Investigation of heat production deficiencies during cold stress and a nonshivering thermogenesis challenge in mice with a deletion in N2A titin
Authors: Miyano Carissa Ashley , Buck Charles Loren , Nishikawa Kiisa 
Published: June 2021

Homeothermic animals strive to maintain core body temperature across varying ambient temperatures. Studies have demonstrated that mice with the muscular dystrophy with myositis (mdm) mutation shiver at a lower than expected frequency, are heterothermic, and have reduced active muscle stiffness in vivo compared to wildtype mice. Mdm mice are characterized by a deletion in the N2A region of titin that results in a lower body mass, stiffer gait, and reduced lifespan. The underlying defect that leads to a lower shivering frequency is not known but could be caused by muscle properties. The titin protein is known to behave like a spring and contribute to passive muscle stiffness. Recent studies have shown that titin contributes to active muscle stiffness as well. Shivering thermogenesis can be modeled as simple harmonic motion, where the spring constant k represents muscle stiffness. This impairment in heat production could be due to the N2A deletion in the titin protein, leading to a more compliant spring and a lower shivering frequency as explained by simple harmonic motion. An alternative hypothesis is that the ability of mdm mice to use nonshivering thermogenesis and/or uncoupling protein 1 (UCP1), found in brown adipose tissue (BAT) to generate heat during cold stress is impaired. The purpose of this study was to evaluate potential causes for the inability of mdm mice to maintain homeothermy during cold stress. We hypothesize that the inability of mdm mice to thermoregulate properly is due primarily to an impairment in heat production (e.g. shivering thermogenesis and nonshivering thermogenesis). Mice underwent a 4‐day cold stress experiment during which they lived in a mouse cage that used a continuous system for indirect calorimetry. The cages were placed in an incubator so that ambient temperature could be controlled. During each experiment, temperature was controlled to either 34°C, 29°C, 25°C, 19°C. Upon conclusion of the cold stress experiment, a nonshivering thermogenesis challenge was employed by administering 1.5 mg/kg of norepinephrine. As expected, wildtype mice increased their metabolic rate during cold stress and during the nonshivering thermogenesis challenge. RQ values reached 0.7, representing pure fat oxidation and utilization of uncoupling protein 1, present in BAT. Core body temperature for an mdm mouse decreased with decreasing ambient temperature within a range of 28C to 37C. During the nonshivering thermogenesis challenge, both wildtype and mdm mice increased their oxygen consumption. Both mdm and wildtype mice increased their oxygen consumption by 37% during the nonshivering thermogenesis challenge. These results suggest that the failure to maintain homeothermy is due to reduced shivering frequency and not a reduced capacity for nonshivering thermogenesis, pending additional experiments.Support or Funding InformationOur funding sources include NSF IOS‐0742483, NSF IOS‐1025806, NSF IOS‐1456868, and the W.M. Keck Foundation.