Atrial-like phenotype is associated with embryonic ventricular failure in retinoid X receptor alpha -/- mice.

We have recently characterized a cardiac model of ventricular chamber defects in retinoid X receptor alpha (RXR alpha) homozygous mutant (-/-) gene-targeted mice. These mice display generalized edema, ventricular chamber hypoplasia, and muscular septal defects, and they die at embryonic day 15. To substantiate our hypothesis that the embryos are dying of cardiac pump failure, we have used digital bright-field and fluorescent video microscopy and in vivo microinjection of fluorescein-labeled albumin to analyze cardiac function. The affected embryos showed depressed ventricular function (average left ventricular area ejection fraction, 14%), ventricular septal defects, and various degrees of atrioventricular block not seen in the RXR alpha wild-type (+/+) and heterozygous (+/-) littermates (average left ventricular area ejection fraction, 50%). The molecular mechanisms involved in these ventricular defects were studied by evaluating expression of cardiac-specific genes known to be developmentally regulated. By in situ hybridization, aberrant, persistent expression of the atrial isoform of myosin light chain 2 was identified in the ventricles. We hypothesize that retinoic acid provides a critical signal mediated through the RXR alpha pathway that is required to allow progression of development of the ventricular region of the heart from its early atrial-like form to the thick-walled adult ventricle. The conduction system disturbances found in the RXR alpha -/- embryos may reflect a requirement of the developing conduction system for the RXR alpha signaling pathway, or it may be secondary to the failure of septal development.

Cell-surface-expressed T-cell antigen-receptor zeta chain is associated with the cytoskeleton.

The T-cell antigen receptor zeta chain plays an important role in coupling antigen recognition to several intracellular signal-transduction pathways. zeta chain can associate with certain protein tyrosine kinases and retains the capacity to transduce signals independently of the other receptor subunits. Thus, zeta chain could couple cell-surface-expressed T-cell antigen receptors to the intracellular signal-transduction apparatus by its association with various intracellular molecules in addition to tyrosine kinases. In the process of searching for zeta chain-associated molecules we observed that after lysis of resting T cells with Triton X-100, zeta chain is localized in the detergent-insoluble fraction, in addition to its presence in the detergent-soluble fraction. Treatment of T cells with cytochalasin B, an actin-depolymerizing agent, leads to the complete dissociation of zeta chain from the Triton-insoluble fraction, suggesting a linkage between zeta chain and the cytoskeletal matrix. We have also determined that cytoskeletal-associated zeta chain is expressed on the cell surface. Furthermore, a tyrosine-phosphorylated 16-kDa zeta chain was detected only in the Triton-insoluble cytoskeletal fraction of resting T cells. zeta chain also maintains its association with the cytoskeleton when expressed in COS cells, inferring that the cytoskeletal elements involved in this linkage may be ubiquitous. Finally, we have localized a 42-amino acid region in the intracytoplasmic domain of zeta chain, which is crucial for maximal interaction between zeta chain and the cytoskeleton. Anchorage of cell-surface-expressed zeta chain to the cytoskeleton in resting T cells may facilitate recycling of receptor complexes and/or allow the transduction of external stimuli into the cell.

A serine proteinase inhibitor locus at 18q21.3 contains a tandem duplication of the human squamous cell carcinoma antigen gene.

The squamous cell carcinoma antigen (SCCA) is a member of the ovalbumin family of serine proteinase inhibitors (serpins). A neutral form of the protein is found in normal and some malignant squamous cells, whereas an acidic form is detected exclusively in tumor cells and in the circulation of patients with squamous cell tumors. In this report, we describe the cloning of the SCCA gene from normal genomic DNA. Surprisingly, two genes were found. They were tandemly arrayed and flanked by two other closely related serpins, plasminogen activator inhibitor type 2 (PAI2) and maspin at 18q21.3. The genomic structure of the two genes, SCCA1 and SCCA2, was highly conserved. The predicted amino acid sequences were 92% identical and suggested that the neutral form of the protein was encoded by SCCA1 and the acidic form was encoded by SCCA2. Further characterization of the region should determine whether the differential expression of the SCCA genes plays a causal role in development of more aggressive squamous cell carcinomas.