Fig. 6. Two distinct RNCAM isoform-dependent activities influence segregation of P2 axons. Frontal (left) and side view (right) of olfactory bulb. The location of P2 semi-innervated glomeruli (white circles) relative to major lateral (L) and medial (M) P2 glomeruli (black circles) are shown in ten mice of each genotype. (A) Control mice (cont), (B) TmRNCAM transgenic mice (Tm), (C) GpiRNCAM transgenic mice (Gpi) and (D) double GpiRNCAM/TmRNCAM transgenic mice (Gpi/Tm). Three different domains (broken circles) of the OB, with an approximately diameter of 200 µm, contained semi-innervated P2 glomeruli. The trajectories of P2 axons innervating incorrect glomeruli within the domains are indicated with arrows. Note that overexpression of TmRNCAM results in targeting errors in close proximity to both the medial and the lateral P2 glomeruli within a domain in which spontaneous targeting errors can be found at a low frequency in control mice. Overexpression of GpiRNCAM results in targeting errors located caudally, distant from the main lateral P2 glomeruli. The phenotype of mice overexpressing both RNCAM isoforms is similar to that of TmRNCAM transgenic mice. These results indicate that two independent RNCAM isoform-specific mechanisms influence OR-specific axon segregation. The data were obtained from progeny of two transgenic founder lines transgenic for each RNCAM isoform.

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