Mitsunori Kawato

Biochemical characterization of the Rho GTPase-regulated actin assembly by diaphanous-related formins, mDia1 and Daam1, in platelets.

The diaphanous-related formins are actin nucleating and elongating factors. They are kept in an inactive state by an intramolecular interaction between the diaphanous inhibitory domain (DID) and the diaphanous-autoregulatory domain (DAD). It is considered that the dissociation of this autoinhibitory interaction upon binding of GTP-bound Rho to the GTPase binding domain next to DID induces exposure of the FH1-FH2 domains, which assemble actin filaments. Here, we isolated two diaphanous-related formins, mDia1 and Daam1, in platelet extracts by GTP-RhoA affinity column chromatography. We characterized them by a novel assay, where beads coated with the FH1-FH2-DAD domains of either mDia1 or Daam1 were incubated with platelet cytosol, and the assembled actin filaments were observed after staining with rhodamine-phalloidin. Both formins generated fluorescent filamentous structures on the beads. Quantification of the fluorescence intensity of the beads revealed that the initial velocity in the presence of mDia1 was more than 10 times faster than in the presence of Daam1. The actin assembly activities of both FH1-FH2-DADs were inhibited by adding cognate DID domains. GTP-RhoA, -RhoB, and -RhoC, but not GTP-Rac1 or -Cdc42, bound to both mDia1 and Daam1 and efficiently neutralized the inhibition by the DID domains. The association between RhoA and Daam1 was induced by thrombin stimulation in platelets, and RhoA-bound endogenous formins induced actin assembly, which was inhibited by the DID domains of Daam1 and mDia1. Thus, mDia1 and Daam1 are platelet actin assembly factors having distinct efficiencies, and they are directly regulated by Rho GTPases.

Constitutive GDP/GTP Exchange and Secretion-dependent GTP Hydrolysis Activity for Rab27 in Platelets

We have previously demonstrated that Rab27 regulates dense granule secretion in platelets. Here, we analyzed the activation status of Rab27 using the thin layer chromatography method analyzing nucleotides bound to immunoprecipitated Rab27 and the pull-down method quantifying Rab27 bound to the GTP-Rab27-binding domain (synaptotagmin-like protein (Slp)-homology domain) of its specific effector, Slac2-b. We found that Rab27 was predominantly present in the GTP-bound form in unstimulated platelets due to constitutive GDP/GTP exchange activity. The GTP-bound Rab27 level drastically decreased due to enhanced GTP hydrolysis activity upon granule secretion. In permeabilized platelets, increase of Ca2+ concentration induced dense granule secretion with concomitant decrease of GTP-Rab27, whereas in non-hydrolyzable GTP analogue GppNHp (β-γ-imidoguanosine 5′-triphosphate)-loaded permeabilized platelets, the GTP (GppNHp)-Rab27 level did not decrease upon the Ca2+-induced secretion. These data suggested that GTP hydrolysis of Rab27 was not necessary for inducing the secretion. Taken together, Rab27 is maintained in the active status in unstimulated platelets, which could function to keep dense granules in a preparative status for secretion.

Tuberous Sclerosis Tumor Suppressor Complex-like Complexes Act as GTPase-activating Proteins for Ral GTPases

The small GTPases RalA and RalB are multifunctional proteins regulating a variety of cellular processes. Like other GTPases, the activity of Ral is regulated by the opposing effects of guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Although several RalGEFs have been identified and characterized, the molecular identity of RalGAP remains unknown. Here, we report the first molecular identification of RalGAPs, which we have named RalGAP1 and RalGAP2. They are large heterodimeric complexes, each consisting of a catalytic α1 or α2 subunit and a common β subunit. These RalGAP complexes share structural and catalytic similarities with the tuberous sclerosis tumor suppressor complex, which acts as a GAP for Rheb. In vitro GTPase assays revealed that recombinant RalGAP1 accelerates the GTP hydrolysis rate of RalA by 280,000-fold. Heterodimerization was required for this GAP activity. In PC12 cells, knockdown of the β subunit led to sustained Ral activation upon epidermal growth factor stimulation, indicating that the RalGAPs identified here are critical for efficient termination of Ral activation induced by extracellular stimuli. Our identification of RalGAPs will enable further understanding of Ral signaling in many biological and pathological processes.

Regulation of platelet dense granule secretion by the Ral GTPase-exocyst pathway.

Non-hydrolyzable GTP analogues, such as guanosine 5′-(β, γ-imido)triphosphate (GppNHp), induce granule secretion from permeabilized platelets in the absence of increased intracellular Ca2+. Here, we show that the GppNHp-induced dense granule secretion from permeabilized platelets occurred concomitantly with the activation of small GTPase Ral. This secretion was inhibited by the addition of GTP-Ral-binding domain (RBD) of Sec5, which is a component of the exocyst complex known to function as a tethering factor at the plasma membrane for vesicles. We generated an antibody against Sec5-RBD, which abolished the interaction between GTP-Ral and the exocyst complex in vitro. The addition of this antibody inhibited the GppNHp-induced secretion. These data indicate that Ral mediates the GppNHp-induced dense granule secretion from permeabilized platelets through interaction with its effector, the exocyst complex. Furthermore, GppNHp enhanced the Ca2+ sensitivity of dense granule secretion from permeabilized platelets, and this enhancement was inhibited by Sec5-RBD. In intact platelets, the association between Ral and the exocyst complex was induced by thrombin stimulation with a time course similar to that of dense granule secretion and Ral activation. Taken together, our results suggest that the Ral-exocyst pathway participates in the regulation of platelet dense granule secretion by enhancing the Ca2+ sensitivity of the secretion.

Flightless-I (Fli-I) Regulates the Actin Assembly Activity of Diaphanous-related Formins (DRFs) Daam1 and mDia1 in Cooperation with Active Rho GTPase

Eukaryotic cells dynamically reorganize the actin cytoskeleton to regulate various cellular activities, such as cell shape change, cell motility, cytokinesis, and vesicular transport. Diaphanous-related formins (DRFs), such as Daam1 and mDia1, play central roles in actin dynamics through assembling linear actin filaments. It has been reported that the GTP-bound active Rho binds directly to DRFs and partially unleashes the intramolecular autoinhibition of DRFs. However, whether proteins other than Rho involve the regulation of the actin assembly activity of DRFs has been unclear. Here, we show that Flightless-I (Fli-I), a gelsolin family protein essential for early development, binds directly to Daam1 and mDia1. Fli-I enhances the intrinsic actin assembly activity of Daam1 and mDia1 in vitro and is required for Daam1-induced actin assembly in living cells. Furthermore, Fli-I promotes the GTP-bound active Rho-mediated relief of the autoinhibition of Daam1 and mDia1. Thus, Fli-I is a novel positive regulator of Rho-induced linear actin assembly mediated by DRFs.

Kyoto Congestive Heart Failure (KCHF) Study: Rationale and Design.

Over the last decade, major developments in medicine have led to significant changes in the clinical management of heart failure patients. This study was designed to evaluate the recent trends in clinical characteristics, management, and short‐term and long‐term prognosis of patients with acute decompensated heart failure (ADHF) in Japan.

Demographics, Management, and In-Hospital Outcome of Hospitalized Acute Heart Failure Syndrome Patients in Contemporary Real Clinical Practice in Japan ― Observations From the Prospective, Multicenter Kyoto Congestive Heart Failure (KCHF) Registry ―

BACKGROUND There is a scarcity of reports on the clinical characteristics and management practice in contemporary all-comer patients with acute decompensated heart failure (ADHF). Methods and Results: The Kyoto Congestive Heart Failure (KCHF) registry is a prospective observational cohort study enrolling 4,056 consecutive patients who had hospital admission due to ADHF without any exclusion criteria between October 2014 and March 2016 in the 19 participating hospitals in Japan. Baseline characteristics, clinical presentations, management, and in-hospital outcomes were compared between heart failure (HF) with reduced left ventricular ejection fraction (LVEF; HFrEF, LVEF <40%), HF with mid-range LVEF (HFmrEF, LVEF 40-49%), and HF with preserved LVEF (HFpEF, LVEF ≥50%). Of the 4,041 patients with documented LVEF, 1,744 (43%) had HFpEF; 746 (19%), HFmrEF; and 1,551 (38%), HFrEF. The median age was 80 years (IQR, 72-86 years) in the entire population, and was higher with increasing LVEF (P<0.001). The in-hospital mortality rate was higher in the HFrEF than in the HFmrEF and HFpEF groups (9.2%, 4.8%, and 5.1%, respectively, P<0.001). CONCLUSIONS This registry elucidated the clinical features and clinically relevant in-hospital outcomes in contemporary consecutive patients with ADHF in real-world clinical practice in Japan. When classified by LVEF, significant differences in characteristics and in-hospital outcomes existed between patients with HFrEF, HFmrEF, and HFpEF.

Elucidation of the molecular mechanism of platelet activation: Dense granule secretion is regulated by small guanosine triphosphate‐binding protein Rab27 and its effector Munc13‐4

Cardiovascular diseases such as myocardial and cerebral infarction are common critical diseases occurring more frequently in the elderly. The trigger of the diseases is platelet activation following plaque rupture or erosion. Investigation of the molecular mechanism in platelet activation has been exclusively performed pharmacologically. We have succeeded in establishing the granule secretion and aggregation assays using permeabilized platelets. These systems enabled us to examine the molecular mechanism in platelet activation with molecular biological and biochemical methods. Using these assay systems, we have been investigating the molecular mechanism of platelet activation. With a support grant from the Novartis Foundation for Gerontological Research, we found several molecules involved in the regulation. In this report, I present the progress in the research of the granule secretion mechanism in activated platelets, which was reported in the Japanese Geriatric Society Meeting in 2005.