SUMMARY Non-coding RNA biogenesis in higher eukaryotes has not been fully characterized. Here, we studied the Drosophila melanogaster Rexo5 (CG8368) protein, a metazoan-specific member of the DEDDh 3′-5′ single-stranded RNA exonucleases, by genetic, biochemical, and RNA sequencing approaches. Rexo5 is required for small nucleolar RNA (snoRNA) and ribosomal RNA (rRNA) biogenesis, and is essential in D. melanogaster. Loss-of-function mutants accumulate improperly 3′-end-trimmed 28S rRNA, 5S rRNA, and snoRNA precursors in vivo. Rexo5 is ubiquitously expressed at low levels in somatic metazoan cells, but extremely elevated in male and female germ cells. Loss of Rexo5 leads to increased nucleolar size, genomic instability, defective ribosome subunit export and larval death. Loss-of-germline expression compromises gonadal growth and meiotic entry during germline development.

The molecular basis for odor perception in humans remains a black box as odorant receptors (ORs) are notoriously difficult to study outside of their native environment. Efforts toward OR expression and functional profiling in heterologous systems have been met with limited success due to poor efficiency of cell surface expression and consequently reduced G-protein signal amplification. We previously reported a genetic strategy in mice to increase the number of sensory neurons expressing specific ORs, which transforms the 10 million neurons of the mouse nose into a bioreactor producing large quantities of fully functional OR protein. We now describe the isolation of cilia from these bioreactors for two human ORs. Cilia are known to contain all components of the olfactory signal transduction machinery and can be placed into an ex vivo well-plate assay to rapidly measure robust, reproducible odor-specific responses. Our OR1A1 and OR5AN1 isolated cilia reveal 10-100fold more sensitivity than existing assays. Tissue from a single animal produces up to 4,000 384-well assay wells, and isolated olfactory cilia can be stored frozen and thus preserved for long term usage. This pipeline offers a sensitive, highly scalable ex vivo odor screening platform that opens the door for decoding human olfaction.

The molecular basis for odor perception in humans remains enigmatic because of the difficulty in studying odorant receptors (ORs) outside their native environment. Efforts toward OR expression and functional profiling have been met with limited success because of the poor efficiency of their cell surface expression in vitro. Structures protruding from the surface of olfactory sensory neurons called cilia contain all of the components of the olfactory signal transduction machinery and can be placed in an ex vivo plate assay to rapidly measure odor-specific responses. Here, we describe an approach using cilia isolated from the olfactory sensory neurons of mice expressing two human ORs, OR1A1 and OR5AN1, that showed 10- to 100-fold more sensitivity to ligands as compared to previous assays. A single mouse can produce enough olfactory cilia for up to 4000 384-well assay wells, and isolated cilia can be stored frozen and thus preserved. This pipeline offers a sensitive and highly scalable ex vivo odor-screening platform that has the potential to decode human olfaction.