Strain Dependence of Myeloid Cell-Associated Retinal Dysplasia in Crb1^rd8 Mice

Abstract

Purpose: To determine the genetic background dependence and myeloid cell status of retinal lesions associated with the Crb1^rd8 allele, which in mice is linked to retinal dysplasia and degeneration and occurs in many C57BL/6-derived strains.

Methods: Retinal phenotypes were assessed in a Crb1^rd8 inbred stock, which has been fixed on a mixed genetic background; in a B6.C3-Crb1^rd8 congenic strain on the C57BL/6J background; and in strain C57BL/6NJ, which is homozygous for the Crb1^rd8 mutation. To detect myeloid cells, these strains were also crossed with B6.129P-Cx3cr1^GFP; Crb1^rd8 mice, which have a mixed C57BL/6J and C57BL/6N background. Pathological changes at 1-3 months of age were detected in live mice by fundus imaging and optical coherence tomography (OCT), and in stained retinal sections or whole mounts by brightfield or fluorescence microscopy, respectively.

Results: Fundus and OCT imaging of Crb1^rd8 homozygous mice revealed punctate lesions at the external limiting membrane throughout the fundus, and larger dysplastic lesions extending from the subretinal space to the inner nuclear layer in the inferior retina. Punctate lesions were increasingly apparent at 2-3 months of age, and corresponded to patches of photoreceptor nuclei displaced sclerally through the external limiting membrane. These patches lacked F-actin staining, suggesting a cytoskeletal defect in photoreceptor inner segments or Müller cell microvilli. Dysplastic lesions were present at all ages and were >20-fold less abundant in the C57BL/6 strain backgrounds than in the inbred stock. Histology showed possible myeloid cells with multilobed nuclei and eosinophilic cytosol in lesion cores or in the underlying subretinal space. Live fluorescence imaging of myeloid-GFP mice suggested myeloid cell clustering near dysplastic lesions in Crb1^rd8 homozygous strains. Analysis of one-month whole mounts showed activated microglial cells in dysplastic lesion cores and alterations of the surrounding vasculature. Retinas of wild-type C57BL/6J mice showed no lesions.

Conclusions: These results show that C57BL/6 strain backgrounds attenuate Crb1^rd8-associated retinal dysplasia without diminishing Crb1^rd8-dependent perturbation of the external limiting membrane, and establish myeloid cell activation as an early dysplastic event. Differences in myeloid cell activation may explain the genetic background dependence of Crb1^rd8 retinal dysplasia.

Keywords: 695 retinal degenerations: cell biology • 696 retinal degenerations: hereditary • 595 microglia