Possible role of microtubules in vesicular transport of matrix protein during sea urchin larval biomineralization

Background: Biomineralization is a vital biological process through which organisms produce mineralized structures such as shells, skeletons, and teeth. Microtubules are essential for biomineralization in various eukaryotic species; however, their specific roles in this process remain unclear. Results: Here, we investigated the structure and function of microtubule filaments and their co-localization with matrix and focal adhesion proteins during the elongation of the calcite spicules of the sea urchin larva. First, we show that inhibiting microtubule polymerization using Nocodazole in whole embryos and isolated skeletogenic cell cultures results in a significant reduction of skeletal growth and affects skeletal morphology. Next, we demonstrate that microtubule filaments elongate from around the skeletogenic nuclei to the biomineralization compartment where they overlap with active focal adhesion kinase. The expression of spicule matrix proteins overlaps with microtubule filaments around the nuclei and with microtubule filaments that elongate to the spicule cavity. Conclusions: We propose that vesicles bearing matrix proteins are trafficked on microtubules to the spicule cavity where their exocytosis is assisted by focal adhesions. The role of microtubules in biomineralization from unicellular algae to human bones suggests that the proposed microtubule-guided vesicle transport into the biomineralization compartment could be a common mechanism in Eukaryotes' biomineralization.