Pleiotropic consequences of misexpression of the developmentally active and stress-inducible non-coding hsrω gene in Drosophila

Abstract

The non-coding hsrω gene of Drosophila melanogaster is expressed in nearly all cell types and developmental stages. However, in the absence of conventional mutant alleles of this gene, its developmental functions remain largely unknown. In the present study, we used a variety of GAL4 drivers to overexpress or ablate this gene’s transcripts in specific tissues and examined the developmental consequences thereof. Our results show that a balanced expression of these non-coding transcripts is critical for survival and normal development in all the tissue types tested, since any change in cellular levels of these transcripts in a given cell type generally has detrimental effects, with extreme cases resulting in organismal lethality, although in a few cases the misexpression of these transcripts also suppresses the mutant phenotype due to other genetic conditions. Evidence is also presented for existence of a new spliced variant of the hsrω-n nuclear transcript. Following the RNAi-mediated down-regulation of hsrω transcripts, the omega speckles disappear so that the nucleoplasmic hnRNPs get diffusely distributed, while upregulation of these transcripts results in greater sequestration of these proteins into omega speckle clusters; either of these conditions would affect activities of the hnRNPs and other hsrω-RNA interacting proteins, which is likely to have cascading consequences. The present findings, together with our earlier observations on effects of altered levels of the hsrω transcripts on induced apoptosis and expanded polyQ-mediated neurodegeneration, further confirm that ncRNA species like the hsrω, far from being evolutionary hangovers, provide critical information for important functions in normal cells.

The large noncoding hsrω-n transcripts are essential for thermotolerance and remobilization of hnRNPs, HP1 and RNA polymerase II during recovery from heat shock in Drosophila

“…However, survival of a small proportion of eggs to the fertile adult stage, in spite of the hsrω-null condition or global RNAi is intriguing. To explain survival of the individuals with Act5C-GAL4-driven globally downregulated levels of hsrω transcripts, Mallik and Lakhotia (2011;also see, Lakhotia 2011) suggested that an as-yet unidentified "backup" machinery gets activated in the surviving individuals, which allows them to continue as weak individuals. We suggest that a proportion of the hsrω-null individuals likewise survive to adulthood at 24°C as weak individuals because their cells could developmentally adapt to the absence of omega speckles by successfully activating the presumed "backup" system.…”

“…Among the various heatshock-induced genes in Drosophila melanogaster, the 93D or hsrω gene is unusual since, while it is developmentally active (Mukherjee and Lakhotia 1979;Bendena et al 1989Bendena et al , 1991Mutsuddi and Lakhotia 1995;Lakhotia et al 2001), it is singularly induced by amides and is also one of the most actively transcribed genes following heat shock (Mukherjee and Lakhotia 1979;Tapadia and Lakhotia 1997;Lakhotia and Tapadia 1998), although it does not code for any protein (Lakhotia and Mukherjee 1982;Garbe et al 1986;Lakhotia 2003). The hsrω gene produces multiple noncoding transcripts (Garbe et al 1986), of which the large (>10 kb) nucleus-limited hsrω-n transcripts organize the nucleoplasmic omega speckles (Prasanth et al 2000;Mallik and Lakhotia 2011), which sequester the various hnRNPs and certain other RNAprocessing proteins that are not actively engaged at a given time (Lakhotia et al 1999;Prasanth et al 2000;Jolly and Lakhotia 2006). Our recent studies (Mallik and Lakhotia 2009a, b, 2010, 2011Onorati et al 2011) have revealed that the hsrω RNA also affects, directly or indirectly, activities of DIAP1, CBP, JNK pathway, proteasome, ISWI, lamin C, etc., highlighting roles of these noncoding RNAs in multiple cellular networks (Lakhotia 2011).…”

AAGAG repeat RNA is an essential component of nuclear matrix inDrosophila

“…[42][43][44][45][46][47][48] Similarly, lncRNA are shown to be involved in organization of heterochromatin, 38,41 paraspeckles, 39,49 nuclear stress bodies and multiple chromatin features. [50][51][52][53] In several other instances, lncRNAs act as guides for chromatin-modifying machinery and mediate the formation of a defined active/inactive chromatin compartment.…”

Emerging Roles for hnRNPs in post-transcriptional regulation: what can we learn from flies?

“…In fact, loss of hsrω function causes the disorganization of omega speckles and a diffused distribution of hnRNPs in the nucleoplasm (Prasanth et al 2000;Lakhotia et al 2012;Mallik and Lakhotia 2011). Intriguingly, the heat-induced non-coding transcripts named Sat III identified in human cells display striking functional similarities with the fly hsrω transcripts (Jolly and Lakhotia 2006).…”