cells Article Osteocytic Connexin43 Channels Regulate Bone–Muscle Crosstalk Guobin Li 1 , Lan Zhang 1 , Kaiting Ning 1 , Baoqiang Yang 1 , Francisca M. Acosta 2 , Peng Shang 3 , Jean X. Jiang 2 and Huiyun Xu 1, *   Citation: Li, G.; Zhang, L.; Ning, K.; Yang, B.; Acosta, F.M.; Shang, P.; Jiang, J.X.; Xu, H. Osteocytic Connexin43 Channels Regulate Bone–Muscle Crosstalk. Cells 2021, 10, 237. https://doi.org/10.3390/ cells10020237 Academic Editor: Gabriel Mbalaviele Received: 23 December 2020 Accepted: 23 January 2021 Published: 26 January 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Key Laboratory for Space Bioscience and Biotechnology, School of Life Sciences, Northwestern Polytechnical University, Xi’an 710072, Shaanxi, China; guobinl@mail.nwpu.edu.cn (G.L.); zhanglan@mail.nwpu.edu.cn (L.Z.); ningkaiting@mail.nwpu.edu.cn (K.N.); yangbq@mail.nwpu.edu.cn (B.Y.) 2 Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX 78229, USA; acostafm@uthscsa.edu (F.M.A.); jiangj@uthscsa.edu (J.X.J.) 3 Key Laboratory for Space Bioscience and Biotechnology, Research & Development Institute in Shenzhen, Northwestern Polytechnical University, Shenzhen 518057, Guangdong, China; shangpeng@nwpu.edu.cn * Correspondence: celldon@nwpu.edu.cn Abstract: Bone–muscle crosstalk plays an important role in skeletal biomechanical function, the progression of numerous pathological conditions, and the modulation of local and distant cellular environments. Previous work has revealed that the deletion of connexin (Cx) 43 in osteoblasts, and consequently, osteocytes, indirectly compromises skeletal muscle formation and function. However, the respective roles of Cx43-formed gap junction channels (GJs) and hemichannels (HCs) in the bone–muscle crosstalk are poorly understood. To this end, we used two Cx43 osteocyte-specific transgenic mouse models expressing dominant negative mutants, Δ130–136 (GJs and HCs functions are inhibited), and R76W (only GJs function is blocked), to determine the effect of these two types of Cx43 channels on neighboring skeletal muscle. Blockage of osteocyte Cx43 GJs and HCs in Δ130–136 mice decreased fast-twitch muscle mass with reduced muscle protein synthesis and increased muscle protein degradation. Both R76W and Δ130–136 mice exhibited decreased muscle contractile force accompanied by a fast-to-slow fiber transition in typically fast-twitch muscles. In vitro results further showed that myotube formation of C2C12 myoblasts was inhibited after treatment with the primary osteocyte conditioned media (PO CM) from R76W and Δ130–136 mice. Additionally, prostaglandin E2 (PGE2) level was significantly reduced in both the circulation and PO CM of the transgenic mice. Interestingly, the injection of PGE2 to the transgenic mice rescued fast-twitch muscle mass and function; however, this had little effect on protein synthesis and degradation. These findings indicate a channel-specific response: inhibition of osteocytic Cx43 HCs decreases fast-twitch skeletal muscle mass alongside reduced protein synthesis and increased protein degradation. In contrast, blockage of Cx43 GJs results in decreased fast-twitch skeletal muscle contractile force and myogenesis, with PGE2 partially accounting for the measured differences. Keywords: osteocytes; Cx43; gap junctions; hemichannels; bone–muscle crosstalk 1. Introduction There is an intimate relationship between bone and skeletal muscle from development, through growth, and into aging [1,2]. While such a relationship has long been considered to be primarily mechanical in nature, recently, it has been proposed that bone and muscle biochemically communicate through the actions of secreted factors [3]. On the one hand, many muscle-derived factors (termed “myokines”) such as insulin-like growth factor- 1 (IGF-1), fibroblast growth factor-2 (FGF-2) [4], myostatin [5], β-aminoisobutyric acid (BAIBA) [6], and irisin [7] affect bone metabolism. On the other hand, bone cells are known to produce factors, for example, osteocalcin [8], prostaglandin E2 (PGE2) [9], Wnt3a [10], and sclerostin [11], which also influence the function of skeletal muscle. These findings Cells 2021, 10, 237. https://doi.org/10.3390/cells10020237 https://www.mdpi.com/journal/cells