Axial skeleton anterior-posterior patterning is regulated through feedback regulation between Meis transcription factors and retinoic acid

Abstract

Vertebrate axial skeletal patterning is controlled by coordinated collinear expression ofHox genes and axial level-dependent activity of Hox protein combinations. Transcription factors of the Meis family act as cofactors of Hox proteins and profusely bind to Hox complex DNA, however their roles in mammalian axial patterning have not been established. Similarly, retinoic acid (RA) is known to regulate axial skeletal element identity through the transcriptional activity of its receptors, however whether this role is related to Meis/Hox regulation or functions in axial patterning remains unknown. Here we study the role of Meis factors in axial skeleton formation and its relationship to the RA pathway by characterizing Meis1, Meis2 and Raldh2 mutant mice. We report that Meis and Raldh2 regulate each other in a positive feedback regulatory loop that controls axial skeletal identity. Meis elimination produces homeotic transformations similar to those found in Raldh2 and anterior-Hox mutants and disrupts the expression of Hox target genes without changing the transcriptional profiles of Hox complexes. We propose that Meis regulates vertebrate axial skeleton patterning by exclusively affecting Hox protein function, and that alterations in RA levels can produce homeotic transformations without altering Hox transcription through regulating Meis expression.Hox gene transcription is activated sequentially in axial precursors during gastrulation (13). Expression of Hox genes located at the 3'-most region of the complexes starts in axial progenitors in the posterior epiblast and is maintained in their descendants as they gastrulate through the primitive streak and colonize the embryonic AP axis. 3'-to-5' sequential transcriptional activation of Hox complexes progresses continuously in axial progenitors, whereas their daughter cells fix their Hox expression code as they exit the progenitor region and colonize the embryonic axis. As cells colonize the different AP segments, they carry the successive Hox expression combinations to the progressively forming body axis, resulting in an AP nested patterns [8]. Thus, temporal information is translated into spatial domains during axial elongation [9]. Hox proteins bind DNA through a 60 amino acid region called the homeodomain [10]. The homeodomain is highly conserved and diversified in several transcription factor families in animals and plants. Hox proteins alone show limited DNA-binding ability, but they gain specificity and affinity for target sequences through interactions with cofactors of the PBC and MEINOX families, both belonging to the three amino acid loop extension (TALE) class of homeodomains [11]. PBC and MEINOX proteins form heterodimers and heterotrimers with Hox proteins, conferring them with increased target sequence selectivity and affinity [12]. In fact, mutants for the single members of the PBC and MEINOX families in Drosophila show AP phenotypes compatible with a generalized Hox loss of function, without affecting Hox AP expression [13, 14]. In mammals, redundancy ...

Retinoic acid signaling in heart development: Application in the differentiation of cardiovascular lineages from human pluripotent stem cells

“…Indeed, putative enhancer regions identified in pSHF cells revealed enrichment for HOX and TALE binding motifs ( Stefanovic et al., 2020 ). While HOX and MEIS genes appear to be direct targets of RA itself ( Mercader et al., 2000 ; Berenguer et al., 2020 ), studies have shown that they can also regulate Raldh2 expression and thereby RA levels through a positive feedback loop ( Vitobello et al., 2011 ; López-Delgado et al., 2021 ). Interestingly, misexpression of Hoxb1 in the aSHF domain resulted in an upregulation of Tbx5 and Nr2f2 ( Stefanovic et al., 2020 ), both of which are implicated in the differentiation of atrial and sinoatrial nodal cells in vivo and in vitro ( Liberatore et al., 2000 ; Wu et al., 2013 ; Devalla et al., 2015 ; Protze et al., 2017 ).…”

Analysis of lamprey meis genes reveals that conserved inputs from Hox, Meis and Pbx proteins control their expression in the hindbrain and neural tube

“…It is also important to note that many domains of meis expression do not overlap with hox expression, as illustrated by expression in anterior regions, such as the midbrain and PA1. Furthermore, in gnathostomes meis genes have also been shown to display hox-independent functions (Lopez-Delgado et al, 2021;Moens and Selleri, 2006), suggesting that the expression domains in lamprey which do not overlap with hox expression are associated with other roles in embryonic and tissue development. Future functional perturbation studies will be required to address the nature of meis gene functions in the pharynx and potential regulatory interactions with Hox genes and other factors.…”

“…Meis proteins are a highly conserved family of transcription factors that have key roles in many aspects of embryonic and axial patterning across metazoans (Burglin, 1997;Lopez-Delgado et al, 2021). In vertebrates, this includes roles in the gene regulatory networks involved in specifying anterior-posterior (A-P) patterning in the developing hindbrain and cranial neural crest (NC) via the clustered Hox genes (Choe et al, 2014;Choe et al, 2009;Choe et al, 2002;Deflorian et al, 2004;Machon et al, 2015;Moens and Selleri, 2006;Stedman et al, 2009).…”

“…The characterization of Hox-responsive cis-regulatory elements that are involved in these interactions in both vertebrate and invertebrate species has revealed crucial roles for Pbx/Exd and Meis/Hth as Hox cofactors (Chan et al, 1994;Ferretti et al, 2000;Merabet et al, 2007;Popperl et al, 1995;Rieckhof et al, 1997;Ryoo and Mann, 1999). Pbx and Meis are members of the TALE (three amino-acid loop extension) family of transcription factors (Burglin, 1997) and have both independent and general roles as cofactors for a variety of transcription factors, with diverse inputs into cell and developmental processes (Laurent et al, 2008;Lopez-Delgado et al, 2021;Merabet and Mann, 2016;Moens and Selleri, 2006;Schulte and Frank, 2014).…”

Identification of bipotent progenitors that give rise to myogenic and connective tissues in mouse

“…The top regulons of this analysis revealed active transcription factors underlying myogenic and non-myogenic cell fates in the EOM at E11.5. Notably, Myf5, Pitx1, Mef2a and Six1, transcription factors known to be implicated in myogenic development (Buckingham and Rigby, 2014), appeared among the top regulons in myogenic cells whereas Fli1, Ebf1, Ets1, Foxc1, Meis1 and Six2, genes known for their involvement in adipogenic, vascular, mesenchymal and tendon development (Jimenez et al, 2006;López-Delgado et al, 2020;Noizet et al, 2016;Truong and Ben-David, 2000;Whitesell et al, 2019;Yamamoto-Shiraishi and Kuroiwa, 2013), constituted some of the highly active non-myogenic transcription factors (Figure 5B). Given that Myf5 appeared as a top regulatory factor of the myogenic program, we interrogated the fate of Myf5-expressing progenitors in a Myf5 nlacZ/nlacZ null genetic background.…”