Expression of mRNAs coding for the transforming growth factor-beta receptors in brain regions of euthyroid and hypothyroid neonatal rats and in adult brain.

The TGF-beta family of peptides has been postulated to play a role in control of the cell cycle but also may act in the developing brain to influence neuronal differentiation and survival. Because reception of TGF-beta signals requires the simultaneous expression of all three known receptor subtypes, we examined two neonatal rat brain regions in which neurogenesis has been largely completed. mRNA coding for all three receptors was detectable in both the forebrain and brainstem but only the type II receptor in brainstem showed a difference from adult levels of expression. Animals given perinatal PTU treatment to achieve congenital cretinism did not show significant differences in expression of any of the receptor subtypes in either of the regions, despite the fact that the treatment is known to cause anomalies of neuronal differentiation. These results indicate that regions in which neurons are undergoing axonogenesis and synaptogenesis rather than neurogenesis, nevertheless express the mRNAs coding for TGF-beta receptors and are thus likely to be receptive to trophic signals mediated through TGF-beta. However, synthesis and release of TGF-beta, rather than receptor expression per se, is more likely to be the major point for regulation of signaling. The potential roles of TGF-beta in developmental events outside of the cell cycle, such as synaptogenesis and apoptosis, need to be examined.

Regulation of apoptosis in transgenic mice by simian virus 40 T antigen-mediated inactivation of p53.

Several proteins encoded by DNA tumor viruses are thought to disrupt cellular growth control by interacting with key cellular proteins, such as p53 and pRB, that normally function to regulate cell growth. However, the biological consequences of intracellular complexing between the viral oncoproteins and cellular proteins have remained unclear. Such complexes could either facilitate functional inactivation of the cellular proteins, leading to a loss-of-function phenotype, or could activate new functions, leading to a gain-of-function phenotype. Here we demonstrate that the simian virus 40 large tumor (T) antigen produces a loss-of-p53-function phenotype when introduced into the thymocytes of transgenic mice. Like thymocytes from the recently characterized p53-null mice, thymocytes from transgenic mice expressing a T-antigen variant capable of binding to p53 are resistant to irradiation-induced apoptosis. Thymocytes from transgenic mice expressing a mutant T antigen that is unable to complex p53, but retains the ability to complex the pRB and p107 proteins, retain sensitivity to irradiation. We further demonstrate that although irradiation-induced apoptosis is impaired by T antigen, clonal deletion of autoreactive thymocytes via p53-independent apoptosis is not perturbed by T antigen. These results provide convincing evidence that T antigen inactivates p53 in thymocytes in vivo and suggest a mechanism by which T antigen predisposes thymocytes to tumorigenesis in T antigen-transgenic mice.

Use of transgenic mice reveals cell-specific transformation by a simian virus 40 T-antigen amino-terminal mutant.

We have used the multifunctional transforming protein, simian virus 40 T antigen, as a probe to study the mechanisms of cell growth regulation in the intact organism. T antigen appears to perturb cell growth, at least in part, by stably interacting with specific cellular proteins that function to maintain normal cell growth properties. Experiments in cultured cells indicate that at least three distinct regions of simian virus 40 T antigen have roles in transformation. Two regions correlate with the binding of known cellular proteins, p53, pRB, and p107. A third activity, located near the amino terminus, has been defined genetically but not biochemically. By targeting expression of wild-type and mutant forms of T antigen to distinct cell types in transgenic mice, we have begun to systematically determine which activities play a role in tumorigenesis of each cell type. In this study, we sought to determine the role of the amino-terminal transformation function with such an analysis of the T-antigen mutant dl1135. This protein, which lacks amino acids 17 to 27, retains the p53-, pRB-, and p107-binding activities yet fails to transform cells in culture. To direct expression in transgenic mice, we used the lymphotropic papovavirus transcriptional signals that are specific for B and T lymphocytes and the choroid plexus epithelium of the brain. We show here that although defective in cell culture, dl1135 specifically induced the development of thymic lymphomas in the mouse. Expression of the protein was routinely observed in B- and T-lymphoid cells, although B-cell abnormalities were not observed. Choroid plexus tumors were observed only infrequently; however, dl1135 was not consistently expressed in this tissue. Within a given transgenic line, the penetrance of T-cell tumorigenesis was 100% but appeared to require secondary events, as judged from the clonal nature of the tumors. These experiments suggest that the amino-terminal region of T antigen has a role in the transformation of certain cell types (such as fibroblasts in culture and B lymphocytes) but is dispensable for the transformation of T lymphocytes.