Neuroligin-1 dependent phosphotyrosine signaling in excitatory synapse differentiation.

Introduction: The synaptic adhesion molecule neuroligin-1 (NLGN1) is involved in the differentiation of excitatory synapses, but the precise underlying molecular mechanisms are still debated. Here, we explored the role of NLGN1 tyrosine phosphorylation in this process, focusing on a subset of receptor tyrosine kinases (RTKs), namely FGFR1 and Trks, that were previously described to phosphorylate NLGN1 at a unique intracellular residue (Y782). Methods: We used pharmacological inhibitors and genetic manipulation of those RTKs in dissociated hippocampal neurons, followed by biochemical measurement of NLGN1 phosphorylation and immunocytochemical staining of excitatory synaptic scaffolds. Results: This study shows that: (i) the accumulation of PSD-95 at de novo NLGN1 clusters induced by neurexin crosslinking is reduced by FGFR and Trk inhibitors; (ii) the increase in PSD-95 puncta caused by NLGN1 over-expression is impaired by FGFR and Trk inhibitors; (iii) TrkB activation by BDNF increases NLGN1 phosphorylation; and (iv) TrkB knock-down impairs the increase of PSD-95 puncta caused by NLGN1 over-expression, an effect which is not seen with the NLGN1 Y782A mutant. Discussion: Together, our data identify TrkB as one of the major RTKs responsible for NLGN1 tyrosine phosphorylation, and reveal that TrkB activity is necessary for the synaptogenic effects of NLGN1.

Protein kinase D promotes activity-dependent AMPA receptor endocytosis in hippocampal neurons.

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type glutamate receptors (AMPARs) mediate the majority of fast excitatory neurotransmission in the brain. The continuous trafficking of AMPARs into and out of synapses is a core feature of synaptic plasticity, which is considered as the cellular basis of learning and memory. The molecular mechanisms underlying the postsynaptic AMPAR trafficking, however, are still not fully understood. In this work, we demonstrate that the protein kinase D (PKD) family promotes basal and activity-induced AMPAR endocytosis in primary hippocampal neurons. Pharmacological inhibition of PKD increased synaptic levels of GluA1-containing AMPARs, slowed down their endocytic trafficking and increased neuronal network activity. By contrast, ectopic expression of constitutive active PKD decreased the synaptic level of AMPARs, while increasing their colocalization with early endosomes. Our results thus establish an important role for PKD in the regulation of postsynaptic AMPAR trafficking during synaptic plasticity.

A unique intracellular tyrosine in neuroligin-1 regulates AMPA receptor recruitment during synapse differentiation and potentiation.

To better understand the molecular mechanisms by which early neuronal connections mature into synapses, we examined the impact of neuroligin-1 (Nlg1) phosphorylation on synapse differentiation, focusing on a unique intracellular tyrosine (Y782), which differentially regulates Nlg1 binding to PSD-95 and gephyrin. By expressing Nlg1 point mutants (Y782A/F) in hippocampal neurons, we show using imaging and electrophysiology that Y782 modulates the recruitment of functional AMPA receptors (AMPARs). Nlg1-Y782F impaired both dendritic spine formation and AMPAR diffusional trapping, but not NMDA receptor recruitment, revealing the assembly of silent synapses. Furthermore, replacing endogenous Nlg1 with either Nlg1-Y782A or -Y782F in CA1 hippocampal neurons impaired long-term potentiation (LTP), demonstrating a critical role of AMPAR synaptic retention. Screening of tyrosine kinases combined with pharmacological inhibitors point to Trk family members as major regulators of endogenous Nlg1 phosphorylation and synaptogenic function. Thus, Nlg1 tyrosine phosphorylation signaling is a critical event in excitatory synapse differentiation and LTP.

Ras and Rab interactor 1 controls neuronal plasticity by coordinating dendritic filopodial motility and AMPA receptor turnover.

Ras and Rab interactor 1 (RIN1) is predominantly expressed in the nervous system. RIN1-knockout animals have deficits in latent inhibition and fear extinction in the amygdala, suggesting a critical role for RIN1 in preventing the persistence of unpleasant memories. At the molecular level, RIN1 signals through Rab5 GTPases that control endocytosis of cell-surface receptors and Abl nonreceptor tyrosine kinases that participate in actin cytoskeleton remodeling. Here we report that RIN1 controls the plasticity of cultured mouse hippocampal neurons. Our results show that RIN1 affects the morphology of dendritic protrusions and accelerates dendritic filopodial motility through an Abl kinase-dependent pathway. Lack of RIN1 results in enhanced mEPSC amplitudes, indicating an increase in surface AMPA receptor levels compared with wild-type neurons. We further provide evidence that the Rab5 GEF activity of RIN1 regulates surface GluA1 subunit endocytosis. Consequently loss of RIN1 blocks surface AMPA receptor down-regulation evoked by chemically induced long-term depression. Our findings indicate that RIN1 destabilizes synaptic connections and is a key player in postsynaptic AMPA receptor endocytosis, providing multiple ways of negatively regulating memory stabilization during neuronal plasticity.

Protein kinase D promotes plasticity-induced F-actin stabilization in dendritic spines and regulates memory formation.

Actin turnover in dendritic spines influences spine development, morphology, and plasticity, with functional consequences on learning and memory formation. In nonneuronal cells, protein kinase D (PKD) has an important role in stabilizing F-actin via multiple molecular pathways. Using in vitro models of neuronal plasticity, such as glycine-induced chemical long-term potentiation (LTP), known to evoke synaptic plasticity, or long-term depolarization block by KCl, leading to homeostatic morphological changes, we show that actin stabilization needed for the enlargement of dendritic spines is dependent on PKD activity. Consequently, impaired PKD functions attenuate activity-dependent changes in hippocampal dendritic spines, including LTP formation, cause morphological alterations in vivo, and have deleterious consequences on spatial memory formation. We thus provide compelling evidence that PKD controls synaptic plasticity and learning by regulating actin stability in dendritic spines.