Figure 4. Loading induced chromatin condensation is regulated by calcium signaling.

(A) Representative [Ca²⁺]_i oscillations (red arrows) in MSCs as a function of time (bar = 100 μm). Addition of ATP (B) or application of 30s DL (C) decreased the time between peeks and increased number of peaks observed in 10 min (n = ~15, *p < 0.05 vs. CM control (0% strain/0mM ATP, mean ± s.d.). (D,E) Pretreatment with BAPTA or KN62 blocked load-induced chromatin condensation, whereas pretreatment with thapsigargin (TG, F) had no effect and Verapamil (VP, G) blocked only the short term increase in CCP (at 600s of DL) (red line: CM control, green line: 600s DL, blue line: 3 h DL, n = ~20 per condition, *p < 0.05 vs. CM control, mean ± s.e.m.). (H) Schematic illustration outlining the operative signaling pathways controlling chromatin condensation with short term (600s) or long term (3h) loading (⊗ a component that is not on the critical path for load induced chromatin condensation at that time point. ATP: Adenosine triphosphate, HMCLs: hemichannels, P2YR: P2Y purinergic receptors, P2XR: P2X purinergic receptors, ER: endoplasmic reticulum, Ca: calcium. CBPs: calcium binding proteins, VGCC: voltage-gated calcium channels, TRPV4: Transient receptor potential cation channel subfamily V member 4, PIEZO: Piezo-type mechanosensitive ion channels, HDAC: Histone deacetylase, MTF: Histone methyltransferase, LICC: load induced chromatin condensation).