June 13, 2014. Ankyrin-G, the protein product of the gene ANK3 that has been linked to bipolar disorder and schizophrenia, has a hand in regulating the birth of new neurons in the developing brain. So says a study published online May 13 in Molecular Psychiatry, led by Li-Huei Tsai of the Massachusetts Institute of Technology, in Cambridge. The study reports that loss of ANK3 resulted in a relocation of β-catenin to the nucleus, where it promoted neural proliferation in mouse embryonic neurons. This proliferation could be pulled back to normal levels, however, by degrading β-catenin with the enzyme glycogen synthase kinase 3 (GSK3β). This suggests that bipolar disorder stems from disruptions to brain development – already a prominent theme for schizophrenia.
The study marks a step toward figuring out the function of the genes filtering in from genetic studies of psychiatric illnesses. ANK3 has been fingered in studies of common and rare variants alike for both schizophrenia and bipolar disorder (see SRF related news report). Moreover, a postmortem study has found abnormalities in ANK3 transcription in schizophrenia (see SRF related news report). ANK3 encodes the scaffolding protein ankyrin-G, which helps hold in place the voltage-gated channels at the axon’s initial segment and nodes of Ranvier – regions critical to action potential propagation. ANK3 is also thought to be important for establishing cell orientation—which part of a neuron is up and which is down.
The new study draws ANK3 into the sphere of influence of the Wnt pathway, which transduces signals from the outside of the cell to trigger transcription inside the nucleus. The Wnt pathway regulates neural proliferation, dendrite formation, and synapse construction in the developing brain. Tsai’s lab previously found that schizophrenia-related variants in Disrupted-in-Schizophrenia-1 (DISC1) decreased neural proliferation through the Wnt pathway (see SRF related news report), while the new study finds that ANK3 loss leads to increased proliferation.
First authors Omer Durak and Froylan Calderon de Anda first noted particularly high levels of ankyrin-G in the ventricular zone, a birthplace for cortical neurons, in embryonic mouse brain. This suggested it may be involved in neural proliferation, and knockdown of ANK3 in these brains using short-hairpin (sh) RNA resulted in a significant increase in newborn neurons two or 24 hours later, compared to those receiving a control shRNA. Similarly, mice lacking one copy of ANK3 also showed signs of increased neural proliferation.
Next, the researchers checked to see if ANK3 knockdown revved up Wnt signaling, given its involvement in neural proliferation, and found that it did: Following ANK3 knockdown, neurons from postnatal day 19 mice kept in culture showed increased levels of TCF/LEF transcription, a sign of Wnt signaling.
ANK3 knockdown did not change the amounts of the different Wnt signaling members, however. Instead, it shifted the localization of β-catenin, a core member of the Wnt pathway. Typically β-catenin binds to E-cadherin, an adhesion protein, and the complex is concentrated along the cell membrane. After ANK3 knockdown, this was less pronounced, and an increase in β-catenin signal emanating from the nucleus also emerged.
The researchers propose that ANK3 knockdown somehow frees β-catenin from E-cadherin, thus increasing the supply of free β-catenin available to enter the nucleus. This suggests that ANK3 levels could fine-tune Wnt signaling. Consistent with this, degrading β-catenin by overexpressing GSK3β, an enzyme that tags β -catenin for removal, reversed ANK3 knockdown’s effects, producing normal levels of neural proliferation.
Knockdown of ANK3 in mice results in behavioral abnormalities (Leussis et al., 2013), and the new results suggest that disruptions to Wnt signaling might contribute to these. Though whether ANK3 variants implicated in bipolar or schizophrenia turn down ANK3 expression remains unclear, the results bolster the role for the Wnt pathway in the neurodevelopment, and argue for paying close attention to the subcellular locations of interesting molecules.—Michele Solis.
Durak O, de Anda FC, Singh KK, Leussis MP, Petryshen TL, Sklar P, Tsai LH.
Ankyrin-G regulates neurogenesis and Wnt signaling by altering the subcellular localization of β-catenin. Mol Psychiatry. 2014 May 13. Abstract