Microglia, the main phagocytes of the central nervous system (CNS), are involved in the surveillance and maintenance of nervous tissue. During normal tissue homeostasis, microglia migrate within the CNS, phagocytose dead cells and tissue debris, and modulate synapse pruning and spine formation via controlled phagocytosis. In the event of an invasion by a foreign body, microglia are able to phagocytose the invading pathogen and process it proteolytically for antigen presentation. Internalized substrates are incorporated and sorted within the endocytic pathway and thereafter transported via complex vesicular routes. When targeted for degradation, substrates are delivered to acidic late endosomes and lysosomes. In these, the enzymatic degradation relies on pH and enzyme content. Endocytosis, sorting, transport, compartment acidification and degradation are regulated by complex signaling mechanisms, and these may be altered during aging and pathology. In this review, we discuss the endocytic pathway in microglia, with insight into the mechanisms controlling lysosomal biogenesis and pH regulation. We also discuss microglial lysosome function associated with Alzheimer’s disease (AD) and the mechanisms of amyloid-beta (Aβ) internalization and degradation. Finally, we explore some therapies currently being investigated to treat AD and their effects on microglial response to Aβ, with insight in those involving enhancement of lysosomal function.

Osmium tetroxide (OsO4) is a commonly used stain for unsaturated lipids in electron and optical microscopy of cells and tissues. In this work, the localization of osmium oxide and specific lipids was independently monitored in mouse adipose tissue by using time-of-flight secondary ion mass spectrometry with Bi cluster primary ions. Substance-specific ion images recorded after OsO4 staining showed that unsaturated C18 fatty acids were colocalized with osmium oxide, corroborating the view that osmium tetroxide binds to unsaturated lipids. In contrast, saturated fatty acids (C14, C16 and C18) and also unsaturated C16 fatty acids show largely complementary localizations to osmium oxide. Furthermore, the distributions of saturated and unsaturated diglycerides are consistent with the specific binding of osmium oxide to unsaturated C18 fatty acids. The abundance of ions, characteristic of phospholipids and proteins, is strongly decreased as a result of the osmium staining, suggesting that a large fraction of these compounds are removed from the tissue during this step, while ions related to fatty acids, di- and triglycerides remain strong after osmium staining. Ethanol dehydration after osmium staining results in more homogeneous distributions of osmium oxide and unsaturated lipids. This work provides detailed insight into the specific binding of osmium oxide to different lipids.

TMEM106B encodes a lysosomal membrane protein and was initially identified as a risk factor for frontotemporal lobar degeneration. Recently, a dominant D252N mutation in TMEM106B was shown to cause hypomyelinating leukodystrophy. However, how TMEM106B regulates myelination is still unclear. Here we show that TMEM106B is expressed and localized to the lysosome compartment in oligodendrocytes. TMEM106B deficiency in mice results in myelination defects with a significant reduction of protein levels of proteolipid protein (PLP) and myelin oligodendrocyte glycoprotein (MOG), the membrane proteins found in the myelin sheath. The levels of many lysosome proteins are significantly decreased in the TMEM106B-deficient Oli-neu oligodendroglial precursor cell line. TMEM106B physically interacts with the lysosomal protease cathepsin D and is required to maintain proper cathepsin D levels in oligodendrocytes. Furthermore, we found that TMEM106B deficiency results in lysosome clustering in the perinuclear region and a decrease in lysosome exocytosis and cell surface PLP levels. Moreover, we found that the D252N mutation abolished lysosome enlargement and lysosome acidification induced by wild-type TMEM106B overexpression. Instead, it stimulates lysosome clustering near the nucleus as seen in TMEM106B-deficient cells. Our results support that TMEM106B regulates myelination through modulation of lysosome function in oligodendrocytes.