Identification of Autophagy-Related Gene 7 and Autophagic Cell Death in the Planarian Dugesia japonica

Abstract

Planarians undergo continuous body size remodeling under starvation or during regeneration. This process likely involves autophagy and autophagic cell death, but this hypothesis is supported by few studies. To test this hypothesis, we cloned and characterized autophagy-related gene 7 (Atg7) from the planarian Dugesia japonica (DjAtg7). The full-length cDNA of DjAtg7 measures 2272 bp and includes a 2082-bp open reading frame encoding 693 amino acids with a molecular weight of 79.06 kDa. The deduced amino acid sequence of DjAtg7 contains a conserved ATP-binding site and a catalytic active site of an E1-like enzyme belonging to the ATG7 superfamily. DjAtg7 transcripts are mainly expressed in intestinal tissues of the intact animals. After amputation, DjAtg7 was highly expressed at the newly regenerated intestinal branch on days 3–7 of regeneration and in the old tissue of the distal intestinal branch on day 10 of regeneration. However, knockdown of DjAtg7 by RNAi did not affect planarian regeneration and did not block autophagosome formation, which indicates that autophagy is more complex than previously expected. Interestingly, TEM clearly confirmed that autophagy and autophagic cell death occurred in the old tissues of the newly regenerated planarians and clearly revealed that the dying cell released vesicles containing cellular cytoplasmic contents into the extracellular space. Therefore, the autophagy and autophagic cell death that occurred in the old tissue not only met the demand for body remodeling but also met the demand for energy supply during planarian regeneration. Collectively, our work contributes to the understanding of autophagy and autophagic cell death in planarian regeneration and body remodeling.

Autophagy-related Djatg8 is required for remodeling in planarians Dugesia japonica

“…Autophagy is an evolutionary conserved mechanism that occurs continuously to maintain the homeostasis of all tissues (González-Estévez and Saló, 2010). During planarian regeneration, they undergo continuous body remodeling and no food can be taken until the formation of a new functional pharynx (Ma et al, 2018a). Therefore, cell death caused by autophagy is necessary as a natural consequence of restricted food availability to maintain tissue homeostasis (González-Estévez, 2008, 2009; González-Estévez and Saló, 2010; Reddien, 2018).…”

“…Recent reports have shown that autophagy occurs in newly regenerated planarians on days 7 and 10 after amputation through TEM investigation (Ma et al, 2018a). Interestingly, the remodeling process can occur by autophagy activation in Polycelis tenuis and only two peaks for acid phosphatase activity (autophagy marker) were observed (Bowen et al, 1982).…”

“…Planarians have been recognized as a classic model to study tissue regeneration due to their amazing characteristics (Saló, 2006; González-Estévez, 2009). During regeneration, planarians undergo continuous body remodeling and no food can be taken until the formation of a new functional pharynx (Ma et al, 2018a). The question in regeneration is how exactly the energy is supplied.…”

“…In Caenorhabditis elegans , the Atg8 homolog ( lgg-1 ) is essential for embyro development, and atg-3 and atg-7 are required for LGG-1 conjugation to PE, which plays a key role in autophagy (Zhang et al, 2015). However, more than 10 years have lapsed since Gonzalez-Estevez (González-Estévez, 2008, 2009) proposed that planarians could serve as a new model organism for studies on autophagy and limited research on autophagy has been performed at the molecular level (Ma et al, 2018a). It has not been shown that Atg in planarians can affect planarian regeneration.…”

Autophagy and its role in regeneration and remodeling within invertebrate

“…Attention has been shifted from higher eukaryotes (e.g. yeast) to invertebrates in identifying the cellular basis of autophagy and the homologs of ATGs [ 52 – 54 ]. During evolution, ATGs have been duplicated and lost, thus resulting in the extinction and expansion of some subfamilies of autophagy-related genes.…”