A potassium channel mutation in weaver mice implicates membrane excitability in granule cell differentiation

The overall sequence similarity between the voltage-activated K+ channels and cyclic nucleotide-gated ion channels from retinal and olfactory neurons suggests that they arose from a common ancestor. On the basis of sequence comparisons, mutations were introduced into the pore of a voltage-activated K+ channel. These mutations confer the essential features of ion conduction in the cyclic nucleotide-gated ion channels; the mutant K+ channels display little selectivity among monovalent cations and are blocked by divalent cations. The property of K+ selectivity is related to the presence of two amino acids that are absent from the pore-forming region of the cyclic nucleotide-gated channels. These data demonstrate that very small differences in the primary structure of an ion channel can account for extreme functional diversity, and they suggest a possible connection between the pore-forming regions of K+, Ca2+, and cyclic nucleotide-gated ion channels.

The high ionic selectivity of K+ channels is a unifying feature of this diverse class of membrane proteins. Though K+ channels differ widely in regulation and kinetics, physiological studies have suggested a common structure: a single file pore containing multiple ion-binding sites and having broader vestibules at both ends. We have used site-directed mutagenesis and single-channel recordings to identify a molecular region that influences ionic selectivity in a cloned A-type K+ channel from Drosophila. Single amino-acid substitutions in H5, the fifth hydrophobic region, enhanced the passage of NH4+ and Rb+, ions with diameters larger than K+, without compromising the ability of the channel to exclude the smaller cation, Na+. The mutations that substantially altered selectivity had little effect on the gating properties of the channel. We conclude that the H5 region is likely to line the pore of the K+ channel.

We have examined the role apoptosis plays during postnatal development of the mouse cerebellum by a new method utilizing T7 DNA polymerase for the in situ detection of DNA fragmentation associated with cell death. Granule cell loss between the third and fifth postnatal weeks, hypothesized to affect the granule cell to Purkinje cell stoichiometry, is not associated with DNA fragmentation. However, cerebellar granule cells undergo extensive nuclear DNA fragmentation between postnatal days 5 and 9. Cell death prior to synaptogenesis may help regulate granule cell number. Our results suggest that different mechanisms of cell death within the same neuronal cell population occur during development.