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 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 variable, 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 show increased PGRN processing to granulin F, correlating with increased activity of AEP, in a region-specific manner. Conclusions: This study demonstrates that multiple lysosomal proteases may work in concert to liberate granulins and implicates both AEP and granulin F in the neurobiology of FTLD-TDP-Pgrn. Modulating progranulin cleavage may represent a new strategy to modulate PGRN and granulin levels in disease.

Mutations in α-synuclein, TDP-43 and tau prolong protein half-life through diminished degradation by lysosomal proteases

“…Regions with a relative paucity of protease processing sites, such as the C-terminal glycine rich domain (amino acids 351 to 414) were signi cant, however, for AEP sites. Interestingly, AEP has previously been shown to exhibit relatively distinctive, non-overlapping cleavage sites in other proteins as well 33,34 .…”

Progranulin as a therapeutic target in neurodegenerative diseases

“…Full-length PGRN has neurotrophic and anti-inflammatory effects, but when PGRN is trafficked into the lysosome by the PSAP-dependent pathway, it is thought to be cleaved into seven individual, highly conserved, disulfide-bond-containing, 6-kDa granulin peptides, designated granulins A through G, and the 3.5-kDa paragranulin [75,76]. While some functions of individual granulins have been identified, their full range of activity is not yet fully understood [77].…”

“…Granulin peptides can be produced as a result of stress and aging and can impair the expression and activity of lysosomal proteases [80]. Of note, regionally localized changes in the protease asparagine endopeptidase (AEP), which cleaves granulin F from PGRN, were observed post-mortem in brain regions with severe neurodegenerative change compared with regions with little degeneration in FTD-GRN cases [75]. Like granulin F, granulin A also takes on a defined 3D structure in solution and has been shown to inhibit cancer cell growth via ENO1 [81].…”

Progranulin Promotes Functional Recovery in Rats With Acute Spinal Cord Injury via Autophagy-induced Anti-inflammatory Microglia Polarization.

“…In patients with frontotemporal dementia due to mutations in the GRN gene, the expression of PGRN was found to be signi cantly reduced [6]. In subsequent studies, it was found that the expression of lysosomes and PGRN is closely related in neurodegenerative diseases [7]. In the PGRN defect model, the function of the lysosome is signi cantly impaired [8].…”