Reinnervation of Motor Endplates and Increased Muscle Fiber Size after Human Insulin-Like Growth Factor I Gene Transfer into the Paralyzed Larynx

Abstract

Current surgical strategies for the treatment of laryngeal paralysis are limited by the muscle atrophy associated with denervation. Moreover, attempts at reinnervation have not effected significant change in surgical outcome. To address this clinical problem, we have developed a rat laryngeal paralysis model to study novel gene transfer strategies. Using this model, the human insulin-like growth factor I (hIGF-I) gene was introduced into paralyzed rat laryngeal muscle to assess the benefit of sustained local hIGF-I production. A muscle-specific nonviral vector containing the α-actin promoter and hIGF-I gene was used in formulation with a polyvinyl-based delivery system and injected into paralyzed adult rat laryngeal muscle. Twenty-eight days after a single injection, gene transfer efficiency, muscle fiber size, motor endplate length, and nerve-to-motor endplate contact were evaluated. Gene transfer was detected in 100% of injected animals by PCR. Gene transfer with expression, as measured by RT-PCR for hIGF-I mRNA, occurred in 81.3% of injected animals. When compared with controls, hIGF-I-transfected animals presented a significant increase in muscle fiber diameter [17.56 (±0.97 SD) μm versus 14.70 (±1.43 SD) μm; p = 0.0002], a significant decrease in motor endplate length [20.88 (±1.42 SD) μm versus 25.41 (±3.19 SD) μm; p = 0.0025], and a significant increase in percentage of endplates with nerve contact (20.3% (±13.9 SD) versus 4.4% (±4.2 SD); p = 0.0079). In the context of laryngeal paralysis, gene therapy represents a tremendous opportunity to augment current surgical treatment modalities by preventing or reversing muscle atrophy, and by enhancing nerve sprouting and reinnervation. A muscle-specific vector containing the hIGF-I gene was used in formulation with a polyvinyl-based delivery system and injected into paralyzed adult rat laryngeal muscle. Twenty-eight days after a single injection, gene transfer efficiency, muscle fiber size, and motor endplate morphology were evaluated. Gene transfer with expression occurred in 81.3% of injected animals. When compared with controls, significant changes in muscle fiber size, motor endplate length, and percentage of endplates with nerve contact were observed. In the context of laryngeal paralysis, gene therapy represents an opportunity to augment current surgical treatment modalities by preventing or reversing muscle atrophy, and by enhancing nerve sprouting and reinnervation.