The guanine nucleotide exchange factor Vav3 modulates oligodendrocyte precursor differentiation and supports remyelination in white matter lesions.

The tightly controlled processes of myelination and remyelination require the participation of the cytoskeleton. The reorganization of the cytoskeleton is controlled by small GTPases of the RhoA family. Here, we report that Vav3, a Rho GTPase regulating guanine nucleotide exchange factor (GEF) is involved in oligodendrocyte maturation, myelination and remyelination. When Vav3 was eliminated by genetic recombination, oligodendrocyte precursor cell (OPC) differentiation toward mature oligodendrocytes was accelerated. In contrast, Vav3-deficient oligodendrocytes displayed a reduced capacity to myelinate synthetic microfibers in vitro. Furthermore, remyelination was impaired in Vav3 knockout cerebellar slice cultures that were demyelinated by the addition of lysolecithin. In agreement with these observations, remyelination was compromised when the cuprizone model of myelin lesion was performed in Vav3-deficient mice. When Vav3-deficient oligodendrocytes were examined with Förster resonance energy transfer (FRET)-based biosensors, an altered activation profile of RhoA GTPases was revealed on the cellular level, which could be responsible for an impaired remyelination. Taken together, this study highlights Vav3 as a novel regulator of oligodendrocyte maturation and remyelination, suggesting that manipulation of the Vav3-dependent signaling pathway could help to improve myelin repair.

Involvement of the guanine nucleotide exchange factor Vav3 in central nervous system development and plasticity.

Small GTP-hydrolyzing enzymes (GTPases) of the RhoA family play manifold roles in cell biology and are regulated by upstream guanine nucleotide exchange factors (GEFs). Herein, we focus on the GEFs of the Vav subfamily. Vav1 was originally described as a proto-oncogene of the hematopoietic lineage. The GEFs Vav2 and Vav3 are more broadly expressed in various tissues. In particular, the GEF Vav3 may play important roles in the developing nervous system during the differentiation of neural stem cells into the major lineages, namely neurons, oligodendrocytes and astrocytes. We discuss its putative regulatory roles for progenitor differentiation in the developing retina, polarization of neurons and formation of synapses, migration of oligodendrocyte progenitors and establishment of myelin sheaths. We propose that Vav3 mediates the response of various neural cell types to environmental cues.

Regulation of oligodendrocyte precursor maintenance by chondroitin sulphate glycosaminoglycans.

Chondroitin sulfate proteoglycans (CSPGs) have been proven to inhibit morphological maturation of oligodendrocytes as well as their myelination capabilities. Yet, it remained unclear, whether CSPGs and/or their respective chondroitin sulfate glycosaminoglycan (CS-GAG) side chains also regulate the oligodendrocyte lineage progression. Here, we initially show that CS-GAGs detected by the monoclonal antibody 473HD are expressed by primary rat NG2-positive oligodendrocyte precursor cells (OPCs) and O4-positive immature oligodendrocytes. CS-GAGs become down-regulated with ongoing oligodendrocyte differentiation. Enzymatic removal of the CS-GAG chains by the bacterial enzyme Chondroitinase ABC (ChABC) promoted spontaneous differentiation of proliferating rat OPCs toward O4-positive immature oligodendrocytes. Upon forced differentiation, the enzymatic removal of the CS-GAGs accelerated oligodendrocyte differentiation toward both MBP-positive and membrane forming oligodendrocytes. These processes were attenuated on enriched CSPG fractions, mainly consisting of Phosphacan/RPTPß/ζ and to less extent of Brevican and NG2. To qualify CS-GAGs as universal regulators of oligodendrocyte biology, we finally tested the effect of CS-GAG removal on OPCs from different sources such as mouse cortical oligospheres, mouse spinal cord neurospheres, and most importantly human-induced pluripotent stem cell-derived radial glia-like neural precursor cells. For all culture systems used, we observed a similar inhibitory effect of CS-GAGs on oligodendrocyte differentiation. In conclusion, this study clearly suggests an important fundamental principle for complex CS-GAGs to regulate the oligodendrocyte lineage progression. Moreover, the use of ChABC in order to promote oligodendrocyte differentiation toward myelin gene expressing cells might be an applicable therapeutic option to enhance white matter repair.

Tenascins Interfere With Remyelination in an Ex Vivo Cerebellar Explant Model of Demyelination.

Oligodendrocytes form myelin membranes and thereby secure the insulation of axons and the rapid conduction of action potentials. Diseases such as multiple sclerosis highlight the importance of this glial cell population for brain function. In the adult brain, efficient remyelination following the damage to oligodendrocytes is compromised. Myelination is characterized by proliferation, migration, and proper integration of oligodendrocyte precursor cells (OPCs). These processes are among others controlled by proteins of the extracellular matrix (ECM). As a prominent representative ECM molecule, tenascin-C (Tnc) exerts an inhibitory effect on the migration and differentiation of OPCs. The structurally similar paralogue tenascin-R (Tnr) is known to promote the differentiation of oligodendrocytes. The model of lysolecithin-induced demyelination of cerebellar slice cultures represents an important tool for the analysis of the remyelination process. Ex vivo cerebellar explant cultures of Tnc -/- and Tnr -/- mouse lines displayed enhanced remyelination by forming thicker myelin membranes upon exposure to lysolecithin. The inhibitory effect of tenascins on remyelination could be confirmed when demyelinated wildtype control cultures were exposed to purified Tnc or Tnr protein. In that approach, the remyelination efficiency decreased in a dose-dependent manner with increasing concentrations of ECM molecules added. In order to examine potential roles in a complex in vivo environment, we successfully established cuprizone-based acute demyelination to analyze the remyelination behavior after cuprizone withdrawal in SV129, Tnc -/- , and Tnr -/- mice. In addition, we documented by immunohistochemistry in the cuprizone model the expression of chondroitin sulfate proteoglycans that are inhibitory for the differentiation of OPCs. In conclusion, inhibitory properties of Tnc and Tnr for myelin membrane formation could be demonstrated by using an ex vivo approach.