Doc2-mediated superpriming supports synaptic augmentation
Proceedings of the National Academy of Sciences of the United States of America
Various forms of synaptic plasticity underlie aspects of learning and memory. Synaptic augmentation is a form of short-term plasticity characterized by synaptic enhancement that persists for seconds following specific patterns of stimulation. The mechanisms underlying this form of plasticity are unclear but are thought to involve residual presynaptic Ca 2+ . Here, we report that augmentation was reduced in cultured mouse hippocampal neurons lacking the Ca 2+ sensor, Doc2; other forms of
... er forms of short-term enhancement were unaffected. Doc2 binds Ca 2+ and munc13 and translocates to the plasma membrane to drive augmentation. The underlying mechanism was not associated with changes in readily releasable pool size or Ca 2+ dynamics, but rather resulted from superpriming a subset of synaptic vesicles. Hence, Doc2 forms part of the Ca 2+ -sensing apparatus for synaptic augmentation via a mechanism that is molecularly distinct from other forms of shortterm plasticity. short-term plasticity | synaptic augmentation | Doc2 | munc13 | superpriming N eurons communicate with one another using chemical synapses that typically display plasticity; that is, their strength is modulated in an activity-dependent manner (1). Short-term synaptic plasticity (STP) occurs on timescales of milliseconds to minutes and contributes to a wide range of neuronal functions ranging from working memory to motor control (2-4). Shortterm enhancement (STE) refers to an increase, and short-term depression refers to a decrease, in the strength of transmission. Subtypes of STE are typically classified, according to their timescales, into the following: paired pulse facilitation (PPF; milliseconds), augmentation (seconds), and posttetanic potentiation (PTP; minutes) (1, 5-7). These forms of plasticity are thought to depend on residual Ca 2+ , which accumulates in presynaptic nerve terminals during bouts of synaptic activity (1, 8). Hence, Ca 2+ -binding proteins play crucial roles in different forms of STP. For example, synaptotagmin 7 has been shown to mediate PPF (9). Although significant progress has been made, it is still unclear how various presynaptic proteins utilize Ca 2+ signals to execute different forms of synaptic plasticity. The focus of the current study is synaptic augmentation. Like other forms of STE, this form of plasticity has been actively studied for decades in cultured neurons (10-13), hippocampal slices (14-16), and the neuromuscular junction (17), yet the underlying mechanisms remain elusive. The synaptic vesicle (SV) priming factor munc13 (18) has been shown to play a role in augmentation (11-13), in part by interacting with calmodulin (12). However, disruption of the calmodulin-binding site of munc13 only partially eliminated augmentation (12), suggesting that additional, unidentified mechanisms exist. The double C2 domain protein (Doc2) is one such possible contributor to augmentation, as it is a Ca 2+ -binding protein that also interacts with munc13 via its munc13 interaction (MID) domain (19). Two of the three known isoforms of Doc2 (α and β) bind Ca 2+ and interact with phospholipids and target membrane soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) in a Ca 2+ -dependent manner. These interactions are mediated via tandem C2 domains (C2A and C2B) (20, 21). Doc2α/β have been proposed to function as Ca 2+ sensors for asynchronous (22) and spontaneous (23) SV release (but see also refs. 24 and 25). Moreover, Doc2 has been implicated in synaptic plasticity: loss of Doc2α leads to altered synaptic depression during train stimulation (22, 26) and disrupts long-term potentiation (26). However, a role for Doc2 in synaptic augmentation has not been explored. In the present study, we systematically tested the role of Doc2 in three types of STE in cultured mouse hippocampal neurons. We found that synaptic augmentation, but not PPF or PTP, was impaired in Doc2α/β double knockout (DKO) neurons. Moreover, the ability to bind Ca 2+ and munc13 underlies the function of Doc2 in augmentation. Finally, we determined that augmentation in cultured hippocampal neurons results from superpriming of a subset of SVs and is not due to an effect on the size of the readily releasable pool (RRP) of SVs or presynaptic Ca 2+ dynamics. These observations reveal a previously unidentified function for Doc2 in presynaptic nerve terminals and provide insights into the molecular mechanisms that underlie synaptic plasticity. Significance Plastic changes in synaptic connections constitute the basis of learning and memory. Different forms of synaptic plasticity are generally distinguished experimentally by their timescales, but it is unclear whether each form of plasticity corresponds to a distinct biological process with a dedicated molecular mechanism. In the present study, we show that the Ca 2+ -binding protein, Doc2, "superprimes" a subset of already primed synaptic vesicles to make them more likely to release, and this process selectively contributes to augmentation (on the scale of seconds). The underlying molecular mechanism does not mediate other forms of short-term enhancement (that occur on the timescale of milliseconds or minutes). This work establishes a function of Doc2 in maintaining synaptic plasticity within a narrow time window.