Tendons Under Attack: The Role of Homogentisic Acid in Connective Tissue Degeneration

About the Project

This PhD project explores how a rare disease - Alkaptonuria - damages tendons, offering unique insights into oxidative stress and tissue degeneration. Using various tendon models, it aims to uncover mechanisms behind pigmentation, rupture, and repair, with implications for both rare and common tendon disorders.

Background

Tendons and ligaments are essential components of the musculoskeletal system, formed of dense connective tissue. Tendinopathy is a degenerative process in which tendons become inflamed and painful and can lead to rupture. Alkaptonuria (AKU) is an example of a rare genetic, degenerative condition in which the body tissues are exposed to a pathological metabolite called homogentisic acid (HGA) that circulates in the blood. In AKU, HGA-derived pigment deposits in connective tissues across the body with a high affinity for cartilage [1], but also includes tendon, ligament and blood vessels. AKU individuals consistently develop osteoarthritis and are also prone to spontaneous tendon and ligament rupture [2].

Due to joint arthropathy being so severe, the tendon disease that manifests in AKU is understudied. AKU mice are limited to early-stage cartilage pigmentation, with no evidence of tendon or ligament pigmentation, except within the enthesis [3]. Tendon pigmentation and rupture is documented in very few studies [2,4-5] and a handful of case reports, in addition to tendon calcification (Hughes, unpublished). Unpublished data from Dr Hughes’ lab provides preliminary evidence that pigmented tendon has altered biomechanical properties. Recently there is also a growing body of evidence that oxidative stress is a major part of AKU disease pathophysiology. The Liverpool AKU Research Group has shown that HGA is undergoes auto-oxidation and that HGA-pigment is a source of free-radicals, and that antioxidant pathways are markedly altered in AKU biofluids and tissues [6-7].

Objectives

This project aims to study the process of HGA exposure and pigmentation on the biology and function of tendons. To do this, novel in vitro and ex vivo models of AKU tendon will be established and used to study the pathophysiology of tendon disease. Although collagen is suspected as a binding site for HGA, it is unknown where in the extracellular matrix HGA binds, and how it causes degenerative changes, tendinopathy, calcification, and rupture. In addition to AKU tendinopathy, the unique properties of HGA also provide the opportunity to study the effect of oxidative stress on tendon biology.

Aim 1: Establish in vitro and ex vivo models of tendon with and without HGA exposure.

Aim 2: Determine the effect that HGA has on tendon biology and function.

Aim 3: Assess reactive oxygen species (ROS) production, mitochondrial health and redox balance in HGA-treated tendon models.

Experimental approach

This project will encompass a range of skills and techniques including tissue culture (2D, 3D and explant culture) [8], biomechanical testing, histology and omics. Students from related fields (e.g., molecular biology, bioengineering, oxidative stress, cell biology) are welcome and training will be provided.

Potential impact

By studying this rare condition, you’ll contribute to knowledge that could benefit AKU patients and also those affected by common tendon disease. By studying what goes wrong in a rare disease, key biological pathways can be studied and mechanisms often shared with more common diseases can be uncovered. The unique properties of the HGA molecule allow the impact of oxidative stress to be studied, as other small molecules likely attack tendon in a similiar way.

Supervisors:

Dr Juliette Hughes - jhhughes@liverpool.ac.uk

Dr Elizabeth Laird - elizabeth.laird@liverpool.ac.uk

Dr Riaz Akhtar - rakhtar@liverpool.ac.uk

Professor Eithne Comerford - ejc@liverpool.ac.uk

Please send an initial CV and covering letter to Dr Juliette Hughes (jhhughes@liverpool.ac.uk) to express your interest in this project, and for informal enquiries.