Sensory epithelium of the vomeronasal organ express TrkA-like and epidermal growth factor receptor in adulthood. An immunohistochemical study in the horse.

Abstract: The medial wall of the vomeronasal organ (VNO) is lined with a sensory epithelium that is closely related to the olfactory epithelium, which is developed from the olfactory placode. It undergoes continuous replacement during its life span. In other sensory epithelia, cell proliferation is under the control of some trophic factors. Whether these proteins are involved in the continuous turnover of the VNO epithelium is unknown. This study approaches this topic by analyzing the occurrence of signal-transducing receptor proteins for neurotrophins (Trk proteins) and epidermal growth factor (EGFr). VNO samples were obtained from adult horses (n = 9) and processed for Western blot or immunohistochemical detection of TrkA, TrkB, TrkC, and EGFr. For immunohistochemistry, both frozen and formalin-fixed, paraffin-embedded sections were used. Antibodies against Trk proteins were polyclonal antibodies that map within the intracytoplasmic domain. Antibodies against EGFr were monoclonal antibodies that map within the external (clone EGFR1) or the cytoplasmic (clone F4) domains. TrkA-like, but not TrkB- or TrkC-like, protein was detected in the VNO. By using immunoblotting, protein bands of TrkA-like protein with estimated molecular weights of 43-45, 55, and 60 kDa were found. In agreement with these findings, the sensory epithelium lining the VNO displayed strong TrkA-like immunoreactivity. On the other hand, regular protein bands with estimated molecular weights of 100 and 170 kDa, corresponding with immature and full-length EGFr, respectively, were found with the clone F4, whereas the clone EGFR1 was ineffective in detecting EGFr with Western blot analysis. Positive EGFr immunolabelling was observed regularly in the supranuclear pole of the sensory epithelial cells, and the pattern was identical with both antibodies used. The present results provide evidence for the occurrence of EGFr in the VNO of the adult horse, suggesting a role for their ligands (EGF and transforming growth factor-alpha) in this organ, probably in continuous cell replacement, during the adult life span. However, although immunoreactivity for TrkA-like protein was regularly observed, because the full-length protein was not found, whether or not its putative ligands (nerve growth factor and neurotrophin-3) act on these cells remains to be demonstrated.