Abstract
Abstract: :
Purpose:Math5 is expressed during early retinal histogenesis and is necessary for retinal ganglion cell (RGC) fate specification. In principle, Math5 may act as an instructive factor, irreversibly directing competent progenitor cells to differentiate as RGCs. Conversely, Math5 may act as a permissive factor, establishing an RGC competence state within a large set of progenitors, some of which will be selected to become RGCs by other agents. Methods:To test these hypotheses, we traced the lineage of Math5 expressing cells during development. We first created BAC transgenic mouse lines expressing Cre recombinase under strict Math5 regulatory control. We then crossed these mice to Z/AP (alkaline phosphatase) and R26–GFP reporter strains to reveal the fate of Math5 expressing cells. To determine when Math5 is expressed during the cell cycle, we co–stained developing retinas of heterozygous Math5–lacZ knock–in mice for lacZ activity and phase–specific markers. Results:The Math5–Cre transgenes accurately reveal the cumulative pattern of Math5 expression during retinal histogenesis. Math5–expressing progenitors contribute to every major cell type in the adult retina, but are heavily skewed toward cells with early birthdates, consistent with the time course of Math5 expression. Math5–Cre labels approximately 60% of RGCs, 30% of cones and horizontal cells, 10% of amacrines, 1.5% of rods, and <0.1% of bipolar and Müller cells. When AP and GFP reporters were tested simultaneously, the concordance of labeling was >90% for every cell type. During retinogenesis, activated AP reporter and perdurant Math5–lacZ are each excluded from M, S and G2 phases of the cell cycle. Math5 is therefore expressed by postmitotic cells. Conclusions:Math5 acts as a permissive factor. During early retinal development, a specific subpopulation of progenitors exits the cell cycle, expresses Math5, and becomes competent to form RGCs. These cells may also adopt other fates, with a probability that depends on the timing of the terminal mitosis. A general mechanism of fate specification is proposed.
Keywords: ganglion cells • retinal development • transcription factors