Neuroregeneration

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Neuroregeneration refers to the regrowth or repair of nervous tissues, cells or cell products. Such mechanisms may include generation of new neurons, glia, axons, myelin, or synapses. Neuroregeneration differs between the peripheral nervous system (PNS) and the central nervous system (CNS) by the functional mechanisms involved, especially in the extent and speed of repair. When an axon is damaged, the distal segment undergoes Wallerian degeneration, losing its myelin sheath. The proximal segment can either die by apoptosis or undergo the chromatolytic reaction, which is an attempt at repair. In the CNS, synaptic stripping occurs as glial foot processes invade the dead synapse. [1]

Contents

Nervous system injuries affect over 90,000 people every year. [2] It is estimated that spinal cord injuries alone affect 10,000 each year. [3] As a result of this high incidence of neurological injuries, nerve regeneration and repair, a subfield of neural tissue engineering, is becoming a rapidly growing field dedicated to the discovery of new ways to recover nerve functionality after injury. The nervous system is divided into two parts: the central nervous system, which consists of the brain and spinal cord, and the peripheral nervous system, which consists of cranial and spinal nerves along with their associated ganglia. While the peripheral nervous system has an intrinsic ability for repair and regeneration, the central nervous system is, for the most part, incapable of self-repair and regeneration. There is currently no treatment for recovering human nerve function after injury to the central nervous system. [4] In addition, multiple attempts at nerve re-growth across the PNS-CNS transition have not been successful. [4] There is simply not enough knowledge about regeneration in the central nervous system. In addition, although the peripheral nervous system has the capability for regeneration, much research still needs to be done to optimize the environment for maximum regrowth potential. Neuroregeneration is important clinically, as it is part of the pathogenesis of many diseases, including multiple sclerosis.

Peripheral nervous system regeneration

Guillain-Barre syndrome - nerve damage Guillain-barre syndrome - Nerve Damage.gif
Guillain–Barré syndrome – nerve damage

Neuroregeneration in the peripheral nervous system (PNS) occurs to a significant degree. [5] [6] After an injury to the axon, peripheral neurons activate a variety of signaling pathways which turn on pro-growth genes, leading to reformation of a functional growth cone and regeneration. The growth of these axons is also governed by chemotactic factors secreted from Schwann cells. Injury to the peripheral nervous system immediately elicits the migration of phagocytes, Schwann cells, and macrophages to the lesion site in order to clear away debris such as damaged tissue which is inhibitory to regeneration. When a nerve axon is severed, the end still attached to the cell body is labeled the proximal segment, while the other end is called the distal segment. After injury, the proximal end swells and experiences some retrograde degeneration, but once the debris is cleared, it begins to sprout axons and the presence of growth cones can be detected. The proximal axons are able to regrow as long as the cell body is intact, and they have made contact with the Schwann cells in the endoneurium (also known as the endoneurial tube or channel). Human axon growth rates can reach 2 mm/day in small nerves and 5 mm/day in large nerves. [4] The distal segment, however, experiences Wallerian degeneration within hours of the injury; the axons and myelin degenerate, but the endoneurium remains. In the later stages of regeneration the remaining endoneurial tube directs axon growth back to the correct targets. During Wallerian degeneration, Schwann cells grow in ordered columns along the endoneurial tube, creating a band of Büngner cells that protects and preserves the endoneurial channel. Also, macrophages and Schwann cells release neurotrophic factors that enhance re-growth.

Central nervous system regeneration

Unlike peripheral nervous system injury, injury to the central nervous system is not followed by extensive regeneration. It is limited by the inhibitory influences of the glial and extracellular environment. The hostile, non-permissive growth environment is, in part, created by the migration of myelin-associated inhibitors, astrocytes, oligodendrocytes, oligodendrocyte precursors, and microglia. The environment within the CNS, especially following trauma, counteracts the repair of myelin and neurons. Growth factors are not expressed or re-expressed; for instance, the extracellular matrix is lacking laminins. Glial scars rapidly form, and the glia actually produce factors that inhibit remyelination and axon repair; for instance, NOGO and NI-35. [6] [7] [8] The axons themselves also lose the potential for growth with age, due to a decrease in GAP43 expression, among others.

Slower degeneration of the distal segment than that which occurs in the peripheral nervous system also contributes to the inhibitory environment because inhibitory myelin and axonal debris are not cleared away as quickly. All these factors contribute to the formation of what is known as a glial scar, which axons cannot grow across. The proximal segment attempts to regenerate after injury, but its growth is hindered by the environment. It is important to note that central nervous system axons have been proven to regrow in permissive environments; therefore, the primary problem to central nervous system axonal regeneration is crossing or eliminating the inhibitory lesion site. [4] Another problem is that the morphology and functional properties of central nervous system neurons are highly complex, for this reason a neuron functionally identical cannot be replaced by one of another type (Llinás' law). [9]

Inhibition of axonal regrowth

Glial cell scar formation is induced following damage to the nervous system. In the central nervous system, this glial scar formation significantly inhibits nerve regeneration, which leads to a loss of function. Several families of molecules are released that promote and drive glial scar formation. For instance, transforming growth factors B-1 and -2, interleukins, and cytokines play a role in the initiation of scar formation. The accumulation of reactive astrocytes at the site of injury and the up regulation of molecules that are inhibitory for neurite outgrowth contribute to the failure of neuroregeneration. [10] The up-regulated molecules alter the composition of the extracellular matrix in a way that has been shown to inhibit neurite outgrowth extension. This scar formation involves several cell types and families of molecules.

Chondroitin sulfate proteoglycan

In response to scar-inducing factors, astrocytes up regulate the production of chondroitin sulfate proteoglycans. Astrocytes are a predominant type of glial cell in the central nervous system that provide many functions including damage mitigation, repair, and glial scar formation. [11] The RhoA pathway is involved. Chondroitin sulfate proteoglycans (CSPGs) have been shown to be up regulated in the central nervous system (CNS) following injury. Repeating disaccharides of glucuronic acid and galactosamine, glycosaminoglycans (CS-GAGs), are covalently coupled to the protein core CSPGs. CSPGs have been shown to inhibit regeneration in vitro and in vivo, but the role that the CSPG core protein vs. CS-GAGs had not been studied until recently.

Keratan sulfate proteoglycans

Like the chondroitin sulfate proteoglycans, keratan sulfate proteoglycan (KSPG) production is up regulated in reactive astrocytes as part of glial scar formation. KSPGs have also been shown to inhibit neurite outgrowth extension, limiting nerve regeneration. Keratan sulfate, also called keratosulfate, is formed from repeating disaccharide galactose units and N-acetylglucosamines. It is also 6-sulfated. This sulfation is crucial to the elongation of the keratan sulfate chain. A study was done using N-acetylglucosamine 6-O-sulfotransferase-1 deficient mice. The wild type mouse showed a significant up regulation of mRNA expressing N-acetylglucosamine 6-O-sulfotransferase-1 at the site of cortical injury. However, in the N-acetylglucosamine 6-O-sulfotransferase-1 deficient mice, the expression of keratan sulfate was significantly decreased when compared to the wild type mice. Similarly, glial scar formation was significantly reduced in the N-acetylglucosamine 6-O-sulfotransferase-1 mice, and as a result, nerve regeneration was less inhibited. [10]

Other inhibitory factors

Proteins of oligodendritic or glial debris origin that influence neuroregeneration:

Clinical treatments

Surgery

Surgery can be done in case a peripheral nerve has become cut or otherwise divided. This is called peripheral nerve reconstruction. The injured nerve is identified and exposed so that normal nerve tissue can be examined above and below the level of injury, usually with magnification, using either loupes or an operating microscope. If a large segment of nerve is harmed, as can happen in a crush or stretch injury, the nerve will need to be exposed over a larger area. Injured portions of the nerve are removed. The cut nerve endings are then carefully reapproximated using very small sutures. The nerve repair must be covered by healthy tissue, which can be as simple as closing the skin or it can require moving skin or muscle to provide healthy padded coverage over the nerve. [15] The type of anesthesia used depends on the complexity of the injury. A surgical tourniquet is almost always used. [15]

Prognosis

The expectations after surgical repair of a divided peripheral nerve depends on several factors:

  • Age: Recovery of a nerve after surgical repair depends mainly on the age of the patient. Young children can recover close-to-normal nerve function. In contrast, a patient over 60 years old with a cut nerve in the hand would expect to recover only protective sensation; that is, the ability to distinguish hot/cold or sharp/dull. [15]
  • The mechanism of injury: Sharp injuries, such as a knife wound, damage only a very short segment of the nerve, availing for direct suture. In contrast, nerves that are divided by stretch or crush may be damaged over long segments. These nerve injuries are more difficult to treat and generally have a poorer outcome. In addition, associated injuries, like injury to bone, muscle and skin, can make nerve recovery more difficult. [15]
  • The level of injury: After a nerve is repaired, the regenerating nerve endings must grow all the way to their target. For example, a nerve injured at the wrist that normally provides sensation to the thumb must grow to the end of the thumb in order to provide sensation. The return of function decreases with increased distance over which a nerve must grow. [15]

Autologous nerve grafting

Currently, autologous nerve grafting, or a nerve autograft, is known as the gold standard for clinical treatments used to repair large lesion gaps in the peripheral nervous system. It is important that nerves are not repaired under tension, [15] which could otherwise happen if cut ends are reapproximated across a gap. Nerve segments are taken from another part of the body (the donor site) and inserted into the lesion to provide endoneurial tubes for axonal regeneration across the gap. However, this is not a perfect treatment; often the final outcome is only limited function recovery. Also, partial de-innervation is frequently experienced at the donor site, and multiple surgeries are required to harvest the tissue and implant it.

When appropriate, a nearby donor may be used to supply innervation to lesioned nerves. Trauma to the donor can be minimized by utilizing a technique known as end-to-side repair. In this procedure, an epineurial window is created in the donor nerve and the proximal stump of the lesioned nerve is sutured over the window. Regenerating axons are redirected into the stump. Efficacy of this technique is partially dependent upon the degree of partial neurectomy performed on the donor, with increasing degrees of neurectomy giving rise to increasing axon regeneration within the lesioned nerve, but with the consequence of increasing deficit to the donor. [16]

Some evidence suggests that local delivery of soluble neurotrophic factors at the site of autologous nerve grafting may enhance axon regeneration within the graft and help expedite functional recovery of a paralyzed target. [17] [18] Other evidence suggests that gene-therapy induced expression of neurotrophic factors within the target muscle itself can also help enhance axon regeneration. [19] [20] Accelerating neuroregeneration and the reinnervation of a denervated target is critically important in order to reduce the possibility of permanent paralysis due to muscular atrophy.

Allografts and xenografts

Variations on the nerve autograft include the allograft and the xenograft. In allografts, the tissue for the graft is taken from another person, the donor, and implanted in the recipient. Xenografts involve taking donor tissue from another species. Allografts and xenografts have the same disadvantages as autografts, but in addition, tissue rejection from immune responses must also be taken into account. Often immunosuppression is required with these grafts. Disease transmission also becomes a factor when introducing tissue from another person or animal. Overall, allografts and xenografts do not match the quality of outcomes seen with autografts, but they are necessary when there is a lack of autologous nerve tissue.

Nerve guidance conduit

Because of the limited functionality received from autografts, the current gold standard for nerve regeneration and repair, recent neural tissue engineering research has focused on the development of bioartificial nerve guidance conduits in order to guide axonal regrowth. The creation of artificial nerve conduits is also known as entubulation because the nerve ends and intervening gap are enclosed within a tube composed of biological or synthetic materials. [21]

Immunisation

A direction of research is towards the use of drugs that target remyelinating inhibitor proteins, or other inhibitors. Possible strategies include vaccination against these proteins (active immunisation), or treatment with previously created antibodies (passive immunisation). These strategies appear promising on animal models with experimental autoimmune encephalomyelitis (EAE), a model of MS. [22] Monoclonal antibodies have also been used against inhibitory factors such as NI-35 and NOGO. [23]

See also

Related Research Articles

Axon Long projection on a neuron that conducts signals to other neurons

An axon, or nerve fiber, is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action potentials away from the nerve cell body. The function of the axon is to transmit information to different neurons, muscles, and glands. In certain sensory neurons, such as those for touch and warmth, the axons are called afferent nerve fibers and the electrical impulse travels along these from the periphery to the cell body and from the cell body to the spinal cord along another branch of the same axon. Axon dysfunction has caused many inherited and acquired neurological disorders which can affect both the peripheral and central neurons. Nerve fibers are classed into three types – group A nerve fibers, group B nerve fibers, and group C nerve fibers. Groups A and B are myelinated, and group C are unmyelinated. These groups include both sensory fibers and motor fibers. Another classification groups only the sensory fibers as Type I, Type II, Type III, and Type IV.

Myelin Fatty substance that surrounds nerve cell axons to insulate them and increase transmission speed

Myelin is a lipid-rich (fatty) substance that surrounds nerve cell axons to insulate them and increase the rate at which electrical impulses are passed along the axon. The myelinated axon can be likened to an electrical wire with insulating material (myelin) around it. However, unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire length of the axon. Rather, each myelin sheath insulates the axon over a single long section and, in general, each axon comprises multiple long myelinated sections separated from each other by short myelin sheath-gaps called nodes of Ranvier.

Schwann cell

Schwann cells or neurolemmocytes are the principal glia of the peripheral nervous system (PNS). Glial cells function to support neurons and in the PNS, also include satellite cells, olfactory ensheathing cells, enteric glia and glia that reside at sensory nerve endings, such as the Pacinian corpuscle. The two types of Schwann cells are myelinating and nonmyelinating. Myelinating Schwann cells wrap around axons of motor and sensory neurons to form the myelin sheath. The Schwann cell promoter is present in the downstream region of the human dystrophin gene that gives shortened transcript that are again synthesized in a tissue-specific manner.

Nervous tissue Main component of the nervous system

Nervous tissue, also called neural tissue, is the main tissue component of the nervous system. The nervous system regulates and controls bodily functions and activity and consists of two parts: the central nervous system (CNS) comprising the brain and spinal cord, and the peripheral nervous system (PNS) comprising the branching peripheral nerves. It is composed of neurons, also known as nerve cells, which receive and transmit impulses, and neuroglia, also known as glial cells or glia, which assist the propagation of the nerve impulse as well as provide nutrients to the neurons.

Motor nerve

A motor nerve is a nerve located in the central nervous system (CNS), usually the spinal cord, that sends motor signals from the CNS to the muscles of the body. This is different from the motor neuron, which includes a cell body and branching of dendrites, while the nerve is made up of a bundle of axons. Motor nerves act as efferent nerves which carry information out from the CNS to muscles, as opposed to afferent nerves, which send signals from sensory receptors in the periphery to the CNS. Efferent nerves can also connect to glands or other organs/issues instead of muscles. In addition, there are nerves that serve as both sensory and motor nerves called mixed nerves.

Wallerian degeneration Biological process of axonal degeneration

Wallerian degeneration is an active process of degeneration that results when a nerve fiber is cut or crushed and the part of the axon distal to the injury degenerates. A related process of dying back or retrograde degeneration known as 'Wallerian-like degeneration' occurs in many neurodegenerative diseases, especially those where axonal transport is impaired such as ALS and Alzheimer's disease. Primary culture studies suggest that a failure to deliver sufficient quantities of the essential axonal protein NMNAT2 is a key initiating event.

Astrogliosis Increase in number of astrocytes due to central nervous system injury

Astrogliosis is an abnormal increase in the number of astrocytes due to the destruction of nearby neurons from central nervous system (CNS) trauma, infection, ischemia, stroke, autoimmune responses or neurodegenerative disease. In healthy neural tissue, astrocytes play critical roles in energy provision, regulation of blood flow, homeostasis of extracellular fluid, homeostasis of ions and transmitters, regulation of synapse function and synaptic remodeling. Astrogliosis changes the molecular expression and morphology of astrocytes, in response to infection for example, in severe cases causing glial scar formation that may inhibit axon regeneration.

Axonotmesis is an injury to the peripheral nerve of one of the extremities of the body. The axons and their myelin sheath are damaged in this kind of injury, but the endoneurium, perineurium and epineurium remain intact. Motor and sensory functions distal to the point of injury are completely lost over time leading to Wallerian degeneration due to ischemia, or loss of blood supply. Axonotmesis is usually the result of a more severe crush or contusion than neurapraxia.

Gliosis is a nonspecific reactive change of glial cells in response to damage to the central nervous system (CNS). In most cases, gliosis involves the proliferation or hypertrophy of several different types of glial cells, including astrocytes, microglia, and oligodendrocytes. In its most extreme form, the proliferation associated with gliosis leads to the formation of a glial scar.

Myelin-associated glycoprotein

Myelin-associated glycoprotein is a type 1 transmembrane protein glycoprotein localized in periaxonal Schwann cell and oligodendrocyte membranes, where it plays a role in glial-axonal interactions. MAG is a member of the SIGLEC family of proteins and is a functional ligand of the NOGO-66 receptor, NgR. MAG is believed to be involved in myelination during nerve regeneration in the PNS and is vital for the long-term survival of the myelinated axons following myelinogenesis. In the CNS MAG is one of three main myelin-associated inhibitors of axonal regeneration after injury, making it an important protein for future research on neurogenesis in the CNS.

Nerve injury Medical condition

Nerve injury is injury to nervous tissue. There is no single classification system that can describe all the many variations of nerve injury. In 1941, Seddon introduced a classification of nerve injuries based on three main types of nerve fiber injury and whether there is continuity of the nerve. Usually, however, (peripheral) nerve injury is classified in five stages, based on the extent of damage to both the nerve and the surrounding connective tissue, since supporting glial cells may be involved. Unlike in the central nervous system, neuroregeneration in the peripheral nervous system is possible. The processes that occur in peripheral regeneration can be divided into the following major events: Wallerian degeneration, axon regeneration/growth, and nerve reinnervation. The events that occur in peripheral regeneration occur with respect to the axis of the nerve injury. The proximal stump refers to the end of the injured neuron that is still attached to the neuron cell body; it is the part that regenerates. The distal stump refers to the end of the injured neuron that is still attached to the end of the axon; it is the part of the neuron that will degenerate but that remains in the area toward which the regenerating axon grows. The study of peripheral nerve injury began during the American Civil War and has greatly expanded to the point of using growth-promoting molecules.

A nerve guidance conduit is an artificial means of guiding axonal regrowth to facilitate nerve regeneration and is one of several clinical treatments for nerve injuries. When direct suturing of the two stumps of a severed nerve cannot be accomplished without tension, the standard clinical treatment for peripheral nerve injuries is autologous nerve grafting. Due to the limited availability of donor tissue and functional recovery in autologous nerve grafting, neural tissue engineering research has focused on the development of bioartificial nerve guidance conduits as an alternative treatment, especially for large defects. Similar techniques are also being explored for nerve repair in the spinal cord but nerve regeneration in the central nervous system poses a greater challenge because its axons do not regenerate appreciably in their native environment.

Nerve tissue is a biological molecule related to the function and maintenance of normal nervous tissue. An example would include, for example, the generation of myelin which insulates and protects nerves. These are typically calcium-binding proteins.

Glial scar Mass formed in response to injury to the nervous system

Glial scar formation (gliosis) is a reactive cellular process involving astrogliosis that occurs after injury to the central nervous system. As with scarring in other organs and tissues, the glial scar is the body's mechanism to protect and begin the healing process in the nervous system.

LINGO1

Leucine rich repeat and Immunoglobin-like domain-containing protein 1 also known as LINGO-1 is a protein which is encoded by the LINGO1 gene in humans. It belongs to the family of leucine-rich repeat proteins which are known for playing key roles in the biology of the central nervous system. LINGO-1 is a functional component of the Nogo receptor also known as the reticulon 4 receptor.

Chondroitin sulfate proteoglycan

Chondroitin sulfate proteoglycans (CSPGs) are proteoglycans consisting of a protein core and a chondroitin sulfate side chain. They are known to be structural components of a variety of human tissues, including cartilage, and also play key roles in neural development and glial scar formation. They are known to be involved in certain cell processes, such as cell adhesion, cell growth, receptor binding, cell migration, and interaction with other extracellular matrix constituents. They are also known to interact with laminin, fibronectin, tenascin, and collagen. CSPGs are generally secreted from cells.

Adult mesenchymal stem cells (MSCs) are being used by researchers in the fields of regenerative medicine and tissue engineering, to artificially reconstruct human tissue which has been previously damaged. Mesenchymal stem cells have the capacity to become any type of fully developed cell, which can contribute to replacing muscle tissues or internal organs. To help discover the therapeutic uses of these stem cells they are grown in laboratories or by using medication to stimulate new cell growth within the human body. In MSC therapy the cells are extracted from the adult patient’s bone marrow via a procedure called bone marrow aspiration. This usually involves inserting a needle into the back of the patients hip bone and removing the sample from there. These cells are then grown under controlled in vitro conditions in a lab, so that they can multiply and same time mature( also referred to as differentiated. This process may take two to three weeks. The kind of mature, fully differentiated cell phenotype and the number of those cells created though this can be influenced in three ways. Firstly by varying the initial seed density in the culture medium, secondly through changing the conditions of the medium during expansion, and lastly through the addition of additives such as proteins or growth hormones to the culture medium. They are then harvested and put back into the patient through local delivery or systemic infusion.

Endogenous regeneration in the brain is the ability of cells to engage in the repair and regeneration process. While the brain has a limited capacity for regeneration, endogenous neural stem cells, as well as numerous pro-regenerative molecules, can participate in replacing and repairing damaged or diseased neurons and glial cells. Another benefit that can be achieved by using endogenous regeneration could be avoiding an immune response from the host.

Epineurial repair

Epineurial repair is a common surgical procedure to repair a nerve laceration via the epineurium, the connective tissue surrounding nerve fibers originating from the spinal cord. It is intended to allow the restoration of sensory function. When a nerve is lacerated or cut, repair is done by sewing the cut ends together through the epineurium to increase the potential of the proximal part growing correctly along the route the degrading distal part leaves behind. Usual sensation and mobility will not be an immediate result because nerves grow at a rate of approximately 1 millimeter per day, so it will take a few months to notice the final outcome. Research in use of nerve grafts and nerve growth factors is being done to speed recovery time.

Spinal cord injury research seeks new ways to cure or treat spinal cord injury in order to lessen the debilitating effects of the injury in the short or long term. There is no cure for SCI, and current treatments are mostly focused on spinal cord injury rehabilitation and management of the secondary effects of the condition. Two major areas of research include neuroprotection, ways to prevent damage to cells caused by biological processes that take place in the body after the insult, and neuroregeneration, regrowing or replacing damaged neural circuits.

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