Exploring Extracellular Matrix Crosslinking as a Therapeutic Approach to Fibrosis

Abstract

The extracellular matrix (ECM) provides structural support for tissues and regulatory signals for resident cells. ECM requires a careful balance between protein accumulation and degradation for homeostasis. Disruption of this balance can lead to pathological processes such as fibrosis in organs across the body. Post-translational crosslinking modifications to ECM proteins such as collagens alter ECM structure and function. Dysregulation of crosslinking enzymes as well as changes in crosslinking composition are prevalent in fibrosis. Because of the crucial roles these ECM crosslinking pathways play in disease, the enzymes that govern crosslinking events are being explored as therapeutic targets for fibrosis. Here, we review in depth the molecular mechanisms underlying ECM crosslinking, how ECM crosslinking contributes to fibrosis, and the therapeutic strategies being explored to target ECM crosslinking in fibrosis to restore normal tissue structure and function.

Keywords: collagen; crosslinking; extracellular matrix; fibrosis.

Figure 1

Collagen processing pathway, from transcription to crosslinked fibril formation. After transcription in the nucleus and subsequent translation, three alpha chains assemble into a procollagen triple helix. Upon export into the extracellular space, N- and C-terminal pro-peptides are removed from procollagen, converting it to tropocollagen. For fibrillar collagens, these tropocollagens then form fibrils, which are subject to LOX-mediated crosslinking. LH hydroxylation precedes LOX activity and has been reported to occur both intra- and extracellularly. Ovals indicate N- and C-terminal pro-peptides. Stars are indicative of crosslinks. Created with BioRender.com, accessed on 30 January 2024.

Figure 2

Transglutaminase crosslinking. Transglutaminases (TGs) mediate a crosslinking reaction between lysine and glutamine residues. Created with BioRender.com, accessed on 26 January 2024.

Figure 3

Enzymatic reactions driven by lysine hydroxylase (LH) and lysyl oxidase (LOX) enzymes. (A) LHs convert lysine to hydroxylysine through hydroxylation reaction. (B) LOXs convert hydroxylysine or lysine into aldehyde form via oxidative deamination. Created with BioRender.com, accessed on 30 January 2024.

Figure 4

Condensation reactions drive formation of immature collagen crosslinks. A lysine or hydroxylysine aldehyde from a LOX modification reacts with unmodified lysine or a hydroxylysine derived from an LH modification to form three different immature crosslinks. Immature collagen crosslinks vary by number of OH groups (green). Created with BioRender.com, accessed on 30 January 2024.

Figure 5

Immature collagen crosslinks go through additional reactions to form mature crosslinks. Pyrroles are formed through a reaction between dehydro-hydroxylysino-norleucine (deH-HLNL) or dehydro-dihydroxylysino-norleucine (deH-DHLNL) with a lysine aldehyde. Pyridinolines are formed through a reaction between deH-HLNL or deH-DHLNL with a hydroxylysine aldehyde. Reactions with deH-HLNL produce the deoxy forms deoxy-pyridinoline and deoxy-pyrrole. Created with BioRender.com, accessed on 26 January 2024.