Blocking exosome release from atrophic skeletal muscle to strengthen bone: a novel therapeutic strategy for muscle atrophy-associated bone loss?

Skeletal muscle (SM) atrophy induced by mechanical unloading or related to aging (sarcopenia) is always accompanied by substantial bone loss, which is associated with increased risk of fragility fracture, leading to serious health consequences to elder people and individuals with physical inactivity. However, medical countermeasure for combatting the muscle atrophy (MA)-associated bone loss is still lacking because of our incomplete mechanistic understanding.

Noteworthily, MA develops along with reduced contraction and the altered production of myokines that could influence both SM and bone metabolism, and importantly, ahead of apparent bone loss. Thus, it advocates the indispensable role of SM-driven regulation in MA-associated bone loss. Nevertheless, clinical studies showed that various exercise regimens, albeit partly preserved muscle strength, were insufficient to mitigate MA-associated bone loss. Additionally, preclinical studies showed that inhibition of myokines with detrimental effect on both SM and bone only abrogated MAwithout interrupting bone loss. All these findings suggest that other SM-derived factors beyond mechanical force and myokine may also contribute to MA-associated bone loss.

Mounting evidence indicates that SM also regulates other organs by muscle miRNAs (myomiR) encapsulated within their exosomes (SMExo). To determine the role of SMExo and myomiR in muscle-bone crosstalk, we generated a transgenic mouse line expressing the GFP-tagged CD63 (a general exosome marker) specifically in SM (SMCD63-GFP) for in vivo tracking of SMExo and myomiR under conditions of hindlimb unloading (HU) by tail suspension and natural aging, respectively. We found that MA developed along with the aberrantly elevated levels of sera myomiRs in SMExo, which preceded the apparent bone loss in HU mice and were associated with substantial bone lossin aged mice, respectively. These sera myomiRs were consistently upregulated in both bedridden patients and elder individuals. Impressively, SMExos were detected intraosseously in both HU and aged mice. Moreover, either genetic blockade of the Rab27amediated SMExo release (SMRab27aKO) or knockdown of myomiRs in SM (SMmiRKD) blunted the upregulation of sera myomiRs related to HU or aging and mitigated the MA-associated bone loss as well. From mechanistic aspect, myomiRs in SMExos from HU and aged mice not only restrained osteogenic activity and enhanced osteoclastogenic activity in vitro, but also stimulated bone loss in grounded mice. We have further developed a novel SM-targeted exosome inhibitor (SMExoInhib), which exhibited inhibitory potency on SMExo release and posed beneficial effect on retarding bone loss in HU mice and aged mice, respectively. Taken together, we hypothesized that blocking SMExo release from atrophic SM for interrupting muscle-bone crosstalk could be an effective therapeutic strategy for MA-associated bone loss.

We have the following specific aims for testing our hypothesis:
Aim 1: To test if replenishing myomiR-encapsulating SMExos from HU mice could exacerbate the MA-associated bone loss in young SMRab27aKO mice with HU.
Aim 2: To examine if replenishing myomiR-encapsulating SMExos from aged mice could exacerbate the MA-associated bone loss in aged SMRab27aKO mice.
Aim 3: To evaluate the therapeutic effect of SMExoInhib on retarding the MA-associated bone loss in young C57BL/6 mice with HU and aged C57BL/6 mice, respectively.

This study will provide new mechanistic insight into muscle-bone crosstalk. It will also facilitate developing new therapeutic adjuncts for counteracting the MA-associated bone loss in either sarcopenic elders or individuals with physical inactivity.