Acute in vivo genetic rescue demonstrates that phosphorylation of RIM1alpha serine 413 is not required for mossy fiber long-term potentiation

cAMP-EPAC-PKCε-RIM1α signaling regulates presynaptic long-term potentiation and motor learning

The cerebellum is involved in learning of fine motor skills, yet whether presynaptic plasticity contributes to such learning remains elusive. Here, we report that the EPAC-PKCε module has a critical role in a presynaptic form of long-term potentiation in the cerebellum and motor behavior in mice. Presynaptic cAMP-EPAC-PKCε signaling cascade induces a previously unidentified threonine phosphorylation of RIM1α, and thereby initiates the assembly of the Rab3A-RIM1α-Munc13-1 tripartite complex that facilitates docking and release of synaptic vesicles. Granule cell-specific blocking of EPAC-PKCε signaling abolishes presynaptic long-term potentiation at the parallel fiber to Purkinje cell synapses and impairs basic performance and learning of cerebellar motor behavior. These results unveil a functional relevance of presynaptic plasticity that is regulated through a novel signaling cascade, thereby enriching the spectrum of cerebellar learning mechanisms.

Active Zone Proteins RIM1αβ Are Required for Normal Corticostriatal Transmission and Action Control

Dynamic regulation of synaptic transmission at cortical inputs to the dorsal striatum is considered critical for flexible and efficient action learning and control. Presynaptic mechanisms governing the properties and plasticity of glutamate release from these inputs are not fully understood, and the corticostriatal synaptic processes that support normal action learning and control remain unclear. Here we show in male and female mice that conditional deletion of presynaptic proteins RIM1αβ (RIM1) from excitatory cortical neurons impairs corticostriatal synaptic transmission in the dorsolateral striatum. Key forms of presynaptic G-protein-coupled receptor-mediated short- and long-term striatal plasticity are spared following RIM1 deletion. Conditional RIM1 KO mice show heightened novelty-induced locomotion and impaired motor learning on the accelerating rotarod. They further show heightened self-paced instrumental responding for food and impaired learning of a habitual instrumental response strategy. Together, these findings reveal a selective role for presynaptic RIM1 in neurotransmitter release at prominent basal ganglia synapses, and provide evidence that RIM1-dependent processes help to promote the refinement of skilled actions, constrain goal-directed behaviors, and support the learning and use of habits.SIGNIFICANCE STATEMENT Our daily functioning hinges on the ability to flexibly and efficiently learn and control our actions. How the brain encodes these capacities is unclear. Here we identified a selective role for presynaptic proteins RIM1αβ in controlling glutamate release from cortical inputs to the dorsolateral striatum, a brain structure critical for action learning and control. Behavioral analysis of mice with restricted genetic deletion of RIM1αβ further revealed roles for RIM1αβ-dependent processes in the learning and refinement of motor skills and the balanced expression of goal-directed and habitual actions.

14-3-3 Proteins in Glutamatergic Synapses

The 14-3-3 proteins are a family of proteins that are highly expressed in the brain and particularly enriched at synapses. Evidence accumulated in the last two decades has implicated 14-3-3 proteins as an important regulator of synaptic transmission and plasticity. Here, we will review previous and more recent research that has helped us understand the roles of 14-3-3 proteins at glutamatergic synapses. A key challenge for the future is to delineate the 14-3-3-dependent molecular pathways involved in regulating synaptic functions.

A Combined Optogenetic-Knockdown Strategy Reveals a Major Role of Tomosyn in Mossy Fiber Synaptic Plasticity

Neurotransmitter release probability (P(r)) largely determines the dynamic properties of synapses. While much is known about the role of presynaptic proteins in transmitter release, their specific contribution to synaptic plasticity is unclear. One such protein, tomosyn, is believed to reduce P(r) by interfering with the SNARE complex formation. Tomosyn is enriched at hippocampal mossy fiber-to-CA3 pyramidal cell synapses (MF-CA3), which characteristically exhibit low P(r), strong synaptic facilitation, and pre-synaptic protein kinase A (PKA)-dependent long-term potentiation (LTP). To evaluate tomosyn's role in MF-CA3 function, we used a combined knockdown (KD)-optogenetic strategy whereby presynaptic neurons with reduced tomosyn levels were selectively activated by light. Using this approach in mouse hippocampal slices, we found that facilitation, LTP, and PKA-induced potentiation were significantly impaired at tomosyn-deficient synapses. These findings not only indicate that tomosyn is a key regulator of MF-CA3 plasticity but also highlight the power of a combined KD-optogenetic approach to determine the role of presynaptic proteins.

14-3-3 proteins are required for hippocampal long-term potentiation and associative learning and memory

14-3-3 is a family of regulatory proteins highly expressed in the brain. Previous invertebrate studies have demonstrated the importance of 14-3-3 in the regulation of synaptic functions and learning and memory. However, the in vivo role of 14-3-3 in these processes has not been determined using mammalian animal models. Here, we report the behavioral and electrophysiological characterization of a new animal model of 14-3-3 proteins. These transgenic mice, considered to be a 14-3-3 functional knock-out, express a known 14-3-3 inhibitor in various brain regions of different founder lines. We identify a founder-specific impairment in hippocampal-dependent learning and memory tasks, as well as a correlated suppression in long-term synaptic plasticity of the hippocampal synapses. Moreover, hippocampal synaptic NMDA receptor levels are selectively reduced in the transgenic founder line that exhibits both behavioral and synaptic plasticity deficits. Collectively, our findings provide evidence that 14-3-3 is a positive regulator of associative learning and memory at both the behavioral and cellular level.

Presynaptic long-term plasticity

Long-term synaptic plasticity is a major cellular substrate for learning, memory, and behavioral adaptation. Although early examples of long-term synaptic plasticity described a mechanism by which postsynaptic signal transduction was potentiated, it is now apparent that there is a vast array of mechanisms for long-term synaptic plasticity that involve modifications to either or both the presynaptic terminal and postsynaptic site. In this article, we discuss current and evolving approaches to identify presynaptic mechanisms as well as discuss their limitations. We next provide examples of the diverse circuits in which presynaptic forms of long-term synaptic plasticity have been described and discuss the potential contribution this form of plasticity might add to circuit function. Finally, we examine the present evidence for the molecular pathways and cellular events underlying presynaptic long-term synaptic plasticity.

Synaptotagmin-12 phosphorylation by cAMP-dependent protein kinase is essential for hippocampal mossy fiber LTP

Synaptotagmin-12 (Syt12) is an abundant synaptic vesicle protein that--different from other synaptic vesicle-associated synaptotagmins--does not bind Ca(2+). Syt12 is phosphorylated by cAMP-dependent protein kinase-A at serine-97 in an activity-dependent manner, suggesting a function for Syt12 in cAMP-dependent synaptic plasticity. To test this hypothesis, we here generated (1) Syt12 knock-out mice and (2) Syt12 knockin mice carrying a single amino-acid substitution [the serine-97-to-alanine- (S97A)-substitution]. Both Syt12 knock-out mice and Syt12 S97A-knockin mice were viable and fertile, and exhibited no measurable change in basal synaptic strength or short-term plasticity as analyzed in cultured cortical neurons or in acute hippocampal slices. However, both Syt12 knock-out and Syt12 S97A-knockin mice displayed a major impairment in cAMP-dependent mossy-fiber long-term potentiation (LTP) in the CA3 region of the hippocampus. This impairment was observed using different experimental configurations for inducing and monitoring mossy-fiber LTP. Moreover, although the Syt12 knock-out had no effect on the short-term potentiation of synaptic transmission induced by the adenylate-cyclase activator forskolin in cultured cortical neurons and in the CA1 region of the hippocampus, both the Syt12 knock-out and the Syt12 S97A-knockin impaired the long-term increase in mossy-fiber synaptic transmission induced by forskolin. Thus, Syt12 is essential for cAMP-dependent presynaptic LTP at mossy-fiber synapses, and a single amino-acid substitution that blocks the cAMP-dependent phosphorylation of Syt12 is sufficient to impair the function of Syt12 in mossy-fiber LTP, suggesting that cAMP-dependent phosphorylation of Syt12 on serine-97 contributes to the induction of mossy-fiber LTP.

Regulation of presynaptic anchoring of the scaffold protein Bassoon by phosphorylation-dependent interaction with 14-3-3 adaptor proteins

The proper organization of the presynaptic cytomatrix at the active zone is essential for reliable neurotransmitter release from neurons. Despite of the virtual stability of this tightly interconnected proteinaceous network it becomes increasingly clear that regulated dynamic changes of its composition play an important role in the processes of synaptic plasticity. Bassoon, a core component of the presynaptic cytomatrix, is a key player in structural organization and functional regulation of presynaptic release sites. It is one of the most highly phosphorylated synaptic proteins. Nevertheless, to date our knowledge about functions mediated by any one of the identified phosphorylation sites of Bassoon is sparse. In this study, we have identified an interaction of Bassoon with the small adaptor protein 14-3-3, which depends on phosphorylation of the 14-3-3 binding motif of Bassoon. In vitro phosphorylation assays indicate that phosphorylation of the critical Ser-2845 residue of Bassoon can be mediated by a member of the 90-kDa ribosomal S6 protein kinase family. Elimination of Ser-2845 from the 14-3-3 binding motif results in a significant decrease of Bassoon's molecular exchange rates at synapses of living rat neurons. We propose that the phosphorylation-induced 14-3-3 binding to Bassoon modulates its anchoring to the presynaptic cytomatrix. This regulation mechanism might participate in molecular and structural presynaptic remodeling during synaptic plasticity.

Presynaptic LTP and LTD of excitatory and inhibitory synapses

Ubiquitous forms of long-term potentiation (LTP) and depression (LTD) are caused by enduring increases or decreases in neurotransmitter release. Such forms or presynaptic plasticity are equally observed at excitatory and inhibitory synapses and the list of locations expressing presynaptic LTP and LTD continues to grow. In addition to the mechanistically distinct forms of postsynaptic plasticity, presynaptic plasticity offers a powerful means to modify neural circuits. A wide range of induction mechanisms has been identified, some of which occur entirely in the presynaptic terminal, whereas others require retrograde signaling from the postsynaptic to presynaptic terminals. In spite of this diversity of induction mechanisms, some common induction rules can be identified across synapses. Although the precise molecular mechanism underlying long-term changes in transmitter release in most cases remains unclear, increasing evidence indicates that presynaptic LTP and LTD can occur in vivo and likely mediate some forms of learning.

Region-specific deletions of RIM1 reproduce a subset of global RIM1α(-/-) phenotypes

The presynaptic protein RIM1α mediates multiple forms of presynaptic plasticity at both excitatory and inhibitory synapses. Previous studies of mice lacking RIM1α (RIM1α(-/-) throughout the brain showed that deletion of RIM1α results in multiple behavioral abnormalities. In an effort to begin to delineate the brain regions in which RIM1 deletion mediates these abnormal behaviors, we used conditional (floxed) RIM1 knockout mice (fRIM1). By crossing these fRIM1 mice to previously characterized transgenic cre lines, we aimed to delete RIM1 selectively in the dentate gyrus (DG), using a specific preproopiomelanocortin promoter driving cre recombinase (POMC-cre) line , and in pyramidal neurons of the CA3 region of hippocampus, using the kainate receptor subunit 1 promoter driving cre recombinase (KA-cre). Neither of these cre driver lines was uniquely selective to the targeted regions. In spite of this, we were able to reproduce a subset of the global RIM1α(-/-) behavioral abnormalities, thereby narrowing the brain regions in which loss of RIM1 is sufficient to produce these behavioral differences. Most interestingly, hypersensitivity to the pyschotomimetic MK-801 was shown in mice lacking RIM1 selectively in the DG, arcuate nucleus of the hypothalamus and select cerebellar neurons, implicating novel brain regions and neuronal subtypes in this behavior.

Munc13-1 is required for presynaptic long-term potentiation

Long-lasting forms of synaptic plasticity involve modification of presynaptic strength in many brain regions. Although a presynaptic site for expression is well established, the detailed molecular mechanisms that lead to sustained changes in neurotransmitter release remain unclear. Here, we use acute in vivo genetic manipulation of synaptic proteins to investigate the molecular basis for presynaptic long-term potentiation (LTP) at hippocampal mossy fiber synapses. Munc13 proteins are active zone proteins that are essential for synaptic vesicle priming and neurotransmitter release. Munc13 proteins also interact with RIM1α, an active zone protein required for presynaptic long-term plasticity. By taking advantage of the observation that the RIM-binding domain of Munc13 is separable from the domain that is required for neurotransmitter release, we selectively tested whether Munc13-1 is an effector for RIM1α in presynaptic LTP. Our results provide the first evidence for the involvement of Munc13-1 in presynaptic long-term synaptic plasticity. We further demonstrate that the interaction between RIM1α and Munc13-1 is required for this plasticity. These results advance our understanding of the molecular mechanisms of presynaptic plasticity and suggest that modulation of vesicle priming may provide the cellular substrate for expression of LTP at mossy fiber synapses.

Rab3B protein is required for long-term depression of hippocampal inhibitory synapses and for normal reversal learning

Rab3B, similar to other Rab3 isoforms, is a synaptic vesicle protein that interacts with the Rab3-interacting molecule (RIM) isoforms RIM1α and RIM2α as effector proteins in a GTP-dependent manner. Previous studies showed that at excitatory synapses, Rab3A and RIM1α are essential for presynaptically expressed long-term potentiation (LTP), whereas at inhibitory synapses RIM1α is required for endocannabinoid-dependent long-term depression (referred to as "i-LTD"). However, it remained unknown whether i-LTD also involves a Rab3 isoform and whether i-LTD, similar to other forms of long-term plasticity, is important for learning and memory. Here we show that Rab3B is highly enriched in inhibitory synapses in the CA1 region of the hippocampus. Using electrophysiological recordings in acute slices, we demonstrate that knockout (KO) of Rab3B does not alter the strength or short-term plasticity of excitatory or inhibitory synapses but does impair i-LTD significantly without changing classical NMDA receptor-dependent LTP. Behaviorally, we found that Rab3B KO mice exhibit no detectable changes in all basic parameters tested, including the initial phase of learning and memory. However, Rab3B KO mice did display a selective enhancement in reversal learning, as measured using Morris water-maze and fear-conditioning assays. Our data support the notion that presynaptic forms of long-term plasticity at excitatory and inhibitory synapses generally are mediated by a common Rab3/RIM-dependent pathway, with various types of synapses using distinct Rab3 isoforms. Moreover, our results suggest that i-LTD contributes to learning and memory, presumably by stabilizing circuits established in previous learning processes.

Structural and functional plasticity of the cytoplasmic active zone

The presynaptic active zone (AZ) membrane is the site where vesicle fusion mediates information transfer between connected neurons. Reaching into the cytoplasm, an electron-dense cytomatrix (CAZ) is found to decorate the AZ membranes. CAZ architectures are meant not only to regulate the synaptic vesicle exocycle/endocycle, but also to structurally stabilize the presynaptic site. The CAZ is composed of a set of large scaffold proteins, many of which are evolutionarily conserved. Recently, several signaling factors controlling the developmental assembly of CAZs were found by unbiased genetics in Drosophila and Caenorhabditis elegans. At the same time, post-translational modification of CAZ proteins was implicated in changing the strength of mammalian brain synapses. Studying how processes of structural and functional CAZ plasticity get integrated within circuit remodeling remains an important challenge.