Muscle and bone are two tightly connected systems on both an anatomic and functional level. Bone and muscle diseases like osteoporosis and sarcopenia have been found to show an association with each other. These two organs form a complex musculoskeletal system and have been found to secrete hormone-like factors called myokines and osteokines that can influence and affect each other. Indeed, the crosstalk between bone and muscle plays an important role during development and aging. For example, myostatin, also known as growth differentiation factor 8 (GDF-8), a cytokine secreted by muscle cells, is a negative regulator of muscle and bone mass. Over expression or loss of function mutations of myostatin in mice have led to muscle atrophy and hypertrophy respectively. Interleukin-6 (IL-6) is expressed abundantly in muscle and is released during exercise and muscle contraction. It has been shown to increase osteoclast (bone cells that break down bone) formation. In the bone, osteocytes make up the majority of all cells and are terminally differentiated osteoblasts. Osteocytes control the balance between bone resorption by osteoclasts and bone formation by osteoblasts. Osteocytes are also known to express receptors for various hormones, including parathyroid hormone (PTH) receptor. As osteocytes comprise more than 90% of all bone cells in adult bone, we hypothesize that osteocytes might secrete factors capable of controlling muscle cells and that PTH might control the expression of these factors. To test this hypothesis, we used an osteocytic cell line Ocy454-12H as well as C2C12 cells, which are a well-accepted model of myocyte differentiation. To investigate the effects of osteocyte-derived factors on myocytes, C2C12 cells were treated with conditioned medium (CM) from osteocytes during specific times. We found that during C2C12 proliferation, when compared to the αMEM control, mRNA expression of MSS51 was decreased for both cells that were treated with CM of osteocytes treated with PTH (PTH CM, p=0.00570) and cells that were treated with CM of osteocytes treated with vehicle only (CM control, p=0.0178). During C2C12 differentiation, mRNA expression of myostatin was significantly (p=0.0387) decreased in cells that were treated with PTH CM compared to cells that were treated with CM control. Considering the importance of mitochondrial respiration in cells, we next analyzed oxygen consumption and metabolism in C2C12 myocytes treated with CM from Ocy454-12H using a Seahorse XF Cell Mito Stress Test. Metabolic analysis revealed that during proliferation, PTH CM led to higher basal respiration, ATP production, and coupling efficiency in C2C12 cells while lowering spare respiratory capacity. In differentiation, there was a trend in which CM control would cause a decrease across all parameters compared to the control group and the PTH CM group. Interestingly, PTH CM-treated C2C12 cells were shown to have a higher oxygen consumption rate (OCR) than the CM-control treated group and would have similar values to that of the control group (C2C12 not treated with CM). Taken together these results suggest that osteocytes might control muscle cells differentiation and metabolism via a PTH-mediated signaling pathway.
| Identifer | oai:union.ndltd.org:bu.edu/oai:open.bu.edu:2144/43852 |
| Date | 09 February 2022 |
| Creators | Huang, Tim |
| Contributors | Divieti Pajevic, Paola, Deeney, Jude T. |
| Source Sets | Boston University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
| Rights | Attribution 4.0 International, http://creativecommons.org/licenses/by/4.0/ |
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