Scalers hypothesis and its confirmation in sea urchin embryos
The phenomenon of embryonic scaling, the ability of embryos to regulate their structure in proportion to size, is a fascinating yet underexplored area of study. First described in sea urchin embryos by Hans Driesch, this phenomenon is now recognized as a striking example of how living organisms use non-equilibrium self-organization to generate pattern-determining morphogen gradients that scale with embryo size. Although specific molecular mechanisms for scaling morphogens gradients have been described in some cases, a general approach for the targeted identification  such mechanism had not been elaborated until recently. In search of a solution, we hypothesized the obligatory participation in the scaling mechanisms of special genes, which we named scalers, with their expression being sensitive to the size of the embryo and their protein products regulating the scale of morphogen gradients. As proof of principle, we recently identified scalers by detecting differentially expressing genes in wild-type and half-size Xenopus laevis gastrula embryos. We also described a novel mechanism by which one of the identified scalers, the gene encoding metalloproteinase 3 (Mmp3), modulates gradients of the morphogenic protein Bmp and its antagonists Chordin and Noggin1/2 based on Xenopus laevis embryo size. To test the universality of the scalers hypothesis, we now applied our method of identifying scalers that adjust Bmp/Chordin gradients to the size of the sea urchin embryo, Strongylocentrotus droebachiensis. Our results show that at least two members of the gene cluster encoding astacin proteinases of the Span family, namely bp10 and Span, exhibit properties characteristic of scalers in the embryos of this species. Notably, their expression increases in half-size embryos, and their protein products specifically digest Chordin. Additionally, we found that the loss of function of bp10 and span lead to contraction of the ventral domain of the Bmp signaling nuclear effector, pSmad1/5. These findings not only validate the scalers hypothesis but also uncover a novel mechanism by which Span proteinases fine-tune Chordin and Bmp morphogen gradients in sea urchins, offering promising avenues for future research into scaling mechanisms across biological systems. Overall design: Three biological replicates were analyzed for 2 conditions (intact embryos vs their siblings divided at two-blastomere stage and grown to mesenchyme blastula stage)