Schwann cell differentiation.

Recent studies of Schwann cell differentiation in vivo and in vitro have provided new insights into determinative signal transduction events both at the cell surface and in the nucleus. Several polypeptide growth factors and their receptors, most notably the neuregulins and receptors of the ErbB family, have been implicated in the specification of cell fate, the control of precursor cell proliferation, and the regulation of programmed cell death during both early and late Schwann cell differentiation. Our understanding of the transcriptional control of Schwann cell development, particularly by the POU protein SCIP and the zinc-finger protein Krox-20, has been advanced by transgenic, knockout, and expression studies.

High serum endostatin levels in Down syndrome: implications for improved treatment and prevention of solid tumours.

We report here a comparison of serum endostatin levels in Down syndrome patients to normal control subjects. We analysed serum samples from 35 patients with Down syndrome and 54 normal control subjects and found that although serum levels of endostatin vary widely in a normal human population, serum endostatin levels are significantly elevated in patients with Down syndrome. This result may explain the relative decrease in incidence of various solid tissue tumours observed in Down syndrome, given the role of endostatin as a potent inhibitor of tumour-induced angiogenesis in both human and animal models. Based upon these data, we propose that an increase of about one-third of normal endostatin serum levels may represent an effective therapeutic dose to significantly inhibit many solid tumours.

Membrane localization of the HflA regulatory protease of Escherichia coli by immunoelectron microscopy.

The hflA locus of Escherichia coli specifies a multisubunit protease that selectively degrades the cII transcriptional activator of phage lambda. The regulated turnover of cII is critical for the choice between the lytic and lysogenic pathways of viral development. Previous cell fractionation work has indicated that HflA is associated with the inner membrane fraction. We have sought to demonstrate that the HflA protease is localized in the cell membrane of intact cells. To achieve this goal, we have combined electron microscopy of thin-sectioned E. coli cells with antibody tagging by a colloidal gold label. Using antibody to purified HflA protein, we have found preferential membrane labeling for hflA+ cells but not for hflA mutant cells. We conclude that HflA protease is localized in the cell membrane. The membrane location for HflA protein may serve as a component of a targeting mechanism to limit the action of the regulatory protease to selected cytoplasmic proteins.

The transcription factors SCIP and Krox-20 mark distinct stages and cell fates in Schwann cell differentiation.

We have studied the transcription factors SCIP and Krox-20 in differentiating Schwann cells-during normal development, in experimentally induced degenerating and regenerating peripheral nerves, and in cell culture-and have compared the expression of these regulators to a battery of genes that mark distinct stages in Schwann cell differentiation. In the myelinating Schwann cell lineage, we find that SCIP is initially induced by contact with axons and first appears near the last round of cell division in immature cells. This expression is transient--it is maximal in "promyelinating" cells and is then extinguished as Schwann cells overtly differentiate and myelinate axons. In contrast, Krox-20 appears in cells 24-36 h after they become SCIP+ and continues to be expressed in mature myelinating cells. These differences in regulation are seen in normal development, in regenerating nerves following nerve crush, and in cultured Schwann cells stimulated to adopt a myelination phenotype by elevation of intracellular cyclic AMP. Importantly, transient SCIP expression is also observed in the nonmyelinating Schwann cell lineage, but Krox-20 expression is not. Together with the myelination phenotypes exhibited by SCIP and Krox-20 mutant mice, these results suggest that SCIP preferentially acts during the predifferentiated phases of Schwann cell development, while in contrast, Krox-20 is associated with the later commitment to myelination and may therefore function as a direct transactivator of myelination genes.

Krox-20 controls SCIP expression, cell cycle exit and susceptibility to apoptosis in developing myelinating Schwann cells.

The transcription factors Krox-20 and SCIP each play important roles in the differentiation of Schwann cells. However, the genes encoding these two proteins exhibit distinct time courses of expression and yield distinct cellular phenotypes upon mutation. SCIP is expressed prior to the initial appearance of Krox-20, and is transient in both the myelinating and non-myelinating Schwann cell lineages; while in contrast, Krox-20 appears approximately 24 hours after SCIP and then only within the myelinating lineage, where its expression is stably maintained into adulthood. Similarly, differentiation of SCIP-/- Schwann cells appears to transiently stall at the promyelinating stage that precedes myelination, whereas Krox-20(-/-) cells are, by morphological criteria, arrested at this stage. These observations led us to examine SCIP regulation and Schwann cell phenotype in Krox-20 mouse mutants. We find that in Krox-20(-/-) Schwann cells, SCIP expression is converted from transient to sustained. We further observe that both Schwann cell proliferation and apoptosis, which are normal features of SCIP+ cells, are also markedly increased late in postnatal development in Krox-20 mutants relative to wild type, and that the levels of cell division and apoptosis are balanced to yield a stable number of Schwann cells within peripheral nerves. These data demonstrate that the loss of Krox-20 in myelinating Schwann cells arrests differentiation at the promyelinating stage, as assessed by SCIP expression, mitotic activity and susceptibility to apoptosis.

A role for insulin-like growth factor-I in the regulation of Schwann cell survival.

During postnatal development in the peripheral nerve, differentiating Schwann cells are susceptible to apoptotic death. Schwann cell apoptosis is regulated by axons and serves as one mechanism through which axon and Schwann cell numbers are correctly matched. This regulation is mediated in part by the provision of limiting axon-derived trophic molecules, although neuregulin-1 (NRG-1) is the only trophic factor shown to date to support Schwann cell survival. In this report, we identify insulin-like growth factor-I (IGF-I) as an additional trophin that can promote Schwann cell survival in vitro. We find that IGF-I, like NRG-1, can prevent the apoptotic death of postnatal rat Schwann cells cultured under conditions of serum withdrawal. Moreover, we show that differentiating Schwann cells in the rat sciatic nerve express both the IGF-I receptor (IGF-I R) and IGF-I throughout postnatal development. These results indicate that IGF-I is likely to control Schwann cell viability in the developing peripheral nerve and, together with other findings, raise the interesting possibility that such survival regulation may switch during postnatal development from an axon-dependent mechanism to an autocrine and/or paracrine one.