Paper #39 - Reduced muscle degeneration and decreased fatty infiltration after rotator cuff tear in a PARP-1 knock-out mouse model

Abstract

Introduction: Disturbed muscular architecture, fatty infiltration and muscular atrophy remain irreversible in chronic rotator cuff tears (RCT) even after repair. Poly-[ADP-ribose]-polymerase 1 (PARP1), a nuclear factor involved in DNA damage repair, has shown to be a key element in the up-regulation of early muscle inflammation, atrophy and fat deposition. We therefore hypothesized that the absence of PARP1 would lead to a reduction in muscular architectural damage, early inflammation, atrophy and fatty infiltration subsequent to combined tenotomy and neurotomy in a PARP1 knock-out mouse model. Methods: PARP1 knock-out (KO group) and standard wild type C57BL/6 (WT group) mice were randomly allocated into three different time points (1, 6 and 12 weeks, total n = 72). In all mice the supraspinatus (SSP) and infraspinatus (ISP) tendons of the left shoulder were detached and the SSP muscle was denervated according to a recently established model. Macroscopic muscle weight analysis, retraction documentation using macroscopic suture, magnetic resonance imaging, immunohistochemistry gene expression analysis using real time qPCR (RTqPCR) and histology were used to assess the differences in muscle architecture, early inflammation, fatty infiltration and atrophy between knock out and wild type mice in the supraspinatus muscle. Results: The SSP did retract in both groups, however; the KO muscles and tendons retracted less than the WT muscles (2.1 ± .21 mm vs 3.4 ± .41 mm;P = .02). Further assessment of muscle architecture demonstrated that the pennation angle was significantly lower in the KO groups at 6 and 12 weeks (28 ± 5 vs 36 ± 5 and 29 ± 4 vs 34 ± 3; P < .0001). Combined Tenotomy and neurotomy resulted in a significant loss of muscle mass in both groups compared to the contralateral unoperated side (KO group 62 ± 11% and WT group 52 ± 11%, P = .04) at 6 weeks. But at 12 weeks postoperatively, there was a significant increase in muscle mass to near normal levels in KO group compared to the WT group (14 ± 6% and 42 ± 7% lower muscle mass respectively; P < .0001). Immunohistochemistry revealed a significant decrease in the expression of inflammatory, apoptotic, adipogenic and muscular atrophy genes at both the 1 week and 6 weeks time points, but not at 12 weeks in the KO group compared to the WT group. This was confirmed by histology. Discussion: Our study is the first to show that knocking out PARP1 leads to decreased loss of muscle architecture, early inflammation, fatty infiltration and atrophy after combined tenotomy and neurotomy of the rotator cuff muscle. Although KO and WT mice developed severe atrophy and fatty infiltration 6 weeks after surgery, the muscles of the KO mice regenerated to almost normal size after 12 weeks. We conclude that PARP1 has an influence on the muscular deterioration after RCT.