Analysis of intronless genes involved in oscillation and differentiation

The genomes of higher eukaryotes are replete with intron-containing genes. Transcription of these genes produces precursor mRNAs containing intervening sequences, which are subsequently removed and the exons spliced together to form the mature mRNA. However, a small proportion of eukaryotic protein-coding genes are intronless and therefore bypass post-transcriptional splicing events. Although a large proportion of intronless genes are known to code for certain types of proteins, their specific role in the genome of higher organism is perplexing. This research set out to elucidate the functions of intronless genes in humans by studying their involvement in the expression pattern of oscillatory gene that occurs in the pre-somitic mesoderm of developing embryo. Twenty-seven (27) human homologs of mouse oscillatory genes were analysed to determine the number of exons present in them using various bioinformatics databases. The result obtained identified two intronless genes –NRARP and ID1 – which are associated with the Notch signalling pathway of the segmentation clock. This represented 7.4% of the total oscillatory genes analysed. No intronless gene was found in the Wnt and FGF signalling pathways – two other pathways famous for oscillatory gene expression. The proteins encoded by the intronless genes are involved in several important biological processes including angiogenesis, cell cycle control and in the regulation of cellular senescence. Although oscillatory genes had fewer numbers of introns compared to the non-oscillatory genes, the intronless genes were not implicated in the regulation of the precise timing events of the segmentation clock. This result may also point to the fact that the rapid expression rate of the oscillatory genes in the PSM may favour the reduced intron length of the oscillatory genes.

The BLMP-1 transcription factor promotes oscillatory gene expression to achieve timely molting

Gene expression oscillators can coordinate developmental events in space and time. In C. elegans, a gene expression oscillator directs rhythmic accumulation of ~25% of the transcriptome, and thus thousands of transcripts, presumably to control molting, a process of rhythmic skin regeneration. The mechanism and organization of the oscillator are not known. Here, we report that rhythmic RNA polymerase II recruitment to promoters produces transcript level oscillations. We identify BLMP-1, orthologous to the mammalian transcription repressor PRDM1, as a rhythmically accumulating transcription factor that is required for timely molting, and oscillatory gene expression. We propose a dual function for BLMP-1 in shaping oscillatory gene expression and coupling it to a set of direct targets, which ensures cuticular integrity. With mammalian PRDM1/BLIMP1 promoting regular cycles of postnatal hair follicle regeneration, our findings point to the possible existence of a fundamentally conserved clock mechanism in control of rhythmic skin regeneration.

Temporal scaling in C. elegans larval development

It is essential that correct temporal order of cellular events is maintained during animal development. During post-embryonic development, the rate of development depends on external conditions, such as food availability, diet and temperature. How timing of cellular events is impacted when the rate of development is changed at the organism-level is not known. We used a novel time-lapse microscopy approach to simultaneously measure timing of oscillatory gene expression, hypodermal stem cell divisions and cuticle shedding in individual animals, during C. elegans larval development from hatching to adulthood. This revealed strong variability in timing between isogenic individuals under the same conditions. However, this variability obeyed ‘temporal scaling’, meaning that events occurred at the same time when measured relative to the duration of development in each individual. We also observed pervasive changes in population-averaged timing when temperature, diet or genotype were varied, but with larval development divided in ‘epochs’ that differed in how the timing of events was impacted. Yet, these variations in timing were still explained by temporal scaling when timing was rescaled by the duration of the respective epochs in each individual. Surprisingly, timing obeyed temporal scaling even in mutants lacking lin-42/Period, presumed a core regulator of timing of larval development, that exhibited strongly delayed, heterogeneous timing and growth arrest. Timing of larval development is likely controlled by timers based on protein degradation or protein oscillations, but such mechanisms do not inherently generate temporal scaling. Hence, our observations will put strong constraints on models to explain timing of larval development.

A novel mathematical method for disclosing oscillations in gene transcription: a comparative study

Circadian rhythmicity, the 24-hour cycle responsive to light and dark, is determined by periodic oscillations in gene transcription. This phenomenon has broad ramifications in physiologic function. Recent work has disclosed more cycles in gene transcription, and to the uncovering of these we apply a novel signal processing methodology known as the pencil method and compare it to conventional parametric, nonparametric, and statistical methods. Methods: In order to assess periodicity of gene expression over time, we analyzed a database derived from livers of mice entrained to a 12-hour light/12-hour dark cycle. We also analyzed artificially generated signals to identify differences between the pencil decomposition and other alternative methods. Results: The pencil decomposition revealed hitherto-unsuspected oscillations in gene transcription with 12-hour periodicity. The pencil method was robust in detecting the 24-hour circadian cycle that was known to exist, as well as confirming the existence of shorter-period oscillations. A key consequence of this approach is that orthogonality of the different oscillatory components can be demonstrated. thus indicating a biological independence of these oscillations, that has been subsequently confirmed empirically by knocking out the gene responsible for the 24-hour clock. Conclusion: System identification techniques can be applied to biological systems and can uncover important characteristics that may elude visual inspection of the data. Significance: The pencil method provides new insights on the essence of gene expression and discloses a wide variety of oscillations in addition to the well-studied circadian pattern. This insight opens the door to the study of novel mechanisms by which oscillatory gene expression signals exert their regulatory effect on cells to influence human diseases.