Processing of progranulin into granulins involves multiple lysosomal proteases and is affected in frontotemporal lobar degeneration

Abstract

Background Progranulin loss-of-function mutations are linked to frontotemporal lobar degeneration with TDP-43 positive inclusions (FTLD-TDP-Pgrn). Progranulin (PGRN) is an intracellular and secreted pro-protein that is proteolytically cleaved into individual granulin peptides, which are increasingly thought to contribute to FTLD-TDP-Pgrn disease pathophysiology. Intracellular PGRN is processed into granulins in the endo-lysosomal compartments. Therefore, to better understand the conversion of intracellular PGRN into granulins, we systematically tested the ability of different classes of endo-lysosomal proteases to process PGRN at a range of pH setpoints. Results In vitro cleavage assays identified multiple enzymes that can process human PGRN into multi- and single-granulin fragments in a pH-dependent manner. We confirmed the role of cathepsin B and cathepsin L in PGRN processing and showed that these and several previously unidentified lysosomal proteases (cathepsins E, G, K, S and V) are able to process PGRN in distinctive, pH-dependent manners. In addition, we have demonstrated a new role for asparagine endopeptidase (AEP) in processing PGRN, with AEP having the unique ability to liberate granulin F from the pro-protein. Brain tissue from individuals with FTLD-TDP-Pgrn showed increased PGRN processing to granulin F and increased AEP activity in degenerating brain regions but not in regions unaffected by disease. Conclusions This study demonstrates that multiple lysosomal proteases may work in concert to liberate multi-granulin fragments and granulins. It also implicates both AEP and granulin F in the neurobiology of FTLD-TDP-Pgrn. Modulating progranulin cleavage and granulin production may represent therapeutic strategies for FTLD-Pgrn and other progranulin-related diseases.

Multi-modal Proteomic Characterization of Lysosomal Function and Proteostasis in Progranulin-Deficient Neurons

“…We found that several lysosomal enzymes were upregulated both in the mouse and human dataset, notably cathepsins. We showed decreased cathepsin B activity in live neurons, a phenomenon only shown in in vitro assays before 52,[54][55][56] . Similar perturbations of lysosomal acidification have been reported in non-neuron cells and other neurodegenerative diseases 57,58 .…”

Conserved gene signatures shared among MAPT mutations reveal defects in calcium signaling

“…These gene signatures were largely specific for tauopathy; however, we identified a number of lysosomal genes that were altered in iPSC-neurons from MAPT mutations and in brains from GRN mutation carriers. GRN has been implicated in lysosomal function and neuronal integrity ( Martens et al, 2012 ; Kao et al, 2017 ; Logan et al, 2021 ; Mohan et al, 2021 ; Simon et al, 2022 ). Thus, our stem cell model revealed several genes and pathways that are also altered in primary tauopathy patients.…”

Autophagy in the Neuronal Ceroid Lipofuscinoses (Batten Disease)

“…Progranulin (PGRN, officially known as GRN) is a soluble glycoprotein that is proteolytically processed into 6 kDa granulins within endo-lysosomes by cathepsins ( Table 1 ) ( Toh et al, 2011 ; Zhou et al, 2017 ; Mohan et al, 2021 ). Although its function is not well defined, GRN is known to localize within lysosomes and extracellularly, and has been linked to cell migration, inflammation, and processes related to cancer progression ( Table 1 ) ( Zhou et al, 1993 ; Tian et al, 2016 ; Daya et al, 2018 ; Paushter et al, 2018 ; Voshtani et al, 2019 ).…”