CNS Schwann-like glia and functional restoration of damaged spinal cord

Axonal Growth and Fasciculation of Spinal Neurons Promoted by Aldynoglia in Alkaline Fibrin Hydrogel: Influence of Tol-51 Sulfoglycolipid

Traumatic spinal cord injury (tSCI) has complex pathophysiological events that begin after the initial trauma. One such event is fibroglial scar formation by fibroblasts and reactive astrocytes. A strong inhibition of axonal growth is caused by the activated astroglial cells as a component of fibroglial scarring through the production of inhibitory molecules, such as chondroitin sulfate proteoglycans or myelin-associated proteins. Here, we used neural precursor cells (aldynoglia) as promoters of axonal growth and a fibrin hydrogel gelled under alkaline conditions to support and guide neuronal cell growth, respectively. We added Tol-51 sulfoglycolipid as a synthetic inhibitor of astrocyte and microglia in order to test its effect on the axonal growth-promoting function of aldynoglia precursor cells. We obtained an increase in GFAP expression corresponding to the expected glial phenotype for aldynoglia cells cultured in alkaline fibrin. In co-cultures of dorsal root ganglia (DRG) and aldynoglia, the axonal growth promotion of DRG neurons by aldynoglia was not affected. We observed that the neural precursor cells first clustered together and then formed niches from which aldynoglia cells grew and connected to groups of adjacent cells. We conclude that the combination of alkaline fibrin with synthetic sulfoglycolipid Tol-51 increased cell adhesion, cell migration, fasciculation, and axonal growth capacity, promoted by aldynoglia cells. There was no negative effect on the behavior of aldynoglia cells after the addition of sulfoglycolipid Tol-51, suggesting that a combination of aldynoglia plus alkaline fibrin and Tol-51 compound could be useful as a therapeutic strategy for tSCI repair.

Schwann cell precursors in health and disease

Schwann cell precursors (SCPs) are frequently regarded as neural crest-derived cells (NCDCs) found in contact with axons during nerve formation. Nevertheless, cells with SCPs properties can be found up to the adulthood. They are well characterized with regard to both gene expression profile and cellular behavior -for instance, proliferation, migratory capabilities and survival requirements-. They differ in origin regarding their anatomic location: even though most of them are derived from migratory NCCs, there is also contribution of the boundary cap neural crest cells (bNCCs) to the skin and other tissues. Many functions are known for SCPs in normal development, including nerve fasciculation and target innervation, arterial branching patterning and differentiation, and other morphogenetic processes. In addition, SCPs are now known to be a source of many neural (glia, endoneural fibroblasts, melanocytes, visceral neurons, and chromaffin cells) and non-neural-like (mesenchymal stromal cells, able e.g., to generate dentine-producing odontoblasts) cell types. Until now no reports of endoderm-like derivatives were reported so far. Interestingly, in the Schwann cell lineage only early SCPs are likely able to differentiate into melanocytes and bone marrow mesenchymal stromal cells. We have also herein discussed the literature regarding their role in repair as well as in disease mechanisms, such as in diverse cancers. Moreover, many caveats in our knowledge of SCPs biology are highlighted all through this article. Future research should expand more into the relevance of SCPs in pathologies and in other regenerative mechanisms which might bring new unexpected clinically-relevant knowledge.

Increased p75 neurotrophin receptor expression in the canine distemper virus model of multiple sclerosis identifies aldynoglial Schwann cells that emerge in response to axonal damage

Gliogenesis under pathophysiological conditions is of particular clinical relevance since it may provide regeneration-promoting cells recruitable for therapeutic purposes. There is accumulating evidence that aldynoglial cells with Schwann cell-like growth-promoting properties emerge in the lesioned CNS. However, the characterization of these cells and the signals triggering their in situ generation have remained enigmatic. In the present study, we used the p75 neurotrophin receptor (p75(NTR) ) as a marker for Schwann cells to study gliogenesis in the well-defined canine distemper virus (CDV)-induced demyelination model. White matter lesions of CDV-infected dogs contained bi- to multipolar, p75(NTR) -expressing cells that neither expressed MBP, GFAP, BS-1, or P0 identifying oligodendroglia, astrocytes, microglia, and myelinating Schwann cells nor CDV antigen. Interestingly, p75(NTR) -expression became apparent prior to the onset of demyelination in parallel to the expression of β-amyloid precursor protein (β-APP), nonphosphorylated neurofilament (n-NF), BS-1, and CD3, and peaked in subacute lesions with inflammation. To study the role of infiltrating immune cells during differentiation of Schwann cell-like glia, organotypic slice cultures from the normal olfactory bulb were established. Despite the absence of infiltrating lymphocytes and macrophages, a massive appearance of p75(NTR) -positive Schwann-like cells and BS-1-positive microglia was noticed at 10 days in vitro. It is concluded that axonal damage as an early signal triggers the differentiation of tissue-resident precursor cells into p75(NTR) -expressing aldynoglial Schwann cells that retain an immature pre-myelin state. Further studies have to address the role of microglia during this process and the regenerative potential of aldynoglial cells in CDV infection and other demyelinating diseases.

In vitro characterization and preferential infection by canine distemper virus of glial precursors with Schwann cell characteristics from adult canine brain

AIMS

Canine distemper virus (CDV)-induced demyelinating leukoencephalomyelitis is a naturally occurring model for multiple sclerosis. The aim of this study was to establish primary glial cell cultures from adult canine brain for the analysis of CDV spread and cell tropism.

METHODS

Cultures were inoculated with the CDV-R252 and a CDV-Onderstepoort strain expressing the green fluorescent protein (CDV-OndeGFP). CDV antigen expression was studied using cell type-specific antibodies at different days post infection. Glial cells expressing p75(NTR) were purified using antibody-based techniques and characterized with regard to antigen expression and proliferation.

RESULTS

Three weeks after seeding, cultures contained spindle-shaped cells expressing p75(NTR), oligodendrocytic cells, astrocytes, microglia and fibroblasts. Both CDV strains induced a mild to moderate cytopathic effect that consisted of single necrotic and few syncytial giant cells, but displayed in part a differential cell tropism. Whereas CDV-OndeGFP expression in microglia and astrocytes did not exceed 1% and 50%, respectively, CDV-R252 infected 100% and 80% of both cell types, respectively. The cells most early infected by both CDV strains expressed p75(NTR) and may correlate to cells previously identified as aldynoglia. Treatment of p75(NTR+) cells with Schwann cell mitogens and serum deprivation increased proliferation and A2B5 expression, respectively, indicating common properties compared with Schwann cells and oligodendrocyte precursors.

CONCLUSIONS

Infection of adult canine astrocytes and microglia revealed CDV strain-specific cell tropism. Moreover, this is the first identification of a glial cell type with Schwann cell-like properties in adult canine brain and, more importantly, these cells displayed a high susceptibility to CDV infection.

The function of p75NTR in glia

The p75 neurotrophin receptor (p75(NTR)) is expressed on many cell types and can influence a variety of cellular functions. This receptor can mediate cell survival or cell death, can promote or inhibit axonal growth and can facilitate or attenuate proliferation, depending on the cell context. The emerging picture regarding p75(NTR) indicates that it can partner with different coreceptors to dictate specific responses. It then signals by recruiting intracellular binding proteins to activate different signaling pathways. The function of p75(NTR) has mainly been studied in neurons; however, it is also expressed in a variety of glial populations, especially during development and after injury, where its roles have been poorly defined. In this review, we will examine the potential roles for p75(NTR) in glial function.

Genetic Manipulation of Neural Stem Cells for Transplantation into the Injured Spinal Cord

The injured adult spinal cord is not conducive for neuronal regeneration and neurogenesis. Engrafted neural precursor cells (NPCs) differentiate largely into astroglia, with only a very small percentage becoming neurons (which might replace injured neurons) or oligodendroglia (which might improve injury induced demyelination of spared neurons). Several recent attempts have been made to enhanced neurogenesis or oligodendroglia differentiation of transplanted NPCs by genetic manipulation. These include exogenous expression of noggin, with the idea of antagonizing the astroglia differentiation promoting Bone Morphogenetic Proteins (BMPs). Direct attempts to enhance neurogenesis have also been made in transgenic over-expression of neurogenic basic helix-loop-helix transcription factors. These experiments resulted in some interesting observations, which we discuss here in the light of recent advances in development of cell-based engraftment therapy for spinal cord injuries.