Retinal Sheet Transplants Benefit Rats with Rod Degeneration, Revealed By Optokinetic Testing And Manganese-Enhanced MRI (MEMRI)

Abstract

Purpose: : To test the hypothesis that retinal sheet transplants in rats with rod photoreceptor degeneration in vivo improved visual and retinal function.

Methods: : Donor retinal progenitor sheets (size 1 mm²) of rat fetuses expressing human placental alkaline phosphatase (hPAP) were transplanted unilaterally to the subretinal space of pigmented RD S334ter line-3 rats (25-34 d). Ocular coherence tomography (OCT) 2-4 weeks after surgery was used to select rats with good transplants for further optokinetic tracking (OKT) and manganese-enhanced MRI (MEMRI) examination. At 2-4 months post surgery, treated and control eyes of dark adapted rats were studied: (a) spatial frequency thresholds were determined by OKT (15 transplants and 7 no-surgery controls) and (b) retinal function was evaluated using high resolution MEMRI (25 μm in plane), followed one month later by MEMRI of light adapted rats. After MEMRI, eyes were fixed and retinal sections stained with antibodies against hPAP (donor marker), together with antibodies against various retinal cell markers.

Results: : Rats selected by OCT showed large transplants close to the optic disk and indications of lamination; no correlation was noted between transplant extent or location and OKT thresholds. OKT thresholds (0.40 + 0.05 cycles/degree (c/d, mean + SD, non-implanted eye) and 0.44 + 0.03 c/d (implanted eye)) were both lower (P < 0.05) than normal control Long-Evans rats (0.51 + 0.01 c/d). However, implanted eyes showed an overall improvement in acuity versus non-implanted eyes (P < 0.05). The greatest improvements were found when fellow eyes had the lowest acuities. Transplant areas could be identified by MEMRI by their increased retinal thickness. In a subset of animals, rats with an OKT difference from non-implanted eyes more than 10% also had relatively larger dark and light differences on MEMRI than rats with less than 10% OKT differences. In histology, rod photoreceptors were only found in the transplants, but the host retina still contained many cones inside and outside the transplant area.

Conclusions: : These data support our hypothesis that retinal transplants improve visual function, even at a relatively early stage of retinal degeneration. The combination of OKT and MRI provided a useful and informative approach for non-invasively and analytically evaluating benefits of retinal sheet transplants in a rat model of retinal degeneration.

Keywords: imaging/image analysis: non-clinical • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • retinal degenerations: hereditary