Activin/TGFβ and BMP crosstalk determines digit chondrogenesis

Abstract

The progress zone (PZ) is a specialized area at the distal margin of the developing limb where mesodermal cells are kept in proliferation and undifferentiated, allowing limb outgrowth. At stages of digit morphogenesis the PZ cells can undergo two possible fates, either aggregate initiating chondrogenic differentiation to configure the digit blastemas, or to die by apoptosis if they are incorporated in the interdigital mesenchyme. While both processes are controlled by bone morphogenetic proteins (BMPs) the molecular basis for such contrasting differential behavior of the autopodial mesoderm remains unknown. Here we show that a well-defined crescent domain of high BMP activity located at the tip of the forming digits, which we termed the digit crescent (DC), directs incorporation and differentiation of the PZ mesenchymal cells into the digit aggregates. The presence of this domain does not correlate with an exclusive expression domain of BMP receptors and its abrogation by surgical approaches or by local application of BMP antagonists is followed by digit truncation and cell death. We further show that establishment of the DC is directed by Activin/TGFbeta signaling, which inhibits Smad 6 and Bambi, two specific BMP antagonists expressed in the interdigits and progress zone mesoderm. The interaction between Activin/TGFbeta and BMP pathways at the level of DC promotes the expression of the chondrogenic factor SOX9 accompanied by a local decrease in cell proliferation. Characteristically, the DC domain is asymmetric, it being extended towards the posterior interdigit. The presence of the DC is transitorily dependent of the adjacent posterior interdigit and its maintenance requires also the integrity of the AER.

Mechanisms of digit formation: Human malformation syndromes tell the story

“…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. Montero et al (2008) furthermore showed that the integrity of the DC/ PFR required AER signaling, which links the DC/PFR to the elongation model proposed by Sanz-Ezquerro and Tickle (2003). These studies altogether suggest that the outgrowth of digit condensations depends on a timely and spatially defined input of strong BMP/Smad1/5/8 signaling.…”

“…Two landmark studies have shed light on the molecular mechanisms responsible for this lineage commitment. The labs of Juan Hurle and John Fallon both demonstrated that a population of cells immediately in front of the growing digit condensation, the digit crescent (DC, Montero et al, 2008) or phalanx-forming region (PFR, Suzuki et al, 2008) controlled the incorporation of mesenchymal precursors into the condensation. This population was positive for the BMP receptor BmpR1b and the chondrogenic lineage marker Sox9 and showed high activity of the canonical BMP signaling pathway as it was positive for phosphorylated Smads 1, 5, and 8 (pSmad1/5/8).…”

“…Disturbance of BMP signaling at this phase leads to reduced size of the initial condensation resulting in brachydactyly syndromes. After establishment of the initial condensation, the DC/PFR assumes an organizer-like function and ''active'' elongation of the condensation is driven by means of strong BMP/pSmad1/5/8 signaling in this structure (Montero et al, 2008;Suzuki et al, 2008). This is performed in interplay with AER/Fgf signaling to keep the balance between maintenance of progenitors and chondrogenic differentiation.…”

Receptor tyrosine kinase-like orphan receptor 2 (ROR2) and Indian hedgehog regulate digit outgrowth mediated by the phalanx-forming region

“…Evidence from the chick indicates that bone morphogenetic protein (BMP)/pSMAD1/5/8 signaling in a population of cells in front of the growing condensation, referred to as the phalanxforming region (PFR) or digit crescent (3,4), is involved in the elongation of the digital rays. This work suggests that the PFR acts as a signaling center to drive distal elongation of the digit and thus determines the number of phalanges via commitment of distal mesenchymal cells to the cartilage condensation.…”

An Investigative Study Into the Relationship of Bone Morphogenetic Protein Antagonist Expression and Osteocyte Density by Region and Quadrant

“…As with many cell signaling factors, BMPs function's can be regulated along any step of the signaling cascade -either extracellularly, at the cell membrane or intracellularly (Montero, et al, 2008). One such regulation method is another family of cytokines referred to as bone morphogenetic protein antagonists, examples of which include noggin, gremlin, follistatin, and sclerostin.…”