Maria Christophorou

Research Summary

How do the cells of an organism, all of which have exactly the same genetic code, adopt such different fates, morphologies and functions? And how do they then respond to the signals and stresses around them in order to make up a living, growing, healthy organism that can adapt to its environment?

Seminal work in the field of Epigenetics has taught us that the answer to the first question lies in the fact that our genome is subject to epigenetic regulation, which ensures its stability and determines when and where genes produce their transcript and protein products. And the answer to the second question lies largely within the fact that these proteins, which then go on to execute most of the cell’s functions, are themselves subject to regulatory mechanisms which determine when, where and how a protein will function. Our lab combines these two fascinating biological questions to understand how genome-regulating proteins are themselves regulated during development.

We employ biochemistry, cell and molecular biology, genomic and epigenetic approaches and mouse model systems to understand the mechanisms that modulate the function of epigenetic regulators, how these mechanisms are perturbed in disease and how they may be targeted for therapeutic effect. We have a particular interest in protein post-translational modifications (PTMs). These are small chemical changes that happen on proteins as a result of cell signalling changes and can quickly alter the activity, stability and sub-cellular localisation of these proteins, as well as their affinity for other molecules. As a result, PTMs add an enormous degree of sophistication to biological systems, beyond what can be achieved by gene regulation.

Our favourite PTM is citrullination, the conversion of an arginine residue to the non-coded amino acid citrulline. Exciting developments in this classically under-explored field have shown that citrullination and the enzymes that catalyse it, the peptidylarginine deiminases (PADIs or PADs), regulate many aspects of cell physiology, while their deregulation contributes to the development of pathologies such as autoimmunity, neurodegeneration and cancer. Understanding the mechanisms that control PADIs and other epigenetic regulators in response to developmental cues and cellular stresses can offer valuable insights into human health, which can be exploited towards therapeutic benefit in a variety of disease conditions.

Latest Publications

Citrullination was introduced into animals by horizontal gene transfer from cyanobacteria.
Cummings TFM, Gori K, Sanchez-Pulido L, Gavriilidis G, Moi D, Wilson AR, Murchison E, Dessimoz C, Ponting CP, Christophorou MA

Protein post-translational modifications (PTMs) add great sophistication to biological systems. Citrullination, a key regulatory mechanism in human physiology and pathophysiology, is enigmatic from an evolutionary perspective. Although the citrullinating enzymes peptidylarginine deiminases (PADIs) are ubiquitous across vertebrates, they are absent from yeast, worms and flies. Based on this distribution PADIs were proposed to have been horizontally transferred, but this has been contested. Here, we map the evolutionary trajectory of PADIs into the animal lineage. We present strong phylogenetic support for a clade encompassing animal and cyanobacterial PADIs that excludes fungal and other bacterial homologues. The animal and cyanobacterial PADI proteins share functionally relevant primary and tertiary synapomorphic sequences that are distinct from a second PADI type present in fungi and actinobacteria. Molecular clock calculations and sequence divergence analyses using the fossil record estimate the last common ancestor of the cyanobacterial and animal PADIs to be less than one billion years old. Additionally, under an assumption of vertical descent, PADI sequence change during this evolutionary time frame is anachronistically low, even when compared to products of likely endosymbiont gene transfer, mitochondrial proteins and some of the most highly conserved sequences in life. The consilience of evidence indicates that PADIs were introduced from cyanobacteria into animals by horizontal gene transfer (HGT). The ancestral cyanobacterial PADI is enzymatically active and can citrullinate eukaryotic proteins, suggesting that the PADI HGT event introduced a new catalytic capability into the regulatory repertoire of animals. This study reveals the unusual evolution of a pleiotropic protein modification.

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Molecular biology and evolution, 1, 1, 03 Nov 2021

PMID: 34730808

Citrullination Alters the Antiviral and Immunomodulatory Activities of the Human Cathelicidin LL-37 During Rhinovirus Infection.
Casanova V, Sousa FH, Shakamuri P, Svoboda P, Buch C, D'Acremont M, Christophorou MA, Pohl J, Stevens C, Barlow PG

Human rhinoviruses (HRV) are the most common cause of viral respiratory tract infections. While normally mild and self-limiting in healthy adults, HRV infections are associated with bronchiolitis in infants, pneumonia in immunocompromised patients, and exacerbations of asthma and COPD. The human cathelicidin LL-37 is a host defense peptide (HDP) with broad immunomodulatory and antimicrobial activities that has direct antiviral effects against HRV. However, LL-37 is known to be susceptible to the enzymatic activity of peptidyl arginine deiminases (PAD), and exposure of the peptide to these enzymes results in the conversion of positively charged arginines to neutral citrullines (citrullination). Here, we demonstrate that citrullination of LL-37 reduced its direct antiviral activity against HRV. Furthermore, while the anti-rhinovirus activity of LL-37 results in dampened epithelial cell inflammatory responses, citrullination of the peptide, and a loss in antiviral activity, ameliorates this effect. This study also demonstrates that HRV infection upregulates PAD2 protein expression, and increases levels of protein citrullination, including histone H3, in human bronchial epithelial cells. Increased gene expression and HDP citrullination during infection may represent a novel viral evasion mechanism, likely applicable to a wide range of pathogens, and should therefore be considered in the design of therapeutic peptide derivatives.

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Frontiers in immunology, 11, 1, 2020

PMID: 32117246

Open Access

Citrullination of HP1γ chromodomain affects association with chromatin.
Wiese M, Bannister AJ, Basu S, Boucher W, Wohlfahrt K, Christophorou MA, Nielsen ML, Klenerman D, Laue ED, Kouzarides T

Stem cell differentiation involves major chromatin reorganisation, heterochromatin formation and genomic relocalisation of structural proteins, including heterochromatin protein 1 gamma (HP1γ). As the principal reader of the repressive histone marks H3K9me2/3, HP1 plays a key role in numerous processes including heterochromatin formation and maintenance.

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Epigenetics & chromatin, 12, 1, 2019

DOI: 10.1186/s13072-019-0265-x

PMID: 30940194

Open Access