Inhibitory regulation of calcium transients in prefrontal dendritic spines is compromised by a nonsense Shank3 mutation

Introduction:

The basic computational units of the brain are glutamatergic and GABAergic neurons, which work together to process and relay data in a coherent manner. This cooperation relies on striking a fine balance between excitation and inhibition, too much of one or the other is thought to lead to neurological disorders such as autism spectrum disorder (ASD), epilepsy, and schizophrenia^1,2,3. Many studies on ASD focus heavily on the effects of mutation on pyramidal neurons (PNs), while a smaller fraction place their focus on interneurons (INs). One high-profile ASD candidate gene is Shank3, a gene encoding a post-synaptic scaffold protein, which when mutated disrupts synapse formation and maturation in PNs and may lead to ASD, schizophrenia or Phelan-McDermid syndrome. Numerous studies have outlined though its effects on INs remain poorly understood. In this preprint, Ali et al. give some attention to the minority cell types of the cortex, the good ole IN. Specifically, they highlight the effects of Shank3 mutations on IN function and how this in turn results in behavioral phenotypes analogous to symptoms of ASDs.

 

Key findings:

Elevated synaptic calcium transients in prefrontal cortex of R1117X mice

To study the effects of Shank3 mutations on cortical circuitry the authors use R1117X knock-in mice which carry a truncating mutation on exon 21. Using the genetically-encoded calcium indicator GCaMP6f and two-photon calcium imaging, the authors found increased rate of calcium events at the dendritic spines of PNs in the prefrontal cortex of R1117X mutant mice. These events were greater in both frequency and amplitude compared to control animals and pointed to a hyperexcitability phenotype in PNs. But that’s only half the story…

Reduced dendritic inhibition by SST+ INs

Previous literature⁴ suggests a significant role for somatostatin-positive (SST+) INs in regulating calcium signaling at dendritic spines. To test whether this may be a factor in the observed aberrant dendritic calcium transients, the authors generated double-transgenic R1117X mice which express GCaMP6s exclusively in SST+ INs. They observed a reduction in calcium events and reduced NMDAR signaling in mutant SST+ INs. This reduction in activity likely affects the overall control which INs are able to exert within the prefrontocortical microcircuitry and subsequently manifests as changes in the animal’s behavior.

Selective NMDAR overexpression in SST+ INs rescues behavioral deficits

The behavioral symptoms observed in R1117X mice include increased anxiety, reduced motor activity, and in the case of the prefrontal cortex, a severe deficit in associative learning. Electrophysiological data pointed to NMDAR as a possible target for rescue as they found reduced, but not eliminated, NMDAR currents. The authors elegantly parse out the phenotypes associated with IN dysfunction by selectively rescuing NMDAR currents in SST+ INs using lentiviral vectors to overexpress NMDAR subunits GluN1 and GluN2B. The overexpression of these in SST+ INs was sufficient to restore the rate of dendritic calcium events to a rate comparable to control animals. Additionally, this treatment rescued certain behavioral deficits, specifically trace fear learning and sensorimotor gating, suggesting these were mediated by reduced dendritic inhibition within the prefrontal cortex.

 

Why this preprint?

I chose this preprint because of my general interest in IN development and their roles in disease. Studies indicate that ASD affects approximately 1:160 children worldwide, though numbers vary by country⁵. Government health agencies and NGOs spend significant resources on research to elucidate the underlying mechanisms of this group of heterogeneous disorders. It is through these efforts that numerous candidate genes have been implicated in ASD, but many studies to date place an emphasis on the effects of mutations in excitatory neurons. The authors of this study used robust assays to convincingly link the underlying interneuronopathy to behavioral symptoms found in mouse models of ASD. They achieved this by succinctly describing how altered inhibitory signaling contributes to increase in calcium signaling at the dendritic spines and ultimately in behavioral changes.

 

My questions for the authors:

1. You mentioned Shank3B het mice had a decreased rate of calcium events (Fig. 2i). Do you have any clues as to which cell type would be primarily responsible for this? Is the interneuron hyperactive or is the pyramidal neuron less sensitive to stimuli? Maybe both?
2. All of your experiments are done in adolescent mice and in the discussion you mention that calcium homeostasis during adolescent periods is critical for circuit maturation in the prefrontal cortex. A big question in the ASD field is whether or not certain deficits are reversible. Did you attempt any of the rescue experiments in older animals? At what point do the R1117X mutant mice stop responding to the GluN2B treatments?

 

References

1. Rubenstein JL, Merzenich MM. Model of autism: increased ratio of excitation/inhibition in key neural systems. Genes Brain Behav. 2003;2(5):255–267. doi:10.1034/j.1601-183x.2003.00037.x
2. Žiburkus J, Cressman JR, Schiff SJ. Seizures as imbalanced up states: excitatory and inhibitory conductances during seizure-like events. J Neurophysiol. 2013;109(5):1296–1306. doi:10.1152/jn.00232.2012
3. Dong Z, Chen W, Chen C, et al. CUL3 Deficiency Causes Social Deficits and Anxiety-like Behaviors by Impairing Excitation-Inhibition Balance through the Promotion of Cap-Dependent Translation. Neuron. 2020;105(3):475–490.e6. doi:10.1016/j.neuron.2019.10.035
4. Marlin JJ, Carter AG. GABA-A receptor inhibition of local calcium signaling in spines and dendrites. J Neurosci. 2014;34(48):15898–15911. doi:10.1523/JNEUROSCI.0869-13.2014
5. Elsabbagh M, Divan G, Koh YJ, et al. Global prevalence of autism and other pervasive developmental disorders. Autism Res. 2012;5(3):160–179. doi:10.1002/aur.239

Tags: cell biology, developmental biology, neuroscience, physiology

doi: https://doi.org/10.1242/prelights.17014

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