Role of BK gamma subunits in renal K secretion

University Of Pittsburgh At Pittsburgh, Pittsburgh PA

Investigators

Abstract

ABSTRACT: Maintenance of whole-body K+ homeostasis is a vital biological function carried out primarily by the aldosterone sensitive distal nephron. Detailed understanding of the physiology underlying this process is critical to the management of patients with dysregulated K+ balance, an outcome arising from a number of different pathological conditions. In general, the kidney must eliminate a surplus of dietary K+ to maintain circulating levels between 3.5 and 5 mM. This is achieved, in part, by the activity of large conductance, Ca2+-activated K+ (BK) channels expressed on the apical membranes of intercalated cells (ICs). Under conditions of high tubular flow rates, such as with volume expansion or excess K+ intake, BK channels open to allow bulk transport of K+ into the tubule lumen in a process termed “flow induced K+ secretion.” BK activation in this context is unusual because these channels are typically only activated by significant membrane depolarization and/or intracellular Ca2+ elevations. Given that ICs are not electrically excitable, and that sustained intracellular Ca2+ elevations are usually cytotoxic, we propose that another mechanism allowing for BK activation is likely to be involved in flow induced K+ secretion. This project is focused on investigating the role of a newly described class of auxiliary γ subunits in regulating BK-mediated K+ secretion in the distal nephron. This subunit family (BKγ1-4), encoded by members of the leucine rich repeat containing protein family (lrrc26, 52, 55, and 38, respectively), has received recent attention for their ability to shift the channel’s activation threshold to near resting membrane potential, even at low Ca2+ levels. This allows for sustained BK activity without the need for extensive depolarizing or calcinergic stimuli. BKγ subunits have been described as having profound effects on channel function in a variety of cell types throughout the body and, in particular, by epithelial cells that carry out vectorial K+ transport. We provide novel evidence that one of these auxiliary subunits (BKγ2; lrrc52) is highly expressed in ICs and is upregulated by dietary K+ loading, which is associated with flow induced K+ secretion. We propose to investigate how BKγ2 is regulated by K+ intake and define its functional role in distal nephron K+ handling. In Aim 1, we will quantify changes in BKγ2 expression in response to various dietary K+ challenges, delineating the cell-specific expression pattern and exploring the potential influence of sex as a variable. We will further identify the role of aldosterone signaling in regulating BK subunit expression in ICs by using an IC-specific knockout of the mineralocorticoid receptor. In Aim 2, we will combine patch clamp electrophysiology, isolated tubular perfusion, and metabolic cage approaches to assess BK channel function, and K+ handling in general, in WT versus BKγ2-/- mice. These studies will allow us to define, for the first time, the role of these subunits in renal K+ transport, as well as to explore the physiological mechanisms that regulate their expression in a cell-specific manner. Our findings will significantly advance the current understanding not only of renal K+ handling, but also of the broader importance of BKγ subunits as a novel class of BK channel regulatory proteins.