The effect of pH on collagen structure and permeability

The extracellular matrix (ECM) consists of a complex mixture of structural and functional macromolecules, the major one is collagen. The ECM has an essential role in tissue and organ morphogenesis, and its structure and permeability affect cell migration, cell morphology, cell function, and drug delivery. In some diseases, the ECM pH can change. For example, the ECM pH adjacent to solid tumors and wounds is often acidic. The pH can affect ECM structure, e.g., the fiber density and porosity of collagen, affecting the ECM permeability. We hypothesize that acidity-induced changes in the collagen structure and function are reversible. We control the ECM pH to investigate whether structural and functional (permeability) changes caused by increased acidity are reversible once neutral pH is reached in the tissue. We use type I collagen hydrogel to model the stromal ECM and control the pH using buffer solutions and liposomes containing alkaline solutions. We characterize the collagen structure as a function of pH, utilizing methodology ranging from the molecular scale to the nano and micro scales. We use Fourier Transform Infra-Red Spectroscopy (FTIR) to determine the collagen secondary structure and intermolecular crosslinking. We utilize Cryo-Scanning Electron Microscopy (cryo-SEM) and Cryo-Transmission Electron Microscopy (cryo-TEM) imaging of collagen to characterize its structure as a function of pH while still hydrated. To link collagen permeability to pH-induced structural changes, we use a permeability test by instilling fluorescently labeled particles. Furthermore, we examine whether acidity-induced changes in collagen structure and permeability are reversible by increasing the pH of the acidic ECM back to neutral pH. A better understanding of the connection between the ECM pH, structure, and permeability may improve drug transport through the ECM, drug uptake by cells, and decrease cancer cell migration from solid primary tumors.