Multi-modal proteomic characterization of lysosomal function and proteostasis in progranulin-deficient neurons

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Authors
Hasan, Saadia 
Fernandopulle, Michael S. 
Humble, Stewart W. 
Frankenfield, Ashley M. 
Li, Haorong 
Abstract

Background: Progranulin (PGRN) is a lysosomal glycoprotein implicated in various neurodegenerative diseases, including frontotemporal dementia and neuronal ceroid lipofuscinosis. Over 70 mutations discovered in the GRN gene all result in reduced expression of the PGRN protein. Genetic and functional studies point toward a regulatory role for PGRN in lysosome functions. However, the detailed molecular function of PGRN within lysosomes and the impact of PGRN deficiency on lysosomes remain unclear. Methods: We developed multifaceted proteomic techniques to characterize the dynamic lysosomal biology in living human neurons and fixed mouse brain tissues. Using lysosome proximity labeling and immuno-purification of intact lysosomes, we characterized lysosome compositions and interactome in both human induced pluripotent stem cell (iPSC)-derived glutamatergic neurons (i3Neurons) and mouse brains. Using dynamic stable isotope labeling by amino acids in cell culture (dSILAC) proteomics, we measured global protein half-lives in human i3Neurons for the first time. Results: Leveraging the multi-modal proteomics and live-cell imaging techniques, we comprehensively characterized how PGRN deficiency changes the molecular and functional landscape of neuronal lysosomes. We found that PGRN loss impairs the lysosome’s degradative capacity with increased levels of v-ATPase subunits on the lysosome membrane, increased hydrolases within the lysosome, altered protein regulations related to lysosomal transport, and elevated lysosomal pH. Consistent with impairments in lysosomal function, GRN-null i3Neurons and frontotemporal dementia patient-derived i3Neurons carrying GRN mutation showed pronounced alterations in protein turnover, such as cathepsins and proteins related to supramolecular polymerization and inherited neurodegenerative diseases. Conclusion: This study suggested PGRN as a critical regulator of lysosomal pH and degradative capacity, which influences global proteostasis in neurons. Beyond the study of progranulin deficiency, these newly developed proteomic methods in neurons and brain tissues provided useful tools and data resources for the field to study the highly dynamic neuronal lysosome biology. Graphical Abstract:

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Acknowledgements: We would like to thank Maia Parsadanian for her assistance with mouse brain immunostaining, Dr. Dimitri Krainc for providing the FTD patient iPSC lines, and Dr. Lingnan Lin for generating Python codes for data analysis.

Keywords
Proteomics, iPSC, Half-life, Turnover, PGRN, Neuron, dSILAC, Progranulin, Lysosome, Frontotemporal dementia
Journal Title
Molecular Neurodegeneration
Conference Name
Journal ISSN
1750-1326
Volume Title
18
Publisher
BioMed Central
Sponsorship
National Institutes of Health (R01NS121608)
National Institute of Neurological Disorders and Stroke (Intramural Research Program)
Dementias Platform UK (MR/M024962/1)