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D. Bacic (1,2) , P. Capuano(1),
C. A. Wagner(1), J. Biber(1), B. Kaissling(2), H Murer(1).
Institutes of (1) Physiology and (2) Anatomy, University of Zurich,
Switzerland |
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Renal handling of inorganic
phosphate (Pi) is essentiell for the control of body Pi-homeostasis.
Pi reabsorption in the renal proximal tubule (PT) occurs mostly via
the Na+/Pi cotransporter type IIa (NaPi-IIa) located in the brush border
membrane (BBM), and regulated, among other factors, by dietary Pi intake
and parathyroid hormone (PTH). The PTH-induced inhibition of Pi-reabsorption
is mediated by endocytosis of Na/Pi-cotransporters from the BBM and
subsequent lysosomal degradation. Internalization of NaPi-IIa occurs
at the invaginated membrane regions at the base of brush border.
Megalin, an integral membrane protein of the LDL receptor family is
involved in receptor-mediated endocytosis of proteins in the renal proximal
tubule. Megalin is expressed at the base of the BBM microvilli and throughout
the subapical endocytotic apparatus, similar to NaPi-IIa. The recently
identified receptor associated protein (RAP) is a novel type of chaperone
responsible for the biosynthesis and intracellular transport of endocytic
receptors such as megalin. Gene disruption of RAP leads to a decrease
of megalin in the BBM and to a disturbed proximal tubular endocytotic
machinery. This prompted us to investigate whether the distribution
of NaPi-IIa and/or its regulation by dietary Pi intake and PTH is affected
in the proximal tubules of RAP kockout mice as a model for megalin deficiency.
In RAP deficient mice NaPi-IIa protein distribution in the kidney was
normal but the abundance was slightly reduced. Also the distribution
of MAP17, CAP1, and NHE-RF-1, proteins interacting with NaPi-IIa, was
normal whereas the abundance of NHE-RF1 was reduced. In whole animals
and in isolated fresh kidney slices the PTH-induced internalization
of NaPI-IIa was strongly delayed in RAP deficient mice compared to wildtype.
However, PTH receptor expression in the proximal tubule was not affected
by the RAP knock-out as well as the ability of cAMP, cGMP or PKC activators
to induce internalization. The cAMP, cGMP and PKC activation induced
internalization was also delayed in RAP deficient mice. In contrast,
both wildtype and RAP deficient mice were able to adapt to high, normal
and low Pi containing diets appropriately as indicated by urinary Pi
excretion and NaPi-IIa protein abundance.
In conclusion, the loss of RAP reduces the expression of megalin, NHE-RF1
and to a lesser extent of NaPi-IIa. The PTH induced internalization
of NaPi-IIa is slowed down in RAP deficient mice. The role of megalin
and NHE-RF1 in this process will need further investigation to clarify
their contribution to the PTH-induced endocytosis of NaPi-IIa.
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