(73d) Substrate Stiffness Regulates Microglial Phenotype and Function

Neurological disorders and aging have been shown to result in changes in the mechanical environment of the brain. The brain is one of the softest organs in the human body, which is attributed to the extracellular matrix (ECM) comprising of hydrated scaffolding proteins instead of fibrous proteins. Over time, malfunction in ECM remodeling during aging and neurological disorders leads to sustained tissue repair and results in a progressive accumulation of dense, stiff brain ECM molecules. Microglia serve as the resident macrophages that play a significant role in nonimmunological functions in the brain, including sensing, monitoring, and regulating features such as ECM repair. Much is known about the biochemical signals via which microglia regulate the brain ECM. However, the role of the brain's dynamic changes in biophysical cues (stiffness) in regulating microglial biology is underexplored. In our study, we hypothesize that stiffness is one key regulator of microglial function and phenotype. To test this, we examined the effect of physiological and pathological stiffness on microglia phenotype and function. We developed a mechanically tunable stiffness platform called “BEASTS”: Bio-Engineered Adhesive Siloxane-based Tunable Stiffness platform. Our platform is comprised of mechanically tunable polydimethylsiloxane (PDMS) substrates that are capable of mimicking physiological (2 kPa) and pathological brain stiffness (8, 15, 25 kPa). To facilitate cell adhesion, the PDMS gels were coated with polyelectrolyte multilayer (PEM) films to engineer a protein-free environment. We found that increased stiffness regulates microglia to adopt an inflammatory phenotype and a dystrophic phenotype found in aged microglia. In addition, microglia cultured on pathological stiffness showed 1) increased inflammatory cytokines, 2) impaired mitochondrial respiration and function, and 3) redox dysfunction. These data suggest a plausible mechanism that increased stiffness modulates microglial dysfunction. Understanding the impact of stiffness on microglial biology will provide significantly nuanced data to intervene in the aging-related loss in brain function.