Utilization of zebrafish larvae to monitor gastric motility in diabetes mellitus: targeting GLP‐1 and GIP receptors
Authors: Hui Chung Man Jessica , Du Peng , Liu Yuen Hang Julia , Lu Zengbing , Cui Dexuan , Jiang Bin , Ngan Man Piu , Ng Shuk Han , Yang Lingqing , Khalid Aleena , Liu Luping , Rudd John A. 
Published: May 2022

Background

Diabetes mellitus (DM) is a prominent chronic disease associated with mortality and morbidity. The impact of DM is extensive, impacting with several major organs systems, including the gastrointestinal (GI) tract. For instance, the prevalence of DM in gastroparetic patients is approximately 30%. Glucagon‐like peptide‐1 (GLP‐1) and gastric inhibitory peptide (GIP) are hormones involved in glucose homeostasis but may also affect gastric motility. GLP‐1 and GIP receptors belong to class B G‐protein coupled receptors (GPCRs) and zebrafish GPCRs (zGPCRs) share structural similarities with the human GLP‐1 (hGLP‐1) and human glucagon receptors (hGCGR). Studies found that zebrafish GLP‐1 (zGLP‐1) receptors exhibit all the structural features of class B GPCRs and have ~50% homology with human GLP‐1 receptors. The aim of the studies is to establish experimental models of DM and investigate if GLP‐1 and GIP receptor agonists alter gastric function.

Methodology

Diet‐induced DM zebrafish larvae models (from 5 days post fertilization (dpf) to 7 dpf) were established by (i) overfeeding, (ii) feeding with a high‐fat diet (egg yolk), (iii) exposure to high levels of glucose (30 mmol/L) or (iv) a knockdown of the pdx1 (insulin promoter factor 1) gene mutant. A body mass index (BMI) analyses and enzyme‐linked immunosorbent assays (ELISA) for insulin levels and glucose assay were used to grade DM. As obesity is associated with development of diabetes, adipose deposits in the abdominal area was also determined by an inverted microscope. Exendin‐4 and tirzepatide (100 µM – 1 nM) were administered via microgavage. Peristalsis was captured in anaesthetised animals using light microscopy and quantified in MATLAB using imaging velocimetry and pixel differences.

Results

Except for the overfed and pdx1 mutated models, significant differences in BMI, insulin and glucose levels were observed relative to controls. The excess diet model showed high level of adipose deposits (2.4 ± 0.17 mm) and at 7 dpf an 11% increase in adipose area is seen but without DM properties (i.e. increase glucose levels and altered insulin levels). There were significant differences between DM models and control‐fed larvae in terms of mixing power, anterograde and retrograde peristalsis of several parameters including contractile intervals and velocity. Preliminary data indicated that larvae at7 dpf fed a standard diet, exendin‐4 (≥ 100 nM/larvae) and tirzepatide (≥100 µM/larvae) reduced motility in the anterior and mid‐intestine.

Conclusion

Three DM models and an obesity model using zebrafish larvae were successfully established. Advanced imaging and processing techniques revealed intricate subtypes of dysmotility in DM and obesity. Exendin‐4 and tirzepatide at high doses reduced motility in normal fish which may be consistent with their clinical side effect profile.