Researchers based at the Institute of Ophthalmology, University College London and at the ICORG Clinical Trials Unit, Western Eye Hospital London, have developed a clinical retinal imaging tool to identify stressed and apoptotic cells in the living eye. The research, supported by the Wellcome Trust (UK), uses a fluorescein isothiocyanate-labelled annexin A5 protein (FITC-annexin A5 or 488-annexin A5) capable to detect apoptotic cells by binding to phosphatidylserine on the bilayer plasma membrane. This biomarker, termed “Detection of Apoptosing Retinal Cells” (DARC) allow cells to distinguish necrosis from apoptosis helping to objectively measure the severity of a range of inherited retinal degenerations (IRDs).
Apoptosis is a genetically controlled mechanism of cell death in which the cell activates a specific set of instructions which lead to the deconstruction of the cell from within. Such cell death contrasts markedly with the more familiar mechanism known as necrosis. Necrosis occurs when a cell is injured or receives some shock whereby it is unable to continue carrying out the activities of life. Though the end result of both apoptosis and necrosis are the same, that is, the death of the cell, the mechanisms leading to such death are crucially different. Necrosis is characterised by swelling, rupture, leakage and inflammation. Apoptosis appears as a more deliberate and choreographed affair. Cells dying by apoptosis replace swelling with shrinkage and rupture with an elegant packaging of cellular contents into a convenient size for disposal. There is no leakage of cellular material and no inflammation. The remaining fragments of an “apoptosed” cell are neatly and quietly disposed of by either neighbouring healthy cells or by the body’s household staff – the macrophages. Externalization of phosphatidylserine (PS) on the outer plasma membrane of damaged cells change the PS from the inner membrane to the outer plasma membrane and these early stages of apoptosis signal immune cells to “mop up” the debris from the apoptotic process. Annexin 5 can be used to discriminate healthy from apoptotic cells and this can be applied for a range of disorders including AMD, geographic atrophy, glaucoma, neurodegenerations and cancers.
Annexin 5 using DARC labelling may be colocalised to photoreceptors and inner nuclear cells undergoing apoptosis. Externalisation of PS in stressed endothelial cells has been highlighted as the very early sign of angiogenesis and neovascular AMD. In addition, analysis has also been performed in a Phase 2 study with geographic atrophy (GA) indicating that the level of DARC analysis was predictive of GA activity. Commenting on their recent paper in Progress in Retinal and Eye Research (https://doi.org/10.1016/j.preteyeres.2021.100976), researchers stated that, “DARC until earlier this year has received the majority of its funding from the Wellcome Trust. This has enabled DARC to get to Phase 2 in an academic setting. As DARC continues to develop, commercially-oriented funding through Novai Ltd has allowed DARC to continue on its journey, with the ultimate aim of helping patients to avoid functional disability through early identification and treatment”.