Early born lineage of retinal neurons express class III beta-tubulin isotype

Class III beta-tubulin, a constituent of neuronal microtubules, has been frequently used as a marker for the neuronal lineage in developmental biology. In retina, it is often used as a marker for ganglion cells. We investigated the developmental expression of this protein in retina and identified the cell types expressing it to gain a better understanding of whether preferred expression of this isotype in certain retinal neurons plays a cell specific role, or whether it is only a part of an intrinsic developmental program.

Immunohistochemistry was done using an antibody against class III beta-tubulin and other retinal cell specific markers in adult retinae of mice. Rabbit and human retinae were used to investigate if there are any species-specific differences.

Class III beta-tubulin was found in ganglion cells, certain amacrine cells, some horizontal cell processes and cone photoreceptors. Class III beta-tubulin was already expressed in the earliest developmental stage studied (Embryonic day 14) in developing nerve fiber layer but became distinct at the day of birth when immunoreactive cells were located in the ganglion cell layer (ganglion and displaced amacrine cells), proximal parts of neuroblastic/inner nuclear layer (amacrine cells) and distal part of neuroblastic/outer nuclear layer (photoreceptors). In one animal, class III beta-tubulin containing bodies were found in the retinal pigment epithelium cells.

Class III beta-tubulin is not solely expressed by ganglion cells and, therefore, cannot be used as an exclusive marker for these cells. Results show that the expression of class III beta-tubulin was not related to cell morphology or cell function, but rather to the cell lineage (early born retinal neurons) suggesting that the expression of class III beta-tubulin in certain cell types may be due to the cell specific developmental program.