Autophagy-related genes atg7 and beclin1 are essential for energy metabolism and survival during the larval-to-juvenile transition stage of zebrafish

High mortality is usually observed during the transition from larvae to juvenile in teleost which is related to the transition from endogenous to exogenous feeding. Autophagy is an evolutionary regulated cellular mechanism highly conserved in eukaryotic organisms to maintain energy homeostasis against stress including starvation. To investigate whether autophagy plays a role during the larval-juvenile transition, we generated atg7 and beclin1 zebrafish mutant lines using CRISPR/Cas9 technology. In this study, both atg7 and beclin1 null zebrafish exhibited a normal body confirmation; nevertheless, they completely died around 15 dpf and 9 dpf respectively. During larval-juvenile transition period, atg7 and beclin1 mutants were unable to cope with the metabolic stress after yolk absorption at 5 dpf and fail to activate autophagy in response to nutrient restriction, and without external feeding, all mutants died nearly at 8 dpf. Dramatic defects in the intestine architecture and metabolic functions in the liver were observed even though providing larvae with an external food supply, suggesting that autophagy isnt only important during yolk depletion but also within food plenty. Treatment with rapamycin, an activator of autophagy, could effectively extend the survival time of both atg7 and beclin1 null zebrafish through lowering the metabolic rate while it couldnt activate autophagy in mutants via the canonical pathway. Our findings provided a molecular evidence for the physiological, histological and metabolic changes that occur during the transition process from the larval to the juvenile stages and the chief role of autophagy on the body metabolism during these turning milestones.\n\nAuthor summaryZebrafish Danio rerio has emergrd one of the most powerful research models for studying genes expression during early embryogenesis and postnatal development. On the basis of the cell mechanisms, Macroautphagy, a natural regulated pathway disassembles unnecessary or dysfunctional components orchestrated by more than 36 autophagy related-genes conserved from yeast to mammals. Among those genes are atg7 and beclin1 which have been proved to play an important role in regulating post natal development in some mammals however their roles during zebrafish development still unedited. During this research, CRISPER/CAS9 were adopted to know atg7 and beclin1 knockout effects on the mutants metabolism during shifting from maternal yolk acquisition to exogenous feeding and the role of autophagy during the larvae to pre-juvenile development. Herein, we found out that larvae couldnt abandon autophagy in both fasting and feeding conditions as larvae died earlier before pre-juvenile development despite feeding declaring the importance of autophagy not only to provide the cell with essential nutrients during starvation but also to get rid of cargos inside the eukaryotic cells. Briefly, if the larvae didnt recycle those cargos due to autophagy perturbations, they will die despite providing suitable conditions including food and acclimatization.