For vertebrate olfactory signal transduction, a calcium-activated chloride conductance serves as a major amplification step. However, the molecular identity of the olfactory calcium-activated chloride channel (CaCC) is unknown. Here we report a proteomic screen for cilial membrane proteins of mouse olfactory sensory neurons (OSNs) that identified all the known olfactory transduction components as well as Anoctamin 2 (ANO2). Ano2 transcripts were expressed specifically in OSNs in the olfactory epithelium, and ANO2::EGFP fusion protein localized to the OSN cilia when expressed in vivo using an adenoviral vector. Patch-clamp analysis revealed that ANO2, when expressed in HEK-293 cells, forms a CaCC and exhibits channel properties closely resembling the native olfactory CaCC. Considering these findings together, we propose that ANO2 constitutes the olfactory calciumactivated chloride channel.Anoctamin ͉ cilia ͉ olfaction ͉ signal transduction ͉ TMEM16B

SUMMARY Nonsense-mediated decay (NMD) degrades both normal and aberrant transcripts harboring stop codons in particular contexts. Mutations that perturb NMD cause neurological disorders in humans, suggesting that NMD has roles in the brain. Here, we identify a brain-specific microRNA—miR-128—that represses NMD and thereby controls batteries of transcripts in neural cells. miR-128 represses NMD by targeting the RNA helicase UPF1 and the exon-junction complex core component MLN51. The ability of miR-128 to regulate NMD is a conserved response occuring in frogs, chickens, and mammals. miR-128 levels are dramatically increased in differentiating neuronal cells and during brain development, leading to repressed NMD and upregulation of mRNAs normally targeted for decay by NMD; overrepresented are those encoding proteins controlling neuron development and function. Together, these results suggest the existence of a conserved RNA circuit linking the microRNA and NMD pathways that induces cell type-specific transcripts during development.

SUMMARY Nonsense-mediated mRNA decay (NMD) is a conserved RNA decay pathway that degrades aberrant mRNAs and directly regulates many normal mRNAs. This dual role for NMD raises the possibility that its magnitude is buffered to prevent the potentially catastrophic alterations in gene expression that would otherwise occur if NMD were perturbed by environmental or genetic insults. In support of this, here we report the existence of a negative feedback regulatory network that directly acts on seven NMD factors. Feedback regulation is conferred by different branches of the NMD pathway in a cell type-specific and developmentally regulated manner. We identify feedback-regulated NMD factors that are rate limiting for NMD and demonstrate that reversal of feedback regulation in response to NMD perturbation is crucial for maintaining NMD. Together, our results suggest the existence of an intricate feedback network that maintains both RNA surveillance and the homeostasis of normal gene expression in mammalian cells.

SUMMARY Gene duplication is a major evolutionary force driving adaptation and speciation, as it allows for the acquisition of new functions and can augment or diversify existing functions. Here, we report a gene duplication event that yielded another outcome – the generation of antagonistic functions. One product of this duplication event – UPF3B – is critical for the nonsense-mediated RNA decay (NMD) pathway, while its autosomal counterpart – UPF3A – encodes an enigmatic protein previously shown to have trace NMD activity. Using loss-of-function approaches in vitro and in vivo, we discovered that UPF3A acts primarily as a potent NMD inhibitor that stabilizes hundreds of transcripts. Evidence suggests that UPF3A acquired repressor activity through simple impairment of a critical domain, a rapid mechanism that may have been widely used in evolution. Mice conditionally lacking UPF3A exhibit “hyper” NMD and display defects in embryogenesis and gametogenesis, consistent with UPF3A serving as a molecular rheostat that directs developmental events.

SUMMARY The mechanisms dictating whether a cell proliferates or differentiates have undergone intense scrutiny but remain poorly understood. Here, we report that a central component in the nonsense-mediated RNA decay (NMD) pathway—UPF1—plays a key role in this decision by promoting the proliferative, undifferentiated cell state. UPF1 acts, in part, by destabilizing the NMD substrate encoding the TGFβ inhibitor, SMAD7, and stimulating TGFβ signaling. UPF1 also promotes the decay of mRNAs encoding many other proteins that oppose the proliferative, undifferentiated cell state. Neural differentiation is triggered when NMD is downregulated by neurally expressed microRNAs (miRNAs). This UPF1-miRNA circuitry is highly conserved and harbors negative feedback loops that act as a molecular switch. Our results suggest that the NMD RNA decay pathway collaborates with the TGF-β signaling pathway to lock-in the stem-like state, a cellular state that is stably reversed when neural differentiation signals that induce NMD-repressive miRNAs are received.