The mechanisms by which neural stem cells give rise to neurons, astrocytes, or oligodendrocytes are beginning to be elucidated. However, it is not known how the specification of one cell lineage results in the suppression of alternative fates. We find that in addition to inducing neurogenesis, the bHLH transcription factor neurogenin (Ngn1) inhibits the differentiation of neural stem cells into astrocytes. While Ngn1 promotes neurogenesis by functioning as a transcriptional activator, Ngn1 inhibits astrocyte differentiation by sequestering the CBP-Smad1 transcription complex away from astrocyte differentiation genes, and by inhibiting the activation of STAT transcription factors that are necessary for gliogenesis. Thus, two distinct mechanisms are involved in the activation and suppression of gene expression during cell-fate specification by neurogenin.

Members of the E2F transcription factor family (E2F-1-E2F-5) are believed to be critical positive regulators of cell cycle progression in eukaryotes although the in vivo functions of the individual E2Fs have not been elucidated. Mice were generated that lack E2F-1 and, surprisingly, these mice develop and reproduce normally. However, E2F-1-/- mice exhibit a defect in T lymphocyte development leading to an excess of mature T cells due to a maturation stage-specific defect in thymocyte apoptosis. As E2F-1-/- mice age they exhibit a second phenotype marked by aberrant cell proliferation. These findings suggest that while certain members of the E2F family may positively regulate cell cycle progression, E2F-1 functions to regulate apoptosis and to suppress cell proliferation.

Labile mRNAs that encode cytokine and immediate-early gene products often contain AU-rich sequences within their 3 untranslated region (UTR). These AU-rich sequences appear to be key determinants of the short half-lives of these mRNAs, although the sequence features of these elements and the mechanism by which they target mRNAs for rapid decay have not been fully defined. We have examined the features of AU-rich elements (AREs) that are crucial for their function as determinants of mRNA instability in mammalian cells by testing the ability of various mutant c-fos AREs and synthetic AREs to direct rapid mRNA deadenylation and decay when inserted within the 3 UTR of the normally stable ␤-globin mRNA. Evidence is presented that the pentamer AUUUA, which previously was suggested to be the minimal determinant of instability present in mammalian AREs, cannot direct rapid mRNA deadenylation and decay. Instead, the nonamer UUAUUUAUU is the elemental AU-rich sequence motif that destabilizes mRNA. Removal of one uridine residue from either end of the nonamer (UUAUUUAU or UAUUUAUU) results in a decrease of potency of the element, while removal of a uridine residue from both ends of the nonamer (UAUUUAU) eliminates detectable destabilizing activity. The inclusion of an additional uridine residue at both ends of the nonamer (UUUAUUUAUUU) does not further increase the efficacy of the element. Taken together, these findings suggest that the nonamer UUAUUUAUU is the minimal AU-rich motif that effectively destabilizes mRNA. Additional ARE potency is achieved by combining multiple copies of this nonamer in a single mRNA 3 UTR. Furthermore, analysis of poly(A) shortening rates for ARE-containing mRNAs reveals that the UUAUUUAUU sequence also accelerates mRNA deadenylation and suggests that the UUAUUUAUU motif targets mRNA for rapid deadenylation as an early step in the mRNA decay process.

IntroductionDendritic cells (DC) are professional antigen-presenting cells that are critically involved in the initiation of T cell-dependent immune responses as a consequence of their high expression of major histocompatibility complex (MHC) and costimulatory molecules. 1 DC are sparsely distributed throughout the body and, in most tissues, are present in an immature state, showing a high capacity for antigen uptake and processing but unable to stimulate T cells. 1,2 Once activated by inflammatory stimuli or infectious agents, DC undergo a maturation process whose hallmarks are up-regulated expression of costimulatory (CD40, CD80, and CD86) and adhesion (CD54 and CD58) molecules, migration into lymphoid organs, and subsequent acquisition of the capacity to activate quiescent, naïve, and memory lymphocytes. [1][2][3][4] In vitro, DC can be derived from either precursor cells or peripheral blood monocytes 5-8 when the appropriate cytokine signals are provided. Immature monocytederived DC (MDDC) can be obtained from peripheral blood monocytes in the presence of granulocyte-macrophage colonystimulating factor (GM-CSF) and interleukin-4 (IL-4). Addition of lipopolysaccharide (LPS) or tumor necrosis factor ␣ (TNF-␣) leads to the appearance of MDDC with all the morphologic, phenotypic, and functional characteristics of mature DC, 5,6 including de novo expression of CD83, 9 up-regulated expression of adhesion and costimulatory molecules, 1-4,10 loss of mannose-receptor-mediated endocytosis, synthesis and release of IL-12, and enhanced antigen presentation capacity. 11 Thus, in vitro maturation of MDDC represents a useful system for analyzing the molecular and functional changes that take place during acquisition of optimal T-cell-stimulating activity by DC.At least 3 distinct mitogen-activated protein kinase (MAPK) signaling cascades exist in mammals, including the extracellular signal-regulated kinase (ERK), the c-Jun N-terminal kinase (JNK), and the p38 MAPK pathways. 12 These kinases are activated by phosphorylation by distinct upstream MAPK kinases. The ERK signaling cascade regulates cell proliferation and differentiation in response to mitogens and growth factors, whereas the JNK and p38 MAPK pathways are preferentially activated by stress-inducing agents. 12 The availability of specific inhibitors for the ERK and p38 MAPK pathways allows evaluation of their respective involvement in cellular responses to extracellular stimuli. The ERK pathway inhibitors PD98059 13 and U0126 14 prevent activation of mitogenactivated protein kinase kinase (MEK) 1/2, 15 upstream activators of ERK 1/2, whereas the pyridinyl imidazole SB203580 inhibits p38 MAPK activity. 15,16 The intracellular signaling pathways implicated in maturation of MDDC are just beginning to be explored. TNF-␣ stimulation of immature MDDC initiates activation of several MAPKs, including For personal use only. on May 11, 2018. by guest www.bloodjournal.org From ERK 2, stress-activated protein kinase-JNK, and p38 MAPK. 10,17,18 Several reports have suggested t...