Chihiro Hisatsune

Behavioral challenges prompt alternating vigorous and reduced mobility – active, passive coping – that optimize energy investment. Here, we show that disrupting astrocytes calcium signaling in the mouse lateral habenula (LHb) prolongs active coping. This state manifests through calcium elevations in both mouse and zebrafish habenular astrocytes. Presynaptic tracing approaches integrate LHb astrocytes within aversion–related neuronal circuits. Thus, astrocytes regulate state transitions highlighting their computational contribution to behaviors across species.

Tuberous sclerosis complex (TSC) is a multisystem developmental disorder characterized by hamartomas in various organs such as the brain, lungs, and kidneys. Epilepsy, along with autism and intellectual disability, is one of the neurological impairments associated with TSC that has an intimate relationship with developmental outcomes and quality of life. Sustained activation of the mammalian target of rapamycin (mTOR) via TSC1 or TSC2 mutations is known to be involved in the onset of epilepsy in TSC. However, the mechanism by which mTOR causes seizures remain unknown. In this study, we showed that, human induced pluripotent stem cell-derived TSC2-deficient (TSC2^-/-) neurons exhibited elevated neuronal activity with highly synchronized Ca²⁺ spikes. Notably, TSC2^-/- neurons presented enhanced Ca²⁺ influx via L-type Ca²⁺ channels (LTCCs), which contributed to the abnormal neurite extension and sustained activation of cAMP response element binding protein (CREB), a critical mediator of synaptic plasticity. Expression of Cav1.3, a subtype of LTCCs, was increased in TSC2^-/- neurons, but long-term rapamycin treatment suppressed this increase and reversed the altered neuronal activity and neurite extensions. Thus, we identified Cav1.3 LTCC as a critical downstream component of TSC-mTOR signaling that would trigger enhanced neuronal network activity of TSC2^-/- neurons. We suggest that LTCCs could be potential novel targets for the treatment of epilepsy in TSC. Significant statement There is a close relationship between elevated mTOR activity and epilepsy in tuberous sclerosis complex (TSC). However, the underlying mechanism by which mTOR causes epilepsy remains unknown. In this study, using human TSC2^-/- neurons, we identified elevated Ca²⁺ influx via LTCCs as a critical downstream component of TSC-mTOR signaling and a potential cause of both elevated neuronal activity and neurite extension in TSC2^-/- neurons. Our findings demonstrate a previously unrecognized connection between sustained mTOR activation and elevated Ca²⁺ signaling via LTCC in human TSC neurons, which could cause epilepsy in TSC.

Signal transduction in sensory neurons of the mammalian vomeronasal organ (VNO) involves the opening of the canonical transient receptor potential channel Trpc2, a Ca²⁺-permeable cation channel that is activated by diacylglycerol and inhibited by Ca²⁺-calmodulin. There has been a long-standing debate about the extent to which the second messenger inositol 1,4,5-trisphosphate (InsP₃) and type 3 InsP₃ receptor (InsP₃R3) are involved in the opening of Trpc2 channels and in sensory activation of the VNO. To address this question, we investigated VNO function of mice carrying a knockout mutation in the Itpr3 locus causing a loss of InsP₃R3. We established a new method to monitor Ca²⁺ in the endoplasmic reticulum of vomeronasal sensory neurons (VSNs) by employing the GFP-aequorin protein sensor erGAP2. We also performed simultaneous InsP₃ photorelease and Ca²⁺ monitoring experiments, and analysed Ca²⁺ dynamics, sensory currents, and action potential or field potential responses in InsP₃R3-deficient VSNs. Disruption of Itpr3 abolished or minimized the Ca²⁺ transients evoked by photoactivated InsP₃, but there was virtually no effect on sensory activation of VSNs. Therefore, InsP₃R3 is dispensable for primary chemoelectrical transduction in mouse VNO. We conclude that InsP₃R3 is not required for gating of Trpc2 in VSNs.

We report three affected members, a mother and her two children, of a non-consanguineous Irish family who presented with a suspected autosomal dominant spinocerebellar ataxia characterized by early motor delay, poor coordination, gait ataxia, and dysarthria. Whole exome sequencing identified a novel missense variant (c.106C>T; p.[Arg36Cys]) in the suppressor domain of type 1 inositol 1,4,5-trisphosphate receptor gene (ITPR1) as the cause of the disorder, resulting in a molecular diagnosis of spinocerebellar ataxia type 29. In the absence of grandparental DNA, microsatellite genotyping of healthy family members was used to confirm the de novo status of the ITPR1 variant in the affected mother, which supported pathogenicity. The Arg36Cys variant exhibited a significantly higher IP₃-binding affinity than wild-type (WT) ITPR1 and drastically changed the property of the intracellular Ca²⁺ signal from a transient to a sigmoidal pattern, supporting a gain-of-function disease mechanism. To date, ITPR1 mutation has been associated with a loss-of-function effect, likely due to reduced Ca²⁺ release. This is the first gain-of-function mechanism to be associated with ITPR1-related SCA29, providing novel insights into how enhanced Ca²⁺ release can also contribute to the pathogenesis of this neurological disorder.

Significance: The cerebellar cortex, and its sole output, Purkinje cells, is essential for motor coordination and learning. Dendritic spines of Purkinje cells are the primary sites of cerebellar synaptic plasticity. Therefore, maintenance of spine structure of mature Purkinje cells is a critical aspect of cerebellar functions, but the underlying mechanisms remain unclear. Here we described an activity-dependent regulatory mechanism of spines in Purkinje cells. We found that F-actin cross-linking activity of Ca ²⁺ /calmodulin-dependent protein kinase II β isoform is controlled by protein kinase C-mediated phosphorylation, which is triggered by group I metabotropic glutamate receptor signaling. Defective function of the phosphorylation leads to excess spine development in mature Purkinje cells. Our findings revealed a mechanism for proper maintenance of cerebellar Purkinje cell spines.

The inositol 1,4,5‐trisphosphate receptor (IP₃R) is a huge Ca²⁺ channel that is localized at the endoplasmic reticulum. The IP₃R releases Ca²⁺ from the endoplasmic reticulum upon binding to IP₃, which is produced by various extracellular stimuli through phospholipase C activation. All vertebrate organisms have three subtypes of IP₃R genes, which have distinct properties of IP₃‐binding and Ca²⁺ sensitivity, and are differently regulated by phosphorylation and by their associated proteins. Each cell type expresses the three subtypes of IP₃R in a distinct proportion, which is important for creating and maintaining spatially and temporally appropriate intracellular Ca²⁺ level patterns for the regulation of specific physiological phenomena. Of the three types of IP₃Rs, the type 1 receptor (IP₃R1) is dominantly expressed in the brain and is important for brain function. Recent emerging evidence suggests that abnormal Ca²⁺ signals from the IP₃R1 are closely associated with human brain pathology. In this review, we focus on the recent advances in our knowledge of the regulation of IP₃R1 and its functional implication in human brain diseases, as revealed by IP₃R mutation studies and analysis of human disease‐associated genes.This article is part of the mini review series “60th Anniversary of the Japanese Society for Neurochemistry”. image