Genetic Analysis of the Roles of BMP2, BMP4, and BMP7 in Limb Patterning and Skeletogenesis

Abstract

Bone morphogenetic protein (BMP) family members, including BMP2, BMP4, and BMP7, are expressed throughout limb development. BMPs have been implicated in early limb patterning as well as in the process of skeletogenesis. However, due to complications associated with early embryonic lethality, particularly for Bmp2 and Bmp4, and with functional redundancy among BMP molecules, it has been difficult to decipher the specific roles of these BMP molecules during different stages of limb development. To circumvent these issues, we have constructed a series of mouse strains lacking one or more of these BMPs, using conditional alleles in the case of Bmp2 and Bmp4 to remove them specifically from the limb bud mesenchyme. Contrary to earlier suggestions, our results indicate that BMPs neither act as secondary signals downstream of Sonic Hedghog (SHH) in patterning the anteroposterior axis nor as signals from the interdigital mesenchyme in specifying digit identity. We do find that a threshold level of BMP signaling is required for the onset of chondrogenesis, and hence some chondrogenic condensations fail to form in limbs deficient in both BMP2 and BMP4. However, in the condensations that do form, subsequent chondrogenic differentiation proceeds normally even in the absence of BMP2 and BMP7 or BMP2 and BMP4. In contrast, we find that the loss of both BMP2 and BMP4 results in a severe impairment of osteogenesis.

Mechanisms of digit formation: Human malformation syndromes tell the story

“…However, the digit having the highest number of phalanges (digit 4) did not show the highest level of BMP/Smad1/5/8 signaling. This corroborated findings from Bandyopadhyay et al (2006), suggesting that an anterior-posterior gradient of interdigital BMPs is not involved in the determination of digit identities. Rather, BMP signaling is required for initial chondrogenesis as outlined above and BMP/pSmad signaling appears to be the driving force for the outgrowth of the digit condensations by means of distally directed chondrogenesis.…”

“…There are several genes encoding members of the BMP superfamily expressed in the autopod mesenchyme, with Bmps 2, 4, and 7 being most prominent, but also expression of Bmp5 and Bmp6 was reported (Zuzarte-Luis et al, 2004;Bandyopadhyay et al, 2006). Genetic experiments indicate that Bmp2, 4, and 7 appear to act in partial redundancy in the mouse limb; however a critical threshold of BMP dosage appears to be required to allow chondrogenesis (Bandyopadhyay et al, 2006). Bmps are expressed mainly in the interdigital mesenchyme and the perichondrium whereas their receptors, in particular BmpR1b, are expressed in the condensing chondrocytes.…”

A second wave of Sonic hedgehog expression during the development of the bat limb

“…These mice also exhibit an expanded Shh expression domain and display posteriorly expanded limb buds. This phenotype is explained in terms of an enhanced Shh-Fgf interaction (16). A similar enhancement occurring naturally during bat limb development may result in a relative increase in cell proliferation and cell survival in the posterior as compared with the anterior autopod.…”

Unique SMAD1/5/8 activity at the phalanx-forming region determines digit identity

“…However, BMP4 and BMP7 mouse mutants show no digit identity changes, and BMP2 null is embryonic-lethal (18,19). A further study (20) reported that the limb-specific conditional BMP2:BMP4 and BMP2:BMP7 compound mouse mutants show no losses or shifts in digit identity. These authors concluded that these ligands are not the interdigital signal(s) affecting digit identity, and the issue of the interdigital ligand remains unresolved.…”