Generation of mice with conditional ablation of the Cd40lg gene: new insights on the role of CD40L.

CD40 ligand (CD40L) acts as an immune modulator in activated T cells, and mutations in the extracellular domain are associated to X-linked hyper IgM syndrome. A role for platelet CD40L in mediating thrombotic and inflammatory processes in atherosclerosis has also been reported. Using the Cre/loxP recombination technology we generated four knockout lines of mice with deletion of the Cd40lg gene restricted to the hematopoietic system. Mouse lines with expression of Cre recombinase driven by the Tie2, Vav1, or CD4 promoters showed in vivo ablation of CD40L in leukocytes and platelets. In contrast, in mice with Cre expression driven by the megakaryocyte lineage-restricted Pf4 promoter, abolition of CD40L expression was observed in megakaryocytes cultured in vitro, but not in circulating platelets. Characterization of these animals revealed reduced in vivo thrombogenesis and defective activation of washed CD40L-deficient platelets, suggesting that membrane-bound CD40L is involved in the control of haemostasis acting as a platelet co-activator. In addition, we report the practically absence of CD40L in mouse and human endothelial cells, as well as the detection of an exon 3-deleted CD40L transcript in both platelets and leukocytes of mouse and human origin. Finally, compared with their corresponding littermate floxed controls, Cre+ mice carrying CD40-deficient leukocytes did not exhibit increased IgM levels, and reduction of IgA and IgG levels was statistically significant only in Tie2-Cre+ mice, suggesting that expression of CD40L in an earlier developmental step may be determinant in the regulation of the class switch recombination process.

Turning on the machine: genetic control of axon regeneration by c-Jun.

Upregulation of the transcription factor c-Jun has been correlated with axon regeneration after injury in multiple types of neurons. In this issue of Neuron, Raivich et al. use a nervous system-specific mutant to provide genetic evidence that c-Jun is necessary for efficient axon regeneration.

Atm and c-Abl cooperate in the response to genotoxic stress during nervous system development.

The c-Abl proto-oncogene is a target of the ATM kinase after DNA double strand breaks, although the physiological significance of these signaling events is not clear. Therefore, to delineate the roles of c-Abl and Atm during mouse development we generated mice with combinations of c-Abl and Atm mutant alleles. We found that dual inactivation of Atm and c-Abl usually resulted in midgestational lethality. However, mice with three mutant alleles, c-Abl(-/-)Atm(+/-) or c-Abl(+/-)Atm(-/-), were viable but predisposed to neuro-developmental abnormalities after genotoxic insult. Thus, these genetic data link Atm and c-Abl signaling and underscore a significant interrelationship between the two during neural development.

Physiological levels of tumstatin, a fragment of collagen IV alpha3 chain, are generated by MMP-9 proteolysis and suppress angiogenesis via alphaV beta3 integrin.

We demonstrate a physiological role for tumstatin, a cleavage fragment of the alpha3 chain of type IV collagen (Col IValpha3), which is present in the circulation. Mice with a genetic deletion of Col IValpha3 show accelerated tumor growth associated with enhanced pathological angiogenesis, while angiogenesis associated with development and tissue repair are unaffected. Supplementing Col IValpha3-deficient mice with recombinant tumstatin to a normal physiological concentration abolishes the increased rate of tumor growth. The suppressive effects of tumstatin require alphaVbeta3 integrin expressed on pathological, but not on physiological, angiogenic blood vessels. Mice deficient in matrix metalloproteinase-9, which cleaves tumstatin efficiently from Col IValpha3, have decreased circulating tumstatin and accelerated growth of tumor. These results indicate that MMP-generated fragments of basement membrane collagen can have endogenous function as integrin-mediated suppressors of pathologic angiogenesis and tumor growth.

Knockout mice as model systems for studying nm23/NDP kinase gene functions. Application to the nm23-M1 gene.

Mice carrying a homozygous germ-line mutation in the nm23-M1 gene that eliminates its protein expression and drives expression of beta-galactosidase by nm23-M1 promoter have been generated. nm23-M1 gene inactivation is not teratogenic and the pups can grow to adult age without apparent health problems. However, they undergo a growth retardation and knocked out females cannot feed their pups. Both effects are background dependent. Beta-galactosidase mapping of nm23-M1 promoter activation during embryogenesis shows that the nm23-M1 gene is principally expressed in epithelial layer of tissues which require inductive epithelial-mesenchymal interactions for their formation. In conclusion, invalidated mice could be interesting models to analyze the role of nm23-M1 on signal transduction pathway regulation, or cancer induction and proliferation.

Growth retardation in mice lacking the proteasome activator PA28gamma.

The proteasome activator PA28 binds to both ends of the central catalytic machine, known as the 20 S proteasome, in opposite orientations to form the enzymatically active proteasome. The PA28 family is composed of three members designated alpha, beta, and gamma; PA28alpha and PA28beta form the heteropolymer mainly located in the cytoplasm, whereas PA28gamma forms a homopolymer that predominantly occurs in the nucleus. Available evidence indicates that the heteropolymer of PA28alpha and PA28beta is involved in the processing of intracellular antigens, but the function of PA28gamma remains elusive. To investigate the role of PA28gamma in vivo, we generated mice deficient in the PA28gamma gene. The PA28gamma-deficient mice were born without appreciable abnormalities in all tissues examined, but their growth after birth was retarded compared with that of PA28gamma(+/-) or PA28gamma(+/+) mice. We also investigated the effects of the PA28gamma deficiency using cultured embryonic fibroblasts; cells lacking PA28gamma were larger and displayed a lower saturation density than their wild-type counterparts. Neither the expression of PA28alpha/beta nor the subcellular localization of PA28alpha was affected in PA28gamma(-/-) cells. These results indicate that PA28gamma functions as a regulator of cell proliferation and body growth in mice and suggest that neither PA28alpha nor PA28beta compensates for the PA28gamma deficiency.