Abstract
Abstract: :
Purpose: The survival of adult retinal ganglion cells (RGC) may depend on neurotrophins, such as BDNF and NT4/5. In glaucoma, obstructed axonal transport, due to elevated intraocular pressure (IOP), is thought to prevent the retrograde transport of these neurotrophins to RGC soma. The purpose of this study is to determine the effect of elevated IOP on endogenous retinal mRNAs for neurotrophins, Trk receptors and Trk signaling pathway intermediates.Methods: Brown Norway rats (N=29) received unilateral episcleral vein injections of hypertonic saline and IOP was measured 3 times weekly on awake animals by Tonopen. Unilateral optic nerve transection was performed on six additional animals. At 5 weeks post–injection and 10 days post–transection, animals were sacrificed and retinal mRNA was prepared, reverse transcribed and quantitated relative to GAPDH using real–time PCR. Optic nerve injury was determined by grading nerve cross–sections. Linear regression and ANOVA were used for statistical analysis. Proteins were localized by immunohistochemistry. Results: Only transection significantly reduced the RGC specific mRNA Brn3b(p<0.01). Retinal mRNA levels for neurotrophins BDNF, NT4/5, NT3 and NGF were not significantly altered by elevated IOP or transection. TrkB mRNA levels were reduced in retinas from injected eyes with the highest IOP levels or the greatest pressure–induced nerve injury (p<0.05). TrkC mRNA levels were also significantly reduced in these same retinas (p<0.01), as well as in retinas with more moderate IOP elevation (p<0.05). TrkA, the truncated forms of TrkB and TrkB survival pathway intermediates AKT, BCL2 and BCLX mRNAs were not significantly altered. With the exceptions of TrkA and BCL2, the proteins associated with these mRNAs are predominately localized to the inner retina. Conclusions: This in vivo study yields two important conclusions. First, the retina does not upregulate endogenous neurotrophins following either elevated IOP or transection. This reduces the likelihood that the retina can compensate for the loss of retrograde transported, target–derived neurotrophins. It also makes less likely that excess neurotrophins may activate ligand–dependent p75NTR pathways which promote RGC apoptosis. Second, elevated IOP, in contrast to transection, also results in the downregulation of TrkB and TrkC mRNA levels. This implies that exposure to elevated IOP reduces the capacity of the retina to activate Trk–mediated survival signaling pathways following either restored transport or therapeutically delivered neurotrophins.
Keywords: intraocular pressure • retina: neurochemistry • growth factors/growth factor receptors