Reciprocal regulation by Elm1 and Gin4 controls septin hourglass assembly and remodeling.

The septin cytoskeleton is extensively regulated by posttranslational modifications, such as phosphorylation, to achieve the diversity of architectures including rings, hourglasses, and gauzes. While many of the phosphorylation events of septins have been extensively studied in the budding yeast Saccharomyces cerevisiae, the regulation of the kinases involved remains poorly understood. Here, we show that two septin-associated kinases, the LKB1/PAR-4-related kinase Elm1 and the Nim1/PAR-1-related kinase Gin4, regulate each other at two discrete points of the cell cycle. During bud emergence, Gin4 targets Elm1 to the bud neck via direct binding and phosphorylation to control septin hourglass assembly and stability. During mitosis, Elm1 maintains Gin4 localization via direct binding and phosphorylation to enable timely remodeling of the septin hourglass into a double ring. This mutual control between Gin4 and Elm1 ensures that septin architecture is assembled and remodeled in a temporally controlled manner to perform distinct functions during the cell cycle.

Elucidating the Synergistic Role of Elm1 and Gin4 Kinases in Regulating Septin Hourglass Assembly.

The septin cytoskeleton is extensively regulated by post-translational modifications such as phosphorylation to achieve the diversity of architectures including rings, hourglass, and gauzes. While many of the phosphorylation events of septins have been extensively studied in the budding yeast Saccharomyces cerevisiae, the regulation of the kinases involved remains poorly understood. Here we show that two septin-associated kinases, the LKB1/PAR-4-related kinase Elm1 and the Nim1/PAR-1-related kinase Gin4, regulate each other at two discrete points of the cell cycle. During bud emergence, Gin4 targets Elm1 to the bud neck via direct binding and phosphorylation to control septin hourglass assembly and stability. During mitosis, Elm1 maintains Gin4 localization via direct binding and phosphorylation to enable timely remodeling of the septin hourglass into a double ring. This unique synergy ensures that septin architecture is assembled and remodeled in a temporally controlled manner to perform distinct functions during the cell cycle.

Bni5 tethers myosin-II to septins to enhance retrograde actin flow and the robustness of cytokinesis.

The collaboration between septins and myosin-II in driving processes outside of cytokinesis remains largely uncharted. Here, we demonstrate that Bni5 in the budding yeast S. cerevisiae interacts with myosin-II, septin filaments, and the septin-associated kinase Elm1 via distinct domains at its N- and C-termini, thereby tethering the mobile myosin-II to the stable septin hourglass at the division site from bud emergence to the onset of cytokinesis. The septin and Elm1-binding domains, together with a central disordered region, of Bni5 control timely remodeling of the septin hourglass into a double ring, enabling the actomyosin ring constriction. The Bni5-tethered myosin-II enhances retrograde actin cable flow, which contributes to the asymmetric inheritance of mitochondria-associated protein aggregates during cell division, and also strengthens cytokinesis against various perturbations. Thus, we have established a biochemical pathway involving septin-Bni5-myosin-II interactions at the division site, which can inform mechanistic understanding of the role of myosin-II in other retrograde flow systems.

Septin Assembly and Remodeling at the Cell Division Site During the Cell Cycle.

The septin family of proteins can assemble into filaments that further organize into different higher order structures to perform a variety of different functions in different cell types and organisms. In the budding yeast Saccharomyces cerevisiae, the septins localize to the presumptive bud site as a cortical ring prior to bud emergence, expand into an hourglass at the bud neck (cell division site) during bud growth, and finally "split" into a double ring sandwiching the cell division machinery during cytokinesis. While much work has been done to understand the functions and molecular makeups of these structures, the mechanisms underlying the transitions from one structure to another have largely remained elusive. Recent studies involving advanced imaging and in vitro reconstitution have begun to reveal the vast complexity involved in the regulation of these structural transitions, which defines the focus of discussion in this mini-review.

The LKB1-like Kinase Elm1 Controls Septin Hourglass Assembly and Stability by Regulating Filament Pairing.

Septins form rod-shaped hetero-oligomeric complexes that assemble into filaments and other higher-order structures, such as rings or hourglasses, at the cell division site in fungal and animal cells [1-4] to carry out a wide range of functions, including cytokinesis and cell morphogenesis. However, the architecture of septin higher-order assemblies and their control mechanisms, including regulation by conserved kinases [5, 6], remain largely unknown. In the budding yeast Saccharomyces cerevisiae, the five mitotic septins (Cdc3, Cdc10, Cdc11, Cdc12, and Shs1) localize to the bud neck and form an hourglass before cytokinesis that acts as a scaffold for proteins involved in multiple processes as well as a membrane-diffusible barrier between the mother and developing bud [7-9]. The hourglass is remodeled into a double ring that sandwiches the actomyosin ring at the onset of cytokinesis [10-13]. How septins are assembled into a highly ordered hourglass structure at the division site [13] is largely unexplored. Here we show that the LKB1-like kinase Elm1, which has been implicated in septin organization [14], cell morphogenesis [15], and mitotic exit [16, 17], specifically associates with the septin hourglass during the cell cycle and controls hourglass assembly and stability, especially for the daughter half, by regulating filament pairing and the functionality of its substrate, the septin-binding protein Bni5. This study illustrates how a protein kinase regulates septin architecture at the filament level and suggests that filament pairing is a highly regulated process during septin assembly and remodeling in vivo.

Architecture, remodeling, and functions of the septin cytoskeleton.

The septin family of proteins has fascinated cell biologists for decades due to the elaborate architecture they adopt in different eukaryotic cells. Whether they exist as rings, collars, or gauzes in different cell types and at different times in the cell cycle illustrates a complex series of regulation in structure. While the organization of different septin structures at the cortex of different cell types during the cell cycle has been described to various degrees, the exact structure and regulation at the filament level are still largely unknown. Recent advances in fluorescent and electron microscopy, as well as work in septin biochemistry, have allowed new insights into the aspects of septin architecture, remodeling, and function in many cell types. This mini-review highlights many of the recent findings with an emphasis on the budding yeast model.

Balloon-Occluded Retrograde Transvenous Obliteration of a Gastric Vascular Malformation: An Innovative Approach to Treatment of a Rare Condition.

Arteriovenous malformations (AVMs) are a high-flow form of a vascular malformation, which can be found anywhere in the body. While historically treated surgically, a multidisciplinary approach utilizing multiple specialties and treatment modalities is now commonly employed. In order to effectively treat an AVM, the nidus must be targeted and eradicated, which can be done via multiple approaches. We present the case of a 43-year-old male with a gastric wall AVM, which was initially incompletely treated using a percutaneous transarterial approach. The gastric AVM was noted to have dominant drainage through a gastrorenal shunt; therefore, Balloon-occluded Retrograde Transvenous Obliteration (BRTO) was utilized to eradicate the AVM nidus. This case illustrates the utility of Interventional Radiology, specifically BRTO, as another treatment option for challenging AVMs.

Modular elements of the TPR domain in the Mps1 N terminus differentially target Mps1 to the centrosome and kinetochore.

Faithful segregation of chromosomes to two daughter cells is regulated by the formation of a bipolar mitotic spindle and the spindle assembly checkpoint, ensuring proper spindle function. Here we show that the proper localization of the kinase Mps1 (monopolar spindle 1) is critical to both these processes. Separate elements in the Mps1 N-terminal extension (NTE) and tetratricopeptide repeat (TPR) domains govern localization to either the kinetochore or the centrosome. The third TPR (TPR3) and the TPR-capping helix (C-helix) are each sufficient to target Mps1 to the centrosome. TPR3 binds to voltage-dependent anion channel 3, but although this is sufficient for centrosome targeting of Mps1, it is not necessary because of the presence of the C-helix. A version of Mps1 lacking both elements cannot localize to or function at the centrosome, but maintains kinetochore localization and spindle assembly checkpoint function, indicating that TPR3 and the C-helix define a bipartite localization determinant that is both necessary and sufficient to target Mps1 to the centrosome but dispensable for kinetochore targeting. In contrast, elements required for kinetochore targeting (the NTE and first two TPRs) are dispensable for centrosomal localization and function. These data are consistent with a separation of Mps1 function based on localization determinants within the N terminus.

ARHGEF17 sets the timer for retention of Mps1 at kinetochores.

The kinetochore-associated kinase Mps1 controls the spindle assembly checkpoint, but the regulation of its kinetochore recruitment and activity is unclear. In this issue, Isokane et al. (2016. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201408089) show that interaction with and phosphorylation of its substrate, ARHGEF17, regulates Mps1 kinetochore retention, suggesting an autoregulated, timer-like mechanism.

Bronchial carcinoid imaged with cardiac gated, 128-slice, dual-source, flash CT scanner to direct operative management.

Carcinoid is a rare lung cancer that typically presents with a relatively indolent clinical behavior. We present the case of a 32-year-old male with progressive respiratory symptoms, which resulted in the diagnosis of typical bronchial carcinoid. This case shows a novel imaging technique for staging a bronchial carcinoid for determination of optimal management. This case also shows the multidisciplinary approach required for management of patients with carcinoid tumors.

VDAC3 regulates centriole assembly by targeting Mps1 to centrosomes.

Centrioles are duplicated during S-phase to generate the two centrosomes that serve as mitotic spindle poles during mitosis. The centrosomal pool of the Mps1 kinase is important for centriole assembly, but how Mps1 is delivered to centrosomes is unknown. Here we have identified a centrosome localization domain within Mps1 and identified the mitochondrial porin VDAC3 as a protein that binds to this region of Mps1. Moreover, we show that VDAC3 is present at the mother centriole and modulates centriole assembly by recruiting Mps1 to centrosomes.