Molecular regulation of NKCC2 in the thick ascending limb

Abstract

The kidney plays an essential role in blood pressure regulation by controlling short-term and long-term NaCl and water balance. The thick ascending limb of the loop of Henle (TAL) reabsorbs 25-30% of the NaCl filtered by the glomeruli in a process mediated by the apical Na(+)-K(+)-2Cl(-) cotransporter NKCC2, which allows Na(+) and Cl(-) entry from the tubule lumen into TAL cells. In humans, mutations in the gene coding for NKCC2 result in decreased or absent activity characterized by severe salt and volume loss and decreased blood pressure (Bartter syndrome type 1). Opposite to Bartter's syndrome, enhanced NaCl absorption by the TAL is associated with human hypertension and animal models of salt-sensitive hypertension. TAL NaCl reabsorption is subject to exquisite control by hormones like vasopressin, parathyroid, glucagon, and adrenergic agonists (epinephrine and norepinephrine) that stimulate NaCl reabsorption. Atrial natriuretic peptides or autacoids like nitric oxide and prostaglandins inhibit NaCl reabsorption, promoting salt excretion. In general, the mechanism by which hormones control NaCl reabsorption is mediated directly or indirectly by altering the activity of NKCC2 in the TAL. Despite the importance of NKCC2 in renal physiology, the molecular mechanisms by which hormones, autacoids, physical factors, and intracellular ions regulate NKCC2 activity are largely unknown. During the last 5 years, it has become apparent that at least three molecular mechanisms determine NKCC2 activity. As such, membrane trafficking, phosphorylation, and protein-protein interactions have recently been described in TALs and heterologous expression systems as mechanisms that modulate NKCC2 activity. The focus of this review is to summarize recent data regarding NKCC2 regulation and discuss their potential implications in physiological control of TAL function, renal physiology, and blood pressure regulation.

Fig. 1.

Schematic structure of the Na⁺-K⁺-2Cl⁻ (NKCC2) protein, highlighting established phosphorylation sites (rat sequence) and some of the protein-protein interaction identified to date.

Fig. 2.

Physiological regulatory pathways of NKCC2 in thick ascending limbs of Henle's loop (TALs). This figures highlights some of the known hormonal and signaling pathways known to modulate NKCC2 activity and trafficking in TALs via the cAMP/PKA pathway. See text for additional definitions.

Fig. 3.

Model for NKCC2 trafficking in TALs. This figure shows known trafficking processes in the TAL, indicated with solid lines. Predicted trafficking process, not fully studied or not validated in TALs, are shown with dashed lines. TGN, trans-Golgi network.

Fig. 4.

Sequence alignment indicating reported NKCC2 phosphorylation sites in different species. (see text and Table 1 for references).