title: Normal Molecular Specification and Neurodegenerative Disease-Like Death of Spinal Neurons Lacking the SNARE-Associated Synaptic Protein Munc18-1.
abstract: The role of synaptic activity during early formation of neural circuits is a topic of some debate; genetic ablation of neurotransmitter release by deletion of the Munc18-1 gene provides an excellent model to answer the question of whether such activity is required for early circuit formation. Previous analysis of Munc18-1(-/-) mouse mutants documented their grossly normal nervous system, but its molecular differentiation has not been assessed. Munc18-1 deletion in mice also results in widespread neurodegeneration that remains poorly characterized. In this study, we demonstrate that the early stages of spinal motor circuit formation, including motor neuron specification, axon growth and pathfinding, and mRNA expression, are unaffected in Munc18-1(-/-) mice, demonstrating that synaptic activity is dispensable for early nervous system development. Furthermore, we show that the neurodegeneration caused by Munc18-1 loss is cell autonomous, consistent with apparently normal expression of several neurotrophic factors and normal GDNF signaling. Consistent with cell-autonomous degeneration, we demonstrate defects in the trafficking of the synaptic proteins Syntaxin1a and PSD-95 and the TrkB and DCC receptors in Munc18-1(-/-) neurons; these defects do not appear to cause ER stress, suggesting other mechanisms for degeneration. Finally, we demonstrate pathological similarities to Alzheimer's disease, such as altered Tau phosphorylation, neurofibrillary tangles, and accumulation of insoluble protein plaques. Together, our results shed new light upon the neurodegeneration observed in Munc18-1(-/-) mice and argue that this phenomenon shares parallels with neurodegenerative diseases.
  
  
  CONCLUSIONS
  In this work, we demonstrate the absence of a requirement for regulated neurotransmitter release in the assembly of early neuronal circuits by assaying transcriptional identity, axon growth and guidance, and mRNA expression in Munc18-1-null mice. Furthermore, we characterize the neurodegeneration observed in Munc18-1 mutants and demonstrate that this cell-autonomous process does not appear to be a result of defects in growth factor signaling or ER stress caused by protein trafficking defects. However, we find the presence of various pathological hallmarks of Alzheimer's disease that suggest parallels between the degeneration in these mutants and neurodegenerative conditions.
authors.affiliation: Institut de Recherches Cliniques de Montréal, Montreal, Quebec H2W 1R7, Canada.
authors.forename: Chris
authors.initials: C
authors.lastname: Law
authors.affiliation: Centre de Recherche de l'Institut Universitaire en Santé Mentale de Québec, Université Laval, Quebec G1J 2G3, Canada.
authors.forename: Marcos
authors.initials: M
authors.lastname: Schaan Profes
authors.affiliation: Centre de Recherche de l'Institut Universitaire en Santé Mentale de Québec, Université Laval, Quebec G1J 2G3, Canada.
authors.forename: Martin
authors.initials: M
authors.lastname: Levesque
authors.affiliation: Developmental Biology Program, Sloan Kettering Institute and Neuroscience Program, Weill Cornell Medical College, New York, New York 10065.
authors.forename: Julia A
authors.initials: JA
authors.lastname: Kaltschmidt
authors.affiliation: Department of Functional Genomics and Department of Clinical Genetics, Center for Neurogenomics and Cognitive Research, Neuroscience Campus Amsterdam, Vrije Universiteit and VU Medical Center, 1081 HV Amsterdam, The Netherlands.
authors.forename: Matthijs
authors.initials: M
authors.lastname: Verhage
authors.affiliation: Institut de Recherches Cliniques de Montréal, Montreal, Quebec H2W 1R7, Canada, Departments of Anatomy and Cell Biology and Biology, Division of Experimental Medicine, Integrated Program in Neuroscience, McGill University Montréal, Quebec H3A 2B2, Canada, and Faculté de Médecine, Université de Montréal, Montreal, Quebec H3C 3J7, Canada artur.kania@ircm.qc.ca.
authors.forename: Artur
authors.identifier: http://orcid.org/0000-0002-5209-2520
authors.initials: A
authors.lastname: Kania
mesh_terms: D000818:Animals; D051792:Basic Helix-Loop-Helix Proteins / Q000378:metabolism; D016923:Cell Death / Q000235:genetics; D000075389:DCC Receptor; D000076122:Disks Large Homolog 4 Protein; D004622:Embryo, Mammalian; D005260:Female; D018507:Gene Expression Regulation, Developmental / Q000235:genetics*; D051100:Glial Cell Line-Derived Neurotrophic Factor / Q000378:metabolism; D051528:Guanylate Kinases / Q000378:metabolism; D008297:Male; D008565:Membrane Proteins / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D009046:Motor Neurons / Q000473:pathology*; D051938:Munc18 Proteins / Q000172:deficiency* / Q000235:genetics; D009419:Nerve Tissue Proteins / Q000378:metabolism; D019636:Neurodegenerative Diseases / Q000235:genetics* / Q000473:pathology*; D000076104:Oligodendrocyte Transcription Factor 2; D021381:Protein Transport / Q000235:genetics; D020813:Receptor, trkB / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000378:metabolism; D015398:Signal Transduction / Q000235:genetics; D013116:Spinal Cord / Q000196:embryology / Q000473:pathology*; D050827:Syntaxin 1 / Q000378:metabolism; D025521:Tumor Suppressor Proteins / Q000378:metabolism
chemical_list: D051792:Basic Helix-Loop-Helix Proteins; D000075389:DCC Receptor; D000076122:Disks Large Homolog 4 Protein; D051100:Glial Cell Line-Derived Neurotrophic Factor; D051528:Guanylate Kinases; D008565:Membrane Proteins; D051938:Munc18 Proteins; D009419:Nerve Tissue Proteins; D000076104:Oligodendrocyte Transcription Factor 2; D020813:Receptor, trkB; D011956:Receptors, Cell Surface; D050827:Syntaxin 1; D025521:Tumor Suppressor Proteins; C501910:Dcc protein, mouse; C501447:Dlg4 protein, mouse; C512773:Olig2 protein, mouse; C497938:Stxbp1 protein, mouse
keywords: Munc18-1; developmental; early neuronal activity; motor neurons; neurodegeneration; neurosecretion