Chrebp Deletion and Mild Protein Restriction Additively Decrease Muscle and Bone Mass and Function

<b>Background/Objectives</b>: Carbohydrate and protein restriction are associated with sarcopenia and osteopenia, but the underlying mechanisms remain unclear. We aimed to determine whether mild protein restriction affects muscle and bone function in wild-type (WT) and homozygous carbohydrate response element binding protein (<i>Chrebp</i>) knockout (KO) mice. <b>Methods</b>: Eighteen-week-old male wild-type and homozygous carbohydrate response element binding protein (<i>Chrebp</i>) knockout (KO) mice were fed a control diet (20% protein) or a low-protein diet (15% protein) for 12 weeks. We estimated the muscle weight and limb grip strength as well as the bone mineral density, bone structure, and bone morphometry. <b>Results</b>: <i>Chrebp</i> deletion and a low-protein diet additively decreased body weight (WT control-KO low-protein: mean difference with 95% CI, 8.7 [6.3, 11.0], <i>p</i> < 0.0001) and epidydimal fat weight (1.0 [0.7, 1.2], <i>p</i> < 0.0001). <i>Chrebp</i> deletion and a low-protein diet additively decreased tibialis anterior muscle weight (0.03 [0.01, 0.05], <i>p</i> = 0.002) and limb grip strength (63.9 [37.4, 90.5], <i>p</i> < 0.0001) due to a decrease in insulin/insulin-like growth factor 1 mRNA and an increase in myostatin mRNA. In contrast, <i>Chrebp</i> deletion increased bone mineral density (BMD) (WT control-KO control: -6.1 [-1.0, -2.3], <i>p</i> = 0.0009), stiffness (-21.4 [-38.8, -4.1], <i>p</i> = 0.011), cancellous bone BV/TV (-6.517 [-10.99, -2.040], <i>p</i> = 0.003), and the number of trabeculae (-1.1 [-1.8, -0.5], <i>p</i> = 0.0008). However, in KO mice, protein restriction additively decreased BMD (KO control-KO low-protein: 8.1 [4.3, 11.9], <i>p</i> < 0.0001), bone stiffness (38.0 [21.3, 54.7], <i>p</i> < 0.0001), cancellous bone BV/TV (7.7 [3.3, 12.2], <i>p</i> = 0.006), and the number of trabeculae (1.2 [0.6, 1.9], <i>p</i> = 0.0004). The effects of mild protein restriction on bone formation parameters (osteoid volume (WT control-WT low-protein: -1.7 [-2.7, -0.7], <i>p</i> = 0.001) and the osteoid surface (-11.2 [-20.8, -1.5], <i>p</i> = 0.02) were observed only in wild-type (WT) mice. The levels of bone resorption markers, such as the number of osteoclasts on the surface, the number of osteoclasts, and surface erosion, did not differ between the groups. <b>Conclusions</b>: Both <i>Chrebp</i> deletion and protein restriction led to a decrease in muscle and bone function; therefore, an adequate intake of carbohydrates and proteins is important for maintaining muscle and bone mass and function. Further studies will be needed to elucidate the mechanisms by which ChREBP deletion and a low-protein diet cause osteosarcopenia.