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.
ABSTRACT
Heat shock induced gene expression and other cellular responses help limit the damage caused by stress and thus facilitate cellular recovery. Cellular damage also triggers apoptotic cell death through several pathways. This paper briefly reviews interactions of the major heat shock proteins with components of the apoptotic pathways. Hsp90, which acts as a chaperone for unstable signal transducers to keep them poised for activation, interacts with RIP and Akt and promotes NF-kappa B mediated inhibition of apoptosis; in addition it also blocks some steps in the apoptotic pathways. Hsp70 is mostly anti-apoptotic and acts at several levels like inhibition of translocation of Bax into mitochondria, release of cytochrome c from mitochondria,formation of apoptosome and inhibition of activation of initiator caspases. Hsp70 also modulates JNK,NF-kappa B and Akt signaling pathways in the apoptotic cascade. In contrast, Hsp60 has both anti-and pro-apoptotic roles. Cytosolic Hsp60 prevents translocation of the pro-apoptotic protein Bax into mitochondria and thus promotes cell survival but it also promotes maturation of procaspase-3,essential for caspase mediated cell death. Our recent in vivo studies show that RNAi for the Hsp60D in Drosophila melanogaster prevents induced apoptosis. Hsp27 exerts its anti-apoptotic influence by inhibiting cytochrome c and TNF-mediated cell death. alpha beta crystallin suppresses caspase-8 and cytochrome c mediated activation of caspase-3. Studies in our laboratory also reveal that absence or reduced levels of the developmentally active as well as stress induced non-coding hsr omega transcripts, which are known to sequester diverse hnRNPs and related nuclear RNA-binding proteins,block induced apoptosis in Drosophila.Modulation of the apoptotic pathways by Hsps reflects their roles as "weak links" between various "hubs" in cellular networks. On the other hand, non-coding RNAs, by virtue of their potential to bind with multiple proteins,can act as "hubs" in these networks. In view of the integrative nature of living systems, it is not surprising that stress-induced genes,generally believed to primarily function in cell survival pathways, inhibit or even promote cell death pathways at multiple levels to ensure homeostasis at cell and/or organism level. The heat shock genes obviously do much more than merely help cells survive stress.