Molecular Pathways and Animal Models of Atrial Septal Defect.

The relative simplicity of the clinical presentation and management of an atrial septal defect belies the complexity of the developmental pathogenesis. Here, we describe the anatomic development of the atrial septum and the venous return to the atrial chambers. Experimental models suggest how mutations and naturally occurring genetic variation could affect developmental steps to cause a defect within the oval fossa, the so-called secundum defect, or other interatrial communications, such as the sinus venosus defect or ostium primum defect.

Distinct enhancers at the Pax3 locus can function redundantly to regulate neural tube and neural crest expressions.

Pax3 is a transcription factor expressed in somitic mesoderm, dorsal neural tube and pre-migratory neural crest during embryonic development. We have previously identified cis-acting enhancer elements within the proximal upstream genomic region of Pax3 that are sufficient to direct functional expression of Pax3 in neural crest. These elements direct expression of a reporter gene to pre-migratory neural crest in transgenic mice, and transgenic expression of a Pax3 cDNA using these elements is sufficient to rescue neural crest development in mice otherwise lacking endogenous Pax3. We show here that deletion of these enhancer sequences by homologous recombination is insufficient to abrogate neural crest expression of Pax3 and results in viable mice. We identify a distinct enhancer in the fourth intron that is also capable of mediating neural crest expression in transgenic mice and zebrafish. Our analysis suggests the existence of functionally redundant neural crest enhancer modules for Pax3.

Tie2Cre-mediated inactivation of plexinD1 results in congenital heart, vascular and skeletal defects.

PlexinD1 is a membrane-bound receptor that mediates signals derived from class 3 secreted semaphorins. Although semaphorin signaling in axon guidance in the nervous system has been extensively studied, functions outside the nervous system including important roles in vascular patterning have also been demonstrated. Inactivation of plexinD1 leads to neo-natal lethality, structural defects of the cardiac outflow tract, peripheral vascular abnormalities, and axial skeletal morphogenesis defects. PlexinD1 is expressed by vascular endothelial cells, but additional domains of expression have also been demonstrated including in lymphocytes, osteoblasts, neural crest and the central nervous system. Hence, the cell-type specific functions of plexinD1 have remained unclear. Here, we describe the results of tissue-specific gene inactivation of plexinD1 in Tie2 expressing precursors, which recapitulates the null phenotype with respect to congenital heart, vascular, and skeletal abnormalities resulting in neonatal lethality. Interestingly, these mutants also have myocardial defects not previously reported. In addition, we demonstrate functions for plexinD1 in post-natal retinal vasculogenesis and adult angiogenesis through the use of inducible cre-mediated deletion. These results demonstrate an important role for PlexinD1 in embryonic and adult vasculature.

Cre reporter mouse expressing a nuclear localized fusion of GFP and beta-galactosidase reveals new derivatives of Pax3-expressing precursors.

A new Cre-reporter strain of mouse has been developed that expresses a fusion protein derived from the lacZ gene fused to GFP with a nuclear localization signal. This construct is expressed from the ROSA26 locus upon Cre-mediated recombination that removes a loxP-flanked PGK-neo cassette, thus allowing for detection of Cre activity in all tissues. This reporter strain, which is similar to prior R26R and R26EGFP strains, has certain advantages related to the nuclear expression and the combined expression of both beta-galactosidase and GFP activities. We show that the use of this newly developed reporter line allows for enhanced resolution, detection and co-localization. Thus, we report a previously unrecognized subset of venous endothelial cells derived from Pax3 expressing precursors.