Neurons are highly specialized cells of the nervous system that receive, process and transmit electrical signals critical for normal brain function. Here, we review the intricate organization of axonal membrane domains that facilitate rapid action potential conduction underlying communication between complex neuronal circuits. Two critical excitable domains of vertebrate axons are the axon initial segment (AIS) and the nodes of Ranvier, which are characterized by the high concentrations of voltage-gated ion channels, cell adhesion molecules and specialized cytoskeletal networks. The AIS is located at the proximal region of the axon and serves as the site of action potential initiation, while nodes of Ranvier, gaps between adjacent myelin sheaths, allow rapid propagation of the action potential through saltatory conduction. The AIS and nodes of Ranvier are assembled by ankyrins, spectrins and their associated binding partners through the clustering of membrane proteins and connection to the underlying cytoskeleton network. Although the AIS and nodes of Ranvier share similar protein composition, their mechanisms of assembly are strikingly different. Here we will cover the mechanisms of formation and maintenance of these axonal excitable membrane domains, specifically highlighting the similarities and differences between them. We will also discuss recent advances in super resolution fluorescence imaging which have elucidated the arrangement of the submembranous axonal cytoskeleton revealing a surprising structural organization necessary to maintain axonal organization and function. Finally, human mutations in axonal domain components have been associated with a growing number of neurological disorders including severe cognitive dysfunction, epilepsy, autism, neurodegenerative diseases and psychiatric disorders. Overall, this review highlights the assembly, maintenance and function of axonal excitable domains, particularly the AIS and nodes of Ranvier, and how abnormalities in these processes may contribute to disease.
Genome-wide association studies have implicated the ANK3 locus in bipolar disorder, a major human psychotic illness. ANK3 encodes ankyrin-G, which organizes the neuronal axon initial segment (AIS).We generated a mouse model with conditional disruption of ANK3 in pyramidal neurons of the adult forebrain (Ank-G cKO). This resulted in the expected loss of pyramidal neuron AIS voltage-gated sodium and potassium channels. There was also dramatic loss of markers of afferent GABAergic cartridge synapses, resembling the cortical microcircuitry changes in brains from psychotic patients, and suggesting disinhibition. Expression of c-fos was increased in cortical pyramidal neurons, consistent with increased neuronal activity due to disinhibition. The mice showed robust behavioral phenotypes reminiscent of aspects of human mania, ameliorated by antimania drugs lithium and valproate. Repeated social defeat stress resulted in repeated episodes of dramatic behavioral changes from hyperactivity to "depressionlike" behavior, suggestive of some aspects of human bipolar disorder. Overall, we suggest that this Ank-G cKO mouse model recapitulates some of the core features of human bipolar disorder and indicates that cortical microcircuitry alterations during adulthood may be involved in pathogenesis. The model may be useful for studying disease pathophysiology and for developing experimental therapeutics.B ipolar disorder and schizophrenia are major psychiatric disorders. Bipolar patients experience both mania, with elevated mood and activity, and depression (often triggered by stress), with low mood and activity. Bipolar disorder is often uniquely responsive to lithium (Li). Many loci have been identified by genome-wide association studies (GWASs) for schizophrenia (1). By contrast, fewer loci with genome-wide significance have been identified for bipolar disorder (2), with some overlap, suggesting some shared mechanisms.In a recent large GWAS of bipolar disorder (3), the most significant signal was detected at the ANK3 locus. This finding has been replicated in many (if not all) GWASs of bipolar disorder (3-5). The ANK3 locus is also found to be associated, to a lesser extent, with schizophrenia (6, 7).The ANK3 gene encodes ankyrin-G, a large scaffold protein highly expressed in neurons in the brain (8). Three main brainspecific splice variants encode 190, 270, and 480 kDa polypeptides. The 480-kDa peptide is the major isoform responsible for organization of the components of the axon initial segment (AIS), including the clustering of voltage-gated sodium and potassium channels important for generation of the action potential (9, 10). The 190-kDa isoform is present at dendritic spines (11,12). Many of the risk alleles for ANK3 are in the five-prime upstream region, suggestive of changes of expression (13, 14). Ankyrin-G protein expression was reduced in pyramidal AISs in superficial layers of schizophrenia cortex (15). These data suggest that ANK3 risk alleles can cause ankyrin-G loss of function.In cortex, the AISs of pyramidal n...
GABAergic circuits are critical for the synchronization and higher order function of brain networks. Defects in this circuitry are linked to neuropsychiatric diseases, including bipolar disorder, schizophrenia, and autism. Work in cultured neurons has shown that ankyrin-G plays a key role in the regulation of GABAergic synapses on the axon initial segment and somatodendritic domain of pyramidal neurons where it interacts directly with the GABA A receptor associated protein (GABARAP) to stabilize cell surface GABA A receptors. Here, we generated a knock-in mouse model expressing a mutation that abolishes the ankyrin-G/GABARAP interaction ( Ank3 W1989R) to understand how ankyrin-G and GABARAP regulate GABAergic circuitry in vivo. We found that Ank3 W1989R mice exhibit a striking reduction in forebrain GABAergic synapses resulting in pyramidal cell hyperexcitability and disruptions in network synchronization. In addition, we identified changes in pyramidal cell dendritic spines and axon initial segments consistent with compensation for hyperexcitability. Finally, we identified the ANK3 W1989R variant in a family with bipolar disorder, suggesting a potential role of this variant in disease. Our results highlight the importance of ankyrin-G in regulating forebrain circuitry and provide novel insights into how ANK3 loss-of-function variants may contribute to human disease.
Genetic variants in SCN2A, encoding the NaV1.2 voltage-gated sodium channel, are associated with a range of neurodevelopmental disorders with overlapping phenotypes. Some variants fit into a framework wherein gain-of-function missense variants that increase neuronal excitability lead to developmental and epileptic encephalopathy, while loss-of-function variants that reduce neuronal excitability lead to intellectual disability and/or autism spectrum disorder with or without co-morbid seizures. One unique case less easily classified using this framework is the de novo missense variant SCN2A-p.K1422E, associated with infant-onset developmental delay, infantile spasms, and features of autism spectrum disorder. Prior structure–function studies demonstrated that K1422E substitution alters ion selectivity of NaV1.2, conferring Ca2+ permeability, lowering overall conductance, and conferring resistance to tetrodotoxin (TTX). Based on heterologous expression of K1422E, we developed a compartmental neuron model incorporating variant channels that predicted reductions in peak action potential speed. We generated Scn2aK1422E mice and characterized effects on neurons and neurological/neurobehavioral phenotypes. Cultured cortical neurons from heterozygous Scn2aK1422E/+ mice exhibited lower current density with a TTX-resistant component and reversal potential consistent with mixed ion permeation. Recordings from Scn2aK1442E/+ cortical slices demonstrated impaired action potential initiation and larger Ca2+ transients at the axon initial segment during the rising phase of the action potential, suggesting complex effects on channel function. Scn2aK1422E/+ mice exhibited rare spontaneous seizures, interictal EEG abnormalities, altered induced seizure thresholds, reduced anxiety-like behavior and alterations in olfactory-guided social behavior. Overall, Scn2aK1422E/+ mice present with phenotypes similar yet distinct from other Scn2a models, consistent with complex effects of K1422E on NaV1.2 channel function.
Objective Interactions between pre-existing differences in mesolimbic function and neuroadaptations induced by consumption of fatty, sugary foods are thought to contribute to human obesity. This study examined basal and cocaine-induced changes in striatal neurotransmitter levels without diet manipulation and D2/D3 dopamine receptor-mediated transmission prior to and after consumption of “junk-foods” in obesity-prone and obesity-resistant rats. Methods Microdialysis and liquid chromatography-mass spectrometry were used to determine basal and cocaine-induced changes in neurotransmitter levels in real time with cocaine-induced locomotor activity. Sensitivity to the D2/D3 dopamine receptor agonist quinpirole was examined before and after restricted junk-food exposure. Selectively bred obesity-prone and obesity-resistant rats were used. Results Cocaine-induced locomotion was greater in obesity-prone rats versus obesity-resistant rats prior to diet manipulation. Basal and cocaine-induced increases in dopamine and serotonin levels did not differ. Obesity-prone rats were more sensitive to the D2 receptor-mediated effects of quinpirole, and junk-food produced modest alterations in quinpirole sensitivity in obesity-resistant rats. Conclusions These data show that mesolimbic systems differ prior to diet manipulation in susceptible versus resistant rats, and that consumption of fatty, sugary foods produce different neuroadaptations in these populations. These differences may contribute to enhanced food craving and an inability to limit food intake in susceptible individuals.
Dysfunction in sodium channels and their ankyrin scaffolding partners have both been implicated in neurodevelopmental disorders, including autism spectrum disorder (ASD). In particular, the genes SCN2A, which encodes the sodium channel NaV1.2, and ANK2, which encodes ankyrin-B, have strong ASD association. Recent studies indicate that ASD-associated haploinsufficiency in Scn2a impairs dendritic excitability and synaptic function in neocortical pyramidal cells, but how NaV1.2 is anchored within dendritic regions is unknown. Here, we show that ankyrin-B is essential for scaffolding NaV1.2 to the dendritic membrane of mouse neocortical neurons, and that haploinsufficiency of Ank2 phenocopies intrinsic dendritic excitability and synaptic deficits observed in Scn2a+/- conditions. Thus, these results establish a direct, convergent link between two major ASD risk genes and reinforce an emerging framework suggesting that neocortical pyramidal cell dendritic dysfunction can be etiological to neurodevelopmental disorder pathophysiology.
The majority of autism spectrum disorder (ASD) risk genes are associated with ASD due to haploinsufficiency, where only one gene copy is functional. Here, using SCN2A haploinsufficiency, a major risk factor for ASD, we show that increasing the expression of the existing functional SCN2A allele with CRISPR activation (CRISPRa) can provide a viable therapeutic approach. We first demonstrate therapeutic potential by showing that restoring Scn2a expression in adolescent heterozygous Scn2a conditional knock-in mice rescues electrophysiological deficits associated with Scn2a haploinsufficiency. Next, using an rAAV-CRISPRa based treatment, we restore electrophysiological deficits in both Scn2a heterozygous mice and human stem-cell-derived neurons. Our results provide a novel therapeutic approach for numerous ASD-associated genes and demonstrate that rescue of Scn2a haploinsufficiency, even at adolescent stages, can ameliorate neurodevelopmental phenotypes.
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