Elevated MMP Expression in the MRL Mouse Retina Creates a Permissive Environment for Retinal Regeneration

The MRL/MpJ (healer) mouse is an established model for autoimmune studies and was recently identified as having a profound ability to undergo scarless regeneration of the tissue in the ear and heart. This regenerative capacity has been linked to elevated matrix metalloproteinase (MMP)-2 and -9 expression, giving this mouse the ability to degrade and remove inhibitory basement membrane molecules. Although elevated MMP expression has been reported in somatic tissues in this strain, little is known about MMP expression and the response to injury in the MRL/MpJ mouse retina. The purpose of this study was to investigate whether increased MMP expression and subsequent decreased inhibitory extracellular matrix molecule deposition in the MRL/MpJ mouse retina produces a permissive regenerative environment.

Experiments were performed using 3- to 4-week-old MRL/MpJ, retinal degenerative (rd1), and C57BL/6 (wild-type) mice. Western blotting, oligo-microarray, and immunohistochemical analyses were used to determine the level and location of MMP and extracellular matrix (ECM) protein expression. Retinal responses to injury were modeled by retinal detachment in vivo and in retinal explantation in vitro. The capacity of the retinal environment to support photoreceptor cell migration, integration, or regeneration was analyzed using hematoxylin-eosin, immunohistochemical staining, and cell counting.

Compared with C57BL/6J animals, MRL/MpJ mice exhibit elevated levels of MMP-2, -9, and -14 and decreased levels of the inhibitory proteins neurocan and CD44 within the retina. Although similar increases in MMP-2, -9, and CD44s (CD44 degradation product) were observed in the rd1 retina, elevated levels of the inhibitory ECM molecules (neurocan and CD44) remained. Thus, the MRL retinal environment, which expresses lower levels of inhibitory ECM molecules after injury, was more conducive to regeneration and enhanced photoreceptor integration in vitro than C57BL/6J or rd1 controls.

The MRL mouse retina shows elevated MMP expression and decreased levels of scar-related inhibitory molecules, which leads to a retinal environment that is more permissive for neural regeneration and cell integration after in vitro retinal explantation.