A Small Molecule TrkB Neurotrophin Receptor Partial Agonist as Possible Treatment for Experimental Nonarteritic Anterior Ischemic Optic Neuropathy

ABSTRACT

Purpose

Brain-derived neurotrophic factor (BDNF) and activation of its high affinity receptor tropomyosin kinase (Trk) B promote retinal ganglion cells (RGCs) survival following injury. In this study, we tested the effects of LM22A-4, a small molecule TrkB receptor-specific partial agonist, on RGC survival in vitro and in experimental nonarteritic anterior ischemic optic neuropathy (AION), the most common acute optic neuropathy in those older than 50 years.

Methods

We assessed drug effects on immunopanned, cultured RGCs and calculated RGC survival and assessed TrkB receptor activation by mitogen-activated protein (MAP) kinase translocation. To assess effects in vivo, we induced murine AION and treated the animals with one intravitreal injection and three-week systemic treatment. We measured drug effects using serial spectral-domain optical coherence tomography (OCT) and quantified retinal Brn3A⁺ RGC density three weeks after ischemia.

Results

In vitro, LM22A-4 significantly increased the survival of cultured RGCs at day 2 (95% CI control: 8.4–13.6; LM22A-4: 23.7–30.3; BDNF: 24.3–29.9; P ≤ 0.0001), similar to the effect of the endogenous TrkB receptor ligand BDNF. There was also significant nuclear and cytoplasmic translocation of MAP kinase (95% CI control: 0.9–6.8; LM22A-4: 38.8–84.4; BDNF: 64.0–93.0; P = 0.0002), a known downstream event of TrkB receptor activation. Following AION, LM22A-4 treatment led to significant preservation of the ganglion cell complex (95% CI: AION-PBS: 66.8–70.7%; AION-LM22A-4: 70.0–73.1; P = 0.03) and total retinal thickness (95% CI: AION-PBS: 185–196%; AION-LM22A-4: 195–203; P = 0.002) as measured by OCT compared with non-treated eyes. There was also significant rescue of the Brn3A⁺ RGC density on morphometric analysis of whole mount retinae (95% CI control: 2360–2629; AION-PBS: 1647–2008 cells/mm²; AION-LM22A-4: 1958–2216 cells/mm²; P = 0.02).

Conclusions

TrkB receptor partial agonist LM22A-4 promoted survival of cultured RGCs in vitro by TrkB receptor activation, and treatment in vivo led to increased survival of RGCs after optic nerve ischemia, providing support that LM22A-4 may be effective therapy to treat ischemic optic neuropathy.

Abbreviations

AION: anterior ischemic optic neuropathy, BDNF: Brain-derived neurotrophic factor, GCC: ganglion cell complex, MAP: mitogen-activated protein, OCT: spectral-domain optical coherence tomography, OD: right eye, ON: optic nerve, ONH: optic nerve head, OS: left eye, RGC: retinal ganglion cell; Trk: tropomyosin kinase

KEYWORDS:

• AION
• BDNF
• TrkB
• OCT
• retinal ganglion cell
• optic neuropathy
• pharmacophore

Acknowledgments

We thank Gun Ho Lee and Jeffrey Ma for their help with experiments.

Conflict of Interest

F.M.L. is listed as an inventor on patents relating to LM22A-4, which is assigned to the University of North Carolina and the University of California, and is entitled to royalties distributed by university standard agreements. F.M.L. is a principal of, and has a financial interest in PharmatrophiX, a company focused on the development of small molecule ligands for neurotrophin receptors which has licensed several of these patents. All other authors had no conflict of interest.

Additional information

Funding

Y.J.L. was supported by the Career Award in Biomedical Sciences from the Burroughs Wellcome Foundation, Western Havens Foundation, and the North American Neuro-Ophthalmology Society Pilot Grant, National Eye Institute (P30-026877), and Research to Prevent Blindness, Inc.