Mechanotransduction in red blood cells

Abstract

Red blood cells (RBC), the oxygen-carriers within blood, eject their nuclei and other organelles to optimise cellular mechanics for gas exchange in capillary networks. Lack of organelles, however, strictly limits circulatory longevity of these cells, due to the inability to repair damaged cellular components. Given the turnover of RBC, the cell population within blood is inherently heterogenous, comprising RBC across the whole spectrum of in vivo age. Moreover, surrender of translational capacity restricts cellular signalling within RBC to modifications of existing proteins and/or flux of ions through membrane-embedded channels, rather than alterations in protein expression. The traversal of the cardiovascular system for the purpose of gas exchange exposes RBC to varying mechanical forces. Exposure to mechanical force physically deforms the RBC membrane, which, upon cessation of force exposure, readopts its native bi-concave disc chape. Novel observations support that these mechanical forces also activate biochemical pathways that may acutely and transiently alter RBC mechanics. The molecular machinery facilitating these mechanotransduction processes in RBC, however, is largely undescribed. The aim of the present body of work was thus to elucidate i. mechanotransductive pathways in mature, enucleated RBC; ii. the contribution of mechanically-activated signalling to the regulation of RBC mechanics; and iii. the impact of sub-populations of RBC with abnormal mechanical properties on blood fluid behaviour. The salient findings of the present dissertation support the presence of a relevant post-translational signalling network in circulating, enucleated RBC, some of which is sensitive to activation by mechanical forces. The cation channel Piezo1 appears to be a central mechanism of ‘force sensing’ in these cells. That is, opening of Piezo1 in response to mechanical force facilitates influx of calciumions, which regulate RBC mechanics via diverse mechanisms, including acute shifts in cell volume, selective removal of susceptible cells within a given RBC population, and initiation of nitric oxide production. Collectively, the herein presented results enhance the current understanding of fundamental RBC physiology by elucidating hitherto unrecognised signalling pathways. Given the demonstrated relevance of these processes to the regulation of RBC mechanical properties, which determine blood fluid properties and effective gas exchange, components of mechanically-activated signalling in these cells may provide novel therapeutic targets. Moreover, adverse complications arising in scenarios where blood is exposed to mechanical forces far exceeding those investigated here, for example during transit of mechanical circulatory support devices or dialysis machines, may be linked to overactivation of mechanically-sensitive signalling.