Systems biology of the secretory pathway: what have we learned so far?

Rab7-harboring vesicles are carriers of the transferrin receptor through the biosynthetic secretory pathway

The biosynthetic secretory pathway is particularly challenging to investigate as it is underrepresented compared to the abundance of the other intracellular trafficking routes. Here, we combined the retention using selective hook (RUSH) to a CRISPR-Cas9 gene editing approach (eRUSH) and identified Rab7-harboring vesicles as an important intermediate compartment of the Golgi-to-plasma membrane transport of neosynthesized transferrin receptor (TfR). These vesicles did not exhibit degradative properties and were not associated to Rab6A-harboring vesicles. Rab7A was transiently associated to neosynthetic TfR-containing post-Golgi vesicles but dissociated before fusion with the plasma membrane. Together, our study reveals a role for Rab7 in the biosynthetic secretory pathway of the TfR, highlighting the diversity of the secretory vesicles' nature.

Mechanisms of secretion and spreading of pathological tau protein

Accumulation of misfolded and aggregated forms of tau protein in the brain is a neuropathological hallmark of tauopathies, such as Alzheimer's disease and frontotemporal lobar degeneration. Tau aggregates have the ability to transfer from one cell to another and to induce templated misfolding and aggregation of healthy tau molecules in previously healthy cells, thereby propagating tau pathology across different brain areas in a prion-like manner. The molecular mechanisms involved in cell-to-cell transfer of tau aggregates are diverse, not mutually exclusive and only partially understood. Intracellular accumulation of misfolded tau induces several mechanisms that aim to reduce the cellular burden of aggregated proteins and also promote secretion of tau aggregates. However, tau may also be released from cells physiologically unrelated to protein aggregation. Tau secretion involves multiple vesicular and non-vesicle-mediated pathways, including secretion directly through the plasma membrane. Consequently, extracellular tau can be found in various forms, both as a free protein and in vesicles, such as exosomes and ectosomes. Once in the extracellular space, tau aggregates can be internalized by neighboring cells, both neurons and glial cells, via endocytic, pinocytic and phagocytic mechanisms. Importantly, accumulating evidence suggests that prion-like propagation of misfolding protein pathology could provide a general mechanism for disease progression in tauopathies and other related neurodegenerative diseases. Here, we review the recent literature on cellular mechanisms involved in cell-to-cell transfer of tau, with a particular focus in tau secretion.

Efficient Golgi Forward Trafficking Requires GOLPH3-Driven, PI4P-Dependent Membrane Curvature

Vesicle budding for Golgi-to-plasma membrane trafficking is a key step in secretion. Proteins that induce curvature of the Golgi membrane are predicted to be required, by analogy to vesicle budding from other membranes. Here, we demonstrate that GOLPH3, upon binding to the phosphoinositide PI4P, induces curvature of synthetic membranes in vitro and the Golgi in cells. Moreover, efficient Golgi-to-plasma membrane trafficking critically depends on the ability of GOLPH3 to curve the Golgi membrane. Interestingly, uncoupling of GOLPH3 from its binding partner MYO18A results in extensive curvature of Golgi membranes, producing dramatic tubulation of the Golgi, but does not support forward trafficking. Thus, forward trafficking from the Golgi to the plasma membrane requires the ability of GOLPH3 both to induce Golgi membrane curvature and to recruit MYO18A. These data provide fundamental insight into the mechanism of Golgi trafficking and into the function of the unique Golgi secretory oncoproteins GOLPH3 and MYO18A.

Genome-wide identification of alternative splicing events that regulate protein transport across the secretory pathway

Alternative splicing (AS) strongly increases proteome diversity and functionality in eukaryotic cells. Protein secretion is a tightly-controlled process, especially in a tissue-specific and differentiation-dependent manner. While previous work has focussed on transcriptional and post-translational regulatory mechanisms, the impact of AS on the secretory pathway remains largely unexplored. Here we integrate a published screen for modulators of protein transport and RNA-Seq analyses to identify over 200 AS events as secretion regulators. We confirm that splicing events along all stages of the secretory pathway regulate the efficiency of membrane trafficking using Morpholinos and CRISPR/Cas9. We furthermore show that these events are highly tissue-specific and adapt the secretory pathway during T-cell activation and adipocyte differentiation. Our data substantially advance the understanding of AS functionality, add a new regulatory layer to a fundamental cell biological process and provide a resource of alternative isoforms that control the secretory pathway.

The link between autophagy and secretion: a story of multitasking proteins

The secretory and autophagy pathways can be thought of as the biosynthetic (i.e., anabolic) and degradative (i.e., catabolic) branches of the endomembrane system. In analogy to anabolic and catabolic pathways in metabolism, there is mounting evidence that the secretory and autophagy pathways are intimately linked and that certain regulatory elements are shared between them. Here we highlight the parallels and points of intersection between these two evolutionarily highly conserved and fundamental endomembrane systems. The intersection of these pathways may play an important role in remodeling membranes during cellular stress.

Mitotic inheritance of the Golgi complex and its role in cell division

The Golgi apparatus plays essential roles in the processing and sorting of proteins and lipids, but it can also act as a signalling hub and a microtubule-nucleation centre. The Golgi complex (GC) of mammalian cells is composed of stacks connected by tubular bridges to form a continuous membranous system. In spite of this structural complexity, the GC is highly dynamic, and this feature becomes particularly evident during mitosis, when the GC undergoes a multi-step disassembly process that allows its correct partitioning and inheritance by daughter cells. Strikingly, different steps of Golgi disassembly control mitotic entry and progression, indicating that cells actively monitor Golgi integrity during cell division. Here, we summarise the basic mechanisms and the molecular players that are involved in Golgi disassembly, focussing in particular on recent studies that have revealed the fundamental signalling pathways that connect Golgi inheritance to mitotic entry and progression.

Alteration of Golgi Structure by Stress: A Link to Neurodegeneration?

The Golgi apparatus is well-known for its role as a sorting station in the secretory pathway as well as for its role in regulating post-translational protein modification. Another role for the Golgi is the regulation of cellular signaling by spatially regulating kinases, phosphatases, and GTPases. All these roles make it clear that the Golgi is a central regulator of cellular homeostasis. The response to stress and the initiation of adaptive responses to cope with it are fundamental abilities of all living cells. It was shown previously that the Golgi undergoes structural rearrangements under various stress conditions such as oxidative or osmotic stress. Neurodegenerative diseases are also frequently associated with alterations of Golgi morphology and many stress factors have been described to play an etiopathological role in neurodegeneration. It is however unclear whether the stress-Golgi connection plays a role in neurodegenerative diseases. Using a combination of bioinformatics modeling and literature mining, we will investigate evidence for such a tripartite link and we ask whether stress-induced Golgi arrangements are cause or consequence in neurodegeneration.

Phospholipase C γ1 regulates early secretory trafficking and cell migration via interaction with p115

The role of early secretory trafficking in the regulation of cell motility remains incompletely understood. Here we used a small interfering RNA screen to monitor the effects on structure of the Golgi apparatus and cell migration. Two major Golgi phenotypes were observed-fragmented and small Golgi. The latter exhibited a stronger correlation with a defect in cell migration. Among the small Golgi hits, we focused on phospholipase C γ1 (PLCγ1). We show that PLCγ1 regulates Golgi structure and cell migration independently of its catalytic activity but in a manner that depends on interaction with the tethering protein p115. PLCγ1 regulates the dynamics of p115 in the early secretory pathway, thereby controlling trafficking from the endoplasmic reticulum to the Golgi. Our results uncover a new function of PLCγ1 that is independent of its catalytic function and link early secretory trafficking to the regulation of cell migration.