GRN and frontotemporal dementia: Moreover, although upregulation of GPNMB in young (5-to-6-month-old) Grn-deficient mice is associated with improved lysosomal function, we still do not know how age modulates these phenotypes, or how the protective role that upregulation of GPNMB may afford in the context of FTD-GRN mutant carriers is modified by an ageing immune system.