Reduced TrkB Expression Potentiates Retinal Ganglion Cell Loss in a Mouse Glaucoma Model

Abstract

Purpose: : Retrograde transport blockage of the neurotrophin receptor TrkB has been demonstrated in optic nerves using animal models of elevated IOP. It is not well established as to what levels adult RGCs are dependent on TrkB for survival, or if deprivation of axonally derived TrkB is a significant factor in RGC death in glaucoma. To determine the dependence of adult RGCs on neurotrophin support in glaucoma, we have begun investigations into the role of TrkB deprivation using a mouse model of chronic ocular hypertension (COH).

Methods: : We obtained heterozygous TrkB hypomorphic mice (TrkB^fbz/+) that express full-length TrkB at 62.5% of their respective wild-type littermates, and mature and reproduce normally. 24 C57BL/6 and 23 TrkB^fbz/+ mice were treated in the right eye to induce COH by the Morrison method of limbal injection of hypertonic (1.5 M) saline. IOP was recorded weekly for 17 weeks by TonoLab tonometry. After sedation, treated and control eyes were enucleated, and optic nerve cross-sections stained with toluidine blue. Axon counts were obtained in a masked fashion using the KS400 imaging system (Zeiss), and % axon survival (treated/control axon counts) for each mouse were calculated. % axon survivals for the two groups were tested for significance with Student’s t-test.

Results: : 21 C57BL/6 and 17 TrkB^fbz/+ mice converted to COH (IOP integral > 200 mm-days), for a conversion rate of 81% (38/47). Mean IOP integrals for the C57BL/6 and TrkB^fbz/+ groups were 426 and 531 mm-days, respectively. One-way ANOVA showed no significant difference between the two groups in IOP integral distribution (P=0.38), indicating equivalent levels of IOP exposure between the two groups. The two groups showed average axon counts of: C57BL/6: 15250 ± 4897 (right eye; mean ± SD), 22396 ± 8231 (left eye); TrkB^fbz/+: 17755 ± 9747 (right eye), 32885 ± 7419 (left eye). The two groups showed average % axon survivals (treated/control axon counts) of: C57BL/6: 75.8% ± 29.1% (mean ± SD); TrkB^fbz/+: 52.5% ± 31.0%. One-way ANOVA showed a significant difference between the two groups in % axon survival (P=0.044).

Conclusions: : In a mouse model of COH, reduction of full-length TrkB expression by 37.5% results in a 30.8% greater loss of optic nerve axons when compared to mice with normal TrkB expression levels. Levels of TrkB-related neurotrophic support are directly correlated with RGC survival in a mouse glaucoma model.

Keywords: ganglion cells • apoptosis/cell death • neuroprotection