*3.2. Klotho, Wnt*/β*-Catenin, and Renal Damage*

Albuminuria downregulates tubular expression of Klotho even in the early stages of CKD [17]. Klotho deficiency induces Wnt activation, which, in turn, is associated with podocyte injury in mice models of diabetic nephropathy and patients with diabetes [18]. Podocyte injury is reduced after the deletion of β-catenin [18]. Although not fully elucidated, Snail-1, a transcription factor induced by Wnt through GSK3β, reduces nephrin expression playing a pivotal role in podocyte injury [19]. The opposite effect is observed with the overexpression of the Wilms Tumor 1 (WT1), which actively suppresses the Wnt pathway through the inhibition of Disheveled protein in podocytes [20].

In the mature kidney, Wnt is suppressed, allowing the podocyte to perform its physiological function. Interestingly, Wnt/β-catenin activity may be affected by high glucose; thus, a pathogenic role of Wnt/β-catenin in diabetic nephropathy cannot be ruled out [21]. In this regard, in a mice model of diabetic nephropathy, Klotho seems to protect glomerular and podocyte injury by inhibiting glomerular hypertrophy and reducing albuminuria [22]. Klotho may also reduce proteinuria by blocking the transient receptor potential cation channel (TRPC6) in podocytes [23], and in the heart, Klotho attenuates stress-induced cardiac hypertrophy via inhibition of TRPC6 [24].

### *3.3. Klotho, Wnt*/β*-Catenin, and Polycystic Kidney Disease (PKD)*

The Wnt/β-catenin pathway activation is also involved in the development of polycystic kidney disease (ADPKD) [25,26]. These patients show increased plasma levels of FGF23 as compared to GFR-matched CKD patients or healthy volunteers irrespective of the renal function, age, and serum levels of PTH and vitamin D [27]. Furthermore, ADPKD patients exhibit resistance to the renal effect of FGF23 that could be explained by the reduction in Klotho [28]. ADPKD patients show significantly reduced circulating sKlotho, as well as higher FGF23 with a superior FGF23-to-Klotho ratio as compared to healthy volunteers and GFR-matched CKD 1 and 2 patients [28]. Interestingly, sKlotho levels inversely correlate with the total cys<sup>t</sup> volume and the annual growth of the kidney.

Cardiovascular disease (CVD) is apparent in ADPKD patients. Indeed, a high incidence of intracranial aneurysms, mitral and aortic valvular prolapse, aortic regurgitation, left ventricular hypertrophy (LVH), and coronary artery disease has been reported, all of which may culminate in heart failure (HF) [29–31]. Although some of these manifestations have a genetic background, such as aneurysms, it is also possible that the dysregulation of the FGF23–Klotho complex could play a role in the development of these complications, mainly those that directly a ffect the heart. However, to our knowledge, no study has described crosstalk between FGF23 increase and Klotho reduction as responsible for ADPKD cardiac and vascular disease.

### *3.4. Consequences of the Close Relationship between Wnt and Klotho*

Since Klotho is downregulated in CKD, it is important to delineate this relationship. The fall of renal Klotho is postulated as one of the most important e ffects of Wnt signaling activation. Therefore, the design of new strategies directed to increase Klotho levels should be considered as a strategy to reduce morbidity and mortality associated with kidney and heart diseases.

sKlotho binds to multiple Wnt ligands suppressing a variety of gene transcription. The upregulation of Klotho halts the activation of Wnt, which reduces the deposition of the extracellular matrix and decreases the transcription of cytokines [32]. The contrary is observed in Klotho heterozygous mutant mice in which Wnt is overexpressed together with an increment of Transforming Growth Factor-β (TGF-β) and collagen type III (Col3); the extracellular matrix deposition and interstitial fibrosis are remarkable as compared to the wild type. Hence, in vivo models have demonstrated that sKlotho attenuates renal fibrosis by halting Wnt signaling [33].
