Expression Patterns in the Developing Zebrafish of Two Paralogs of Activating Transcription Factor 5 (atf5a and atf5b) Roberto Rodríguez-Morales1, Viveca Vélez-Negrón1, Aranza Torrado-Tapias1, Gaurav Varshney2 and Martine Behra1 1. Department of Anatomy and Neurobiology, University of Puerto Rico, Medical Sciences Campus , San Juan, PR 2. Oklahoma Medical Research Foundation (OMRF), Oklahoma City, USA The Activating Transcription Factor 5 (ATF5) is a basic Zipper (bZIP) transcription factor (TF) with a proposed stress-protective, anti-apoptotic and oncogenic role. In Atf5 KO-mice, olfactory sensory neurons (OSNs) are immature and instable, and the olfactory bulb (OB) to where they project axons, is smaller. In the ventricular zone (VZ), a neurogenic brain region, Atf5 is strongly expressed and restricted to neuroprogenitors suggesting that it is necessary for their proliferation. However, a more systematic expression analysis found abundant Atf5 expression in various mature neuron populations and surviving Atf5-/- homozygotes are smell defective, but also less territorial and display several anxiety-like behaviors, suggesting additional roles in the CNS for Atf5. Furthermore, it is also expressed in other organs (liver, bones) in which it promotes differentiation of progenitor cells. Taken together, Atf5 function appears strongly context-dependent and additional work in physiological environments is needed. ATF5 is well conserved with two highly identical paralogs in Zebrafish: atf5a and atf5b. Here, we present the developmental expression patterns for both genes from 6-hour post fertilization (hpf) to 5-day post-fertilization (dpf), established by whole-mount in-situ hybridization (WISH) and semi-quantitative retro-transcription PCR (RT-PCR). We found ubiquitous maternal expression for both genes. From 2dpf onward, we found both genes expressed in the sensory epithelia of the inner ear. However, only atf5a was strongly expressed in the olfactory placodes and pits, with a weak and transient expression in the lateral line (LL). On the contrary, atf5b was not expressed in the nose and strongly expressed in the LL. Discrete bilateral regions in the midbrain and hindbrain were stained and partially overlapping at later developmental stages, with an earlier onset and broader staining for atf5a. Our work will facilitate future functional analyses in a genetic tractable animal model. Fig.2. Expression of atf5a and atf5b in the sensory lateral line (LL). (A, B, C) atf5a antisense mRNA probe hybridization in lateral view of whole-larvae at 2, 3 and 5dpf, with high- magnification (D,E,F) of NM corresponding to each developmental stage. (G,H,I) atf5b antisense RNA probe hybridization in lateral view of whole-larvae at 2, 3 and 5dpf, with high magnification (J,K,L) of NM to the left. SB: A,G= 200um, D,E=15um. Conclusions and Perspectives  atf5a/atf5b mRNAs are maternally contributed at 6hpf. atf5a is strongly expressed in olfactory organs, specifically in OSNs, while weakly expressed in LL. atf5b is strongly expressed in LL while not found in olfactory organs. Both genes are expressed in the developing brain, presumably neurogenic areas. Single and double-knock outs will shed light into the function of each in tissues described here. Fig.3. atf5a/atf5b expression in developing inner ear. (A,B,C) atf5a antisense RNA hybridization in the three cristae and maculae of the inner ear. (D,E,F) atf5b antisense RNA hybridization. SB: O,P=40um Fig.5. atf5a/atf5b expression in developing brain. (A,B,C) atf5a antisense RNA hybridization in dorsal view of undissected (A) and dissected (B,C) larval brains. (D,E,F) Same for atf5b. SB: A,B, = 150um. We performed whole-mount in-situ hybridization using antisense RNA probes for both paralogous genes (atf5a and atf5b). Their expression profile in zebrafish larvae, starting at 2dpf, is described below. Fig.4. atf5a expression in olfactory organ. (A,B,C) atf5a antisense RNA hybridization in dorsal view o