Omegasome

Omegasomes were first discovered by researchers in 2008 as specialized structures that play a key role in the formation of autophagosomes,^[5] the central organelle in macroautophagy.^[6] In a study observing the movement and localization of double FYVE domain-containing protein 1 (DFCP1), a phosphatidylinositol 3-phosphate (PI(3)P) binding marker, researchers found that this protein would localize to a ring-shaped structure on the endoplasmic reticulum.^[1] Because of their ring-shaped morphology, which resembled the Greek capital letter Omega (Ω), researchers named these structures omegasomes. It was also found that omegasomes are enriched with PI(3)P, a phospholipid that plays an important role in membrane signaling and trafficking.^[7] PI(3)P is produced by class III phosphatidylinositol 3-kinase complexes (PI3KC3), primarily involving Vps34 and Beclin1, and researchers found that its recognition is crucial for the formation of omegasomes.^[8] Further examination of the omegasomes using live imaging indicated that there is a dynamic connection between the omegasomes and the endoplasmic reticulum, leading researchers to conclude that the omegasomes serve as an interface between the ER and autophagic machinery.^[1] The discovery of omegasomes has provided a missing link in the understanding of autophagosome biogenesis and autophagy initiation and has opened the door to new avenues of research where autophagy may be involved. Subsequent studies have already revealed more about omegasome maturation, their interactions with other organelles, and their roles in more selective autophagy processes, such as mitophagy and xenophagy.^[7][9] Additionally, omegasomes will be of particular interest in future research of conditions such as cancer, neurodegenerative diseases, and various infections where autophagy plays an important role.^[10]

Omegasomes have a very distinct morphology, as they appear as ring-like or cup-shaped protrusions that resemble the Greek capital letter Omega. This unique structure is what gives omegasomes their name. These protrusions extend from the ER membrane, and the structures typically have a diameter of about 1.0 micrometers.^[11] DFCP1 can be used to visualize these structures as it binds to PI(3)P through its FYVE domains and accumulates at the omegasome formation site, outlining its shape.^[11] The formation of omegasomes is a very dynamic process that occurs within three minutes of autophagy induction.^[5] Before omegasome formation begins, the mechanistic target of rapamycin complex 1 (mTORC1) must be inactivated due to amino acid starvation or the activation of AMP-activated protein kinase (AMPK) due to glucose starvation. The inactivation of mTORC1 leads to the activation of the UNC51-like kinase (ULK1) complex, and the activity of ULK1 promotes autophagy.^[5] ULK1 and PI3KC complexes are recruited to the ER and catalyze the production of PI(3)P, initiating the formation of omegasomes.^[5] The growth of the omegasome is sustained by the production of PI(3)P,^[12] and this step in omegasome formation is why they are so enriched with PI(3)P. Once formed, omegasomes serve as a scaffolding for the nucleation of phagophores, which are a precursor to autophagosomes.^[12]

Omegasomes serve as an important intermediate in the formation of autophagosomes, which are the primary organelle in a process known as autophagy. Autophagy (from Greek words for "self" and "eating") is a process of digesting or degrading cytoplasmic molecules (proteins, lipids, sugars and organelles). Macroautophagy is the main autophagic pathway, used primarily to eradicate damaged cell organelles such as mitochondria,^[13] ribosomes, etc., which helps in supplying amino acids and energy to the cells, and maintains longevity.^[14] Omegasomes, enriched with PI(3)P and PI(3)P-binding proteins, are positioned on the ER to serve as a scaffolding for the nucleation of phagophores, a double-membraned sequestering structure that matures into an autophagosome.^[15][16] Additionally, omegasomes attract the effectors needed to target PI3P, while also ensuring that the autophagosomal membranes fuse with the double membrane vesicles and promote autophagosome formation.^[4] ATG9 vesicles, which come from the Golgi apparatus, have been proposed as the membrane seed for the phagophore formation, and the contact between the omegasome and the phagophore are initiated by these ATG9-positive seed vesicles.^[11] The omegasome remains present at the opening of the sack-like phagophore while items destined for degradation by macroautophagy are loaded into the phagophore. There are specific receptor proteins that recruit items to the phagophore.^[17] The phagophore expands to accommodate the items, until the omegasome is closed to produce the roughly spherical autophagosome. DFCP1's ATPase activity is believed to play a role in detaching the mature autophagosome from the omegasome, and autophagocytosis associated protein Atg3 and other proteins appear to be required as well.^[5] Additionally, collections of thin tubules at the junction between omegasome and phagophore and actin appear to be involved.^[18][19][20] Ultimately, omegasomes play a vital role in autophagy, and the omegasome regulation of this pathway provides a smooth transition of autophagosome formation and enrichment of nutrients in the cells.^[16]