Author(s): CHERUBINI, J., MIKHAIL, A.I., BADOUR, M.I., STONE, J.C., MCALPINE, C.S., LJUBICIC, V.L., MACDONALD, M.J., Institution: MCMASTER UNIVERSITY, Country: CANADA, Abstract-ID: 2515

INTRODUCTION:
Vascular endothelial cells (ECs) influence hematopoiesis and atherogenic adaptations in disease. The extent to which ECs may influence hematopoiesis during exercise has not been investigated. Here, we show that ECs within the marrow may respond to exercise-induced increases in hemodynamic forces and may influence neighboring hematopoietic cells.
METHODS:
We initiated our experimentation by examining if bone marrow ECs experience increases in shear stress during exercise. Using a unilateral limb model, shear stress was modulated within the bone marrow vasculature using surgical femoral artery ligation in mice which partially abrogated blood flow into the tibia. Shear stress sensors were measured at the transcriptional level using qPCR. For human translation, we enumerated leukocyte egress into circulation within in a convenience sample of one asplenic and one splenic human during maximal exercise. Endothelial paracrine signalling was examined using cultured ECs that were exposed to shear stress or static conditions in vitro, wherein primary mouse bone marrow cells were subsequently exposed to the conditioned media. In addition, we examined mice, and accelerated hematopoiesis via sleep disruption induced by whole body genetic deletion of hypocretin (Hcrt－/－). Hcrt－/－ mice were kept sedentary, or allowed voluntary exercise for 18 weeks, at which point marrow shear sensing was modulated using femoral artery ligation for three weeks thereafter. Bone marrow hematopoietic cell abundances were enumerated using flow cytometry.
RESULTS:
We found that bone marrow shear sensing transcripts were upregulated in the contralateral unligated compared to the ipsilateral ligated limb after treadmill exercise to exhaustion (mRNA transcripts Notch1, Dll4, Jag1, Klf4; all P < 0.05). That CD31+ bone marrow ECs increased in the ligated (2142 ± 161.9 ECs), compared to unligated (629 ± 771 ECs), limb after exercise indicated further endothelial responsivity. The magnitude of leukocyte egress into systemic circulation was similar between one asplenic and one splenic human after maximal exercise. The marrow, and not other hematopoietic organs such as the spleen, may thus sense shear during exercise. We next examined paracrine signalling. Trends towards increased angiogenic (mRNA transcripts Hif1⍺ and Vegf; P < 0.05) and decreased inflammatory transcriptional programming (mRNA transcript Tnf⍺; P = 0.052) were noted among primary bone marrow cells that were exposed to shear-conditioned EC media. In sleep-disrupted Hcrt－/－ mice, femoral artery ligation abrogated exercise-induced decreases in hematopoietic progenitor cell (MPP2) abundances in the ipsilateral flow-restricted leg compared to the contralateral leg.
CONCLUSION:
Together, these data indicate that modulating the bone marrow hemodynamic environment may alter ECs shear stress sensing during exercise and may influence hematopoiesis in a paracrine manner. This work was funded by an NSERC Discovery Grant awarded to MJM: 20011033.