Microglia deploy TAM receptors to kill motor neurons in a mouse model of amyotrophic lateral sclerosis.

Activation of microglia is a prominent feature of amyotrophic lateral sclerosis (ALS), a neurodegenerative disease that leads to the death of motor neurons. A key component of this activation is elevated expression of the TAM receptor tyrosine kinases Axl and Mer (gene name Mertk). Here we show that germline and microglial-restricted inactivation of the Axl and Mertk genes in the SOD1G93A mouse model of ALS leads to an extension of lifespan, which is tied to the preservation of cholinergic motor neurons and neuromuscular synapses. Also elevated on SOD1G93A neuronal surfaces is the essential TAM co-ligand phosphatidylserine, a potent 'eat-me' signal through which apoptotic cells are engulfed by microglia. Correspondingly, we find that microglial lysosomes are filled with the remains of cholinergic neurons in the SOD1G93A spinal cord, whereas this accumulation is markedly reduced in the SOD1G93AAxl-/-Mertk-/- cord. Together, these results suggest that microglia phagocytically kill living neurons, and thereby hasten death in ALS.