Proteasome inhibition suppress microgravity elevated RANK signaling during osteoclast differentiation

Bioinformatic analysis of the RNA expression patterns in microgravity-induced bone loss

Researchers have linked microgravity in space to the significant imbalance between bone formation and bone resorption that induces persistent bone loss in load-bearing bones. However, the underlying molecular mechanisms are still unclear, which hinders the development of therapeutic measures. The aim of this study was to identify hub genes and explore novel molecular mechanisms underlying microgravity-induced bone loss using transcriptome datasets obtained from the GEO and SRA databases. In summary, comparative RNA expression pattern studies that differ in species (Homo or Mus), models (in vitro or in vivo), microgravity conditions (real microgravity or ground-based simulators) and microgravity duration showed that it is difficult to reach a consistent conclusion about the pathogenesis of microgravity-induced bone loss across these studies. Even so, we identified 11 hub genes and some miRNA-mRNA interactions mainly based on the GSE100930 dataset. Also, the expression of CCL2, ICAM1, IGF1, miR-101-3p and miR-451a markedly changed under clinorotation-microgravity condition. Remarkedly, ICAM1 and miR-451a were key mediators of the osteogenesis of hMSCs under clinorotation-microgravity condition. These findings provide novel insights into the molecular mechanisms of bone loss during microgravity and could indicate potential targets for further countermeasures against this condition.

Microgravity-Related Changes in Bone Density and Treatment Options: A Systematic Review

Space travelers are exposed to microgravity (µg), which induces enhanced bone loss compared to the age-related bone loss on Earth. Microgravity promotes an increased bone turnover, and this obstructs space exploration. This bone loss can be slowed down by exercise on treadmills or resistive apparatus. The objective of this systematic review is to provide a current overview of the state of the art of the field of bone loss in space and possible treatment options thereof. A total of 482 unique studies were searched through PubMed and Scopus, and 37 studies met the eligibility criteria. The studies showed that, despite increased bone formation during µg, the increase in bone resorption was greater. Different types of exercise and pharmacological treatments with bisphosphonates, RANKL antibody (receptor activator of nuclear factor κβ ligand antibody), proteasome inhibitor, pan-caspase inhibitor, and interleukin-6 monoclonal antibody decrease bone resorption and promote bone formation. Additionally, recombinant irisin, cell-free fat extract, cyclic mechanical stretch-treated bone mesenchymal stem cell-derived exosomes, and strontium-containing hydroxyapatite nanoparticles also show some positive effects on bone loss.

Interaction Network Provides Clues on the Role of BCAR1 in Cellular Response to Changes in Gravity

When culturing cells in space or under altered gravity conditions on Earth to investigate the impact of gravity, their adhesion and organoid formation capabilities change. In search of a target where the alteration of gravity force could have this impact, we investigated p130cas/BCAR1 and its interactions more thoroughly, particularly as its activity is sensitive to applied forces. This protein is well characterized regarding its role in growth stimulation and adhesion processes. To better understand BCAR1′s force-dependent scaffolding of other proteins, we studied its interactions with proteins we had detected by proteome analyses of MCF-7 breast cancer and FTC-133 thyroid cancer cells, which are both sensitive to exposure to microgravity and express BCAR1. Using linked open data resources and our experiments, we collected comprehensive information to establish a semantic knowledgebase and analyzed identified proteins belonging to signaling pathways and their networks. The results show that the force-dependent phosphorylation and scaffolding of BCAR1 influence the structure, function, and degradation of intracellular proteins as well as the growth, adhesion and apoptosis of cells similarly to exposure of whole cells to altered gravity. As BCAR1 evidently plays a significant role in cell responses to gravity changes, this study reveals a clear path to future research performing phosphorylation experiments on BCAR1.

Inhibition of NFAM1 suppresses phospho-SAPK/JNK signaling during osteoclast differentiation and bone resorption

We have recently demonstrated NFAT activating protein with ITAM motif 1 (NFAM1) signaling increases osteoclast (OCL) formation/bone resorption associated with the Paget's disease of bone, however, the underlying molecular mechanisms of the NFAM1 regulation of OCL differentiation and bone resorption remains unclear. Here, we showed that RANK ligand stimulation enhances NFAM1 expression in preosteoclast cells. Conditioned media collected from RANKL stimulated RAW264.7 NFAM1 knockdown (KD) stable cells showed inhibition of interleukin-6 (2.5-fold), tumour necrosis factor-α (2.2-fold) and CXCL-5 (3-fold) levels compared to wild-type (WT) cells. Further, RANKL stimulation significantly increased p-STAT6 expression (5.5-fold) in WT cells, but no significant effect was observed in NFAM1-KD cells. However, no changes were detected in signal transducer and activator of transcription 3 levels in either of cell groups. Interestingly, NFAM1-KD suppressed the RANKL stimulated c-fos, p-c-Jun and c-Jun N-terminal kinase (JNK) activity in preosteoclasts. We further showed that the suppression of JNK activity is through inhibition of p-SAPK/JNK in these cells. In addition, NFATc1 expression, a critical transcription factor associated with osteoclastogenesis is significantly inhibited in NFAM1-KD preosteoclast cells. Interestingly, NFAM1 inhibition suppressed the OCL differentiation and bone resorption capacity in mouse bone marrow cell cultures. We also demonstrated inhibition of tartrate-resistant acid phosphatase expression in RANKL stimulated NFAM1-KD preosteoclast cells. Thus, our results suggest that NFAM1 control SAPK/JNK signaling to modulate osteoclast differentiation and bone resorption.

Implications of Altered Endosome and Lysosome Biology in Space Environments

Space exploration poses multiple challenges for mankind, not only on a technical level but also to the entire physiology of the space traveller. The human system must adapt to several environmental stressors, microgravity being one of them. Lysosomes are ubiquitous to every cell and essential for their homeostasis, playing significant roles in the regulation of autophagy, immunity, and adaptation of the organism to changes in their environment, to name a few. Dysfunction of the lysosomal system leads to age-related diseases, for example bone loss, reduced immune response or cancer. As these conditions have been shown to be accelerated following exposure to microgravity, this review elucidates the lysosomal response to real and simulated microgravity. Microgravity activates the endo-lysosomal system, with resulting impacts on bone loss, muscle atrophy and stem cell differentiation. The investigation of lysosomal adaptation to microgravity can be beneficial in the search for new biomarkers or therapeutic approaches to several disease pathologies on earth as well as the potential to mitigate pathophysiology during spaceflight.