GRASP65 controls Golgi position and structure during G2/M transition by regulating the stability of microtubules

The Golgi checkpoint: Golgi unlinking during G2 is necessary for spindle formation and cytokinesis

Entry into mitosis requires not only correct DNA replication but also extensive cell reorganization, including the separation of the Golgi ribbon into isolated stacks. To understand the significance of pre-mitotic Golgi reorganization, we devised a strategy to first block Golgi segregation, with the consequent G2-arrest, and then force entry into mitosis. We found that the cells forced to enter mitosis with an intact Golgi ribbon showed remarkable cell division defects, including spindle multipolarity and binucleation. The spindle defects were caused by reduced levels at the centrosome of the kinase Aurora-A, a pivotal spindle formation regulator controlled by Golgi segregation. Overexpression of Aurora-A rescued spindle formation, indicating a crucial role of the Golgi-dependent recruitment of Aurora-A at the centrosome. Thus, our results reveal that alterations of the pre-mitotic Golgi segregation in G2 have profound consequences on the fidelity of later mitotic processes and represent potential risk factors for cell transformation and cancer development.

Activation of Ca2+ phosphatase Calcineurin regulates Parkin translocation to mitochondria and mitophagy in flies

Selective removal of dysfunctional mitochondria via autophagy is crucial for the maintenance of cellular homeostasis. This event is initiated by the translocation of the E3 ubiquitin ligase Parkin to damaged mitochondria, and it requires the Serine/Threonine-protein kinase PINK1. In a coordinated set of events, PINK1 operates upstream of Parkin in a linear pathway that leads to the phosphorylation of Parkin, Ubiquitin, and Parkin mitochondrial substrates, to promote ubiquitination of outer mitochondrial membrane proteins. Ubiquitin-decorated mitochondria are selectively recruiting autophagy receptors, which are required to terminate the organelle via autophagy. In this work, we show a previously uncharacterized molecular pathway that correlates the activation of the Ca2+-dependent phosphatase Calcineurin to Parkin translocation and Parkin-dependent mitophagy. Calcineurin downregulation or genetic inhibition prevents Parkin translocation to CCCP-treated mitochondria and impairs stress-induced mitophagy, whereas Calcineurin activation promotes Parkin mitochondrial recruitment and basal mitophagy. Calcineurin interacts with Parkin, and promotes Parkin translocation in the absence of PINK1, but requires PINK1 expression to execute mitophagy in MEF cells. Genetic activation of Calcineurin in vivo boosts basal mitophagy in neurons and corrects locomotor dysfunction and mitochondrial respiratory defects of a Drosophila model of impaired mitochondrial functions. Our study identifies Calcineurin as a novel key player in the regulation of Parkin translocation and mitophagy.

Split but merge: Golgi fragmentation in physiological and pathological conditions

The Golgi complex is a highly dynamic and tightly regulated cellular organelle with essential roles in the processing as well as the sorting of proteins and lipids. Its structure undergoes rapid disassembly and reassembly during normal physiological processes, including cell division, migration, polarization, differentiation, and cell death. Golgi dispersal or fragmentation also occurs in pathological conditions, such as neurodegenerative diseases, infectious diseases, congenital disorders of glycosylation diseases, and cancer. In this review, current knowledge about both structural organization and morphological alterations in the Golgi in physiological and pathological conditions is summarized together with the methodologies that help to reveal its structure.

Divergent Contribution of the Golgi Apparatus to Microtubule Organization in Related Cell Lines

Membrane trafficking in interphase animal cells is accomplished mostly along the microtubules. Microtubules are often organized radially by the microtubule-organizing center to coordinate intracellular transport. Along with the centrosome, the Golgi often serves as a microtubule-organizing center, capable of nucleating and retaining microtubules. Recent studies revealed the role of a special subset of Golgi-derived microtubules, which facilitates vesicular traffic from this central transport hub of the cell. However, proteins essential for microtubule organization onto the Golgi might be differentially expressed in different cell lines, while many potential participants remain undiscovered. In the current work, we analyzed the involvement of the Golgi complex in microtubule organization in related cell lines. We studied two cell lines, both originating from green monkey renal epithelium, and found that they relied either on the centrosome or on the Golgi as a main microtubule-organizing center. We demonstrated that the difference in their Golgi microtubule-organizing activity was not associated with the well-studied proteins, such as CAMSAP3, CLASP2, GCC185, and GMAP210, but revealed several potential candidates involved in this process.

Structural Organization and Function of the Golgi Ribbon During Cell Division

The Golgi complex has a central role in the secretory traffic. In vertebrate cells it is generally organized in polarized stacks of cisternae that are laterally connected by membranous tubules, forming a structure known as Golgi ribbon. The steady state ribbon arrangement results from a dynamic equilibrium between formation and cleavage of the membrane tubules connecting the stacks. This balance is of great physiological relevance as the unlinking of the ribbon during G2 is required for mitotic entry. A block of this process induces a potent G2 arrest of the cell cycle, indicating that a mitotic “Golgi checkpoint” controls the correct pre-mitotic segregation of the Golgi ribbon. Then, after mitosis onset, the Golgi stacks undergo an extensive disassembly, which is necessary for proper spindle formation. Notably, several Golgi-associated proteins acquire new roles in spindle formation and mitotic progression during mitosis. Here we summarize the current knowledge about the basic principle of the Golgi architecture and its functional relationship with cell division to highlight crucial aspects that need to be addressed to help us understand the physiological significance of the ribbon and the pathological implications of alterations of this organization.

Functional Coordination among the Golgi Complex, the Centrosome and the Microtubule Cytoskeleton during the Cell Cycle

The Golgi complex of mammalian cells is organized in a ribbon-like structure often closely associated with the centrosome during interphase. Conversely, the Golgi complex assumes a fragmented and dispersed configuration away from the centrosome during mitosis. The structure of the Golgi complex and the relative position to the centrosome are dynamically regulated by microtubules. Many pieces of evidence reveal that this microtubule-mediated dynamic association between the Golgi complex and centrosome is of functional significance in cell polarization and division. Here, we summarize findings indicating how the Golgi complex and the centrosome cooperate in organizing the microtubule network for the directional protein transport and centrosome positioning required for cell polarization and regulating fundamental cell division processes.

Resurrecting Golgi proteins to grasp Golgi ribbon formation and self-association under stress

The Golgi complex is an essential organelle of the eukaryotic exocytic pathway. A subfamily of Golgi matrix proteins, called GRASPs, is central in stress-induced unconventional secretion, Golgi dynamics during mitosis/apoptosis, and Golgi ribbon formation. The Golgi ribbon is vertebrate-specific and correlates with the appearance of two GRASP paralogues and two Golgins (GM130/Golgin45), which form specific GRASP-Golgin pairs. The molecular details of their appearance only in Metazoans are unknown. Moreover, despite new functionalities supported by GRASP paralogy, little is known about their structural and evolutionary differences. Here, we used ancestor sequence reconstruction and biophysical/biochemical approaches to assess the evolution of GRASPs structure/dynamics, fibrillation, and how they started anchoring their Golgin partners. Our data showed that a GRASP ancestor anchored Golgins before gorasp gene duplication in Metazoans. After gene duplication, variations within the GRASP binding pocket determined which paralogue would recruit which Golgin. These interactions are responsible for their specific Golgi location and Golgi ribbon appearance. We also suggest that GRASPs have a long-standing capacity to form supramolecular structures, affecting their participation in stress-induced processes.

Cytoskeleton and Associated Proteins: Pleiotropic JNK Substrates and Regulators

This review extensively reports data from the literature concerning the complex relationships between the stress-induced c-Jun N-terminal kinases (JNKs) and the four main cytoskeleton elements, which are actin filaments, microtubules, intermediate filaments, and septins. To a lesser extent, we also focused on the two membrane-associated cytoskeletons spectrin and ESCRT-III. We gather the mechanisms controlling cytoskeleton-associated JNK activation and the known cytoskeleton-related substrates directly phosphorylated by JNK. We also point out specific locations of the JNK upstream regulators at cytoskeletal components. We finally compile available techniques and tools that could allow a better characterization of the interplay between the different types of cytoskeleton filaments upon JNK-mediated stress and during development. This overview may bring new important information for applied medical research.

The Golgi ribbon: mechanisms of maintenance and disassembly during the cell cycle

The Golgi complex (GC) has an essential role in the processing and sorting of proteins and lipids. The GC of mammalian cells is composed of stacks of cisternae connected by membranous tubules to create a continuous network, the Golgi ribbon, whose maintenance requires several core and accessory proteins. Despite this complex structural organization, the Golgi apparatus is highly dynamic, and this property becomes particularly evident during mitosis, when the ribbon undergoes a multistep disassembly process that allows its correct partitioning and inheritance by the daughter cells. Importantly, alterations of the Golgi structure are associated with a variety of physiological and pathological conditions. Here, we review the core mechanisms and signaling pathways involved in both the maintenance and disassembly of the Golgi ribbon, and we also report on the signaling pathways that connect the disassembly of the Golgi ribbon to mitotic entry and progression.