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Plexin (previously referred to as B2) is a neuronal cell surface molecule that has been identified in Xenopus. cDNA cloning reveals that plexin has no homology to known neuronal cell surface molecules but possesses, in its extracellular segment, three internal repeats of cysteine clusters that are homologous to the cysteine-rich domain of the c-met proto-oncogene protein product. The exogenous plexin proteins expressed on the surfaces of L cells by cDNA transfection mediate cell adhesion via a homophilic binding mechanism, under the presence of calcium ions. Plexin is expressed in the receptors and neurons of particular sensory systems. These findings indicate that plexin is a novel calcium-dependent cell adhesion molecule and suggest its involvement in specific neuronal cell interaction and/or contact.
Figure6. Expression of Plexin in the Accessory Olfactory System
(A, B, and D) Localization of the plexin proteins,as detected by immunofluorescent staining with MAbB2.The epithelium of the vomeronasal organ
(VNE in [A]), the vomeronasal nerve fibers (arrows in [A] and [B]), and the neuropileof the accessory olfactory bulb (AOB in [D]) are positively stained with MAbB2.
(C, E, and F) Localization of the plexin mRNAs,as detected by in situ hybridization.(C) and (E) show dark-field micrographs;(F) shows a bright-field micrograph of (E). In situ hybridization signals are detected in the epithelial cells of the vomeronasal organ (VNEin [C]) and the target neurons for the vomeronasal nerve fibers (arrows in [E] and [F])within the accessory olfactory bulb (AOB). Asterisks in [B] indicate the Bowman's glands. All images are at stage51. Bars, 100 rim (A-C); 200 pm (D-F).
Figure 7. Expression of Plexin in the Lateral Line System and the Vestibular System
(A-D) Localization of the plexin protein, as detected by immunofluorescent staining with MAbB2. The neuromasts of the lateral line receptors (LLR in [A] and [B]), the lateral line nerve fibers (arrows in [A] and [B]), and the lateral line ganglion neurons (LLG in [B]) located at the dorsal part of the trigeminal ganglion (TG) are stained with MAbB2. Neither the epidermal cells (small arrows in [A] and [B]) nor the trigeminal ganglion neurons (B) are stained with MAbB2. The antibody also binds to the vestibular receptor (VR in [C]), the eighth (the vestibular) nerve fibers (arrow in [C]), the vestibular ganglion neurons (VG in [C]), and the common terminal neuropile for the lateral line and the eighth nerves within the medulla (see text; asterisk in [D]). TB/BT in (D) indicates the MAbB2-positive tectobulbal and bulbotectal tracts, which are the efferent and afferent of the optic tectum, respectively (see Ohta et al, 1992).
(E and F) Localization of the plexin mRNAs, as detected by in situ hybridization. Dark-field and light-field micrographs ([E] and IF], respectively) of a frontal section of the medulla. In situ hybridization signals are detected in the cells of the lateral line nucleus (dotted area; LLN) and of the nucleus for the eighth nerve (nVIII). Asterisks indicate the neuoropile corresponding to that in (D).
Images are at stage 43 (A and B) and at stage 48 (C-F). Bars, 50 p~m(A), 100 I~m (B and C), 200 I~m (D-F).
