Requirement of the RNA helicase-like protein PRP22 for release of messenger RNA from spliceosomes

The product of the yeast PRP22 gene acts late in the splicing of yeast pre-messenger RNA, mediating the release of the spliced mRNA from the spliceosome. The predicted PRP22 protein sequence shares extensive homology with that of PRP2 and PRP16 proteins, which are also involved in nuclear pre-mRNA splicing. The homologous region contains sequence elements characteristic of several demonstrated or putative ATP-dependent RNA helicases. A putative RNA-binding motif originally identified in bacterial ribosomal protein S1 and Escherichia coli polynucleotide phosphorylase has also been found in PRP22.     

Histone H2AX-dependent GABA(A) receptor regulation of stem cell proliferation

Stem cell self-renewal implies proliferation under continued maintenance of multipotency. Small changes in numbers of stem cells may lead to large differences in differentiated cell numbers, resulting in significant physiological consequences. Proliferation is typically regulated in the G1 phase, which is associated with differentiation and cell cycle arrest. However, embryonic stem (ES) cells may lack a G1 checkpoint. Regulation of proliferation in the 'DNA damage' S/G2 cell cycle checkpoint pathway is known for its role in the maintenance of chromatin structural integrity. Here we show that autocrine/paracrine gamma-aminobutyric acid (GABA) signalling by means of GABA(A) receptors negatively controls ES cell and peripheral neural crest stem (NCS) cell proliferation, preimplantation embryonic growth and proliferation in the boundary-cap stem cell niche, resulting in an attenuation of neuronal progenies from this stem cell niche. Activation of GABA(A) receptors leads to hyperpolarization, increased cell volume and accumulation of stem cells in S phase, thereby causing a rapid decrease in cell proliferation. GABA(A) receptors signal through S-phase checkpoint kinases of the phosphatidylinositol-3-OH kinase-related kinase family and the histone variant H2AX. This signalling pathway critically regulates proliferation independently of differentiation, apoptosis and overt damage to DNA. These results indicate the presence of a fundamentally different mechanism of proliferation control in these stem cells, in comparison with most somatic cells, involving proteins in the DNA damage checkpoint pathway.     

15-Deoxy-Δ12,14-prostaglandin-J2 reveals a new pVHL-independent, lysosomal-dependent mechanism of HIF-1α degradation

Hypoxia-inducible factor-1α (HIF-1α) protein is degraded under normoxia by its association to von Hippel-Lindau protein (pVHL) and further proteasomal digestion. However, human renal cells HK-2 treated with 15-deoxy-Δ^12,14-prostaglandin-J₂ (15d-PGJ₂) accumulate HIF-1α in normoxic conditions. Thus, we aimed to investigate the mechanism involved in this accumulation. We found that 15d-PGJ₂ induced an over-accumulation of HIF-1α in RCC4 cells, which lack pVHL and in HK-2 cells treated with inhibitors of the pVHL-proteasome pathway. These results indicated that pVHL-proteasome-independent mechanisms are involved, and therefore we aimed to ascertain them. We have identified a new lysosomal-dependent mechanism of HIF-1α degradation as a target for 15d-PGJ₂ based on: (1) HIF-1α colocalized with the specific lysosomal marker Lamp-2a, (2) 15d-PGJ₂ inhibited the activity of cathepsin B, a lysosomal protease, and (3) inhibition of lysosomal activity did not result in over-accumulation of HIF-1α in 15d-PGJ₂-treated cells. Therefore, expression of HIF-1α is also modulated by lysosomal degradation.