. Gap junctions are composed of a family of structural proteins called connexins, which oligomerize into intercellular channels and function to exchange low molecular weight metabolites and ions between adjacent cells . We have cloned a new member of the connexin family from lens cDNA, with a predicted molecular mass of 46 kD, called rat connexin46 (Cx46) . Since a full-length cDNA corresponding to the 2.8-kb mRNA was not obtained, the stop codon and surrounding sequences were confirmed from rat genomic DNA . The RNA coding for this protein is abundant in lens fibers and detectable in both myocardium and kidney. Western analysis of both rat and bovine lens membrane proteins, using the anti-MP70 monoclonal antibody 6-4-B2-C6 and three anti-peptide antibodies against Cx46 demonstrates that Cx46 and MP70 are different proteins. Immunocytochemistry demonstrates that both proteins are localized in the same lens fiber junctional maculae. Synthesis of Cx46 in either reticulocyte lysate or Xenopus oocytes yields a 46-kD polypeptide ; P junctions are composed of a family of membrane structural proteins, called connexins, which oligo-

A reliable, nontoxic method of inducing transplantation tolerance is needed to overcome the problems of chronic organ graft rejection and immunosuppression-related toxicity. Treatment of mice with single injections of an anti-CD40 ligand antibody and CTLA4Ig, a low dose (3 Gy) of whole body irradiation, plus fully major histocompatibility complex–mismatched allogeneic bone marrow transplantation (BMT) reliably induced high levels (>40%) of stable (>8 mo) multilineage donor hematopoiesis. Chimeric mice permanently accepted donor skin grafts (>100 d), and rapidly rejected third party grafts. Progressive deletion of donor-reactive host T cells occurred among peripheral CD4+ lymphocytes, beginning as early as 1 wk after bone marrow transplantation. Early deletion of peripheral donor-reactive host CD4 cells also occurred in thymectomized, similarly treated marrow recipients, demonstrating a role for peripheral clonal deletion of donor-reactive T cells after allogeneic BMT in the presence of costimulatory blockade. Central intrathymic deletion of newly developing T cells ensued after donor stem cell engraftment had occurred. Thus, we have shown that high levels of chimerism and systemic T cell tolerance can be reliably achieved without myeloablation or T cell depletion of the host. Chronic immunosuppression and rejection are avoided with this powerful, nontoxic approach to inducing tolerance.

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Gap junction communication in some cells has been shown to be inhibited by pp60v-src, a protein tyrosine kinase encoded by the viral oncogene v-src. The gap junction protein connexin43 (Cx43) has been shown to be phosphorylated on serine in the absence of pp60v-src and on both serine and tyrosine in cells expressing pp60v-src. However, it is not known if the effect of v-src expression on communication results directly from tyrosine phosphorylation of the Cx43 or indirectly, for example, by activation of other second-messenger systems. In addition, the effect of v-src expression on communication based on other connexins has not been examined. We have used a functional expression system consisting of paired Xenopus oocytes to examine the effect of v-src expression on the regulation of communication by gap junctions comprised of different connexins. Expression of pp60v-src completely blocked the communication induced by Cx43 but had only a modest effect on communication induced by connexin32 (Cx32). Phosphoamino acid analysis showed that pp60v-src induced tyrosine phosphorylation of Cx43, but not Cx32. A mutation replacing tyrosine 265 of Cx43 with phenylalanine abolished both the inhibition of communication and the tyrosine phosphorylation induced by pp60v-src without affecting the ability of this protein to form gap junctions. These data show that the effect of pp60v-src on gap junctional communication is connexin specific and that the inhibition of Cx43-mediated junctional communication by pp60v-src requires tyrosine phosphorylation of Cx43.

The actin-myosin cytoskeleton is generally accepted to produce the contractile forces necessary for cellular processes such as cell rounding and migration. All vertebrates examined to date are known to express at least two isoforms of non-muscle myosin II, referred to as myosin IIA and myosin IIB. Studies of myosin IIA and IIB in cultured cells and null mice suggest that these isoforms perform distinct functions. However, how each myosin II isoform contributes individually to all the cellular functions attributed to "myosin II" has yet to be fully characterized. Using isoform-specific small-interfering RNAs, we found that depletion of either isoform resulted in opposing migration phenotypes, with myosin IIA-and IIB-depleted cells exhibiting higher and lower wound healing migration rates, respectively. In addition, myosin IIA-depleted cells demonstrated impaired thrombin-induced cell rounding and undertook a more motile morphology, exhibiting decreased amounts of stress fibers and focal adhesions, with concomitant increases in cellular protrusions. Cells depleted of myosin IIB, however, were efficient in thrombin-induced cell rounding, displayed a more retractile phenotype, and maintained focal adhesions but only in the periphery. Last, we present evidence that Rho kinase preferentially regulates phosphorylation of the regulatory light chain associated with myosin IIA. Our data suggest that the myosin IIA and IIB isoforms are regulated by different signaling pathways to perform distinct cellular activities and that myosin IIA is preferentially required for Rho-mediated contractile functions.

Donor-specific tolerance induced by bone marrow transplantation (BMT) would allow organ allografting without chronic immunosuppressive therapy. However, the toxicity of conditioning regimens used to achieve marrow engraftment has precluded the clinical use of BMT for tolerance induction. We have developed a BMT strategy that achieves alloengraftment without toxic or myelosuppressive host conditioning. B6 mice received depleting anti-CD4 and anti-CD8 monoclonal antibodies, local thymic irradiation, and a high-dose (174 x 10(6)) of major histocompatibility (MHC)-mismatched B10.A bone marrow cells (BMCs) divided over days 0 through 4. High levels of donor cells were observed among white blood cells (WBCs) of all lineages. Permanent, multilineage mixed chimerism; donor-specific skin-graft tolerance; and in vitro tolerance were observed in most animals. Large numbers of donor class II(high) cells were detected in thymuses of long-term chimeras, and their presence was associated with intrathymic deletion of donor-reactive host thymocytes. The treatment was not associated with significant myelosuppression, toxicity, or graft-versus-host disease (GVHD). Thus, high levels of allogeneic stem-cell engraftment can be achieved without myelosuppressive host conditioning. As stem-cell mobilization and in vitro culture techniques have increased the feasibility of administering high doses of hematopoietic cells to humans, this approach brings hematopoietic cell transplantation closer to clinical use for the induction of central deletional T-cell tolerance.

Abstract. In clams, fertilization is followed by the prominent synthesis of two cyclins, A and B. During the mitotic cell cycles, the two cyclins are accumulated and then destroyed near the end of each metaphase. Newly synthesized cyclin B is complexed with a small set of other proteins, including a kinase that phosphorylates cyclin B in vitro. While both cyclins can act as general inducers of entry into M phase, the two are clearly distinguished by their amino acid sequences (70% nonidentity) and by their different modes of expression in oocytes and during meiosis. In contrast to cyclin A, which is stored solely as maternal mRNA, oocytes contain a stockpile of cyclin B protein, which is stored in large, rapidly sedimenting aggregates. Fertilization results in the release of cyclin B to a more disperse, soluble form. Since the first meiotic division in clams can proceed even when new protein synthesis is blocked, these results strongly suggest it is the fertilization-triggered unmasking of cyclin B protein that drives cells into meiosis I. We propose that the unmasking of maternal cyclin B protein allows it to interact with cdc2 protein kinase, which is also stored in oocytes, and that the formation of this cyclin B/cdc2 complex generates active M phase-promoting factor.