PAR3 and aPKC regulate Golgi organization through CLASP2 phosphorylation to generate cell polarity

MARK2 regulates Golgi apparatus reorientation by phosphorylation of CAMSAP2 in directional cell migratio

The reorientation of the Golgi apparatus is crucial for cell migration and is regulated by multipolarity signals. A number of non-centrosomal microtubules anchor at the surface of the Golgi apparatus and play a vital role in the Golgi reorientation, but how the Golgi are regulated by polarity signals remains unclear. Calmodulin-regulated spectrin-associated protein 2 (CAMSAP2) is a protein that anchors microtubules to the Golgi, a cellular organelle. Our research indicates that CAMSAP2 is dynamically localized at the Golgi during its reorientation processing. Further research shows that CAMSAP2 is potentially regulated by a polarity signaling molecule called MARK2, which interacts with CAMSAP2. We used mass spectrometry to find that MARK2 phosphorylates CAMSAP2 at serine-835, which affects its interaction with the Golgi-associated protein USO1 but not with CG-NAP or CLASPs. This interaction is critical for anchoring microtubules to the Golgi during cell migration, altering microtubule polarity distribution, and aiding Golgi reorientation. Our study reveals an important signaling pathway in Golgi reorientation during cell migration, which can provide insights for research in cancer cell migration, immune response, and targeted drug development.

CLASP2 safeguards hematopoietic stem cell properties during mouse and fish development

Hematopoietic stem cells (HSCs) express a large variety of cell surface receptors that are associated with acquisition of self-renewal and multipotent properties. Correct expression of these receptors depends on a delicate balance between cell surface trafficking, recycling, and degradation and is controlled by the microtubule network and Golgi apparatus, whose roles have hardly been explored during embryonic/fetal hematopoiesis. Here we show that, in the absence of CLASP2, a microtubule-associated protein, the overall production of HSCs is reduced, and the produced HSCs fail to self-renew and maintain their stemness throughout mouse and zebrafish development. This phenotype can be attributed to decreased cell surface expression of the hematopoietic receptor c-Kit, which originates from increased lysosomal degradation in combination with a reduction in trafficking to the plasma membrane. A dysfunctional Golgi apparatus in CLASP2-deficient HSCs seems to be the underlying cause of the c-Kit expression and signaling imbalance.

Molecular mechanisms of cell polarity in a range of model systems and in migrating neurons

Cell polarity is defined as the asymmetric distribution of cellular components along an axis. Most cells, from the simplest single-cell organisms to highly specialized mammalian cells, are polarized and use similar mechanisms to generate and maintain polarity. Cell polarity is important for cells to migrate, form tissues, and coordinate activities. During development of the mammalian cerebral cortex, cell polarity is essential for neurogenesis and for the migration of newborn but as-yet undifferentiated neurons. These oriented migrations include both the radial migration of excitatory projection neurons and the tangential migration of inhibitory interneurons. In this review, I will first describe the development of the cerebral cortex, as revealed at the cellular level. I will then define the core molecular mechanisms - the Par/Crb/Scrib polarity complexes, small GTPases, the actin and microtubule cytoskeletons, and phosphoinositides/PI3K signaling - that are required for asymmetric cell division, apico-basal and front-rear polarity in model systems, including C elegans zygote, Drosophila embryos and cultured mammalian cells. As I go through each core mechanism I will explain what is known about its importance in radial and tangential migration in the developing mammalian cerebral cortex.

Identification of CDC42 Effectors Operating in FGD1-Dependent Trafficking at the Golgi

Loss of function mutations in the FGD1 gene cause a rare X-linked disease, faciogenital dysplasia (FGDY, also known as Aarskog-Skott syndrome), which is associated with bone and urogenital abnormalities. The FGD1 gene encodes à CDC42-specific guanine nucleotide exchange factor. The mutations are frequently located in the DH module of FGD1 preventing its transformation to the active form. We previously reported that Golgi-associated FGD1 regulates post-Golgi transport of some conventional and bone-specific proteins in a CDC42-dependent manner. However, the downstream targets of FGD1/CDC42 signaling that operate to support transport from the Golgi remain elusive. Here, we demonstrate that Golgi-localized CDC42 effectors might be involved in FGD1-mediated post-Golgi transport, probably through coordination of Golgi membrane and cytoskeleton dynamics. Overexpression of effector-specific CDC42 mutants (exhibiting preferential affinities for PAK1, IQGAP1, N-WASP, or PAR6) only partially rescue membrane trafficking in FGD1-deficient cells, indicating that the orchestrated activities of several downstream targets of CDC42 are required to support FGD1-mediated export from the Golgi. Our findings provide new insights into understanding the molecular mechanisms behind FGD1/CDC42-dependent transport events and uncover new targets whose potential might be explored for correction of membrane trafficking in FGDY.

Distinct Functions for Mammalian CLASP1 and -2 During Neurite and Axon Elongation

Mammalian cytoplasmic linker associated protein 1 and -2 (CLASP1 and -2) are microtubule (MT) plus-end tracking proteins that selectively stabilize MTs at the edge of cells and that promote MT nucleation and growth at the Golgi, thereby sustaining cell polarity. In vitro analysis has shown that CLASPs are MT growth promoting factors. To date, a single CLASP1 isoform (called CLASP1α) has been described, whereas three CLASP2 isoforms are known (CLASP2α, -β, and -γ). Although CLASP2β/γ are enriched in neurons, suggesting isoform-specific functions, it has been proposed that during neurite outgrowth CLASP1 and -2 act in a redundant fashion by modulating MT dynamics downstream of glycogen synthase kinase 3 (GSK3). Here, we show that in differentiating N1E-115 neuroblastoma cells CLASP1 and CLASP2 differ in their accumulation at MT plus-ends and display different sensitivity to GSK3-mediated phosphorylation, and hence regulation. More specifically, western blot (WB) analysis suggests that pharmacological inhibition of GSK3 affects CLASP2 but not CLASP1 phosphorylation and fluorescence-based microscopy data show that GSK3 inhibition leads to an increase in the number of CLASP2-decorated MT ends, as well as to increased CLASP2 staining of individual MT ends, whereas a reduction in the number of CLASP1-decorated ends is observed. Thus, in N1E-115 cells CLASP2 appears to be a prominent target of GSK3 while CLASP1 is less sensitive. Surprisingly, knockdown of either CLASP causes phosphorylation of GSK3, pointing to the existence of feedback loops between CLASPs and GSK3. In addition, CLASP2 depletion also leads to the activation of protein kinase C (PKC). We found that these differences correlate with opposite functions of CLASP1 and CLASP2 during neuronal differentiation, i.e., CLASP1 stimulates neurite extension, whereas CLASP2 inhibits it. Consistent with knockdown results in N1E-115 cells, primary Clasp2 knockout (KO) neurons exhibit early accelerated neurite and axon outgrowth, showing longer axons than control neurons. We propose a model in which neurite outgrowth is fine-tuned by differentially posttranslationally modified isoforms of CLASPs acting at distinct intracellular locations, thereby targeting MT stabilizing activities of the CLASPs and controlling feedback signaling towards upstream kinases. In summary, our findings provide new insight into the roles of neuronal CLASPs, which emerge as regulators acting in different signaling pathways and locally modulating MT behavior during neurite/axon outgrowth.

Rosiglitazone affects lumen formation in MDCKII cell through regulating apico-basal polarity

This study aimed to investigate the potential mechanisms underlying the effects of Rosiglitazone on the apico-basal polarity in renal epithelial cells. 3D-MDCK model was used to study the lumen formation and localization of polarity proteins at the early stage of the establishment of the apico-basal polarity. The calcium switch model, immunofluorescence staining and measurement of transmembrane electrical impedance are employed to investigate the epithelial apico-basal polarity including the development and maintenance of apical domains and the formation of tight junction. MDCKII cells were cultured with 20 uM rosiglitazone or DMSO. Results showed Rosiglitazone reduced the percentage of single central lumen cysts, but the percentage of multiple lumen cysts increased. At the early stage of MDCKII cysts (2-5 cells), Rosiglitazone induced mislocalization of apical and basolateral membrane proteins. In the repolarization process of MDCKII cell induced by a calcium switch (CS), Rosiglitazone delayed the apical membrane domain development in the early phase of cell polarization; while during the maintenance phase of cell polarity, the apical domain retention was significantly affected by Rosiglitazone. Rosiglitazone significantly delayed the formation of tight junctions (TJs); 24 h after CS, however, there were no apparent differences between control group and Rosiglitazone group; the development of transepithelial electrical resistance (TER) was significantly disturbed in Rosiglitazone group. This study shows Rosiglitazone may affect the development and maintenance of apical domains and the formation of TJs disturbs apical protein delivery to the plasma membrane, eventually leading to the abnormal apico-basal polarity, which affects lumen formation in MDCKII cells.

Characterization of the CLASP2 Protein Interaction Network Identifies SOGA1 as a Microtubule-Associated Protein

CLASP2 is a microtubule-associated protein that undergoes insulin-stimulated phosphorylation and co-localization with reorganized actin and GLUT4 at the plasma membrane. To gain insight to the role of CLASP2 in this system, we developed and successfully executed a streamlined interactome approach and built a CLASP2 protein network in 3T3-L1 adipocytes. Using two different commercially available antibodies for CLASP2 and an antibody for epitope-tagged, overexpressed CLASP2, we performed multiple affinity purification coupled with mass spectrometry (AP-MS) experiments in combination with label-free quantitative proteomics and analyzed the data with the bioinformatics tool Significance Analysis of Interactome (SAINT). We discovered that CLASP2 coimmunoprecipitates (co-IPs) the novel protein SOGA1, the microtubule-associated protein kinase MARK2, and the microtubule/actin-regulating protein G2L1. The GTPase-activating proteins AGAP1 and AGAP3 were also enriched in the CLASP2 interactome, although subsequent AGAP3 and CLIP2 interactome analysis suggests a preference of AGAP3 for CLIP2. Follow-up MARK2 interactome analysis confirmed reciprocal co-IP of CLASP2 and revealed MARK2 can co-IP SOGA1, glycogen synthase, and glycogenin. Investigating the SOGA1 interactome confirmed SOGA1 can reciprocal co-IP both CLASP2 and MARK2 as well as glycogen synthase and glycogenin. SOGA1 was confirmed to colocalize with CLASP2 and with tubulin, which identifies SOGA1 as a new microtubule-associated protein. These results introduce the metabolic function of these proposed novel protein networks and their relationship with microtubules as new fields of cytoskeleton-associated protein biology.

Par3 integrates Tiam1 and phosphatidylinositol 3-kinase signaling to change apical membrane identity

Pathogens can alter epithelial polarity by recruiting polarity proteins to the apical membrane, but how a change in protein localization is linked to polarity disruption is not clear. In this study, we used chemically induced dimerization to rapidly relocalize proteins from the cytosol to the apical surface. We demonstrate that forced apical localization of Par3, which is normally restricted to tight junctions, is sufficient to alter apical membrane identity through its interactions with phosphatidylinositol 3-kinase (PI3K) and the Rac1 guanine nucleotide exchange factor Tiam1. We further show that PI3K activity is required upstream of Rac1, and that simultaneously targeting PI3K and Tiam1 to the apical membrane has a synergistic effect on membrane remodeling. Thus, Par3 coordinates the action of PI3K and Tiam1 to define membrane identity, revealing a signaling mechanism that can be exploited by human mucosal pathogens.

Emerging role of Cdc42-specific guanine nucleotide exchange factors as regulators of membrane trafficking in health and disease

It is widely accepted that the Golgi complex operates as a main sorting station in the biosynthetic pathway. On the other hand, the Golgi complex harbors numerous signaling molecules that generate the platform for the coordination of the transduction of specific signals and of membrane transport events. A part of these processes, which require the complex integration of transport-, cytoskeleton- and polarity-associated mechanisms, is tightly regulated by molecular machineries comprising guanine nucleotide exchange factors (GEF) and their down-stream effectors, such as the small GTPase Cdc42. Dysfunction of several Cdc42-specific GEFs has been shown to cause a number of human diseases, which are associated with impaired intracellular trafficking at the level of the Golgi complex as well as in other compartments. Here we briefly overview how mutations in Cdc42-specific GEFs have an impact on the organization of intracellular trafficking fluxes and how such trafficking aberrations could be associated with a number of human disorders.

Transport Vesicle Tethering at the Trans Golgi Network: Coiled Coil Proteins in Action

The Golgi complex is decorated with so-called Golgin proteins that share a common feature: a large proportion of their amino acid sequences are predicted to form coiled-coil structures. The possible presence of extensive coiled coils implies that these proteins are highly elongated molecules that can extend a significant distance from the Golgi surface. This property would help them to capture or trap inbound transport vesicles and to tether Golgi mini-stacks together. This review will summarize our current understanding of coiled coil tethers that are needed for the receipt of transport vesicles at the trans Golgi network (TGN). How do long tethering proteins actually catch vesicles? Golgi-associated, coiled coil tethers contain numerous binding sites for small GTPases, SNARE proteins, and vesicle coat proteins. How are these interactions coordinated and are any or all of them important for the tethering process? Progress toward understanding these questions and remaining, unresolved mysteries will be discussed.

PAR3-aPKC regulates Tiam1 by modulating suppressive internal interactions

The Rac1 activator Tiam1 is inhibited by internal interactions. Phosphorylation of Tiam1 by aPKC lends bias to an “open” conformation, potentiating its activation and allowing for its localization through protein–protein interactions.

Tiam1 is one of the most extensively analyzed activators of the small GTPase Rac. However, fundamental aspects of its regulation are poorly understood. Here we demonstrate that Tiam1 is functionally suppressed by internal interactions and that the PAR complex participates in its full activation. The N-terminal region of Tiam1 binds to the protein-binding and catalytic domains to inhibit its localization and activation. Atypical PKCs phosphorylate Tiam1 to relieve its intramolecular interactions, and the subsequent stabilization of its interaction with PAR3 allows it to exert localized activity. By analyzing Tiam1 regulation by PAR3-aPKC within the context of PDGF signaling, we also show that PAR3 directly binds PDGF receptor β. Thus we provide the first evidence for the negative regulation of Tiam1 by internal interactions, elucidate the nature of Tiam1 regulation by the PAR complex, and reveal a novel role for the PAR complex in PDGF signaling.

Nucleation and Dynamics of Golgi-derived Microtubules

Integrity of the Golgi apparatus requires the microtubule (MT) network. A subset of MTs originates at the Golgi itself, which in this case functions as a MT-organizing center (MTOC). Golgi-derived MTs serve important roles in post-Golgi trafficking, maintenance of Golgi integrity, cell polarity and motility, as well as cell type-specific functions, including neurite outgrowth/branching. Here, we discuss possible models describing the formation and dynamics of Golgi-derived MTs. How Golgi-derived MTs are formed is not fully understood. A widely discussed model implicates that the critical step of the process is recruitment of molecular factors, which drive MT nucleation (γ-tubulin ring complex, or γ-TuRC), to the Golgi membrane via specific scaffolding interactions. Based on recent findings, we propose to introduce an additional level of regulation, whereby MT-binding proteins and/or local tubulin dimer concentration at the Golgi helps to overcome kinetic barriers at the initial nucleation step. According to our model, emerging MTs are subsequently stabilized by Golgi-associated MT-stabilizing proteins. We discuss molecular factors potentially involved in all three steps of MT formation. To preserve proper cell functioning, a balance must be maintained between MT subsets at the centrosome and the Golgi. Recent work has shown that certain centrosomal factors are important in maintaining this balance, suggesting a close connection between regulation of centrosomal and Golgi-derived MTs. Finally, we will discuss potential functions of Golgi-derived MTs based on their nucleation site location within a Golgi stack.

Par-3 modulates intestinal epithelial barrier function through regulating intracellular trafficking of occludin and myosin light chain phosphorylation

BACKGROUND

Tight junctions play a critical role in the maintenance of intestinal barrier function. Partitioning-defective protein 3 (Par-3) can regulate intestinal barrier function through the modulation of tight junction assembly and cell polarity. However, the mechanisms are still not fully understood.

METHODS

Adult C57BL/6 mice were treated with dextran sulfate sodium for 7 days, and segments of colon were harvested for immunofluorescent staining of Par-3. Caco-2 intestinal epithelial cells were treated with tumor necrosis factor α (TNF-α) for 24 h, and Par-3 expression was detected by Western blot analysis and immunofluorescence. Additionally, Caco-2 cells were treated with Par-3 small interfering RNA, and altered expression and subcellular localization of tight junction proteins were studied by Western blot analysis and immunofluorescence. Furthermore, the interaction between Par-3 and myosin light chain (MLC) was detected by immunoprecipitation.

RESULTS

Par-3 was downregulated in murine dextran sulfate sodium induced acute inflammation and TNF-α-treated Caco-2 cells. Depletion of Par-3 expression by small interfering RNA delayed intestinal epithelial barrier development in Caco-2 cells. This regulation was due to the redistribution of the tight junction protein occludin rather than the altered total levels of tight junction proteins. Par-3 silencing blocked the trafficking of occludin from or through the Golgi complex to the cell surface, and dramatically induced occludin accumulated at the Golgi complex. Importantly, Par-3 can interact with MLC, and loss of Par-3 upregulated MLC kinase expression and MLC phosphorylation, which contributed to intestinal epithelial barrier dysfunction.

CONCLUSIONS

These results indicate that Par-3 plays an important role in the modulation of intestinal barrier function by regulating delivery of occludin as well as suppression of MLC phosphorylation.