Dystroglycan is a transmembrane heterodimeric complex of ␣ and ␤ subunits that links the extracellular matrix to the cell cytoskeleton. It was originally identified in skeletal muscle, where it anchors dystrophin to the sarcolemma. Dystroglycan is also highly expressed in nonmuscle tissues, including brain. To investigate the molecular interactions of dystroglycan in the CNS, we fractionated a digitonin-soluble extract from bovine brain synaptosomes by laminin-affinity chromatography and characterized the protein components. The 120-kDa ␣-dystroglycan was the major 125 I-laminin-labeled protein detected by overlay assay. This complex, in addition to ␤-dystroglycan, was also found to contain Grb2 and focal adhesion kinase p125 FAK (FAK). Anti-FAK antibodies co-immunoprecipitated Grb2 with FAK. However, no direct interaction between ␤-dystroglycan and FAK was detected by co-precipitation assay. Grb2, an adaptor protein involved in signal transduction and cytoskeleton organization, has been shown to bind ␤-dystroglycan. We isolated both FAK and Grb2 from synaptosomal extracts by chromatography on immobilized recombinant ␤-dystroglycan. In the CNS, FAK phosphorylation has been linked to membrane depolarization and neurotransmitter receptor activation. At the synapses, the adaptor protein Grb2 may mediate FAK-␤-dystroglycan interaction, and it may play a role in transferring information between the dystroglycan complex and other signaling pathways. Key Words: Dystroglycan-Focal adhesion kinase-Laminin-CytoskeletonGrb2-Central nervous system.

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Poly(ADP-ribosyl)ation, catalysed by a family of poly (ADP-ribose) polymerases (PARPs), plays an important role in a large variety of physiological processes, including cell proliferation, but its role in cell cycle progression is not yet completely defined. As reported here, the examination of early times following serum stimulation of quiescent fibroblasts suggests that poly(ADP-ribosyl) ation is necessary for the transition from the G0 phase to the G1 phase. We show that PARP activity is involved in this step through the regulation of immediate-early response genes, such as c-Fos and c-Myc. This is supported by the finding that exogenous Myc expression substantially restores cell cycle reactivation in the absence of polymer synthesis. Furthermore, using RNA interference, we show that PARP-1 is the PARP family member playing the most prominent role in the upregulation of c-Fos and c-Myc during G0-G1 transition. We report that even in lectin-stimulated peripheral blood mononucleated cells, the inhibition of PARP activity interferes with the upregulation of immediate-early genes and delays the induction of proliferation, suggesting a general role for PARP-1 in linking growth factor signaling with cell cycle entry.

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Murine polyomavirus (MPyV) infection occurs through recognition of sialic acid (SA) residues present on the host cell membrane, but the nature of the molecules involved and the exact role of this interaction in virus cell entry still need to be clarified. In this work, mutations at residues R 77 or H 298 of the MPyV VP1 protein were shown to lead to a complete loss of virus infectivity, which, however, could be restored by lipofection of virus particles into the cytoplasm of the host cells. Using virus-like particles (VLPs), it was demonstrated that the non-infectivity of these mutants was due to impaired cell entry caused by total abrogation of SA-dependent cell binding. This indicates that SA residues are essential primary cell receptors for MPyV. As the a4b1 integrin has been identified recently as a cell receptor for MPyV, the relationship, if any, was investigated between SA-containing and a4b1 integrin receptors. The ability of mutants R 77 Q and H 298 Q and wt VLPs to bind to cells overexpressing the a4b1 integrin was studied in SA-positive (BALB/c 3T3 cells and Pro-5 cells) and SA-deficient (Pro5-derived Lec-2 cells) backgrounds. Overexpression of a4b1 integrin did not restore binding of mutant VLPs in any of these cell lines or, indeed, that of wt VLPs in a SA-deficient background. Moreover, evidence is provided that overexpression of the sialylated a4b1 integrin enhances wt VLP cell binding, suggesting that, in addition to its function at a post-attachment level, a4b1 integrin acts also as one of the SA-containing receptors for initial cell binding.