The lysosomal protein cathepsin L is a progranulin protease

Chris W Lee,Yari Carlomagno,Sandra Almeida,Kwok-Fai Lau,Jeannette N Stankowski,Jeannie Chew,Matthew Bogyo,Fen-Biao Gao,Ying-Wai Lam,Dennis W Dickson,Casey N Cook,Mercedes Prudencio,Leonard Petrucelli

doi:10.1186/s13024-017-0196-6

Abstract

Haploinsufficiency of GRN, the gene encoding progranulin (PGRN), causes frontotemporal lobar degeneration (FTLD), the second most common cause of early-onset dementia. Receptor-mediated lysosomal targeting has been shown to regulate brain PGRN levels, and complete deficiency of PGRN is a direct cause of neuronal ceroid lipofuscinosis (NCL), a lysosomal storage disease. Here we show that the lysosomal cysteine protease cathepsin L (Cat L) can mediate the proteolytic cleavage of intracellular PGRN into poly-granulin and granulin fragments. Further, PGRN and Cat L co-localize in lysosomes of HEK293 cells, iPSC-derived neurons and human cortical neurons from human postmortem tissue. These data identify Cat L as a key intracellular lysosomal PGRN protease, and provides an intriguing new link between lysosomal dysfunction and FTLD.

Highlights

Frontotemporal dementia (FTD) is a neurodegenerative disorder resulting from frontotemporal lobar degeneration (FTLD), and is characterized by progressive changes in personality, behavior, and/or language
Cathepsin L is a specific lysosomal PGRN protease that cleaves PGRN into granulins We first confirmed that PGRN is localized to the lysosome (Additional file 1: Figure S1a, b) and that the PGRN antibody was specific as shown by siRNAmediated PGRN knockdown (Additional file 1: Figure S1a)
While cathepsin B (Cat B)-OE or cathepsin D (Cat D)-OE cells showed only minor, non-specific, reductions in PGRN levels, cathepsin L (Cat L)-OE cells showed a shift from the predominantly 70 kDa PGRN to lower-molecular weight species (40 to 55 kDa) (Fig. 1a). This shift to lower molecular weight PGRN species upon exposure to Cat L-OE was confirmed using a second full-length PGRN antibody (Fig. 1b). These results strongly suggest that Cat L is a specific intracellular PGRN protease involved in regulating the maturation and turnover of PGRN within the lysosome

Summary

Introduction

Frontotemporal dementia (FTD) is a neurodegenerative disorder resulting from frontotemporal lobar degeneration (FTLD), and is characterized by progressive changes in personality, behavior, and/or language. PGRN contains seven and a half tandem granulin motifs separated by linker regions and proteolytic processing of PGRN results in the formation of several distinct granulin proteins [8,9,10]. Extracellular proteases, such as neutrophil elastase [18] and MMP12 [19], have been shown to cleave the secreted form of PGRN into mature, individual granulins A-G, as well as intermediate poly-granulins. As specific biological roles for full-length PGRN and/or the cleaved granulins remain unclear, it is important to further our understanding of the mechanisms involved in PGRN proteolytic processing

Methods

Results

Conclusion