Understanding the Role GPRC6a During Tau Metabolism

Project summary Tauopathies including Alzheimer’s disease (AD) consist of age-associated neurodegenerative diseases for which no disease-modifying treatments exist. Our group has uncovered a unique interaction between the arginine metabolism and tauopathies. Arginine metabolism affects multiple biological processes that show considerable influence upon tau biology. We hypothesize that L-arginine acts as a sensor for amino acid abundance and that the depletion of arginine initiates autophagy to replenish amino acid levels. Arginase 1 (Arg1) remains one of several different enzymes capable of metabolizing the semi-essential amino acid L- arginine. In fact numerous isoforms exist for Arg and nitric oxide synthases (NOS). We demonstrate in cells and animal models of tauopathy the benefits of increasing Arg1 in reducing many aspects of the tau phenotype (hallmarks comprised of tau effects). Several seminal findings show lysosomal and cytoplasmic arginine sensors that modulate mechanistic target of rapamycin complex 1 (mTORC1). GPRC6a is a G-protein coupled receptor that binds L-α amino acids including L-arginine and may serve as an extracellular arginine sensor for mTORC1. Our preliminary data indicates increased activation of the arginine-sensing mTORC1 pathway in AD brains and animal models of tauopathies. We show that arginine producing and metabolizing enzymes, arginine sensors, and mTORC1 complexes increase in the hippocampus of AD patients compared to control aged matched brains. We also find that tau increased total arginine levels in the brain by 35%, increased basal levels of extracellular arginine and arginine release following neuronal stimulation in the brain. Furthermore, novel allosteric antagonist to GPRC6a clears various forms of tau in cell lines, primary neurons, and mice. Genetic reduction of GPRC6a also reduced tau expression and seemingly regulates several intracellular arginine sensors. We posit that tauopathies (including AD) cause impaired arginine metabolism and uncoupling of arginine-sensing mTORC1 signaling, which lead to hyper-mTORC1 activation creating a positive feed- forward loop to augment the tau phenotype. We will test the hypothesis that decreased GPRC6a signaling from allosteric antagonists or shRNA to GPRC6a modulate arginine-sensing mTORC1 signaling, induces autophagy and increases tau clearance in vitro and in vivo. We will use tetracycline inducible tau cell lines with a photo- switchable fluorescent tracer to measure tau metabolism, split tau-GFP constructs that measure tau oligomerization, and primary neurons to determine the role of GPRC6a and tau fate. We will determine if genetic knockdown and pharmacological inhibition of GPRC6a impacts tau biology in vitro and in vivo. Success in this application would provide a new receptor target that may impact arginine-sensing mTORC1 signaling. Furthermore, we aim to identify an entirely new class of drugs that targets GPCR6a to mitigate the tau phenotype provide new strategies for tauopathies and AD.