Description

	• Several intrinsic factors influence regeneration in the CNS.
	• Pharmaceutical as well as biological therapies such as stem cells are used to facilitate, repair, and regenerate the damaged nervous system.

Primary sensory neurons with cell bodies in the dorsal root ganglia have 2 branches: (1) a peripheral axon that regenerates itself if injured; and (2) a central axon that enters the CNS and does not regenerate after injury. The local environment of these branches explains the difference in regeneration: The peripheral axon contains Schwann cells, whereas the central axon contains oligodendrocytes and astrocytes. In the peripheral nervous system, myelin debris is cleared promptly, and Schwann cells dedifferentiate and downregulate expression of the myelin protein, thus, facilitating regeneration. The composition and organization of extracellular matrix in CNS lesions is a hindrance to regeneration.

Intrinsic factors that influence regeneration in the CNS. Various intrinsic factors that modulate regeneration in the CNS are listed in Table 1 and described in the following text. Neurotrophic factors are the most important of all the factors influencing regeneration.

Table 1. Factors that Influence Regeneration in the Central Nervous System

Factors	Role in regeneration and recovery
Neurotrophic factors	• Cell survival, axon growth-cone stimulation, synapse regeneration.
Neuroprotective gene expression	• Expression of genes such as Bcl-2 and c-fos/jun may occur within minutes of an acute brain injury and are a determinant of eventual recovery.
Neural stem cells	• Neural stem cells can migrate to the site of injury in the brain and participate in regeneration.
Oligodendrocytes	• Oligodendrocyte precursor cells in CNS are activated in response to myelin injury, migrate to the site of damage, and differentiate into mature oligodendrocytes for remyelination; failure of this process leads to demyelination, which requires repair (06).
Cadherins	• These are involved in synaptogenesis in the CNS.
Intracellular levels of cyclic nucleotide in the neurons	• These influence the capacity of mature CNS neurons to initiate and maintain a regrowth response.
Innate immune system, represented by activated macrophages	• This can facilitate the processes of regeneration in the severed spinal cord.
Inducible nitric oxide synthase	• This is not usually present in the brain but can be detected in the brain following injury and may be required for adequate repair.
Activin	• Strong expression of activin is seen in repair processes of the brain and may have a role in neuroprotection. Although a transient overexpression of activin after tissue injury might be beneficial for the repair process, sustained expression of activin could be detrimental to regeneration.
Glial scar	• Physical barrier to regeneration apart from being the source of various neurite outgrowth inhibitors.
Neurite outgrowth inhibitors	• These are found in the gliotic tissue and inhibit regeneration.
Humoral autoantibodies	• Usually pathogenic in nature, humoral autoantibodies can also promote tissue repair following central nervous system injury and disease.
Enriched environments	• Facilitate regeneration of brain in experimental animals

Causes of lack of regeneration in the CNS. The CNS regenerative process is unsuccessful for 3 reasons: (1) neurons are highly susceptible to death after CNS injury; (2) multiple inhibitory factors in the CNS environments hinder regeneration; and (3) the intrinsic growth capacity of postmitotic neurons is reduced. Research is providing an insight into these areas and will form the basis of strategies to promote regeneration of the CNS.

Role of glial cells in CNS injury and regeneration. Glial cells consist of microglia, which have a phagocytic function, and macroglia (astrocytes and oligodendrocytes). Astrocytes provide structural, trophic, and metabolic support to neurons and modulate synaptic activity. Therefore, impairment of astrocyte functions in brain injuries can compromise neuron survival. Functions of astrocytes that are known to influence neuronal survival include glutamate metabolism, free-radical scavenging, and the production of cytokines and nitric oxide. Neuron regeneration after injury is influenced by the release of neurotrophic factors by astrocytes. Degenerative disorders may affect both neurons and glial cells; the latter may contribute to impairment of learning and memory. Therapeutic approaches to neurodegenerative disorders should be aimed at restoring the functions of both neurons and glial cells.

To overcome the inhibitory environment of the glial scar, treatments should enhance the ability of neurons to elongate and manipulate the extrinsic inhibitors that block growth in the immediate environment of the glial scar. This combined approach may induce functional regeneration after CNS injury.

Astroglia can be directly converted into the 2 main classes of cortical neurons, excitatory as well as inhibitory, by the selective transduction of transcription factors, specific proteins that regulate the transcription of DNA. This approach may provide new therapies for neurodegenerative diseases.

There is a crosstalk between endothelial progenitor cell and reactive astrocytes, which can release a damage-associated molecular-pattern molecule called high-mobility-group-box-1 that promotes endothelial progenitor cell-mediated neurovascular remodeling during recovery after stroke and brain injury (13).

Role of neurotrophic factors in neuronal regeneration. The role of neurotrophic factors during neuronal regeneration differs little from their role during neuronal development in the expression of cytoskeletal genes or cellular protein synthesis, suggesting that regulatory events during regeneration recapitulate the patterns found during development.

An experimental study in mouse models has shown that plasma membrane protein, called Efr3, regulates brain-derived neurotrophic factor-tropomyosin-related kinase B signaling pathway (BNDF-TrkB) and affects the generation of new neurons in the hippocampus of adult brains (28). In turn, this generation of new neurons plays a significant role in learning and memory.

Nerve growth factor. Within the central nervous system, the main neuronal system regulated by nerve growth factor is that of basal forebrain cholinergic neurons, which send topographically organized projections to the hippocampus and cerebral neocortex. Because of their involvement in neurodegenerative disorders such as Alzheimer disease, many studies have looked at the effect of nerve growth factor on these neurons. Cholinergic neurons receive nerve growth factor from the cortex and hippocampus where the neurotrophin is synthesized.

Regeneration in the adult mammalian central nervous system has been viewed pessimistically in the past. Rapid progress of concepts and tools in developmental biology has been applied to approach the questions of regeneration. Important aims are cell survival, reinitiation of axon growth, target finding, and formation of functional connections. Significant developments include the availability of recombinant neurotrophic factors and stem cells for repair of the nervous system.

Neurotransmitters and synapse regeneration. Synapses are the final common pathway for information exchange in the nervous system. They mediate a wide range of activities from a simple reflex arc to learning and memory. Synapse formation plays an important role in neuronal regeneration and survival. The presynaptic and postsynaptic parts of the synapse are separated by a synaptic cleft. At the neuromuscular junction, a specialized extracellular matrix known as the synaptic basal lamina occupies this cleft. Neurotransmitters such as acetylcholine or glutamate are released from the presynaptic vesicles, traverse the cleft, and bind to their receptors on the postsynaptic membrane. The signal is terminated by the reuptake or enzymatic destruction of the neurotransmitters. Synapses are formed by the expression of specific gene products such as synaptic vesicle proteins and neurotransmitter receptors. This process is regulated to some extent by a combination of neurotrophic factors and electrical activity.

Role of stem cells in regeneration. Neural stem cells migrate through the parenchyma along various routes in a precise, directed manner across great distances to injury sites in the central nervous system, where they might engage niches harboring local transiently expressed reparative signals. Activation of endogenous neural stem cells is being investigated along with stem cell transplantation for regeneration of the injured spinal cord.

Mesenchymal stem cells secrete several neuroregulatory factors that promote neurogenesis, inhibition of glial scar formation, and neuroprotection, all of which improve the integration of local progenitor cells in neuroregeneration processes (33). Mesenchymal stem cells have potential clinical application for neuroregeneration.

Cadherins. These are found in the synaptic cleft near the transmitter release zone and may provide a molecular basis for the adhesive interactions between opposing synaptic membranes. Thus, they play a role in the formation and maintenance of synapses. Cadherins might directly regulate cell signaling to modulate synaptic connectivity.

Glial scar. A glial scar containing extracellular matrix molecules including chondroitin sulfate proteoglycans develops at the site of injury and prevents axonal regeneration. Degradation of chondroitin sulfate proteoglycans after spinal cord injury by application of chondroitinase ABC to the lesioned dorsal columns of adult rats promotes regeneration of corticospinal tract axons.

Chondroitin sulfate proteoglycans also have multiple inhibitory actions mediated by the protein tyrosine phosphatase sigma receptor that result in incomplete remyelination following CNS injury and provide potential therapeutic targets to enhance white matter repair in the damaged CNS (27). An experimental study has demonstrated that activity of the Wnt/β-catenin pathway in fibroblast-like cells in the lesion site is pivotal for axon regrowth and functional recovery by inducing expression of col12a1a/b and deposition of collagen XII, which is necessary for axons to actively navigate the nonneural lesion site environment (38). Transplantation of Wnt3a-secreting fibroblasts into a spinal lesion site in rats enhances functional recovery and provides strategies to modulate the composition of the lesion site to tip the balance towards a growth-conducive extracellular matrix environment.

The classical view that a glial scar hinders regeneration has been challenged by genetically targeted loss-of-function manipulations in adult mice, which show that prevention or ablation of astrocyte scar failed to result in spontaneous regrowth of transected axons in spinal cord injury lesions. Local delivery of axon-specific growth factors missing in spinal cord injury lesions stimulates axon regrowth past scar-forming astrocytes, and prevention of astrocytic scar formation significantly reduces this stimulated axon regrowth as cells in these lesions express multiple molecules to support axon regrowth (02).

Neurite outgrowth inhibitors. Various growth inhibitors are found in a glial scar. These include the following:

• Myelin-associated inhibitors of axonal regeneration. • Astrocytes produce tenascin, brevican, and neurocan. • Meningeal cells produce NG2 and other proteoglycans. • Activated microglia produce free radicals, nitric oxide, and arachidonic acid derivatives.

Three inhibitors of axonal regeneration have been identified in myelin: Nogo, myelin-associated glycoprotein, and oligodendrocyte myelin glycoprotein. All these proteins induce growth cone collapse and inhibit neurite outgrowth. These inhibitors and their receptors can be blocked with antibodies or peptides to facilitate regeneration. Some of the specific antagonists are commercially available for experimental investigation. The combination of anti-Nogo-A, chondroitinase ABC, and rehabilitation shows promise for enhancing functional recovery after spinal cord injury (47).

Humoral autoantibodies. Autoimmune responses directed against the central nervous system are generally considered pathogenic in nature, but autoreactive antibodies can also enhance endogenous myelin repair. All of the antibodies that promote remyelination bind to antigens on the surfaces of oligodendrocytes, suggesting that these antibodies might function through direct stimulation of the myelin-producing cells. An understanding of these mechanisms should open significant new areas for the development of antibody-based therapeutics and perhaps also for small-molecule-based therapeutics and vaccines for induction of the reparative response.

Various factors that influence plasticity in the CNS. Plasticity in the CNS following injury or damage from a pathological process is influenced by several factors that include age of the subject as well as the nature and severity of the pathological process.

Age. The brain possesses a certain degree of biological plasticity that diminishes with age. It can be maintained unless a progressive neurodegenerative disease or severe damage to the brain occurs.

Location of lesions. There is less room for plasticity in spinal cord lesions than in the cerebral hemispheres.

Type of lesion. Cerebral ischemia has a better scope of regeneration and recovery than progressive neurodegenerative diseases do. Another determining factor is the secondary damage that results from ischemic and traumatic lesions of the central nervous system, which may reduce the plasticity recovery by enlarging the extent of damage by the original lesion. Plasticity may not be able to keep up with progressive CNS degeneration. In diseases such as Alzheimer, the clinical manifestations may become obvious when decompensation of plasticity occurs.

Neurotrophic factors and cortical plasticity. The adult cortex undergoes plastic changes that are dependent on neuronal activity. Neurotrophins and their receptors play a role in this plasticity. Rapid and opposing effects of brain-derived neurotrophic factor and nerve growth factor on the functional organization of the adult cortex in the rat indicate that neurotrophins can modulate stimulus-dependent activity in the adult cortex. They also suggest a role for neurotrophins in regulating adult cortical plasticity. Current knowledge enables some manipulation of plasticity and the induction of functional changes beneficial for vision.

Molecular basis of axonal plasticity. GAP-43 plays an important role in axonal plasticity by guiding growth cones rather than supporting axonal elongation. The molecule GAP-43 is key to initiating axon growth, whereas other genes are necessary to develop a full regenerative program. Addition of GAP-43 gene can induce the formation of branched plexuses typical of sprouting growth.

Approaches to regeneration and repair of the CNS. Several methods are being used to facilitate regeneration and repair of the CNS. These include use of biological therapies such as stem cells, gene therapy, vaccines, and antibodies. Other approaches include nanobiotechnology, application of electromagnetic fields, optogenetics, pharmaceuticals, and hyperbaric oxygenation. Tissue engineering may involve delivery of encapsulated cells and biomolecule delivery in polymeric nano/microspheres and hydrogels for the nerve regeneration (08). Stem cells may be combined with other biomolecules. Stimulation of neurovascular remodeling by enhancing angiogenesis, neurogenesis, oligodendrogenesis, and axonal sprouting—all acting in concert—may improve functional recovery after traumatic brain injury (44).

Biological therapies to facilitate CNS regeneration are shown in Table 2, and pharmaceutical approaches are shown in Table 3.

Table 2. Biological Therapies for CNS Regeneration

Transplantation of living biological scaffolds Cells therapy using non-neural somatic cells Stem cell therapy
	• Embryonic stem cells (ESCs) • Adult stem cells: hematopoietic stem cells (HSCs), mesenchymal stem cells (MSCs) • Induced pluripotent stem cells (iPSCs) • Neural stem cells (NSCs)
Retinal progenitor cells (RPCs) Gene therapy
	• Vector-mediated gene transfer for delivery of neurotrophic factors • Transplantation of genetically modified cells to release neurotrophic factors
Vaccines
	• Vaccine against inhibitors of neurite outgrowth after spinal cord injury • Immunotherapy to inhibit inflammatory response to spinal cord injury

Table 3. Pharmaceutical Approaches to Facilitate Regeneration of the CNS

	• Anti-Nogo-A antibodies • Rho-ROCK inhibitors
Glial scar inhibitors
	• Local application of chondroitinase at the site of injury
Axon guidance molecules Pharmacological modulation of the signal transduction pathways
	• Cyclic AMP enhancers • Inhibitors of the phosphoinositide 3-kinase pathway • Inhibitors of the inositol triphosphate receptor • Nitric oxide-cyclic guanosine-monophosphate transduction pathway
Agents that promote regeneration
	• Bone morphogenetic protein 7 • Immunophilin ligands • Neurotrophic factors • Retinoic acid • Simvastatin
Agents that improve remyelination
	• Fampridine • Monoclonal antibodies
Drugs that mobilize intrinsic stem cells Drug combinations with devices and biological therapies
From (16)

Transplantation of living biological scaffolds. Microtissue engineering has enabled construction of transplantable hydrogel-collagen scaffolds that mimic the glial tubes, which guide neuronal progenitor cell migration in vivo along regenerative pathways and have the potential for central nervous system repair (40). Use of biomaterials can not only increase the survival of grafts and their integration with the host in the injured CNS, but also facilitate delivery of therapeutic bioproducts to the injured area (10). A phase I trial to determine the safety and efficacy of functional neural regeneration collagen scaffold transplantation in complete acute spinal cord injury patients is in progress in China (ClinicalTrials.gov identifier: NCT02510365).

Cell therapy for CNS repair. Cell therapy for CNS disorders involves the use of cells of neural or non-neural origin to replace, repair, or enhance the function of the damaged nervous system and is usually achieved by transplantation of the cells, which are isolated and may be modified, eg, by genetic engineering, when it may be referred to as gene therapy.

The olfactory ensheathing cells play an important role in CNS regeneration. In clinical trials, olfactory ensheathing cells have produced some of the most promising results, including a functional recovery in humans following CNS injury (19).

Stem cells for CNS repair. Stem cells are involved both in endogenous repair and in proposed therapies for human neurodegenerative diseases, multiple sclerosis, and spinal cord injury (39).

Stem cell-based cellular replacement strategies have a potential therapeutic role following traumatic brain injury, but the mechanism by which stem cells produce their effect (eg, via integration into surviving neuronal circuits, local neurotrophic support, or modification of the local microenvironment to enhance endogenous regeneration and neuroprotection) remains to be assessed further. Trials gauge the safety and potential of treating children suffering from traumatic brain injury using intravenous hematopoietic stem cells derived from their own bone marrow were based on laboratory and animal research indicating that hematopoietic stem cells can migrate to an injured area of the brain, differentiate into new neurons and support cells, and induce brain repair. Results showed that autologous bone marrow–derived hematopoietic stem cell therapy reduces the treatment intensity required to manage intracranial pressure and corroborates preclinical data that it attenuates the effects of inflammation in the early posttraumatic brain injury period (23).

Progress in stem cell biology has made it feasible to induce the regeneration of injured axons after spinal cord injury in experimental animals by transplanting neural stem cells. Neural stem cells generated from the embryonic stem cells can be developed into motor neurons by using special techniques and culture media with growth factors. In an open trial, intravenous injection of autologous bone marrow cells in conjunction with the administration of granulocyte macrophage-colony stimulating factor led to improvement in patients with complete spinal cord injury. Cell transplantation alone may not suffice for regeneration of the spinal cord and may need to be combined with other methods such as neurotrophic factors, blocking of inhibitors of neural regeneration, and modulation of inflammatory response following injury.

Human mesenchymal stem cells are easily obtained from adipose tissue and have neuronal differentiation potential similar to human bone marrow-derived stem cells, but with a better proliferation capacity that is an advantage for regeneration of the CNS (12).

A nonrandomized, single group, open-label, phase I clinical trial to evaluate the safety and efficacy of intrathecal delivery of autologous bone marrow mesenchymal stem cells for the treatment of spinal cord injury was completed in 2016 in Pakistan, but the results have not yet been published (ClinicalTrials.gov identifier: NCT02482194).

Human pluripotent stem cells (hPSC), with their capacity to be differentiated from almost all cells of the body, have a better translational potential than other types of cells. Combination of hPSCs with gene editing technologies for modification, eg, reduction of immunogenic response, has enabled promising clinical trials that will eventually demonstrate their therapeutic potential in tissue regeneration (01). Application of hPSCs for regeneration in neurologic disorders is still in animal experimental stage, but clinical trials in age-related macular dystrophy are promising.

Vaccines for neuroregeneration. Inability of neurons and axons to regenerate following injury to the nervous system is due mostly to the presence of myelin and oligodendrocyte-related inhibitors of neurite outgrowth. A vaccine-based approach can be used to circumvent this issue and promote axonal regeneration and repair following traumatic injury and other neurodegenerative disorders.

Gene therapy approaches for repair of CNS injuries. Neuronal regeneration can be induced by transgenic integrin expression. Integrins are nerve cell receptors that have been linked to the growth of nerve cells. Nerve cells taken from developing animals typically have high levels of integrins compared with those taken from adult animals. In experimental studies, the regenerative performance of adult neurons can be restored to that of young neurons by the gene transfer-mediated expression of a single alpha-integrin.

Gene therapy has the potential to overcome many of the difficulties associated with the delivery of antiscarring and neurotrophic substances to the site of an injury. Suitable and safe vectors for the delivery of genes need to be developed. Although there are several obstacles to making gene therapy practical and effective in humans, it has the potential to provide a different approach to the treatment of traumatic brain injury.

Pharmaceutical approaches. Examples of these are:

Simvastatin. In experimental studies, simvastatin was shown to enhance angiogenesis in the lesion boundary zone and hippocampus, which may occur in response to traumatic brain injury, and improve functional recovery (42). These beneficial effects on angiogenesis may be related to simvastatin-induced activation of the vascular endothelial growth factor receptor-2/Akt/endothelial nitric oxide synthase signaling pathway.

Carbon monoxide. Carbon monoxide is an endogenous biological molecule that transiently upregulates HO-1 expression and facilitates cellular communications in signaling pathways. It is nontoxic in healthy individuals and induces CNS regeneration by stimulating endogenous neural stem cells and endothelial cells to produce neurotrophic factors. This regenerative potential of carbon monoxide leads to the formation of functional adult neural circuits in CNS injury, multiple sclerosis, and Alzheimer disease (18).

Nanobiotechnology for regeneration and repair of the CNS. Nanotechnology is the creation and utilization of materials, devices, and systems through the control of matter on the nanometer (one billionth of a meter) scale. Nanobiotechnology is the application of nanotechnology in biotechnology leading to the development of nanomedicine (15). Various nanomaterials have been designed to self-assemble into nanofibers and provide the framework for regeneration of nerve fibers in experimental studies on animal models of spinal cord injury. In a nanofiber network, progenitor cells develop into neurons, not astrocytes, which form scar tissue and hinder regeneration. Silica or chitosan nanoparticles facilitate repair and functional recovery in spinal cord injury from breaches in neural membranes via rapid sealing and reassembly of the compromised (04).

Role of enriched environments. Considerable published evidence shows that living in an enriched environment alters dendrites and synapses in the brains of adult rodents. Studies on adult primates show that the brain is highly sensitive to experiential complexity, and living in laboratory housing may induce reversible decreases of synapses in brain regions important for cognition. Currently, enriched environment is the single most efficient plasticity and regeneration promoting paradigm. This has important implications for neurorehabilitation.

Role of electrical fields in CNS regeneration. Electrical fields have been generated over the brain and the spinal cord for diagnostic and therapeutic purposes. Cranial electrotherapy or transcranial direct current stimulation (tDCS) and repetitive transcranial magnetic stimulation (rTMS), ie, noninvasive stimulation of the cerebral cortex using externally applied magnetic fields, are used for treatment of neurologic disorders.

Exogenous application of electrical fields to cultured neurons to supplement endogenous results in enhanced sprouting of neurites and directed growth along the fields. This is the basis for suggesting the use of electrical fields in a regenerative therapeutic setting (12). The mechanisms of effect of electrical field are not well understood. However, the likely effect is the stimulation of release and/or production of various neurotrophic factors with AC stimulation, providing a nonspecific supportive environment for the regeneration of nervous cells. In contrast, direct current can provide directional attractive cues for regeneration, alter gene expression, and accelerate reinnervation. Axonal regeneration and improved quality of life may be achieved in spinal cord injury by using electrical field stimulation. A phase I clinical trial of human oscillating field stimulator in patients with spinal cord injury has shown that it is safe, reliable, and easy to use. The stimulation provides significant improvement in sensation and improved motor scores in some cases compared to historical data for untreated patients. A physiological level of electric stimulation can be used to control directional recruitment of neural stem cells in a spinal cord slice culture model, suggesting potential for clinical use of electrical fields to optimize stem cell grafting in vivo for CNS injuries (09).

rTMS and tDCS show promise for repairing injured neural circuits. rTMS presents a unique opportunity to modulate brain excitability and plasticity in a precisely controlled manner, but there is need for determining how rTMS can be applied following neurotrauma to promote regeneration and rehabilitation of neural circuits (30).

Optogenetics for CNS repair. After axonal injury, the conserved second messenger cyclic adenosine monophosphate (cAMP), which is produced by an enzyme called adenylyl cyclase, has the capacity to promote axonal regrowth but pharmacological strategies have failed to activate it. An experimental study has shown that this can be achieved by optogenetics, ie, light inducible protein activation using a special form of adenylyl cyclase to modulate the production of cAMP in cells expressing this enzyme by the use of blue light (43). This optogenetic method can promote the selective regeneration of refractory axons in a living vertebrate.

Clinical applications

	• Various neurophysiological methods, clinical neuropsychological assessments, and brain imaging studies can be used for assessing regeneration, recovery, and plasticity in the CNS.
	• The knowledge gained from study of regeneration and repair in the CNS can be used for treatment of TBI, SCI, stroke, and neurodegenerative disorders.

Methods of assessment of regeneration and plasticity. These include the following:

Neurophysiological techniques. These techniques are useful for evaluating spontaneous recovery from damage and the therapeutic benefits of training, as well as other therapies. Transcranial magnetic stimulation has been used for this purpose.

Brain imaging techniques. Positron emission tomography and functional magnetic resonance imaging can be used to monitor the recovery and plasticity of the brain following injury. PET has been used to demonstrate changes in the activation of cortical and subcortical brain areas in response to altered spinothalamic and spinocerebellar input in paraplegic patients. These techniques have also been used to map clinically relevant plasticity after a stroke.

Applications of the knowledge of regeneration, repair, and plasticity in the CNS. Various clinical situations in which knowledge of regeneration, repair, and plasticity of CNS can be used are listed in Table 4. Spontaneous partial myelin repair in multiple sclerosis is an example of the inherent capacity of the CNS for regenerative tissue repair. This repair is limited, however, and the disease progresses with resulting disability. Study of these mechanisms also provides an insight into strategies that may be used to promote regeneration in this disease.

Table 4. Potential Applications of the Knowledge of Regeneration, Repair, and Plasticity of the CNS

• For devising therapies for neurodegenerative diseases • For improving neural transplant procedures for Parkinson disease • Repair strategies for demyelinating diseases • For neuroprotection • For promoting regeneration in CNS trauma: spinal cord injury, traumatic brain injury • For restoring function following a stroke • Use of neuroplasticity for neurorehabilitation

Neuroprotection and neuroregeneration. Neuroprotection is required in the acute phase, and neuroregeneration is the long-term goal for restoring function following CNS injury, stroke, or neurodegenerative diseases. No clear-cut line of demarcation exists between neuroprotection and neuroregeneration. The measures for achieving these 2 goals overlap in accordance with the generally accepted concept that rehabilitation should start in the acute phase of neurologic disease. No acute phase is identifiable in some chronic neurologic disorders with an insidious onset and neuroregeneration supplements neuroprotective strategies to counteract the progressive deterioration of neurologic function. Cell transplants and gene therapy methods for regeneration of the CNS may not be practical for application in the acute phase of injury, but both can have a long-term neuroprotective function.

Repair strategies for demyelinating diseases. Myelin disorders form an important group of neurologic disorders with no known cure. Experimental studies indicate that it is feasible to repair the demyelinating lesions either by enhancing endogenous repair or by transplanting appropriate cells (see separate article on cell therapy for neurologic disorders). Future therapies could involve both transplantation and promotion of endogenous repair. It is also possible to combine these with ex vivo manipulation of donor tissue.

Spinal cord injury. Significant spontaneous functional recovery may occur over several years following incomplete spinal cord injuries. Possible mechanisms involved are synaptic plasticity in preexisting pathways and the formation of new circuits through collateral sprouting of lesioned as well as intact fibers. Some evidence shows that plasticity can be facilitated by activity or experimental manipulations. These studies form a basis for the development of new rehabilitation approaches for spinal cord injury.

Several approaches being pursued for acute spinal cord injury with complete functional transection include the following:

	• Neuroprotection. Various strategies for neuroprotection are described in the separate article on neuroprotection for central nervous system disorders.
	• Careful modulation of the inflammatory response following trauma as it is both beneficial and detrimental to recovery of function.
	• Repair by transplanting cells (see separate article on cell therapy for neurologic disorders).
	• Bridging the injured area by peripheral nerve transplantation.
	• Manipulation of the extracellular matrix composition as a measure to provide a supportive environment for sprouting and regenerating neurons and to reduce glial scarring.
	• Synthetic gel implants to support nerve regrowth after spinal cord injury. The implants could serve as substrates for neurite outgrowth and synapse formation in tissue engineering of the spinal cord injury. Biodegradable polymer grafts may have significant therapeutic potential in the surgical repair of the injured spinal cord.
	• Multiple channel poly(lactide-co-glycolide) bridges support cell infiltration, create a favorable environment for spinal cord regeneration, and direct the growth of neural fibers (45).
	• Vaccines to stimulate nerve regeneration.
	• Anti-Nogo-A antibodies for neutralizing the inhibitory effect of Nogo-A have been shown to enhance fiber growth, regeneration, and functional recovery in primate models of spinal cord injury and are being considered for development as therapeutics for human spinal cord injury. Ablation of NgR1 gene, which is responsible for the growth restrictive environment, may lead to significant recovery in locomotor function following experimental spinal cord injury (36).
	• Artemin is a naturally occurring growth factor, which is expressed on both myelinated and unmyelinated sensory neurons, consistent with artemin’s ability to promote regeneration of large and small sensory neurons. In experimental studies, systemic treatment with artemin is shown to promote regeneration of sensory axons away from their site of interruption in dorsal roots to the brainstem where they reestablish functional connections (41). Artemin is a potential therapy for restoring sensory function after spinal cord injury.
	• An experimental study showed that the regrowth potential of corticospinal tract axons is lost in the fully grown mouse, and this is accompanied by downregulation of mTOR activity in corticospinal neurons, which further diminishes following axonal injury (24). Forced upregulation of mTOR activity in these experiments by deletion of PTEN, a negative regulator of mTOR, enables successful regeneration of corticospinal axons past a spinal cord lesion.
	• Neuroprosthetic applications. Neuroprostheses are machines designed to artificially restore lost neurologic function. Emphasis is on development of neuroprosthetic devices that utilize the information recorded directly from the CNS. Development of neurorobots for simple walking movements may require elaborate systems for timed interaction between sensory input and rhythmic motor output programs.
	• A porous bioresorbable polymer scaffold has been implanted in a spinal cord injury patient during a pilot study testing the safety and feasibility of this technique (clinicaltrials.gov NCT02138110). At 6-month postoperative follow-up, there were no procedural complications or apparent safety issues related to the scaffold implantation (34). This is the first human implantation, and future outcomes of other patients in this clinical trial will further assess the safety and possible efficacy of the scaffold.

Traumatic brain injury. Traumatic brain injury or neurosurgical procedures may cause extensive loss of cerebral parenchyma. Reconstruction and regeneration is desirable, not only to replace the lost brain substance, but also to restore lost function and prevent formation of scar tissue. However, no clinically effective method is available as yet. Cell therapy is expected to play an important role in the repair of traumatic brain injury (14). It is important that cells are transplanted into an environment that is favorable for extended survival and integration within the host tissue. Extracellular matrix proteins such as fibronectin and laminin are involved in neural development and may mediate subsequent cell signaling events. Enhanced cell survival was demonstrated following transplantation of a neural stem cell construct containing laminin-based scaffold into the traumatically injured mouse brain.

The peptide nanofiber scaffold is an effective technology for tissue repair and restoration and is a promising treatment for traumatic brain injury. This peptide nanofiber scaffold has several advantages over currently available polymer biomaterials. The network of nanofibers is similar in scale to the native extracellular matrix and, thus, provides an environment for cell growth, migration, and differentiation. This peptide disintegrates and is immunologically inert. Self-assembling peptide nanofiber scaffold may help to reconstruct the acutely injured brain and reduce the glial reaction and inflammation in the surrounding brain tissue (37). Gene therapy, by selecting appropriate gene targets and promoting neuronal regeneration, may also prove to be effective in the repair of traumatic brain injury.

Stroke. The aim of regenerative strategies following stroke is to repair the damage to the brain and extends beyond that of neuroprotection. These can be applied weeks or months after the stroke. Therapies under investigation for neuroregeneration following stroke include stem cells, neurotrophic factors, pharmaceuticals, anti-Nogo-A antibodies, CCR5 gene deletion, and electromagnetic stimulation. Intensive physical therapy can improve motor recovery following stroke through development of plasticity.

Application of an immune-modulating angiogenic biomaterial directly to the stroke cavity promotes tissue formation de novo and results in axonal networks along the generated blood vessels, resulting in regenerated tissue that produces functional recovery (26). An experimental study has shown that genetic deletion of the neurite outgrowth inhibitor Nogo-A or one of its corresponding receptors, S1PR2, improves vascular sprouting in the periinfarct zone, which correlates with tissue recovery and reduction of neurologic deficits after cerebral ischemia in mice (31). These findings have been reproduced in a therapeutic approach using intrathecal anti–Nogo-A antibody ATI355, which was shown to be safe in a clinical trial on patients with spinal cord injury (20).

Combination therapies in animal models for regeneration in stroke include other methods besides cell transplantation. Mechanisms underlying the beneficial effects of transplanted cells include the "bystander" effects, paracrine mechanisms, or extracellular vesicles-mediated restorative effects (32). Mitochondria transfer also appears to be a powerful strategy for regenerative processes. Studies in humans are currently limited to a small number of studies using autologous stem cells mainly aimed to assess tolerability and side effects of human stem cells.

CCR5 gene, which is uniquely expressed in cortical neurons after stroke, plays a role in recovery after stroke and traumatic brain injury because poststroke neuronal knockdown of CCR5 in premotor cortex leads to early recovery of motor control. Administration of Maraviroc, a CCR5 antagonist approved by the FDA for HIV treatment, produces similar effects on poststroke motor recovery, and in a clinical trial on stroke patients, carriers for a naturally occurring loss-of-function mutation in CCR5 showed greater recovery of neurologic deficits and cognitive function (17). CCR5 is the first gene reported to be associated with enhanced recovery in human stroke and is a translational target for neural repair in stroke and traumatic brain injury.

Although glial cells can be converted into neurons, the total number of neurons generated is limited and the therapeutic potential is unclear. NeuroD1-mediated in situ astrocyte-to-neuron conversion can regenerate a large number of functional new neurons after ischemic injury. Specifically, using NeuroD1 adeno-associated virus-based gene therapy, a study was able to regenerate one third of the total lost neurons caused by ischemic injury and simultaneously protect another one third of injured neurons, leading to a significant neuronal recovery (03). RNA sequencing and immunostaining confirmed neuronal recovery after cell conversion at both the mRNA level and protein level. Brain slice recordings found that the astrocyte-converted neurons showed robust action potentials and synaptic responses at 2 months after NeuroD1 expression. Anterograde and retrograde tracing revealed long-range axonal projections from astrocyte-converted neurons to their target regions in a time-dependent manner. Behavioral analyses showed a significant improvement of both motor and cognitive functions after cell conversion. These results demonstrate that in vivo cell conversion technology through NeuroD1-based gene therapy can regenerate a large number of functional new neurons to restore lost neuronal functions after injury.

Blindness due to damage to optic pathways. Damage can occur to retinal ganglion cells, which connect the eyes to the brain, and failure of regeneration leads to blindness. Certain types of neurons respond to optic nerve crush by maintaining their mammalian target of rapamycin (mTOR) signaling activation, which provides an opportunity for regenerative capacity of axons of injured neurons. An experimental study found that the light-responsive G protein-coupled receptor (GPCR) melanopsin could promote axonal regeneration after optic nerve crush by activating mTOR (22). This provides a strategy to promote axon regeneration after injury by modulating neuronal activity through GPCR signaling. Thus, new technologies have the potential to enable regeneration/repair of the optic system by reactivation of intrinsic developmental growth programs in retinal ganglion cells to enhance their regeneration, and reformation of functional eye-to-brain connections, even in the adult brain (21). Retinal ganglion cell transplantation and gene therapy with light-activated channels may serve to replace or resurrect dead or injured retinal neurons. Introduction of light-sensitive ion-gated channels to restore light sensitivity can drive retinal ganglion cell firing in response to light. Retinal prostheses that can restore vision in animal models may be applicable in the human clinical setting. Restoration of sight in some forms of blindness is feasible in human patients in the future.