The evolution of autophagy proteins – diversification in eukaryotes and potential ancestors in prokaryotes

ABSTRACT Autophagy is a degradative pathway for cytoplasmic constituents, and is conserved across eukaryotes. Autophagy-related (ATG) genes have undergone extensive multiplications and losses in different eukaryotic lineages, resulting in functional diversification and specialization. Notably, even though bacteria and archaea do not possess an autophagy pathway, they do harbor some remote homologs of Atg proteins, suggesting that preexisting proteins were recruited when the autophagy pathway developed during eukaryogenesis. In this Review, we summarize our current knowledge on the distribution of Atg proteins within eukaryotes and outline the major multiplication and loss events within the eukaryotic tree. We also discuss the potential prokaryotic homologs of Atg proteins identified to date, emphasizing the evolutionary relationships and functional differences between prokaryotic and eukaryotic proteins.

Sequential dynein effectors regulate axonal autophagosome motility in a maturation-dependent pathway

Autophagy is a degradative pathway required to maintain neuronal homeostasis. Neuronal autophagosomes form constitutively at the axon terminal and mature via lysosomal fusion during dynein-mediated transport to the soma. How the dynein-autophagosome interaction is regulated during maturation is unknown. Here, we identify a series of handoffs between dynein effectors as autophagosomes transit along the axons of primary neurons. In the distal axon, JIP1 initiates autophagosomal transport, while autophagosomes in the mid-axon require HAP1 and Huntingtin for motility. We demonstrate that HAP1 is a bonafide dynein activator, binding the dynein-dynactin complex via canonical and noncanonical interactions. JIP3 is found on most axonal autophagosomes but specifically regulates the transport of acidified autolysosomes. Inhibiting autophagosomal transport disrupts maturation, while inhibiting autophagosomal maturation perturbs the association and function of dynein effectors. Thus maturation and transport are tightly linked. These results describe a novel maturation-based dynein effector handoff on neuronal autophagosomes that is key to autophagosomal motility, cargo degradation, and the maintenance of axonal health.SummaryNeuronal autophagosomes form in the distal axon and mature via fusion with lysosomes during their dynein-driven transport to the soma. Dynein is regulated on autophagosomes by distinct effector proteins—JIP1, HAP1, and JIP3—depending on location and autophagosomal maturity. In this sequential pathway, transport and maturation state are tightly linked to maintain neuronal health.

The HMGB1-RAGE axis modulates the growth of autophagy-deficient hepatic tumors

Autophagy is an intracellular lysosomal degradative pathway important for tumor surveillance. Autophagy deficiency can lead to tumorigenesis. Autophagy is also known to be important for the aggressive growth of tumors, yet the mechanism that sustains the growth of autophagy-deficient tumors is not known. We previously reported that progression of hepatic tumors developed in autophagy-deficient livers required high mobility group box 1 (HMGB1) that is released from autophagy-deficient hepatocytes. However, the mechanism by which HMGB1 promotes hepatic tumorigenesis is not understood. In this study we examined the pathological features of the hepatic tumors and the mechanism of HMGB1-mediated tumorigenesis using liver-specific autophagy-deficient (Atg7-/-) and Atg7-/-/Hmgb1-/- mice. We found that in Atg7-/- mice the tumors cells were still deficient in autophagy and could also release HMGB1. Histological analysis using cell-specific markers suggested that fibroblast and ductular cells were present only outside the tumor whereas macrophages were present both inside and outside the tumor. Genetic deletion of HMGB1 or one of its receptors, receptor for advanced glycated end product (Rage), retarded liver tumor development. In addition, we found that expression of RAGE was only on ductual cells and Kupffer’s cells but not on hepatoctyes, which suggested that HMGB1 might promote hepatic tumor growth through a paracrine mode that altered the tumor microenvironment. Furthermore, HMGB1 and RAGE enhanced the proliferation capability of the autophagy-deficient hepatocytes and tumors. Finally, RNAseq analysis of the tumors indicated that HMGB1 induced a much broad changes in tumors. In particular, genes related to mitochondrial structures or functions were enriched among those differentially expressed in tumors in the presence or absence of HMGB1, revealing a potential key role of mitochondria in sustaining the growth of autophagy-deficient liver tumors via HMGB1 stimulation.