The crescent-like Golgi ribbon is shaped by the Ajuba/PRMT5/Aurora-A complex-modified HURP.

BACKGROUND: Golgi apparatus (GA) is assembled as a crescent-like ribbon in mammalian cells under immunofluorescence microscope without knowing the shaping mechanisms. It is estimated that roughly 1/5 of the genes encoding kinases or phosphatases in human genome participate in the assembly of Golgi ribbon, reflecting protein modifications play major roles in building Golgi ribbon. METHODS: To explore how Golgi ribbon is shaped as a crescent-like structure under the guidance of protein modifications, we identified a protein complex containing the scaffold proteins Ajuba, two known GA regulators including the protein kinase Aurora-A and the protein arginine methyltransferase PRMT5, and the common substrate of Aurora-A and PRMT5, HURP. Mutual modifications and activation of PRMT5 and Aurora-A in the complex leads to methylation and in turn phosphorylation of HURP, thereby producing HURP p725. The HURP p725 localizes to GA vicinity and its distribution pattern looks like GA morphology. Correlation study of the HURP p725 statuses and GA structure, site-directed mutagenesis and knockdown-rescue experiments were employed to identify the modified HURP as a key regulator assembling GA as a crescent ribbon. RESULTS: The cells containing no or extended distribution of HURP p725 have dispersed GA membranes or longer GA. Knockdown of HURP fragmentized GA and HURP wild type could, while its phosphorylation deficiency mutant 725A could not, restore crescent Golgi ribbon in HURP depleted cells, collectively indicating a crescent GA-constructing activity of HURP p725. HURP p725 is transported, by GA membrane-associated ARF1, Dynein and its cargo adaptor Golgin-160, to cell center where HURP p725 forms crescent fibers, binds and stabilizes Golgi assembly factors (GAFs) including TRIP11, GRASP65 and GM130, thereby dictating the formation of crescent Golgi ribbon at nuclear periphery. CONCLUSIONS: The Ajuba/PRMT5/Aurora-A complex integrates the signals of protein methylation and phosphorylation to HURP, and the HURP p725 organizes GA by stabilizing and recruiting GAFs to its crescent-like structure, therefore shaping GA as a crescent ribbon. Therefore, the HURP p725 fiber serves a template to construct GA according to its shape. Video Abstract.

The mitosis-regulating and protein-protein interaction activities of astrin are controlled by aurora-A-induced phosphorylation.

Cells display dramatic morphological changes in mitosis, where numerous factors form regulatory networks to orchestrate the complicated process, resulting in extreme fidelity of the segregation of duplicated chromosomes into two daughter cells. Astrin regulates several aspects of mitosis, such as maintaining the cohesion of sister chromatids by inactivating Separase and stabilizing spindle, aligning and segregating chromosomes, and silencing spindle assembly checkpoint by interacting with Src kinase-associated phosphoprotein (SKAP) and cytoplasmic linker-associated protein-1α (CLASP-1α). To understand how Astrin is regulated in mitosis, we report here that Astrin acts as a mitotic phosphoprotein, and Aurora-A phosphorylates Astrin at Ser(115). The phosphorylation-deficient mutant Astrin S115A abnormally activates spindle assembly checkpoint and delays mitosis progression, decreases spindle stability, and induces chromosome misalignment. Mechanistic analyses reveal that Astrin phosphorylation mimicking mutant S115D, instead of S115A, binds and induces ubiquitination and degradation of securin, which sequentially activates Separase, an enzyme required for the separation of sister chromatids. Moreover, S115A fails to bind mitosis regulators, including SKAP and CLASP-1α, which results in the mitotic defects observed in Astrin S115A-transfected cells. In conclusion, Aurora-A phosphorylates Astrin and guides the binding of Astrin to its cellular partners, which ensures proper progression of mitosis.

Vascular hyperpermeability in response to inflammatory mustard oil is mediated by Rho kinase in mice systemically exposed to arsenic.

The mechanisms underlying vascular dysfunction and cardiovascular disease induced by chronic arsenic exposure are not completely understood. We have previously shown that mice chronically fed sodium arsenite are hypersensitive to the permeability-increasing effects of inflammatory mustard oil. The aim of this study was to investigate whether RhoA/Rho kinase (ROCK)-mediated vascular leakage (hyperpermeability) is induced by mustard oil in mice systemically exposed to arsenic. Animals were orally fed water (control group) or sodium arsenite for 8weeks. We compared the blood pressure and microvessel density of the ears between these two groups. Both control and arsenic groups exhibited a similar mean arterial pressure and microvessel density. Microvessel permeability changes that occurred following mustard oil treatment in the presence of Y-27632, a ROCK inhibitor, were quantified using the Evans blue (EB) technique and vascular labeling with carbon particles. Both the excessive leakiness of EB and the high density of carbon-labeled microvessels upon stimulation with mustard oil in the arsenic-fed mice were reduced by Y-27632 treatment. However, RhoA and ROCK2 expression levels were similar between control and arsenic-fed mice. We further investigated ROCK2 levels and ROCK activity in the ears following mustard oil challenge. ROCK2 levels in mouse ears treated with mustard oil were higher in the arsenic group as compared with the control group. Following mustard oil application, ROCK activity was significantly higher in the arsenic-fed mice compared with the control mice. These findings indicate that increased ROCK2 levels and enhanced ROCK activity are responsible for mustard oil-induced vascular hyperpermeability in arsenic-fed mice.

A rare case of multiple neuromuscular variations in the axilla and arm.

During the dissection of a 73-year-old embalmed male cadaver, we noted unusual variations in the flexor compartment of the upper limb-bilateral axillary arch muscles, a three-headed biceps brachii muscle with two supernumerary bellies-and variations in the origin of the musculocutaneous and median nerves from the brachial plexus. The morphological and clinical significance of this unique coexistence of multiple neuromuscular variations are discussed.