Defeating peripheral neuropathy: The mechanisms underlying peripheral neuropathies are not well understood. Spaulding et al . studied mouse models of the inherited Charcot-Marie-Tooth (CMT) disease, which is caused by mutations in transfer RNA (tRNA) synthetases. Changes in gene expression and the rate of protein synthesis in neurons in the spinal cord triggered the cell stress response activated by the protein sensor GCN2. When GCN2 was genetically deleted or inhibited with drugs, the stress response was blocked, and the neuropathy was much milder. Zuko et al . found that mutant glycyl-tRNA synthetases bind tRNA _Gly but fail to release it, thus depleting the cellular tRNA _Gly pool. This process caused stalling of translating ribosomes on glycine codons and activated the integrated stress response. Transgenic tRNA _Gly overexpression prevented peripheral neuropathy and protein synthesis defects in mouse and fruit fly models. Thus, elevating tRNA _Gly levels or targeting GCN2 may have therapeutic potential for this currently untreatable disease (see the Perspective by Mellado and Willis). —SMH

The complexity in composition and function of the eukaryotic nucleus is achieved through its organization in specialized nuclear compartments. The Drosophila chromatin remodeling ATPase ISWI plays evolutionarily conserved roles in chromatin organization. Interestingly, ISWI genetically interacts with the hsrω gene, encoding multiple non-coding RNAs (ncRNA) essential, among other functions, for the assembly and organization of the omega speckles. The nucleoplasmic omega speckles play important functions in RNA metabolism, in normal and stressed cells, by regulating availability of hnRNPs and some other RNA processing proteins. Chromatin remodelers, as well as nuclear speckles and their associated ncRNAs, are emerging as important components of gene regulatory networks, although their functional connections have remained poorly defined. Here we provide multiple lines of evidence showing that the hsrω ncRNA interacts in vivo and in vitro with ISWI, regulating its ATPase activity. Remarkably, we found that the organization of nucleoplasmic omega speckles depends on ISWI function. Our findings highlight a novel role for chromatin remodelers in organization of nucleoplasmic compartments, providing the first example of interaction between an ATP-dependent chromatin remodeler and a large ncRNA. Chromatin structure and function are regulated by the concerted activity of covalent modifiers of chromatin, nucleosome remodeling factors, and several emerging classes of non-coding RNAs. ISWI is an evolutionarily conserved ATP-dependent chromatin remodeler playing essential roles in chromosome condensation, gene expression, and DNA replication. ISWI activity has been involved in a variety of nuclear functions including telomere silencing, stem cell renewal, neural morphogenesis, and epigenetic reprogramming. Using an in vivo assay to identify factors regulating ISWI activity in the model system Drosophila melanogaster, we recovered a genetic interaction between ISWI and hsrω. The hsrω gene encodes multiple non-coding RNAs (ncRNAs), of which the >10 kb nuclear hsrω-n RNA, with functional homolog in mammals, is essential for the assembly and organization of hnRNP-containing nucleoplasmic omega speckles. These special nuclear compartments play essential roles in the storage/sequestration of hnRNP family and other proteins, thus playing important roles in mRNA maturation and other regulatory processes. Here we show that the hsrω-n ncRNA interacts in vivo and in vitro with ISWI to directly regulate its ATPase activity. We also provide in vivo data showing that omega speckle nuclear organization depends on ISWI function, highlighting a novel role for chromatin remodelers in nuclear speckles organization.

Polyglutamine diseases are a class of inherited neurodegenerative disorders, characterized by expansion of CAG trinucleotide repeats translated into elongated glutamine tracts within the mutant proteins. Over-expression of the non-coding hsrω transcripts has been shown to dominantly enhance polyQ induced cytotoxicity in Drosophila. In the present study we demonstrate that RNA interference mediated down regulation of hsrω-n transcripts is sufficient to suppress pathogenesis in several Drosophila models of human polyQ neurodegenerative diseases. Loss of hsrω-n RNA not only suppresses the eye-specific degeneration mediated by GMR-GAL4 driven expression of the 127Q or MJDtr-Q78 or ataxin1 82Q or httex1p Q93 transgene, but also rescues premature death of flies expressing the expanded polyQ proteins pan-neuronally using the elav-GAL4 driver. We further demonstrate that the morphological and functional rescue of polyQ toxicity observed upon hsrω-n RNAi is associated with substantial reduction of polyQ protein aggregation without affecting transcription of the 127Q transgene. Unlike in the polyQ expressing cells, co-expression of hsrω-n RNAi also abolishes the induction of Hsp70. These results suggest that the hsrω transcripts have a role in early stages of polyQ aggregate formation. Interestingly, hsrω-RNAi has, at best, only a marginal effect on neuropathy following over-expression of normal or mutant tau protein in flies. Functional analogues of the large non-coding hsrω transcripts in human thus appear to be promising candidates as therapeutic targets for the polyQ-mediated neurodegenerative diseases.

The large nucleus limited noncoding hsrω-n RNA of Drosophila melanogaster is known to associate with a variety of heterogeneous nuclear RNA-binding proteins (hnRNPs) and certain other RNA-binding proteins to assemble the nucleoplasmic omega speckles. In this article, we show that RNAi-mediated depletion of this noncoding RNA dominantly suppresses apoptosis, in eye and other imaginal discs, triggered by induced expression of Rpr, Grim, or caspases (initiator as well as effector), all of which are key regulators/effectors of the canonical caspase-mediated cell death pathway. We also show, for the first time, a genetic interaction between the noncoding hsrω transcripts and the c-Jun N-terminal kinase (JNK) signaling pathway since downregulation of hsrω transcripts suppressed JNK activation. In addition, hsrω-RNAi also augmented the levels of Drosophila Inhibitor of Apoptosis Protein 1 (DIAP1) when apoptosis was activated. Suppression of induced cell death following depletion of hsrω transcripts was abrogated when the DIAP1-RNAi transgene was coexpressed. Our results suggest that the hsrω transcripts regulate cellular levels of DIAP1 via the hnRNP Hrb57A, which physically interacts with DIAP1, and any alteration in levels of the hsrω transcripts in eye disc cells enhances association between these two proteins. Our studies thus reveal a novel regulatory role of the hsrω noncoding RNA on the apoptotic cell death cascade through multiple paths. These observations add to the diversity of regulatory functions that the large noncoding RNAs carry out in the cells' life.