A developmentally regulated switch directs regenerative growth of Schwann cells through cyclin D1.
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Sciatic nerve axons in cyclin D1 knockout mice develop normally, become properly ensheathed by Schwann cells, and appear to function normally. However, in the Wallerian degeneration model of nerve injury, the mitotic response of Schwann cells is completely inhibited. The mitotic block is Schwann cell autonomous and developmentally regulated. Rescue analysis (by "knockin" of cyclin E) indicates that D1 protein, rather than regulatory elements of the D1 gene, provides the essential Schwann cell function. Genetic inhibition of the Schwann cell cycle shows that neuronal responses to nerve injury are surprisingly independent of Schwann cell mitotic responses. Even axonal regrowth into the distal zone of a nerve crush injury is not markedly impaired in cyclin D1-/- mice. (+info)
Peripheral demyelination and neuropathic pain behavior in periaxin-deficient mice.
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The Prx gene in Schwann cells encodes L- and S-periaxin, two abundant PDZ domain proteins thought to have a role in the stabilization of myelin in the peripheral nervous system (PNS). Mice lacking a functional Prx gene assemble compact PNS myelin. However, the sheath is unstable, leading to demyelination and reflex behaviors that are associated with the painful conditions caused by peripheral nerve damage. Older Prx-/- animals display extensive peripheral demyelination and a severe clinical phenotype with mechanical allodynia and thermal hyperalgesia, which can be reversed by intrathecal administration of a selective NMDA receptor antagonist We conclude that the periaxins play an essential role in stabilizing the Schwann cell-axon unit and that the periaxin-deficient mouse will be an important model for studying neuropathic pain in late onset demyelinating disease. (+info)
Cell cycle control of Schwann cell proliferation: role of cyclin-dependent kinase-2.
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Schwann cell proliferation is regulated by multiple growth factors and axonal signals. However, the molecules that control growth arrest of Schwann cells are not well defined. Here we describe regulation of the cyclin-dependent kinase-2 (CDK2) protein, an enzyme that is necessary for the transition from G1 to S phase. Levels of CDK2 protein were elevated in proliferating Schwann cells cultured in serum and forskolin. However, when cells were grown with either serum-free media or at high densities, CDK2 levels declined to low levels. The decrease in CDK2 levels was associated with growth arrest of Schwann cells. The modulation of CDK2 appears to be regulated at the transcriptional level, because CDK2 mRNA levels and its promoter activity both decline during cell cycle arrest. Furthermore, analysis of the CDK2 promoter suggests that Sp1 DNA binding sites are essential for maximal activation in Schwann cells. Together, these data suggest that CDK2 may represent a significant target of developmental signals that regulate Schwann cell proliferation and that this regulation is mediated, in part, through regulation of Sp1 transcriptional activity. (+info)
Axonal regulation of Schwann cell proliferation and survival and the initial events of myelination requires PI 3-kinase activity.
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In this report, we have investigated the signaling pathways that are activated by, and mediate the effects of, the neuregulins and axonal contact in Schwann cells. Phosphatidylinositol 3-kinase (PI 3-kinase) and mitogen-activated protein kinase kinase (MAPK kinase) are strongly activated in Schwann cells by glial growth factor (GGF), a soluble neuregulin, and by contact with neurite membranes; both kinase activities are also detected in Schwann cell-DRG neuron cocultures. Inhibition of the PI 3-kinase, but not the MAP kinase, pathway reversibly inhibited Schwann cell proliferation induced by GGF and neurites. Cultured Schwann cells undergo apoptosis after serum deprivation and can be rescued by GGF or contact with neurites; these survival effects were also blocked by inhibition of PI 3-kinase. Finally, we have examined the role of these signaling pathways in Schwann cell differentiation in cocultures. At early stages of coculture, inhibition of PI 3-kinase, but not MAPK kinase, blocked Schwann cell elongation and subsequent myelination but did not affect laminin deposition. Later, after Schwann cells established a one-to-one relationship with axons, inhibition of PI 3-kinase did not block myelin formation, but the myelin sheaths that formed were shorter, and the rate of myelin protein accumulation was markedly decreased. PI 3-kinase inhibition had no observable effect on the maintenance of myelin sheaths in mature myelinated cocultures. These results indicate that activation of PI 3-kinase by axonal factors, including the neuregulins, promotes Schwann cell proliferation and survival and implicate PI 3-kinase in the early events of myelination. (+info)
Spontaneous age-related peripheral neuropathy in B6C3F1 mice.
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Peripheral neuropathy, which accompanies aging, occurs during the long-term rearing of laboratory animals. The present study set out to delineate the clinical and functional features of this neuropathy. A total of 200 B6C3F1 female mice, in groups of 5 to 20 mice, were sacrificed and autopsied each week beginning at 5 weeks and continuing to 130 weeks of age. Examination for histopathologic changes was conducted on the dorsal nerve roots, sciatic nerves, peroneal nerves, tibial nerves, plantar nerves and brachial nerve plexuses. At 90 weeks of age or later, peripheral neuropathy, characterized by axonal degeneration and Schwann cell proliferation, were observed mainly in the sciatic nerves, brachial nerve plexus and peroneal nerves. These spontaneous age-related nerve lesions appeared in all animals by 100 weeks of age in all nerves, and increased with increasing age. The nerve lesions were most prominent in the distal sciatic nerve. The rectal and hind-limb surface temperatures, motor nerve conduction velocity, blood glucose and HbA1C decreased with increasing age. Elevation of sorbitol contents in sciatic nerves and reduction of myo-inositol levels were also detected in 120-week-old mice. However, except for blood glycemic parameters, no correlation with peripheral nerve lesions could be demonstrated. Spontaneous hypoglycemia (< 40 mg/dL) persisted throughout the day in a small percentage (< 5%) of animals aged 80 weeks or more; these animals had extensive lesions in the peripheral nerves and showed decreased plasma levels of HbA1C and frucutosamines and increased plasma levels of ketones. These results suggest that spontaneous peripheral nerve disorders which accompany aging might worsen if spontaneous age-related hypoglycemia is also present. Such age-related changes must be taken into consideration in experimental studies performed on mice of this age. (+info)
Schwann cells synthesize type V collagen that contains a novel alpha 4 chain. Molecular cloning, biochemical characterization, and high affinity heparin binding of alpha 4(V) collagen.
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Previously, we reported the isolation of a heparan sulfate-binding collagenous protein, p200, that is expressed by Schwann cells in developing peripheral nerves ((1996) J. Biol. Chem. 271, 13844-13853; (1999) J. Neurosci. Res. 56, 284-294). Here, we report the cloning of p200 cDNA from a Schwann cell cDNA library. The deduced amino acid sequence identifies p200 as a novel member of the collagen type V gene family. This polypeptide, which we have named alpha4 type V (alpha4(V)) collagen, contains an uninterrupted Gly-X-X collagen domain of 1011 amino acids that shows 82% sequence identity to human alpha3(V) collagen and 71% identity to rat alpha1(V) collagen. alpha4(V) is secreted by Schwann cells as a collagen heterotrimer that also contains alpha1(V) chains. alpha4(V)-containing collagen molecules synthesized by Schwann cells retain their amino-terminal non-collagenous domains. alpha4(V) mRNA was detected by reverse transcriptase-linked polymerase chain reaction amplification in neonatal and adult brain and neonatal peripheral nerve. alpha4(V) mRNA and protein were not detected in most other tissues, including the placenta and heart, which are known to contain alpha3(V). This pattern of alpha4(V) expression contrasted with that of alpha1(V) mRNA and protein, which were ubiquitously expressed. The isolated alpha4(V) chain demonstrated an unusually high affinity for heparin. The restricted expression and unusual properties of alpha4(V)-containing collagen type V molecules suggest a unique and important role for these molecules in peripheral nerve development. (+info)
A distal Schwann cell-specific enhancer mediates axonal regulation of the Oct-6 transcription factor during peripheral nerve development and regeneration.
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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. (+info)
Active zones on motor nerve terminals contain alpha 3beta 1 integrin.
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Active zones are the sites along nerve terminals where synaptic vesicles dock and undergo calcium-dependent exocytosis during synaptic transmission. Here we show, by immunofluorescent staining with antibodies generated against Xenopus laevis integrins, that alpha3beta1 integrin is concentrated at the active zones of Xenopus motor nerve terminals. Because integrins can link extracellular matrix molecules to cytoskeletal elements and participate in the formation of signaling complexes, the localization of integrin at active zones suggests that it may play a role in the adhesion of the nerve terminals to the synaptic basal lamina, in the formation and maintenance of active zones, and in some of the events associated with calcium-dependent exocytosis of neurotransmitter. Our findings also indicate that the integrin composition of the terminal Schwann cells differs from that of the motor nerve terminals, and this may account at least in part for differences in their adhesiveness to the synaptic basal lamina. (+info)