As cells age, protein quality control becomes increasingly important. Accumulated stressors such as starvation, oxidative damage, and infections lead to organelle dysfunction and protein misfolding. Stem cells, such as those present in the skin, gut, and blood, can dilute these insults through cell division. Neurons and other post-mitotic cells, however, must confront them directly. Autophagy, or lysosome-mediated degradation, is the primary mechanism for clearing large dysfunctional entities within the cell. Neuronal autophagy must be especially robust for two main reasons: 1) the high rates of protein synthesis and ATP generation in neurons entail a higher incidence of misfolded proteins and reactive oxygen species, both of which damage the cell, and 2) high spatial separation of multiple specialized regions (e.g. axons, dendrites, synapses) demands local clearance of damage in those areas to prevent key functional loss. Predictably, errors in autophagy often affect the aging nervous system. Frontotemporal dementia (FTD), a progressive neurocognitive disease, is associated with several single-gene mutations involved in autophagy and broader protein quality control. Many of these genes overlap with those implicated in amyotrophic lateral sclerosis (ALS), a common neuromuscular disease. Progranulin (PGRN), a monogenic cause of FTD and risk modifier for ALS, is a lysosomal glycoprotein that causes defects in autophagy through an unknown mechanism. Recent work by our lab has demonstrated that another ALS- associated protein, annexin A11 (ANXA11), shows decreased recruitment to the lysosome in PGRN deficient neurons. ER exit site (ERES) proteins, which regulate protein export from the endoplasmic reticulum, also show decreased lysosomal recruitment in PGRN deficiency. ANXA11 is known to bind members of ERESs, as well as to associate physically with the lysosome. Furthermore, ERESs may be involved in more than just protein export. Our collaborators recently discovered a phenomenon where lysosomes directly engulf and degrade ERESs bearing misfolded proteins?a clear example of autophagic involvement. I propose to test the hypothesis that PGRN regulates ERES autophagy through ANXA11 action by addressing the following specific aims: 1) determine how PGRN regulates ANXA11 interaction with lysosome-organelle contact sites in iPSC-derived neurons, and 2) determine how PGRN and ANXA11 jointly regulate autophagic activity. I will use a combination of sophisticated microscopy techniques, biochemical protein identification, and targeted genetic manipulation to complete these aims. Uncovering the mechanism of PGRN-related neurodegeneration could lead to a better understanding of the shared pathophysiology of FTD and ALS, providing new drug targets for these incurable and universally devastating diseases of aging.
Age-related neurodegenerative disease often results from deficits in autophagy, the primary mechanism for clearing large dysfunctional entities within the cell. The lack of effective treatments for these diseases, their universally devastating personal and societal effects, and their growing visibility given global demographic aging, necessitates further research into the basic mechanisms that govern autophagy in neurons. This study seeks to characterize a promising lead in the shared molecular pathophysiology of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS), with the hope of identifying novel drug targets that ameliorate autophagic dysfunction in these diseases.
