| TMEM119 is a type I transmembrane protein of unknown enzymatic activity that is highly enriched in parenchymal microglia and has emerged as a robust marker for resident microglia in the central nervous system, where it helps distinguish yolk sac–derived microglia from infiltrating blood‑borne macrophages in many physiological and pathological contexts. The protein contains a luminal/extracellular N‑terminal region, a single transmembrane segment, and a short cytoplasmic tail, and its expression is largely restricted to microglia within brain parenchyma, with minimal or absent expression in neurons, astrocytes, oligodendrocytes, and most peripheral myeloid cells, which allows its use as a relatively specific microglial surface marker in immunohistochemistry, flow cytometry, and transcriptomic targeting approaches. Comparative microglial transcriptome analyses identified TMEM119 as a conserved microglial signature gene whose mRNA and protein are maintained across resting and classically activated states more stably than some other markers, and TMEM119 immunoreactivity labels IBA1⁺CD68⁺ microglia in human neurodegenerative brains while sparsely labeling or failing to label IBA1⁺ macrophages infiltrating multiple sclerosis lesions or infarcts, which supports its use to discriminate resident microglia from recruited monocyte-derived macrophages in human tissue. TMEM119 expression changes with microglial activation state and anatomical location: traumatic brain injury, experimental demyelination, ischemia, and other acute or chronic insults are often accompanied by decreased TMEM119 signal in microglia that adopt amoeboid or DAM‑like phenotypes, and TMEM119 protein is reduced particularly in lesion cores and periplaque regions while being better preserved in more distal or less activated microglial populations, indicating that TMEM119 marks homeostatic microglia and becomes downregulated as microglia transition to strongly reactive states. Recent work shows that TMEM119 can bind amyloid‑β oligomers, participate in recruitment of low-density lipoprotein receptor family members such as LRP1, and influence amyloid clearance and microglial phagocytic capacity in Alzheimer’s disease models, suggesting a functional role in microglial handling of aggregated proteins in addition to its value as a marker. TMEM119 mRNA levels are increased in some Alzheimer’s disease brains while protein levels show region- and context-dependent changes, and TMEM119-positive microglia do not consistently align with classical M1 or M2 polarization markers, which positions TMEM119 as a marker of microglial lineage and homeostatic identity rather than of a simple activation state. Expression outside the CNS has been reported in select peripheral cell types under specific conditions, and TMEM119 can be downregulated in highly stressed or disease-associated microglia, so its specificity and sensitivity depend on disease model and time point; these features are critical when TMEM119 promoter-driven reporter lines or antibody-based quantification are used to define or isolate microglial populations. |
