The Martin lab uses mouse genetics to investigate fundamental questions of vertebrate development. One approach that we have used successfully is to model human disease in mice using gene targeting in mouse embryonic stem (ES) cells. Our goal is to gain insight into the molecular genetic function of the disease gene with the understanding that this will lead to a new understanding of development, as well as potential therapeutic advances in the future. Our work has been greatly facilitated by the convergence of two technologic breakthroughs: the ability to manipulate the mouse genome and large scale sequencing of complex genomes, in particular the human and mouse genomes.

The function of Pitx2 in craniofacial morphogenesis

Studies performed in my laboratory have helped to define the role of the Pitx2 homeobox gene in development. Pitx2 was identified as the gene mutated in Rieger syndrome I, a haploinsufficient disorder that includes tooth, ocular and umbilical abnormalities as its primary features. We generated a Pitx2 loss-of-function mutation in mice by introducing a deletion of the Pitx2 homeobox in ES cells. Pitx2 mutant mice had severe abnormalities in multiple developmental fields including tooth, eye, and umbilical defects, consistent with the role of pitx2 in Rieger syndrome.

We found that Pitx2 regulated the expression of signaling molecules in the oral ectoderm. In Pitx2 null embryos, Fgf8, normally widely expressed in the oral ectoderm, failed to be maintained, while Bmp4 expression in oral ectoderm was expanded. Bmp4 and Fgf8 have overlapping and distinct expression domains in the oral ectoderm prior to tooth formation. Antagonistic interactions between these pathways have been proposed to have an important role in multiple aspects of branchial arch morphogenesis. Thus, our observation that Pitx2 had a role in differentially regulating Bmp4 and Fgf8 expression provided insight into the molecular mechanisms underlying regulation of branchial arch morphogenesis.

Pitx2 in left-right asymmetry and cardiac morphogenesis

Morphologic left-right asymmetry is demonstrated by all internal viscera and is manifested by differential placement of organs, asymmetric shapes of organs and stereotyped rotation of the developing embryo and gut organs. Pitx2 is expressed at the correct time and place to function in the later stages of asymmetric organ morphogenesis. In mouse and chick embryos, pitx2 is expressed asymmetrically in the left lateral plate mesoderm slightly after expression of lefty1, lefty2, and nodal and, at later times, in the left heart tube and left gut mesenchyme. Moreover, right-sided misexpression of pitx2 in the chick resulted in cardiac and gut situs abnormalities. Work from my lab has shown that Pitx2c patterns a prospective secondary heart field that contributes to the conotruncal myocardium. In this work, we showed that Pitx2c is expressed in a secondary heart field within the branchial arch and splanchnic mesoderm that contributes to the aortic sac and conotruncal myocardium. Moreover, we generated a Pitx2 Cre recombinase knock-in allele and crossed this with the Rosa26 reporter line to perform fate mapping studies. Using this approach, we found that Pitx2 daughter cells populated the right and left ventricles, atrioventricular cushions and valves, and pulmonary veins. In Pitx2 mutant embryos, descendents of Pitx2-expressing cells failed to contribute to the atrioventricular cushions and valves, and the pulmonary vein. These findings suggest that the Pitx2-mediated left-right asymmetry pathway plays an extensive role in patterning the heart, including the outflow and inflow tracts and ventricular myocardium. In addition, these data provide evidence that the prospective secondary heart field (SHF) derived from branchial arch and splanchnic mesoderm patterns the forming outflow tract. This paper also reveals a role for Pitx2c and the Nodal-Pitx2 pathway, in asymmetric remodeling of the aortic arch vessels.

Bmp-signaling in cardiac and craniofacial morphogenesis

Our work on Pitx2 led us in the direction of investigating Bmp-signaling in craniofacial and cardiac morphogenesis. In the last few years we have investigate Bmp4 in cardiac, craniofacial, and limb morphogenesis. Our approach to initiate these studies was to generate a Bmp4 conditional null allele. Subsequently, we have used this allele to investigate Bmp4 function in the mouse embryo. More recently, we have generated a Bmp2 conditional null allele to extend our studies into Bmp-signaling in mouse embryogenesis. Currently, we are investigating Bmp2 and Bmp4 function in craniofacial morphogenesis and cardiovascular development.

Recent publications

• Ma L, Martin JF (2005) Generation of a conditional null Bmp2 allele. genesis 42:203–206.
  
  
• Liu W, Selever J, Murali D, Sun X, Brugger SM, Schwartz RJ, Maxson R, Furuta Y, Martin JF (2005) Threshold-specific requirements for Bmp4 in mandibular development. Developmental Biology 283:282–293.
  
  
• Liu W, Sun X, Braut A, Mishina Y, Behringer RR, Mina M, Martin JF (2005) Distinct functions for Bmp signaling in lip and palate fusion in mice. Development 132:1453–1461.
  
  
• Lee CT, Li L, Takamoto N, Martin JF, DeMayo FJ, Tsai M-J, Tsai SY (2004) The nuclear orphan receptor COUP-TFII is required for limb and skeletal muscle development. Molecular and Cellular Biology 24:10835–10843.
  
  
• Selever J, Liu W, Behringer R, Martin JF (2004) Bmp4 in limb bud mesoderm regulates digit pattern by controlling AER development. Developmental Biology 276:268–279.
  
  
• Wei Liu, Jennifer Selever, Degang Wang, Mei-Fang Lu , Kelvin A. Moses,
  Robert J. Schwartz , and James F. Martin (2004) Bmp4 signaling is required
  for outflow tract septation and branchial arch artery remodeling. PNAS 101:4489–94.
  
  
• Liu, W., Selever, J, Wang D., Lu, M.F., and Martin, J.F. (2003) Genetic
  dissection of Pitx2 in craniofacial development uncovers new functions in
  branchial arch morphogenesis, late aspects of tooth morphogenesis, and cell
  migration. Development 130:6375–6385.
  
  
• Liu, C., Liu, W., Palie, J, Lu, M.F., Brown N, and Martin, J.F. (2002) Pitx2c patterns anterior myocardium and aortic arch vessels and is required for local cell movement into atrioventricular cushions. Development, 129:5081–5091.
  
  
• Malcolm Logan, James F. Martin, Andras Nagy, Corrinne Lobe, Eric N. Olson, Clifford J. Tabin (2002). Expression of Cre Recombinase in the Developing Mouse Limb Bud Driven by a Prxl Enhancer. genesis 33:77–80.
  
  
• Liu C, Liu W, Lu MF, Brown N, Martin JF (2001) Regulation of left-right asymmetry by thresholds of pitx2c activity. Development 128:2039–2048.
  
  
• Lu MF, Pressman C, Dyer R, Johnson RL, Martin JF (1999) Function of Rieger syndrome gene in left-right asymmetry and craniofacial development. Nature 401:276–278.
  
  
• Lu MF, Cheng HT, Kern MJ, Potter SS, Tran B, Diekwisch TG, Martin JF (1999) prx-1 functions cooperatively with another paired-related homeobox gene, prx-2, to maintain cell fates within the craniofacial mesenchyme. Development 126:495–504.
  
  
• Lu MF, Cheng HT, Lacy AR, Kern MJ, Argao EA, Potter SS, Olson EN, Martin JF (1999) Paired-related homeobox genes cooperate in handplate and hindlimb zeugopod morphogenesis. Dev Biol 205:145–157.
  
  
• ten Berge D, Brouwer A, Korving J, Martin JF, Meijlink F (1998) Prx1 and Prx2 in skeletogenesis: roles in the craniofacial region, inner ear and limbs. Development 125:3831–3842.
  
  
• Martin JF, Bradley A, Olson EN (1995) The paired-like homeobox gene MHox is required for early events of skeletogenesis in multiple lineages. Genes Dev 9:1237–1249.

Last updated 09/08/2006