TREM2 and neurodegenerative disease: Microglia underlie different neurodegenerative disease processes, including lipomembranous osteodysplasia with sclerosing leukoencephalopathy (Nasu-Hakola disease) caused by mutations in TREM2 and adult onset leukoencephalopathy with axonal spheroids and pigmented glia due to CSF1R haploinsufficiency (reviewed in [44]).