Spatial distribution of cellular protein binding to retinoic acid in the chick limb bud

Abstract

Retinoic acid may be the natural morphogen used to generate digit pattern in the chick limb bud. It has been proposed that retinoic acid acts by binding to a cellular retinoic acid-binding protein (CRABP) and then entering the nucleus to alter the pattern of gene activity. High-affinity receptors that bind both retinoic acid and DNA and are analogous to the steroid receptors have been identified. But the concentration of endogenous retinoic acid in the limb and the binding coefficient of the nuclear receptors indicate that they are saturated throughout the limb. Here we investigate the CRABP distribution in the developing chick limb bud. We find CRABP in the area of intense morphogenetic activity at the tip, with a differential distribution across the anteroposterior axis, the high point being at the anterior margin. Retinoic acid also forms a concentration gradient across the limb bud, but is highest on the posterior side. We propose that CRABP could be reducing the effective concentration of retinoic acid reaching the nucleus to a level appropriate for the differential regulation of gene transcription, providing a spatially modulated morphogenetic gradient of information for digit formation.

Spatial and temporal pattern of expression of the cellular retinoic acid-binding protein and the cellular retinol-binding protein during mouse embryogenesis.

“…It has been proposed that CRABP and the nuclear retinoic acid receptors may cooperate in the mechanism of action of retinoic acid (24). Interestingly, CRABP has been detected in the developing chicken limb and forms a gradient that is opposite in direction to the gradient formed by retinoic acid (25).…”

Stage‐ and region‐dependent chondrogenesis and growth of chick wing‐bud mesenchyme in serum‐containing and defined tissue culture media

“…Members of the homeobox-containing Hoxa (formerly Hox-1 ) complex are expressed in partially overlapping domains along the limb bud's proximodistal axis (Yokouchi et al, 1991) and may regulate, to some extent, patterns of signalling and responsiveness along this axis. Other pattern-regulating genes with differential expression along this axis include the nuclear retinoic acid-binding protein, RAR-f% (Smith and Eichele, 1991;Schofield et al, 1992), and cellular retinoic acid-binding protein (Maden et al, 1988;Dolle et al, 1990).…”

Localization of CRABP‐I and CRABP‐II mRNA in the early mouse embryo by whole‐mount in situ hybridization: Implications for teratogenesis and neural development

“…However, their expression in structures known to be specifically vulnerable to excess RA (Ruberte et al, 19921, in a rhombomere specific fashion in the developing hindbrain (Maden et al, 1992;Ruberte et al, 1992), and in a gradient across the developing limb bud (Maden et al, 1988) suggests that these proteins may be crucial for maintaining the proper concentration of free RA available to the receptors. It has been proposed that CRABPs may modulate free RA levels by simply sequestering it and/or by facilitating its catabolism to polar metabolites (Fiorella and Napoli, 1991).…”