Abstract
Stratospheric ozone depletion, climate warming and acidification of aquatic ecosystems have resulted in elevated levels of solar radiation reaching many aquatic environments with an increased deleterious impact on a wide range of living organisms. While detrimental effects on living organisms are thought to occur primarily through DNA damage, solar UV can also damage cellular proteins, lipids and signalling pathways. Cryptosporidium, a member of the eukaryotic phylum Apicomplexa, contain numerous vesicular secretory organelles and their discharge via regulated exocytosis is essential for the successful establishment of infection. Using flow cytometric techniques we demonstrate that solar UV rapidly induces sporozoite exocytosis resulting in a significant reduction in the ability of sporozoites to attach and invade host cells. We found that solar UV induced sporozoite membrane depolarization, resulting in reduced cellular ATP and increased cytosolic calcium. These changes were accompanied by a reduction in the internal granularity of sporozoites, indicative of apical organelle discharge, which was confirmed by analysis of sporozoites with an exocytosis-sensitive dye. The precise timing of apical organelle discharge in the presence of a compatible host cell is critical for sporozoite attachment and invasion. Our results demonstrate for the first time how solar UV radiation can interfere with exocytosis, a fundamental cellular process in all eukaryotic cells. We contend that not only may the forecast increases in solar radiation in both aquatic and terrestrial environments significantly affect members of the Apicomplexa, solar UV-induced membrane depolarizations resulting in cytosolic calcium perturbation may affect a wider range of eukaryotic organisms through antagonistic effects on a myriad of calcium dependant cellular functions.
Highlights
The eukaryotic phylum Apicomplexa comprises more than 5000 species of pathogenic protozoa, members of which cause considerable morbidity and mortality in humans, livestock and wildlife [1]
We undertook outdoor microcosm studies to investigate the formation of Cyclobutane pyrimidine dimers (CPDs) in C. parvum oocysts and relate CPD formation to reductions witnessed in oocyst infectivity
Both immunoblot and quantitative sequence detection (QSD) assays were identified as suitable methods for detecting DNA lesions in irradiated oocysts inactivated to varying degrees over a range of UV-C dosages (Figs. 1, 2)
Summary
The eukaryotic phylum Apicomplexa comprises more than 5000 species of pathogenic protozoa, members of which cause considerable morbidity and mortality in humans, livestock and wildlife [1]. Within this phylum the largest group of parasites, the coccidians, maintain their lifecycle by shedding infective oocysts within the host faeces, with the aquatic environment serving as an excellent vehicle for transmission and survival of this stage. While UV exposure has been identified as detrimental to a wide range of organisms, scant attention has been paid to its affect on parasites in aquatic systems. Cyclobutane pyrimidine dimers (CPDs) are the major aberrant DNA photoproduct induced by solar UV [12], making up approximately 75% of all UV-induced photoproducts, and their accumulation in populations has been shown to be highly toxic and mutagenic [13]
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