Synaptic dysfunction and abnormal behaviors in mice lacking major isoforms of Shank3.

SHANK3 is a synaptic scaffolding protein enriched in the postsynaptic density (PSD) of excitatory synapses. Small microdeletions and point mutations in SHANK3 have been identified in a small subgroup of individuals with autism spectrum disorder (ASD) and intellectual disability. SHANK3 also plays a key role in the chromosome 22q13.3 microdeletion syndrome (Phelan-McDermid syndrome), which includes ASD and cognitive dysfunction as major clinical features. To evaluate the role of Shank3 in vivo, we disrupted major isoforms of the gene in mice by deleting exons 4-9. Isoform-specific Shank3(e4-9) homozygous mutant mice display abnormal social behaviors, communication patterns, repetitive behaviors and learning and memory. Shank3(e4-9) male mice display more severe impairments than females in motor coordination. Shank3(e4-9) mice have reduced levels of Homer1b/c, GKAP and GluA1 at the PSD, and show attenuated activity-dependent redistribution of GluA1-containing AMPA receptors. Subtle morphological alterations in dendritic spines are also observed. Although synaptic transmission is normal in CA1 hippocampus, long-term potentiation is deficient in Shank3(e4-9) mice. We conclude that loss of major Shank3 species produces biochemical, cellular and morphological changes, leading to behavioral abnormalities in mice that bear similarities to human ASD patients with SHANK3 mutations.

Advances in understanding visual cortex plasticity.

Visual cortical plasticity can be either rapid, occurring in response to abrupt changes in neural activity, or slow, occurring over days as a homeostatic process for adapting neuronal responsiveness. Recent advances have shown that the magnitude and polarity of rapid synaptic modifications are regulated by neuromodulators, while homeostatic modifications can occur through regulation of cytokine actions or N-methyl-d-aspartate (NMDA) receptor subunit composition. Synaptic and homeostatic plasticity together produce the normal physiological response to monocular impairments. In vivo studies have now overturned the dogma that robust plasticity is limited to an early critical period. Indeed, rapid physiological plasticity in the adult can be enabled by prior, experience-driven anatomical rearrangements or through pharmacological manipulations of the epigenome.

Coactivation of M(1) muscarinic and alpha1 adrenergic receptors stimulates extracellular signal-regulated protein kinase and induces long-term depression at CA3-CA1 synapses in rat hippocampus.

Intact cholinergic innervation from the medial septum and noradrenergic innervation from the locus ceruleus are required for hippocampal-dependent learning and memory. However, much remains unclear about the precise roles of acetylcholine (ACh) and norepinephrine (NE) in hippocampal function, particularly in terms of how interactions between these two transmitter systems might play an important role in synaptic plasticity. Previously, we reported that activation of either muscarinic M(1) or adrenergic alpha1 receptors induces activity- and NMDA receptor-dependent long-term depression (LTD) at CA3-CA1 synapses in acute hippocampal slices, referred to as muscarinic LTD (mLTD) and norepinephrine LTD (NE LTD), respectively. In this study, we tested the hypothesis that mLTD and NE LTD are independent forms of LTD, yet require activation of a common Galphaq-coupled signaling pathway for their induction, and investigated the net effect of coactivation of M(1) and alpha1 receptors on the magnitude of LTD induced. We find that neither mLTD nor NE LTD requires phospholipase C activation, but both plasticities are prevented by inhibiting the Src kinase family and extracellular signal-regulated protein kinase (ERK) activation. Interestingly, LTD can be induced when M(1) and alpha1 agonists are coapplied at concentrations too low to induce LTD when applied separately, via a summed increase in ERK activation. Thus, because ACh and NE levels in vivo covary, especially during periods of memory encoding and consolidation, cooperative signaling through M(1) and alpha1 receptors could function to induce long-term changes in synaptic function important for cognition.

Layer 2/3 synapses in monocular and binocular regions of tree shrew visual cortex express mAChR-dependent long-term depression and long-term potentiation.

Acetylcholine is an important modulator of synaptic efficacy and is required for learning and memory tasks involving the visual cortex. In rodent visual cortex, activation of muscarinic acetylcholine receptors (mAChRs) induces a persistent long-term depression (LTD) of transmission at synapses recorded in layer 2/3 of acute slices. Although the rodent studies expand our knowledge of how the cholinergic system modulates synaptic function underlying learning and memory, they are not easily extrapolated to more complex visual systems. Here we used tree shrews for their similarities to primates, including a visual cortex with separate, defined regions of monocular and binocular innervation, to determine whether mAChR activation induces long-term plasticity. We find that the cholinergic agonist carbachol (CCh) not only induces long-term plasticity, but the direction of the plasticity depends on the subregion. In the monocular region, CCh application induces LTD of the postsynaptic potential recorded in layer 2/3 that requires activation of m3 mAChRs and a signaling cascade that includes activation of extracellular signal-regulated kinase (ERK) 1/2. In contrast, layer 2/3 postsynaptic potentials recorded in the binocular region express long-term potentiation (LTP) following CCh application that requires activation of m1 mAChRs and phospholipase C. Our results show that activation of mAChRs induces long-term plasticity at excitatory synapses in tree shrew visual cortex. However, depending on the ocular inputs to that region, variation exists as to the direction of plasticity, as well as to the specific mAChR and signaling mechanisms that are required.

Muscarinic receptor dependent long-term depression in rat visual cortex is PKC independent but requires ERK1/2 activation and protein synthesis.

Intact cholinergic innervation of visual cortex is critical for normal processing of visual information and for spatial memory acquisition and retention. However, a complete description of the mechanisms by which the cholinergic system modifies synaptic function in visual cortex is lacking. Previously it was shown that activation of the m1 subtype of muscarinic receptor induces an activity-dependent and partially N-methyl-d-aspartate receptor (NMDAR)-dependent long-term depression (LTD) at layer 4-layer 2/3 synapses in rat visual cortex slices in vitro. The cellular mechanisms downstream of the Galphaq coupled m1 receptor required for induction of this LTD (which we term mLTD) are currently unknown. Here, we confirm a role for m1 receptors in mLTD induction and use a series of pharmacological tools to study the signaling molecules downstream of m1 receptor activation in mLTD induction. We found that mLTD is prevented by inhibitors of L-type Ca(2+) channels, the Src kinase family, and the mitogen-activated kinase/extracellular kinase. mLTD is also partially dependent on phospholipase C but is unaffected by blocking protein kinase C. mLTD expression can be long-lasting (>2 h) and its long-term maintenance requires translation. Thus we report the signaling mechanisms underlying induction of an m1 receptor-dependent LTD in visual cortex and the requirement of protein synthesis for long-term expression. This plasticity could be a mechanism by which the cholinergic system modifies glutamatergic synapse function to permit normal visual system processing required for cognition.