2.3.2. NEDD4-2-Mediated Ubiquitylation and NCC

Accumulating data also indicate that the ubiquitin ligase NEDD4-2 regulates NCC. Arroyo et al. demonstrated that in cultured cells, NEDD4-2 interacts with NCC, resulting in its ubiquitylation and reduced cell surface expression [43]. They also observed that serum/glucocorticoid-regulated kinase 1 (Sgk1) prevented the NEDD4-2-mediated deactivation of NCC in a kinase-dependent manner, indicating that Sgk1 is also involved in the NEDD4-2-mediated NCC regulation [43]. The role of Sgk1 in regulating NEDD4-2 and NCC has been demonstrated in vivo in Sgk1 knockout mice [44]. In another study, tetracycline-inducible, nephron-specific NEDD4-2 knockout mice exhibited increased NCC protein levels and salt-sensitive hypertension [45]. The mRNA expression of NCC remained unchanged, suggesting that NEDD4-2 regulates NCC abundance at the post-transcriptional level. Roy et al. reported that NEDD4-2 regulates NCC function through WNK1 [46]. They identified two alternatively spliced exons within a proline-rich region of WNK1 that contain PY motifs. NEDD4-2 binds to the PY motifs of WNK1, ubiquitylating WNK1 and targeting it for proteasomal degradation [46]. Dysregulation of NEDD4-2 has been implicated in the pathophysiology of salt-sensitive hypertension in a model of chronic kidney disease, which resulted in NCC activation through WNK1/SPAK [47]. In a recent study, Wu et al. reported that NEDD4-2 modulated NCC levels through a mechanism involving basolateral K<sup>+</sup> channel Kir4.1 (KCNJ10) [48]. The authors observed that kidney-specific deletion of NEDD4-2 hyperpolarized the DCT membrane, accompanied by the increase in NCC abundance. These changes were abolished in kidney-specific NEDD4-2/KCNJ10 double-knockout mice, leading to the suppression of NCC and blunted thiazide-induced natriuresis [48]. These data demonstrate a role of Kir4.1 in the NEDD4-2-mediated regulation of NCC.
