Roman J. Giger - US grants

DESCRIPTION (provided by applicant): Traumatic injury of the spinal cord in humans leads to permanent paralysis and other serious medical complications. Paralysis is a result of lost neuronal connectivity between the brain and spinal cord motor units. The failure of severed spinal axons to recover, however, is not primarily due to an intrinsic inability to regenerate, but is a result of the central nervous system (CNS) environment that is highly refractory to axonal growth. When provided with a suitable environment, injured CNS axons do recover, extending processes over long distances and partially restoring function in animal models of spinal cord injury (SCI). Multiple CNS myelin constituents are thought to directly contribute to the regenerative failure of damaged spinal axons, including proteins called Nogo, myelin-associated glycoprotein (MAG), and oligodendrocyte-myelin glycoprotein (OMgp). The main objective of this study is to gain insights into the molecular and cellular mechanisms of myelin-mediated inhibition of axonal growth. A detailed understanding of the biology of axon-glia interaction is a prerequisite to devising strategies aimed at lowering the growth inhibitory barrier of adult CNS myelin and to promote neuronal repair following traumatic injury of the CNS. The identification, of a novel family of receptor proteins comprised of members with distinct binding specificities toward established myelin inhibitors of axonal growth is at the heart of our investigations. A major goal of the proposed study is to define the role of these receptors in neuronal responses to CNS myelin inhibitors. To functionally characterize members of this gene family, we will engineer recombinant viral vectors for gain-of-function studies in neurons. Mouse genetics will be used for loss-of-function studies in vivo. In a parallel approach, we will develop mutated receptors with antagonistic function. Mutated soluble receptors that still bind ligand but no longer possess the ability to signal axonal growth inhibition will be assessed for their potential to promote axonal growth on myelin substrate in vitro. Coupling our biochemical approaches with in vitro neurite outgrowth assays and in vivo functional studies will provide a strong basis to elucidate the role played by novel receptor-ligand interactions in neurite outgrowth inhibition. Together, this family of receptor proteins may provide new molecular handles for the design of therapeutic interventions for CNS injuries.

DESCRIPTION (provided by applicant): In vertebrates, including humans, rapid neuronal communication in the peripheral (PNS) and central nervous system (CNS) is dependent on proper myelination. The myelin-forming cell in the PNS is the Schwann cell (SC) and in the CNS the oligodendrocyte (OL). These specialized cells ensheath neuronal processes and thereby facilitate rapid propagation of electrical impulses. PNS myelin is defective in several types of Charcot-Marie-Tooth (CMT) disease, one of the most common inherited neurological disorders. Abnormal development of myelin in the CNS results in disorders known as leukodystrophies. We previously described the severe peripheral neuropathy CMT4J, caused by mutation of the human FIG4/SAC3 gene encoding an evolutionarily conserved lipid phosphatase that regulates intracellular vesicle trafficking along the endo-lysosomal pathway. The main objectives of our research are to understand the molecular mechanisms by which FIG4 deficiency disrupts myelin formation, and to develop treatment strategies for CMT4J in a preclinical model. Mutant mice with global loss of Fig4 expression (Fig4-/-) exhibit dramatic reduction of myelin in the CNS and PNS, severe tremor, and juvenile lethality. Electrophysiological recordings revealed slowed conduction of electrical impulses in sciatic and optic nerves. Surprisingly, the myelin defects in Fig4-/- mice can be rescued by neuron-specific expression of wildtype Fig4. Based on these observations, we hypothesize that loss of Fig4 disrupts neuron-specific signaling mechanisms required for myelination. In Specific Aim 1 and Aim 2 we use a combination of mouse genetics and proteomics to identify the neuronal myelination signals that are disrupted in Fig4 mutant mice and to determine the temporal requirement for Fig4 in vivo. These experiments will provide new mechanistic insights into the neuronal signals that direct myelinogenesis. To model human CMT4J, we developed transgenic mice that ubiquitously express low levels of the human disease allele Fig4-I41T on a Fig4-/- background (CMT4J mice). These mice exhibit hypomyelination comparable to that of Fig4-/- mice, but survive to adulthood with many neurologic features of the human disease. Since we have shown that transgenic expression of Fig4 in neurons is sufficient to drive myelination, we propose a gene therapy study in Specific Aim 3. Dorsal root ganglion neurons (PNS) and retinal ganglion cells (CNS) of CMT4J mice will be transduced with viral vectors to express wildtype Fig4. Myelination, nodal structure, and nerve conduction velocity in sciatic or optic nerve will be monitored as indicators of efficacy. Restoration of myelination by Fig4 gene therapy in mice would demonstrate a new therapeutic avenue for patients suffering from myelination disorders. PUBLIC HEALTH RELEVANCE: Malformation, degeneration or acute damage to myelin in the nervous system is observed in a broad spectrum of white matter disorders including leukodystrophies and multiple sclerosis in the central nervous system and Charcot-Marie-Tooth disease in the peripheral nervous system. The research described in this project uses a novel mouse genetic model to probe the molecular and biochemical processes involved in white matter disease. A better understanding of these processes will lay the foundation for the development of treatment strategies for nervous system white matter disorders.

Mounting evidence suggests that many neuropsychiatric disorders have a developmental origin with a strong genetic underpinning. A large number of allelic variants has been identified that associate with mental illness. While genetic discoveries are a crucial first step, the next major challenge is to define the biochemical pathways altered by disease alleles and to develop a more nuanced understanding of how these dysfunctional pathways disrupt brain function relevant to disease symptomatology. Of interest, mutations in members of the SEMAPHORIN (SEMA) family of axon guidance molecules, and their receptors, the PLEXINS (PLXN) carry varying genetic risks for mental illness. Allelic variants of SEMA5A and its receptor PLXNA2 associated with mood disorders cause reduced gene expression. To explore the underlying biology of how reduced SEMA5A/PLXNA2 expression may contribute to mental illness, we use Plxna2 transgenic mice as an experimental system. Morphological studies with Sema5a and Plxna2 mutant mice identified distinct anatomical phenotypes: defects in migration of early-born dentate granule cells (GCs) progenitors from the primary neuroepithelium to the dentate gyrus (DG) and an increase in spine synapse density in mature GCs in the DG. Adult Plxna2 mice exhibit gene-dosage and sex-specific behavioral defects in episodic memory, sensorimotor gating, and sociability; traits commonly observed in psychiatric disorders including Schizophrenia. For a deeper understanding of how Plxna2 deficiency may contribute to behavioral phenotypes, we used gene editing to disrupt the GTPase-activating protein (GAP) domain in the PlxnA2 cytoplasmic region. Loss of PlxnA2-GAP enzymatic activity impairs Rap1-GTPase dependent GC progenitor migration and leads to supernumerary spine synapses in mature GC. Moreover, PlxnA2-GAP deficiency disrupts episodic memory and sensorimotor gating. To probe deeper into how impaired Sema5A/PlxnA2-GAP/Rap1 signaling leads to behavioral defects, we used a proteomics-based approach and identified novel PlxnA2 interacting proteins. In the current application we propose a multidisciplinary approach comprised of recently developed biochemical tools, electrophysiological techniques, conditional gene ablation and mouse behavior. SPECIFIC AIM 1 is aimed at the characterization of novel mechanisms that regulate Sema5A-PlxnA2 signaling, including the guanine nucleotide exchange factors (GEFs) that antagonize PlxnA2-GAP activity. SPECIFIC AIM 2 builds on our observation that forebrain specific ablation of Plxna2 mimics behavioral defects observed in Plxna2 germline null mice. We pursue a mouse genetic approach to identify developmental epochs of vulnerability and the neural substrate associated with impaired behaviors in Plxna2-/- and Plxna2-GAP deficient mice. Genetic studies will be complemented by electrophysiological recordings, histological analyses and high- resolution magnetic resonance imaging. Studies are aimed at determining where and when Plxna2-GAP function is required during brain development to ensure normal behavior in adulthood.