An Vanden Bosch

TRPV4-Mediated Calcium Influx Regulates Terminal Differentiation of Osteoclasts

Calcium signaling controls multiple cellular functions and is regulated by the release from internal stores and entry from extracellular fluid. In bone, osteoclast differentiation is induced by RANKL (receptor activator of NF-kappaB ligand)-evoked intracellular Ca(2+) oscillations, which trigger nuclear factor-activated T cells (NFAT) c1-responsive gene transcription. However, the Ca(2+) channels involved remain largely unidentified. Here we show that genetic ablation in mice of Trpv4, a Ca(2+)-permeable channel of the transient receptor potential (TRP) family, increases bone mass by impairing bone resorption. TRPV4 mediates basolateral Ca(2+) influx specifically in large osteoclasts when Ca(2+) oscillations decline. TRPV4-mediated Ca(2+) influx hereby secures intracellular Ca(2+) concentrations, ensures NFATc1-regulated gene transcription, and regulates the terminal differentiation and activity of osteoclasts. In conclusion, our data indicate that Ca(2+) oscillations and TRPV4-mediated Ca(2+) influx are sequentially required to sustain NFATc1-dependent gene expression throughout osteoclast differentiation, and we propose TRPV4 as a therapeutic target for bone diseases.

Androgen Signaling in Myocytes Contributes to the Maintenance of Muscle Mass and Fiber Type Regulation But Not to Muscle Strength or Fatigue

Muscle frailty is considered a major cause of disability in the elderly and chronically ill. However, the exact role of androgen receptor (AR) signaling in muscle remains unclear. Therefore, a postmitotic myocyte-specific AR knockout (mARKO) mouse model was created and investigated together with a mouse model with ubiquitous AR deletion. Muscles from mARKO mice displayed a marked reduction in AR protein (60-88%). Interestingly, body weights and lean body mass were lower in mARKO vs. control mice (-8%). The weight of the highly androgen-sensitive musculus levator ani was significantly reduced (-46%), whereas the weights of other peripheral skeletal muscles were not or only slightly reduced. mARKO mice had lower intra-abdominal fat but did not demonstrate a cortical or trabecular bone phenotype, indicating that selective ablation of the AR in myocytes affected male body composition but not skeletal homeostasis. Furthermore, muscle contractile performance in mARKO mice did not differ from their controls. Myocyte-specific AR ablation resulted in a conversion of fast toward slow fibers, without affecting muscle strength or fatigue. Similar results were obtained in ubiquitous AR deletion, showing lower body weight, whereas some but not all muscle weights were reduced. The percent slow fibers was increased, but no changes in muscle strength or fatigue could be detected. Together, our findings show that myocyte AR signaling contributes to the maintenance of muscle mass and fiber type regulation but not to muscle strength or fatigue. The levator ani weight remains the most sensitive and specific marker of AR-mediated anabolic action on muscle.

NuSAP is essential for chromatin-induced spindle formation during early embryogenesis.

Mitotic spindle assembly is mediated by two processes: a centrosomal and a chromosomal pathway. RanGTP regulates the latter process by releasing microtubule-associated proteins from inhibitory complexes. NuSAP, a microtubule- and DNA-binding protein, is a target of RanGTP and promotes the formation of microtubules near chromosomes. However, the contribution of NuSAP to cell proliferation in vivo is unknown. Here, we demonstrate that the expression of NuSAP highly correlates with cell proliferation during embryogenesis and adult life, making it a reliable marker of proliferating cells. Additionally, we show that NuSAP deficiency in mice leads to early embryonic lethality. Spindle assembly in NuSAP-deficient cells is highly inefficient and chromosomes remain dispersed in the mitotic cytoplasm. As a result of sustained spindle checkpoint activity, the cells are unable to progress through mitosis, eventually leading to caspase activation and apoptotic cell death. Together, our findings demonstrate that NuSAP is essential for proliferation of embryonic cells and, simultaneously, they underscore the importance of chromatin-induced spindle assembly.