Purpose

This study evaluates glucose metabolism as a pre-clinical biomarker in the golden retriever muscular dystrophy (GRMD) model of Duchenne muscular dystrophy (DMD). Given the metabolic dysfunctions in DMD (e.g., reduced glycolytic and oxidative enzymes, mitochondrial abnormalities, and increased oxidative stress), this research explores molecular, biochemical, and in vivo imaging techniques to track disease progression and therapeutic response.

Methods

• Molecular and Biochemical Analyses:
  • mRNA Profiling: Expression levels of GLUT4 were analyzed in cranial sartorius (CS), vastus lateralis (VL), and long digital extensor (LDE) muscles.
  • qPCR and Western Blotting: Quantified GLUT4 and Hexokinase-1 (HK1) expression levels.
  • Immunofluorescence Microscopy: Assessed GLUT4 localization in muscle tissues.
• Physiologic Glucose Handling:
  • Fasting Glucose Tolerance Test (GTT): Measured basal glucose, insulin levels, and tissue glucose uptake.
  • PET/CT Imaging: Used 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) to visualize glucose uptake in skeletal and cardiac muscles.

Key Findings

• GLUT4 Expression and Localization:
  • GLUT4 mRNA levels were significantly decreased in CS, VL, and LDE muscles of GRMD dogs.
  • Western blotting showed no significant differences in GLUT4 protein levels in CS and VL, but a significant increase in the LDE of GRMD dogs.
  • Confocal microscopy revealed diffuse membrane localization of GLUT4 in GRMD skeletal muscles.
• Glucose Tolerance and Insulin Response:
  • GRMD dogs exhibited higher basal glucose and insulin levels compared to normal dogs.
  • Glucose tolerance tests (GTT) showed rapid glucose clearance in GRMD dogs.
• PET/CT Imaging:
  • Higher mean standard uptake values (SUV) in GRMD skeletal muscle post-[18F]FDG injection.
  • Lower mean and max SUV in GRMD cardiac muscle, indicating altered glucose metabolism.

Conclusions

• Glucose Metabolism as a Biomarker: Altered glucose metabolism in GRMD skeletal and cardiac muscles can be effectively monitored using molecular, biochemical, and imaging techniques.
• Potential for Clinical Application: Rapid glucose clearance and increased GLUT4 membrane localization in GRMD dogs suggest the potential of glucose metabolism assays as non-invasive biomarkers for tracking DMD progression and treatment efficacy.
• Future Directions: Further research is needed to understand differential glucose uptake in skeletal and cardiac muscles, and to explore additional imaging modalities.