Taken together, our results indicate that replacement of BDNF by NT4 can, at least partly, rescue disrupted taste target innervationin vivo. == Figure 4. was disrupted, and gustatory axons failed to reach their targets. However, disrupted innervation was rescued and target innervation is normal when NT4 replaced BDNF. Genome wide expression analyses revealed that BDNF and NT4 mutant mice exhibited different gene expression profiles in gustatory (geniculate) ganglion. Compared to wild type, the expression of differentiation-, ZD-1611 apoptosis- and axon guidance-related genes was changed in BDNF mutant mice, which is consistent with their different roles during taste development. However, replacement of BDNF by NT4 rescued these gene expression changes. These findings indicate that the functions of BDNF and NT4 in taste development are interchangeable. Spatial and temporal differences in BDNF and NT4 expression can regulate differential gene expressionin vivoand determine their specific roles during development. Keywords:Taste, neurotrophins, brain derived neurotrophic factor, neurotrophin 4, geniculate ganglion neurons == Introduction == During development, taste neurons innervate specific regions of gustatory epithelium with a precise number of neurons. These connections are mediated by neurotrophins, including brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT4). Although both BDNF and NT4 activate the same receptors, TrkB and p75, they play different regulatory roles in taste development. BDNF is expressed in epithelial placodes that eventually form fungiform papillae and taste buds and it functions as a chemoattractant allowing gustatory fibers to distinguish their fungiform papilla targets from non-gustatory epithelium, such as filiform papillae (Hoshino et al., 2010;Krimm et al., 2001;Lopez and Krimm, 2006a;Ma et al., 2009;Nosrat et al., 1997;Nosrat et al., 1996;Nosrat and Olson, 1995;Nosrat et al., 2012;Ringstedt et al., 1999). NT4 is not necessary for target innervation in the taste system (Ma et al., 2009), but is essential for geniculate Itgb8 ganglion neuronal survival during development (Liebl et al., 1997;Liu et al., 1995;Patel and Krimm, 2012). In mutant mice lacking either BDNF or NT4, approximately half of the neurons are lost from the geniculate ganglion (Liebl et al., 1997;Liu et al., 1995). In mice lacking both of these factors, almost all geniculate ganglion neurons are lost (Liu et al., ZD-1611 1995). Although both BDNF and NT4 regulate geniculate ganglion neuron number, they exert influence at different developmental stages ZD-1611 via different mechanisms. Geniculate neurons become BDNF-dependent at E13.5, and BDNF regulates neuronal apoptosis by preventing ZD-1611 caspase-3 activation (Patel and Krimm, 2010). Geniculate neurons become NT4-dependent by E11.5, but NT4 does not prevent caspase-3 activation (Patel and Krimm, 2012). It is unclear how BDNF and NT4 differentially regulate geniculate neuron development through the same receptors. The diverse functions of BDNF and NT4 could be due to their distinct temporal and spatial expression patterns in the peripheral taste system during development. NT4 expression levels are the highest at embryonic day 12.5 (E12.5) and then decrease quickly, while BDNF steadily increases in the ganglia throughout embryonic development (Huang and Krimm, 2010). In the tongue, BDNF but not NT4 is specifically expressed in developing taste placodes, and expression remains high through E16.5 (Huang and Krimm, 2010;Nosrat et al., 1996). The distinct expression patterns of BDNF and NT4 may determine their different roles in taste development. Alternatively, neurotrophins can achieve diverse functions by activating different internal signaling pathways downstream of Trk receptor binding. It has been shown that Shc- and phospholipase C1-dependent pathways downstream of TrkB, play distinct roles in neuronal survival, targeting innervation and synaptic plasticity (Gartner et al., 2006;Minichiello et al., 2002;Musumeci et al., 2009;Postigo et al., 2002;Sciarretta et al., 2010). BDNF and NT4 could exert different functions by activating distinct signaling pathways downstream of TrkB. This issue of locationvs.signaling was examined previously by replacing BDNF with NT4 (Fan et al., 2000). However, no unique roles for either BDNF or NT4 were examined in any system that utilizes both factors during development. For example, numbers of neurons were examined in the geniculate, nodose, and petrosal ganglion; but both BDNF and NT4 regulate neuron number in these ganglia. Because these knock-in mice over-express NT4, the increased NT4 expression could enhance neuron number via a variety of mechanisms without reversing the effects of BDNF removal on cell death. In fact, the neurons rescued in these mixed sensory ganglia by knocking-in NT4, may not even be the same subpopulation of neurons as those lost inBdnf/mice. In addition, BDNFs unique functions like targeting were never examined. Therefore, it was still not clear if these two factors can function interchangeably during.