Identification of Intron Retention in the Slc16a3 Gene Transcript Encoding the Transporter MCT4 in the Brain of Aged and Alzheimer-Disease Model (APPswePS1dE9) Mice

The monocarboxylate transporter 4 (MCT4; Slc16a3) is expressed in the central nervous system, notably by astrocytes. It is implicated in lactate release and the regulation of glycolytic flux. Whether its expression varies during normal and/or pathological aging is unclear. As the presence of its mature transcript in the brain of young and old mice was determined, an unexpectedly longer RT-PCR fragment was detected in the mouse frontal cortex and hippocampus at 12 vs. 3 months of age. Cultured astrocytes expressed the expected 516 base pair (bp) fragment but treatment with IL-1β to mimic inflammation as can occur during aging led to the additional expression of a 928 bp fragment like that seen in aged mice. In contrast, cultured pericytes (a component of the blood-brain barrier) only exhibited the 516 bp fragment. Intriguingly, cultured endothelial cells constitutively expressed both fragments. When RT-PCR was performed on brain subregions of an Alzheimer mouse model (APPswePS1dE9), no fragment was detected at 3 months, while only the 928 bp fragment was present at 12 months. Sequencing of MCT4 RT-PCR products revealed the presence of a remaining intron between exon 2 and 3, giving rise to the longer fragment detected by RT-PCR. These results unravel the existence of intron retention for the MCT4 gene in the central nervous system. Such alternative splicing appears to increase with age in the brain and might be prominent in neurodegenerative diseases such as Alzheimer's disease. Hence, further studies in vitro and in vivo of intron 2 retention in the Slc16a3 gene transcript are required for adequate characterization concerning the biological roles of Slc16a3 isoforms in the context of aging and Alzheimer's disease pathology.

Residues in the alternative reading frame tumor suppressor that influence its stability and p53-independent activities

The Alternative Reading Frame (ARF) protein suppresses tumorigenesis through p53-dependent and p53-independent pathways. Most of ARF's anti-proliferative activity is conferred by sequences in its first exon. Previous work showed specific amino acid changes occurred in that region during primate evolution, so we programmed those changes into human p14ARF to assay their functional impact. Two human p14ARF residues (Ala(14) and Thr(31)) were found to destabilize the protein while two others (Val(24) and Ala(41)) promoted more efficient p53 stabilization and activation. Despite those effects, all modified p14ARF forms displayed robust p53-dependent anti-proliferative activity demonstrating there are no significant biological differences in p53-mediated growth suppression associated with simian versus human p14ARF residues. In contrast, p53-independent p14ARF function was considerably altered by several residue changes. Val(24) was required for p53-independent growth suppression whereas multiple residues (Val(24), Thr(31), Ala(41) and His(60)) enabled p14ARF to block or reverse the inherent chromosomal instability of p53-null MEFs. Together, these data pinpoint specific residues outside of established p14ARF functional domains that influence its expression and signaling activities. Most intriguingly, this work reveals a novel and direct role for p14ARF in the p53-independent maintenance of genomic stability.

A novel nuclear interactor of ARF and MDM2 (NIAM) that maintains chromosomal stability

The ARF tumor suppressor signals through p53 and other poorly defined anti-proliferative pathways to block carcinogenesis. In a search for new regulators of ARF signaling, we discovered a novel nuclear protein that we named NIAM (nuclear interactor of ARF and MDM2) for its ability to bind both ARF and the p53 antagonist MDM2. NIAM protein is normally expressed at low to undetectable levels in cells because of, at least in part, MDM2-mediated ubiquitination and proteasomal degradation. When reintroduced into cells, NIAM activated p53, caused a G1 phase cell cycle arrest, and collaborated with ARF in an additive fashion to suppress proliferation. Notably, NIAM retains growth inhibitory activity in cells lacking ARF and/or p53, and knockdown experiments revealed that it is not essential for ARF-mediated growth inhibition. Thus, NIAM and ARF act in separate anti-proliferative pathways that intersect mechanistically and suppress growth more effectively when jointly activated. Intriguingly, silencing of NIAM accelerated chromosomal instability, and microarray analyses showed reduced NIAM mRNA expression in numerous primary human tumors. This study identifies a novel protein with tumor suppressor-like behaviors and functional links to ARF-MDM2-p53 signaling.

Spinophilin: from partners to functions

Spinophilin/neurabin 2 has been isolated independently by two laboratories as a protein interacting with protein phosphatase 1 (PP1) and F-actin. Gene analysis and biochemical approaches have contributed to define a number of distinct modular domains in spinophilin that govern protein-protein interactions such as two F-actin-, three potential Src homology 3 (SH3)-, a receptor- and a PP1-binding domains, a PSD95/DLG/zo-1 (PDZ) and three coiled-coil domains, and a potential leucine/isoleucine zipper (LIZ) motif. More than 30 partner proteins of spinophilin have been discovered, including cytoskeletal and cell adhesion molecules, enzymes, guanine nucleotide exchange factors (GEF) and regulator of G-protein signalling protein, membrane receptors, ion channels and others proteins like the tumour suppressor ARF. The physiological relevance of some of these interactions remains to be demonstrated. However, spinophilin structure suggests that the protein is a multifunctional protein scaffold that regulates both membrane and cytoskeletal functions. Spinophilin plays important functions in the nervous system where it is implicated in spine morphology and density regulation, synaptic plasticity and neuronal migration. Spinophilin regulates also seven-transmembrane receptor signalling and may provide a link between some of these receptors and intracellular mitogenic signalling events dependent on p70(S6) kinase and Rac G protein-GEF. Strikingly a role for spinophilin in cell growth was demonstrated and this effect was enhanced by its interaction with ARF. Here we review the current knowledge of the protein partners of spinophilin and present the available data that are contributing to the appreciation of spinophilin functions.

Dynamics of the hobo transposable element in transgenic lines of Drosophila melanogaster

The impact of the hobo transposable element in global reorganization of the Drosophila melanogaster genome has been investigated in transgenic lines generated by injection of hobo elements into the Hikone strain, which lacked them. In the present extensive survey, the chromosomal distribution of hobo insertion sites in the line 28 was found to be homogeneous and similar for all chromosomal arms, except 3L, when compared with other transgenic lines. However, some original features were observed in this line at the genetic and chromosomal levels. Several hotspots of insertion sites were observed on the X, second and third chromosomes. Five sites with a high frequency of hobo insertions were present on the 3L arm in most individuals tested, suggesting the action of selection for hobo element in some sites. The presence of doublets or triplet was also observed, implying that hobo inserts can show local jumps or insertions in preferred regions. This local transposition occurred independently in 11 specific genomic regions in many individuals and generations. The dynamics of this phenomenon were analysed across generations. These results support the use of the hobo system as an important tool in fundamental and applied Drosophila genetics.

Transmission pattern of hobo transposable element in transgenic lines of Drosophila melanogaster

This study is an attempt to trace the fate of hobo elements in the genomes of E strains of Drosophila melanogaster that have been transfected with pHFL1, a plasmid containing an autonomous hobo. Such long-term population studies (over 105 generations) could be very useful for better understanding the population and genomic dynamics of transposable elements and their pattern of insertions. Molecular analyses of hobo elements in the transfected lines were performed using Southern blots of XhoI-digested genomic DNAs. The complete element was observed in all six injected lines. In two lines we observed, at generation 100, two deleted elements, which did not correspond to Th1 and Th2. The results obtained by the in situ method show that the number of hybridization sites increases in each line and prove that the hobo element may be amplified in an RM genome. The hobo activity does not seem to be systematically correlated with the number of hobo elements. After generation 85, the evolution of the hobo element's insertion site number depends on the injected line. In all lines, the total number of insertions remains quite small, between 0 and 11. Hobo elements are located on each of the chromosomal arms. We describe 'hotspots'-insertion sites present in all lines and in all generations. On the 3R arm, a short inversion appeared once at generation 85.

Sterol pathway in yeast. Identification and properties of mutant strains defective in mevalonate diphosphate decarboxylase and farnesyl diphosphate synthetase

Yeast mutant strains auxotrophic for ergosterol and blocked in mevalonate diphosphate decarboxylase (erg19) and farnesyl diphosphate (FPP) synthetase (erg20) were isolated. The main feature of the mutants blocked in FPP synthetase is their ability to excrete prenyl alcohols, such as geraniol and farnesol. The isolation of the functional ERG20 gene allowed us to show that farnesyl diphosphate synthetase could be a rate limiting enzyme in ergosterol biosynthesis in yeast.

Isolation and properties of yeast mutants affected in farnesyl diphosphate synthetase

Two yeast mutant strains auxotrophic for ergosterol and blocked in farnesyl diphosphate synthetase (EC 2.5.1.1) were isolated. Genetic analysis has shown that these mutant strains carry additional mutations in the ergosterol pathway besides erg20-1 and erg20-2 which affect FPP synthetase. The novel feature of these mutants is their ability to excrete prenyl alcohols (farnesol and geraniol). As geraniol is toxic for yeast cells, the above leaky mutations in FPP synthetase have to be associated with others in the sterol pathway, in order to slow down geraniol synthesis.