Dose-Dependent Effects of Radiation on Mitochondrial Morphology and Clonogenic Cell Survival in Human Microvascular Endothelial Cells.

To better understand radiation-induced organ dysfunction at both high and low doses, it is critical to understand how endothelial cells (ECs) respond to radiation. The impact of irradiation (IR) on ECs varies depending on the dose administered. High doses can directly damage ECs, leading to EC impairment. In contrast, the effects of low doses on ECs are subtle but more complex. Low doses in this study refer to radiation exposure levels that are below those that cause immediate and necrotic damage. Mitochondria are the primary cellular components affected by IR, and this study explored their role in determining the effect of radiation on microvascular endothelial cells. Human dermal microvascular ECs (HMEC-1) were exposed to varying IR doses ranging from 0.1 Gy to 8 Gy (~0.4 Gy/min) in the AFRRI 60-Cobalt facility. Results indicated that high doses led to a dose-dependent reduction in cell survival, which can be attributed to factors such as DNA damage, oxidative stress, cell senescence, and mitochondrial dysfunction. However, low doses induced a small but significant increase in cell survival, and this was achieved without detectable DNA damage, oxidative stress, cell senescence, or mitochondrial dysfunction in HMEC-1. Moreover, the mitochondrial morphology was assessed, revealing that all doses increased the percentage of elongated mitochondria, with low doses (0.25 Gy and 0.5 Gy) having a greater effect than high doses. However, only high doses caused an increase in mitochondrial fragmentation/swelling. The study further revealed that low doses induced mitochondrial elongation, likely via an increase in mitochondrial fusion protein 1 (Mfn1), while high doses caused mitochondrial fragmentation via a decrease in optic atrophy protein 1 (Opa1). In conclusion, the study suggests, for the first time, that changes in mitochondrial morphology are likely involved in the mechanism for the radiation dose-dependent effect on the survival of microvascular endothelial cells. This research, by delineating the specific mechanisms through which radiation affects endothelial cells, offers invaluable insights into the potential impact of radiation exposure on cardiovascular health.

Abh and AbrB control of Bacillus subtilis antimicrobial gene expression.

The Bacillus subtilis abh gene encodes a protein whose N-terminal domain has 74% identity to the DNA-binding domain of the global regulatory protein AbrB. Strains with a mutation in abh showed alterations in the production of antimicrobial compounds directed against some other Bacillus species and gram-positive microbes. Relative to its wild-type parental strain, the abh mutant was found deficient, enhanced, or unaffected for the production of antimicrobial activity. Using lacZ fusions, we examined the effects of abh upon the expression of 10 promoters known to be regulated by AbrB, including five that transcribe well-characterized antimicrobial functions (SdpC, SkfA, TasA, sublancin, and subtilosin). For an otherwise wild-type background, the results show that Abh plays a negative regulatory role in the expression of four of the promoters, a positive role for the expression of three, and no apparent regulatory role in the expression of the other three promoters. Binding of AbrB and Abh to the promoter regions was examined using DNase I footprinting, and the results revealed significant differences. The transcription of abh is not autoregulated, but it is subject to a degree of AbrB-afforded negative regulation. The results indicate that Abh is part of the complex interconnected regulatory system that controls gene expression during the transition from active growth to stationary phase.