Microglial activation occurs in response to peripheral nerve injury and affects survival of injured neurons. For the communication between microglia and nerve injured neurons, some receptors expressed on microglial surfaces are crucial to initiate signaling cascades leading to the microglial activation. Although purinoceptors and colony stimulating factor-1 receptor are well referred as mediators for the activation of microglia, additional receptors have been scarcely addressed. We recently identified a transmembrane protein DNAX-activating protein of 12 kDa (DAP12) as another key molecule regulating microglial activation after neuronal injury. DAP12 forms complexes with several other receptors and transduce their signals into the cytoplasm by functioning as an adaptor. Although DAP12 is reported to regulate an activation of myeloid cells such as macrophages and osteoclasts, its functions in microglia have been largely unknown. We therefore set out to analyze DAP12 functions in microglial activation using hypoglossal nerve transection model of mice. Histological analyses revealed that DAP12 was specifically expressed in activated microglia surrounding injured motor neurons in response to nerve injury. DAP12 deficiency resulted in shortening of microglial activation duration after nerve injury and suppressed expression of cytotoxic cytokines. Consequently in DAP12 knockout mice, nerve injury-induced motor neuron death was dramatically prevented. These results suggest that DAP12-mediated signals accelerate microglial activation to enhance neurotoxicity after neuronal injury.