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Channel function: Klotho increases life span by cutting the glycan thread

Functional Glycomics (14 August 2008) | doi:10.1038/fg.2008.37

The transmembrane protein Klotho aids the ion maintenance function of the TRPV5 channel by cleaving sialic acids from the TRPV5 N-glycan, thereby exposing lactosamines that anchor the channel in the cell membrane.

Growth retardation of klotho knockout (right) mice. From Kuoro-o, M. et al. Mutation of the mouse klotho gene leads to a syndrome resembling ageing. Nature 390, 45–51 (1997).

Mice lacking the transmembrane protein Klotho die significantly younger than their wild-type littermates and develop severe defects in ion homeostasis. It has been shown that Klotho enhances the cell surface presence of the TRPV5 (transient receptor potential cation V5) channel, which regulates both Ca2+ reabsorption in the kidney and total body calcium homeostasis, by modifying the channel's N-glycan. Now Cha et al. report in PNAS that the extracellular portion of Klotho is shed into extracellular fluid where it cleaves sialic acid from TRPV5 N-glycan.

Sialic acid often caps the terminal lactosamines of N-glycans, blocking the binding between lactosamine branches and cell surface galectins. This interaction anchors transmembrane proteins to the cell membrane and prevents their endocytosis, thus leading to increased Ca2+ currents in the case of TRPV5. Cha et al. noticed that the increased current found after incubation of TRPV5-expressing human embryonic kidney (HEK) cells with the extracellular domain of murine Klotho was abrogated when an unglycosylatable mutant TRPV5 was expressed. Knockdown of the ST6Gal-1 sialyltransferase in HEK cells neutralized the effects of Klotho; likewise, Klotho was without effect when applied to TRPV5-expressing Chinese Hamster ovary (CHO) cells which lack ST6Gal-1. These findings suggested that Klotho specifically modifies the alpha2,6-linked sialic acid that caps the TRPV5 glycan.

N-acetylglucosaminyltransferase V (Mgat5) acts on N-glycans that carry two or three lactosamine branches to introduce a maximum number of four branches. Cha et al. found that TRPV5-expressing mutant CHO cells that lacked Mgat5 did not respond to Klotho, but became responsive upon co-expression of both Mgat5 and ST6Gal-1. Incubation of the co-expressing cells with N-acetyllactosamine which disrupts lactosamine binding to galectin-1, or knockdown of galectin-1 in HEK cells prevented the regulation of TRPV5 by Klotho. These results indicated that when Klotho removes alpha2,6-linked sialic acids it exposes underlying lactosamines, allowing binding to galectin-1 and increasing the time that TRPV5 resides on the membrane.

The findings of Cha et al. reveal how Klotho regulates Ca2+ reabsorption in the kidney and add evidence to the hypothesis that N-glycan branching increases the residence time of cell surface proteins — an important feature of the regulation of growth factor receptor and immune cell receptor function. However, unlike the known mechanisms of N-glycan branching regulation via the intracellular modulation of N-glycan synthesis, Klotho acts by modifying mature N-glycans at the cell surface. This action of Klotho represents a novel mechanism for the regulation of glycoprotein activity at the cell surface and may lead to the identification of further cell surface molecules that are modified by Klotho activity.

Mirko von Elstermann

Original paper:

  1. Cha, S. K., Ortega, B., Kurosu, H., Rosenblatt, K. P., Kuro-O, M. & Huang, C. L. Removal of sialic acid involving Klotho causes cell-surface retention of TRPV5 channel via binding to galectin-1. PNAS 105, 9805–9810 (2008). doi: 10.1073/pnas.0803223105 | Article |