Greg Lemke

Neuregulin 1 and Susceptibility to Schizophrenia

The cause of schizophrenia is unknown, but it has a significant genetic component. Pharmacologic studies, studies of gene expression in man, and studies of mouse mutants suggest involvement of glutamate and dopamine neurotransmitter systems. However, so far, strong association has not been found between schizophrenia and variants of the genes encoding components of these systems. Here, we report the results of a genomewide scan of schizophrenia families in Iceland; these results support previous work, done in five populations, showing that schizophrenia maps to chromosome 8p. Extensive fine-mapping of the 8p locus and haplotype-association analysis, supplemented by a transmission/disequilibrium test, identifies neuregulin 1 (NRG1) as a candidate gene for schizophrenia. NRG1 is expressed at central nervous system synapses and has a clear role in the expression and activation of neurotransmitter receptors, including glutamate receptors. Mutant mice heterozygous for either NRG1 or its receptor, ErbB4, show a behavioral phenotype that overlaps with mouse models for schizophrenia. Furthermore, NRG1 hypomorphs have fewer functional NMDA receptors than wild-type mice. We also demonstrate that the behavioral phenotypes of the NRG1 hypomorphs are partially reversible with clozapine, an atypical antipsychotic drug used to treat schizophrenia.

TAM Receptors Are Pleiotropic Inhibitors of the Innate Immune Response

The activation of Toll-like receptors (TLRs) in dendritic cells (DCs) triggers a rapid inflammatory response to pathogens. However, this response must be tightly regulated because unrestrained TLR signaling generates a chronic inflammatory milieu that often leads to autoimmunity. We have found that the TAM receptor tyrosine kinases-Tyro3, Axl, and Mer-broadly inhibit both TLR and TLR-induced cytokine-receptor cascades. Remarkably, TAM inhibition of inflammation is transduced through an essential stimulator of inflammation-the type I interferon receptor (IFNAR)-and its associated transcription factor STAT1. TLR induction of IFNAR-STAT1 signaling upregulates the TAM system, which in turn usurps the IFNAR-STAT1 cassette to induce the cytokine and TLR suppressors SOCS1 and SOCS3. These results illuminate a self-regulating cycle of inflammation, in which the obligatory, cytokine-dependent activation of TAM signaling hijacks a proinflammatory pathway to provide an intrinsic feedback inhibitor of both TLR- and cytokine-driven immune responses.

The anticoagulation factor protein S and its relative, Gas6, are ligands for the Tyro 3/Axl family of receptor tyrosine kinases

We report the identification of ligands for Tyro 3 (alternatively called Sky, rse, brt, or tif) and Axl (alternatively, Ark or UFO), members of a previously orphan family of receptor-like tyrosine kinases. These ligands correspond to protein S, a protease regulator that is a potent anticoagulant, and Gas6, a protein related to protein S but lacking any known function. Our results are reminiscent of recent findings that the procoagulant thrombin, a protease that drives clot formation by cleaving fibrinogen to form fibrin, also binds and activates intracellular signaling via a G protein-coupled cell surface receptor. Proteases and protease regulators that also activate specific cell surface receptors may serve to integrate coagulation with associated cellular responses required for tissue repair and growth, as well as to coordinate protease cascades and associated cellular responses in other systems, such as those involved in growth and remodeling of the nervous system.

Immunobiology of the TAM receptors

Recent studies have revealed that the TAM receptor protein tyrosine kinases — TYRO3, AXL and MER — have pivotal roles in innate immunity. They inhibit inflammation in dendritic cells and macrophages, promote the phagocytosis of apoptotic cells and membranous organelles, and stimulate the maturation of natural killer cells. Each of these phenomena may depend on a cooperative interaction between TAM receptor and cytokine receptor signalling systems. Although its importance was previously unrecognized, TAM signalling promises to have an increasingly prominent role in studies of innate immune regulation.

Mouse P0 gene disruption leads to hypomyelination, abnormal expression of recognition molecules, and degeneration of myelin and axons

We have used homologous recombination in embryonic stem cells to generate mice carrying a mutation in the gene encoding P0, an immunoglobulin-related recognition molecule and the major protein of peripheral nervous system myelin. These mice are deficient in normal motor coordination and exhibit tremors and occasional convulsions. Axons in their peripheral nerves are severely hypomyelinated and a subset of myelin-like figures and axons degenerate. The mutation leads to an abnormal regulation of some, but not all, molecules involved in myelination. These results demonstrate that P0 is essential for the normal spiraling, compaction, and maintenance of the peripheral myelin sheath and the continued integrity of associated axons. They further suggest that this protein conveys a signal that regulates Schwann cell gene expression.

An extended family of protein-tyrosine kinase genes differentially expressed in the vertebrate nervous system

We have used PCR to identify 13 novel protein-tyrosine kinase genes (tyro-1 to -13), six of which (tyro-1 to -6) are preferentially expressed in the developing vertebrate nervous system. The tyro-2 and tyro-9 genes encode kinase domains that exhibit strong amino acid sequence similarity to the equivalent regions of the receptors for EGF and FGF, respectively, and may encode novel receptors for these or related polypeptide ligands. The tyro-1 to -6 genes are all expressed during central nervous system neurogenesis and exhibit distinct and highly regionalized patterns of expression in the adult brain. Together with recent studies in invertebrates, these data are consistent with the hypothesis that protein-tyrosine kinases play a central role in neural development.

Tyro-3 family receptors are essential regulators of mammalian spermatogenesis

We have generated and analysed null mutations in the mouse genes encoding three structurally related receptors with tyrosine kinase activity: Tyro 3, Axl, and Mer. Mice lacking any single receptor, or any combination of two receptors, are viable and fertile, but male animals that lack all three receptors produce no mature sperm, owing to the progressive death of differentiating germ cells. This degenerative phenotype appears to result from a failure of the tropic support that is normally provided by Sertoli cells of the seminiferous tubules, whose function depends on testosterone and additional factors produced by Leydig cells. Tyro 3, Axl and Mer are all normally expressed by Sertoli cells during postnatal development, whereas their ligands, Gas6 and protein S, are produced by Leydig cells before sexual maturity, and by both Leydig and Sertoli cells thereafter. Here we show that the concerted activation of Tyro 3, Axl and Mer in Sertoli cells is critical to the role that these cells play as nurturers of developing germ cells. Additional observations indicate that these receptors may also be essential for the tropic maintenance of diverse cell types in the mature nervous, immune and reproductive systems.

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.

Topographic Mapping from the Retina to the Midbrain Is Controlled by Relative but Not Absolute Levels of EphA Receptor Signaling

Topographic maps are a fundamental feature of sensory representations in nervous systems. The formation of one such map, defined by the connection of ganglion cells in the retina to their targets in the superior colliculus of the midbrain, is thought to depend upon an interaction between complementary gradients of retinal EphA receptors and collicular ephrin-A ligands. We have tested this hypothesis by using gene targeting to elevate EphA receptor expression in a subset of mouse ganglion cells, thereby producing two intermingled ganglion cell populations that express distinct EphA receptor gradients. We find that these two populations form separate maps in the colliculus, which can be predicted as a function of the net EphA receptor level that a given ganglion cell expresses relative to its neighbors.

Isolation and analysis of the gene encoding peripheral myelin protein zero

We have isolated the gene encoding the Schwann cell glycoprotein P0, the major structural protein of the peripheral myelin sheath. In rats and mice, this gene is split into six exons distributed over 7 kb of DNA. The segregation of these exons is consistent with the functional segregation of the P0 protein into extracellular, membrane-spanning, and cytoplasmic domains. We find that the P0 extracellular domain is similar in structure to a single immunoglobulin variable region domain. In contrast to prototypical immunoglobulin domains, however, this P0 domain is encoded by two exons, the partitioning of which provides genetic evidence for the evolution of immunoglobulin-related domains from an ancestral half-domain. We also describe procedures for transfection of cultures of nontransformed rat Schwann cells and use these procedures to show that the Schwann cell-specific expression of the P0 gene is controlled by cis-acting elements localized upstream of exon I.