P16 is an essential regulator of skeletogenesis in the sea urchin embryo

Abstract

The primary mesenchyme cells (PMCs) of the sea urchin embryo undergo a dramatic sequence of morphogenetic behaviors that culminates in the formation of the larval endoskeleton. Recent studies have identified components of a gene regulatory network that underlies PMC specification and differentiation. In previous work, we identified novel gene products expressed specifically by PMCs (Illies, M.R., Peeler, M.T., Dechtiaruk, A.M., Ettensohn, C.A., 2002. Identification and developmental expression of new biomineralization proteins in the sea urchin, Strongylocentrotus purpuratus. Dev. Genes Evol. 212, 419-431). Here, we show that one of these gene products, P16, plays an essential role in skeletogenesis. P16 is not required for PMC specification, ingression, migration, or fusion, but is essential for skeletal rod elongation. We have compared the predicted sequences of P16 from two species and show that this small, acidic protein is highly conserved in both structure and function. The predicted amino acid sequence of P16 and the subcellular localization of a GFP-tagged form of the protein suggest that P16 is enriched in the plasma membrane. It may function to receive signals required for skeletogenesis or may play a more direct role in the deposition of biomineral. Finally, we place P16 downstream of Alx1 in the PMC gene network, thereby linking the network to a specific "effector" protein involved in biomineralization.

Par6 regulates skeletogenesis and gut differentiation in sea urchin larvae

“…Furthermore, skeletogenic cells expressed p16 that is essential for skeletal rod elongation (Fig. 3n,o;Cheers and Ettensohn 2005). However, the size of the PMC ring in Mpar6 larvae was smaller than in controls, probably due to dwarfish ectoderm patterning and/or a lack of accompanying growth of skeletal rods in the experimental larvae.…”

“…Spicule matrix proteins, such as SM30 and SM50, are tethered to calcite deposited in the vacuole (Seto et al, 2004), whereas MSP130 and P16 are localized on the surface of PMCs and their filopodia Cheers and Ettensohn 2005).…”

“…3), suggesting that Par6 may play a more immediate role in the deposition of biomineral. Like Par6, P16 acts at relatively late step in skeletogenesis; P16 is not required for PMC specification, ingression, migration, or fusion, but essential for skeletal rod elongation (Cheers and Ettensohn 2005). Cheers and Ettensohn (2005) suggest that P16 may be a receptor for ectoderm-derived signals required for skeletal rod elongation; alternatively, P16 may function in the plasma membrane or an intracellular membrane compartment to regulate calcium transport.…”

“…Like Par6, P16 acts at relatively late step in skeletogenesis; P16 is not required for PMC specification, ingression, migration, or fusion, but essential for skeletal rod elongation (Cheers and Ettensohn 2005). Cheers and Ettensohn (2005) suggest that P16 may be a receptor for ectoderm-derived signals required for skeletal rod elongation; alternatively, P16 may function in the plasma membrane or an intracellular membrane compartment to regulate calcium transport. Since phenotypes of p16 knockdown resemble those of chimeric embryos with Mpar6 present only in PMCs, it seems possible that Par6 relays signals transduced by P16 or cooperates with P16 to deposit biomineral.…”

Lessons from a gene regulatory network: echinoderm skeletogenesis provides insights into evolution, plasticity and morphogenesis

“…Spicule matrix proteins have a characteristic structure that consists of a single C-lectin domain and often (but not always) a region of short repeats that are rich in proline, glycine, glutamine and/or asparagine residues (Illies et al, 2002). A small family of acidic, serine-rich, PMC-specific transmembrane proteins has also been identified, and the founding member (P16) has been shown to play an important role in skeletal rod elongation (Cheers and Ettensohn, 2005). Other biomineralization-related proteins include several collagens and a PMC-specific carbonic anhydrase (Livingston et al, 2006).…”