Methodological aspects of in vivo axial loading in rodents: a systematic review.

Axial loading in rodents provides a controlled setting for mechanical loading, because load and subsequent strain, frequency, number of cycles and rest insertion between cycles, are precisely defined. These methodological aspects as well as factors, such as ovariectomy, aging, and disuse may affect the outcome of the loading test, including bone mass, structure, and bone mineral density. This review aims to overview methodological aspects and modifying factors in axial loading on bone outcomes. A systematic literature search was performed in bibliographic databases until December 2021, which resulted in 2183 articles. A total of 144 articles were selected for this review: 23 rat studies, 74 mouse studies, and 47 knock out (KO) mouse studies. Results indicated that peak load, frequency, and number of loading cycles mainly affected the outcomes of bone mass, structure, and density in both rat and mouse studies. It is crucial to consider methodological parameters and modifying factors such as age, sex-steroid deficiency, and disuse in loading protocols for the prediction of loading-related bone outcomes.

Mechanical loading modulates phosphate related genes in rat bone.

Mechanical loading determines bone mass and bone structure, which involves many biochemical signal molecules. Of these molecules, Mepe and Fgf23 are involved in bone mineralization and phosphate homeostasis. Thus, we aimed to explore whether mechanical loading of bone affects factors of phosphate homeostasis. We studied the effect of mechanical loading of bone on the expression of Fgf23, Mepe, Dmp1, Phex, Cyp27b1, and Vdr. Twelve-week old female rats received a 4-point bending load on the right tibia, whereas control rats were not loaded. RT-qPCR was performed on tibia mRNA at 4, 5, 6, 7 or 8 hours after mechanical loading for detection of Mepe, Dmp1, Fgf23, Phex, Cyp27b1, and Vdr. Immunohistochemistry was performed to visualise FGF23 protein in tibiae. Serum FGF23, phosphate and calcium levels were measured in all rats. Four-point bending resulted in a reduction of tibia Fgf23 gene expression by 64% (p = 0.002) and a reduction of serum FGF23 by 30% (p<0.001), six hours after loading. Eight hours after loading, Dmp1 and Mepe gene expression increased by 151% (p = 0.007) and 100% (p = 0.007). Mechanical loading did not change Phex, Cyp27b1, and Vdr gene expression at any time-point. We conclude that mechanical loading appears to provoke both a paracrine as well as an endocrine response in bone by modulating factors that regulate bone mineralization and phosphate homeostasis.