Inhibition of Bruton’s Tyrosine Kinase Modulates Microglial Phagocytosis: Therapeutic Implications for Alzheimer’s Disease

Abstract

Bruton’s tyrosine kinase (BTK), a critical component of B cell receptor signaling, has recently been implicated in regulation of the peripheral innate immune response. However, the role of BTK in microglia, the resident innate immune cells of the central nervous system, and its involvement in the pathobiology of neurodegenerative disease has not been explored. Here we found that BTK is a key regulator of microglial phagocytosis. Using potent BTK inhibitors and small interfering RNA (siRNA) against BTK, we observed that blockade of BTK activity decreased activation of phospholipase gamma 2, a recently identified genetic risk factor in Alzheimer’s disease (AD), and reduced phagocytosis in rodent microglia and human monocyte-derived macrophages. Inhibition of BTK signaling also decreased microglial uptake of synaptosomes but did not have major impacts on other key microglial functions such as migration and cytokine release. Similarly, blocking BTK function ex vivo in acute brain slices reduced microglial phagocytosis and maintained numbers of resting microglia. In brain tissues from the 5xFAD mouse model of AD, levels of microglial BTK were elevated while in two gene expression datasets of post-mortem AD patient brain tissues, upregulation of BTK transcript was observed. Our study provides novel insights into the role of BTK in regulating microglial phagocytosis and uptake of synaptic structures and suggests that inhibiting microglial BTK may improve cognition in AD by preventing microglial activation and synaptic loss. Graphical Abstract Microglial-mediated synapse loss has been implicated in AD pathogenesis. Inhibition of BTK decreases activation of PLCγ2, a genetic risk factor in AD, and reduces microglial phagocytosis and uptake of synaptic structures. As such BTK inhibition may represent a therapeutic route to prevent microglial activation and synapse loss in AD

Ibrutinib modulates Aβ/tau pathology, neuroinflammation, and cognitive function in mouse models of Alzheimer's disease

“…However, the potential of ibrutinib to modulate the pathology of neurodegenerative diseases such as AD has not been fully addressed. BTK expression is upregulated postmortem in the brains of AD patients and 5xFAD mice (mouse model of AD) (Keaney et al, 2019), and inhibition of BTK suppresses microglial activation and synaptic loss by deactivating PLCγ2 (Keaney et al, 2019). Given that reactive microglia are critical in the pathogenesis of AD, these observations imply that suppression of BTK might be a potential target for regulating AD pathology via inhibition of microglial activation.…”

Strategies and Tools for Studying Microglial-Mediated Synapse Elimination and Refinement

“…To visualize their engulfment by microglia or astrocytes, SYNs can be stained with dyes sensitive to acidic pH ( 102 – 104 , 147 ). These dyes (one of the most widely employed is pHrodo) show little or no fluorescent signal at neutral pH, while they fluoresce brightly when in acidic environments, thus allowing SYN visualization only when engulfed by acidic phagosomes.…”

“…Using a similar approach, Keaney et al showed that the blockade of Bruton's Tyrosine Kinase (BTK), a protein involved in different processes such as B cell receptor signaling, pro-inflammatory cytokine release and phagocytosis, reduces the uptake of pHrodo-labeled SYNs by microglia ( 102 ). PHrodo labeled SYNs were also used by Madore et al, to show the relevance of poly-unsaturated omega-3 fatty acids (n-3 PUFAs) in controlling microglial phagocytosis in the developing brain.…”

The Emerging Therapeutic Potential of Nitro Fatty Acids and Other Michael Acceptor-Containing Drugs for the Treatment of Inflammation and Cancer

“…Of note, additional studies pointed on the use of Bruton tyrosine kinase inhibitors in the treatment of AD as well as in MS (Montalban et al, 2019;Keaney et al, 2019). In addition, cysteine-targeting compounds such as ICE-like cysteine protease inhibitors (caspase I inhibitors) have been recently suggested as anti-apoptotic and anti-inflammatory agents to treat AD and PD patients, in which progressive neuronal death seems to be associated with caspase overactivation (LoPachin et al, 2008).…”