Aging results in segmental reduction in contractile smooth muscle coverage on retinal arterioles

Abstract

Purpose : In vivo functional measurements using dynamic retinal vessel analyzers in human subjects have shown that aging is associated with decreased retinal arteriolar dilatory and contractile responsiveness. Importantly, aging also results in non-uniform irregularities in retinal arteriolar profiles as a function of vessel length (Kotliar et al., 2008, IOVS 49:2094-2102). However, a cellular/molecular mechanism for such observations has not been definitively identified. In this study, we provide evidence of segmental loss of contractile smooth muscle cells (SMCs) and concomitant arteriolar diameter changes in a mouse model of aging.

Methods : Retinal whole mounts from young (6-7 months old) and aged (24-27 months old) C57Bl/6j mice were prepared and immunolabeled for vascular markers including PECAM-1/CD31 (endothelium), alpha smooth muscle actin (aSMA; vascular SMCs), caveolin-1 (Cav-1, endothelium and mural cells) and/or NG2 (mural cells). Immunoreactivity was detected by appropriate fluorophore-conjugated secondary antibodies. Whole mounts were imaged by confocal microscopy and mural cell coverage and retinal vessel diameters were assessed.

Results : Retinal arterioles from aged mice displayed significantly reduced immunoreactivity for aSMA and generalized increases in arteriolar diameter. Most dramatically, all aged mice had patchy segmental loss of aSMA immunoreactivity that was usually associated with focal increases in arteriolar diameter compared to regions with aSMA coverage. Arteriolar regions that were devoid of aSMA also lacked NG2 immunoreactivity suggesting loss of smooth muscle cells as opposed to a loss of expression of the contractile protein. Dramatic morphological changes in retinal venules were not apparent in aged retinae.

Conclusions : Our data suggest that normal aging results in reduced coverage of SMCs on retinal resistance vessels. The segmental loss of SMCs and focal increases in vessel diameters are consistent with functional studies in human subjects that indicate that normal aging results in focal irregularities in arterial contractile responsiveness. The consequences of these findings or retinal blood flow and age-related retinal pathologies remain to be determined.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.