Expression and function of Bapx1 during chick limb development

Abstract

The homeobox-containing transcription factor Bapx1 (also known as Nkx3.2) is crucial for development of the axial skeleton and parts of the chondrocranium. Here we describe the detailed expression of Bapx1 during chick limb development and show that in contrast to its expression in the axial skeleton, Bapx1 is expressed after the commitment to chondrogenesis. Bapx1 is initially expressed throughout the developing skeletal elements prior to the overt differentiation of the distinct chondrogenic layers. Once distinct layers (proliferating, prehypertrophic and hypertrophic) have formed, Bapx1 expression is restricted to the proliferating chondrocytes. Bapx1 transcripts are excluded from the articular cartilage. A second homeobox-containing transcription factor, Barx1, is expressed in a complementary fashion in the developing joint and articular cartilage. Interestingly, in vitro functional analyses showed that Bapx1 overexpression in micromass cultures increased both matrix production and nodule number suggesting that Bapx1 is sufficient to promote chondrogenesis in the limb. In contrast, Barx1 had the opposite effect on nodule number suggesting that it has an inhibitory effect on chondrogenic initiation consistent with its expression in the developing joint. A slight increase in matrix levels was also observed consistent with its expression in the articular chondrocytes. Finally, we show that Bapx1 is also expressed in the soft tissues such as the developing tendons, muscle sheaths and surrounding mesenchyme, and therefore may have additional as yet uncharacterized roles in limb morphogenesis.

Nkx3.2/Bapx1 acts as a negative regulator of chondrocyte maturation

“…Its expression is restricted to the immature, proliferative chondrocytes within the developing cartilage element, and is specifically downregulated as cells begin to express the prehypertrophic marker Ihh, and to withdraw from the cell cycle. This expression pattern is consistent with that very recently described by Francis-West and colleagues, who noted that this gene is also expressed in soft tissues, such as developing tendons, muscle sheaths and surrounding mesenchyme, in developing chicken limbs (Church et al, 2005).…”

“…Nkx3.2/Bapx1 expression, in the skeletal elements of chick wings and mouse limbs, coincides with regions of immature and proliferative chondrocytes; its expression declines in chondrocytes that become prehypertrophic, and is either undetectable or is only detected at trace levels in hypertrophic chondrocytes (see also Church et al, 2005). Retroviral misexpression of Nkx3.2 in chicken wings resulted in a severe delay of chondrocyte maturation, indicating that Nkx3.2 expression needs to be downregulated to allow chondrocyte maturation to proceed.…”

Identification of Fibroblast Growth Factor-18 as a Molecule to Protect Adult Articular Cartilage by Gene Expression Profiling

“…Barx1, a homeobox-containing transcription factor, is known to be expressed in the developing joint interzone cells (15), which give rise to articular chondrocytes later (16,17). High expression of Barx1 in the adult articular cartilage shown by the present study implies that it may also play an important role not only in the formation but also in the maintenance of articular cartilage.…”

Indian Hedgehog signalling triggers Nkx3.2 protein degradation during chondrocyte maturation

“…Nkx3.2-deficient mice are perinatal lethal and exhibit significant skeletal phenotypes, including major hypoplasia in vertebrae [8-11]. Interestingly, during cartilage development, Nkx3.2 expression is maintained at high levels in early-stage chondrocytes, while it is down-regulated in terminal-stage chondrocytes [1, 2, 12, 13]. Consistent with these expression patterns, Nkx3.2 has been reported to support cell viability for early-stage chondrocytes [13, 14], whereas chondrocyte hypertrophy can be inhibited by Nkx3.2 [2, 15, 16].…”