Co-option of Lysosomal Machinery for Biomineralization
Biomineralization evolved repeatedly across animals, resulting in novel strategies for buoyancy, locomotion, and defense. Sponges are the only metazoans that build a silica-based skeleton and the mechanisms underlying their biosilicification are relatively understudied compared to other major silicifiers. Here, we show that in freshwater sponges, cells specialized for biosilicification, termed sclerocytes, highly express lysosomal-associated genes, including the V-H+-ATPase (VHA) proton pump, oculocutaneous albinism type 2 (OCA2) anion channels, cathepsins, silicateins, TMEM55A/B, TMEM192, TMEM199, and other proteins involved in lysosomal maintenance and degradation. We demonstrate that VHA protein localizes to the sclerocyte silica deposition vesicle (SDV) and that VHA-dependent SDV acidification is essential for silica spicule formation. This function mirrors the role of VHA in diatom biosilicification and in biocalcification across a broad sweep of taxa. We also found that genes homologous to the plant silicon efflux transporter Lsi2 are highly upregulated in sclerocytes and confirmed their localization using Hybridization Chain Reaction Fluorescence In Situ Hybridization (HCR-FISH). These results corroborate phylogenetic and transcriptomic evidence that Lsi2 homologs are involved in sponge silicic acid transport. Data mining of previous studies revealed many of the sponge sclerocyte lysosomal-associated genes have been correlated with biomineralization across eukaryotes, including coccolithophores, bryozoans, crustaceans, mollusks, and mammals. As detoxification was a likely driver for the evolution of biomineralization, we propose that lysosomal machinery aided in detoxification by allowing sponges to sequester Si in acidified compartments to prevent its catastrophic intracellular precipitation. This lysosomal toolkit was likely independently co-opted for the formation of biomineralized structures in multiple species.