The nuclear matrix is defined as the insoluble framework of the nucleus and has been implicated in the regulation of gene expression, the cell cycle, and nuclear structural integrity via linkage to intermediate filaments of the cytoskeleton. We have discovered a novel nuclear matrix protein, NRP/B (nuclear restricted protein/brain), which contains two major structural elements: a BTB domain–like structure in the predicted NH2 terminus, and a “kelch motif” in the predicted COOH-terminal domain. NRP/B mRNA (5.5 kb) is predominantly expressed in human fetal and adult brain with minor expression in kidney and pancreas. During mouse embryogenesis, NRP/B mRNA expression is upregulated in the nervous system. The NRP/B protein is expressed in rat primary hippocampal neurons, but not in primary astrocytes. NRP/B expression was upregulated during the differentiation of murine Neuro 2A and human SH-SY5Y neuroblastoma cells. Overexpression of NRP/B in these cells augmented neuronal process formation. Treatment with antisense NRP/B oligodeoxynucleotides inhibited the neurite development of rat primary hippocampal neurons as well as the neuronal process formation during neuronal differentiation of PC-12 cells. Since the hypophosphorylated form of retinoblastoma protein (p110RB) is found to be associated with the nuclear matrix and overexpression of p110RB induces neuronal differentiation, we investigated whether NRP/B is associated with p110RB. Both in vivo and in vitro experiments demonstrate that NRP/B can be phosphorylated and can bind to the functionally active hypophosphorylated form of the p110RB during neuronal differentiation of SH-SY5Y neuroblastoma cells induced by retinoic acid. Our studies indicate that NRP/B is a novel nuclear matrix protein, specifically expressed in primary neurons, that interacts with p110RB and participates in the regulation of neuronal process formation.

Human artificial chromosome (HAC)-based vectors offer a promising system for delivery and expression of full-length human genes of any size. HACs avoid the limited cloning capacity, lack of copy number control, and insertional mutagenesis caused by integration into host chromosomes that plague viral vectors. We previously described a synthetic HAC that can be easily eliminated from cell populations by inactivation of its conditional kinetochore. Here, we demonstrate the utility of this HAC, which has a unique gene acceptor site, for delivery of full-length genes and correction of genetic deficiencies in human cells. A battery of functional tests was performed to demonstrate expression of NBS1 and VHL genes from the HAC at physiological levels. We also show that phenotypes arising from stable gene expression can be reversed when cells are "cured" of the HAC by inactivating its kinetochore in proliferating cell populations, a feature that provides a control for phenotypic changes attributed to expression of HAC-encoded genes. This generation of human artificial chromosomes should be suitable for studies of gene function and therapeutic applications.

The cytoskeleton plays an important role in neuronal morphogenesis. We have identified and characterized a novel actin-binding protein, termed Mayven, predominantly expressed in brain. Mayven contains a BTB (broad complex, tramtrack, bric-a-brac)/POZ (poxvirus, zinc finger) domain-like structure in the predicted N terminus and "kelch repeats" in the predicted C-terminal domain. Mayven shares 63% identity (77% similarity) with the Drosophila ring canal ("kelch") protein. Somatic cell-hybrid analysis indicated that the human Mayven gene is located on chromosome 4q21.2, whereas the murine homolog gene is located on chromosome 8. The BTB/POZ domain of Mayven can self-dimerize in vitro, which might be important for its interaction with other BTB/POZcontaining proteins. Confocal microscopic studies of endogenous Mayven protein revealed a highly dynamic localization pattern of the protein. In U373-MG astrocytoma/ glioblastoma cells, Mayven colocalized with actin filaments in stress fibers and in patchy cortical actin-rich regions of the cell margins. In primary rat hippocampal neurons, Mayven is highly expressed in the cell body and in neurite processes. Binding assays and far Western blotting analysis demonstrated association of Mayven with actin. This association is mediated through the "kelch repeats" within the C terminus of Mayven. Depolarization of primary hippocampal neurons with KCl enhanced the association of Mayven with actin. This increased association resulted in dynamic changes in Mayven distribution from uniform to punctate localization along neuronal processes. These results suggest that Mayven functions as an actin-binding protein that may be translocated along axonal processes and might be involved in the dynamic organization of the actin cytoskeleton in brain cells.

Vascular endothelial growth factor (VEGF) plays a significant role in blood-brain barrier breakdown and angiogenesis after brain injury. VEGF-induced endothelial cell migration is a key step in the angiogenic response and is mediated by an accelerated rate of focal adhesion complex assembly and disassembly. In this study, we identified the signaling mechanisms by which VEGF regulates human brain microvascular endothelial cell (HBMEC) integrity and assembly of focal adhesions, complexes comprised of scaffolding and signaling proteins organized by adhesion to the extracellular matrix. We found that VEGF treatment of HBMECs plated on laminin or fibronectin stimulated cytoskeletal organization and increased focal adhesion sites. Pretreating cells with VEGF antibodies or with the specific inhibitor SU-1498, which inhibits Flk-1/KDR receptor phosphorylation, blocked the ability of VEGF to stimulate focal adhesion assembly. VEGF induced the coupling of focal adhesion kinase (FAK) to integrin ␣ v ␤ 5 and tyrosine phosphorylation of the cytoskeletal components paxillin and p130cas . Additionally, FAK and related adhesion focal tyrosine kinase (RAFTK)/Pyk2 kinases were tyrosine-phosphorylated by VEGF and found to be important for focal adhesion sites. Overexpression of wild type RAFTK/Pyk2 increased cell spreading and the migration of HBMECs, whereas overexpression of catalytically inactive mutant RAFTK/Pyk2 markedly suppressed HBMEC spreading (ϳ70%), adhesion (ϳ82%), and migration (ϳ65%). Furthermore, blocking of FAK by the dominant-interfering mutant FRNK (FAK-related non-kinase) significantly inhibited HBMEC spreading and migration and also disrupted focal adhesions. Thus, these studies define a mechanism for the regulatory role of VEGF in focal adhesion complex assembly in HBMECs via activation of FAK and RAFTK/Pyk2.

The integrity of the blood-brain barrier (BBB) is critical for normal brain function. Neuropathological abnormalities in AIDS patients have been associated with perivascular HIV-infected macrophages, gliosis, and abnormalities in the permeability of the BBB. The processes by which HIV causes these pathological conditions are not well understood. To characterize the mechanism by which HIV-1 Tat protein modulates human brain microvascular endothelial cell (HBMEC) functions, we studied the effects of HIV-1 Tat in modulating HBMEC apoptosis and permeability. Treatment of HBMEC with HIV-1 Tat led to Flk-1/KDR and Flt-4 receptor activation and the release of NO. The protein levels of endothelial NO synthase (NOS) and inducible NOS were increased by HIV-1 Tat stimulation. Importantly, HIV-1 Tat caused apoptosis of HBMEC, as evidenced by changes in the cleavage of poly(A)DP-ribose polymerase, DNA laddering, and incorporation of fluorescein into the nicked chromosomal DNA (TUNEL assay). HIV-1 Tat-mediated apoptosis in HBMEC was significantly inhibited in the presence of N-nitro-l-arginine methyl ester (an inhibitor of NOS) and wortmannin (a phosphoinositol 3-kinase inhibitor). Furthermore, HIV-1 Tat treatment significantly increased HBMEC permeability, and pretreatment with both N-nitro-l-arginine methyl ester and wortmannin inhibited the Tat-induced permeability. Taken together, these results indicate that dysregulated production of NO by HIV-1 Tat plays a pivotal role in brain endothelial injury, resulting in the irreversible loss of BBB integrity, which may lead to enhanced infiltration of virus-carrying cells across the BBB.

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Rearrangement of the cytoskeleton leading to the extension of cellular processes is essential for the myelination of axons by oligodendrocytes. We observed that the actin-binding protein, Mayven, is expressed during all stages of the oligodendrocyte lineage, and that its expression is up-regulated during oligodendrocyte differentiation. Mayven is localized in the cytoplasm and along the cell processes. Mayven also binds actin, and is involved in the cytoskeletal reorganization in oligo

Smad3, a major intracellular mediator of TGFb signaling, functions as both a positive and negative regulator in carcinogenesis. In response to TGFb, the TGFb receptor phosphorylates serine residues at the Smad3 C-tail. Cancer cells often contain high levels of the MAPK and CDK activities, which can lead to the Smad3 linker region becoming highly phosphorylated. Here, we report, for the first time, that mutation of the Smad3 linker phosphorylation sites markedly inhibited primary tumor growth, but significantly increased lung metastasis of breast cancer cell lines. In contrast, mutation of the Smad3 C-tail phosphorylation sites had the opposite effect. We show that mutation of the Smad3 linker phosphorylation sites greatly intensifies all TGFb-induced responses, including growth arrest, apoptosis, reduction in the size of putative cancer stem cell population, epithelialmesenchymal transition, and invasive activity. Moreover, all TGFb responses were completely lost on mutation of the Smad3 C-tail phosphorylation sites. Our results demonstrate a critical role of the counterbalance between the Smad3 C-tail and linker phosphorylation in tumorigenesis and metastasis. Our findings have important implications for therapeutic intervention of breast cancer. Cancer Res; 74(21);