In neural development, genetic programs endow each neuron with a distinct cellular identity. This identity includes a repertoire of molecules on the cell surface that dictate how that neuron will respond to the environment it encounters as it projects to form neural circuits. A large body of work has focused on how the interactions between cell surface identity molecules and ligands in the extracellular space guide neural projections, yet it remains comparatively unclear how mammals generate the stunning diversity of neuronal identities that underly these intricate networks of connectivity. The mammalian olfactory system offers a perfect microcosm of this question. Here, developing olfactory sensory neurons (OSN) chose to express a single olfactory receptor (OR) from roughly 1,500 possibilities in the mouse genome. OR choice sets the components of OSN cellular identity that direct targeting, endowing an OSN with a ?barcode? of cell surface molecules that specifies a precise location in the olfactory bulb (OB) to which all cells choosing that OR will project. The composition of this barcode is known and includes neural activity-independent molecules (Neuropilin-1/Semaphorin-3A, Neuropilin- 2/Semaphorin-3F) as well as activity-dependent molecules (Kirrel2-3, Ephrin-A5 and its receptor, non-canonical Protocadherins). However, the mechanisms mapping OR choice to a specific identity barcode are incompletely understood. We previously reported that OR choice during OSN development triggers the unfolded protein response (UPR), a genome-wide mRNA translation-regulatory program essential for complete neuronal maturation and stable OR expression. Preliminary data suggests that the UPR is differentially active in OSNs depending on the OR that they chose. Remarkably, these differences are intimately linked to expression patterns for several neuronal activity-dependent components of the OSN cell surface axon targeting barcode, as well as three transcription factors with possible roles in organizing the barcode. These results suggest an entirely novel role for the UPR as a molecular determinant of neuronal identity in the context of axon guidance. We will test this hypothesis in three specific aims.
In aim 1, we will use two mouse genetic approaches to demonstrate that differential activation of the UPR causally affects OSN cellular identity and axon targeting.
Aim 2 will define the molecular cascade linking the UPR to these identity molecules by identifying master regulator transcription factors (mrTFs) controlling UPR-mediated cellular identity. Finally, aim 3 will determine how hierarchical OR-dependent and OR-independent roles for the UPR work together to shape the whole of OSN identity. We anticipate that these experiments will unveil a previously undescribed role for the UPR as a molecular determinant of neuronal identity relevant for axon guidance in the olfactory system, offering a new paradigm with which to study neural development in the context of health and disease.
Defining the molecular genetic processes that establish neuronal identity remains a critical prerequisite to fully understanding how the developing nervous system specifies its intricate cellular connectivity. Using developing mouse olfactory sensory neurons as a model system, we will evaluate the role for a global mRNA translation- regulatory process called the unfolded protein response (UPR) in shaping cellular identity as it relates to axon targeting and neural circuit formation. We aim to delineate a novel genetic mechanism underlying the remarkable organization of the mammalian olfactory system, while revealing a new paradigm in axon guidance with potential relevance to the rest of the nervous system in health and disease.
