Ribonuclease Activity of Dis3 Is Required for Mitotic Progression and Provides a Possible Link between Heterochromatin and Kinetochore Function

BACKGROUND: Cellular RNA metabolism has a broad range of functional aspects in cell growth and division, but its role in chromosome segregation during mitosis is only poorly understood. The Dis3 ribonuclease is a key component of the RNA-processing exosome complex. Previous isolation of the dis3-54 cold-sensitive mutant of fission yeast Schizosaccharomyces pombe suggested that Dis3 is also required for correct chromosome segregation. METHODOLOGY/PRINCIPAL FINDINGS: We show here that the progression of mitosis is arrested in dis3-54, and that segregation of the chromosomes is blocked by activation of the mitotic checkpoint control. This block is dependent on the Mad2 checkpoint protein. Double mutant and inhibitor analyses revealed that Dis3 is required for correct kinetochore formation and function, and that this activity is monitored by the Mad2 checkpoint. Dis3 is a member of the highly conserved RNase II family and is known to be an essential subunit of the exosome complex. The dis3-54 mutation was found to alter the RNaseII domain of Dis3, which caused a reduction in ribonuclease activity in vitro. This was associated with loss of silencing of an ura4(+) reporter gene inserted into the outer repeats (otr) and central core (cnt and imr) regions of the centromere. On the other hand, centromeric siRNA maturation and formation of the RITS RNAi effector complex was normal in the dis3-54 mutant. Micrococcal nuclease assay also suggested the overall chromatin structure of the centromere was not affected in dis3-54 mutant. CONCLUSIONS/SIGNIFICANCE: RNase activity of Dis3, a core subunit of exosome, was found to be required for proper kinetochore formation and establishment of kinetochore-microtubule interactions. Moreover, Dis3 was suggested to contribute to kinetochore formation through an involvement in heterochromatic silencing at both outer centromeric repeats and within the central core region. This activity is likely monitored by the mitotic checkpoint, and distinct from that of RNAi-mediated heterochromatin formation directly targeting outer centromeric repeats

The transcription factor FBI-1/OCZF/LRF is expressed in osteoclasts and regulates RANKL-induced osteoclast formation in vitro and in vivo

OBJECTIVE.: Since transcription factors expressed in osteoclasts are possible targets for regulation of bone destruction in bone disorders, we investigated the expression of the transcription factor FBI-1/OCZF/LRF (FBI-1 in humans, OCZF in rats, and LRF, formerly also called Pokemon, in mice) in RA and its role in osteoclastogenesis in vivo. METHODS.: The expression of FBI-1 and OCZF in subchondral osteoclasts was investigated using human RA and rat adjuvant arthritis with immunostaining and in situ hybridization, respectively. Transgenic (Tg) mice were generated which overexpress OCZF under the control of the cathepsin K promoter. Bone mineral density and bone histomorphometric analysis in OCZF Tg mice was determined by peripheral quantitative computed tomography, calcein double-labeling and specific staining for osteoclasts and osteoblasts. LRF/OCZF expression and the consequence of LRF inhibition were assessed in vitro with RANKL-induced osteoclast differentiation. RESULTS.: FBI-1/OCZF was detected in the nuclei of osteoclasts in rat adjuvant arthritis and human RA. RANKL increased LRF mRNA and nuclear localized LRF protein in both primary and established macrophages. OCZF Tg mice showed significantly decreased bone volume. The number of osteoclasts but not osteoblasts was increased in long bones of OCZF Tg mice. In addition, osteoclast survival was promoted in OCZF Tg mice. Conversely, inhibition of LRF expression suppressed the formation of osteoclasts from macrophages in vitro. CONCLUSION.: These data suggest that FBI-1/OCZF/LRF regulates osteoclast formation and apoptosis in vivo, and may become a useful marker and target in treating disorders leading to reduced bone density, including chronic arthritis

A complex gene regulatory mechanism that operates at the nexus of multiple RNA processing decisions.

Expression of crs1 pre-mRNA, encoding a meiotic cyclin, is blocked in actively growing fission yeast cells by a multifaceted mechanism. The most striking feature is that crs1 transcripts are continuously synthesized in vegetative cells, but are targeted for degradation rather than splicing and polyadenylation. Turnover of crs1 RNA requires the exosome, similar to previously described nuclear surveillance and silencing mechanisms, but does not involve a non-canonical poly(A) polymerase. Instead, crs1 transcripts are targeted for destruction by a factor previously implicated in turnover of meiotic RNAs in growing cells. Like exosome mutants, mmi1 mutants splice and polyadenylate vegetative crs1 transcripts. Two regulatory elements are located at the 3′ end of the crs1 gene, consistent with the increased accumulation of spliced RNA in polyadenylation factor mutants. This highly integrated regulatory strategy may ensure a rapid response to adverse conditions, thereby guaranteeing survival