Abstract
Efficient pH regulation is a fundamental requisite of all calcifying systems in animals and plants but with the underlying pH regulatory mechanisms remaining largely unknown. Using the sea urchin larva, this work identified the SLC4 HCO3- transporter family member SpSlc4a10 to be critically involved in the formation of an elaborate calcitic endoskeleton. SpSlc4a10 is specifically expressed by calcifying primary mesenchyme cells with peak expression during de novo formation of the skeleton. Knock-down of SpSlc4a10 led to pH regulatory defects accompanied by decreased calcification rates and skeleton deformations. Reductions in seawater pH, resembling ocean acidification scenarios, led to an increase in SpSlc4a10 expression suggesting a compensatory mechanism in place to maintain calcification rates. We propose a first pH regulatory and HCO3- concentrating mechanism that is fundamentally linked to the biological precipitation of CaCO3. This knowledge will help understanding biomineralization strategies in animals and their interaction with a changing environment.
Highlights
Sea urchin larvae with their elaborate calcareous endoskeleton have been studied by embryologists for over a century to promote our understanding of calcification in biological systems (Boveri, 1901; Decker and Lennarz, 1988; Wilt, 2002)
Similar to mammalian osteoblasts that arise from mesenchymal stem cells (MSC), the sea urchin larval skeleton is produced by a specific cell line – the primary mesenchyme cells (PMCs) (Wilt, 2002)
Among the four SLC4 transporters identified in the sea urchin genome (SpSlc4a3, SpSlc4a2, SpSlc4a11 and SpSlc4a10) the PMC specific SpSlc4a10 clusters within the clade of the Slc4a7-10, electroneutral Na+-coupled HCO3- transporters found in vertebrates (Figure 1—figure supplement 2)
Summary
Sea urchin larvae with their elaborate calcareous endoskeleton have been studied by embryologists for over a century to promote our understanding of calcification in biological systems (Boveri, 1901; Decker and Lennarz, 1988; Wilt, 2002). This process involves at least 40 distinct skeletal matrix proteins supporting the formation of the mature calcite spicules within this extracellular space (Beniash et al, 1997; Benson et al, 1986) Some of these matrix proteins are present in intracellular compartments where they may play a role in the stabilization of ACC (Urry et al, 2000; Wilt, 2002). Recent findings suggest that Ca2+ enters calcification vesicles by endocytosis of seawater into a vesicular network within the PMCs (Vidavsky et al, 2016)
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