Biophysical analysis of the zebrafish morphogen Squint.

The Nodal signal Squint (Sqt) acts as a morphogen during early zebrafish embryogenesis. Quantitative live imaging of fluorescently tagged Sqt was used to investigate the dynamics of Sqt distribution. Sqt-GFP was localized extracellularlly and formed punctate intracellular accumulations. Sqt-GFP generated from a local source distributed rapidly and formed a concentration gradient following a single exponential profile with a length constant of 52.5 + 20.8 mum. This range of Sqt-GFP distribution was consistent with observed ranges of downstream target gene induction. FRAP (Fluorescence Recovery after Photobleaching) analysis revealed a diffusion coefficient of 2.2 + 0.7 mum2/sec for Sqt-GFP. A degradation rate of 7.5 x 10-5 + 6.2 x 10-6 sec-1 was measured by using Sqt fused to the photoconvertible protein Dendra. Simulations using a model based on diffusion restricted by endocytosis revealed general consistency between length constant, diffusion coefficient and degradation rate. Remarkably, the Sqt-GFP diffusion coefficient is ∼20 fold larger than the diffusion coefficient of Dpp-GFP in the Drosophila wing imaginal disc. These results, in conjunction with studies by others, suggest that different diffusion properties may underlie the spatial and temporal scaling of morphogen distribution in different developmental contexts.