The "olfactosome" as a biomolecular condensate

Summary Biomolecular condensates (BMCs) represent an ingenious biological solution to the problem of organizing and streamlining biochemical processes without generating distinct organelles. From P bodies in the cytoplasm to the nucleolus and heterochromatic compartments in the nucleus, the assembly of nucleoprotein condensates provides the means of efficiently co-regulating gene expression and mRNA processing and translation. Further, stimulus-induced assembly of condensates at the cell membrane enhances the specificity and robustness of signal transduction cascades. Thus, it is without doubt that BMCs play critical roles in neuronal functions and that their misregulation contributes to neurodegeneration and other neurological disorders. Here, we take advantage of the unique properties of the mammalian olfactory system to establish a powerful paradigm for the rigorous dissection of BMC assembly and function in the nervous system. Specifically, we propose that the “olfactosome,” the multi-chromosomal enhanceosome that assembles upon a randomly chosen olfactory receptor (OR) locus, represents a BMC in which molecular crowding induces the activation of 1/2800 OR alleles. To explore this hypothesis, we will take advantage of a recent technical breakthrough that allows us to culture neuronal progenitors and to differentiate them into olfactory sensory neurons that express the same OR allele in a singular fashion, ex vivo. With this remarkable tool, we propose to genetically label the nascent OR RNA, the converging inter-chromosomal enhancers, and the proteins residing in the olfactosome. Then, we will perform live imaging single particle tracking (SPT) experiments that will determine the kinetic properties of the nucleoprotein components of the olfactosome inside and outside this multi-enhancer hub and at variable nuclear concentrations. Our experiments will provide mechanistic insight to a regulatory process that is essential for the function of olfactory neurons and for sensory perception, and will generate widely applicable principles for the role of molecular crowding in highly cooperative transcriptional processes. Importantly, with the emerging role of olfactory deficits in a plethora of pathological human conditions, from COVID-19 infection to Alzheimer’s disease, which according to our observations, stem from the disruption of the olfactosome, our studies will link basic biology to translationally important molecular changes that impair neuronal function.