Microtubule polarity and distribution in teleost photoreceptors

We have characterized the polarity orientation of microtubules in teleost retinal photoreceptors. The highly polarized rods and cones contain large numbers of paraxially aligned microtubules and exhibit dramatic cell shape changes. The myoid portion of the inner segments of both rods and cones undergoes contraction and elongation in response to light or circadian signals. Previous studies in our laboratory have demonstrated that in cones but not rods myoid elongation is microtubule-dependent. To determine polarity orientation, we decorated microtubules in photoreceptors of the green sunfish Lepomis cyanellus, with hooks formed from either exogenous or endogenous tubulin subunits. The direction of curvature of the attached hooks in cross section indicates microtubule polarity orientation by allowing one to determine the relative positions within the cell of the plus (fast-growing) and minus (slow-growing) ends of the microtubules. We found that virtually all cytoplasmic microtubules in photoreceptors are oriented with plus ends directed toward the synapse and minus ends toward the basal body at the base of the outer segment. Axonemal microtubules in photoreceptor outer segments are oriented with minus ends toward the basal body as in cilia and flagella. We have suggested previously that cone myoid elongation is mediated by mechanochemical sliding between microtubules. The polarity observations reported here indicate that if microtubules do slide in cones, sliding would necessarily occur between microtubules of parallel orientation as is observed in cilia and flagella.