Abelson interactor 1 (ABI1) and its interaction with Wiskott-Aldrich syndrome protein (wasp) are critical for proper eye formation in Xenopus embryos

Bioelectric signalling via potassium channels: a mechanism for craniofacial dysmorphogenesis in KCNJ2-associated Andersen-Tawil Syndrome

KEY POINTS

Xenopus laevis craniofacial development is a good system for the study of Andersen-Tawil Syndrome (ATS)-associated craniofacial anomalies (CFAs) because (1) Kcnj2 is expressed in the nascent face; (2) molecular-genetic and biophysical techniques are available for the study of ion-dependent signalling during craniofacial morphogenesis; (3) as in humans, expression of variant Kcnj2 forms in embryos causes a muscle phenotype; and (4) variant forms of Kcnj2 found in human patients, when injected into frog embryos, cause CFAs in the same cell lineages. Forced expression of WT or variant Kcnj2 changes the normal pattern of Vmem (resting potential) regionalization found in the ectoderm of neurulating embryos, and changes the normal pattern of expression of ten different genetic regulators of craniofacial development, including markers of cranial neural crest and of placodes. Expression of other potassium channels and two different light-activated channels, all of which have an effect on Vmem , causes CFAs like those induced by injection of Kcnj2 variants. In contrast, expression of Slc9A (NHE3), an electroneutral ion channel, and of GlyR, an inactive Cl(-) channel, do not cause CFAs, demonstrating that correct craniofacial development depends on a pattern of bioelectric states, not on ion- or channel-specific signalling. Using optogenetics to control both the location and the timing of ion flux in developing embryos, we show that affecting Vmem of the ectoderm and no other cell layers is sufficient to cause CFAs, but only during early neurula stages. Changes in Vmem induced late in neurulation do not affect craniofacial development. We interpret these data as strong evidence, consistent with our hypothesis, that ATS-associated CFAs are caused by the effect of variant Kcnj2 on the Vmem of ectodermal cells of the developing face. We predict that the critical time is early during neurulation, and the critical cells are the ectodermal cranial neural crest and placode lineages. This points to the potential utility of extant, ion flux-modifying drugs as treatments to prevent CFAs associated with channelopathies such as ATS.

ABSTRACT

Variants in potassium channel KCNJ2 cause Andersen-Tawil Syndrome (ATS); the induced craniofacial anomalies (CFAs) are entirely unexplained. We show that KCNJ2 is expressed in Xenopus and mouse during the earliest stages of craniofacial development. Misexpression in Xenopus of KCNJ2 carrying ATS-associated mutations causes CFAs in the same structures affected in humans, changes the normal pattern of membrane voltage potential regionalization in the developing face and disrupts expression of important craniofacial patterning genes, revealing the endogenous control of craniofacial patterning by bioelectric cell states. By altering cells' resting potentials using other ion translocators, we show that a change in ectodermal voltage, not tied to a specific protein or ion, is sufficient to cause CFAs. By adapting optogenetics for use in non-neural cells in embryos, we show that developmentally patterned K(+) flux is required for correct regionalization of the resting potentials and for establishment of endogenous early gene expression domains in the anterior ectoderm, and that variants in KCNJ2 disrupt this regionalization, leading to the CFAs seen in ATS patients.

The NESH/Abi-3-based WAVE2 complex is functionally distinct from the Abi-1-based WAVE2 complex

BACKGROUND

Abl interactor (Abi) family proteins play significant roles in actin cytoskeleton organization through participation in the WAVE complex. Mammals possess three Abi proteins: Abi-1, Abi-2, and NESH/Abi-3. Abi-1 and Abi-2 were originally identified as Abl tyrosine kinase-binding proteins. It has been disclosed that Abi-1 acts as a bridge between c-Abl and WAVE2, and c-Abl-mediated WAVE2 phosphorylation promotes actin remodeling. We showed previously that NESH/Abi-3 is present in the WAVE2 complex, but neither binds to c-Abl nor promotes c-Abl-mediated phosphorylation of WAVE2.

RESULTS

In this study, we characterized NESH/Abi-3 in more detail, and compared its properties with those of Abi-1 and Abi-2. NESH/Abi-3 was ectopically expressed in NIH3T3 cells, in which Abi-1, but not NESH/Abi-3, is expressed. The expression of NESH/Abi-3 caused degradation of endogenous Abi-1, which led to the formation of a NESH/Abi-3-based WAVE2 complex. When these cells were plated on fibronectin-coated dishes, the translocation of WAVE2 to the plasma membrane was significantly reduced and the formation of peripheral lamellipodial structures was disturbed, suggesting that the NESH/Abi-3-based WAVE2 complex was unable to help produce lamellipodial protrusions. Next, Abi-1, Abi-2, or NESH/Abi-3 was expressed in v-src-transformed NIH3T3 cells. Only in NESH/Abi-3-expressed cells did treatment with an Abl kinase inhibitor, imatinib mesylate, or siRNA-mediated knockdown of c-Abl promote the formation of invadopodia, which are ventral membrane protrusions with extracellular matrix degradation activity. Structural studies showed that a linker region between the proline-rich regions and the Src homology 3 (SH3) domain of Abi-1 is crucial for its interaction with c-Abl and c-Abl-mediated phosphorylation of WAVE2.

CONCLUSIONS

The NESH/Abi-3-based WAVE2 complex is functionally distinct from the Abi-1-based one, and NESH/Abi-3 may be involved in the formation of ventral protrusions under certain conditions.

Sox21 regulates the progression of neuronal differentiation in a dose-dependent manner

Members of the SoxB transcription factor family play critical roles in the regulation of neurogenesis. The SoxB1 proteins are required for the induction and maintenance of a proliferating neural progenitor population in numerous vertebrates, however the role of the SoxB2 protein, Sox21, is less clear due to conflicting results. To clarify the role of Sox21 in neurogenesis, we examined its function in the Xenopus neural plate. Here we report that misexpression of Sox21 expands the neural progenitor domain, and represses neuron formation by binding to Neurogenin (Ngn2) and blocking its function. Conversely, we found that Sox21 is also required for neuron formation, as cells lacking Sox21 undergo cell death and thus are unable to differentiate. Together our data indicate that Sox21 plays more than one role in neurogenesis, where a threshold level is required for cell viability and normal differentiation of neurons, but a higher concentration of Sox21 inhibits neuron formation and instead promotes progenitor maintenance.