Relationship of and cross-talk between physical and biologic properties of the glomerulus

Abstract

Purpose of review: Cells and tissues must respond to physical stresses. Cells exist in an elastic environment determined by their matrix, matrix contacts, cell-cell contacts, and cytoskeletal structure. We discuss the determinants of the elastic environment of cells and its potential roles in glomerular disease.

Recent findings: Control of the mechanical environment is sufficient to induce and maintain the differentiated state of cells including myofibroblasts. New experimental techniques permit precise measurement of the elastic characteristics of normal and diseased tissues and cells, and analysis of cell behavior and cytoskeletal structure in response to mechanical and elastic stimuli. Glomeruli become soft early in the course of several disease models, yet late stages are characterized by increased stiffness and fibrosis with loss of organ function. Work in hepatic fibrosis, arterial disease, and oncology demonstrate that increased collagen crosslinking by lysyl oxidase, an early step in the diseases, can result in a sufficient increase in tissue stiffness to alter cell behavior, leading to disease progression.

Summary: The elastic environment of cells and tissues provides essential signals in development, differentiation, and disease. Identifying the mechanisms that determine the mechanical environment of glomerular cells will complement other approaches to reduce pathologic fibrosis and loss of tissue function.

Figure 1. Comparison of the elastic moduli (E_Mod) of several normal and diseased tissues

The elastic moduli of a number of normal adult tissues is compared to diseased tissues using a log scale of elastic modulus (E_Mod) that ranges from 0 to 10⁹ Pa. Normal tissues are shown in black above the scale, and diseased tissues are shown in blue below the scale. The values were obtained using rheometry, atomic force microscopy, or micro-indentation.

Figure 2. Essentials of mechanical signaling

This figure shows the basic molecular machinery that senses and responds to matrix-generated mechanical signals. When a cell encounters a matrix, integrins bind proteins in the matrix, and additional proteins on the cytoplasmic surface aggregate forming a focal complex (Top). A focal complex contains integrins that connect the extracellular matrix to actin fibers as well as additional proteins that lead to activation and aggregation of integrins and that link them to actin fibers. These proteins include talin, vinculin, and paxillin. Kinases and phosphatases, also important for these processes, are not shown for simplicity. In the presence of force, probably generated by actin polymerization, additional integrins aggregate and bind to F-actin fibers and non-muscle myosins leading to formation of a focal adhesion (Middle). The cell surveys its mechanical environment with periodic contraction of actin and non-muscle myosin stress fibers, which are attached to the integrins that pull against the matrix. To sense force, the complexes must be separated by > 2 μm. Over time, and as a result of mechanical force arising from the actin fibers and non-smooth muscle myosins, the aggregation of these additional proteins results in development of mature focal adhesions (Bottom). Additionally, proteins, such as α-actinin, filamin, and cortactin, cross-link actin fibers thereby adding mechanical strength to the actin cytoskeleton and consequently the cell overall. Filamin and α-actinin also participate in linking actin fibers to integrin β subunits.

Figure 3. Responses of normal cells to matrix elasticity

The top two images show the responses of normal cells to soft and stiff matrices. An abnormal cell (bottom image) that is unable to sense matrix stiffness. In the top image, a cell is shown on a soft matrix, which is represented by a wavy black line, indicating that the cell can contract against the matrix and deform it. This cell has only few focal adhesions (red squares) and actin fibers (green arrows). When the same cell is placed on a stiffer matrix, which the contractile apparatus of the cell cannot deform (second image), the number of focal adhesions increases. The number of actin-non-muscle myosin stress fibers and their thickness also increases, leading to cell spreading and stiffening. In disease states such as cancer and scarring, cells might encounter abnormally stiff matrix and therefore take on abnormal mechanical and cell biologic characteristics. The third image shows a cell that cannot sense or respond to matrix-generated mechanical signals on a stiff matrix. The cell remains soft with only a few focal adhesions and actin fibers. Cells with these characteristics are for example found in filamin-null M2 melanoma cells, cells that lack integrins, in glomerular podocytes from a mouse model of HIV-associated nephropathy, α-actinin-4-null mice, and in cells without functional non-muscle myosins. These cells all have defects in adhesion, migration, and show increased susceptibility to injury by mechanical force. The bottom image shows a normal cell on a soft matrix that is composed of fibronectin (FN) and hyaluronic acid (HA). In this case, the matrix induces the cell to develop focal adhesions and stress fibers and become stiffer than its matrix. This situation could arise in a wound where FN and HA are present. In cells on a stiff matrix (stiff cells) YAP is localized to the nucleus, In all other conditions, soft matrix, soft cells, or stiff cells on a soft matrix composed of fibronectin and hyaluronic acid, YAP is cytoplasmic.