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Male hypogonadic (hgn/hgn) rats show male sterility, reduced female fertility, progressive renal insufficiency and body growth retardation. These defects are associated with loss-of-function mutation of astrin and appear to be related to organ hypoplasia resulting from abnormal cell proliferation and increased cell death during embryonic and early postnatal development. As targeted disruption of mouse spag5 (astrin ortholog) has been reported to show no phenotype, we performed rescue experiments based on the introduction of rat astrin cDNA transgene into hgn/hgn rats to determine whether astrin is actually necessary for the establishment of normal male fertility and renal function. Astrin transgenic (Tg) rats were mated with hgn/+ rats of the HGN strain, and Tg-hgn/+ rats were then crossed to obtain Tg-hgn/hgn. Tg-hgn/hgn males showed recovery of body growth, fertility and renal function. Testis size was smaller in these transgenic animals than normal controls, but showed an increase by 16.5-fold compared with hgn/hgn males. Spermatogenesis occurred in Tg-hgn/hgn testes, and their accessory reproductive organs were of approximately normal size. hgn/hgn males show hypergonadotropic hypogonadism. Increased testosterone and decreased LH levels in Tg-hgn/hgn serum indicated the recovery of Leydig cells' function. Tg-hgn/hgn males showed normal reproductive behaviour, and their mating with Tg-hgn/hgn females produced pups in normal litter size. Their renal sizes and glomerular numbers showed complete recovery, and renal function assayed by biochemical parameters was normal. These results indicated that the transgene is functional in the testis and kidney development as well as body growth. In conclusion, astrin is necessary for the establishment of normal size (cell number) and function of the testis and kidney in rats., (© 2012 American Society of Andrology and European Academy of Andrology.)

Murine double minute 2 (MDM2) binding protein (MTBP) has been implicated in tumor cell proliferation, but the underlying mechanisms remain unclear. The results of MTBP expression analysis during cell cycle progression demonstrated that MTBP protein was rapidly degraded during mitosis. Immunofluorescence studies revealed that a portion of MTBP was localized at the kinetochores during prometaphase. MTBP overexpression delayed mitotic progression from nuclear envelope breakdown (NEB) to anaphase onset and induced abnormal chromosome segregation such as lagging chromosomes, chromosome bridges, and multipolar chromosome segregation. Conversely, MTBP downmodulation caused an abbreviated metaphase and insufficient mitotic arrest, resulting in abnormal chromosome segregation, aneuploidy, decreased cell proliferation, senescence, and cell death, similar to that of Mad2 (mitotic arrest-deficient 2) downmodulation. Furthermore, MTBP downmodulation inhibited the accumulation of Mad1 and Mad2, but not BubR1 (budding uninhibited by benzimidazoles related 1), on the kinetochores, whereas MTBP overexpression inhibited the release of Mad2 from the metaphase kinetochores. These results may imply that MTBP has an important role in recruiting and/or retaining the Mad1/Mad2 complex at the kinetochores during prometaphase, but its degradation is required for silencing the mitotic checkpoint. Together, this study indicates that MTBP has a crucial role in proper mitotic progression and faithful chromosome segregation, providing new insights into regulation of the mitotic checkpoint.

Mdm2 inhibits the function of the p53 tumor suppressor. Mdm2 is overexpressed in many tumors with wild-type p53 suggesting an alternate mechanism of loss of p53 activity in tumors. An Mdm2-binding protein (MTBP) was identified using a yeast two-hybrid screen. In tissue culture, MTBP inhibits Mdm2 self-ubiquitination, leading to stabilization of Mdm2 and increased degradation of p53. To address the role of MTBP in the regulation of the p53 pathway in vivo, we deleted the Mtbp gene in mice. Homozygous disruption of Mtbp resulted in early embryonic lethality, which was not rescued by loss of p53. Mtbp+/- mice were not tumor prone. When mice were sensitized for tumor development by p53 heterozygosity, we found that the Mtbp+/-p53+/- mice developed significantly more metastatic tumors (18.2%) as compared to p53+/- mice (2.6%). Results of in vitro migration and invasion assays support the in vivo findings. Downmodulation of Mtbp in osteosarcoma cells derived from p53+/- mice resulted in increased invasiveness, and overexpression of Mtbp in Mtbp+/-p53+/- osteosarcoma cells inhibited invasiveness. These results suggest that MTBP is a metastasis suppressor. These results advance our understanding of the cellular roles of MTBP and raise the possibility that MTBP is a novel therapeutic target for metastasis.