Morphological multivariate cluster analysis of murine retinal ganglion cells selectively expressing yellow fluorescent protein.

Optic neuropathies, such as glaucoma, lead to retinal ganglion cell (RGC) death. Transgenic mouse strains that express fluorescent proteins under the control of the Thy1 promoter have permitted single RGC imaging. Specifically, in one strain of mice expressing yellow fluorescent protein (Thy1-YFP), fluorescence is expressed in only 0.2% of RGCs. This reduced expression allows visualization of the full dendritic arbour of YFP-expressing RGCs, facilitating the investigation of structural changes. As susceptibility amongst RGCs varies with morphology and subtype, labelling methods should ideally non-discriminately label RGCs to accurately determine the effects of experimental glaucoma. This study therefore sought to determine morphological subtypes of RGCs in the Thy1-YFP mouse strain. Retinas from Thy1-YFP mice were imaged ex vivo with fluorescence microscopy. With Sholl analysis, a technique for quantifying the morphology of individual neurons, the dendritic field (DF), area under the curve (AUC), normalized AUC (N), peak number of intersections (PNI), and skew for single RGCs were computed. The distance of the RGC from the optic nerve head (dONH) was also measured. These morphological parameters were inputted into a multivariate cluster analysis to determine the optimal number of clusters to group all RGCs analyzed, which were then grouped into "Small", "Medium", and "Large" sized cluster groups according to increasing DF size. A total of 178 RGCs from 10 retinas of 8 mice were analyzed from which the cluster analysis identified 13 clusters. Eighty-eight (49%), 77 (43.2%), and 13 (7.3%) RGCs were grouped into small, medium and large clusters, respectively. Clusters 1-6 had small DFs. Clusters 1 and 3 had the lowest AUC and N. Clusters 2, 3, and 5 had asymmetric DFs while Clusters 3, 5, and 6 were distal to the ONH. Clusters 7-11 had medium DFs; of these, Clusters 7 and 10 had the lowest AUC, Clusters 8 and 10 had the highest skew, and Clusters 7 and 11 were closest to the ONH. Clusters 12 and 13 had large DFs. Both had low skew and high AUC. High PNI and dONH distinguished Cluster 12 from Cluster 13. We present the largest study to date examining YFP expression in RGCs of transgenic Thy1-YFP mice. Among the 13 clusters, there was a wide range of morphological features with further variation within size categories. Our findings support the notion that YFP is expressed non-discriminatingly in RGCs of Thy1-YFP transgenic mice and this strain is a valuable tool for studies of experimental optic neuropathies.