An interdigit signalling centre instructs coordinate phalanx-joint formation governed by 5′Hoxd–Gli3 antagonism

Abstract

The number of phalanges and joints are key features of digit ‘identity' and are central to limb functionality and evolutionary adaptation. Prior chick work indicated that digit phalanges and their associated joints arise in a different manner than the more sparsely jointed long bones, and their identity is regulated by differential signalling from adjacent interdigits. Currently, there is no genetic evidence for this model, and the molecular mechanisms governing digit joint specification remain poorly understood. Using genetic approaches in mouse, here we show that functional 5′Hoxd–Gli3 antagonism acts indirectly, through Bmp signalling from the interdigital mesenchyme, to regulate specification of joint progenitors, which arise in conjunction with phalangeal precursors at the digit tip. Phalanx number, although co-regulated, can be uncoupled from joint specification. We propose that 5′Hoxd genes and Gli3 are part of an interdigital signalling centre that sets net Bmp signalling levels from different interdigits to coordinately regulate phalanx and joint formation.

John Saunders' ZPA, Sonic hedgehog and digit identity – How does it really all work?

“…As mentioned above, such a relay has been suggested by certain experiments in chick demonstrating potentiation of Shh by Bmp signaling (Drossopoulou et al, 2000). In addition, work in both chick and mouse has provided clear evidence for ongoing late regulation of digit identity that is non-autonomous but occurs well after normal Shh expression has ceased (Dahn and Fallon, 2000; Huang et al, 2016; Suzuki et al, 2008), indicating the likelihood of relay mechanisms involving a signal cascade, but the nature of the factors involved remains contentious.…”

“…This distinction has complicated analysis and interpretation of ZPA/Shh function despite its attractiveness as a model. The readout, of necessity, is a late, very delayed assay of how Shh may function to provide positional information that cannot be assessed until well after Shh signaling has ceased (several days later in both mouse and chick), and after late-stage regulatory signals have intervened to regulate final identity (Dahn and Fallon, 2000; Huang et al, 2016; Sanz-Ezquerro and Tickle, 2003; Suzuki et al, 2008). Any early reporter would require the use of marker gene expression, since relying strictly on the position of early condensations ignores potential transformations in identity.…”

Expression analysis of limb element markers during mouse embryonic development

“…Meech et al, (2005) reported Gdf5 expression in digit rays at E11.5 and in developing cartilage condensations by E13.5; however, we do not see Gdf5 expression exclusively across developing joint sites until E14.5 (Meech et al, 2005; Ota et al, 2007; Seemann et al, 2005). This difference may be due to strain differences in embryonic development, which can exhibit differences in gestation periods (Degenkolbe et al, 2013; Huang et al, 2016). Gdf5 is expressed across developing joint sites, including interphalangeal joints, from E14.5 (Chen et al, 2016; Houweling et al, 2001; Huang et al, 2016; Sohaskey et al, 2009).…”

“…This difference may be due to strain differences in embryonic development, which can exhibit differences in gestation periods (Degenkolbe et al, 2013; Huang et al, 2016). Gdf5 is expressed across developing joint sites, including interphalangeal joints, from E14.5 (Chen et al, 2016; Houweling et al, 2001; Huang et al, 2016; Sohaskey et al, 2009). Our findings for Scleraxis expression are in agreement with previously published analyses of Scleraxis , indicating expression in the autopod and in developing digit rays and cartilage condensations at E12.5, as well as defined expression in developing tendons at E13.5 (Huang et al, 2015; McGlinn et al, 2005; Schweitzer et al, 2001) and along developing digits, with strongest expression across sites of developing interzones at E14.5 (Murchison et al, 2007; Schweitzer et al, 2001).…”

“…Distinct aspects of Gdf5 expression have been reported in several publications that combined have shown Gdf5 expression between E11.5 and E14.5 in the presumptive sites of joint development and the forming joint interzone (Chen et al, 2016; Degenkolbe et al, 2013; Hellman et al, 2012; Houweling et al, 2001; Huang et al, 2016; Meech et al, 2005; Ota et al, 2007; Perez et al, 2010; Seemann et al, 2005; Sohaskey et al, 2008). Uniquely, we have shown side‐by‐side Gdf5 expression across a range of key stages of development in both the forelimb and hindlimb.…”