HuR and post-transcriptional regulation in vascular aging

HuR(ELAV11(embryonic lethal,abnormal vision)-like 1),a ubiquitously expressed member of the ELAV-like RNA-binding protein family,has been shown to regulate the stability and translation of mRNAs that encode factors regulating cellular senescence,thereby impacting on aging.In this review,we discuss the current knowledge of HuR’s role in vascular cell senescence and vascular aging.     

Translational regulation in growing clonal human astrocytoma cells in culture

The in vivo and in vitro protein synthesis by polysomes prepared from Cox astrocytoma cells grown in the presence of 100 mM ethanol were examined during transition from exponential to stationary growth phase. A sharp decline of translational activities of Cox poly (A)+messenger RNAs (mRNAs) occurred during this transition. This decline was accentuated when cells were grown in the presence of ethanol. The observed decline in mRNA translational activity was investigated in vitro in a micrococcal nuclease treated, mRNA depleted postmitochondrial supernatant (PMS) fraction containing [³⁵S]methionine. The formation of the³⁵S-labeled 40S ternary complex in the absence of mRNA and of the³⁵S-labeled 80S initiation complex in the presence of Cox or brain poly (A)+mRNAs were reduced substantially when the source of PMS was from stationary phase or ethanol exposed cells. The sedimentation of peaks containing 40S ternary and 80S initiation complexes following sucrose density gradient analysis showed marked reductions in [³⁵S] methionine labeling during the transition to stationary phase and also following ethanol exposure. The reduced formation of initiation complexes suggests possible functional modifications of eukaryotic initiation factor-2 (eIF-2) present in the PMS fraction and of mRNAs under these conditions. Data suggest that cells initiate adaptive or protective mechanisms by reducing the rate of the initiation reaction following environmental alterations produced by ethanol.     

Functions of maternal mRNA in early development

In this review, the types of mRNAs found in oocytes and eggs of several animal species, particularly Drosophila, marine invertebrates, frogs, and mice, are described. The roles that proteins derived from these mRNAs play in early development are discussed, and connections between maternally inherited information and embryonic pattern are sought. Comparisons between genetically identified maternally expressed genes in Drosophila and maternal mRNAs biochemically characterized in other species are made when possible. Regulation of the meiotic and early embryonic cell cycles is reviewed, and translational control of maternal mRNA following maturation and/or fertilization is discussed with regard to specific mRNAs.     

RACK1 mRNA translation is regulated via a rapamycin-sensitive pathway and coordinated with ribosomal protein synthesis

RACK1 has been shown to interact with several proteins, this suggesting that it may play a central role in cell growth regulation. Some recent articles have described RACK1 as a component of the small ribosomal subunit. To investigate the relationship between RACK1 and ribosome, we analyzed RACK1 mRNA structure and regulation. Translational regulation was studied in HeLa cells subjected to serum or amino acid deprivation and stimulation. The results show that RACK1 mRNA has a 5' terminal oligopyrimidine sequence and that its translation is dependent on the availability of serum and amino acids in exactly the same way as any other vertebrate ribosomal protein mRNA.     

CiYB1 is a major component of storage mRNPs in ascidian oocytes: implications in translational regulation of localized mRNAs

In ascidian eggs, the existence of several localized maternal cytoplasmic determinants has been proposed and the importance of localized mRNAs for tissue differentiation has been demonstrated. We previously identified the ascidian Y-box proteins (CiYB1, 2 and 3), homologues of which are known to be involved in the storage of maternal mRNA in oocytes of other organisms. In this study, we found that CiYB1 protein is abundant in the gonad, egg, and embryo. Purification of messenger ribonucleoprotein (mRNP) particles from the gonad revealed that CiYB1 was one of their major components. A significant change in the distribution of CiYB1 protein from stored mRNP particles in the gonad to the ribosome fraction in eggs and embryos was observed. This change correlates most likely with the shift of stored maternal mRNAs to polyribosomes. Moreover, we found that CiYB1 colocalized with Cipem and Ci-macho1 mRNAs, which are localized at the posterior end of the embryo at the cleavage stage. Cipem and Ci-macho1 mRNAs were co-immunoprecipitated with CiYB1 in the oocyte and embryo lysates. The formation of a complex between Cipem mRNA and CiYB1 protein resulted in translational repression in the in vitro translation system. Our results indicate that associating with CiYB1 protein contributes to the translational control of the localized mRNA in eggs and embryos.     

uORFs, reinitiation and alternative translation start sites in human mRNAs

It is known that eukaryotic ribosomes are able to translate small ORFs and reinitiate translation at downstream start codons. However, this mechanism is widely considered to be inefficient and it is not commonly taken into account. We compiled a sample of human mRNAs containing small upstream ORFs overlapping with annotated protein coding sequences. Statistical analysis supported the hypothesis on reinitiation of translation at downstream AUG codons and functional significance of potential alternative ORFs. It may be assumed that some 5'UTR-located upstream ORFs can deliver ribosomes to alternative translation starts, and they should be taken into consideration in the prediction of human mRNA coding potential.     

Phosphoinositide-binding domains: Functional units for temporal and spatial regulation of intracellular signalling

Inositol phospholipid (phosphoinositide) is a versatile lipid characterized by its isomer-specific localization, as well as its molecular diversity attributable to phosphorylation events. Phosphoinositides act as signal mediators in a spatially and temporally controlled manner. Information about the timing and location of their production is received by phosphoinositide-binding proteins and transmitted to multiple lines of intracellular events such as signal transduction, cytoskeletal rearrangement, and membrane trafficking. Among those proteins, a significant portion possess globular structural units, called domains, which are specialized for phosphoinositide binding. The pleckstrin homology (PH) domain was the first phosphoinositide-binding domain identified. It contains the largest number of members and is associated with the formation of signalling complexes on the plasma membrane. Recent studies identified other novel phosphoinositide-binding domains (Fab1p, YOTB, Vps27p, EEA1 (FYVE), Phox homology (PX), and epsin N-terminal homology (ENTH)), thus extending our knowledge of how the functional versatility of phosphoinositides is achieved.     

Macromolecular crowding and its role as intracellular signalling of cell volume regulation.

Macromolecular crowding has been proposed as a mechanism by means of which a cell can sense relatively small changes in volume or, more accurately, the concentration of intracellular solutes. According to the macromolecular theory, the kinetics and equilibria of enzymes can be greatly influenced by small changes in the concentration of ambient, inert macromolecules. A 10% change in the concentration of intracellular proteins can lead to changes of up to a factor of ten in the thermodynamic activity of putative molecular regulatory species, and consequently, the extent to which such regulator(s) may bind to and activate membrane-associated ion transporters. The aim of this review is to examine the concept of macromolecular crowding and how it profoundly affects macromolecular association in an intact cell with particular emphasis on its implication as a sensor and a mechanism through which cell volume is regulated.