Doc2 Supports Spontaneous Synaptic Transmission by a Ca²⁺-Independent Mechanism

Two families of Ca²⁺-binding proteins have been proposed as Ca²⁺ sensors for spontaneous release: synaptotagmins and Doc2s, with the intriguing possibility that Doc2s may represent high-affinity Ca²⁺ sensors that are activated by deletion of synaptotagmins, thereby accounting for the increased spontaneous release in synaptotagmin-deficient synapses. Here, we use an shRNA-dependent quadruple knockdown of all four Ca²⁺-binding proteins of the Doc2 family to confirm that Doc2-deficient synapses exhibit a marked decrease in the frequency of spontaneous release events. Knockdown of Doc2s in synaptotagmin-1-deficient synapses, however, failed to reduce either the increased spontaneous release or the decreased evoked release of these synapses, suggesting that Doc2s do not constitute Ca²⁺ sensors for asynchronous release. Moreover, rescue experiments revealed that the decrease in spontaneous release induced by the Doc2 knockdown in wild-type synapses is fully reversed by mutant Doc2B lacking Ca²⁺-binding sites. Thus, our data suggest that Doc2s are modulators of spontaneous synaptic transmission that act by a Ca²⁺-independent mechanism.