8.A.49 The Klotho Auxiliary Protein (Klotho) Family
The Klotho protein in mammals activates a number of transporters, but also affects aging and aging-related conditions such as neurodegeneration and muscle wasting. It is a large membrane-embeded protein with two TMSs, one N-terminal and one C-terminal. Its extracellular domain has β-glucuronidase activity with two domains of the BglB (Glyco_hydro_1) superfamily which comprise virtually all of the extracellular domain between the two TMSs, and these may play a role in enhancing or inhibiting various transport activities. It activates CreaT (SLC6A8) by stabilizing the carrier in the plasma membrane (Almilaji et al. 2014). The direct role of alphaKlotho and FGF23 in the maintenance of phosphate homeostasis is partly mediated by modulation of type II Na+-dependent phosphate co-transporters in target organs. alphaKlotho and FGF23 are principal phosphotropic hormones (Hu et al. 2018).
Klotho also regulates diverse calcium and potassium ion channels as well as several carriers including the Na+-coupled excitatory amino acid transporters EAAT1, 2, 3 and 4 which are up-regulated (Warsi et al. 2015), the Na+-coupled phosphate cotransporters, NaPi-IIa and NaPi-IIb, and the Na+/K+-ATPase. All these cellular transport regulations contribute to the aging suppressor role of Klotho (Sopjani et al. 2014). Klotho expressed in the proximal tubules has a defined but limited role in renal phosphate handling in vivo (Ide et al. 2016). The expression of Ca2+ channels, including Orai3, but not Orai1, Orai2, TRPV5 and TRPV6 was reduced in KL-silenced as compared to control cells (Xuan and Hai 2018).
Klotho in humans is encoded by the hKL gene. This aging suppressor protein is predominantly expressed in the distal convoluted tubule of the kidney, parathyroid glands, and choroid plexus of the brain. The Klotho protein exists in both a full-length membrane form and a soluble secreted form. The extracellular domain can be enzymatically cleaved off and released into the systemic circulation where it functions as both a beta-glucuronidase and a hormone. Soluble Klotho (sKlotho) is present in biological fluids including blood, urine and cerebrospinal fluids. Klotho deficiency leads to multi-organ failure accompanied by early appearance of multiple age-related disorders and early death, whereas overexpression results in the opposite effects. Klotho regulates plasma membrane transport either indirectly through inhibiting calcitriol (1,25(OH)2D3) formation (or another mechanism), or by directly binding to and affecting transporter proteins. As noted above, these include ion channels, cellular carriers, and the Na+/K+-ATPase (Sopjani and Dërmaku-Sopjani 2016).
alpha-Klotho, a type 1 transmembrane protein, exhibits aging suppression function. The large amino-terminal extracellular domain of α-klotho is shed as soluble klotho (sKlotho) and functions as a circulating cardioprotective hormone as noted above. Diacylglycerol (DAG)-activated calcium-permeable TRPC6 channels play a critical role in stress-induced cardiac remodeling. DAG activates TRPC6 by acting directly on the channel to increase its activity and by stimulation of channel exocytosis. sKlotho protects the heart by inhibiting DAG stimulation of TRPC6 exocytosis. Using a compound that directly activates TRPC6 without affecting channel exocytosis, Xie et al. 2020 showed that sKlotho selectively blocks DAG stimulation of channel exocytosis, and that DAG stimulates exocytosis of TRPC6-containing vesicles pre-docked to the plasma membrane. Mnuc13 family proteins play roles in the proper assembly of SNARE proteins and priming the vesicle competent for fusion. DAG stimulates TRPC6 exocytosis by targeting to the C1 domain of Munc13-2 (Xie et al. 2020).