Role of mTOR in autophagic and lysosomal reactions to environmental stressors in molluscs

Abstract

Lysosomal membrane stability (LMS) has been used in various organisms as a very sensitive biomarker of stress. However, despite the abundance of data about regulation of the autophagic process in mammals, in the invertebrates there is only limited mechanistic understanding. Marine mussels (Mytilus galloprovincialis Lam.) are bivalve molluscs, widely used as models in ecotoxicology and as environmental bioindicators of sea water quality. In order to elucidate this fundamental process, in the present study, mussels were exposed for 3 days to a "priority", ubiquitous environmental contaminant, benzo[a]pyrene (B[a]P) at different concentrations (i.e. 5, 50, 100 μg/L seawater). B[a]P accumulated in lysosomes of digestive tubule epithelial cells (digestive cells) and in enlarged lipid-rich lysosomes (autolysosomes) as detected by immunofluorescence and UV-fluorescence. B[a]P also activated the autophagic process with a marked decrease of LMS and concurrent increase in lysosomal/cytoplasmic volume ratio. Dephosphorylation of mTOR contributes to increased lysosomal membrane permeability and induced autophagy. B[a]P induced a decrease in phosphorylated (active form) mTOR. The probable role of mTOR in cell signalling and the regulation of the cellular responses to the contaminants has been also confirmed in a field study, where there was significant inactivation of mTOR in stressed animals. Statistical and network modelling supported the empirical investigations of autophagy and mTOR; and was used to integrate the mechanistic biomarker data with chemical analysis and DNA damage.

Lysosomes, Autophagy, and Hormesis in Cell Physiology, Pathology, and Age-Related Disease

“…Thus, CMA can be designated a stress-induced pathway that is activated not only by nutrient deprivation but also by mild oxidative stress 44 and exposure to toxic compounds. 14,21,31 This was clearly demonstrated by Massey et al 45 who impaired CMA in mouse cells. The survival rates of these cells were unaffected when maintained under optimum conditions and it was hypothesized that the cells were able to upregulate macro-and microautophagic pathways to sustain protein turnover homeostasis.…”

“…Increasing stress resulted in a decline in connectance, indicative of a loss of complexity and increasing cell injury. 21,29,30 Connectivity associated with autophagy was a significant factor in the increase in connectance induced by mild stress. The outputs from this simulation have since been validated in experimental studies with molluscan hepatopancreas or digestive gland.…”

Identification of the autophagy pathway in a mollusk bivalve, Crassostrea gigas

“…Mollusk bivalves are generally exposed to numerous biotic (pathogens) and abiotic (tide, pollution) stressors. Initial studies demonstrated that the autophagy pathway can play an important role in the response to these different stress factors [12,[15][16][17], highlighting the importance of elucidating the autophagy pathway in bivalve mollusks. In this study, we identified the autophagy pathway by data mining of the Pacific oyster genome [14].…”

Application of a new targeted low density microarray and conventional biomarkers to evaluate the health status of marine mussels: A field study in Sardinian coast, Italy

“…These results may indicate an alteration in the growth of mussels in this area in particular and in highly polluted zones in general. The LMS correlates with the scope for growth (Moore et al, 2006); LMS is now considered in different model organisms to be a prognostic biomarker capable of providing early evidence of future noxious consequences at the population level (Moore et al, 2006;Sforzini et al, 2018).A possible interpretation of these results involves the role of mechanistic target of rapamycin (mTOR), a key element in the regulation of autophagy, protein synthesis, and energy metabolism (Antikainen et al, 2017). In mussels exposed to toxic chemicals, the level of the phosphorylated active form of mTOR was recently demonstrated to decrease strongly; in the same animals, a drastic increase in autophagy was observed (Sforzini et al,2018).…”