Diabetic Retinopathy: premature cellular senescence and microvascular dysfunction

By Pietro Maria Bertelli, PhD Candidate, Queens University Belfast

Eye complications are very frequent in patients with diabetes, and this can lead to vision impairment and blindness 1,2. A better understanding of the pathophysiology of diabetic vascular complications is needed to facilitate the development of novel treatments. Diabetes has been previously associated with ageing and cellular senescence 3, 4. Retinal blood vessels, composed of endothelial cells and pericytes, are significantly affected by diabetic conditions 5–7. We are evaluating the impact of diabetes on endothelial cell function at the cellular and molecular level in relation to senescence to provide new insights into the biology of diabetic retinopathy.

Diabetic Retinopathy (DR) is considered the most common microvascular complication occurring in the working-age population in western countries 8. The diabetic milieu, such as hyperglycaemia, hyperlipidaemia and hypertension, triggers several pathological pathways, which result in vascular dysfunction associated with deficit in blood perfusion, increased vascular permeability, vessel loss and ischaemia 9. This can lead to diabetic macular oedema (DMO) and/or pre-retinal pathological neovascularisation as two frequent complications of DR that cause visual impairment and blindness 9,10. 

Senescence was firstly defined by Hayflick and colleagues as the limited capability of cells to divide and propagate in culture conditions 11,12. Three subtypes can be described: replicative, stress-induced and oncogene-induced senescence.

Figure 1: Hallmarks of senescent cells. Senescent cells are characterised by an irreversible growth arrest, resistance to apoptosis, SA-βGal expression, secretion of growth factors, cytokines, chemokines and others (SASP) and changes in the molecular and gene expression profiles. Scale bars=50 μm.

Senescent cells enter a state of permanent cell cycle arrest13 but, they remain metabolically active. Furthermore, they are characterised by a senescence-associated secretory phenotype (SASP)14, increased lysosomal β-galactosidase expression15, morphological changes and a changed molecular and gene expression profile16–18 (Figure 1 and 2).

Figure 2: Morphological changes and SA- βGal expression in senescent cells. (A) Senescent cells are flat and bigger in size compared to early passage cells. (B) Senescent have an increased expression of SA- β Galactosidase, due to an increase of the lysosomal content. SA- β Galactosidase is detected by a colorimetric assay performed at pH 615. In figure A and B human retinal endothelial cells in a monolayer cell culture are shown. Scale bars =150 μm

Some studies, mainly performed in cultured human endothelial cells exposed to high glucose and diabetic mouse models, have suggested development of premature endothelial senescence under diabetic conditions 19–21. More recently, SASP was further confirmed to be a significant contributor in the pathological aspects of DR22. Our group at Queen’s University Belfast has characterised the senescence programme in human endothelial progenitors and its association with SASP components such as interleukin 823.

My PhD project at Queen’s University Belfast funded by the National Eye Research Centre, and our research group supported by JDRF and the Leverhulme Trust , are investigating the role that cellular senescence plays in the pathogenesis of diabetic retinopathy with a particular interest in endothelial cells for the development of therapeutic strategies for the condition: senotherapies24.

References

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