slowpoke
slowpoke expression throughout development has been examined. It is expressed in
muscle cells, neurons of the CNS and PNS, mushroom bodies, a limited number of cells in embryonic
and larval midgut and in epithelial-derived tracheal cells. The promoter has been cloned and shown to
direct expression in the same pattern as the endogenous gene in both neural and epithelial-derived cells.
During pupariation and embryogenesis, slo is expressed in muscles many hours prior to the appearance
of functional channels (Becker, 1995).
The entire developmental history of muscle membrane electrogenesis can be observed in the embryonic myotubes of Drosophila. The development of ionic currents and muscle properties was examined using whole-cell patch-clamp techniques throughout embryonic
myogenesis. In the early stages of myogenesis, from myoblast fusion through to establishing epidermal insertions, the myotubes are
electrically inert and are electrically and dye coupled to adjacent myotubes. Membrane electrogenesis begins in the mid-embryonic stages
(early stage 16), when the myotubes abruptly uncouple, revealing the first of five prominent extrajunctional currents: a small, inward,
voltage-gated calcium current (ICa). The uncoupling of the embryonic myotubes heralds the onset of extremely rapid electrogenesis;
within several minutes both the fast, inactivating (IA; Shaker) and delayed, noninactivating (IK) outward potassium currents, the
stretch-activated outward potassium current, and the junctional glutamate-gated inward current all appear and begin to develop in a
current-specific manner. Very late in embryogenesis (late stage 17), the calcium-dependent, outward potassium currents [rapid,
inactivating (ICF; slowpoke) then delayed, noninactivating (ICS)] develop, completing the complement of macroscopic currents in the
mature larval muscle. Hence, the voltage-gated currents (ICa, IA, and IK, respectively) appear relatively early, and the
calcium-dependent currents (ICF, ICS) appear only very late during myogenesis. This developmental progression of current maturation
is reflected in dynamic changes in the voltage responses of the embryonic membrane, from wholly passive response to current injection
in the early, coupled myotubes to regenerating, overshooting action potentials in the mature embryonic muscle. The earliest embryonic IA
current has a midpoint of inactivation 40 mV more negative than the IA current in the mature embryo. As myogenesis proceeds, the
inactivation curve develops a biphasic character, suggesting that a low-inactivation IA channel is present in early development and
progressively replaced by the mature form as development proceeds. The current at all stages can be completely eliminated in Shaker
mutants (ShKS133). These findings suggest that an embryonic form of the Shaker IA channel is present during early myogenesis. The
prominent IA current present in early development is almost entirely inactivated at the physiological resting potential; the significance and
mechanism of this developmental shift are unclear (Broadie, 1993).
The slowpoke gene of Drosophila encodes a pore-forming subunit of a BK-type CaCa2+-activated K+ channel. The gene is expressed in
neurons, muscles, tracheal cells and in the midgut. The P1 transgene gene contains the entire slowpoke transcriptional control region and
drives the expression of a reporter protein comprised of slowpoke amino terminal sequences fused to beta-galactosidase. Midgut expression is limited to the copper cell and iron cell regions. The copper cell region is composed of two cell types: the copper
cells and the interstitial cells. The P1 transgene is expressed in the interstitial cells but not the copper cells. Furthermore, it is shown that the
reporter protein is apically localized in the interstitial cells. In these cells, the slowpoke CaCa2+-activated K+ channel is thought to
participate in the transport of ions between the hemolymph and the lumen of the gut. Subcellularly localized BK channels may be
involved in the secretion of acid into the gut lumen. An analogous role for basolaterally localized BK channels has been proposed in the
acid-secreting intercalating cells of the human kidney (Brenner, 1997).
slowpoke:
Biological Overview
| Evolutionary Homologs
| Regulation
| Developmental Biology
| References
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