HuR-dependent expression of RyR2 contributes to calcium-mediated thermogenesis in brown adipocytes

Several uncoupling protein 1 (UCP1)-independent thermogenic pathways have been described in thermogenic adipose tissue, including calcium-mediated thermogenesis in beige adipocytes via sarco/endoplasmic reticulum ATPase (SERCA). We have previously shown that adipocyte-specific deletion of the RNA binding protein human antigen R (HuR) results in thermogenic dysfunction independent of UCP1 expression. RNA sequencing revealed the downregulation of several genes involved in calcium ion transport upon HuR deletion. The goal of this work was to define the HuR-dependent mechanisms of calcium driven thermogenesis in brown adipocytes. We generated (BAT)-specific HuR-deletion (BAT-HuR ^-/- ) mice and show that their body weight, glucose tolerance, brown and white adipose tissue weights, and total lipid droplet size were not significantly different compared to wild-type. Similar to our initial findings in Adipo-HuR ^-/- mice, mice with BAT-specific HuR deletion are cold intolerant following acute thermal challenge at 4°C, demonstrating specificity of acute HuR-dependent thermogenesis to BAT. We also found decreased expression of ryanodine receptor 2 (RyR2), but no changes in RyR2, SERCA1, SERCA2, or UCP1 expression, in BAT from BAT-HuR ^-/- mice. Next, we used Fluo-4 calcium indicator dye to show that genetic deletion or pharmacological inhibition of HuR blunts the increase in cytosolic calcium concentration in SVF-derived primary brown adipocytes. Moreover, we saw a similar blunting in β-adrenergic-mediated heat generation, as assessed by ERtherm AC fluorescence, in SVF-derived brown adipocytes following HuR inhibition or deletion. Mechanistically, we show that HuR directly binds and reduces the decay rate of RyR2 mRNA in brown adipocytes, and stabilization of RyR2 via S107 rescues β-adrenergic-mediated cytosolic calcium increase and heat generation in HuR deficient brown adipocytes. In conclusion, our results suggest that HuR-dependent control of RyR2 expression plays a significant role in the thermogenic function of brown adipose tissue through modulation of SR calcium cycling.