Activation of TRPV4 promotes osteoclasts differentiation
- Ritsuko Masuyama1
© Masuyama; licensee BioMed Central Ltd. 2012
Published: 29 February 2012
Osteoclast differentiation is critically dependent on cellular calcium (Ca2+) signaling. Intracellular Ca2+ concentration ([Ca2+]i) is regulated by two flux pathways; Ca2+ oscillations evoked by the release of Ca2+ from the endoplasmic reticulum, and/or Ca2+ entry from the extracellular fluid. The latter is carried out by the plasmamembrane localized Ca2+ permeable channel such as "transient receptor potentials (Trps)". Trpv4-deficient mice show an increased bone mass due to impaired osteoclast maturation, because Trpv4 mediates Ca2+ influx at the late stage of osteoclast differentiation and hereby regulates Ca2+ signaling . Furthermore, substitutions of amino acids R616Q/V620I of Trpv4 have been discovered as gain of function mutations resulting in increased Ca2+ transport . Since the region of these substitutions at the trans-membrane pore domain is perfectly conserved between species, we created a mutant of the mouse Trpv4 (Trpv4R616Q/V620I) and characterized it on Ca2+ signaling especially in the occurrences of oscillations at the initial step of osteoclast differentiation.
Intact Trpv4 and Trpv4R616Q/V620I were equally transduced by retroviral infection into bone marrow derived hematopoietic cells isolated from WT mice, and mock-transfection was used as control. The resorptive activity was significantly increased in Trpv4R616Q/V620I-expressing osteoclasts when treated with RANKL for 7 days, associating increased NFATc1 and calcitonin receptor mRNA expression. Noteworthy, the expression of these differentiation markers was already elevated in Trpv4R616Q/V620I cells before RANKL treatment, suggesting that the activation of Trpv4 advances osteoclast differentiation through Ca2+-NFATc1 pathway. Accordingly, basal [Ca2+]i, analyzed in progenitor cells treated with RANKL for 24 hr, increased 2 fold in intact Trpv4 (p < 0.05) and 3 fold in Trpv4R616Q/V620I (p < 0.01) compared to controls. Although spontaneous Ca2+ oscillations were absent in control progenitor cells, Trpv4R616Q/V620I progenitor cells already displayed irregular oscillatory pattern.
In summary, our findings provide evidences that the activation of Ca2+ permeable channel supports Ca2+ oscillations in progenitor cells and therefore promotes the potential of osteoclast differentiation.
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