The invariant chain (Ii, also known as CD74) is a multifunctional regulator of adaptive immune responses and is responsible for sorting major histocompatibility complex class I and class II (MHCI... Show moreThe invariant chain (Ii, also known as CD74) is a multifunctional regulator of adaptive immune responses and is responsible for sorting major histocompatibility complex class I and class II (MHCI and MHCII, respectively) molecules, as well as other Ii-associated molecules, to a specific endosomal pathway. When Ii is expressed, endosomal maturation and proteolytic degradation of proteins are delayed and, in non-antigen presenting cells, the endosomal size increases, but the molecular mechanisms underlying this are not known. We identified that a SNARE, Vti1b, is essential for regulating these Ii-induced effects. Vti1b binds to Ii and is localized at the contact sites of fusing Ii-positive endosomes. Furthermore, truncated Ii lacking the cytoplasmic tail, which is not internalized from the plasma membrane, relocates Vti1b to the plasma membrane. Knockout of Ii in an antigen-presenting cell line was found to speed up endosomal maturation, whereas silencing of Vti1b inhibits the Ii-induced maturation delay. Our results suggest that Ii, by interacting with the SNARE Vti1b in antigen-presenting cells, directs specific Ii-associated SNARE-mediated fusion in the early part of the endosomal pathway that leads to a slower endosomal maturation for efficient antigen processing and MHC antigen loading. Show less
Late endosomal transport is disrupted in several diseases such as Niemann-Pick type C, ARC syndrome and Alzheimer__s disease. This thesis describes the regulation of late endosomal dynamics by... Show moreLate endosomal transport is disrupted in several diseases such as Niemann-Pick type C, ARC syndrome and Alzheimer__s disease. This thesis describes the regulation of late endosomal dynamics by cholesterol and multi-protein complexes. We find that cholesterol acts as a cellular tomtom that steers the direction of late endosomal transport by dynein-motors, and that this process is disrupted in Niemann-Pick disease. Furthermore cholesterol also regulates tethering via the multi-subunit HOPS complex. Homologues of this complex are mutated in ARC syndrome patients and the cellular consequences are described in this thesis. We find that the cholesterol sensing proteins ORP1L and MLN64 are major regulators of late endosomal transport, and that this transport is further regulated by the endoplasmatic reticulum. Show less
