Stanislav S. Zakharenko

Primary cilia, which are present in most brain cells, are essential for brain development and function. During early brain development, dysfunction of the primary cilia can lead to a broad spectrum of disorders, collectively termed ciliopathies, that include brain malformations and intellectual disability. Although the role of primary cilia in brain development is well-established, cilia-mediated signaling in mature neurons and the contribution of cilia to neuronal circuit function remain controversial. Using mouse genetic and behavioral studies, single-cell electrophysiology, and 2-photon imaging, we show that deletion of primary cilia from adult hippocampal neurons is not required for hippocampal circuit function or behavior. Chronic genetic deletion or acute laser ablation of primary cilia from mature pyramidal neurons in the CA1 or CA3 regions of the hippocampus did not affect neuronal excitability, basal synaptic transmission, or long-term synaptic plasticity at excitatory CA3–CA1 hippocampal synapses. Moreover, the loss of primary cilia did not affect hippocampal-dependent learning and memory or anxiety-like behaviors. These results challenge the prevailing view of cilia function in mature hippocampal neurons and suggest that neuronal cilia in the adult hippocampus do not serve as major signaling hubs for pathways essential for neuronal function or behavior.

SUMMARY: Synaptic vesicle cycling, if not properly constrained, can result in excessive neurotransmitter release and subsequent neural pathology. Rab GTPases orchestrate synaptic vesicle trafficking through GTP-dependent interactions with effector proteins, but the restraining mechanism of these interactions is unknown. Here we identify LRRC57 (or RABIN for RAB INhibitor), a conserved brain-enriched protein in glutamatergic synapses that binds multiple GTP-loaded synaptic Rabs and competitively blocks access to their effectors. Loss of Rabin increased glutamate release, expanded vesicle pools, accelerated vesicle turnover, and produced circuit hyperexcitability with epileptiform activity, which was mitigated by an antiepileptic agent that targets presynaptic function. Conversely, overexpression of the Rabin gene suppressed neurotransmitter release and protected against induced seizures and persistent epileptiform discharges. Together, these findings define a noncanonical decoy–effector mechanism that constrains presynaptic Rab signaling to preserve excitatory circuit stability.

Summary: Schizophrenia is a complex neurodevelopmental disorder characterized by cognitive dysfunction, hallucinations, and negative symptoms such as amotivation. Negative symptoms are largely resistant to current antipsychotic treatments, and the neural circuits underlying amotivational states remain poorly defined. Here, using a mouse model of schizophrenia-associated 22q11.2 deletion syndrome (22q11DS), we report amotivation and weakened glutamatergic synaptic transmission between the thalamic parafascicular nucleus (Pf) and the dorsomedial striatum (DMS). Thalamostriatal disruption is attributed to hyperactivity of striatal cholinergic interneurons (CHIs), which is associated with enhanced Trpc3 and Pex51 ( Trip8b ) gene expression. Elevated acetylcholine levels in the DMS act on presynaptic M2 muscarinic receptors to weaken Pf-DMS glutamatergic transmission. Importantly, disruption of Pf-DMS synaptic transmission or hyperactivation of CHIs are each sufficient to cause amotivation in wild-type mice. These results identify a striatal hypercholinergic state and subsequent thalamostriatal disruption as core pathogenic events causing amotivation in 22q11DS, providing potential therapeutic targets.

Neurodevelopmental disorders are thought to arise from intrinsic brain abnormalities. Alternatively, they may arise from disrupted crosstalk among tissues. Here we show the local reduction of two vestibulo-cerebellar lobules, the paraflocculus and flocculus, in mouse models and humans with 22q11.2 deletion syndrome (22q11DS). In mice, this paraflocculus/flocculus dysplasia is associated with haploinsufficiency of the Tbx1 gene. Tbx1 haploinsufficiency also leads to impaired cerebellar synaptic plasticity and motor learning. However, neural cell compositions and neurogenesis are not altered in the dysplastic paraflocculus/flocculus. Interestingly, 22q11DS and Tbx1^+/– mice have malformations of the subarcuate fossa, a part of the petrous temporal bone, which encapsulates the paraflocculus/flocculus. Single-nuclei RNA sequencing reveals that Tbx1 haploinsufficiency leads to precocious differentiation of chondrocytes to osteoblasts in the petrous temporal bone autonomous to paraflocculus/flocculus cell populations. These findings suggest a previously unrecognized pathogenic structure/function relation in 22q11DS in which local skeletal deformity and cerebellar dysplasia result in behavioral deficiencies.

During the preparation of this manuscript, a text error was introduced during copyediting by JCI staff. Specifically, the word "mice" was inadvertently introduced into a sentence in the introductory paragraph describing published reports of HNRNPH2 mutations in humans (references 5-7). The correct sentence is below: Although all 6 individuals in the initial report were female, subsequent studies have identified males carrying missense mutations in HNRNPH2 associated with a range of overlapping phenotypes (5-7). The JCI regrets the error.

Synaptic plasticities, such as long-term potentiation (LTP) and depression (LTD), tune synaptic efficacy and are essential for learning and memory. Current studies of synaptic plasticity in humans are limited by a lack of adequate human models. Here, we modeled the thalamocortical system by fusing human induced pluripotent stem cell–derived thalamic and cortical organoids. Single-nucleus RNA-sequencing revealed that most cells in mature thalamic organoids were glutamatergic neurons. When fused to form thalamocortical assembloids, thalamic and cortical organoids formed reciprocal long-range axonal projections and reciprocal synapses detectable by light and electron microscopy, respectively. Using whole-cell patch-clamp electrophysiology and two-photon imaging, we characterized glutamatergic synaptic transmission. Thalamocortical and corticothalamic synapses displayed short-term plasticity analogous to that in animal models. LTP and LTD were reliably induced at both synapses; however, their mechanisms differed from those previously described in rodents. Thus, thalamocortical assembloids provide a model system for exploring synaptic plasticity in human circuits. Highlights Human thalamic organoids consist of mostly glutamatergic projection neurons. Thalamocortical assembloids form reciprocal glutamatergic synapses. Synapses are functional and undergo short-term plasticity resembling animal models. Long-term potentiation and depression reveal mechanisms distinct from rodents. eTOC Human organoids are often used to model diseases with synaptic pathology; however, few studies have examined synaptic function via single-cell or single-synapse recordings. Patton et al. fused human thalamic and cortical organoids into assembloids to examine synaptic transmission and short- and long-term synaptic plasticity in human thalamocortical and corticothalamic circuits.