Martine Jaegle

The POU Factor Oct-6 and Schwann Cell Differentiation

The POU transcription factor Oct-6, also known as SCIP or Tst-1, has been implicated as a major transcriptional regulator in Schwann cell differentiation. Microscopic and immunochemical analysis of sciatic nerves of Oct-6−/− mice at different stages of postnatal development reveals a delay in Schwann cell differentiation, with a transient arrest at the promyelination stage. Thus, Oct-6 appears to be required for the transition of promyelin cells to myelinating cells. Once these cells progress past this point, Oct-6 is no longer required, and myelination occurs normally.

Neural crest stem cells undergo multilineage differentiation in developing peripheral nerves to generate endoneurial fibroblasts in addition to Schwann cells

Neural crest stem cells (NCSCs) persist in peripheral nerves throughout late gestation but their function is unknown. Current models of nerve development only consider the generation of Schwann cells from neural crest, but the presence of NCSCs raises the possibility of multilineage differentiation. We performed Cre-recombinase fate mapping to determine which nerve cells are neural crest derived. Endoneurial fibroblasts, in addition to myelinating and non-myelinating Schwann cells, were neural crest derived, whereas perineurial cells, pericytes and endothelial cells were not. This identified endoneurial fibroblasts as a novel neural crest derivative, and demonstrated that trunk neural crest does give rise to fibroblasts in vivo, consistent with previous studies of trunk NCSCs in culture. The multilineage differentiation of NCSCs into glial and non-glial derivatives in the developing nerve appears to be regulated by neuregulin, notch ligands, and bone morphogenic proteins, as these factors are expressed in the developing nerve, and cause nerve NCSCs to generate Schwann cells and fibroblasts, but not neurons, in culture. Nerve development is thus more complex than was previously thought, involving NCSC self-renewal, lineage commitment and multilineage differentiation.

Targeted mutation of Cyln2 in the Williams syndrome critical region links CLIP-115 haploinsufficiency to neurodevelopmental abnormalities in mice

Williams syndrome is a neurodevelopmental disorder caused by the hemizygous deletion of 1.6 Mb on human chromosome 7q11.23. This region comprises the gene CYLN2, encoding CLIP-115, a microtubule-binding protein of 115 kD. Using a gene-targeting approach, we provide evidence that mice with haploinsufficiency for Cyln2 have features reminiscent of Williams syndrome, including mild growth deficiency, brain abnormalities, hippocampal dysfunction and particular deficits in motor coordination. Absence of CLIP-115 also leads to increased levels of CLIP-170 (a closely related cytoplasmic linker protein) and dynactin at the tips of growing microtubules. This protein redistribution may affect dynein motor regulation and, together with the loss of CLIP-115–specific functions, underlie neurological alterations in Williams syndrome.

The claw paw mutation reveals a role for Lgi4 in peripheral nerve development.

Peripheral nerve development results from multiple cellular interactions between axons, Schwann cells and the surrounding mesenchymal tissue. The delayed axonal sorting and hypomyelination throughout the peripheral nervous system of claw paw (clp) mutant mice suggest that the clp gene product is critical for these interactions. Here we identify the clp mutation as a 225-bp insertion in the Lgi4 gene. Lgi4 encodes a secreted and glycosylated leucine-rich repeat protein and is expressed in Schwann cells. The clp mutation affects Lgi4 mRNA splicing, resulting in a mutant protein that is retained in the cell. Additionally, siRNA-mediated downregulation of Lgi4 in wild-type neuron–Schwann cell cocultures inhibits myelination, whereas exogenous Lgi4 restores myelination in clp/clp cultures. Thus, the abnormalities observed in clp mice are attributable to the loss of Lgi4 function, and they identify Lgi4 as a new component of Schwann cell signaling pathway(s) that controls axon segregation and myelin formation.

Adam22 Is a Major Neuronal Receptor for Lgi4-Mediated Schwann Cell Signaling

The segregation and myelination of axons in the developing PNS, results from a complex series of cellular and molecular interactions between Schwann cells and axons. Previously we identified the Lgi4 gene (leucine-rich glioma-inactivated4) as an important regulator of myelination in the PNS, and its dysfunction results in arthrogryposis as observed in claw paw mice. Lgi4 is a secreted protein and a member of a small family of proteins that are predominantly expressed in the nervous system. Their mechanism of action is unknown but may involve binding to members of the Adam (A disintegrin and metalloprotease) family of transmembrane proteins, in particular Adam22. We found that Lgi4 and Adam22 are both expressed in Schwann cells as well as in sensory neurons and that Lgi4 binds directly to Adam22 without a requirement for additional membrane associated factors. To determine whether Lgi4-Adam22 function involves a paracrine and/or an autocrine mechanism of action we performed heterotypic Schwann cell sensory neuron cultures and cell type-specific ablation of Lgi4 and Adam22 in mice. We show that Schwann cells are the principal cellular source of Lgi4 in the developing nerve and that Adam22 is required on axons. Our results thus reveal a novel paracrine signaling axis in peripheral nerve myelination in which Schwann cell secreted Lgi4 functions through binding of axonal Adam22 to drive the differentiation of Schwann cells.

N-WASP is required for membrane wrapping and myelination by Schwann cells

N-WASP–deficient Schwann cells sort and ensheath axons but arrest at the promyelinating stage.

A cell type-specific allele of the POU gene Oct-6 reveals Schwann cell autonomous function in nerve development and regeneration

While an important role for the POU domain transcription factor Oct‐6 in the developing peripheral nerve has been well established, studies into its exact role in nerve development and regeneration have been hampered by the high mortality rate of newborn Oct‐6 mutant animals. In this study we have generated a Schwann cell‐specific Oct‐6 allele through deletion of the Schwann cell‐specific enhancer element (SCE) in the Oct‐6 locus. Analysis of mice homozygous for this allele (ΔSCE allele) reveals that rate‐limiting levels of Oct‐6 in Schwann cells are dependent on the SCE and that this element does not contribute to Oct‐6 regulation in other cell types. We demonstrate a Schwann cell autonomous function for Oct‐6 during nerve development as well as in regenerating nerve. Additionally, we show that Krox‐20, an important regulatory target of Oct‐6 in Schwann cells, is activated, with delayed kinetics, through an Oct‐6‐independent mechanism in these mice.

Functional dissection of the Oct6 Schwann cell enhancer reveals an essential role for dimeric Sox10 binding.

The POU domain transcription factor Pou3f1 (Oct6/Scip/Tst1) initiates the transition from ensheathing, promyelinating Schwann cells to myelinating cells. Axonal and other extracellular signals regulate Oct6 expression through the Oct6 Schwann cell enhancer (SCE), which is both required and sufficient to drive all aspects of Oct6 expression in Schwann cells. Thus, the Oct6 SCE is pivotal in the gene regulatory network that governs the onset of myelin formation in Schwann cells and provides a link between myelin promoting signaling and activation of a myelin-related transcriptional network. In this study, we define the relevant cis-acting elements within the SCE and identify the transcription factors that mediate Oct6 regulation. On the basis of phylogenetic comparisons and functional in vivo assays, we identify a number of highly conserved core elements within the mouse SCE. We show that core element 1 is absolutely required for full enhancer function and that it contains closely spaced inverted binding sites for Sox proteins. For the first time in vivo, the dimeric Sox10 binding to this element is shown to be essential for enhancer activity, whereas monomeric Sox10 binding is nonfunctional. As Oct6 and Sox10 synergize to activate the expression of the major myelin-related transcription factor Krox20, we propose that Sox10-dependent activation of Oct6 defines a feedforward regulatory module that serves to time and amplify the onset of myelination in the peripheral nervous system.

A distal Schwann cell‐specific enhancer mediates axonal regulation of the Oct‐6 transcription factor during peripheral nerve development and regeneration

The POU domain transcription factor Oct‐6 is a major regulator of Schwann cell differentiation and myelination. During nerve development and regeneration, expression of Oct‐6 is under the control of axonal signals. Identification of the cis‐acting elements necessary for Oct‐6 gene regulation is an important step in deciphering the complex signalling between Schwann cells and axons governing myelination. Here we show that a fragment distal to the Oct‐6 gene, containing two DNase I‐hypersensitive sites, acts as the Oct‐6 Schwann cell‐specific enhancer (SCE). The SCE is sufficient to drive spatially and temporally correct expression, during both normal peripheral nerve development and regeneration. We further demonstrate that a tagged version of Oct‐6, driven by the SCE, rescues the peripheral nerve phenotype of Oct‐6‐deficient mice. Thus, our isolation and characterization of the Oct‐6 SCE provides the first description of a cis‐acting genetic element that responds to converging signalling pathways to drive myelination in the peripheral nervous system.

Role of Oct-6 in Schwann cell differentiation.

Research into the POU transcription factor Oct‐6 has been the focus of much current attention, in particular its role in Schwann cell development and differentiation. Based on published data and data presented here, we propose a model for Oct‐6 function at two distinct stages of Schwann cell maturation. First, Oct‐6 function is required in promyelin cells for their timely differentiation into myelinating cells. Second, Oct‐6 functions during myelination and is required for the proper downregulation of its own gene. While the first function of Oct‐6 is firmly established, the second function is still highly hypothetical. Experiments to establish a distinct role for Oct‐6 in late Schwann cell differentiation are discussed. Microsc. Res. Tech. 41:372–378, 1998.