Opposing actions of adrenal androgens and glucocorticoids on alternative splicing of Slo potassium channels in bovine chromaffin cells

Abstract

Pituitary ablation (hypophysectomy) in rats was previously reported to cause a precipitous change in the relative abundance of two alternative splice variants of the ''BK''-or ''Maxi K''-encoding Slo gene in adrenal chromaffin cells. Inclusion of the optional ''STREX'' exon (STRess axis-regulated EXon) in a C-terminal splice site was reduced, in preference to the variant lacking an insert at this site. Adrenocorticotropic hormone (ACTH) injections prevented the drop in STREX inclusion, implicating stress-axis function, as opposed to other pituitary functions. Because ACTH promotes synthesis and release of glucocorticoids (corticosterone or cortisol, depending on species), we hypothesized that glucocorticoids applied directly would promote STREX inclusion. Contrary to predictions, we report that direct application of glucocorticoids to bovine cells in vitro decreased STREX inclusion. This effect was blocked by the glucocorticoid receptor antagonist RU38486. As with glucocorticoids, synthesis and release of the adrenal androgen dehydroepiandrosterone (DHEA) increases in response to stress or elevated ACTH levels in some species. We report that direct application of DHEA increased expression of the STREX variant in cultured bovine cells. Two other androgens, androstenedione and testosterone, had similar effects. We hypothesize that Slo splicing in adrenal chromaffin cells in vivo is differentially regulated by the integrative, concentration-and time-dependent actions of glucocorticoids and androgens, with potentially important ramifications for stress-evoked catecholamine secretion.A lternative splicing gives rise to ion-channel variety that underlies diversity in the intrinsic firing properties of excitable cells. In adrenal chromaffin cells, calcium-and voltagedependent BK channels are encoded by the Slo gene. Two splice variants at splice site 5 in Slo (Fig. 1B) occur in chromaffin cells, and these exhibit distinct functional properties when characterized in heterologous expression systems (1-3). Thus Slo channels with the optional STREX exon (STREX channels) activate faster and at more negative voltages and deactivate more slowly than those with no extra exon at this site (ZERO channels). Although BK-channel function is multifaceted, the facilitated activation of STREX-type Slo channels enhances repetitive firing in rat chromaffin cells by minimizing action potential duration and speeding recovery of Na channels from inactivation during the brief afterhyperpolarization (4).Although there are many other examples of channelalternative splicing, little is known about mechanisms regulating the decision. The incorporation of STREX into Slo transcripts has been shown in rats to decrease substantially after hypophysectomy. This phenomenon can be effectively prevented by injecting adrenocorticotropin (ACTH) (3). In classic hypothalamic-pituitary-adrenal axis (HPA) function, perceived stress triggers the release of corticotropin-releasing hormone (CRH) and͞or arginine vasopressin (AVP) from the hypothalamus, which stimu...

Hypertension of Kcnmb1 −/− is linked to deficient K secretion and aldosteronism

“…We found that the BK-␤1 subunit is expressed in the adrenal glands of mouse by PCR and western blot. The BK␣ subunit has been identified in the mouse adrenal cortex and the adrenal medulla with immunohistochemical staining (37), findings consistent with previous patch-clamp studies revealing the presence of BK currents in adrenal glomerulosa cells (25,26) and the chromaffin cells of the adrenal medulla (38).…”

Functionally Diverse Complement of Large Conductance Calcium- and Voltage-activated Potassium Channel (BK) α-Subunits Generated from a Single Site of Splicing

“…Alternative splicing of the e21(STREX) exon can be dynamically regulated, for example, in response to circulating hormones (24,27), during stress (23), pregnancy (25), and upon cellular depolarization (58). However, factors that may regulate splicing decisions for the e20, e22, or ⌬e23 splice variants are essentially unknown.…”

“…Even neighboring cells (16,17), or compartments within cells (18,19), may express BK channels with differences in their functional properties. Furthermore, these properties can be modified temporally, for example, during development (20 -22) or following a physiological challenge (23)(24)(25)(26)(27).…”

Control of alternative pre-mRNA splicing by Ca++ signals

“…The exon is included in endocrine cells and neurons and enriched in the high frequency region of cochleae [22][23][24]84,107], contributing to the fine-tuning of hearing frequencies in birds and turtles [23,31]. Its inclusion is also regulated by steroid hormones in animals and by membrane depolarization in cultured cells [24,104,107,[155][156][157]. Thus, the STREX exon inclusion is inducible by external stimuli and the regulation may modulate multiple properties of the BK channels.…”