The Shank3 gene encodes a scaffolding protein that anchors multiple elements of the postsynaptic density at the synapse. Previous attempts to delete the Shank3 gene have not resulted in a complete loss of the predominant naturally occurring Shank3 isoforms. We have now characterized a homozygous Shank3 mutation in mice that deletes exon 21, including the Homer binding domain. In the homozygous state, deletion of exon 21 results in loss of the major naturally occurring Shank3 protein bands detected by C-terminal and N-terminal antibodies, allowing us to more definitively examine the role of Shank3 in synaptic function and behavior. This loss of Shank3 leads to an increased localization of mGluR5 to both synaptosome and postsynaptic density-enriched fractions in the hippocampus. These mice exhibit a decrease in NMDA/AMPA excitatory postsynaptic current ratio in area CA1 of the hippocampus, reduced long-term potentiation in area CA1, and deficits in hippocampus-dependent spatial learning and memory. In addition, these mice also exhibit motor-coordination deficits, hypersensitivity to heat, novelty avoidance, altered locomotor response to novelty, and minimal social abnormalities. These data suggest that Shank3 isoforms are required for normal synaptic transmission/plasticity in the hippocampus, as well as hippocampus-dependent spatial learning and memory.
Shank3 is a multi-domain, synaptic scaffolding protein that organizes proteins in the postsynaptic density of excitatory synapses. Clinical studies suggest that ~0.5% of autism spectrum disorder (ASD) cases may involve SHANK3 mutation/deletion. Patients with SHANK3 mutations exhibit deficits in cognition along with delayed/impaired speech/language and repetitive and obsessive/compulsive-like (OCD-like) behaviors. To examine how mutation/deletion of SHANK3 might alter brain function leading to ASD, we have independently created mice with deletion of Shank3 exons 4–9, a region implicated in ASD patients. We find that homozygous deletion of exons 4–9 (Shank3e4–9 KO) results in loss of the two highest molecular weight isoforms of Shank3 and a significant reduction in other isoforms. Behaviorally, both Shank3e4–9 heterozygous (HET) and Shank3e4–9 KO mice display increased repetitive grooming, deficits in novel and spatial object recognition learning and memory, and abnormal ultrasonic vocalizations. Shank3e4–9 KO mice also display abnormal social interaction when paired with one another. Analysis of synaptosome fractions from striata of Shank3e4–9 KO mice reveals decreased Homer1b/c, GluA2, and GluA3 expression. Both Shank3e4–9 HET and KO demonstrated a significant reduction in NMDA/AMPA ratio at excitatory synapses onto striatal medium spiny neurons. Furthermore, Shank3e4–9 KO mice displayed reduced hippocampal LTP despite normal baseline synaptic transmission. Collectively these behavioral, biochemical and physiological changes suggest Shank3 isoforms have region-specific roles in regulation of AMPAR subunit localization and NMDAR function in the Shank3e4–9 mutant mouse model of autism.
Mutations/deletions in the SHANK3 gene are associated with autism spectrum disorders and intellectual disability. Here, we present electrophysiological and behavioral consequences in novel heterozygous and homozygous mice with a transcriptional stop cassette inserted upstream of the PDZ domain-coding exons in Shank3 (Shank3E13). Insertion of a transcriptional stop cassette prior to exon 13 leads to loss of the two higher molecular weight isoforms of Shank3. Behaviorally, both Shank3E13 heterozygous (HET) and homozygous knockout (KO) mice display increased repetitive grooming, deficits in social interaction tasks, and decreased rearing. Shank3E13 KO mice also display deficits in spatial memory in the Morris water maze task. Baseline hippocampal synaptic transmission and short-term plasticity are preserved in Shank3E13 HET and KO mice, while both HET and KO mice exhibit impaired hippocampal long-term plasticity. Additionally, Shank3E13 HET and KO mice display impaired striatal glutamatergic synaptic transmission. These results demonstrate for the first time in this novel Shank3 mutant that both homozygous and heterozygous mutation of Shank3 lead to behavioral abnormalities with face validity for autism along with widespread synaptic dysfunction.
Many animal models of temporal lobe epilepsy (TLE) begin with status epilepticus (SE) followed by a latency period. Increased hippocampal pyramidal neuron excitability may contribute to seizures in TLE. I h , mediated by h channels, regulates intrinsic membrane excitability by modulating synaptic integration and dampening dendritic calcium signaling. In a rat model of TLE, we found bidirectional changes in h channel function in CA1 pyramidal neurons. 1-2 days after SE, before onset of spontaneous seizures, physiological parameters dependent upon h channels were augmented and h channel subunit surface expression was increased. 28-30 days following SE, after onset of spontaneous seizures, h channel function in dendrites was reduced, coupled with diminished h channel subunit surface expression and relocalization of subunits from distal dendrites to soma. These results implicate h channel localization as a molecular mechanism influencing CA1 excitability in TLE.
Analysis of naturally occurring mutations that cause seizures in rodents has advanced understanding of the molecular mechanisms underlying epilepsy. Abnormalities of I h and h channel expression have been found in many animal models of absence epilepsy. We characterized a novel spontaneous mutant mouse, apathetic (ap/ap), and identified the ap mutation as a 4 base pair insertion within the coding region of Hcn2, the gene encoding the h channel subunit 2 (HCN2). We demonstrated that Hcn2 ap mRNA is reduced by 90% compared to wild type, and the predicted truncated HCN2 ap protein is absent from the brain tissue of mice carrying the ap allele. ap/ap mice exhibited ataxia, generalized spike-wave absence seizures, and rare generalized tonic-clonic seizures. ap/+ mice had a normal gait, occasional absence seizures and an increased severity of chemoconvulsant-induced seizures. These findings help elucidate basic mechanisms of absence epilepsy and suggest HCN2 may be a target for therapeutic intervention.
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