Cellular and Molecular Analysis of Defects at Aging Neuromuscular Synapses

Harvard University, Cambridge MA

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Abstract

DESCRIPTION (provided by applicant): The nervous system undergoes disturbing changes with age. To elucidate key mechanisms underlying age-related decline in neural function, we will examine the neuromuscular system, which is very accessible, relatively simple, and the site of clinically significant age-related functional decline. Our initial analysis has led to four sets of results: 1) Neuromuscular junctions (NMJs) undergo many structural and molecular alterations as they age. 2) Preterminal portions of motor axons exhibit regions of abnormal thinning, distension and sprouting. 3) Atrophy and synaptic changes in aged muscles are correlated on a fiber-by-fiber basis. 4) Although NMJs in most muscles are ravaged by age, those in a few are spared. Here, we propose studies designed to explore the relationships among these changes, identify molecular defects that underlie them, and test one way to reverse them. First, we will follow up preliminary observation of a dramatic and specific decline in levels of three known synaptic organizing molecules at aging NMJs -laminins 4 and 2 and agrin. We will correlate changes in the levels and distribution of these proteins with structural alterations, and ask whether targeted null or conditional mutants with decreased levels of these proteins show premature synaptic aging. Second, we will seek transport defects that underlie axonal dystrophy by correlative light and electron microscopy, along with use of new transgenic mice in which mitochondria and synaptic vesicles are labeled. Third, we will determine the relationship between the synaptic abnormalities and sarcopenia, the clinically significant age-related decline in muscle mass and strength. Using transgenic mice in which single motor axons and muscle fibers of specific types are selectively labeled we will assess myogenic and neurogenic determinants of sarcopenia. Fourth, we will follow up our observation that extraocular muscles are spared from age-related neuromuscular decline. This result is intriguing because extraoculars are also spared in the invariably fatal disease, amyotrophic lateral sclerosis (ALS), suggesting parallel mechanisms underlying age-related and neurodegenerative defects. Finally, we will use transgenic rescue techniques to ask whether reintroduction of laminins or agrin attenuates age- related synaptic disorganization. Together, these studies will provide insights into age- related neural defects that may not only provide ways to ameliorate sarcopenia but also be generally applicable to the nervous system PUBLIC HEALTH RELEVANCE: The nervous system undergoes disturbing changes with age. The investigators propose to use the neuromuscular system to elucidate key mechanisms underlying age-related decline in neural function. This system is very accessible, relatively simple, and the site of clinically significant age-related functional decline. First, they will follow up preliminary observation of a dramatic and specific decline in levels of known developmentally important molecules (laminins and agrin) at aging neuromuscular junction (NMJs), the synapses made by motoneurons on muscle fibers. They will correlate changes in the levels and distribution of these proteins with structural alterations, and ask whether mutant mice with decreased levels of these proteins show premature synaptic aging. Second, they will seek defects in the transport of materials along nerve fibers to the NMJ. Third, they will determine the relationship between the synaptic abnormalities and sarcopenia, the clinically significant age-related decline in muscle mass and strength. Fourth, they will explore intriguing similarities in symptoms and muscle-specific susceptibility between neuromuscular changes in aged mice and those in the invariably fatal disease, amyotrophic lateral sclerosis (ALS). Finally, they will ask whether reintroduction of laminins or agrin attenuates age-related synaptic disorganization. Together, these studies will provide insights into age-related neural defects that may not only provide ways to ameliorate sarcopenia but also be generally applicable to the nervous system.