RNA interference (RNAi) has great potential to treat human disease1–3. However, in vivo delivery of short interfering RNAs (siRNAs), which are negatively charged double-stranded RNA macromolecules, remains a major hurdle4–9. Current siRNA delivery has begun to move away from large lipid and synthetic nanoparticles to more defined molecular conjugates9. Here we address this issue by synthesis of short interfering ribonucleic neutrals (siRNNs) whose phosphate backbone contains neutral phosphotriester groups, allowing for delivery into cells. Once inside cells, siRNNs are converted by cytoplasmic thioesterases into native, charged phosphodiester-backbone siRNAs, which induce robust RNAi responses. siRNNs have favorable drug-like properties, including high synthetic yields, serum stability and absence of innate immune responses. Unlike siRNAs, siRNNs avidly bind serum albumin to positively influence pharmacokinetic properties. Systemic delivery of siRNNs conjugated to a hepatocyte-specific targeting domain induced extended dose-dependent in vivo RNAi responses in mice. We believe that siRNNs represent a technology that will open new avenues for development of RNAi therapeutics.

These studies show that Notch signals can antagonize neurite outgrowth and that repressing endogenous Notch signals enhances neurite outgrowth in neuroblastoma cells. Notch signals therefore act as regulators of neuritic extension in neuroblastoma cells. The response of neuritic processes to Delta1 expressed in the neurite was opposite to that to Delta1 contacted on another cell, however. These results suggest a model in which developing neurons determine their extent of process outgrowth on the basis of the opposing influences on Notch signals of ligands contacted on another cell and ligands expressed in the same cell.

A role for Notch in the elaboration of existing neural processes is emerging that is distinct from the increasingly well understood function of this gene in binary cell-fate decisions. Several research groups, by using a variety of organisms, have shown that Notch is important in the development of neural ultrastructure. Simultaneously, Presenilin (Psn) was identified both as a key mediator of Notch signaling and as a site of genetic lesions that cause earlyonset Alzheimer's disease. Here we demonstrate that Notch loss of function produces memory deficits in Drosophila melanogaster. The effects are specific to long-term memory, which is thought to depend on ultrastructural remodeling. We propose that Notch plays an important role in the neural plasticity underlying consolidated memory.W hereas the Notch protein plays an important role in binary cell-fate decisions during development (1), it is also present in the adult brain (2, 3). This finding is particularly interesting in adult Drosophila because the CNS is established by eclosion (4). Given that Notch is not needed for cell-fate decisions in the adult brain, its role in this tissue is unclear.Several groups have shown that Notch is important in the arborization of neuritic processes in development. In mammals, Notch regulates arborization of cortical neurons in vivo (5, 6) and in tissue culture (7,8). In Drosophila, the Notch pathway is required for the elaboration of processes in the CNS in the developing third-instar larva (9) and in the embryo (10, 11). Recently, this work has been extended to the analysis of the developing neuromuscular junction, a structure that serves as an accessible model for CNS plasticity. In addition, a Psn-mediated role for Notch is required in the development of neural projections mediating learned thermotaxis in Caenorhabditis elegans (12). Recently (3), we demonstrated that prolonged disruptions in Notch function produced an early lethality and an impairment in the coordinated neuromuscular activity of flight. Accordingly, because Notch clearly is involved in the regulation of neural ultrastructure during development, we investigated the possibility that Notch is also required for memory consolidation, a process believed to require remodeling of existing neurons in adults (13).Drosophila is an ideal organism for studying genes influencing behavioral phenotypes. Advances in our understanding of learning and memory mechanisms have been achieved through genetic, transgenic, and genomic studies in the fly (14). To investigate Notch function in adults, it is necessary to use conditional reagents to avoid developmental phenotypes that may kill the fly or compromise behavior. We used a Notch temperature-sensitive allele (N ts1 ) (15) and RNA interference (RNAi) derived from an inducible transgene (16), in combination with two independent behavioral assays for memory. We show that short-term memory is not impaired by conditional manipulations of Notch, but that Notch is required in adults for long-term memory. MethodsDrosophila Strai...

The larval salivary gland of Drosophila melanogaster synthesizes and secretes glue glycoproteins that cement developing animals to a solid surface during metamorphosis. The steroid hormone 20-hydroxyecdysone (20E) is an essential signaling molecule that modulates most of the physiological functions of the larval gland. At the end of larval development, it is known that 20E—signaling through a nuclear receptor heterodimer consisting of EcR and USP—induces the early and late puffing cascade of the polytene chromosomes and causes the exocytosis of stored glue granules into the lumen of the gland. It has also been reported that an earlier pulse of hormone induces the temporally and spatially specific transcriptional activation of the glue genes; however, the receptor responsible for triggering this response has not been characterized. Here we show that the coordinated expression of the glue genes midway through the third instar is mediated by 20E acting to induce genes of the Broad Complex (BRC) through a receptor that is not an EcR/USP heterodimer. This result is novel because it demonstrates for the first time that at least some 20E-mediated, mid-larval, developmental responses are controlled by an uncharacterized receptor that does not contain an RXR-like component.

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