Morphogenesis of Digits in the Avian Limb Is Controlled by FGFs, TGFβs, and Noggin through BMP Signaling

Abstract

In the final stages of limb morphogenesis, autopodial cells leaving the progress zone differentiate into cartilage or undergo apoptotic cell death, depending on whether they are incorporated into the digital rays or interdigital spaces. Most evidence indicates that these two opposite fates of the autopodial mesoderm are controlled by BMP signaling. However, the molecular basis for these two distinct actions of BMPs, including the receptors involved in the process, is controversial. In this study we have addressed this question by exploring the presence in the developing autopod of diffusible signals able to modulate BMP function and by analyzing the effects of their exogenous administration on the pattern of expression of BMP receptor genes. Our findings show that tgfbeta2 and noggin genes are expressed in the condensing region of the developing digital rays in addition to the well-known distribution in the autopodial tissues of FGFs (apical ectodermal ridge, AER) and BMPs (AER, progress zone mesoderm, and interdigital regions). Exogenous administration of all the factors causes changes in the expression of the bmpR-1b gene which are followed by parallel alterations of the skeletal phenotype: FGFs inhibit the expression of bmpR-1b compatible with their function in the maintenance of the progress zone mesoderm in an undifferentiated state; and TGFbetas induce the expression of bmpR-1b and promote ectopic chondrogenesis, compatible with a function in the establishment of the position of the digital rays. In addition we provide evidence for the occurrence of an interactive loop between BMPs and noggin accounting for the spatial distribution of bmpR-1b which may control the size and shape of the skeletal pieces. In contrast to the bmpR-1b gene, the bmpR-1a gene is expressed at low levels in the autopodial mesoderm and its expression is not modified by any of the tested factors regardless of their effects on chondrogenesis or cell death. Finally, the role of BMPs in programmed cell death is confirmed here by the intense inhibitory effect of noggin on apoptosis, but the lack of correlation between changes in the pattern of cell death induced by treatment with the studied factors and the expression of either bmpR-1a or bmpR-1b genes suggest that a still-unidentified BMP receptor may account for this BMP function.

FGF18 Represses Noggin Expression and Is Induced by Calcineurin

“…In a similar mechanism, FGF signaling was shown to suppress noggin expression in cranial sutures (50), thus explaining the premature fusion of cranial suture in individuals with activating mutations of FGF receptors 1 and 2. Experiments of chicken limb development have also shown that the application of FGF bound to beads can suppress noggin expression (51). These results highlight the interplay of BMP and FGF signaling that regulates both the osteoblast and chondrocyte lineage to shape both endochondral and membranous bones.…”

“…Unregulated FGF signaling as in mice or humans with constitutively active FGF receptors may cause excessive inhibition of noggin expression and consequently develop abnormal digits or extremities. Noggin is widely expressed in condensing chondroblast mesenchyme and at the site of presumptive or developing joints (51). Therefore, local antagonism of BMP signaling by noggin may regulate the formation of joints and the shape of developing bone elements.…”

TGF-β Family Signaling in Connective Tissue and Skeletal Diseases

“…Cells within the medial layer express BMP-2 and subsequently undergo cell death, thereby forming the joint cavity, whereas the remaining outer layers will give rise to the articular cartilage (reviewed in Pacifici et al 2005). During joint formation, noggin is expressed within the intermedial layer of the interzone (Brunet et al 1998;Capdevila and Johnson 1998;Merino et al 1998;Goumans and Mummery 2000;Pizette and Niswander 2000;Seemann et al 2005;Lorda-Diez et al 2013), where it counteracts the effect of GDF-5, the key molecule required for defining the future joint interzone within the unpatterned digital ray. Loss of GDF-5 function leads to abnormal joint development with complete or partial fusion of consecutive skeletal elements and structural changes in autopods, along with other skeletal defects, whereas loss of noggin activity results in failure to initiate joint formation and thus symphalangism Kingsley 1996, 1999;Brunet et al 1998).…”

Mechanisms of digit formation: Human malformation syndromes tell the story

“…Bmps are expressed mainly in the interdigital mesenchyme and the perichondrium whereas their receptors, in particular BmpR1b, are expressed in the condensing chondrocytes. Chondrogenesis in the limb is dependent on BMP signaling, since its inhibition in the chicken limb by implantation of Noggin beads, or retroviral overexpression of dominant-negative BMP receptors, equally interfere with chondrogenesis (Zou et al, 1997;Merino et al, 1998). In the mouse ablation of both type I receptors (BmpR1a and BmpR1b) causes a severe generalized chondrodysplasia in which most skeletal elements that form through endochondroal ossification are absent or rudimentary (Yoon et al, 2005;Ovchinnikov et al, 2006).…”