Normal somatic cells of higher organisms do not divide indefinitely. After a finite number of divisions, normal cells irreversibly cease proliferation by a process termed replicative or cellular senescence. Replicative senescence is controlled by multiple, dominant‐acting genes about which very little is known. The only genes known to reactivate DNA synthesis in senescent cells are viral oncogenes encoding proteins that bind and inactivate the p53 and retinoblastoma (pRb) tumor suppressor proteins. SV40 T antigen is the best studied of these viral oncoproteins. T[K1] is a T antigen point mutant that selectively is defective in binding pRb and the pRb‐related proteins p107 and p130. We show that T[K1] stimulated quiescent human fibroblasts to synthesize DNA nearly as well as wild‐type T but was incapable of stimulating senescent cells. We tested several growth regulatory genes that are repressed in senescent cells for ability to restore activity to T[K1]. These included c‐fos, c‐jun, Id‐1, Id‐2, E2F‐1, and cdc2. Only the helix‐loop‐helix (HLH) protein, Id‐1, restored the ability of T[K1] to reactivate DNA synthesis in senescent cells. This activity of Id‐1 was not shared by Id‐2, a related protein, and depended on an intact HLH domain. It did not appear that Id‐1 interacted directly with pRb or p107. Constitutive Id‐1 expression failed to rescue proliferating cells from growth inhibition by pRb, p107, or p130, and failed to interact with pRb in the yeast two hybrid system. Because Id proteins negatively regulate basic‐HLH (bHLH) transcription factors, we suggest that senescent cells express one or more bHLH factor that cooperates with pRb, or pRb‐related proteins, to suppress proliferation. © 1996 Wiley‐Liss, Inc.

This paper describes the development and validation of the PULSE Vision & Change Rubrics, version 1.0, a reliable tool for measuring departmental change. It was found that liberal arts institutions are farther along in implementing the recommendations of Vision and Change and that institutions overall earned the highest scores on the Curriculum rubric and the lowest scores on the Assessment rubric.

Our previous work demonstrated that Xenopus ectoderm cells undergo an alkalinization in response to planar inductive signals during neural induction in explants. We have examined the role of intracellular alkalinization in the establishment of anterior neural fate. First, RT-PCR was used to examine neural-specific gene expression in planar explants in which the alkalinization is prevented by treatment with 4,4'-dihydrodiisothiocyanatostilbene-2,2'-disulfonate (H2DIDS). In explants cultured in the presence of H2DIDS, expression of NCAM and the anterior neural gene otx2 is greatly reduced or absent. Second, neural-specific gene expression was examined in isolates of uninduced animal cap ectoderm cultured in the presence of either methylamine or ammonium chloride. NCAM, otx2, and the anterior neural inducer noggin were expressed in alkalinized ectoderm, while the more posterior neural markers krox-20 and Hox B9 were undetectable. Expression of NCAM, otx2, and noggin was observed at stage 11 in both alkalinized ectoderm and the newly induced neural plate, suggesting that intracellular alkalinization could contribute to propagation of noggin signaling through the dorsal ectoderm. Alkalinization of uninduced ectoderm at stage 10.5 led to an upregulation of otx2 within 15 min. Activation of NCAM expression in alkalinized dissociated cells was identical to that observed in intact animal caps, indicating that alkalinization-mediated changes in gene expression do not require cell-cell contact. Finally, the effects of intracellular alkalinization on protein tyrosine phosphorylation were investigated using 2D gel electrophoresis and immunoblots probed with an antiphosphotyrosine antibody. Several phosphorylated protein detected in induced and alkalinized ectoderm were greatly reduced or absent in uninduced ectoderm, indicating that alkalinization elicits alterations in tyrosine phosphorylation similar to some of those observed during neural induction in vivo. Our results indicate that intracellular alkalinization plays a critical role in the activation of anterior neural-specific gene expression and that alkalinization may act by regulating the activity of a tyrosine kinase or phosphatase.

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