New paradigms in fibroblast-immune cells coordination in matrix homeostasis in health and disease

Collagen is the most abundant protein in the human body (~25% by mass), and is formed into long fibrils that provide structure to organs with very different functions and mechanical properties (e.g. giant parallel bundles in tendons, smooth flat lattices in lungs). This suggests a fine control of the cells in producing and assembling these long fibrils, and dysregulation of this control underpins many pathologies, including fibrosis, heart disease, and many age-related conditions such as proneness to fractures, skin looseness, and osteoarthritis. Convention dictates that different cells have different functions - notably fibroblasts are the key arbiters to collagen matrix formation, and immune cells are the removers; however, recently it has been discovered that immune cells directly contribute to the production of collagen. This highlights how despite collagen's clear fundamental importance, we still do not fully understand the process of its assembly and maintenance, in particular how different types of cells communicate and cooperate with one another in this process. This forms the basis and goals of my research.

My recent work has shown that the circadian rhythm controls collagen production and secretion, and I have also discovered that a specialised compartment of the cells (known as the endosome) holds the key to the fate of collagen within the cells, i.e. whether they will be made into fibrils, simply secreted out of the cells, or degraded. Using new mass spectrometry approaches that I have pioneered, I aim to determine how this control of fibril formation happens at the molecular level, as knowledge of how to direct collagen between the different fates will allow for new therapeutic strategies for fibrosis - a disease characterised by an abundance of collagen fibrils that currently has no effective cure. Using the same mass spectrometry technique, I will further determine the molecular networks in immune cells that control collagen uptake, which may provide further insights to targeting fibrosis from a different angle. This knowledge will also be fundamental to other conditions heavily involving the immune system and matrix, including wound healing and cancer metastasis.

I have also found that immune cells induce fibroblast circadian rhythms, and that fibroblasts and immune cells make more collagen fibrils when they are mixed together. I will interrogate this relationship and determine the proportion of collagen produced by each cell type in the mixtures, using state-of-the-art genetic modification and high throughput biochemical assays, providing important insights into how fibroblasts and immune cells communicate and coordinate in collagen deposition. As such, my work has important and fundamental implications to further our mechanistic understanding of fibrotic responses, in its broadest sense.

Vertebrates are ~25% by mass collagen, mostly in the form of long fibrils that provide tensile strength and tissue structures. Collagen dysregulation underpins many diseases: excess leads to incurable fibrosis, which disrupts organ functions and is associated with 45% of all deaths. Reduced collagen (in amount or fibril functionality) leads to other conditions, e.g. musculoskeletal diseases, poor wound-healing. Thus, collagen pathologies have major health consequences, and understanding the molecular control of collagen is fundamental to therapeutics development. We previously reported that fibroblast collagen homeostasis is under circadian control, and clockless fibroblasts produce abnormal collagen fibrils. I also showed that the endosome is a master regulator, directing collagen (endogenous, or exogenous that was endocytosed) to either fibril formation or monomer secretion, and that distinct machineries are involved; elevated endosomal-recycling may be a disease-mechanism in lung fibrosis. To determine the components of this monomer/fibril switch, I developed a method coupling flow cytometry to mass spectrometry that will allow me to identify collagen-trafficking proteins. I will test how these proteins contribute to the monomer/fibril switch using collagen-reporter cells, and verify their involvement in lung fibrosis using human control/patient samples.
Further, an unexpected collagen-deposition role for macrophages was recently reported; I have found that immune cells synchronise and promote collagen fibril deposition in fibroblasts, and make collagen fibrils themselves, indicating an as yet-unknown immune-fibroblast coordination on collagen deposition. I will define how immune cells contribute to collagen homeostasis, either directly and/or through controlling the fibroblast circadian rhythm.
These mechanistic insights will be clinically important, by establishing collagen-trafficking components and defining the role of immune cells in collagen homeostasis.