A coherent nomenclature for Eph receptors and their ligands.
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Animals↗
Biomedical subjects
G Lemke
Publications and source records attributed to G Lemke.
Neuregulins and neuregulin receptors in neural development.
The neuregulins are a family of closely related proteins that play important roles in neural and cardiac development, as well as in mammary carcinogenesis. The pleiotropic activities of these molecules are transduced by a set of receptor protein tyrosine kinases that exhibit structural similarity to the receptor for epidermal growth factor. Recent results have demonstrated essential roles for the neuregulins and their receptors in regulating cell number, determining cell fate, and establishing pattern in the developing central and peripheral nervous systems.
Animals↗
Sek4 and Nuk receptors cooperate in guidance of commissural axons and in palate formation.
Sek4 and Nuk are members of the Eph-related family of receptor protein-tyrosine kinases. These receptors interact with a set of cell surface ligands that have recently been implicated in axon guidance and fasciculation. We now demonstrate that the formation of the corpus callosum and anterior commissure, two major commissural axon tracts that connect the two cerebral hemispheres, is critically dependent on Sek4 and Nuk. While mice deficient in Nuk exhibit defects in pathfinding of anterior commissure axons, sek4 mutants have defects in corpus callosum formation. The phenotype in both axon tracts is markedly more severe in sek4/nuk1 double mutants, indicating that the two receptors act in a partially redundant fashion. sek4/nuk1 double mutants also exhibit specific guidance and fasciculation defects of diencephalic axon tracts. Moreover, while mice singly deficient in either Sek4 or Nuk are viable, most sek4/nuk1 double mutants die immediately after birth primarily due to a cleft palate. These results demonstrate essential and cooperative functions for Sek4 and Nuk in establishing axon pathways in the developing brain, and during the development of facial structures.
Animals↗
Unwrapping myelination.
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Crystallography, X-Ray↗
Cell death in the Schwann cell lineage and its regulation by neuregulin.
The development of Schwann cells, the myelin-forming glial cells of the vertebrate peripheral nervous system, involves a neonatal phase of proliferation in which cells migrate along and segregate newly formed axons. Withdrawal from the cell cycle, around postnatal days 2-4 in rodents, initiates terminal differentiation to the myelinating state. During this time, Schwann cell number is subject to stringent regulation such that within the first postnatal week, axons and myelinating Schwann cells attain the one-to-one relationship characteristic of the mature nerve. The mechanisms that underly this developmental control remain largely undefined. In this report, we examine the role of apoptosis in the determination of postnatal Schwann cell number. We find that Schwann cells isolated from postnatal day 3 rat sciatic nerve undergo apoptosis in vitro upon serum withdrawal and that Schwann cell death can be prevented by beta forms of neuregulin (NRG-beta) but not by fibroblast growth factor 2 or platelet-derived growth factors AA and BB. This NRG-beta-mediated Schwann cell survival is apparently transduced through an ErbB2/ErbB3 receptor heterodimer. We also provide evidence that postnatal Schwann cells undergo developmentally regulated apoptosis in vivo. Together with other recent findings, these results suggest that Schwann cell apoptosis may play an important role in peripheral nerve development and that Schwann cell survival may be regulated by access to axonally derived NRG.
Animals↗
Expression of the Tyro4/Mek4/Cek4 gene specifically marks a subset of embryonic motor neurons and their muscle targets.
Tyro4 is a member of the eph family of receptor protein-tyrosine kinases. We present sequence analysis that identifies Tyro4 as the rat homolog of mouse Mek4 and chick Cek4. We also present expression studies that demonstrate an evolutionarily conserved pattern of expression for Tyro4, Mek4, and Cek4. Most strikingly, we find this receptor to be specifically expressed, in all three species, in a subset of motor neurons in the medial motor column and in a subset of axial, but not limb, muscles. Mek4 has previously been ascribed a role in guiding retinal axons to their targets in the optic tectum. Our results extend the purported role of Mek4 in axon guidance to include motor neurons of the medial motor column.
Amino Acid Sequence↗
Neuregulins in development.
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Alternative Splicing↗
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.
Animals↗
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.
Animals↗
Aberrant neural and cardiac development in mice lacking the ErbB4 neuregulin receptor.
Various in vitro studies have suggested that ErbB4 (HER4) is a receptor for the neuregulins, a family of closely related proteins implicated as regulators of neural and muscle development, and of the differentiation and oncogenic transformation of mammary epithelia. Here we demonstrate that ErbB4 is an essential in vivo regulator of both cardiac muscle differentiation and axon guidance in the central nervous system (CNS). Mice lacking ErbB4 die during mid-embryogenesis from the aborted development of myocardial trabeculae in the heart ventricle. They also display striking alterations in innervation of the hindbrain in the CNS that are consistent with the restricted expression of the ErbB4 gene in rhombomeres 3 and 5. Similarities in the cardiac phenotype of ErbB4 and neuregulin gene mutants suggest that ErbB4 functions as a neuregulin receptor in the heart; however, differences in the hindbrain phenotypes of these mutants are consistent with the action of a new ErbB4 ligand in the CNS.
Animals↗
Premature Schwann cell differentiation and hypermyelination in mice expressing a targeted antagonist of the POU transcription factor SCIP.
The transcription factor SCIP is expressed by immature neurons and Schwann cells of the developing central and peripheral nervous systems, but this expression is largely extinguished when these cells fully differentiate. In immature Schwann cells in vitro, SCIP acts as a repressor of the myelin-specific genes that mark full differentiation. We have generated transgenic mice that express a dominant-negative antagonist of SCIP, specifically targeted to developing Schwann cells. This antagonist--designated delta SCIP--is transcriptionally inactive, but retains full DNA-binding activity. Mice that express delta SCIP exhibit a debilitating peripheral neuropathy that results from developmentally advanced Schwann cell differentiation, over-expression of myelin-specific gene products, and hypermyelination. These results suggest that SCIP functions as a transcriptional sensor of differentiation cues and thereby regulates the time and place at which Schwann cells differentiate.
Animals↗
B-50/GAP-43 mRNA expression in cultured primary Schwann cells is regulated by cyclic AMP.
Following peripheral nerve crush or transection, B-50 mRNA expression increased dramatically in the distal nerve stump. This increase has been fully attributed to an up-regulation of B-50 synthesis in reactive Schwann cells. Here we describe that B-50 mRNA expression in primary Schwann cell cultures is strongly down-regulated by cyclic AMP. Treatment of neonatal Schwann cell cultures with as low as 20 nM forskolin decreased B-50 mRNA expression. We show that B-50 promoter P2, but not P1, is active in Schwann cells and that the activity of P2 is inhibited 2.5 fold by forskolin. P2 does not contain a consensus sequence of a known cyclic AMP responsive element suggesting that the effect of forskolin is indirect.
Animals↗
Hypomyelinating peripheral neuropathies and schwannomas in transgenic mice expressing SV40 T-antigen.
We have prepared transgenic mice carrying a temperature-sensitive mutant of the SV40 oncogene (tsA-1609) under the control of 5' flanking sequences from the Schwann cell-specific P0 gene. Four of six founder mice showed moderate to severe hypomyelination in peripheral nerves of tail biopsies, with only rare myelinated fibers. Offspring were obtained from three of these founders. Northern blot and immunohistochemical analyses showed that expression of T-antigen was restricted to the PNS. Mice expressing the highest levels of T-antigen exhibited the most severe hypomyelination. Mice expressing lower levels developed transient mild hypomyelination, but after long latencies developed sporadic schwannomas. An immortalized cell line exhibiting properties of Schwann cells at an arrested stage of differentiation, termed "SCT-1," was derived from one of these tumors.
Animals↗
Structure, expression, and activity of Tyro 3, a neural adhesion-related receptor tyrosine kinase.
We have isolated mouse cDNA clones encoding Tyro 3, a receptor protein-tyrosine kinase (PTK) of the mammalin central nervous system (CNS). Expression of the Tyro 3 gene is strongly up-regulated in neurons of the mouse neocortex, cerebellum, and hippocampus after the day of birth, during periods of active synaptogenesis, and high expression is maintained in the adult CNS. The sequence of Tyro 3 cDNAs predicts a glycoprotein receptor with similarity to neural cell recognition and adhesion molecules--the extracellular (ligand binding) region of this receptor is composed of two immunoglobulin-related domains followed by two fibronectin type III repeats. Immunoblot and immunoprecipitation analyses with anti-Tyro 3 antibodies indicate that the 125 kD Tyro 3 protein is abundantly expressed in CNS synaptosomes, and immunohistochemical analysis of cultured hippocampal cells demonstrates that Tyro 3 is a product of neurons. Rat-2 fibroblasts stably transfected with a Tyro 3 expression construct acquire the ability to grow in soft agar, suggesting that Tyro 3 is potentially oncogenic.
Amino Acid Sequence↗
Structure and expression of the Tyro 10 receptor tyrosine kinase.
We have isolated cDNA clones encoding Tyro 10, a novel receptor protein-tyrosine kinase (PTK) whose catalytic domain exhibits significant similarity to the Trk family of neurotrophin receptors (Lai & Lemke, 1991). We find that the Tyro 10 gene is widely expressed, both within and outside the nervous system, and in both developing and mature neural tissue. The primary structure of Tyro 10, deduced from cDNA sequence, defines a new sub-family of receptor PTKs. Although the Tyro 10 kinase domain is more closely related to the equivalent domains of Trk, TrkB and TrkC than to the catalytic domains of other receptor PTKs, it is less closely related to these Trk domains than they are to each other. More significantly, the Tyro 10 extracellular (ligand binding) domain is not structurally related to the extracellular domains of the Trk receptors, but instead bears homology to cell surface mediators of protein-protein interactions, including blood coagulation Factors V and VIII, and the neuronal recognition protein A5. These appear to be structural features of a distinct receptor PTK sub-family, in that they are also found in the recently-described discoidin domain receptor (DDR).
Amino Acid Sequence↗
Axons regulate Schwann cell expression of the POU transcription factor SCIP.
SCIP (suppressed cAMP-inducible POU) is a POU domain transcription factor expressed by Schwann cells. Drugs that elevate intracellular cAMP, such as forskolin, increase the expression of SCIP and partially mimic the inductive effects of axons on Schwann cell gene expression. Thus, SCIP may be involved in a differentiation pathway in Schwann cells that is activated by axons. We have examined this issue by studying SCIP expression in developing, degenerating, and regenerating rat peripheral nerves, and in Schwann cell-neuron cocultures. High levels of SCIP mRNA were detected in developing and regenerating nerves, and axotomy at these times caused the level of SCIP mRNA to plummet. Similarly, there were many SCIP-immunoreactive Schwann cell nuclei in developing and regenerating nerves, and their number fell sharply after axotomy. SCIP-immunoreactive Schwann cells were associated with axons in developing and regenerating nerves, and in Schwann cell-neuron cocultures. These data demonstrate that axons upregulate the expression of SCIP in Schwann cells, and that SCIP is expressed in Schwann cells that ensheathe axons. Thus, SCIP may mediate some of the changes in Schwann cell gene expression that accompany axonal ensheathment.
Animals↗
The human TYRO3 gene and pseudogene are located in chromosome 15q14-q25.
Partial cDNAs of the human TYRO3 gene, encoding a putative receptor tyrosine kinase, and its processed pseudogene (TYRO3P) were cloned from human teratocarcinoma cell, bone marrow and melanocyte cDNA libraries. The tyrosine kinase homologous domains of TYRO3 and TYRO3P were sequenced and compared with each other and with the mouse TYRO3 gene. Abundant levels of the 4.2-kb TYRO3 mRNA were detected in human brain, and lower levels in other human tissues. TYRO3 and TYRO3P were both assigned to human chromosome 15q14-q25 by analysis of DNAs from somatic cell hybrids.
Amino Acid Sequence↗
Cell-specific action and mutable structure of a transcription factor effector domain.
POU proteins are cell-specific transcription factors whose specificity of action has been attributed to protein-DNA and protein-protein interactions mediated by their DNA-binding (POU) domains. Here we report that transcriptional activation by SCIP, a POU protein expressed by developing Schwann cells, is dependent on an amino-terminal effector domain and that this domain mediates cell-specific transactivation in the complete absence of the POU domain. When fused to a heterologous DNA-binding domain, this SCIP domain is a potent transactivator in Schwann cells but is inactive in three heterologous cell types. The primary structure of the SCIP amino-terminal domain is novel but contains a polymorphic string of alanine residues similar to those found in several other transcription factors. Although previously hypothesized to be important for transcription factor activity, we find that the SCIP string is functionally irrelevant. We propose that homopolymers of alanine, and certain other amino acids, do not represent a motif required for transcription factor function but instead reflect regions of unstable DNA related to those associated with four recently characterized human genetic disorders.
Alanine↗