Biophysical properties of normal and diseased renal glomeruli

Abstract

The mechanical properties of tissues and cells including renal glomeruli are important determinants of their differentiated state, function, and responses to injury but are not well characterized or understood. Understanding glomerular mechanics is important for understanding renal diseases attributable to abnormal expression or assembly of structural proteins and abnormal hemodynamics. We use atomic force microscopy (AFM) and a new technique, capillary micromechanics, to measure the elastic properties of rat glomeruli. The Young's modulus of glomeruli was 2,500 Pa, and it was reduced to 1,100 Pa by cytochalasin and latunculin, and to 1,400 Pa by blebbistatin. Cytochalasin or latrunculin reduced the F/G actin ratios of glomeruli but did not disrupt their architecture. To assess glomerular biomechanics in disease, we measured the Young's moduli of glomeruli from two mouse models of primary glomerular disease, Col4a3−/− mice (Alport model) and Tg26HIV/nl mice (HIV-associated nephropathy model), at stages where glomerular injury was minimal by histopathology. Col4a3−/− mice express abnormal glomerular basement membrane proteins, and Tg26HIV/nl mouse podocytes have multiple abnormalities in morphology, adhesion, and cytoskeletal structure. In both models, the Young's modulus of the glomeruli was reduced by 30%. We find that glomeruli have specific and quantifiable biomechanical properties that are dependent on the state of the actin cytoskeleton and nonmuscle myosins. These properties may be altered early in disease and represent an important early component of disease. This increased deformability of glomeruli could directly contribute to disease by permitting increased distension with hemodynamic force or represent a mechanically inhospitable environment for glomerular cells.

Opposing effects of podocin on the gating of podocyte TRPC6 channels evoked by membrane stretch or diacylglycerol

“…This distribution corresponds to the domains of highest cholesterol content in podocytes on the basis of earlier freeze-fracture studies (43). The glomerular capillary is a relatively high-pressure microvascular bed, and its walls are unusually rigid (4,70). For this reason, podocyte foot processes will be exposed to continuous highfrequency mechanical stimuli originating from the cardiac cycle, as well as slower fluctuations associated with myogenic autoregulatory responses and tubuloglomerular feedback (19).…”

Mechanisms of mechanical signaling in development and disease

“…For example, it was shown that conditionally immortalized glomerular podocytes from a mouse model of HIVassociated nephropathy (HIVAN) are substantially softer than normal podocytes (by 25%), as assessed by atomic force microscopy (AFM) and microaspiration (Tandon et al, 2006). At a stage of the disease that showed no detectable pathological changes, the glomeruli from these mice were 30% softer than normal glomeruli (1.5 kPa compared with 2.5 kPa, as measured by AFM) (Wyss et al, 2011). Glomeruli and podocytes in other disease models are also significantly softer than normal cells, indicating that the increased mechanical deformability of podocytes and glomeruli might be a common feature of a number of renal diseases.…”

Alpha-actinin binding kinetics modulate cellular dynamics and force generation

“…Because the moduli of reconstituted actin networks are highly sensitive to cross-linker affinity and concentration (1,2,19), we hypothesized that changes in actin cross-linking will also significantly impact the total force and work exerted by an active cell on its environment. However, the effect of this mutation, or variable cross-linking in general, on the dynamics and forces of cells is largely unknown (20). Understanding how cross-linker binder affinity changes force generation will provide essential insight into how cells modulate their mechanics, and exert forces on their surrounding environment.…”