Introduction

Nerve regeneration has been long studied within the neuroscience field. When there is axonal damage in the Central Nervous System (CNS), it is unable to fully regenerate the axons. 1 The Peripheral Nervous System (PNS), however, has a better ability to regenerate axons after axonal damage mostly due to the ability of the Schwann cells to promote a permissible environment for growth. 1 Scientists study PNS regeneration to understand its mechanisms to apply the knowledge in CNS regeneration and neurodegenerative diseases. 2 The process begins with demyelination of the axons after detection of peripheral nerve injury. 2 The Schwann cells revert back to an undifferentiated state. 2 The previous Schwann cells become precursors to aid in axonal regeneration. 2 Macrophages are recruited to aid the Schwann cells in demyelination. 1 After the removal of myelin and axonal debris, the axon can regenerate. Schwann cells then redifferentiate to fully complete the process. Through inducible and knockout studies, Schwann cells are shown to be important regulators in the process of peripheral nerve regeneration.

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Axon_regeneration.jpg
Fig. 1 : General Overview of Peripheral Nerve Regeneration. Adapted from Gaudet, A. D., et al, 2011

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Degeneration and Demyelination of Axon

Dedifferentiation of Schwann cells
MMP9.jpg
Fig. 2: On the right side, the Raf/MEK/ERK pathway is activated by MMP-9 during dedifferentiation of Schwann cells. On the left side, the Akt/PI3-K pathway is inhibited durin g dedifferentiation of Schwann cells. Adapted from Chattopadhyay, S.et al, 2009

When an injury is detected, the site of injury and the distal end of the axon demyelinates and degenerates. 3 The Schwann cells at the distal axon arrest the production of myelin and beginning to dedifferentiate. 4 After axonal injury, Schwann cells secrete metalloproteinase-9 (MMP-9), a protease. MMP-9 activates the Raf/MEK/ERK pathway in Schwann cells by the binding actions of neuregulin (NRG), insulin-like growth factor-1 (IGF-1), ErbB, and platelet-derived growth factor (PDGF) and their receptors. 5 The Raf/MEK/ERK signalling pathway drive the dedifferentiation of Schwann cells. 6 The importance of the Raf/MEK/ERK pathway has been demonstrated through the use of transgenic mice that allowed for the activation of Raf-kinase in the absence of injury.6 Raf-kinase activity alters the gene expression to control the differentiation state. 6 MMP-9 activates the ERK pathway after activation of Raf-kinase. 5 The ERK signalling increases, dominating the prodifferentiating signals of the Schwann cell, and preventing the Schwann cells from differentiating. 6 The duration of demyelination is determined by the duration of ERK signalling. 6 The ubiquitin-proteasome pathway, where ubiquitinated proteins are degraded, has also been found to have an important role in Schwann cell dedifferentiation and demyelination. 7 The inhibition of proteasome activity delayed the dedifferentiation and demyelination of Schwann cells, which could be due to the inhibition of the degradation activity of the proteasome. 7

Removal of Debris

The separated segment of the axon undergoes a process called Wallerian degeneration. 3 The initial degradation of the myelin sheath is regulated by phospholipase-A2 (PLA2) enzymes, which hydrolyzes phosphatidylcholine to induce myelin degradation. 8 Cytokines in Schwann cells activate PLA2 activity and expression. If PLA2 activity is impaired, degradation of the axon and myelin are delayed. 8 The myelin sheath is degraded into fragments that contain growth-inhibitory molecules. 9 Thus, when the axon degenerates, it creates an environment that inhibits axonal growth. 9 In order to create a growth promoting environment, the debris must be removed. 3 Schwann cells were proven to recruit inflammatory cells, such as macrophages and neutrophils, via chemoattractants and cytokines. 3 MMP-9 also aids in recruitment of macrophages by activating Raf-kinase. 10 Signalling of Raf-kinase allows for the secretion of chemoattractants that recruit the inflammatory cells. 6 Consequently, the blood- brain barrier (BBB) is compromised to allow inflammatory cells to enter. 11 The inflammatory cells and the dedifferentiated Schwann cells synergistically phagocytose the myelin and axonal debris of the separated axon. 12 Toll-Like receptors (TLRs) in Schwann cells also aid in the inflammatory activity of the cells by mediating the transcription of inflammatory pathways. 13 TLRs bind foreign molecules to recognize the axonal injury and then activating the inflammatory response. 13 In the absence of TLRs, a significantly reduced inflammatory response was found. 13 The macrophages and Schwann cells then exit through the permeable BBB to the blood circulation. 3


Regeneration of Axon


Schwann cells guide axon to target tissue

The STAT3 pathway, activated by cytokines secreted by Schwann cells, has been shown to initiate the process of axonal regeneration. 14 During the dedifferentiation process, Schwann cells remain attached to their basal lamina.6 Schwann cells proliferate to create bands of Bungner, a tube-like structure composed of dedifferentiated Schwann cells. 3 The proliferation of Schwann cells is regulated by MMP-9. 15 MMP-9 activity is inhibited, leading to enhanced proliferation of the Schwann cells.15 The bands of Bungner allows the axon to be guided back to its target tissue for the nerve to be reinnervated. 6 The Schwann cells of the bands of Bungner secrete growth factors and Netrin-1, a chemoattractant. 16 The proximal end of the axon forms a growth cone that contains Netrin-1 receptors. 16 Deleted in Colorectal Cancer (DCC) and Uncoordinated (Unc5H2) receptors are both Netrin-1 receptors that are found in the growth cone. 16 DCC receptors are attracted to Netrin-1, while Unc5H2 receptors are repelled by Netrin-1. 16 The Schwann cells guide the regenerating axon using DCC and Unc5H2 receptors to attract and repel the neuron, respectively. 16 The upregulation of DCC receptors and downregulation of Unc5H2 has been found to increase axonal regeneration. 16 Schwann cells proliferate and migrate to front of regenerating axon. 17

Redifferentiation of Schwann Cells

Once the axon reaches its target tissues, the Schwann cells can redifferentiate. Since MMP-9 activity is inhibited, the ERK signals decrease, allowing the prodifferentiating signals to be sensed by the cell. 5,6 The prodifferentiating signals arise from the consistent activation of the Akt/PI3-K pathway through IGF-1, prompting the Schwann cell to redifferentiate. 18 In order for the nerve to function properly, the Schwann cells must redifferentiate to myelinate the axon again. 18 However, the myelin is much thinner and the nodes are much closer than that before the injury. 3 The thickness of the myelin layer is controlled by Neuregulin.18 Neuregulin-1 (NRG-1) Type III was demonstrated to being significant for remyelination. 19 Knockout experiments of NRG-1 showed a reduction in myelin thickness and delayed axonal regeneration.19 Vimentin, an intermediate filament, has been demonstrated to regulated the levels of NRG-1, by inhibiting NRG-1. 20 The enzymes, TACE and BACE1, also regulates remyelination by cleaving NRG-1. 20 The cleaving activity by BACE1 allows NRG-1 to binds to the ErbB2/3 receptors on Schwann cells. The PI3-Kinase pathway is activated by NRG-1 binding to Erb2/3, leading to the myelination process. 20


Clinical Applications of Peripheral Nerve Regeneration


Many pathologies result from abnormalities that occur with the nerve regeneration process. 6 One example is neurofibromatosis, where the inflammatory response of Schwann cells and macrophages was unsuccessful and led to an accumulation of these cells. 6 If the mechanisms that allow for peripheral nerve regeneration can be determined, possible treatments can be designed not only for these types of peripheral neuropathies, but for spinal cord and brain pathologies as well.

In peripheral nerve regeneration, it is difficult for nerves to regenerate across large gaps due to the inability to sustain nerve regeneration for long periods of time. 3, 16 A possible treatment for this is to increase expression of DCC and to completely eliminate the expression of Unc5H2 since DCC is an attracts the nerve. 16 In some neuropathies, the Raf/MEK/ERK signalling pathway had been overactivated, leading to excessive degradation of axon and the myelin sheath. 6 Because of the essential role of Schwann cells in peripheral nerve regeneration, treatment that researchers have been observing is to transplant Schwann cells into the central nervous system to repair damaged nerves. 21 Currently, the solution for large gaps has been to implant tissue grafts to connect the gap. 21 However, this treatment has been known to have negative effects, such as loss of sensation. 21 Another solution is the implantation of artificial nerve conduits that guide axons with neurotrophic factors. 21

The central nervous system does not contain Schwann cells, which has a dominant role in nerve regeneration. By implanting Schwann cells into the CNS, nerves may be able to regenerate due to its growth-promoting properties, such as secretion of neurotrophic factors and recruitment of macrophages. 6, 21 Researchers have examined the Schwann cell gene to observe the differences that allow Schwann cells to mediate nerve regeneration. 22 Further research in the mechanisms of peripheral nerve regeneration and Schwann cells can lead to potential treatments for neurodevelopmental disorders and neurodegenerative disorders, such as Parkinsons or Alzheimer’s disease.











References


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