Functional antagonism of voltage-gated K+ channel α-subunits in the developing brain ventricular system

Abstract

The brain ventricular system is essential for neurogenesis and brain homeostasis. Its neuroepithelial lining effects these functions, but the underlying molecular pathways remain to be understood. We found that the potassium channels expressed in neuroepithelial cells determine the formation of the ventricular system. The phenotype of a novel zebrafish mutant characterized by denudation of neuroepithelial lining of the ventricular system and hydrocephalus is mechanistically linked to Kcng4b, a homologue of the 'silent' voltagegated potassium channel α-subunit K v 6.4. We demonstrated that Kcng4b modulates proliferation of cells lining the ventricular system and maintains their integrity. The gain of Kcng4b function reduces the size of brain ventricles. Electrophysiological studies suggest that Kcng4b mediates its effects via an antagonistic interaction with Kcnb1, the homologue of the electrically active delayed rectifier potassium channel subunit K v 2.1. Mutation of kcnb1 reduces the size of the ventricular system and its gain of function causes hydrocephalus, which is opposite to the function of Kcng4b. This demonstrates the dynamic interplay between potassium channel subunits in the neuroepithelium as a novel and crucial regulator of ventricular development in the vertebrate brain.

Kv2.1 voltage‐gated potassium channels in developmental perspective

“…This process involves numerous other components, such as the soluble NSF attachment protein receptors (SNAREs), notably α‐SNAP (soluble NSF attachment protein). Interestingly, the α‐SNAP mutation in hyh mice causes a loss of integrity of the ependymal layer, resulting in the delamination of ependymal cells followed by hydrocephalus, which is very similar to that observed in kcng4b −/− mutant zebrafish . This suggests that during brain development Kcng4 and α‐SNAP perform closely related functions.…”

“…The biophysical properties of Kcnb1/Kcng4b heterotetramers in zebrafish are very similar to those in mammals. This indicates that when they are co‐expressed in hollow organs in developing zebrafish, Kcng4b may modify the biophysical properties of Kcnb1 similar to human KCNG4 …”

“…For example, a mouse mutant of Kcnb1 presented defects in insulin secretion and rapid progression of induced seizures but did not exhibit spontaneous seizures that characterize most human KCNB1 mutants. Developmental disturbances of brain ventricles seen in Kcnb1‐deficient zebrafish were not observed in mouse mutants . Perhaps, different phenotypes could be attributed to differences in the distribution of bodily fluids in aquatic and terrestrial animals.…”

“…The mutation of kcng4b encoding zebrafish KvS caused the formation of the truncated after S1 transmembrane domain Kcng4b* polypeptide (Figure B). Despite failing to prevent the formation of Kcnb1 homotetramers this mutation triggered a dominant‐negative effect similar to that caused by the truncated N‐terminal Kcna5 and Kcnb1 polypeptides . Tempting speculation is that this effect could be attributed to an interaction between the T1 cytoplasmic domain of the mutant polypeptides with the C‐terminals in the cytoplasmic subunit of Kcnb1 homotetramers.…”

“…Similarly, our recent analysis demonstrated the tissue‐specific expression of kcnb1 in adult zebrafish . In contrast, KCNG4 in human and kcng4b in zebrafish are not expressed in adults, whereas in developing embryos kcnb1 and kcng4b co‐express, in particular, in hollow organs such as the brain ventricular system, ear, eye (; https://www.proteinatlas.org/ENSG00000168418-KCNG4/tissue). This suggests that a regulatory role for Kcng4b over Kcnb1 may be restricted to the development of proliferative nonexcitable stem‐like cells that line cavities of hollow organs such as ependyma.…”