High Mobility of Bicoid Captured by Fluorescence Correlation Spectroscopy: Implication for the Rapid Establishment of Its Gradient

Abstract

The Bicoid (Bcd) morphogen is essential for pattern formation in fruit flies. It forms an exponential concentration gradient along the embryo AP axis and turns on cascades of target genes in distinct anterior domains. The most commonly accepted model for gradient formation assumes that Bcd travels by simple diffusion and is uniformly degraded across syncytial embryos, yet several recent studies have challenged these ideas. Here, the question of Bcd mobility was investigated using fluorescence correlation spectroscopy in live Drosophila melanogaster embryos. Bcd-EGFP molecules were found to be highly mobile in the cytoplasm during cycles 12-14, with a diffusion coefficient approximately 7 microm(2)/s. This value is large enough to explain the stable establishment of the Bcd gradient simply by diffusion before cycle 8, i.e., before the onset of zygotic transcription.

Morphogen transport

“…The morphogens Dpp and Wingless (Wg) have relatively short ranges in the Drosophila wing disc (Entchev et al, 2000;Teleman and Cohen, 2000;Kicheva et al, 2007;Bollenbach et al, 2008), Bicoid (Bcd) has a mid-range in Drosophila embryos (Driever and Nüsslein-Volhard, 1988a;Driever and Nüsslein-Volhard, 1988b;Gregor et al, 2007;Abu-Arish et al, 2010;Castle et al, 2011;Drocco et al, 2011), and Nodal and FGF8 can act over longer distances in zebrafish embryos (Gritsman et al, 2000;Chen and Schier, 2001;Harvey and Smith, 2009;Yu et al, 2009;Müller et al, 2012). Intriguingly, Lefty has a much longer range than Nodal, even though both molecules are TGFβ superfamily members and expressed in zebrafish embryos.…”

“…Low values indicate a short range, whereas large values indicate a long range. For comparison, the absolute range λ= ͱ D/k --is shown in B. λ and 1/φ were calculated from literature values of D and k (Gregor et al, 2007;Kicheva et al, 2007;Yu et al, 2009;Abu-Arish et al, 2010;Castle et al, 2011;Drocco et al, 2011;Wartlick et al, 2011;Müller et al, 2012;Zhou et al, 2012). (Teleman and Cohen, 2000).…”

Characterizing anomalous diffusion in crowded polymer solutions and gels over five decades in time with variable-lengthscale fluorescence correlation spectroscopy

“…In biological systems, however, and in particular in cells, diffusive motions are often found to deviate from simple Fickian diffusion 1 . This deviation can take dif-ferent forms, from an apparent size-dependent viscosity of the medium to a non-Gaussian distribution of displacements or to a mean-squared displacement that is not directly proportional to time [2][3][4][5][6][7][8][9][10][11][12][13] . Similar effects can be recreated in vitro, using for example polymer solutions [14][15][16] , gels [17][18][19][20] or colloidal suspensions 21 .…”

Multiscale diffusion in the mitotic Drosophila melanogaster syncytial blastoderm

“…3). Fluorescence correlation spectroscopy measurements, which essentially measure the cytoplasmic diffusion coefficient and should not be expected to be affected by membrane furrows, have suggested a high cytoplasmic diffusion coefficient for Bicoid (16). Our multiscale modeling provides an explanation for the discrepancy between the physiological diffusion coefficient of Bicoid measured by these different techniques.…”

“…Quantitative assessment of the actual diffusion coefficient of Bicoid has remained controversial, with some recent estimates differing by several orders of magnitude (5,(15)(16)(17). Previous FRAP experiments have suggested that the diffusion coefficient is too small for the simple SDD model to explain the Bicoid gradient (15).…”