Understanding amphisomes

Amphisomes are intermediate/hybrid organelles produced through the fusion of endosomes with autophagosomes within cells. Amphisome formation is an essential step during a sequential maturation process of autophagosomes before their ultimate fusion with lysosomes for cargo degradation. This process is highly regulated with multiple protein machineries, such as SNAREs, Rab GTPases, tethering complexes, and ESCRTs, are involved to facilitate autophagic flux to proceed. In neurons, autophagosomes are robustly generated in axonal terminals and then rapidly fuse with late endosomes to form amphisomes. This fusion event allows newly generated autophagosomes to gain retrograde transport motility and move toward the soma, where proteolytically active lysosomes are predominantly located. Amphisomes are not only the products of autophagosome maturation but also the intersection of the autophagy and endo-lysosomal pathways. Importantly, amphisomes can also participate in non-canonical functions, such as retrograde neurotrophic signaling or autophagy-based unconventional secretion by fusion with the plasma membrane. In this review, we provide an updated overview of the recent discoveries and advancements on the molecular and cellular mechanisms underlying amphisome biogenesis and the emerging roles of amphisomes. We discuss recent developments towards the understanding of amphisome regulation as well as the implications in the context of major neurodegenerative diseases, with a comparative focus on Alzheimer's disease and Parkinson's disease. Macroautophagy, hereafter referred to as autophagy, is a major cytosolic degradative system involving sequestration of damaged cellular components and dysfunctional organelles within autophagosomes for subsequent lysosomal clearance [1]. This mechanism relies on dedicated autophagy regulators - the autophagy-related proteins - and can be induced by various stress stimuli such as nutrient deprivation, energy loss, redox condition, hypoxia, as well as the presence of protein aggregates and intracellular pathogens. Upon autophagy activation, an initial step is to form a pre-autophagosomal membrane called phagophore or isolation membrane that is elongated to facilitate cargo engulfment and finally enclosed to form autophagosomes. The outer membrane of autophagosomes thus fuses

Automated summary

Amphisomes are intermediate organelles formed by the fusion of endosomes and autophagosomes, playing crucial roles in material transport and retrograde movement in cells. This process is highly regulated by various protein machineries, especially in neurons, where it facilitates cargo movement from axonal terminals towards the soma.