Figure 8

Progressive gene silencing following optic nerve crush (ONC) precedes the loss of retinal ganglion cells (RGCs). Analysis of transcript levels of several silenced genes following ONC or optic nerve axotomy. (A) A retrospective analysis of published reports of gene silencing in apoptotic RGCs. Each data point represents the mean of values collected by different investigators to simplify the data set shown. One of the earliest demonstrations of gene silencing in RGCs after optic nerve damage was a quantitative RNase protection assay study for the RGC marker gene Thy1 [11]. These data points follow an exponential decay curve described by the equation, y = -18.441 Ln(x) + 55.703. All other data points are shown relative to this curve and displayed similar kinetics of silencing. Alternatively, during the first 7 days after ONC less than 1% of the cells had been eliminated (data not shown) [11]. Thus, gene silencing precedes cell loss by several days and represents a relatively early event in the apoptotic pathway. (B) Quantitative PCR analysis of silenced genes following ONC, graphed over the exponential decay curve of the original Thy1 studies. A similar decline in transcripts is evident in a more controlled prospective study. Interestingly, after an initial decline, BclX levels recover, consistent with earlier reports. In addition, genes with increases or no change in expression were also examined. Both Gap43 and Bim exhibited increases in transcript level in the experimental eye following ONC (145% and 116%, respectively), while S16 remained unchanged (99.8%, data not shown). In independent qPCR experiments, cJun transcript levels were also observed to increase modestly, shortly after ONC (112%).

^a [11] Transcript abundance quantified by RNase protection assays in mouse ONC.

^b [15] Quantified by qPCR in a rat model of axotomy.

^c[12] Protein levels quantified by ELISA assays in mouse ONC.

^d[14] Transcript abundance quantified by RNase protection assay in rat ONC.

^{e, f, g}[5] Quantified by qPCR following axotomy in rats.

^h[17] Transcript abundance measured by quantitative in situ hybridization studies following axotomy in rats.