COL4A4 and obesity due to melanocortin 4 receptor deficiency: Furthermore, obesity led to the up-regulation of sodium transporters (Slc13a1, Slc22a8, Slc17a1, Slc17a3) and the down-regulation of structural proteins (Col4a3, Col4a4) as well as Na/K-ATPase subunits encoded by Atp1a1, Atp1b1, suggesting impaired sodium metabolism and underlying renal injury (Figure 8F).