The CD39-CD73-adenosine axis: a pivotal regulator of retinal homeostasis and diseases.

Extracellular purinergic signaling, mediated by nucleotides such as adenosine triphosphate (ATP) and adenosine, is a critical regulator of retinal homeostasis. Within the retina, this signaling pathway is indispensable for maintaining physiological functions. Dysregulation of extracellular purine balance is a key driver of retinal pathophysiology, contributing to inflammation, pathological angiogenesis, and neurodegeneration in diseases like diabetic retinopathy (DR), age-related macular degeneration (AMD), glaucoma, and retinopathy of prematurity (ROP). The hydrolysis of extracellular ATP to adenosine, primarily orchestrated by the sequential actions of NTPDase1/ENTPD1 (CD39) and ecto-5'-nucleotidase/NT5E (CD73), represents a fundamental mechanism for calibrating purinergic signals and shifting the microenvironment from pro-inflammatory to anti-inflammatory and protective. CD39 and CD73 are pivotal enzymes in regulating retinal purinergic equilibrium. Despite their well-characterized roles in tumor biology and cerebral pathophysiology, the expression, physiological functions, and pathological implications of CD39/CD73 in the retina remain understudied and lack systematic synthesis. This review synthesizes current understanding of the roles of CD39 and CD73 in retinal physiology and pathology, with a focus on their implications in DR, AMD, glaucoma, and ROP. Critically, this review provides the first systematic integration of their layer-specific spatial compartmentalization, context-dependent functional duality, and emerging metabolic roles in photoreceptor homeostasis. We also discuss emerging therapeutic strategies that target this pathway, addressing the growing need for translational research to preserve vision.