Gremlin is the BMP antagonist required for maintenance of Shh and Fgf signals during limb patterning

Abstract

During limb outgrowth, signaling by bone morphogenetic proteins (BMPs) must be moderated to maintain the signaling loop between the zone of polarizing activity (ZPA) and the apical ectodermal ridge (AER). Gremlin, an extracellular Bmp antagonist, has been proposed to fulfill this function and therefore be important in limb patterning. We tested this model directly by mutating the mouse gene encoding gremlin (Cktsf1b1, herein called gremlin). In the mutant limb, the feedback loop between the ZPA and the AER is interrupted, resulting in abnormal skeletal pattern. We also show that the gremlin mutation is allelic to the limb deformity mutation (ld). Although Bmps and their antagonists have multiple roles in limb development, these experiments show that gremlin is the principal BMP antagonist required for early limb outgrowth and patterning.

Anteroposterior patterning in the limb and digit specification: Contribution of mouse genetics

“…However, Fgf8 expression is only slightly reduced (Kuhlman and Niswander, 1997). Therefore, the defects in limb patterning in the mutant are likely due to Shh down-regulation rather than to AER disruption (Khokha et al, 2003;Michos et al, 2004). Noticeably, Shh induces Gremlin in adjacent cells but not in Shh-producing cells or their descendants, which provides a mechanism to terminate the Shh-Fgf4 loop as ZPA-derived cells expand .…”

“…Shh is necessary to maintain the AER posteriorly (Laufer et al, 1994;Niswander et al, 1994). This maintenance is mediated by the inhibition of BMP activity (Bmp2, Bmp4, Bmp7), an anti-AER factor, through the production of Gremlin Khokha et al, 2003;Michos et al, 2004). As a result, Fgf4 is undetectable in the Gremlin mutant AER.…”

Msx1 and Msx2 in limb mesenchyme modulate digit number and identity

“…We thus analyzed the three Bmp genes expressed in limb development (Bmp2, Bmp4, Bmp7) as well as the BMP antagonist gene Gremlin, which has been shown to play a crucial role during limb development (Khokha et al, 2003). At 11.5 dpc, expression of these four genes was modified at the anterior of the forelimb bud, but in different ways: Bmp4 was down-regulated, whereas Bmp7 was moderately up-regulated and Bmp2 and Gremlin ectopically expressed (Fig.…”

“…Conversely, both Msx1 and Msx2 are up-regulated in the limb mesenchyme by a null mutation of Gremlin, a potent BMP inhibitor expressed in the limb (Khokha et al, 2003;Michos et al, 2004). Nevertheless, although a mesenchyme-specific null mutation of Bmpr1a leads to a drastic down-regulation of the two Msx genes (Ovchinnikov et al, 2006), residual expression is still detectable for both Msx1 and Msx2 anteriorly, raising the possibility that, in this region, Msx genes are controlled by other factors such as, for example, Gli3R (Lallemand et al, 2009).…”

Evolution of vertebrate appendicular structures: Insight from genetic and palaeontological data

“…Pr-D growth Wrist and digit elements greatly reduced (Sun et al, 2002) Upstream Known for Fgf4 (Fraidenraich et al, 1998) Grem1 Pr-D growth/AER maintenance Reduced digit number (Khokha et al, 2003) Downstream Known (Zuniga et al, 2004) Wnt7a Dorsal and posterior limb development Loss of dorsal phenotype and posterior most digits (Adamska et al, 2005) Upstream or parallel…”