Proximodistal identity during vertebrate limb regeneration is regulated by Meis homeodomain proteins

Abstract

The mechanisms by which cells obtain instructions to precisely re-create the missing parts of an organ remain an unresolved question in regenerative biology. Urodele limb regeneration is a powerful model in which to study these mechanisms. Following limb amputation, blastema cells interpret the proximal-most positional identity in the stump to reproduce missing parts faithfully. Classical experiments showed the ability of retinoic acid (RA) to proximalize blastema positional values. Meis homeobox genes are involved in RA-dependent specification of proximal cell identity during limb development. To understand the molecular basis for specifying proximal positional identities during regeneration, we isolated the axolotl Meis homeobox family. Axolotl Meis genes are RA-regulated during both regeneration and embryonic limb development. During limb regeneration, Meis overexpression relocates distal blastema cells to more proximal locations, whereas Meis knockdown inhibits RA proximalization of limb blastemas. Meis genes are thus crucial targets of RA proximalizing activity on blastema cells.

Looking proximally and distally: 100 years of limb regeneration and beyond

“…Left: As the host hindlimb blastema grows and redifferentiates, the wrist and elbow blastemas sort distally to their corresponding tarsal and knee levels, respectively, while the mid-upper arm blastema remains at the amputation level. Furthermore, distal blastema cells are proximalized by overexpression of Meis 2 and relocate more proximally (Mercader et al, 2005). These observations implicate Hoxa 9, Hoxd 10, and Meis 2 in specifying the stylopodium, and Hoxa 9/13 in specifying the autopodium.…”

“…Forelimbs and hindlimbs express distinct T-box genes (Simon et al, 1997). Hoxa and d, and Meis genes are involved in PD patterning (Gardiner et al, 1995;Torok et al, 1998;Mercader et al, 2005). Hoxd 10 is expressed at a 2-3-fold higher level in proximal versus distal blastemas (Simon and Tabin, 1993;Simon et al, 1995Simon et al, , 1997.…”

“…Hoxa 9 is expressed throughout both proximal and distal blastemas, but Hoxa 13 expression is 30% higher in distal versus proximal blastemas (Gardiner et al, 1995). Meis 1 and 2 are expressed preferentially in the stylopodial region of a blastema derived from the upper arm (Mercader et al, 2005). Hoxd 10 and Meis 1 and 2 expression is upregulated in distal blastemas proximalized by RA (Simon and Tabin, 1993;Mercader et al, 2005), while expression of Hoxa 13 is reduced (Gardiner et al, 1995).…”

“…By the cone stage, expression of Hoxa 13 was restricted to a distal domain corresponding to the prospective autopodium, while the prospective zeugopodial and stylopodial regions expressed Hoxa 9 only. The Meis 1 and 2 genes are also expressed at a high level in the prospective stylopodium of conestage blastemas derived from the upper arm (Mercader et al, 2005). Assuming that the Hoxa 9/13 combination is pivotal in specifying an autopodial domain and Hoxa 9 and Meis in specifying a stylopodial domain, how can we reconcile the uniform expression of both Hoxa 9 and 13 in the accumulation blastema with complete PD self-organization (we do not know if Meis genes are expressed earlier than cone stage), and how are these domains separated?…”

Mechanisms of urodele limb regeneration

“…Meis 1 and 2 are two other genes expressed preferentially in the prospective stylopodial region. These genes are upregulated in autopodial blastemas proximalized by RA, while Hoxa13 is downregulated, and autopodial blastemas are also proximalized by overexpression of Meis 2 (Mercader, Tanaka, & Torres, 2005). These observations implicate Hoxa9/Meis 1/2 in specfying the PD pattern of the stylopodium, Hoxa9/11/Meis2 the zeugopodium and Hoxa9/13 the autopodium.…”