Optic Nerve Head (ONH) Gene Expression Following Optic Nerve Transection vs an Ischemic Event

Abstract

Purpose : To identify ONH gene expression changes following optic nerve transection vs partial ischemia (elevated IOP with low perfusion pressure).

Methods : Anesthetized rats underwent either (1) unilateral orbital optic nerve transection or (2) an 8-hour controlled elevation of IOP (CEI) at 60 mm Hg plus experimentally reduced blood pressure to produce partial ONH ischemia (CEI-ischemia). RNA-seq was used to compare expression 3 days following injury to that in naive ONH (N=8/group). RNA was extracted, individual libraries prepared, and sequences mapped to the UC Santa Cruz rat genome. Differentially expressed genes were identified based on trimmed mean of M-value (TMM) normalized reads. Statistical computations used Bioconductor packages and R-statistical language. Expression changes >2 fold with FDR adjusted p-values <0.05 were considered statistically significant. Significant gene clusters (enrichment score > 1.3) were discovered using DAVID Bioinformatics tools.

Results : Transection yielded 713 up- and 384 down-regulated genes. Upregulated clusters were dominated by immune response and cell proliferation. Downregulated gene clusters included synapse, ligand-receptor interaction, and visual perception. CEI-ischemia [baseline mean arterial pressure reduced by 25 ± 5 (SEM) mm Hg (P = .002)], produced 265 up- and 56 down-regulated genes. Upregulated clusters were dominated by cell proliferation, immune response, and angiogenesis. Downregulated clusters included chemical synaptic transmission and ion transmembrane transport. Both models shared many gene clusters. However, compared to transected ONH, CEI-ischemia ONH had: (1) twice as many upregulated cell proliferation clusters (51% to 21%), (2) fewer upregulated immune clusters (6% to 24%) and (3) fewer (8% to 32%) downregulated neural clusters.

Conclusions : While both injuries activate many common gene functions and upregulated genes dominate, there are differences between the responses. The severe optic nerve trauma of transection enhances immune-related ONH injury responses. In contrast, with CEI-ischemia, ONH glial proliferation predominates and the specific astrocytic and axonal functions of membrane transport and synaptic support are reduced. These analyses will aid interpretation of future gene expression responses following CEI events in which apparently adequate capillary perfusion is maintained.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.