Neurodegenerative disorders as well as traumatic and ischemic injuries to the brain are characterized by neuronal cell death and axonal degeneration. Programmed cell death and axonal degenerative are distinct self-destructive programs that are invoked to eliminate damaged cells and/or axons; however, they are activated inappropriately in many neurological disorders. While the components of pathways leading to cell death are relatively well characterized, the mechanisms involved in axonal degeneration are largely unknown. Studies of the wlds mouse revealed that overexpression of Nmnat enzymes, which synthesize NAD, can prevent axonal degeneration and, more recently, a toll-like receptor adaptor protein called Sarm1 was discovered to be an important component of the intrinsic axon degeneration program. Additional studies, including from our own labs, show that Sarm1 can also promote cell death in neurons and other cells. Indeed, loss of Sarm1 protects neurons from metabolic stress and mitochondrial dysfunction. Together, these breakthroughs show that Sarm1 drives a general cell destruction program that we term sarmoptosis. Our molecular analysis of Sarm1 shows that the SAM domains are necessary for its multimerization, whereas the TIR domain is required for its ability to activate cell death and axonal degeneration. To study molecular aspects of sarmoptosis, we have developed a variety of tools including a regulable Sarm1 TIR domain dimerization system that allows us to trigger cell death or, using compartmentalized chambers, axonal degeneration in a controlled fashion. It is our goal to understand sarmoptosis so that therapeutic agents can be devised to block this process, as this could be a useful method for treating many neurological disorders. In this proposal, we outline experiments that utilize these reagents to define the molecular pathways engaged by Sarm1 to promote cell destruction. First, we will identify structural motifs in the Sarm1 TIR domain that activate cell destruction. We will identify these key functional residues by analyzing a series of site-directed TIR domain mutants. Second, we will identify the enzyme(s) involved in mediating the NAD depletion that occurs after Sarm1 activation. Third, we will identify proteins that function downstream in this destructive pathway using a suppressor screen. Mechanistic studies of identified suppressors will be performed to characterize their role in sarmoptosis.
Our research is focused on defining the molecular underpinnings that cause neurons to die and their axons to degenerate in neurodegenerative disorders. We have identified a novel process that plays a role in both cell death and axon destruction. The identification of drugs to inhibit steps in this cell destruction pathway will be helpful in the treatment of a wide variety of disorders ranging from stroke and traumatic brain injury to neurodegenerative diseases like Alzheimer's disease and Parkinson's disease.
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