Regulation of ion channels by secreted Klotho: mechanisms and implications

Galectin-7 modulates the length of the primary cilia and wound repair in polarized kidney epithelial cells

Galectins (Gal) are β-galactoside-binding proteins that function in epithelial development and homeostasis. An overlapping role for Gal-3 and Gal-7 in wound repair was reported in stratified epithelia. Although Gal-7 was thought absent in simple epithelia, it was reported in a proteomic analysis of cilia isolated from cultured human airway, and we recently identified Gal-7 transcripts in Madin-Darby canine kidney (MDCK) cells (Poland PA, Rondanino C, Kinlough CL, Heimburg-Molinaro J, Arthur CM, Stowell SR, Smith DF, Hughey RP. J Biol Chem 286: 6780-6790, 2011). We now report that Gal-7 is localized exclusively on the primary cilium of MDCK, LLC-PK(1) (pig kidney), and mpkCCD(c14) (mouse kidney) cells as well as on cilia in the rat renal proximal tubule. Gal-7 is also present on most cilia of multiciliated cells in human airway epithelia primary cultures. Interestingly, exogenous glutathione S-transferase (GST)-Gal-7 bound the MDCK apical plasma membrane as well as the cilium, while the lectin Ulex europeaus agglutinin, with glycan preferences similar to Gal-7, bound the basolateral plasma membrane as well as the cilium. In pull-down assays, β1-integrin isolated from either the basolateral or apical/cilia membranes of MDCK cells was similarly bound by GST-Gal-7. Selective localization of Gal-7 to cilia despite the presence of binding sites on all cell surfaces suggests that intracellular Gal-7 is specifically delivered to cilia rather than simply binding to surface glycoconjugates after generalized secretion. Moreover, depletion of Gal-7 using tetracycline-induced short-hairpin RNA in mpkCCD(c14) cells significantly reduced cilia length and slowed wound healing in a scratch assay. We conclude that Gal-7 is selectively targeted to cilia and plays a key role in surface stabilization of glycoconjugates responsible for integrating cilia function with epithelial repair.

KLOTHO gene polymorphism of G395A is associated with kidney stones

BACKGROUND/AIMS

KLOTHO, a type-1 transmembrane protein with glucurodinase activity, is expressed in tissues responsible for calcium homeostasis such as the kidney, parathyroid gland and the epithelium of the choroid plexus in the brain. Given the emerging evidence indicating a novel regulatory function for KLOTHO protein in renal calcium and phosphate homeostasis, the present study aims to investigate the association between KLOTHO genetic polymorphisms and kidney stone (KS).

METHODS

KLOTHO gene polymorphisms G395A in the promoter region, F252V in exon 2, and C1818T in exon 4 were investigated in 108 patients with renal calcium stone formation and 51 age-matched healthy volunteers with no history of renal stone formation, using polymerase chain reaction.

RESULTS

GG genotype of G395A KLOTHO polymorphism had approximately 2-fold increased KS risk compared with the homozygous genotype AA and heterozygote GA (OR 1.849, 95% CI 1.016-3.364, p = 0.044). We also found that non-A allele carriers had significantly higher KS risk associated with the KS clinical characteristics including hypercalcemia, hypophosphatemia and phosphaturia.

CONCLUSION

Our findings suggested that the G395A polymorphism of KLOTHO gene is associated with the KSs and may act as a risk factor for the development of KS disease.

Klotho, FGF23, and FGF receptors in chronic kidney disease: a yin-yang situation?

Secondary hyperparathyroidism in chronic kidney disease (CKD) develops in response to disturbances in calcium and phosphate metabolism associated with CKD, including FGF23 and klotho. FGF23 activates its receptor FGFR1, splice variant IIIC, in the parathyroid gland via a klotho-dependent mechanism and suppresses parathyroid hormone (PTH) secretion. Klotho also may regulate PTH secretion in an FGF23-independent mode, by modulating parathyroid Na+/K+-ATPase activity. The persistence of hyperparathyroidism with progressing CKD despite high serum FGF23 is indicative of FGF23 resistance.

Phosphate and Klotho

Klotho is a putative aging suppressor gene encoding a single-pass transmembrane co-receptor that makes the fibroblast growth factor (FGF) receptor specific for FGF-23. In addition to multiple endocrine organs, Klotho is expressed in kidney distal convoluted tubules and parathyroid cells, mediating the role of FGF-23 in bone–kidney–parathyroid control of phosphate and calcium. Klotho^–/– mice display premature aging and chronic kidney disease-associated mineral and bone disorder (CKD-MBD)-like phenotypes mediated by hyperphosphatemia and remediated by phosphate-lowering interventions (diets low in phosphate or vitamin D; knockouts of 1α-hydroxylase, vitamin D receptor, or NaPi cotransporter). CKD can be seen as a state of hyperphosphatemia-induced accelerated aging associated with Klotho deficiency. Humans with CKD experience decreased Klotho expression as early as stage 1 CKD; Klotho continues to decline as CKD progresses, causing FGF-23 resistance and provoking large FGF-23 and parathyroid hormone increases, and hypovitaminosis D. Secreted Klotho protein, formed by extracellular clipping, exerts FGF-23-independent phosphaturic and calcium-conserving effects through its paracrine action on the proximal and distal tubules, respectively. We contend that decreased Klotho expression is the earliest biomarker of CKD and the initiator of CKD-MBD pathophysiology. Maintaining normal phosphate levels with phosphate binders in patients with CKD with declining Klotho expression is expected to reduce mineral and vascular derangements.

Claudin-5a in developing zebrafish brain barriers: another brick in the wall

Claudins serve essential roles in regulating paracellular permeability properties within occluding junctions. Recent studies have begun to elucidate developmental roles of claudins within immature tissues. This work has uncovered an involvement of several claudins in determining tight junction properties that have an effect on embryonic morphogenesis and physiology. During zebrafish brain morphogenesis, Claudin-5a determines the paracellular permeability of tight junctions within a transient neuroepithelial-ventricular barrier that maintains the hydrostatic fluid pressure required for brain ventricular lumen expansion. However, the roles of Claudins in development may well extend beyond being mere junctional components. Several post-translational modifications of Claudins have been characterized that indicate a direct regulation by developmental signals. This review focuses on the involvement of Claudin-5a in cerebral barrier formation in the zebrafish embryo and includes some speculations about possible modes of regulation.

Is fiber-type composition related to daily jaw muscle activity during postnatal development?

AIM

Muscles containing large numbers of slow-contracting fibers are generally more active than muscles largely composed of fast fibers. This relationship between muscle activity and phenotype suggests that (1) changes in fiber-type composition during postnatal development are accompanied by changes in daily activity and (2) individual variations in fiber-type composition are related to similar variations in daily muscle activity.

METHODS

The masseter and digastric muscles of 23 New Zealand White rabbits (young, juvenile and adult) were examined for their phenotype (myosin heavy chain content) and their daily activity (total daily number of activity bursts).

RESULTS

During development, the masseter showed a strong increase in the number of fast-type fibers compared to the number of slow-type fibers. During development, also the number of powerful bursts in the masseter increased. The digastric showed no significant changes in fiber types or burst numbers. Within each muscle, across individual animals, no significant correlations (R < 0.70) were found between any of the fiber types and daily burst numbers in any of the age groups.

CONCLUSIONS

The results suggest that activity-related influences are of relatively minor importance during development and that other factors are dominant in determining fiber-type composition.