Reha S. Erzurumlu - US grants

This proposal addresses fundamental issues regarding the formation of sensory maps in the mammalian brain. Experiments are proposed to sort out the role of factors that are Intrinsic to growing axons vs. of those derived from their environment in a model in vitro system. The rodent trigeminal system is a highly specialized sensory system; it is characterized by a specific arrangement of axonal and neuronal elements which are homeomorphic to the distribution of sensory receptors (around whiskers) on the snout. The patterned distribution of sensory receptors, or the sensory map, is conveyed to the brain by primary sensory neurons which reside in the trigeminal ganglion. In this proposal, embryonic trigeminal ganglia will be co-cultured with a variety of peripheral target tissue and central nervous system explants. This recently developed technical approach will be used to experimentally test the role of target- derived influences, such as cell and substrate adhesion molecules, glial organization within target tissues, possible positional cues embedded in the target, and of growth factors that are manufactured in the targets to reveal many unknown facets of axon-target interactions which lead to the formation of sensory maps in the brain. Understanding cellular and molecular mechanisms which underlie the formation of neural pathways and the spatial order within their tracts and terminal zones is a major endeavor in developmental neurobiology. Such knowledge has powerful implications for our ability to repair damage in specific sensory pathways of the brain.

Description (provided by applicant): Patterned and topographic organization of neural connections is an essential structural substrate for processing sensory information in the brain. The rodent trigeminal pathway is an exceptionally attractive model system to study the cellular and molecular mechanisms underlying the malleability of patterned somatotopic maps following peripheral sensory nerve damage. In this system, the patterned array of whiskers on the snout is represented by neural modules at every level in the brain. Injury to the whisker follicles or the sensory nerve innervating them during a critical period in development leads to irreversible and predictable structural alterations. Accompanying physiological plasticity is largely unknown. The major aim of this proposal is to uncover cellular and molecular mechanisms of neonatal peripheral nerve injury-induced CNS synaptic plasticity in the first- and second-order relay stations of the trigeminal sensory pathway. During the current funding period, we characterized synaptic plasticity within the trigeminal principal sensory nucleus following acute nerve injury in neonates. We found that peripheral denervation induces rapid synaptic plasticity, now we propose to compare its manifestations following successful peripheral nerve regeneration in its thalamic relay station, ventroposteromedial nucleus. The long-term objective of this proposal is to determine cellular mechanisms underlying peripheral nerve injury-induced plasticity along the central somatosensory pathways in neonates. Combined electrophysiological, pharmacological and anatomical techniques will be used to chart out membrane properties, synaptic responses, and NMDA receptor-mediated response characteristics in the trigeminal brainstem and thalamus. A solid understanding of mechanisms underlying development of patterned neural organization and its plasticity following peripheral nerve injury is critical for preventing or repairing often irreversible effect of damage to the developing human nervous system. PUBLIC HEALTH RELEVANCE: This research proposal aims to uncover manifestations and mechanisms of central nervous system plasticity following peripheral sensory nerve injury in neonates. We use the rodent trigeminal system as a model, because much is known about the organization and function of this system. Availability of genetically engineered mice to study molecular loss-of-function also makes this system highly attractive. This proposal is geared towards understanding cellular and molecular mechanisms of neural plasticity in the brain following peripheral nerve injury.

DESCRIPTION (provided by applicant): The role of neural activity in wiring and plasticity of sensory pathways is a major topic of interest in developmental neuroscience. A vast body of literature underscores the importance of N-methyl D Aspartate (NMDA) receptor- mediated neural activity during development of neural connections and their plasticity, learning and memory, as well as during excitotoxicity in pathological states of the mature nervous system. This proposal focuses on the development of somatosensory thalamocortical circuitry in mice with genetically impaired NMDAR function. Rodent somatosensory pathway is an excellent model system to study development of topographic connections and patterning within somatosensory maps. Somatosensory patterns are abolished in the brainstem of mice lacking the critical subunit of the NMDARs. Mice that express lower levels of NMDAR function also show absence of patterning all along the somatosensory pathway. Mice with cortex-restricted disruption of NMDARs in excitatory neurons also display severe defects in cortical patterning within the somatosensory body map region. Aside from axonal and postsynaptic defects, the subdivisions of the somatosensory body map within the neocortex are altered. Thus, somatosensory region-specific knockout mouse models provide an excellent means to dissect out the role of NMDARs and downstream signaling molecules in patterning of pre- and postsynaptic neural elements. The long-term objective of this proposal is to reveal how axon arbors and dendritic processes of postsynaptic cells are altered following impaired NMDAR function. Combined molecular genetic and neuroanatomical approaches will be used to elucidate structural changes in the somatosensory cortex and thalamus of these mice. A clear understanding of such anatomical changes will pave the way for dissecting out molecular mechanisms of pattern formation and plasticity in developing mammalian sensory pathways. These studies could then be expanded to investigate mechanisms of adult cortical plasticity during learning or as a consequence of peripheral nerve damage. PUBLIC HEALTH RELEVANCE N-methyl D Aspartate (NMDA) receptors (NMDARs) play a major role in development of neural connections and their plasticity, learning and memory, as well as during excitotoxicity in pathological states of the mature nervous system. In this proposal we shall test the role of NMDARs in the development and plasticity of somatosensory neural circuits by examining neural defects in NMDAR-deficient mouse models.

[unreadable] DESCRIPTION (provided by applicant): The role of neural activity in wiring and plasticity of sensory pathways is a major topic of interest in developmental neuroscience. During the past decade, considerable attention is directed toward the role of N-methyl D Aspartate (NMDA) receptor-mediated neural activity. A vast body of literature now underscores the importance of NMDARs during development of neural connections and their plasticity, learning and memory, as well as during excitotoxicity in pathological states of the mature nervous system. This supplemental proposal focuses on the development and plasticity of somatosensory thalamocortical circuitry in mice with cortex-specific null mutation of the essential subunit of the NMDAR, NRI. Rodent somatosensory pathway is an excellent model system to study development of topographic connections and patterning within somatosensory maps. Somatosensory patterns are abolished in the brainstem of mice lacking the critical subunit of the NMDARs. Mice that express lower levels of NMDAR function also show absence of patterning all along the somatosensory pathway. However, interpretation of results at the cortical level has been difficult, due to lack of region specificity in genetic manipulation of the NMDA receptor. We now have access to mice, which lack functional NMDARs in the neocortex, hippocampus and olfactory bulbs. These mice will undoubtedly allow us to dissociate cortical effects from subcortical ones. The long-term objective of this proposal is to reveal how axon arbors and dendritic processes of postsynaptic cells are altered following impaired NMDAR function. Combined molecular genetic and neuroanatomical approaches will be used to elucidate structural changes in the somatosensory cortex of these mice. A clear understanding of such anatomical changes will pave the way for dissecting out molecular mechanisms of pattern formation and plasticity in developing mammalian sensory pathways.

laboratory mouse

Trigeminal ganglion axons convey orofacial sensation to the brain via synapses in the brainstem trigeminal nuclei. During wiring of this pathway, peripheral and central trigeminal axons exhibit highly specific target- directed growth, target recognition, and elaboration of synaptic terminals. Molecular mechanisms underlying these processes are largely unknown. In recent years, several families of molecules, which guide growing axons, have been identified. These molecular signals are remarkably conserved between species from flies to mammals. Some of these molecules attract, while others repel growing axons, thus directing their pathway choice. Axon guidance involves not only pathway formation but also recognition of targets and elaboration of terminal branches and formation of synaptic terminals within targets. Molecular signals, which direct this latter phase of axon development, are not well understood. Recently Slit2, a member of the Slit family of proteins have been implicated as a potent branching factor for sensory neurons. During development of the trigeminal pathway mRNA for Slits and Robo receptors are expressed differentially. While Robol and Robo2 expression is prominent in the trigeminal ganglion of the three Slits, Slit-2 mRNA appears in the brainstem trigeminal nuclei just as trigeminal ganglion axons begin arborizing in this target. In the main peripheral target of the trigeminal ganglion, the whisker pad, Slit mRNAs are expressed differentially around the whisker follicles. Our main objective is to elucidate the role of Slit2 in switching trigeminal axons from elongation to branching/arborization phase. We propose to experimentally test the role of Slit2 in trigeminal axon branching and arborization using an in vitro model of the rodent trigeminal pathway. Additionally, we will examine the role of other Slit proteins in guiding trigeminal axons in the periphery and trigeminal lemniscal axons in the CMS.

DESCRIPTION (provided by applicant): Topographic representation of the sensory space in the brain is essential for sensory information processing and perception. The somatosensory and motor cortical maps in each hemisphere represent the contralateral body and the face. This is due to midline crossing of the ascending (sensory) and descending (motor) pathways at the level of the medulla or the pons. Genetic and developmental defects in midline crossing or injury at the crossing site severely affect sensory-motor information processing and actions in both animals and humans. In this proposal we use a region-specific gene deletion mouse model to study the consequences of partial crossing of the ascending somatosensory face pathway. Midline crossing defects in this mouse leads to bilateral face representation in the thalamus and subsequently in the somatosensory cortex. We will use this mouse model to investigate (a) morphological and electrophysiological properties of the pre and postsynaptic elements in the bifacial cortical map; (b) altered thalamocortical and corticocortical connectivity patterns in response to bilateral face representation; (c) behavioral consequences of this genetic mutation. A combination of molecular, morphological, electrophysiological, voltage-sensitive dye imaging and behavioral techniques will be used to elucidate mechanisms underlying the functional organization and behavioral manifestations of developmental injury-related or genetic defects in ascending somatosensory pathways.

? DESCRIPTION (provided by applicant): There is increasing evidence, implicating disruption of excitatory and inhibitory neurotransmission in the etiology of Autism Spectrum Disorders (ASD) and Rett Syndrome (RTT). Specific genetic effects are associated with these developmental disorders. MET receptor and it ligand hepatocyte growth factor (HGF) and both are expressed in the developing brain. The human gene MET, which encodes MET receptor tyrosine kinase has been identified as a prominent risk factor for ASD. RTT is a neurodevelopmental disorder caused by mutations in the MECP2 gene. Met and Mecp2 loss-of-function mouse models provide invaluable opportunities in understanding morphological and physiological changes in various brain regions, and they allow for development of therapeutic strategies. Both ASD and RTT affected children display distinct somatosensory behavioral proclivities suggesting specific defects in somatosensory information processing. We focus on the somatosensory thalamocortical circuit physiology and in vivo functional analyses in development, using region-specific genetic loss of function mouse models to uncover basic scientific mechanisms of thalamocortical circuitry defects following genetic disruption of these two genes associated with ASD and RTT. Our preliminary results in the Bird mouse model of MeCP2 deficiency indicate that the balance of excitation and inhibition in Layer 4 excitatory neurons of barrel cortex is biased toward inhibition. In contrast when Met signaling is disrupted in cortical excitatory neurons, heterozygous mice show loss of inhibition. We focus on the mouse primary somatosensory (whisker barrel) cortex because of its patterned organization and well-characterized development and plasticity. We combine mouse genetics with electrophysiological, functional imaging, biochemical, and behavioral analyses to understand the cellular mechanisms and consequences of thalamocortical circuitry defects following these specific genetic disruptions.