cTAGE5/MEA6 plays a critical role in neuronal cellular components trafficking and brain development

Normal neural development is essential for the formation of neuronal networks and brain function. Cutaneous T cell lymphoma-associated antigen 5 (cTAGE5)/meningioma expressed antigen 6 (MEA6) plays a critical role in the secretion of proteins. However, its roles in the transport of nonsecretory cellular components and in brain development remain unknown. Here, we show that cTAGE5/MEA6 is important for brain development and function. Conditional knockout of cTAGE5/MEA6 in the brain leads to severe defects in neural development, including deficits in dendrite outgrowth and branching, spine formation and maintenance, astrocyte activation, and abnormal behaviors. We reveal that loss of cTAGE5/MEA6 affects the interaction between the coat protein complex II (COPII) components, SAR1 and SEC23, leading to persistent activation of SAR1 and defects in COPII vesicle formation and transport from the endoplasmic reticulum to the Golgi, as well as disturbed trafficking of membrane components in neurons. These defects affect not only the transport of materials required for the development of dendrites and spines but also the signaling pathways required for neuronal development. Because mutations in cTAGE5/MEA6 have been found in patients with Fahr's disease, our study potentially also provides insight into the pathogenesis of this disorder.

Regulation of the Sar1 GTPase Cycle Is Necessary for Large Cargo Secretion from the Endoplasmic Reticulum

Proteins synthesized within the endoplasmic reticulum (ER) are transported to the Golgi via coat protein complex II (COPII)-coated vesicles. The formation of COPII-coated vesicles is regulated by the GTPase cycle of Sar1. Activated Sar1 is recruited to ER membranes and forms a pre-budding complex with cargoes and the inner-coat complex. The outer-coat complex then stimulates Sar1 inactivation and completes vesicle formation. The mechanisms of forming transport carriers are well-conserved among species; however, in mammalian cells, several cargo molecules such as collagen, and chylomicrons are too large to be accommodated in conventional COPII-coated vesicles. Thus, special cargo-receptor complexes are required for their export from the ER. cTAGE5/TANGO1 complexes and their isoforms have been identified as cargo receptors for these macromolecules. Recent reports suggest that the cTAGE5/TANGO1 complex interacts with the GEF and the GAP of Sar1 and tightly regulates its GTPase cycle to accomplish large cargo secretion.

Mechanisms for exporting large-sized cargoes from the endoplasmic reticulum

Cargo proteins exported from the endoplasmic reticulum to the Golgi apparatus are typically transported in coat protein complex II (COPII)-coated vesicles of 60–90 nm diameter. Several cargo molecules including collagens and chylomicrons form structures that are too large to be accommodated by these vesicles, but their secretion still requires COPII proteins. Here, we first review recent progress on large cargo secretions derived especially from animal models and human diseases, which indicate the importance of COPII proteins. We then discuss the recent isolation of specialized factors that modulate the process of COPII-dependent cargo formation to facilitate the exit of large-sized cargoes from the endoplasmic reticulum. Based on these findings, we propose a model that describes the importance of the GTPase cycle for secretion of oversized cargoes. Next, we summarize reports that describe the structures of COPII proteins and how these results provide insight into the mechanism of assembly of the large cargo carriers. Finally, we discuss what issues remain to be solved in the future.

Vesicle-mediated export from the ER: COPII coat function and regulation

Vesicle trafficking from the endoplasmic reticulum (ER) is a vital cellular process in all eukaryotes responsible for moving secretory cargoes from the ER to the Golgi apparatus. To accomplish this feat, the cell employs a set of conserved cytoplasmic coat proteins - the coat protein II (COPII) complex - that recruit cargo into nascent buds and deform the ER membrane to drive vesicle formation. While our understanding of COPII coat mechanics has developed substantially since its discovery, we have only recently begun to appreciate the factors that regulate this complex and, in turn, ER-to-Golgi trafficking. Here, we describe these factors and their influences on COPII vesicle formation. Properties intrinsic to the GTP cycle of the coat, as well as coat structure, have critical implications for COPII vesicle trafficking. Extrinsic factors in the cytosol can modulate COPII activity through direct interaction with the coat or with scaffolding components, or by changing composition of the ER membrane. Further, lumenal and membrane-bound cargoes and cargo receptors can influence COPII-mediated trafficking in equally profound ways. Together, these factors work in concert to ensure proper cargo movement in this first step of the secretory pathway. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.