Denervation Induced Degradation of Motor Endplates can be Delayed Using Local Supplementation of Agrin at the Time of Injury.
Winnie Palispis, MD1, Henry Hoang, BS1, Jennifer Uong, BS1, Justin P. Chan, BA1, James Clune, MD2, Ranjan Gupta, MD1.
1University of California, Irvine, Irvine, CA, USA, 2Yale University, New Haven, CT, USA.
Purpose: Robust muscle atrophy and fibrosis are cited as the prime causes for the permanence of motor deficits following surgical intervention for a denervation injury. Following a peripheral nerve transection, regenerating axons traverse long distances to reinnervate their target organs. However, due to the slow rate of axonal regrowth and a large segmental defect, muscle atrophy and destabilization of the neuromuscular junction (NMJ) usually proceeds before reinnervation occurs. Agrin is characterized as an essential component of NMJ formation and synaptogenesis. Here, we utilize an in vivo agrin deficient mouse nerve denervation model to both characterize the morphology of agrin deficient endplates as well as the response of the NMJ to local delivery of agrin following a denervation event. We found the endplates of agrin-deficient mice in a more degraded states than the endplates of wildtype mice following denervation. We also found that local delivery of agrin following denervation serves to transiently improve motor endplate morphology. We thus demonstrate the efficacy of using a biomolecular therapeutic approach to maintain and preserve the denervated NMJ. Materials and Methods: A denervation model was created in 6-week-old mice from WT and agrin deficient C57BL/6 strains by excising 10 mm right sciatic nerve segment from the mid-thigh of the mice and suturing the proximal nerve stump to the gluteal muscle with 9-0 suture so as to prevent regeneration. Agrin deficient mice were either injected with supplemental agrin or PBS as a control at the site of injury. The downstream denervated and contralateral control soleus, plantaris and gastrocnemius muscles were harvested for immunohistochemistry, cryo-sections with H&E staining, and quantitative western blots at the 1,2,4,8 and 16-week timepoints. Quantification of motor endplate morphology was done using Volocity 3-D image software. Quantification of muscle fiber diameter was done using ImageJ software. Results: Fluorescence confocal imaging of harvested soleus muscles revealed that agrin supplemented animals retained superior motor endplate morphology over control animals in all timepoints. The average surface area of agrin supplemented endplates were significantly greater than control endplates in all analyzed timepoints (p <0.05) Conclusion: Highlighting the importance of agrin, we have shown that even a single supplemental dose of agrin delivered locally at the site of injury is effective in preserving motor endplates in denervated mice hindlimbs even at the latest time points. This is consistent with our previous work that detailed MMP3 knock down maintained agrin at the NMJ and thereby preserved the motor endplates so as to improve function recovery. Taken together, these current experiments support the strategy to preserve motor endplates so as to prolong the window of opportunity for surgical intervention after a traumatic nerve injury.
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