*2.2. Thick Ascending Limb*

NKCC2, a target of loop diuretics such as furosemide, regulates sodium reabsorption in the TAL [22]. Regarding the UPS-mediated modulation of NKCC2, Wu et al. reported a role of UPS in the regulation of NKCC2 abundance in a high-salt condition [23]. They used a cytochrome P450 4F2 transgenic mouse model, which exhibited an increased production of 20-hydroxyeicosatetraenoic acid (20-HETE), a regulator of vascular tone and renal sodium reabsorption, by blocking Ca2+-activated K<sup>+</sup> channels [23]. Compared with wild-type mice, the transgenic mice displayed a profound decrease in renal NKCC2 abundance in response to a high-salt diet. This e ffect was not accompanied by the changes in NKCC2 mRNA expression but increased the abundance of ubiquitylated NKCC2. NKCC2 interacted with NEDD4-2, suggesting a role of this ubiquitin enzyme in the regulation of NKCC2 abundance. Another study reported that dibutyryl cyclic GMP (db-cGMP), a cell-permeable cGMP analog, decreased NKCC2 levels by increasing NKCC2 ubiquitylation and proteasomal degradation in rats [24]. In that study, db-cGMP induced a significant reduction in surface NKCC2 levels in suspensions of rat medullary TALs, which was inhibited in the presence of the proteasome inhibitor MG132. Furthermore, that study reported that NKCC2 levels were constitutively ubiquitylated and that the process was promoted by db-cGMP [24]. Pathways that modulate NKCC2 ubiquitylation at the downstream of cGMP signaling remains to be determined. Given that phosphorylation can regulate the interaction between substrates and ubiquitin ligases, roles of cGMP-dependent kinase might be worth exploring in future studies.
