Endogenous molecular MRI of creatine as a mediator of adrenergic activation of brown adipose tissue - ABSTRACT More than 2 billion people are projected to be overweight and 1 billion to be obese by 2030. In addition to its specific co-morbidities, obesity is a major risk factor for metabolic diseases and cancer. Emerging approaches utilizing brown adipose tissue (BAT) activation and thermogenesis have the potential to be safe, effective, sustainable, and affordable for treating obesity-related metabolic diseases. However, currently imaging methods for BAT metabolic function have limitations. Recent studies found, through the futile creatine cycling, creatine promotes adrenergic activation of BAT and thermogenesis in a manner that is independent of mitochondrial uncoupling protein 1 (UCP1). We have made breakthroughs in extracting the molecular Chemical exchange saturation transfer (CEST) contrasts from high-resolution Z-spectral imaging of adipose tissues, particularly the creatine CEST (CrCEST) as an endogenous and clinically translatable MRI. Our preliminary animal studies demonstrated that dynamic CrCEST MRI detected significant BAT adrenergic activation in response to a β3-AR agonist CL-316243 (CL). Our central hypothesis is that endogenous CrCEST MRI measures metabolic function of adipose tissues through adipocyte creatine, the mediator of adrenergic activation. Aim 1. To investigate the molecular origin of CrCEST MRI for BAT metabolic function, including characterizing the percentage contribution of creatine to CrCEST and the longitudinal repeatability with and without respiratory gating. Aim 2. To validate CrCEST MRI for the assessment of BAT metabolism in genetically modified mice. We will develop fast dynamic CrCEST MRI based on key-hole technique. We will assess the metabolic balance of UCP1-KO mice through futile creatine cycling. Mice will receive daily saline, Cr supplement, or β-GPA (a Cr kinase inhibitor) for 2 weeks, while dynamic CrCEST MRI and 18F-FDG PET/CT will be performed with biopathological measurements in the end for validation. We expect, Cr supplement will boost the adrenergic activation of BAT in UCP1-KO mice while β-GPA blunt the adrenergic activation. Similarly, we will study adipose-specific Cr transporter knockout (CrTKO) mice and expect that CrTKO impairs adrenergic activation in BAT and disable the effect of Cr supplement. Aim 3. To evaluate longitudinal Z-spectral MRI for monitoring BAT activation and WAT beiging during the prevention or treatment of obesity. In the prevention study, young mice on high-fat diet (HFD) will receive saline, Cr supplement, mirabegron (a β3-AR agonist), or the combination in the diet for 8 weeks and be monitored every two weeks with Z-spectral MRI producing CrCEST and amide proton transfer effect. In the treatment study, obese mice will be treated similarly and monitored for 8 weeks followed with biopathological measurements. The successful development and validation of CrCEST MRI in adipose tissues will provide 1) real-time non-invasive functional imaging of adipose tissues; 2) a better understanding of creatine in BAT adrenergic activation and white-fat browning, and its potential for preventing and treating metabolic diseases.
