The cyclin-dependent kinase inhibitor p57kip2 is a negative regulator of Schwann cell differentiation and in vitro myelination

Abstract

The p57kip2 gene encodes a member of the cyclin-dependent kinase inhibitor family, proteins known to block G 1/S transition during the mammalian cell cycle. We observed that expression of p57kip2 in Schwann cells of the developing and injured adult peripheral nervous system is dynamically regulated. Using gene knockdown by means of vector-based RNA interference in cultured primary Schwann cells we found that reduced levels of p57kip2 lead to cell cycle exit, actin filament stabilization, altered cell morphology and growth, and down-regulation of promyelinating markers as well as induction of myelin genes and proteins. In addition, we could demonstrate that in vitro myelination is enhanced by p57kip2-suppressed Schwann cells. Using microarray technology we found that these cellular reactions are specific to lowered p57kip2 expression levels and detected a shift of the transcriptional expression program toward the pattern known from Schwann cells in developing peripheral nerves. Because in the absence of axons primary Schwann cells normally do not display differentiation-associated reactions, we conclude that we have identified a mechanism and an important intrinsic negative regulator of myelinating glia differentiation.glia ͉ immune neuropathies ͉ myelin ͉ peripheral nerve A xons are tightly associated with myelinating glial cells in the vertebrate nervous system, Schwann cells in the peripheral nervous system, and oligodendrocytes in the CNS. These cells wrap around axons in a radial growth and migration process and establish multiple lipid-and protein-rich layers to support and electrically insulate them, thus enabling saltatory propagation of electrical signals. As a result of axonal segregation, myelinating Schwann cells are associated with large-caliber axons in a 1:1 relationship and represent highly specialized and differentiated cells. Schwann cells derive from neural crest, and their differentiation during peripheral nerve development was shown to depend on a variety of genes, such as integrins, the transcription factors Sox10, Oct-6, and Krox20, or neuregulin-, p75-LNGFR, and Lgi4 signaling cascades (1-7).Cell cycle exit is a prerequisite for myelinating glial cell differentiation followed by morphological changes as well as the expression and deposition of myelin proteins within the layers of lipid sheaths. The establishment of such a complex morphology depends on regulated cytoskeletal dynamics, e.g., actin filament turnover. This was demonstrated by means of application of cytochalasin D, a mycotoxin that interferes with actin filament assembly. Differentiation and myelination of Schwann cells cocultured with dorsal root ganglion (DRG) neurons were found to be blocked when this drug was added to the culture medium, indicating that actin filament dynamics is imperative for Schwann cell growth and morphological adaptation during the myelination process (8). Nevertheless, the question as to whether naturally occurring regulators of cytoskeletal dynamics, acting during nerve development and/or after inju...

p57Kip2 is a repressor of Mash1 activity and neuronal differentiation in neural stem cells

“…An expression-level-dependent role for p57Kip2 in Notch-regulated oligodendrocyte specification in the ventral spinal cord has been suggested from elegant experiments in zebrafish, 34 and it was recently shown that proper levels of p57Kip2 is critical for correct proliferation as well as differentiation of primary oligodendrocyte progenitor cells as well as Schwann cells. 36,37 Furthermore, the timing and levels of Mash1 and Dlx1/2 expression, as well as Notch signaling, have been shown to be critical parameters to secure the proper proliferation, differentiation, and migration of ventral telencephalic progenitors. 24,38 It is at present difficult experimentally to yield efficient oligodendrocyte production from embryonic NSCs hampering such investigations in NSCs, but future studies should address this hypothesis in greater detail.…”

Negative regulation of myelination: Relevance for development, injury, and demyelinating disease

“…The cyclin-dependent kinase inhibitor p57kip2, which is downregulated as postnatal nerves myelinate, has also been implicated in negative regulation of myelination, since RNA interference-based knockdown of p57kip2 in cultured Schwann cells induces modest expression of myelin proteins, including P 0 and MBP, and promotes myelination in co-cultures (Heinen et al, 2008).…”

p57kip2 is dynamically regulated in experimental autoimmune encephalomyelitis and interferes with oligodendroglial maturation

“…However, silencing RNA (siRNA) dependent approaches led only to transient suppression of p57kip2 expression. As an alternative, we used a small hairpin RNA (shRNA) cassette (19), which allows long-term p57kip2 suppression and was previously shown to be specific for p57kip2 and free from off-target effects, such as an IFN response induction (19). By using this approach, oligodendroglial p57kip2 expression could be reduced for up to 9 days as shown by quantitative RT-PCR of transfected and sorted OPCs 2 days following transfection ( Fig.…”

“…As a member of the cip/kip family, p57kip2 was originally described as a cyclin-dependent kinase inhibitor (CDKI), but a number of other cellular processes were recently shown to depend on cip/kip proteins (18). For instance, long-term suppression of p57kip2 in Schwann cells, the myelinating glial cells of the PNS, efficiently induces their differentiation in culture, even in the absence of axons (19). We therefore investigated the impact of down-regulated p57kip2 levels on oligodendroglial maturation and report here that this results in acceleration of cellular differentiation in vitro.…”