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

Abstract

The zone of polarizing activity is the primary signaling center controlling anterior-posterior patterning of the amniote limb bud. The autopodial interdigits (IDs) are secondary signaling centers proposed to determine digit identity by acting on the cells of the digital ray. Here, we focus on events accompanying digital fate determination and define a region of the digital ray that expresses Sox9 and Bmpr1b and is phosphorylated-SMAD1/5/8 (p-SMAD1/ 5/8) positive. We name this region the phalanx-forming region (PFR), and show that the PFR cells arise from the distal subridge mesenchyme of digital ray. This phalanx-forming cell lineage is subsequently committed to the cartilage lineage; the fate of these cells is initially labile but becomes fixed as they are incorporated into the condensed cartilage of the digit primordium. Using an in vivo reporter assay, we establish that each digital PFR has a unique p-SMAD1/5/8 activity signature. In addition, we show that changes in this activity correlate with the identity of the digit that forms after experimental manipulation, supporting the idea that threshold signaling levels can lead to different developmental outcomes in a morphogenetic field. Our data define the molecular profile of the PFR, and we propose a model for understanding formation and variation of digits during autopodial development.Bmpr1b ͉ brachydactyly ͉ limb development

Mechanisms of digit formation: Human malformation syndromes tell the story

“…Suzuki et al (2008) elegantly demonstrated that there is a specific signature of BMP/Smad1/5/8 signaling in the DC/PFR for the different digits of the chicken foot, implicating the DC/PFR in the establishment of digit identity. However, the digit having the highest number of phalanges (digit 4) did not show the highest level of 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.…”

TGF-β Family Signaling in Connective Tissue and Skeletal Diseases

“…Interlinked feedback loops between AER, ZPA, and mesenchymal BMP signaling regulate initiation, propagation, and termination of the limb signaling system (reviewed in Benazet et al 2009;Zeller et al 2009). Disruption of these feedback loops result in impaired progression of limb bud development and specification of digit identity (Kawakami et al 1996;Ahn et al 2001;Khokha et al 2003;Ovchinnikov et al 2006;Montero et al 2008;Suzuki et al 2008;Benazet et al 2009). …”

Vertebrate Limb Development: Moving from Classical Morphogen Gradients to an Integrated 4-Dimensional Patterning System

“…The targets of this instructive BMP signaling from the ID mesenchyme are the cells located at the distal tip of the forming digit models. This region of mesenchyme located directly under the AER is called the phalanx-forming region (PFR) (Suzuki et al 2008). Suzuki and coworkers established that PFR of a particular digit is characterized by its unique BMP activity signature (green shaded areas in Fig.…”