Multiple beta cell-independent mechanisms drive hypoglycemia in Timothy syndrome.

The canonical G406R mutation that increases Ca2+ influx through the CACNA1C-encoded CaV1.2 Ca2+ channel underlies the multisystem disorder Timothy syndrome (TS), characterized by life-threatening arrhythmias. Severe episodic hypoglycemia is among the poorly characterized non-cardiac TS pathologies. While hypothesized from increased Ca2+ influx in pancreatic beta cells and consequent hyperinsulinism, this hypoglycemia mechanism is undemonstrated because of limited clinical data and lack of animal models. We generated a CaV1.2 G406R knockin mouse model that recapitulates key TS features, including hypoglycemia. Unexpectedly, these mice do not show hyperactive beta cells or hyperinsulinism in the setting of normal intrinsic beta cell function, suggesting dysregulated glucose homeostasis. Patient data confirm the absence of hyperinsulinism. We discover multiple alternative contributors, including perturbed counterregulatory hormone responses with defects in glucagon secretion and abnormal hypothalamic control of glucose homeostasis. These data provide new insights into contributions of CaV1.2 channels and reveal integrated consequences of the mutant channels driving life-threatening events in TS.

Interneuron FGF13 regulates seizure susceptibility via a sodium channel-independent mechanism.

Developmental and Epileptic Encephalopathies (DEEs), a class of devastating neurological disorders characterized by recurrent seizures and exacerbated by disruptions to excitatory/inhibitory balance in the brain, are commonly caused by mutations in ion channels. Disruption of, or variants in, FGF13 were implicated as causal for a set of DEEs, but the underlying mechanisms were clouded because FGF13 is expressed in both excitatory and inhibitory neurons, FGF13 undergoes extensive alternative splicing producing multiple isoforms with distinct functions, and the overall roles of FGF13 in neurons are incompletely cataloged. To overcome these challenges, we generated a set of novel cell type-specific conditional knockout mice. Interneuron-targeted deletion of Fgf13 led to perinatal mortality associated with extensive seizures and impaired the hippocampal inhibitory/excitatory balance while excitatory neuron-targeted deletion of Fgf13 caused no detectable seizures and no survival deficits. While best studied as a voltage-gated sodium channel (Nav) regulator, we observed no effect of Fgf13 ablation in interneurons on Navs but rather a marked reduction in K+ channel currents. Re-expressing different Fgf13 splice isoforms could partially rescue deficits in interneuron excitability and restore K+ channel current amplitude. These results enhance our understanding of the molecular mechanisms that drive the pathogenesis of Fgf13-related seizures and expand our understanding of FGF13 functions in different neuron subsets.

Epigenetic function during heroin self-administration controls future relapse-associated behavior in a cell type-specific manner.

Opioid use produces enduring associations between drug reinforcement/euphoria and discreet or diffuse cues in the drug-taking environment. These powerful associations can trigger relapse in individuals recovering from opioid use disorder (OUD). Here, we sought to determine whether the epigenetic enzyme, histone deacetylase 5 (HDAC5), regulates relapse-associated behavior in an animal model of OUD. We examined the effects of nucleus accumbens (NAc) HDAC5 on both heroin- and sucrose-seeking behaviors using operant self-administration paradigms. We utilized cre-dependent viral-mediated approaches to investigate the cell-type-specific effects of HDAC5 on heroin-seeking behavior, gene expression, and medium spiny neuron (MSN) cell and synaptic physiology. We found that NAc HDAC5 functions during the acquisition phase of heroin self-administration to limit future relapse-associated behavior. Moreover, overexpressing HDAC5 in the NAc suppressed context-associated and reinstated heroin-seeking behaviors, but it did not alter sucrose seeking. We also found that HDAC5 functions within dopamine D1 receptor-expressing MSNs to suppress cue-induced heroin seeking, and within dopamine D2 receptor-expressing MSNs to suppress drug-primed heroin seeking. Assessing cell-type-specific transcriptomics, we found that HDAC5 reduced expression of multiple ion transport genes in both D1- and D2-MSNs. Consistent with this observation, HDAC5 also produced firing rate depression in both MSN classes. These findings revealed roles for HDAC5 during active heroin use in both D1- and D2-MSNs to limit distinct triggers of drug-seeking behavior. Together, our results suggest that HDAC5 might limit relapse vulnerability through regulation of ion channel gene expression and suppression of MSN firing rates during active heroin use.