Mechanobiology and the microcirculation: cellular, nuclear and fluid mechanics

Abstract

Endothelial cells are stimulated by shear stress throughout the vasculature and respond with changes in gene expression and by morphological reorganization. Mechanical sensors of the cell are varied and include cell surface sensors that activate intracellular chemical signaling pathways. Here, possible mechanical sensors of the cell including reorganization of the cytoskeleton and the nucleus are discussed in relation to shear flow. A mutation in the nuclear structural protein lamin A, related to Hutchinson-Gilford progeria syndrome, is reviewed specifically as the mutation results in altered nuclear structure and stiffer nuclei; animal models also suggest significantly altered vascular structure. Nuclear and cellular deformation of endothelial cells in response to shear stress provides partial understanding of possible mechanical regulation in the microcirculation. Increasing sophistication of fluid flow simulations inside the vessel is also an emerging area relevant to the microcirculation as visualization in situ is difficult. This integrated approach to study--including medicine, molecular and cell biology, biophysics and engineering--provides a unique understanding of multi-scale interactions in the microcirculation.

Figure 1. Processing of lamin A

(A) The lamin A precursor is expressed and transported into the nucleus. Lamin A forms dimers in this process, but is shown as a monomer for simplicity. (B) Once inside the nucleus, farnesyltransferase (FTase) attaches a farnesyl group to the cysteine of the CAAX box at the C-terminus. (C) A protease cleaves the last three amino acids and prelamin A localizes to the inner nuclear membrane. (D) pcCMT carboxymethylates the terminal cysteine. (E) FACE-1 cleaves the last 15 amino acids to produce mature lamin A, which incorporates into the lamin A filaments of the nuclear lamina.

Figure 2. Lamin distribution and DNA in fibroblasts from HGPS patients

Control primary fibroblasts, when fixed and labeled with an antibody for lamin A/C, show homogeneous distribution of lamins at the nuclear envelope with some lamins at the nuclear interior. The DNA, labeled with DAPI, is relatively evenly dispersed throughout the nucleus except for exclusion from the nucleoli. Cells from patients with HGPS (from the Progeria Research Foundation) at passage 12 begin to show redistribution of lamins to the nuclear membrane as well as an altered nuclear morphology. DNA is also redistributed heterogeneously throughout the nucleus. The scale bar is 20 µm.

Figure 3. Imaging of single RBC trajectories in a model microcirculatory device at high hematocrit

RBCs are labeled with a membrane stain and measured using confocal microscopy. Time-lapsed extracellular velocity fields (white vectors) are produced from particle image velocimetry analysis and show the movements induced by a single RBC as it passes through a cell cluster.

Figure 4. Computational fluid dynamics results at the micro-scale

Increasingly sophisticated techniques allow the simulation of complex fluid profiles and complex geometries within the microcirculation. Here, average wall shear stress (WSS; N/m²) at the mid-acceleration phase of the cardiac cycle of early embryonic aortic arch is shown for one simulation. The color map at the right shows the intensity gradients reflecting the diversity of shear stresses within one region.