Activity-dependent and graded BACE1 expression in the olfactory epithelium is mediated by the retinoic acid metabolizing enzyme CYP26B1

It is well established that environmental influences play a key role in sculpting neuronal connectivity in the brain. One example is the olfactory sensory map of topographic axonal connectivity. While intrinsic odorant receptor signaling in olfactory sensory neurons (OSN) determines anterior–posterior counter gradients of the axonal guidance receptors Neuropilin-1 and Plexin-A1, little is known about stimulus-dependent gradients of protein expression, which correlates with the functional organization of the olfactory sensory map along its dorsomedial (DM)–ventrolateral (VL) axis. Deficiency of the Alzheimer’s β-secretase BACE1, which is expressed in a DMlow–VLhigh gradient, results in OSN axon targeting errors in a DM > VL and gene dose-dependent manner. We show that expression of BACE1 and the all-trans retinoic acid (RA)-degrading enzyme Cyp26B1 form DM–VL counter gradients in the olfactory epithelium. Analyses of mRNA and protein levels in OSNs after naris occlusion, in mice deficient in the olfactory cyclic nucleotide-gated channel and in relation to onset of respiration, show that BACE1 and Cyp26B1 expression in OSNs inversely depend on neuronal activity. Overexpression of a Cyp26B1 or presence of a dominant negative RA receptor transgene selectively in OSNs, inhibit BACE1 expression while leaving the DMlow–VLhigh gradient of the axonal guidance protein Neuropilin-2 intact. We conclude that stimulus-dependent neuronal activity can control the expression of the RA catabolic enzyme Cyp26B1 and downstream genes such as BACE1. This result is pertinent to an understanding of the mechanisms by which a topographic pattern of connectivity is achieved and modified as a consequence of graded gene expression and sensory experience.