Microglial TYROBP/DAP12 in Alzheimer’s disease: Transduction of physiological and pathological signals across TREM2

Abstract

TYROBP (also known as DAP12 or KARAP) is a transmembrane adaptor protein initially described as a receptor-activating subunit component of natural killer (NK) cells. TYROBP is expressed in numerous cell types, including peripheral blood monocytes, macrophages, dendritic cells, and osteoclasts, but a key point of recent interest is related to the critical role played by TYROBP in the function of many receptors expressed on the plasma membrane of microglia. TYROBP is the downstream adaptor and putative signaling partner for several receptors implicated in Alzheimer’s disease (AD), including SIRP1β, CD33, CR3, and TREM2. TYROBP has received much of its current notoriety because of its importance in brain homeostasis by signal transduction across those receptors. In this review, we provide an overview of evidence indicating that the biology of TYROBP extends beyond its interaction with these four ligand-binding ectodomain-intramembranous domain molecules. In addition to reviewing the structure and localization of TYROBP, we discuss our recent progress using mouse models of either cerebral amyloidosis or tauopathy that were engineered to be TYROBP-deficient or TYROBP-overexpressing. Remarkably, constitutively TYROBP-deficient mice provided a model of genetic resilience to either of the defining proteinopathies of AD. Learning behavior and synaptic electrophysiological function were preserved at normal physiological levels even in the face of robust cerebral amyloidosis (in APP/PSEN1;Tyrobp−/− mice) or tauopathy (in MAPTP301S;Tyrobp−/− mice). A fundamental underpinning of the functional synaptic dysfunction associated with each proteotype was an accumulation of complement C1q. TYROBP deficiency prevented C1q accumulation associated with either proteinopathy. Based on these data, we speculate that TYROBP plays a key role in the microglial sensome and the emergence of the disease-associated microglia (DAM) phenotype. TYROBP may also play a key role in the loss of markers of synaptic integrity (e.g., synaptophysin-like immunoreactivity) that has long been held to be the feature of human AD molecular neuropathology that most closely correlates with concurrent clinical cognitive function.

TREM2 is associated with tumor immunity and implies poor prognosis in glioma

“…After binding to the ligand, TREM2 transmits intracellular signals through the DNAX-activating protein (DAP12) ( 14 ). DAP12, also known as TYRO protein tyrosine kinase binding protein, is a signal transduction connector protein expressed predominantly in microglia and other cells involved in innate immune responses ( 15 ). DAP12 is phosphorylated on tyrosine by protein kinase SRC, which recruits the tyrosine-protein kinase SYK, and that in turn recruits various signaling mediators and adapters, resulting in intracellular signal transmission ( 16 ).…”

Comparative gene regulatory network analysis in Alzheimer’s disease and major depressive disorder identifies shared core regulatory circuits

“…In addition, there were terms related to phagocytosis. TYROBP, which is an adaptor protein that associates with activating receptors of immune cells, has been implicated in AD before 58 . It associates with TREM2, and both are required for phagocytosis in microglia.…”

The effect of Abi3 locus deletion on the progression of Alzheimer’s disease-related pathologies

“…TYROBP is a key regulator of this gene-network module, driving the expression of other genes. Interestingly, it is also the downstream signaling mediator of several microglial receptors that are implicated in AD, such as TREM2 and CD33 ( 41 , 48 ). One of the downstream pathways of TREM2-TYROBP is small GTPases in the Ras-homologous (Rho) family (e.g., Cdc42, Rac) ( 49 , 50 ).…”