Co-option of Lysosomal Machinery for Biomineralization

Biomineralization first emerged [~]600 million years ago, giving rise to novel strategies for buoyancy, locomotion, and defense.1 Sponges are the only metazoans that build a silica-based skeleton,2 and the cellular and molecular mechanisms underlying their silicification process are much less understood than in diatoms and land plants, the other two major biosilicifiers. Here, we show that the cells specialized for sponge biosilicification, termed sclerocytes, highly express lysosomal-associated genes, including the V-H+-ATPase (VHA) proton pump, oculocutaneous albinism type 2 (OCA2) anion channels, cathepsin-L proteins, silicateins, TMEM55A, TMEM192, TMEM199, and multiple other proteins involved with lysosomal maintenance and degradation. Further, 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 localization and role of VHA in diatom biosilicification,3 while in land plants, acidification occurs extracellularly in the xylem and is driven by a different proton pump, the plasma membrane P-type H+-ATPase.4,5 These results indicate that vesicle acidification is a shared mechanism for silica handling in sponges, diatoms, and plants. We also found that genes homologous to the plant silicon efflux transporter Lsi2,6 which belong to the same gene family as OCA2,7,8 are upregulated in sclerocytes. Using Hybridization Chain Reaction Fluorescence In Situ Hybridization (HCR-FISH), we found that sponge Lsi2 genes are expressed in sclerocytes, corroborating prior phylogenetic and transcriptomic evidence that Lsi2 homologs are involved in sponge and diatom silicic acid transport.7-10 Taking into account previous work showing that VHA, cathepsins, and other lysosomal-associated genes have been associated with calcification across a remarkable range of eukaryotes including coccolitophores,11 foraminifera,12 sea urchins,13 bryozoans,14 crustaceans,15 mollusks,16 and mammals,17 we posit that the various biomineralization mechanisms observed throughout the tree of life arose convergently through the co-option of an ancestral lysosomal gene toolkit.