[Effects of high-fat diet on the morphology and function of the thyroid gland and glycolipid metabolism in thyroid clock gene Bmal1 knockout mice]

Objective: To investigate the changes in thyroid morphology and function in mice with thyroid-specific knockout of the clock gene Bmal1 under high-fat diet (HFD), and to examine its effects on glycolipid metabolism in mice. Methods: Construct a mouse model with specific knockout of the Bmal1 gene in the thyroid (T-Bmal1^-/-) (knockout group, n=10), and use Bmal1^flox/flox mice without thyroid peroxidase-cyclization recombination enzyme (T-Bmal1^+/+) as the control group (non-knockout group, n=10). The mice were fed until 6 weeks (body weight 20-23 g), and then use the random number table method to evenly divide each group of mice into two subgroups. They were then fed either with normal diet (ND) or (HFD), resulting in four final groups: ND non-knockout group, HFD non-knockout group, ND knockout group, and HFD knockout group, with 5 mice in each group. From the 6th week onwards, the body weights of the mice were measured for 10 consecutive weeks. On the first day of the 14th week, the intraperitoneal glucose tolerance test (IPGTT) was conducted, and on the first day of the 15th week, the intraperitoneal insulin tolerance test (IPITT) was performed. On the first day of the 16th week after the mice were born, blood samples (0.2 ml) were taken from the eyes under anesthesia at 7∶00, 13:00, 19:00, and 1∶00 the next day, and the thyroid, liver, kidneys, spleen, inguinal white adipose tissue, and scapular brown adipose tissue were rapidly removed. Thyroid tissue morphology, thyroid function indexes, thyroid clock genes and thyroid hormone synthesis and secretion related genes expression, body weight, IPGTT and IPITT area under the curve (AUC) and lipid levels were detected and compared among all groups. Results: There was no significant difference in the number of thyroid follicles, the area of thyroid follicles and the height of thyroid follicle cavity between ND knockout group and ND non-knockout group (all P>0.05), but the height of thyroid follicle cavity in HFD knockout group was lower than that in HFD knockout group [(25.8±1.6) vs (54.4±9.5) μm, P=0.002]. The level of serum T4 in knockout group after ND or HFD feeding was higher than that in non-knockout group (all P<0.05). There was no significant difference in the mRNA expression level of sodium iodine transporter between ND knockout group and ND non-knockout group (P=0.550), but the mRNA expression level of sodium iodine transporter in HFD knockout group was higher than that in HFD non-knockout group [0.67±0.27 vs 0.20±0.09, P=0.006].There was no significant difference in weight gain and groin white fat weight between ND knockout group and ND non-knockout group (both P<0.05), but weight gain and groin white fat tissue weight in HFD knockout group were lower than those in HFD knockout group (both P<0.05). The IPGTT results showed no statistically significant difference in AUC between the ND knockout and ND non-knockout groups (P=0.226), but the HFD knockout group had a smaller AUC than the HFD non-knockout group (P=0.008). The IPITT revealed that the AUC of the ND knockout group was smaller than that of the ND non-knockout group (P=0.047). Lipid profiles were similar between the ND knockout and ND non-knockout groups (all P>0.05), but the HFD knockout group had lower levels of total cholesterol and high-density lipoprotein cholesterol than the HFD non-knockout group (both P<0.05). Conclusion: Thyroid-specific knockout of the clock gene Bmal1 can affect thyroid hormone levels and glycolipid metabolism in mice, with more pronounced effects after HFD feeding.