Signaling through Arf6 guanine-nucleotide exchange factor cytohesin-1 regulates migration in Schwann cells

Cellular Signal-Regulated Schwann Cell Myelination and Remyelination

Increasing studies have demonstrated multiple signaling molecules responsible for oligodendrocytes and Schwann cells development such as migration, differentiation, myelination, and axo-glial interaction. However, complicated roles in these events are still poorly understood. This chapter focuses on well established intracellular signaling transduction and recent topics that control myelination and are elucidated from accumulating evidences. The underlying molecular mechanisms, which involved in membrane trafficking through small GTPase Arf6 and its activator cytohesins, demonstrate the crosstalk between well established intracellular signaling transduction and a new finding signaling pathway in glial cells links to physiological phenotype and essential role in peripheral nerve system (PNS). Since Arf family proteins affect the expression levels of myelin protein zero (MPZ) and Krox20, which is a transcription factor regulatory factor in early developmental stages of Schwann cells, Arf proteins likely to be key regulator for Schwann cells development. Herein, we discuss how intracellular signaling transductions in Schwann cells associate with myelination in CNS and PNS.

Mutations in dock1 disrupt early Schwann cell development

Background

In the peripheral nervous system (PNS), specialized glial cells called Schwann cells produce myelin, a lipid-rich insulating sheath that surrounds axons and promotes rapid action potential propagation. During development, Schwann cells must undergo extensive cytoskeletal rearrangements in order to become mature, myelinating Schwann cells. The intracellular mechanisms that drive Schwann cell development, myelination, and accompanying cell shape changes are poorly understood.

Methods

Through a forward genetic screen in zebrafish, we identified a mutation in the atypical guanine nucleotide exchange factor, dock1, that results in decreased myelination of peripheral axons. Rescue experiments and complementation tests with newly engineered alleles confirmed that mutations in dock1 cause defects in myelination of the PNS. Whole mount in situ hybridization, transmission electron microscopy, and live imaging were used to fully define mutant phenotypes.

Results

We show that Schwann cells in dock1 mutants can appropriately migrate and are not decreased in number, but exhibit delayed radial sorting and decreased myelination during early stages of development.

Conclusions

Together, our results demonstrate that mutations in dock1 result in defects in Schwann cell development and myelination. Specifically, loss of dock1 delays radial sorting and myelination of peripheral axons in zebrafish.

Electronic supplementary material

The online version of this article (10.1186/s13064-018-0114-9) contains supplementary material, which is available to authorized users.

Physiological functions of the small GTPase Arf6 in the nervous system

The small GTPase ADP-ribosylation factor 6 (Arf6) plays important roles in membrane dynamics-based neuronal cell events such as neurite outgrowth and spine formation. However, physiological functions of Arf6 in the nervous system at whole animal level have not yet been explored. We have recently generated conditional knockout mice lacking Arf6 in neurons or oligodendrocytes of central nervous system (CNS) or both cell lineages, and analyzed them. We found that ablation of Arf6 gene from neurons, but not from oligodendrocytes, caused the defect in axon myelination at the fimbria of hippocampus (Fim) and corpus callosum (CC). We also found that migration of oligodendrocyte precursor cells (OPCs) from the subventricular zone to the Fim and CC in mice lacking Arf6 in neurons was impaired. Finally, it was found that secretion of fibroblast growth factor-2 (FGF-2), a guidance factor for OPC migration, from hippocampi lacking Arf6 was impaired. Collectively, these findings demonstrate that Arf6 in neurons of the CNS plays an important role in OPC migration by regulating secretion of FGF-2 from neurons, thereby contributing to the axon myelination. Here, we discuss our current understanding of physiological functions of Arf6 in the nervous system.

Cytohesin 2/ARF6 regulates preadipocyte migration through the activation of ERK1/2

Preadipocyte migration is vital for the development of adipose tissue, which plays a crucial role in lipid metabolism. ADP-ribosylation factor 6 (ARF6) small GTPase, which regulates membrane trafficking, is activated by GTP-exchange factors (GEFs) such as cytohesin 2. Cytohesin 2 and ARF6 have previously been implicated in the regulation of 3T3-L1 preadipocyte migration. We investigated here the molecular mechanism underlying the cytohesin 2 and ARF6 mediated regulation of preadipocyte migration. Preadipocyte migration and the activation of ARF6 and ERK1/2 were studied by using a number of approaches, including pharmacological inhibitors, siRNA and the inhibitory peptides. The siRNA mediated down regulation of ARF6 and cytohesin 2 expression confirmed the requirement of both for migration of preadipocytes. Phosphatidylinositol 3-kinase (PI3K) and PI 4,5-bisphosphate (PIP2) have also found to be essential for the cytohesin 2/ARF6 induced preadipocyte migration. Pharmacological inhibition of the activation of ARF6, ERK1/2 or dynamin led to significant reduction in migration of 3T3-L1 preadipocytes. Furthermore, our study revealed the activation of ARF6 and ERK1/2 during migration of preadipocytes. In the migrating preadipocytes, ARF6 activation was inhibited with SecinH3 (cytohesin inhibitor) and LY294002 (PI3K inhibitor) whereas the ERK1/2 phosphorylation was inhibited with SecinH3, LY294002, PBP10 (a PIP2 sequester peptide) and PD98059 (MAPKK inhibitor). However, dynosore (dynamin inhibitor) had inhibited neither ARF6 activation nor ERK1/2 phosphorylation during preadipocyte migration. These results together suggest that cytohesin 2 activates ARF6 in a PI3K dependent manner and then the active ARF6 causes phosphorylation of ERK1/2 during preadipocyte migration.

In vivo knockdown of ErbB3 in mice inhibits Schwann cell precursor migration

The myelin sheath insulates neuronal axons and markedly increases the nerve conduction velocity. In the peripheral nervous system (PNS), Schwann cell precursors migrate along embryonic neuronal axons to their final destinations, where they eventually wrap around individual axons to form the myelin sheath after birth. ErbB2 and ErbB3 tyrosine kinase receptors form a heterodimer and are extensively expressed in Schwann lineage cells. ErbB2/3 is thought to be one of the primary regulators controlling the entire Schwann cell development. ErbB3 is the bona fide Schwann cell receptor for the neuronal ligand neuregulin-1. Although ErbB2/3 is well known to regulate both Schwann cell precursor migration and myelination by Schwann cells in fishes, it still remains unclear whether in mammals, ErbB2/3 actually regulates Schwann cell precursor migration. Here, we show that knockdown of ErbB3 using a Schwann cell-specific promoter in mice causes delayed migration of Schwann cell precursors. In contrast, littermate control mice display normal migration. Similar results are seen in an in vitro migration assay using reaggregated Schwann cell precursors. Also, ErbB3 knockdown in mice reduces myelin thickness in sciatic nerves, consistent with the established role of ErbB3 in myelination. Thus, ErbB3 plays a key role in migration, as well as in myelination, in mouse Schwann lineage cells, presenting a genetically conservative role of ErbB3 in Schwann cell precursor migration.

Rab35, acting through ACAP2 switching off Arf6, negatively regulates oligodendrocyte differentiation and myelination

Oligodendrocyte precursor cells differentiate into oligodendrocytes to form myelin sheaths. Rab35/ACAP2 and cytohesin-2 antagonistically control oligodendrocyte differentiation and myelination through Arf6 on/off regulation, presenting a unique way of regulating oligodendrocyte differentiation and myelination by a small GTPase network.

Oligodendrocyte precursor cells differentiate to produce myelin sheaths that insulate axons to ensure fast propagation of action potentials. Many aspects of differentiation are regulated by multiple extracellular signals. However, their intracellular signalings remain elusive. We show that Rab35 and its effector, ACAP2, a GTPase-activating protein that switches off Arf6 activity, negatively regulate oligodendrocyte morphological differentiation. Knockdown of Rab35 or ACAP2 with their respective small interfering RNAs promotes differentiation. As differentiation initiates, the activities of Rab35 and ACAP2 are down-regulated. The activity of Arf6, in contrast, is up-regulated. Arf6 knockdown inhibits differentiation, indicating that Rab35 and ACAP2 negatively regulate differentiation by down-regulating Arf6. Importantly, as differentiation proceeds, the activity of cytohesin-2, a guanine nucleotide exchange factor that switches on Arf6 activity, is up-regulated. Pharmacological inhibition of cytohesin-2 inhibits differentiation, suggesting that cytohesin-2 promotes differentiation by activating Arf6. Furthermore, using oligodendrocyte-neuronal cocultures, we find that knockdown of Rab35 or ACAP2 promotes myelination, whereas inhibition of cytohesin-2 or knockdown of Arf6 inhibits myelination. Thus Rab35/ACAP2 and cytohesin-2 antagonistically control oligodendrocyte differentiation and myelination through Arf6 regulation, presenting a unique small GTPase on/off switching mechanism.