). Although the mechanism of action of pllOR"I remains unknown, several lines of evidence suggest that it plays a role in the regulation of transcription. We now show that overexpression of pllOl causes repression of the adenovirus early promoter EIIaE and the promoters of two cellular genes, c-myc and RBI, both of which contain E2F-binding motifs. Mutation of the E2 element in the c-myc promoter abolishes pllOWl repression.We also demonstrate that a p110"R1 mutant, which is refractory to cell cycle phosphorylation but intact in Ela/large T antigen-binding properties, represses EHaE with 50-to 80-fold greater efficiency than wild-type pllOR11. These data provide evidence that hypophosphorylated pllOIWl actively represses expression of genes with promoters containing the E2F-binding motif (E2 element).Progression through the eukaryotic cell cycle appears to be regulated at a number of restriction points. For example, following mitogenic stimulation of quiescent fibroblasts, progression through G, towards the S phase requires a rapid induction of the proto-oncogene c-myc. Inhibition of c-myc expression with antisense c-myc oligonucleotides prevents cells from entering the S phase (20,28). In addition to positive factors such as c-myc, however, negative factors also play an important role in the regulation of the cell cycle. Furthermore, as with positive factors, progression to the transformed phenotype involves deregulation of these negative factors (52).The first negative regulator of the cell cycle to be identified was the product of the retinoblastoma susceptibility gene (RB1) (18), pllOl, a nuclear phosphoprotein with a relative molecular mass of 110 to 116 kDa (36; for a review, see reference 23). Negative regulation of the cell cycle by RBJ was implied from the model proposed by Knudson (33) and Comings (9), which predicted that retinoblastoma arose because of mutation of both alleles of RBI. This prediction was subsequently verified by characterization of mutations in retinoblastoma tumors (12)(13)(14)29). More recently, a number of observations have supported the model that pllO1l acts, in part, to control progression to the S phase. First, pllOl is modified in a cell cycle-dependent manner (5,7,11,43

The neuroprotective effects of MK-801, a noncompetitive antagonist of N-methyl-D-aspartate (NMDA) receptors, were evaluated in models of cerebral ischemia using Mongolian gerbils. Bilateral occlusion of the carotid arteries for a period of 5 min resulted in a consistent pattern of degeneration of hippocampal CA1 and CA2 pyramidal neurons, which was quantified using an image analyzer. Systemic administration of MK-801 (0.01-10 mg/kg, i.p.) 1 hr prior to the occlusion caused a dose-dependent protection of the CA1 and CA2 neurons. The ED50 value for neuroprotection by MK-801 was calculated to be 0.3 mg/kg, and at doses greater than or equal to 3 mg/kg the majority of animals were completely protected against the ischemic insult. Systemic administration of MK-801 (1 or 10 mg/kg, i.p.) 1 hr prior to unilateral occlusion of the right carotid artery resulted in significant protection against hippocampal neurodegeneration following 10 min of occlusion, and increased the survival rate after 30 min of occlusion. The potent neuroprotective effects of MK-801 in these cerebral ischemia models add further weight to the evidence that NMDA receptors are involved in the mechanism of ischemia-induced neuronal degeneration.

Eukaryotic telomerase contains a telomerase reverse transcriptase (TERT) and an RNA template component that are essential for telomerase catalytic activity and several other telomerase-associated factors of which only a few appear to be integral enzyme components [1-3]. The first essential telomerase protein identified was S. cerevisiae Est1p, whose deletion leads to ever-shorter telomeres despite the persistence of telomerase activity [4-6]. Extensive genetic and biochemical data show that Est1p, via its interaction with the telomerase RNA and telomere end DNA binding complex Cdc13p/Stn1p/Ten1p, promotes the ability of telomerase to elongate telomeres in vivo [7-22]. The characterization of Est1p homologs outside of yeast has not been documented. We report the characterization of two putative human homologs of Est1p, hEST1A and hEST1B. Both proteins specifically associated with telomerase activity in human cell extracts and bound hTERT in rabbit reticulocyte lysates independently of the telomerase RNA. Overproduction of hEST1A cooperated with hTERT to lengthen telomeres, an effect that was specific to cells containing telomerase activity. Like Est1p, hEST1A (but not hEST1B) exhibited a single-stranded telomere DNA binding activity. These results suggest that the telomerase-associated factor Est1p is evolutionarily conserved in humans.

Stroke is one of the most complex diseases, with several subtypes, as well as secondary risk factors, such as hypertension, hyperlipidemia, and diabetes, which, in turn, have genetic and environmental risk factors of their own. Here, we report the results of a genomewide search for susceptibility genes for the common forms of stroke. We cross-matched a population-based list of patients with stroke in Iceland with an extensive computerized genealogy database clustering 476 patients with stroke within 179 extended pedigrees. Linkage to 5q12 was detected, and the LOD score at this locus meets the criteria for genomewide significance (multipoint allele-sharing LOD score of 4.40, P=3.9 x 10(-6)). A 20-cM region on 5q was physically and genetically mapped to obtain accurate marker order and intermarker distances. This locus on 5q12, which we have designated as "STRK1," does not correspond to known susceptibility loci for stroke or for its risk factors and represents the first mapping of a locus for common stroke.