Releasing the brake on presynaptic development: Cdc20-APC triggers NeuroD2 degradation to drive presynaptic differentiation.

The assembly of synapses is important for the establishment of neuronal circuitry in the brain. During development, the morphogenesis of axons and dendrites culminating in synaptogenesis is temporally coordinated1. The initial phases of axon and dendrite growth and patterning are followed by a later phase of synapse formation. The mechanisms that orchestrate the transition from axon and dendrite growth to synaptogenesis in postmitotic neurons are poorly understood. Yang et al. have recently reported a mechanism that inhibits presynaptic differentiation in newly generated neurons, suggesting that presynaptic formation is restrained in immature neurons2. Further, the authors uncovered a Cdc20-APC ubiquitin ligase signaling pathway that releases the presynaptic brake in maturing neurons and hence drives presynaptic differentiation. The anaphase promoting complex (APC) was initially identified in proliferating cells as an important regulator of progression through the cell cycle3. The two APC coactivators, Cdh1 and Cdc20, are active at distinct phases during the cell cycle and target the APC to different substrates3. During early mitosis, Cdc20-APC drives the separation of sister chromatids, moving cells through anaphase (Figure 1A). Cdh1-APC is active in late mitosis and during the G1 phase of the cell cycle and is necessary for mitotic exit and maintenance of the G1 phase. Failure to activate the APC during critical points along the cell cycle leads to cell cycle arrest. Figure 1 Cdc20-APC drives the progression through anaphase in proliferating cells and through presynaptic differentiation in postmitotic neurons Recent evidence has implicated both Cdh1-APC and Cdc20-APC in the development of postmitotic neurons in the mammalian brain4–6. Interestingly, neuronal Cdh1-APC controls the early stages of axon morphogenesis comprised of growth and patterning in the mammalian brain5. In a recent study, Yang et al. found that neuronal Cdc20-APC plays a critical role in a later stage of axon morphogenesis consisting of presynaptic axonal differentiation2. Cdc20-APC coordinately promoted clustering of synaptic vesicle and active zone proteins and the establishment of functional presynaptic sites (Figure 1B)2. However, Cdc20-APC appeared to have little or no effect on axon growth. This suggests that Cdh1-APC and Cdc20-APC may operate at distinct developmental time points in postmitotic neurons to coordinate the morphogenesis of axons in the brain. Since Cdc20-APC harbors E3 ubiquitin ligase activity, the authors searched for potential neuronal Cdc20-APC substrates important for presynaptic differentiation2. A putative target was the transcription factor NeuroD2, which contains the Cdc20 recognition sequence, the destruction-box (D-box) motif. Cdc20 was found to interact with NeuroD2 in a D-box dependent manner, and Cdc20 knockdown led to the accumulation of NeuroD2 in neurons2. Importantly, NeuroD2 suppressed presynaptic differentiation in neurons. In epistasis analyses, NeuroD2 was found to function downstream of Cdc20-APC in the regulation of presynaptic differentiation. Together, these data suggest that NeuroD2 restrains presynaptic formation, and targeting of NeuroD2 by Cdc20-APC for degradation drives presynaptic development (Figure 1B). Yang and colleagues also identified a key function for the NeuroD2 target gene, Cplx2, in the control of presynaptic differentiation2. In genetic experiments, the authors found that Cplx2 operates downstream of Cdc20-APC and NeuroD2 in the suppression of presynaptic differentiation (Figure 1B)2. Since Cplx2 regulates synaptic vesicle membrane fusion and locally controls presynaptic differentiation in diverse organisms7, the identification of a NeuroD2/Cplx2 link suggests that NeuroD2 function is intimately coupled to the control of presynaptic differentiation. In summary, the new results on Cdc20-APC function in neurons suggest that a Cdc20-APC/NeuroD2/Cplx2 signaling pathway may orchestrate the timing of presynaptic differentiation in neurons (Figure 1B)2. The transcription factor NeuroD2 may inhibit the premature connectivity of newly generated neurons with efferent neurons until the proper morphogenesis and patterning of axons is achieved. The degradation of NeuroD2 triggered by Cdc20-APC may thus represent a developmental switch that marks the transition between axon growth and the phase of axon morphogenesis that features presynaptic differentiation. It will important in future studies to identify the extracellular stimuli that interface with the Cdc20-APC/NeuroD2/Cplx2 signaling pathway to drive presynaptic differentiation. Since neuronal activity plays an essential role during the critical periods of synapse development, calcium signaling might be anticipated to regulate the temporal dynamics of presynaptic formation. Calcium entry through voltage-gated calcium channels induce phosphorylation of the transactivation domain of the NeuroD family of transcription factors and consequently activate NeuroD-dependent gene transcription8–9. Thus, it is anticipated that neuronal activity might play a critical role in the transcriptional regulation of the NeuroD2 target gene Cplx2 during development. Intriguingly, abnormalities in the expression of Cplx2 have been reported in diverse neurological and psychiatric diseases, including Huntington’s disease and schizophrenia10. It will be interesting to determine if dysfunction of the Cdc20-APC/NeuroD2 signaling pathway might play a role in the pathogenesis of these brain disorders.