Leukocyte HMGB1 Guides Neoangiogenesis in Regenerating Skeletal Muscle

Campana, Lara (2012). Leukocyte HMGB1 Guides Neoangiogenesis in Regenerating Skeletal Muscle. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f0e2

Abstract

HMGB1 is ubiquitously present in the cell nucleus and highly conserved among species. It is released by necrotic cells and actively secreted by immune cells in the extracellular environment, where it behaves as a DAMP (Damage-Associate Molecular Pattern). Up to now all studies addressing the role of HMGB1 in wound healing have been carried out either in vitro or using pharmacological inhibitors. To assess the relative contribution of leukocyte HMGB1 and of passively released HMGB1 we have set up an ad hoc animal model. In this system, haematopoietic cells selectively lack HMGB1 by transplanting Hmgb1^+/+ or Hmgb1^-/- foetal livers into WT recipients. The role of leukocyte HMGB1 in tissue repair has been analyzed after skeletal muscle damage. I have used histological and magnetic resonance imaging (MRI) to non-invasively evaluate muscle inflammation and tissue architecture. In the absence of Hmgbl, there is a lower number of regenerating fibres at day7 after injury and an aberrant repair with smaller fibres at day 15. In vitro no differential effects of Hmgb1^-/- and Hmgb1^+/+ macrophages are detected on satellite cells differentiation/fusion, suggesting that events taking place in vivo are independent of the direct macrophage/satellite cell cross talk. No obvious differences are observed in the extent of leukocyte infiltration, as assessed both by cytofluorimetric and MRI analysis, suggesting that leukocyte Hmgb1 is not required for the attraction of inflammatory cells. qPCR unveils a role for leukocyte Hmgb1 in the production of angiogenic stimuli such as Ang2 and TNFα, probably in a hypoxia-dependent manner. Accordingly, the number of CD31 cells is dramatically reduced in the absence of leukocyte Hmgb1. Collectively our data suggest that HMGB1 secreted by leukocytes guides muscle regeneration through a paracrine role on vascular bed reconstitution.