Mitogen-activated protein kinase and neural specification in Xenopus

We have investigated the activity and function of mitogen-activated protein kinase (MAPK) during neural specification in Xenopus. Ectodermal MAPK activity increased between late blastula and midgastrula stages. At midgastrula, MAPK activity in both newly induced neural ectoderm and ectoderm overexpressing the anterior neural inducer noggin was 5-fold higher than in uninduced ectoderm. Overexpression of MAPK phosphatase-1 (MKP-1) in ectoderm inhibited MAPK activity and prevented neurectoderm-specific gene expression when the ectoderm was recombined with dorsal mesoderm or treated with fibroblast growth factor (FGF). Neurectoderm-specific gene expression was observed, however, in ectoderm overexpressing both noggin and MKP-1. To evaluate the role of MAPK in posterior regionalization, ectodermal isolates were treated with increasing concentrations of FGF and assayed for MAPK activity and neurectoderm-specific gene expression. Although induction of posterior neural ectoderm by FGF was accompanied by an elevation of MAPK activity, relative MAPK activity associated with posterior neural fate was no higher than that of ectoderm specified to adopt an anterior neural fate. Thus, increasingly posterior neural fates are not correlated with quantitative increases in MAPK activity. Because MAPK has been shown to down-regulate Smad1, MAPK may disrupt bone morphogenetic protein 4 (BMP-4) signaling during neural specification. Our results suggest that MAPK plays an essential role in the establishment of neural fate in vivo.

The role of intracellular alkalinization in the establishment of anterior neural fate in Xenopus

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.

Inhibition of morphogenetic movement during Xenopus gastrulation by injected sulfatase: implications for anteroposterior and dorsoventral axis formation

In order to explore the role of morphogenetic movement in the establishment of anteroposterior and dorsoventral axes, we sought to identify novel in vivo inhibitors of gastrulation movements in Xenopus laevis. Injection of hydrolytic sulfatase into the blastocoels of gastrula stage embryos resulted in severe anteroposterior truncation, without a corresponding truncation of the dorsoventral axis. Confocal microscopy of whole embryos revealed that gastrulation movements are severely disrupted by sulfatase; in addition, sulfatase dramatically inhibited chordomesodermal cell elongation and convergent extension movements in planar dorsal marginal zone explants. The phenotype of anteroposterior reduction elicited by sulfatase is distinctly different from commonly generated dorsoanterior phenotypes (e.g., ultraviolet irradiation of the vegetal cortex prior to cortical rotation or suramin injection), and the two varieties of phenotype appear to result from inhibition of distinct, separable components of the axis-generating machinery.

Disruption of gastrulation and oral-aboral ectoderm differentiation in the Lytechinus pictus embryo by a dominant/negative PDGF receptor

Little is known about the cell signaling involved in forming the body plan of the sea urchin embryo. Previous work suggested that PDGF-like and EGF-like receptor-mediated signaling pathways are involved in gastrulation and spiculogenesis in the Lytechinus pictus embryo. Here we show that expression of the human PDGF receptor-beta lacking the cytoplasmic domain disrupted development in a manner consistent with a dominant/negative mechanism. The truncated PDGF receptor-beta inhibited gut and spicule formation and differentiation along the oral-aboral axis. The most severely affected embryos arrested at a developmental stage resembling mesenchyme blastula. Coinjection into eggs of RNA encoding the entire human PDGF receptor-beta rescued development. The truncated PDGF receptor-beta caused the aboral ectoderm-specific genes LpS1 and LpC2 to be repressed while an oral ectoderm-specific gene, Ecto-V, was expressed in all ectoderm cells. The results support the hypothesis that a PDGF-like signaling pathway plays a key role in the intercellular communication required for gastrulation and spiculogenesis, and in cell commitment and differentiation along the oral-aboral axis.

The helix-loop-helix protein Id-1 and a retinoblastoma protein binding mutant of SV40 T antigen synergize to reactivate DNA synthesis in senescent human fibroblasts

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.

Identification of a transcript that is down-regulated in senescent human fibroblasts. Cloning, sequence analysis, and regulation of the human L7 ribosomal protein gene

Normal eukaryotic cells divide only a limited number of times before proliferation ceases due to cellular senescence. We previously reported that a constitutively expressed, non-cell cycle-regulated transcript of unknown identity declines severalfold when human fibroblasts become senescent. We show here, from the sequence of cDNA and genomic clones, that this transcript encodes L7, a structural protein of the large ribosomal subunit. The human L7 protein shares > 90% amino acid identity with the mouse and rat L7 proteins but is shorter than either rodent protein due to fewer basic repetitive motifs at the amino terminus. The position of the first intron is conserved between the mouse and human genes. The L7 mRNA was abundant, stable (t1/2 > 10 h), and polyadenylated in presenescent and senescent human fibroblasts; however, steady state mRNA levels were 5-10-fold lower in senescent cells, whether derived from fetal lung or neonatal foreskin. Quiescent and senescent cells synthesized protein at similar rates, yet only senescent cultures showed a decline in L7 mRNA. The mRNAs encoding five other ribosomal proteins (L5, P1, S3, S6, and S10) behaved similarly. The results suggest that the senescence-associated decline in L7 and other ribosomal protein mRNAs is unrelated to growth state or protein synthetic rate per se and support the view that senescence and quiescence are dissimilar states.

The role of RNA polymerase initiation and elongation in control of total RNA and histone mRNA synthesis in sea urchin embryos

The involvement of RNA polymerase initiations in regulating total RNA synthesis and the synthesis of the early histone mRNAs was investigated. Nuclei were isolated from developing sea urchin embryos from 4- to 600-cell stages, and the transcription of already initiated polymerase complexes was studied in a "run-off," or elongation, assay; this assay was optimized by using high levels of ribonucleoside triphosphates. Under these conditions the relative levels of RNA synthesis in isolated nuclei from different stages closely paralleled the known rates of synthesis in vivo. However, if sarkosyl is included in the elongation assay, the nuclei of older stages display greatly stimulated synthesis while early cleavage stage nuclei are not stimulated. Sarkosyl does not reveal any elongated transcripts from the early histone genes in nuclei from later stages of development. This has been interpreted to mean that there are many initiated polymerase II complexes that do not elongate rapidly at later stages, but the early histone genes are inactive at later stages because they do not possess any productively initiated polymerases.

Requirement of hydrocortisone and insulin for extended proliferation and passage of rat keratinocytes

A procedure for the preparation and cultivation of rat epidermal basal cells from full thickness skin resulted in greater than 99% viability and 90% plating efficiency. However, attempts to subculture monolayers of these epithelial cells grown in medium with serum as the only supplement were totally unsuccessful. When hydrocortisone and insulin were added to the medium, subcultivation of primary growth was obtained. It was demonstrated that hydrocortisone at concentrations as low as 0.1 micrograms/ml was necessary for at least the initial attachment of the cells to the substrate - an essential step in subcultivation. Increasing concentrations of insulin (0.1 to 50 micrograms/ml) caused the rate of proliferation and the cell density to increase, but insulin alone did not support subcultivation.