Mutation of two Mycoplasma arthritidis surface lipoproteins with divergent functions in cytadherence.

Mycoplasma arthritidis is a natural pathogen of rats, causing an acute polyarthritis. Previous studies identified two membrane-bound lipoproteins, Maa1 and Maa2, thought to be associated with cytadherence of M. arthritidis strain 158p10p9. We have since confirmed that Maa1 is a major adhesin, although the role of Maa2 has proven more elusive. Both proteins were capable of eliciting protective immunity in rats against challenge with the virulent strain 158p10p9, suggesting that they may be important in pathogenesis. The purpose of this study was to better understand the roles of Maa1 and Maa2 in cytadherence in vitro. Insertion mutants were created for both genes by transposon mutagenesis. In vitro adherence of the Maa1 mutant KOMaa1 to rat L2 lung cells was reduced to the level previously reported for a spontaneous low-adherence mutant of 158p10p9 in which Maa1 is truncated and nonfunctional. Surprisingly, adherence of the Maa2 mutant KOMaa2 was approximately fivefold greater than that of the wild type. Complementation of KOMaa1 and KOMaa2 with wild-type alleles of maa1 and maa2, respectively, returned adherence to wild-type levels. This work confirms our earlier observation that Maa1 is a major adhesin for M. arthritidis strain 158p10p9. Maa2, on the other hand, may play a suppressive or modulatory role, possibly serving to release organisms from microcolonies at certain stages of infection.

Upregulation of gamma-catenin compensates for the loss of beta-catenin in adult cardiomyocytes.

Recent progresses in signal transduction have revealed that beta-catenin signaling controls embryonic development, tumorigenesis, cell shape, and polarity. The role of this pathway in myocyte shape regulation during cardiac hypertrophy and failure is, however, not clearly defined. Since homozygous knockout of beta-catenin is embryonically lethal, we have deleted beta-catenin genes specifically in the heart of adult mice by crossing loxP-flanked beta-catenin mice with transgenic mice expressing tamoxifen-activated MerCreMer protein (MCM) driven by the alpha-myosin heavy chain promoter. Administration of tamoxifen to homozygous loxP-flanked beta-catenin mice positive for MCM induces the deletion of beta-catenin only in cardiomyocytes. Immunolabeling with beta-catenin antibody demonstrates that 90% of cardiomyocytes completely lose their beta-catenin expression but maintain normal rod-shaped morphology. The intercalated disk of cardiomyocytes lacking beta-catenin is morphologically unremarkable with normal distribution of vinculin, N-cadherin, desmoplakin, ZO-1, connexin43, and alpha-, gamma-, and p120 catenins. The expression level of these proteins, except that of gamma-catenin, is also similar in tamoxifen-treated and control mice with both homozygous loxP-flanked beta-catenin genes and the MCM transgene. Western blot analyses reveal that gamma-catenin increases in the heart of beta-catenin knockout mice compared with controls. Confocal microscopy also demonstrates that gamma-catenin has significantly increased in the intercalated disk of cardiomyocytes lacking beta-catenin. Echocardiographic data indicate that the knockout mice maintain normal ventricular geometry and cardiac function. The results suggest that upregulation of gamma-catenin can compensate for the loss of beta-catenin in cardiomyocytes to maintain normal cardiac structure and function.