Latrunculin B

Rhizochromulina is a genus of unicellular dictyochophycean algae (Heterokontophyta), comprising a single species R. marina and numerous strains. Recently, we described the first arctic rhizochromuline-Rhizochromulina sp. strain B44. Amoeboid cells of this algae are able to transform into flagellates, and this transition can be triggered by prolonged mechanical disturbance. Thin branching pseudopodia of the neighboring rhizochromuline cells fuse to form a meroplasmodium. The pseudopodia contain microtubules, but do not contain actin microfilaments; actin forms the cytoplasmic cytoskeleton and extends only to the bases of the pseudopodia. Microtubule-driven pseudopodia are characteristic to a plethora of eukaryotes, but the role of microtubular and actin cytoskeleton in locomotion of these organisms remains poorly understood. We conducted a series of experiments where amoeboid cells of Rhizochromulina sp. B44 were treated with either 10 µM nocodazole, 10 µM latrunculin B, or both drugs simultaneously. Cellular locomotion was captured on camera, tracked, and then analyzed with the help of the generalized additive mixed model. The obtained results indicate that both drugs, when applied separately, decrease the motility of the studied cells. Unexpectedly, the combined treatment had the opposite effect, as the cells became more motile. The analysis also revealed a non-linear pattern of relationship between motility of amoeboid cells of rhizochromulines and density of their population.

Actin, the most abundant cellular protein, is essential for maintaining structural organization, mechanical stability and cellular motility. The actin cytoskeleton undergoes continuous ATP-dependent reorganization, incurring significant energy costs through treadmilling. However, experimental quantifications of these energy expenditures, especially in ectotherms, remain scarce. In this study, we assessed the energy costs of actin remodeling in the Pacific oyster Crassostrea [also Magallana] gigas, a marine bivalve, by measuring oxygen consumption in the presence of inhibitors of actin treadmilling (latrunculin B, jasplakinolide and cytochalasin D). Our results indicate that under normal physiological conditions, actin remodeling contributes less than 5% to the cellular energy budget in gill and mantle cells of oysters. Unexpectedly, cytochalasin D induced a marked increase in mitochondrial proton leak, observed both in intact cells and isolated mitochondria, suggesting a connection between actin disorganization and increased mitochondrial maintenance costs. Notably, jasplakinolide and latrunculin B, which inhibit actin treadmilling through different mechanisms from those of cytochalasin D, had no effect on mitochondrial respiration. This suggests that different mechanisms of actin cytoskeleton disruption can lead to distinct cellular outcomes. Given the significant role of proton leak in cellular respiration, these findings suggest that actin dynamics may play a crucial role in regulating mitochondrial metabolism, with broad implications for cellular energy costs. Further studies are needed to elucidate the underlying mechanisms of actin–mitochondria interactions and their broader relevance to the regulation of cellular metabolism in ectothermic species.