Monospecific antibodies reveal reduction of granulins in frontotemporal dementia caused by GRN mutations

Loss-of-function mutations in the progranulin (PGRN) gene ( GRN ) are a major cause of Frontotemporal dementia (FTD), also known as frontotemporal lobar degeneration (FTLD). PGRN is a lysosomal protein implicated in neuroprotection, proteostasis, and regulation of lipids including bis(monoacylglycero)phosphate (BMP). PGRN is proteolytically processed into a half-granulin and seven granulin peptides (GRNs 1–GRN7; GRNs A-G), but the individual roles of granulins in health and disease remain poorly defined due to the lack of granulin-specific antibodies. We generated and validated a panel of polyclonal monospecific antibodies targeting each human granulin (GRN1–GRN7). Granulin antibodies were characterized by immunoblot, immunocytochemistry, and epitope mapping using a peptide microarray. The ability of anti-granulin antibodies to detect endogenous GRNs was evaluated in human cell lines, peripheral tissues, and postmortem brain samples from individuals with and without GRN mutation-associated FTD. Antibodies against GRN1, GRN2, GRN3, GRN4, and GRN7 selectively detected overexpressed and endogenous GRNs in human cell lines, with minimal cross-reactivity. Epitope mapping revealed that anti-GRN2 and GRN4 antibodies target unique linear regions that correlate with their superior performance in complex tissues. GRN2 and GRN4 were readily detectable in human brain and peripheral tissues, with the highest expression in thalamus and white matter. GRN2 and GRN4 levels were significantly reduced in postmortem frontal cortex from FTD- GRN patients ( n = 12) compared to controls ( n = 12) paralleling deficiency of full-length PGRN. Reduced levels of granulins and PGRN occurred alongside astrogliosis, microgliosis, and phospho-TDP-43 accumulation. We report the development of a panel of antibodies for the detection of individual human granulins. These reagents uncover selective reductions of GRN2 and GRN4 in FTD- GRN brain tissue, suggesting that decreased and insufficient levels of granulins contribute to disease pathogenesis and neurodegeneration. This antibody resource opens new avenues for dissecting granulin biology and its role in neurodegeneration.