Abstract
Purpose :
In most mammals, the radial glia and midline neurons of the optic chiasm selectively decussate retinal ganglion cell (RGC) axons into the optic tracts. These specialized glia and neurons adhere to the underlying pial basement membrane (PBM) that is rich in β2-containing laminins. Here we asked if b2-containing laminins present in the PBM control the selective decussation of RGC axons.
Methods :
The morphology of the radial glia and disruption of the PBM was determined by immunolabeling sections from wild-type (WT) and Lamb2-/- mice at the peak (E15.5) and late (E17.5) phases of axonal decussation. The extent of axonal decussation was determined by anterograde labeling of RGC axons with DiI in WT and Lamb2-/- mice at P4. The area of retinal projections was measured and used to calculate the proportional area of each projection. The relative fluorescence intensities of EphrinB2, α-Dystroglycan (α-DG) and Adgrg1 (GPR56) immunolabels in radial glia were measured in at least 3 images of anterior chiasms (2-4 biological replicates) using equal image threshold and background subtraction.
Results :
At P4, the selective decussation of retinal axons is disrupted, with a larger proportion of non-decussating axons entering the ipsilateral tract in Lamb2-/- (46%) than in wild-type (WT) (40%) mice (p<0.0001). The β2 laminin subunit is expressed in the PBM at the time of selective decussation. Analysis of the embryonic chiasm in Lamb2-/- mice showed that the radial glia basal processes are thinner and undulated, while endfeet lose their typical club shape and attachment to the PBM. Concurrent with these changes, the expression of the laminin receptor α-DG is lost from the radial glia endfeet and processes (55%, p=0.01). Also, the expression of the collagen receptor GPR56 is lost from the endfeet and the total cellular expression is reduced (76%, p=0.014). Interestingly, the ipsilateral cue EphrinB2 is abnormally increased in the radial glia during the late phase of axon decussation, when non-decussating axons have ceased entering the ipsilateral tract (70%, p=0.0004).
Conclusions :
These data suggest that b2-containing laminins regulate RGC selective axon decussation by providing cues that confer polarity to the radial glia and that decrease EphrinB2 expression to terminate the growth of axons into the ipsilateral tract.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.