Shiver me titin! Elucidating titin's role in shivering thermogenesis

Abstract

Shivering frequency scales predictably with body mass and is 10 times higher in a mouse than a moose. The link between shivering frequency and body mass may lie in the tuning of muscle elastic properties. Titin functions as a muscle 'spring', so shivering frequency may be linked to titin's structure. The muscular dystrophy with myositis (mdm) mouse is characterized by a deletion in titin's N2A region. Mice that are homozygous for the mdm mutation have a lower body mass, stiffer gait and reduced lifespan compared with their wild-type and heterozygous siblings. We characterized thermoregulation in these mice by measuring metabolic rate and tremor frequency during shivering. Mutants were heterothermic at ambient temperatures of 20-37°C while wild-type and heterozygous mice were homeothermic. Metabolic rate increased at smaller temperature differentials (i.e. the difference between body and ambient temperatures) in mutants than in nonmutants. The difference between observed tremor frequencies and shivering frequencies predicted by body mass was significantly larger for mutant mice than for wild-type or heterozygous mice, even after accounting for differences in body temperature. Together, the heterothermy in mutants, the increase in metabolic rate at low temperature differentials and the decreased tremor frequency demonstrate the thermoregulatory challenges faced by mice with the mdm mutation. Oscillatory frequency is proportional to the square root of stiffness, and we observed that mutants had lower active muscle stiffness in vitro. The lower tremor frequencies in mutants are consistent with reduced active muscle stiffness and suggest that titin affects the tuning of shivering frequency.

Titin force enhancement following active stretch of skinned skeletal muscle fibres

“…corroborates with myofibrils and studies across variable muscle preparations showing that mdm mutant skeletal muscles generate less contractile force than controls (Huebsch et al, 2005;Lopez et al, 2008;Monroy et al, 2012;Taylor-Burt et al, 2015). However, skinned mutant psoas fibres show only a 30% reduction in the average contractile stress compared with control fibres.…”

“…However, no increase in the stiffness of titin was observed in mutant sarcomeres, suggesting that titin-based force regulatory mechanisms are affected by the mdm mutation . Investigations of whole mdm soleus have also demonstrated active compliance of mutant soleus muscles in comparison to wild-type controls (Monroy et al, 2016;Taylor-Burt et al, 2015), further supporting the idea that titinbased force regulation is affected by the mdm mutation. From this work, we speculate that force regulatory sites are deleted from mdm titin.…”

“…Because of the large size of the titin protein, gel analyses are not sensitive enough to detect any change in size of mdm titin (Garvey et al, 2002), which may be missing more amino acids than predicted owing to aberrant post-translational processing (Lopez et al, 2008;Buck et al, 2014). Mdm mice are smaller than wild-type mice, exhibit an abnormal gait, have disrupted thermoregulation and die prematurely (Garvey et al, 2002;Huebsch et al, 2005;Monroy et al, 2016;Taylor-Burt et al, 2015). Mdm mutant muscles show muscle and fibre atrophy (Huebsch et al, 2005;Lopez et al, 2008;Monroy et al, 2016;Taylor-Burt et al, 2015), increased satellite cell populations (Heimann et al, 1996), increased occurrence of centralized nuclei (Heimann et al, 1996;Lopez et al, 2008) and large connective tissue deposits (Lopez et al, 2008).…”

“…Mdm mice are smaller than wild-type mice, exhibit an abnormal gait, have disrupted thermoregulation and die prematurely (Garvey et al, 2002;Huebsch et al, 2005;Monroy et al, 2016;Taylor-Burt et al, 2015). Mdm mutant muscles show muscle and fibre atrophy (Huebsch et al, 2005;Lopez et al, 2008;Monroy et al, 2016;Taylor-Burt et al, 2015), increased satellite cell populations (Heimann et al, 1996), increased occurrence of centralized nuclei (Heimann et al, 1996;Lopez et al, 2008) and large connective tissue deposits (Lopez et al, 2008). At the myofibril level, mutant sarcomeres are indistinguishable from healthy controls Witt et al, 2004), suggesting that the mdm mutation does not affect the structure of the sarcomeres.…”

“…At the myofibril level, mutant sarcomeres are indistinguishable from healthy controls Witt et al, 2004), suggesting that the mdm mutation does not affect the structure of the sarcomeres. Nevertheless, mechanical studies show decreased contractile force and increased passive force in mdm muscles (Huebsch et al, 2005;Lopez et al, 2008;Monroy et al, 2016;Taylor-Burt et al, 2015), suggesting that the deletion in titin and subsequent loss of titin force enhancement may influence the contractile mechanics of mdm muscles.…”