpha, beta, and upsilon; they assemble to form a cruciform protein. The presence of laminin in the extracellular matrix of the developing nervous system has been correlated with the regions where tracts are growing. It is also found in the lesions of patients with Alzheimer disease, where it is apparently involved in the growth of neuronal processes associated with senile plaques.

Acetylcholine receptor-inducing activity. This is a neurotrophic protein that stimulates acetylcholine receptor synthesis and is a member of the Neu ligand family. Acetylcholine receptor-inducing activity is synthesized as a transmembrane precursor. The active epidermal growth factor-like domain is released from the cell surface by proteolytic cleavage to act on the target. Many alternatively spliced variants of ACh-inducing activity, nerve growth factor, and glial growth factor have been described. Sequence analysis has shown that acetylcholine receptor-inducing activity is an isoform of the neuregulins, the ligands for the erbB family of receptor tyrosine kinases. Neuregulins are synthesized by motor neurons and are present at the neuromuscular junction.

Neuroimmunophilins. Immunophilins are "receptors" for major immunosuppressant drugs such as cyclosporin A and FK506; these drugs were designed to overcome rejection of organ transplants. The drug-immunophilin complexes bind to target proteins. Immunophilin concentrations in the brain tissues are 10- to 30-fold higher than those in the immune tissues and have been demonstrated to have a neurotrophic effect. The immunophilin ligands show promise as a novel class of neuroprotective and neuroregenerative agents and have the potential to treat a variety of neurologic disorders.

Interaction of neurotrophic factors with serotonergic system of the brain. BDNF and GDNF stimulate the growth of serotonergic (5-HT) neurons and affects the expression of key genes of the brain 5-HT-system, ie, those coding tryptophan hydroxylase-2 and 5-HT1A and 5-HT2A receptors (30). A feedback mechanism autoregulates neurotransmitter (5-HT) neurotrophic factor interactions. Because of differences in action of these factors and variations in levels of their expression in brain regions (BDNF expression is highest in hippocampus and cortex, GDNF expression in the striatum), reaction of 5-HT2A receptors to BDNF and GDNF administration manifests in different types of behavior.

Role of neurotrophic factors in action of antidepressants. Actions of most antidepressants are attributed to the modulation of serotonergic or noradrenergic transmission in the brain. A study suggests that fluoxetine may contribute to normalize the trophic support to neurons in major depression by increasing the expression of specific astrocyte-derived neurotrophic factors such as VEGF and BDNF from astrocytes (01).

Role of neurotrophic factors in brain inflammation. Increased production of neurotrophic factors during brain inflammation can suppress inflammation by switching the immune response to an anti-inflammatory, suppressive mode in a brain-specific environment. This effect is in addition to the protection of axons and myelin and may be beneficial in chronic inflammatory diseases of the CNS. Interleukin-1, a multifunctional cytokine that plays a key role in mediating inflammation in the brain, interacts with the nerve growth factor in the brain and may have a neuroprotective effect by promoting neuron survival.

Effect of exercise on serum neurotrophic factors. In humans, exercise leads to increase of serum neurotrophic factors, which may be an explanation for the beneficial effects of exercise such as improvement of cognition. Studies on pregnant women during late pregnancy and postpartum period have shown that exercise increases the serum concentrations of IGF-1 and BDNF, indicating that it might be a beneficial activity with implications for maternal mood and cognitive performance (34). A study on healthy young adults has shown that high responders to exercise have better high-interference memory performance when combined with cognitive training compared to exercise alone, suggesting that potential synergistic effects might depend on the availability of neurotrophic factors (13). Exercise has been shown to improve cognitive function by increasing hippocampal BDNF as well as by modification of the expression of BDNF receptors. Exercise also increases histone acetylation by histone deacetylases inhibitors, which is also associated with the increase of hippocampal BDNF. Considering the beneficial effects shared by exercise and histone deacetylases inhibitors, it is reasonable to combine the 2 approaches to improve cognitive function in patients with neurologic disorders (22). A study has shown that BDNF concentration was significantly lower in postmenopausal than in premenopausal obese women, and continuous leisure-time physical activity improved body composition as well as neurotrophic factors and alleviated menopausal symptoms including depression and cognitive impairment (16).

Neurotrophic factors and oxidative stress. In degenerative disorders of the central nervous system such as Alzheimer, Parkinson, and Huntington diseases, oxidative stress is linked to the loss of neurotrophic support (09). Neurotrophic factors can upregulate antioxidant enzyme; on the other hand, oxidative stress can cause downregulation of neurotrophic factors.

Neurotrophic factors and epilepsy. Fibroblast growth factor-2 and brain-derived neurotrophic factor have been delivered by a viral vector in the pilocarpine model of status epilepticus-induced neurodegeneration and epileptogenesis (32). Results showed increased neuronogenesis, limited neuronal damage, and reduction in the occurrence of spontaneous seizures. However, the 2 neurotrophic factors have 2 contrasting effects in status epilepticus and in its sequelae; eg, FGF-2 favors seizures but BDNF protects cells from damage (31). A better understanding of the mechanisms of FGF-2 and BDNF will be necessary for the development of therapeutic strategies that are not compromised by paradoxical side effects.

Clinical applications

The clinical applications of neurotrophic factors in neurologic disorders are shown in Table 3 and are discussed in more detail in other clinical summaries within MedLink Neurology. Most of the applications are in neurodegenerative disorders, cerebral ischemia, and injuries of the central nervous system.

Neurotrophic factors may not correct the primary pathology of most of the diseases. However, symptomatic treatment of neurodegenerative disorders by neurotrophic factors or activation of signal transduction pathways downstream of their receptors will retain a place in the treatment of these disorders in the future. Neurotrophic factors have a potential role in regeneration of the nervous system.

The negative results of several clinical trials with neurotrophic factors have revealed that the manner and site of administration of these substances is of critical importance. Improvements in viral vectors enable the targeted delivery of neurotrophic factors while providing long-lasting supplies and adequate therapeutic doses. Methods of delivery of neurotrophic factors are discussed in summaries dealing with applications for various disorders and in the article on drug delivery to the central nervous system.

Table 3. Potential Clinical Applications of Neurotrophic Factors in Neurologic Disorders

Neurotrophic factors as neuroprotective agents. In cell culture and in vivo, several different neurotrophic factors have been shown to prevent death of cortical and hippocampal neurons induced by excitotoxic and oxidative insult (14). Various studies of neurotrophic factors have shown that they have a neuroprotective action and that they enhance cellular systems involved in the maintenance of Ca2+ homeostasis and free radical metabolism. Several other compounds that mimic the action of neurotrophic factors by activating signal transduction cascades involving tyrosine phosphorylation have proven to be beneficial in animal studies of ischemic brain injury and provide opportunities for the development of therapeutic approaches for ischemic stroke. Further studies are required to determine the relative potency of various neurotrophic factors, therapeutic windows, modes of administration, and doses. Neuroprotective effect of neurotrophic factors is described in the development of treatments for stroke, neurodegenerative disorders, and injuries to the central nervous system. A few examples are as follows:

Activity-dependent neuroprotective protein (ADNP). Activity-dependent neuroprotective protein enhances basal glucose and glutamate transport, and it attenuates oxidative impairment of glucose and glutamate transport induced by amyloid beta-peptide. Activity-dependent neuroprotective protein-9 can act locally in synaptic compartments to suppress oxidative stress and preserve function of glucose and glutamate transporters. Such synapto-protective actions indicate possible therapeutic applications of agents that stimulate local synaptic (transcription-independent) neurotrophic factor signaling pathways.

Angiogenesis growth factor (AF)-1. Angiogenesis growth factor-1 is a naturally occurring proprietary small molecule that has now been demonstrated to stimulate regeneration of the spinal cord following partial transection in a manner like that achieved by inosine, another small molecule growth factor. This target appears to be an enzyme within central nervous system neurons that specifically controls axon growth of all central nervous system nerve cells, whether in the brain or in the spinal cord. Activation of this enzyme by inosine and angiogenesis growth factor-1 is apparently sufficient to overwhelm the natural inhibitory factor(s), such as Nogo, that ordinarily prevent nerve regeneration in the central nervous system. Such regeneration had not been achieved to any substantial degree until angiogenesis growth factor-1 and inosine were demonstrated to stimulate significant axonal regrowth in the corticospinal tract.

Vascular endothelial growth factor (VEGF). VEGF mainly promotes angiogenesis and enhances vascular permeability processes that are effective in hypoxia. VEGF also has neurotrophic properties and exerts neuroprotective action directly through the inhibition of apoptosis and the stimulation of neurogenesis.

Basic fibroblast growth factor. Intravenous basic fibroblast growth factor can cross the damaged blood-brain barrier and exert a direct trophic effect on the ischemic brain tissue. The mechanism of infarct reduction may include direct neuroprotective effect. No angiogenesis has been demonstrated in any of the studies, although basic fibroblast growth factor is known to have angiogenic properties. Although the exact mechanisms of the beneficial effect of basic fibroblast growth factor in ischemic stroke have not been proven, delayed administration can enhance functional recovery. The mechanism of action of basic fibroblast growth factor as a neuroprotective agent involves activation of survival signals that enhance antiapoptotic proteins, antioxidant enzymes, and calcium-binding proteins.

Brain-derived neurotrophic factor. Potential mechanisms of the neuroprotective role of brain-derived neurotrophic factor in focal cerebral ischemia include the following:

BDNF secreted from the endothelium participates in the homeostatic interactions between endothelium and cerebral parenchyma and protects neurons against oxygen-glucose deprivation, oxidative damage, endoplasmic reticulum stress, hypoxia, and amyloid neurotoxicity. Growing evidence indicates that neurotrophic factors BDNF and estrogens significantly prevent neuronal damage caused by oxidative stress (26).

Neurotrophic factors and regeneration of the nervous system. Neurotrophic factors are expressed in nonneural cells as well and play a role in tissue regeneration. Neurotrophic factors play important roles in regulating stem cell behavior and promoting tissue regeneration (35). These properties are relevant to regeneration in nervous system, which is further discussed in articles dealing with role of neurotrophic factors in CNS trauma, peripheral nerve lesions, and neurodegenerative disorders. This effect is under investigation in clinical studies.

Neurotrophic factors and pain. Neurotrophic factors that have been extensively studied for their role in pain include the neurotrophins, nerve growth factor, BDNF, and the GDNF. Nerve growth factor (NGF) and some cytokines have been shown to be mediators involved in various pathways in preclinical chronic pain models (04). NGF and interleukin-6 (IL-6) are potential therapeutic targets, and their inhibitors are in clinical trials as pain therapeutics (15). A randomized, placebo-controlled phase 2 clinical trial of tanezumab, a monoclonal antibody directed against NGF for chronic low back pain, was completed (NCT00876187). This was followed by an uncontrolled, randomized, multicenter study to evaluate long-term safety and effectiveness of tanezumab in patients with chronic low back pain (NCT00924664). Results of this trial showed that tanezumab 10 mg had better tolerability than tanezumab 20 mg and may represent an effective long-term treatment for chronic low back pain (10).

In contrast to NGF, GDNF reduces ectopic discharges within sensory neurons after nerve injury by reversing injury-induced plasticity of several sodium channel subunits, thereby providing a rational basis for the use of GDNF as a novel therapeutic agent for neuropathic pain.

Preclinical and clinical findings of the role of neurotrophic factors in musculoskeletal pain and osteoarthritis are accumulating because the musculoskeletal system is the most prevalent source of chronic pain and disability, and clinical testing of neurotrophic agents, mostly biologics, is most advanced in this area (25).

Clinical trials of neurotrophic factors. As of August 2021, the United States government website for clinical trials, ClinicalTrials.gov, lists 268 clinical trials of neurotrophic factors for neurologic disorders. These trials include those that have been completed or terminated as well as those that have not yet started to recruit patients.

Besides testing therapeutic effects of neurotrophic factors, some trials are investigating them as biomarkers as well as determining their role in various disease processes and response to other treatments.

Future for clinical applications of neurotrophic factors. Although considerable work, both clinical and preclinical, has been done on neurotrophic factors, no approved product for use in clinical neurology is available. Attempts to translate the potential of neurotrophic factors to the clinic have been disappointing, largely due to significant delivery obstacles. Challenges in the delivery of neurotrophic factors include routes of administration and design of appropriate delivery vehicles. Discovery of potential biomarkers that identify the right targets and an appropriate design of clinical trials would help to overcome the challenges in clinical translation of these strategies (27). Delivery to the nervous system is improving with advances in cell and gene therapy, and there are a few promising products in clinical trials. Use of small molecules and peptide mimetics as well as combinatory approaches will further facilitate delivery of therapeutics to the brain. One limitation for clinical use has been invasive procedures required for delivery to the CNS, and this limitation is being overcome with development of techniques for targeted delivery to the lesion following systemic administration. One of the promising but clinically untested route of delivery is transnasal, which has been found useful for delivery of therapeutic proteins into the CSF space in the brain. Neurotrophic factors are involved in the mechanism of action of several therapies for neurologic disorders and improved knowledge of neurotrophic factors increases our understanding of rationale of these therapies and improves them.