ER stress transforms random olfactory receptor choice into axon targeting precision.

Olfactory sensory neurons (OSNs) convert the stochastic choice of one of >1,000 olfactory receptor (OR) genes into precise and stereotyped axon targeting of OR-specific glomeruli in the olfactory bulb. Here, we show that the PERK arm of the unfolded protein response (UPR) regulates both the glomerular coalescence of like axons and the specificity of their projections. Subtle differences in OR protein sequences lead to distinct patterns of endoplasmic reticulum (ER) stress during OSN development, converting OR identity into distinct gene expression signatures. We identify the transcription factor Ddit3 as a key effector of PERK signaling that maps OR-dependent ER stress patterns to the transcriptional regulation of axon guidance and cell-adhesion genes, instructing targeting precision. Our results extend the known functions of the UPR from a quality-control pathway that protects cells from misfolded proteins to a sensor of cellular identity that interprets physiological states to direct axon wiring.

Chromatin Immunoprecipitation from Formaldehyde Cross-Linked Olfactory Sensory Neurons.

Chromatin immunoprecipitation (ChIP) allows a researcher to determine the genomic occupancy of nuclear proteins, providing insight into the roles of transcription factors, chromatin modifiers, histone modifications, and other factors bound to DNA. Protein-DNA interactions are first fixed in vivo by chemical cross-linking, and then a target protein is captured together with any associated DNA by an antibody mediated pull-down. The co-immunoprecipitated DNA can then be assayed by quantitative PCR or deep sequencing. Here, we demonstrate this technique using murine olfactory sensory neurons (OSNs) purified using fluorescence-activated cell sorting (FACS) and antibodies for the ubiquitous chromatin protein CTCF.