Autophagy is the process by which cells degrade and recycle their own components. It helps maintain homeostasis which is crucial to proper cell functioning. Among the different subtypes of autophagy, chaperone-mediated autophagy (CMA), which helps to degrade a wide range of proteins and plays an essential role in the regulation of several cell functions, including gene transcription, DNA repair, cell death and survival, cell metabolism and the cell cycle. CMA defects have been linked to human diseases, including neurodegenerative diseases, certain types of cancer, metabolic disorders, and immune system diseases.
CMA plays a functional role in fish
So far, the absence of any identifiable LAMP2A - a necessary and limiting protein for CMA activity - outside of mammals and birds, led to the paradigm that this cellular function is lacking in other vertebrate lineages. In 2018, French researchers carried out a fine-scale transcriptome analysis in fish using the Phylofish database and discovered that several fish species had transcript sequences displaying high homology with the mammalian version of LAMP2A. This finding suggested that CMA did occur in fish and had appeared much earlier during vertebrate evolution than initially thought.
In a new study, these same researchers have found clear evidence for this assertion. By using a fluorescent reporter previously used to track CMA in mammalian cells, they revealed the existence of a CMA-like pathway in a fibroblast cell line of the fish medaka (Oryzias latipes).
Furthermore, in order to address the physiological role of Lamp2a in fish, they generated medaka knockout for the splice variant lamp2a, and found severe alterations in carbohydrate and fat metabolisms, as previously demonstrated in mice deficient for CMA in liver. Taken together, these results show that CMA is not restricted to birds and mammals: it also occurs in fish, where it plays an essential role in metabolic regulation.
This discovery provides an entirely new understanding of the regulation of metabolism in fish and sheds new light on the evolutionary history of CMA.
In aquaculture research, this scientific breakthrough will lead to a better understanding of the mechanisms underlying nutritional modulation of intermediary metabolism, and will provide new metabolic criteria, which can help apply new feeding / breeding strategies in order to optimise fish growth potential while maintaining aquatic product quality and lowering environmental impacts.
It also offers new perspectives in medical research by enabling the use of complementary genetic models, such as zebrafish or medaka, for studying CMA. Dysfunctions in CMA are known to be involved in several diseases, such as Parkinson's disease, diabetes, immune system disorders and certain types of cancer.
The full study, which has been published in Molecular Biology and Evolution, can be accessed by clicking on “Chaperone-Mediated Autophagy in the light of evolution: insight from fish"