Age- and stress-associated C. elegans granulins impair lysosomal function and induce a compensatory HLH-30/TFEB transcriptional response

The progressive failure of protein homeostasis is a hallmark of aging and a common feature in neurodegenerative disease. As the enzymes executing the final stages of autophagy, lysosomal proteases are key contributors to the maintenance of protein homeostasis with age. We previously reported that expression of granulin peptides, the cleavage products of the neurodegenerative disease protein progranulin, enhance the accumulation and toxicity of TAR DNA binding protein 43 (TDP-43) in Caenorhabditis elegans (C. elegans). In this study we show that C. elegans granulins are produced in an age- and stress-dependent manner. Granulins localize to the endolysosomal compartment where they impair lysosomal protease expression and activity. Consequently, protein homeostasis is disrupted, promoting the nuclear translocation of the lysosomal transcription factor HLH-30/TFEB, and prompting cells to activate a compensatory transcriptional program. The three C. elegans granulin peptides exhibited distinct but overlapping functional effects in our assays, which may be due to amino acid composition that results in distinct electrostatic and hydrophobicity profiles. Our results support a model in which granulin production modulates a critical transition between the normal, physiological regulation of protease activity and the impairment of lysosomal function that can occur with age and disease. Author summary Progressive decline in maintenance of protein homeostasis clearly contributes to the development of neurodegenerative disorders, yet the molecular basis of this decline is poorly understood. Here, we take advantage of molecular genetic techniques available in the model organism C. elegans to investigate the mechanism underlying neurodegenerative disease due to mutations in the progranulin gene. We find that age, gene mutation and physiological stress lead to the accumulation of lysosomal granulins (the cleavage products of the progranulin protein) thereby disrupting cellular protein homeostasis. Granulin expression impairs animal fitness, resistance to stress and neuronal function, and stimulates a lysosomal stress response in an attempt to up-regulate lysosomal genes and restore normal function. Our findings are particularly important because they suggest a new, rational target-inhibition of progranulin cleavage into granulins-for neurodegenerative disease therapy.