Psychiatric Risk Factor ANK3/Ankyrin-G Nanodomains Regulate the Structure and Function of Glutamatergic Synapses

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222

Axon initial segments (AISs) and nodes of Ranvier are sites of clustering of voltage-gated sodium channels (VGSCs) in nervous systems of jawed vertebrates that facilitate fast long-distance electrical signaling. We demonstrate that proximal axonal polarity as well as assembly of the AIS and normal morphogenesis of nodes of Ranvier all require a heretofore uncharacterized alternatively spliced giant exon of ankyrin-G (AnkG). This exon has sequence similarity to I-connectin/Titin and was acquired after the first round of whole-genome duplication by the ancestral ANK2/ANK3 gene in early vertebrates before development of myelin. The giant exon resulted in a new nervous system-specific 480-kDa polypeptide combining previously known features of ANK repeats and β-spectrin-binding activity with a fibrous domain nearly 150 nm in length. We elucidate previously undescribed functions for giant AnkG, including recruitment of β4 spectrin to the AIS that likely is regulated by phosphorylation, and demonstrate that 480-kDa AnkG is a major component of the AIS membrane "undercoat' imaged by platinum replica electron microscopy. Surprisingly, giant AnkG-knockout neurons completely lacking known AIS components still retain distal axonal polarity and generate action potentials (APs), although with abnormal frequency. Giant AnkG-deficient mice live to weaning and provide a rationale for survival of humans with severe cognitive dysfunction bearing a truncating mutation in the giant exon. The giant exon of AnkG is required for assembly of the AIS and nodes of Ranvier and was a transformative innovation in evolution of the vertebrate nervous system that now is a potential target in neurodevelopmental disorders.neuropsychiatric disease | cognitive impairment disorder | axon initial segment | ankyrin-G | axonal polarity

Section: Resultsmentioning

confidence: 99%

Giant ankyrin-G: A critical innovation in vertebrate evolution of fast and integrated neuronal signaling

Jenkins

Kim²,

Jones

et al. 2014

149

222

“…Recent findings implicate ankyrin-G in AMPA glutamate receptor-excitatory synapse structure and function, as well as in GABA synapses onto the cell body (12,52). Future research may be designed to determine the contribution of these different aspects.…”

Section: Discussionmentioning

confidence: 99%

“…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).…”

mentioning

confidence: 99%

Genetic disruption of ankyrin-G in adult mouse forebrain causes cortical synapse alteration and behavior reminiscent of bipolar disorder

Zhu

Cordner²,

Xiong

et al. 2017

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...

“…Thus, assembly of giant ankyrin-G in cell bodies and axons likely is regulated through yet to be defined but distinct sets of protein kinases and/ or phosphatases. An additional level of complexity in ankyrin-G targeting is provided by the recent finding that 190-kDa ankyrin-G lacking the giant exon but otherwise capable of binding to beta spectrins and membrane protein partners localizes to nanodomains in dendritic spines (21). In addition to GABAergic synapses, somatodendritic membranes also harbor ion channels involved in regulating excitability (37).…”

Section: Discussionmentioning

confidence: 99%

“…S1A), indicating the somatodendritic localization of ankyrin-G is neurontype specific. Interestingly, following submission of this paper, 190-kDa ankyrin-G lacking the giant exon was reported to localize to nanodomains in dendritic spines of hippocampal neurons where it contributes to NMDA receptor-dependent plasticity (21).…”

Section: Somatodendritic Gabaergic Synapse Stabilization Requires Giantmentioning

confidence: 96%

Giant ankyrin-G stabilizes somatodendritic GABAergic synapses through opposing endocytosis of GABA _A receptors

Tseng

Jenkins

Tanaka

et al. 2014

GABA A -receptor-based interneuron circuitry is essential for higher order function of the human nervous system and is implicated in schizophrenia, depression, anxiety disorders, and autism. Here we demonstrate that giant ankyrin-G (480-kDa ankyrin-G) promotes stability of somatodendritic GABAergic synapses in vitro and in vivo. Moreover, giant ankyrin-G forms developmentally regulated and cell-type-specific micron-scale domains within extrasynaptic somatodendritic plasma membranes of pyramidal neurons. We further find that giant ankyrin-G promotes GABAergic synapse stability through opposing endocytosis of GABA A receptors, and requires a newly described interaction with GABARAP, a GABA A receptorassociated protein. We thus present a new mechanism for stabilization of GABAergic interneuron synapses and micron-scale organization of extrasynaptic membrane that provides a rationale for studies linking ankyrin-G genetic variation with psychiatric disease and abnormal neurodevelopment.