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

Abstract

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...

Palmitoylation controls the stability of 190 kDa Ankyrin-G in dendritic spines and is regulated by ZDHHC8 and lithium

“…In this study, we demonstrate a role for AnkG-190 in the arborization of pyramidal neuronal dendrites and dendritic spine density in vivo. Multiple Ank3 KO mouse models have severe behavioral deficits related to psychiatric disorders (35,38). Our data support that those phenotypes may, at least in part, depend on alterations of dendrite and spine architecture.…”

“…Analysis of functional SNPs from GWAS data suggest that AnkG may be involved in the lithium response (33). Lithium has been shown to rescue some BD-related behavioral deficits in different Ank3 KO mouse model (35,41,42), suggesting it can alleviate symptoms of mouse models with AnkG loss of function. However, heterozygous and homozygous loss of ANK3 likely represent rare mutations only present in a small subset of patients with ASD and ID, and the majority of individuals with BD are likely to retain AnkG expression or have increased expression (9,10,43).…”

Current understanding of bipolar disorder: Toward integration of biological basis and treatment strategies

“…In addition, the amount of activity was markedly decreased after social defeat stress, however, it was increased after the stress, resembling a manic-switch. 123 As shown above, it is not conclusive whether inhibitory or excitatory neurons play a more important role in BD pathophysiology.…”

“…This is based on the decrease in ANKG‐like immunoreactivity in the axons of pyramidal neurons in the post‐mortem prefrontal cortex of schizophrenic patients . They created excitatory neurons‐specific conditional knockout mice of ANK3 that showed manic‐like behavioral changes, such as decreased immobility time in the forced swimming test, increased activity, and reduced anxiety. Since lithium and valproic acid reversed these changes, these mice may be considered as a model of mania.…”

“…Since lithium and valproic acid reversed these changes, these mice may be considered as a model of mania. In addition, the amount of activity was markedly decreased after social defeat stress, however, it was increased after the stress, resembling a manic‐switch …”