Select neurotrophins promote oligodendrocyte progenitor cell process outgrowth in the presence of chondroitin sulfate proteoglycans.

Axonal damage and the subsequent interruption of intact neuronal pathways in the spinal cord are largely responsible for the loss of motor function after injury. Further exacerbating this loss is the demyelination of neighboring uninjured axons. The post-injury environment is hostile to repair, with inflammation, a high expression of chondroitin sulfate proteoglycans (CSPGs) around the glial scar, and myelin breakdown. Numerous studies have demonstrated that treatment with the enzyme chondroitinase ABC (cABC) creates a permissive environment around a spinal lesion that permits axonal regeneration. Neurotrophic factors like brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), neurotrophic factor-3 (NT-3), and ciliary neurotrophic factor (CNTF) have been used to promote neuronal survival and stimulate axonal growth. CSPGs expressed near a lesion also inhibit migration and differentiation of endogenous oligodendrocyte progenitor cells (OPCs) in the spinal cord, and cABC treatment can neutralize this inhibition. This study examined the neurotrophins commonly used to stimulate axonal regeneration after injury and their potential effects on OPCs cultured in the presence of CSPGs. The results reveal differential effects on OPCs, with BDNF and GDNF promoting process outgrowth and NT-3 stimulating differentiation of OPCs, while CNTF appears to have no observable effect. This finding suggests that certain neurotrophic agents commonly utilized to stimulate axonal regeneration after a spinal injury may also have a beneficial effect on the endogenous oligodendroglial cells as well.

The oligodendrocyte growth cone and its actin cytoskeleton: A fundamental element for progenitor cell migration and CNS myelination.

Cells of the oligodendrocyte (OLG) lineage engage in highly motile behaviors that are crucial for effective central nervous system (CNS) myelination. These behaviors include the guided migration of OLG progenitor cells (OPCs), the surveying of local environments by cellular processes extending from differentiating and pre-myelinating OLGs, and during the process of active myelin wrapping, the forward movement of the leading edge of the myelin sheath's inner tongue along the axon. Almost all of these motile behaviors are driven by actin cytoskeletal dynamics initiated within a lamellipodial structure that is located at the tip of cellular OLG/OPC processes and is structurally as well as functionally similar to the neuronal growth cone. Accordingly, coordinated stoichiometries of actin filament (F-actin) assembly and disassembly at these OLG/OPC growth cones have been implicated in directing process outgrowth and guidance, and the initiation of myelination. Nonetheless, the functional importance of the OLG/OPC growth cone still remains to be fully understood, and, as a unique aspect of actin cytoskeletal dynamics, F-actin depolymerization and disassembly start to predominate at the transition from myelination initiation to myelin wrapping. This review provides an overview of the current knowledge about OLG/OPC growth cones, and it proposes a model in which actin cytoskeletal dynamics in OLG/OPC growth cones are a main driver for morphological transformations and motile behaviors. Remarkably, these activities, at least at the later stages of OLG maturation, may be regulated independently from the transcriptional gene expression changes typically associated with CNS myelination.

Effect of lesion proximity on the regenerative response of long descending propriospinal neurons after spinal transection injury.

BACKGROUND: The spinal cord is limited in its capacity to repair after damage caused by injury or disease. However, propriospinal (PS) neurons in the spinal cord have demonstrated a propensity for axonal regeneration after spinal cord injury. They can regrow and extend axonal projections to re-establish connections across a spinal lesion. We have previously reported differential reactions of two distinct PS neuronal populations-short thoracic propriospinal (TPS) and long descending propriospinal tract (LDPT) neurons-following a low thoracic (T10) spinal cord injury in a rat model. Immediately after injury, TPS neurons undergo a strong initial regenerative response, defined by the upregulation of transcripts to several growth factor receptors, and growth associated proteins. Many also initiate a strong apoptotic response, leading to cell death. LDPT neurons, on the other hand, show neither a regenerative nor an apoptotic response. They show either a lowered expression or no change in genes for a variety of growth associated proteins, and these neurons survive for at least 2 months post-axotomy. There are several potential explanations for this lack of cellular response for LDPT neurons, one of which is the distance of the LDPT cell body from the T10 lesion. In this study, we examined the molecular response of LDPT neurons to axotomy caused by a proximal spinal cord lesion. RESULTS: Utilizing laser capture microdissection and RNA quantification with branched DNA technology, we analyzed the change in gene expression in LDPT neurons following axotomy near their cell body. Expression patterns of 34 genes selected for their robust responses in TPS neurons were analyzed 3 days following a T2 spinal lesion. Our results show that after axonal injury nearer their cell bodies, there was a differential response of the same set of genes evaluated previously in TPS neurons after proximal axotomy, and LDPT neurons after distal axotomy (T10 spinal transection). The genetic response was much less robust than for TPS neurons after proximal axotomy, included both increased and decreased expression of certain genes, and did not suggest either a major regenerative or apoptotic response within the population of genes examined. CONCLUSIONS: The data collectively demonstrate that the location of axotomy in relation to the soma of a neuron has a major effect on its ability to mount a regenerative response. However, the data also suggest that there are endogenous differences in the LDPT and TPS neuronal populations that affect their response to axotomy. These phenotypic differences may indicate that different or multiple therapies may be needed following spinal cord injury to stimulate maximal regeneration of all PS axons.

Cauda equina repair in the rat: Part 3. Axonal regeneration across Schwann cell-Seeded collagen foam.

INTRODUCTION: Treatments for patients with cauda equina injury are limited. METHODS: In this study, we first used retrograde labeling to determine the relative contributions of cauda equina motor neurons to intrinsic and extrinsic rat tail muscles. Next, we transected cauda equina ventral roots and proceeded to bridge the proximal and distal stumps with either a type I collagen scaffold coated in laminin (CL) or a collagen-laminin scaffold that was also seeded with Schwann cells (CLSC). Regeneration was assessed by way of serial retrograde labeling. RESULTS: After accounting for the axonal contributions to intrinsic vs. extrinsic tail muscles, we noted a higher degree of double labeling in the CLSC group (58.0 ± 39.6%) as compared with the CL group (27.8 ± 16.0%; P = 0.02), but not the control group (33.5 ± 18.2%; P = 0.10). DISCUSSION: Our findings demonstrate the feasibility of using CLSCs in cauda equina injury repair. Muscle Nerve 57: E78-E84, 2018.

Nanoparticle Technologies in the Spinal Cord.

Nanoparticles are increasingly being studied within experimental models of spinal cord injury (SCI). They are used to image cells and tissue, move cells to specific regions of the spinal cord, and deliver therapeutic agents locally. The focus of this article is to provide a brief overview of the different types of nanoparticles being studied for spinal cord applications and present data showing the capability of nanoparticles to deliver the chondroitinase ABC (chABC) enzyme locally following acute SCI in rats. Nanoparticles releasing chABC helped promote axonal regeneration following injury, and the nanoparticles also protected the enzyme from rapid degradation. In summary, nanoparticles are viable materials for diagnostic or therapeutic applications within experimental models of SCI and have potential for future clinical use.

Biomaterial Approaches to Enhancing Neurorestoration after Spinal Cord Injury: Strategies for Overcoming Inherent Biological Obstacles.

While advances in technology and medicine have improved both longevity and quality of life in patients living with a spinal cord injury, restoration of full motor function is not often achieved. This is due to the failure of repair and regeneration of neuronal connections in the spinal cord after injury. In this review, the complicated nature of spinal cord injury is described, noting the numerous cellular and molecular events that occur in the central nervous system following a traumatic lesion. In short, postinjury tissue changes create a complex and dynamic environment that is highly inhibitory to the process of neural regeneration. Strategies for repair are outlined with a particular focus on the important role of biomaterials in designing a therapeutic treatment that can overcome this inhibitory environment. The importance of considering the inherent biological response of the central nervous system to both injury and subsequent therapeutic interventions is highlighted as a key consideration for all attempts at improving functional recovery.

Chondroitin sulfate proteoglycans in the nervous system: inhibitors to repair.

Chondroitin sulfate proteoglycans (CSPGs) are widely expressed in the normal central nervous system, serving as guidance cues during development and modulating synaptic connections in the adult. With injury or disease, an increase in CSPG expression is commonly observed close to lesioned areas. However, these CSPG deposits form a substantial barrier to regeneration and are largely responsible for the inability to repair damage in the brain and spinal cord. This review discusses the role of CSPGs as inhibitors, the role of inflammation in stimulating CSPG expression near site of injury, and therapeutic strategies for overcoming the inhibitory effects of CSPGs and creating an environment conducive to nerve regeneration.

The inhibitory effects of chondroitin sulfate proteoglycans on oligodendrocytes.

The formation of the glial scar following a spinal cord injury presents a significant barrier to the regenerative process. It is primarily composed of chondroitin sulfate proteoglycans (CSPGs) that can inhibit axonal sprouting and regeneration. Although the inhibitory effects on neurons are well documented, little is known about their effects on oligodendrocyte progenitor cells (OPCs). In this study, we examined the effects of CSPGs on OPC process outgrowth and differentiation in vitro. The results show that specific CSPGs, in particularly those highly up-regulated following spinal cord injury, inhibit OPC process outgrowth and differentiation, and that treatment with chondroitinase ABC can completely reverse this inhibition. Additionally, treatment with the Rho kinase inhibitor Y-27632 also reverses the observed inhibition, implicating the activation of Rho kinase in the CSPG inhibition of OPC growth. Taken together, these findings demonstrate that the CSPGs found within the glial scar are not only inhibitory to neurons, but also to OPCs. Moreover, this study shows that chondroitinase ABC treatment, having shown promise in promoting axonal regeneration, may also enhance remyelination.

Chondroitinase treatment following spinal contusion injury increases migration of oligodendrocyte progenitor cells.

Following spinal cord injury (SCI), the demyelination of spared intact axons near the lesion site likely contributes to the loss of motor function. This demyelination occurs when oligodendrocytes, the myelinating cells of the central nervous system (CNS), are either destroyed during the initial trauma or die as a result of secondary pathology. In an attempt to remyelinate the affected axons, endogenous oligodendrocyte progenitor cells (OPCs) begin to accumulate at the border of demyelination. However, the differentiation of OPCs into fully myelinating cells is limited. While the reasons for this are unknown, it is well known that the injured spinal cord is rich in inhibitory molecules that block repair. One such family of molecules is the chondroitin sulfate proteoglycans (CSPGs), which are known to be highly inhibitory to the process of axonal elongation. Recent in vitro findings have demonstrated that CSPGs are also highly inhibitory to OPCs, affecting both their migration and differentiation. Treatment with the enzyme chondroitinase ABC (cABC), which removes the glycosaminoglycan side chains of CSPGs, reverses the inhibitory effects of CSPGs on these cells. In the present study, we examined the effects of cABC on the migratory behavior of endogenous OPCs in vivo following a moderate spinal contusion injury. The total number of OPCs surrounding the lesion site was significantly increased after cABC treatment as compared to controls. cABC treatment also enhanced axonal sprouting, but OPC migration occurs along a different time course and appears independent of new process outgrowth. These data suggest that CSPGs in the post-injury environment inhibit the migration of OPCs, as well as axonal regeneration. Therefore, cABC treatment may not only enhance regenerative axonal sprouting, but may also enhance remyelination after SCI.

Oligodendroglial cells express and secrete reelin.

Oligodendrocyte (OL) progenitor cells (OPCs) give rise to the myelinating cells of the central nervous system (CNS), the OL. To examine molecular changes involved in OPC differentiation, a microarray analysis was performed at several time points during OPC maturation. The results revealed significant expression levels of mRNA for reelin, one reelin receptor, very low density lipoprotein receptor (Vldlr), and the cytoplasmic adaptor molecule, disabled homolog 1 (Dab1). The expression of these proteins in oligodendroglial (ODG) cells was confirmed by immunocytochemistry and Western blot analysis. It was also discovered that both progenitors and mature OLs secrete reelin. Although there is no known effect of reelin on ODG cells, the data suggest that these cells may be a source of reelin in the CNS.

The selective RNA-binding protein quaking I (QKI) is necessary and sufficient for promoting oligodendroglia differentiation.

Quaking I (QKI) is a selective RNA-binding protein essential for myelination of the central nervous system. Three QKI isoforms with distinct C termini and subcellular localization, namely QKI-5, QKI-6, and QKI-7, are expressed in oligodendroglia progenitor cells (OPCs) prior to the initiation of myelin formation and implicated in promoting oligodendrocyte lineage development. However, the functional requirement for each QKI isoform and the mechanisms by which QKI isoforms govern OPC development still remain elusive. We report here that exogenous expression of each QKI isoform is sufficient to enhance differentiation of OPCs with different efficiency, which is abolished by a point mutation that abrogates the RNA binding activity of QKI. Reciprocally, small interfering RNA-mediated QKI knockdown blocks OPC differentiation, which can be partly rescued by QKI-5 and QKI-6 but not by QKI-7, indicating the differential requirement of QKI isoform function in advancing OPC differentiation. Furthermore, we found that abrogation of OPC differentiation, as a result of QKI deficiency, is not due to altered proliferation capacity or cell cycle progression. These results indicate that QKI isoforms are necessary and sufficient for promoting OPC development, which must involve direct influence of QKI on differentiation/maturation of OPCs independent of cell cycle exit, likely via regulating the expression of the target mRNAs of QKI that support OPC differentiation.

Developmentally-programmed FMRP expression in oligodendrocytes: a potential role of FMRP in regulating translation in oligodendroglia progenitors.

The fragile X mental retardation protein (FMRP) is a selective RNA-binding protein whose function is implicated in regulating protein synthesis of its mRNA targets. The lack of FMRP leads to abnormal synapse development in the brain and impaired learning/memory. Although FMRP is predominantly expressed in neurons of the adult brain, whether FMRP also functions in glia during early development remains elusive, since expression of FMRP in glia has not been rigorously examined. This is an important question because recent studies revealed important roles of glia in synaptic development. Here we report that in addition to the observed neuronal expression, FMRP expression is detected in oligodendroglia progenitor cells (OPCs), immature oligodendrocytes and oligodendroglia cell lines, where it interacts with a subgroup of oligodendrocyte-specific mRNAs, including the myelin basic protein (MBP) mRNA. FMRP expression gradually declines as oligodendrocytes differentiate in vitro and in the developing brain. The decline of FMRP expression during oligodendrocyte differentiation is associated with a vigorous up-regulation of the MBP protein. In addition, we show that the MBP 3'untranslated region (3'UTR) is necessary and sufficient for binding FMRP, and mediates translation inhibition of a reporter gene by FMRP specifically in oligodendrocytes. These results support the hypothesis that FMRP may participate in regulating translation of its bound mRNAs in oligodendroglia during early brain development.