Myostatin is a member of the bone morphogenetic protein/transforming growth factor-β (BMP/TGFβ) super-family of secreted differentiation factors. Myostatin is a negative regulator of muscle mass as shown by increased muscle mass in myostatin deficient mice. Interestingly, these mice also exhibit increased bone mass suggesting that myostatin may also play a role in regulating bone mass. To investigate the role of myostatin in bone, young adult mice were administered with either a myostatin neutralizing antibody (Mstn-mAb), a soluble myostatin decoy receptor (ActRIIB-Fc) or vehicle. While both myostatin inhibitors increased muscle mass, only ActRIIB-Fc increased bone mass. Bone volume fraction (BV/TV), as determined by microCT, was increased by 132% and 27% in the distal femur and lumbar vertebrae, respectively. Histological evaluation demonstrated that increased BV/TV in both locations was attributed to increased trabecular thickness, trabecular number and bone formation rate. Increased BV/TV resulted in enhanced vertebral maximum compressive force compared to untreated animals. The fact that ActRIIB-Fc, but not Mstn-mAb, increased bone volume suggested that this soluble decoy receptor may be binding a ligand other than myostatin, that plays a role in regulating bone mass. This was confirmed by the significant increase in BV/TV in myostatin deficient mice treated with ActRIIB-Fc. Of the other known ActRIIB-Fc ligands, BMP3 has been identified as a negative regulator of bone mass. However, BMP3 deficient mice treated with ActRIIB-Fc showed similar increases in BV/TV as wild type (WT) littermates treated with ActRIIB-Fc. This result suggests that BMP3 neutralization is not the mechanism responsible for increased bone mass. The results of this study demonstrate that ActRIIB-Fc increases both muscle and bone mass in mice. Therefore, a therapeutic that has this dual activity represents a potential approach for the treatment of frailty.

BackgroundThe aggregation of amyloid-β (Aβ) into insoluble plaques is a hallmark pathology of Alzheimer’s disease (AD). Previous work has shown increasing serotonin levels with selective serotonin re-uptake inhibitor (SSRI) compounds reduces Aβ in the brain interstitial fluid (ISF) in a mouse model of AD and in the cerebrospinal fluid of humans. We investigated which serotonin receptor (5-HTR) subtypes and downstream effectors were responsible for this reduction.ResultsAgonists of 5-HT4R, 5-HT6R, and 5-HT7R significantly reduced ISF Aβ, but agonists of other receptor subtypes did not. Additionally, inhibition of Protein Kinase A (PKA) blocked the effects of citalopram, an SSRI, on ISF Aβ levels. Serotonin signaling does not appear to change gene expression to reduce Aβ levels in acute timeframes, but likely acts within the cytoplasm to increase α-secretase enzymatic activity. Broad pharmacological inhibition of putative α-secretases increased ISF Aβ and blocked the effects of citalopram.ConclusionsIn total, these studies map the major signaling components linking serotonin receptors to suppression of brain ISF Aβ. These results suggest the reduction in ISF Aβ is mediated by a select group of 5-HTRs and open future avenues for targeted therapy of AD.Electronic supplementary materialThe online version of this article (doi:10.1186/s13024-016-0112-5) contains supplementary material, which is available to authorized users.

Accumulation of β-amyloid peptide is a key step in Alzheimer’s disease pathogenesis. Yuede et al. propose a novel method to track β-amyloid levels in vivo.

Rationale:Several neurotransmitter receptors activate signaling pathways that alter processing of the amyloid precursor protein (APP) into amyloid-β (Aβ). Serotonin signaling through a subset of serotonin receptors suppresses Aβ generation. We proposed that escitalopram, the most specific selective serotonin reuptake inhibitor (SSRI) that inhibits the serotonin transporter , SERT, would suppress Aβ levels in mice.Objectives:We hypothesized that acute treatment with ESC would reduce Aβ generation which would be reflected chronically with a significant reduction in Aβ plaque load.Methods:We performed in vivo microdialysis and in vivo two-photon imaging to assess changes in brain interstitial fluid (ISF) Aβ and Aβ plaque size over time, respectively, in the APP/PS1 mouse model of Alzheimer’s disease treated with vehicle or ESC. We also chronically treated mice with ESC to determine the effect on plaques histologically.Results:ESC acutely reduced ISF Aβ by 25% by increasing α-secretase cleavage of APP. Chronic administration of ESC significantly reduced plaque load by 28% and 34% at 2.5 mg/day and 5 mg/day, respectively. ESC at 5mg/kg did not remove existing plaques, but completely arrested individual plaque growth over time.Conclusions:ESC significantly reduced Aβ in mice similar to previous findings in humans treated with acute dosing of an SSRI.