Messages do diffuse faster than messengers: The case of the Bicoid gradient formation

PONCE DAWSON, SILVINA

Conferencia; Dynamics Days South America; 2012

Diffusion is key to many physiologically relevant processes. It is particularly relevant for the formation of morphogen gradients which induce the differentiation of otherwise undifferentiated cells. One of the most widely studied morphogens is Bicoid (Bcd) which distribution is determinant for the organization of the anterior-posterior axis in Drosophila embryos. The exponential dependence of the Bcd concentration with the distance along the embryo is compatible with the so called SDD model in which Bcd is synthesized locally at the anterior pole, and then diffuses and decays into the embryo. Within this model, the actual value of the diffusion coefficient of Bcd is critical to set the timescale of gradient formation. This coefficient has been estimated using two optical techniques, Fluorescence Recovery After Photobleaching (FRAP) and Fluorescence Correlation Spectroscopy (FCS), obtaining estimates in which the FCS value is faster by an order of magnitude than the FRAP one. In this talk I will show the results of a collaborative work with Lorena Sigaut, Alejandro Colman-Lerner and John E. Pearson in which we use a simple biophysical model to deal with this issue. In particular I will show that both the FRAP and FCS estimates may be correct and compatible with the observed timescale of Bcd gradient formation if a significant fraction of Bcd is bound to slowly diffusing partners. The discrepancy arises from the fact that FCS and FRAP report on different effective coefficients, one of which describes the spreading rate of the individual Bcd molecules (the messengers) and the other one that of their concentration (the message).