Biophysical regulation of macrophages in health and disease

Abstract

Macrophages perform critical functions for homeostasis and immune defense in tissues throughout the body. These innate immune cells are capable of recognizing and clearing dead cells and pathogens, and orchestrating inflammatory and healing processes that occur in response to injury. In addition, macrophages are involved in the progression of many inflammatory diseases including cardiovascular disease, fibrosis, and cancer. Although it has long been known that macrophages respond dynamically to biochemical signals in their microenvironment, the role of biophysical cues has only recently emerged. Furthermore, many diseases that involve macrophages are also characterized by changes to the tissue biophysical environment. This review will discuss current knowledge about the effects of biophysical cues including matrix stiffness, material topography, and applied mechanical forces, on macrophage behavior. We will also describe the role of molecules that are known to be important for mechanotransduction, including adhesion molecules, ion channels, as well as nuclear mediators such as transcription factors, scaffolding proteins, and epigenetic regulators. Together, this review will illustrate a developing role of biophysical cues in macrophage biology, and also speculate upon molecular targets that may potentially be exploited therapeutically to treat disease.

Keywords: mechanobiology; mechanotransduction.

Figure 1:. Effect of biophysical cues on macrophage function.

(a, c, e, g) Schematics showing the interaction of macrophages with surfaces of different stiffness (a) or topology (c) and external mechanical forces including stretch (e) and interstitial flow (g). (b) Image of IgG coated latex beads (cyan) uptaken by RAW264.7 macrophages on stiff compared to soft surfaces (left), adapted from [38], and TNFα secretion by primary murine macrophages on surfaces different stiffness (right), adapted from [40]. (d) Immunofluorescence image of Arg1 (M2 marker, red) and nuclei (blue) of macrophages cultured on flat and 1D wrinkled surfaces (left), and quantification of Arg1 expression as measured by Western blot (right), adapted from [59]. (f) Immunofluorescence image of F4/80 (red) and Ki67 (green) cell proliferation marker of murine peritoneal macrophages undergoing cyclic mechanical stretch (4%, biaxial, 0.67 Hz) compared to static controls (left) and quantification of cell number and Ki67 (right), adapted from [71]. (h) Immunofluorescence images of Dapi (blue) and CD206 (M2 marker, red) in BMDM exposed to interstitial flow (3 μm/s) compared to static controls (left), and quantification of CD206 levels (right), adapted from [83].

Figure 2:. Heat map of adhesome gene expression during macrophage differentiation and LPS stimulation.

Transcriptional analysis of differentiating HL-60 cells after 0–120 hrs of treatment with phorbol 12-myristate 13-acetate (PMA) [88]. Genes from integrin and cadherin adhesomes, as previously defined [89, 90], are shown. Data was filtered for minimum expression based on FPKM (fragments per kilobase of transcript per million mapped reads). The remaining genes were then normalized across all conditions and hierarchically clustered using the unweighted pair group method with arithmetic mean (Z-Score range: −2.4 to 2.4).

Figure 3:. Regulation of macrophage adhesome gene expression by stimulation with polarizing signals.

(a) Graph of human macrophage integrin and cadherin adhesome genes that increased or decreased by greater than 2-fold upon stimulation with combinations of interferon-γ (INFγ), standard LPS (sLPS), ultrapure LPS (upLPS), IL-4 and IL-13. Plots were generated using microarray expression data from Xue et al. [5]. Genes expressed below a baseline threshold were filtered out and the remaining genes were compared using a pairwise Log2 fold change analysis relative to an unstimulated control, which was made publicly available by the authors (b) Venn diagrams showing numbers of genes from (a) overlapping across different pro-inflammatory conditions.

Figure 4:. Schematic of potential macrophage mechanotransduction pathways.

Macrophages integrate soluble and physical cues from their microenvironment to regulate their adhesion, phagocytosis, migration, and activation (left). Podosomal type adhesions contain integrins, which physically anchor cells to the ECM, and bind intracellularly to adhesion-associated scaffolding and signaling proteins and the actin cytoskeleton (box 1). Forces transmitted across integrins and cytoskeleton, as well as plasma membrane tension, regulate stretch-activated ion channels (box 2). Biophysical cues trigger nuclear translocation of transcriptional regulators to direct gene expression (box 3), and may also regulate epigenetic enzymes including writers and erasers, which add and remove chromatin modifications respectively, while also enabling readers that detect modifications (box 4). BRDs: bromodomain containing proteins, DNMTs: DNA methyltransferases, ECM: extracellular matrix, FAK: focal adhesion kinase, H3Ac: histone H3 acetylation, HDACs: histone deacetylases, Methyl-C: cytosine methylation, MRTF-A: myocardin–related transcription factor, NPC: nuclear pore complex, Pyk: protein tyrosine kinase, TLRs: toll-like receptors, TRP: transient receptor potential, YAP: yes associated protein.