A mutation associated with Charcot-Marie-Tooth disease enhances the formation of stable dynamin 2 complexes in cells

Mutations in dynamin 2 (DNM2) have been associated with two distinct motor disorders, Charcot-Marie-Tooth neuropathies (CMT) and centronuclear myopathy (CNM). The majority of these mutations are clustered in the pleckstrin homology domain (PHD) which engage in intramolecular interactions that suppress dynamin self-assembly and GTPase activation. CNM mutations in the PHD interferes with these intramolecular interactions, thereby blocking the formation of the auto-inhibited state. CMT mutations are located primarily on the opposite surface of the PHD, which is specialized for lipid PIP2 binding. It has been speculated that the distinct locations and interactions of residues mutated in CMT and CNM explain why each set of mutations cause either one disease or the other, despite their close proximity within the PHD sequence. We show that at least one CMT-causing mutant, lacking residues 555DEE557 (∆DEE), displays this inability to undergo auto-inhibition as observed in CNM-linked mutants. This ∆DEE deletion mutant induces the formation of abnormally large cytoplasmic inclusions similar to those observed for CNM-linked mutant R369W. We also found substantially reduced migration from the membrane of the ∆DEE deletion mutant. These findings call into question the molecular mechanism currently believed to underlie the absence of pathogenic overlap between DNM2-dependent CMT and CNM.

Dynamin-2 Deficiency Causes Neutropenia and Dysplastic Bone Marrow Changes in an Age and Sex Dependent Manner in Mice

The dynamins are a family of ubiquitously expressed proteins with GTPase activity and are known for their role in membrane remodeling and intracellular trafficking. However, their exact role within various hematopoietic lineages is incompletely understood. In humans, most of the clinical cases with cytopenia in Charcot-Marie-Tooth (CMT) disease due to dynamin-2 mutations are associated with neutropenia, and CMT patients may suffer from impaired wound healing. Of interest, pregnancy notably worsens CMT disease, possibly due to hormonal changes. Antiprogesterone treatment was successfully given in a CMT rat model, and similar treatment is being considered for human CMT. We previously reported that inhibiting dynamin (DNM) activity impairs migration capability in mature megakaryocytes. We obtained the conditional deletion of Dnm2 and targeted its deletion in hematopoietic tissues with the vav-cre murine strain. Homozygous deletion of Dnm2 in blood tissues appears embryonic lethal. None of the pups born showed a Vav-cre/Dnm-2 fl/fl genotype, whereas a third of the pups born had a Vav-cre/Dnm-2 fl/wt (Dnm2 het) genotype. Bone marrow cells from the heterozygous female mice (Dnm2 het) had 35% to 50% decreased Dnm2 expression in comparison with age-matched controls (CTRL). Evaluation of the complete blood counts demonstrated that Dnm2 het female mice developed leukopenia which was detected from 40 weeks of age (average granulocyte-monocyte counts: CTRL 532/mm3 vs. Dnm2 het 300/mm3; p=0.0164). Neutropenia was unequivocal at 65 weeks of age (average neutrophil counts, CTRL 700/mm3 vs. Dnm2 het 343/mm3, p=0.016). Dnm2 het showed a trend for higher platelet counts than controls, but this was non-statistically significant. Further analysis of hematopoietic lineage maturation by flow cytometry indicated that lineage-negative cells and granulocyte-monocyte progenitors were decreased in Dnm2 het mice (average bone marrow lineage-negative cells: CTRL 2.8x10E6 vs. Dnm2 het 1.97x10E6, p =0.0056; average granulocyte-monocyte progenitors: CTRL 1.35x10E6 vs. Dnm2 het 0.85x10e6, p=0.01), along with a relative increase of common lymphoid progenitors and of megakaryocyte/erythrocyte progenitors in the bone marrow. Immunohistochemical staining for mature neutrophils with Ly6G showed an overall decreased number of mature granulocytes in the bone marrow of Dnm2 het mice (average Ly6G-positive cells: CTRL 20% vs. Dnm2 het 29%, p=0.0026). A linear pattern of distribution of Ly6G positive bone marrow cells along blood vessels was observed in fewer mice in the Dnm2 het group than in the CTRL group (25% vs. 59%, p=0.02), indicating that the migration pattern within the bone marrow is altered in the Dnm2 het group (see Figure). In addition, Dnm2 het mice developed splenomegaly (average spleen weight: Dnm2 het 146 mg vs. CTRL 99 mg, p=0.006), which was secondary to a marked florid reactive germinal center hyperplasia. Some of the Dnm2 het mice, including 5 mice whose pregnancy occurred in middle-age (p=0.005 when comparing with CTRL or young Dnm2 het mice) and 2 non-pregnant older mice, showed physical signs of distress with markedly reduced activity, poor grooming, ruffled furs, and hunched posture. Both non-pregnant sick mice showed a marked decrease in Ly6G positive mature neutrophils at 0.3% of total marrow cells (Figure), and the bone marrow from one mouse was completely effaced by immature myeloid precursors, consistent with the development of acute myeloid leukemia. In addition, a third of Dnm2 hetmice showed no distress but displayed morphological bone marrow abnormalities including megakaryocytic dysmorphology. In summary, female mice with loss of Dnm2 in the hematopoietic compartment develop persistent neutropenia as they age, with decreased granulocyte progenitor production and with migration defects. These abnormalities are associated with a risk for developing megakaryocytic dysplasia, and acute myeloid leukemia. These findings might also suggest a mechanism for chronic idiopathic neutropenia, which has a predominance in middle-aged women. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Common Pathogenic Mechanisms in Centronuclear and Myotubular Myopathies and Latest Treatment Advances

Centronuclear myopathies (CNM) are rare congenital disorders characterized by muscle weakness and structural defects including fiber hypotrophy and organelle mispositioning. The main CNM forms are caused by mutations in: the MTM1 gene encoding the phosphoinositide phosphatase myotubularin (myotubular myopathy), the DNM2 gene encoding the mechanoenzyme dynamin 2, the BIN1 gene encoding the membrane curvature sensing amphiphysin 2, and the RYR1 gene encoding the skeletal muscle calcium release channel/ryanodine receptor. MTM1, BIN1, and DNM2 proteins are involved in membrane remodeling and trafficking, while RyR1 directly regulates excitation-contraction coupling (ECC). Several CNM animal models have been generated or identified, which confirm shared pathological anomalies in T-tubule remodeling, ECC, organelle mispositioning, protein homeostasis, neuromuscular junction, and muscle regeneration. Dynamin 2 plays a crucial role in CNM physiopathology and has been validated as a common therapeutic target for three CNM forms. Indeed, the promising results in preclinical models set up the basis for ongoing clinical trials. Another two clinical trials to treat myotubular myopathy by MTM1 gene therapy or tamoxifen repurposing are also ongoing. Here, we review the contribution of the different CNM models to understanding physiopathology and therapy development with a focus on the commonly dysregulated pathways and current therapeutic targets.

Dynamin-2 Phosphorylation as A Critical Regulatory Target of Bin1 and GSK3α for Endocytosis in Muscle

Tight regulation of endocytosis ensures accurate control of cellular signaling and membrane dynamics, which are crucial for tissue morphogenesis and functions. Mutations of Bin1 and dynamin-2 (Dyn2), proteins that generate membrane curvature and sever endocytic invaginations, respectively, cause progressive hereditary myopathy. Here, we show that Bin1 inhibits Dyn2 via direct interaction of its SRC Homology 3 (SH3) domain with the proline-rich domain (PRD) of Dyn2. Phosphorylation of S848 of Dyn2 by GSK3α, a kinase downstream of insulin signaling, relieves Dyn2 from the inhibition of Bin1 and promotes endocytosis in muscle. Mutations of Bin1 associated with centronuclear myopathy disrupt its inhibition of Dyn2, thereby exaggerating Dyn2 fission activity and causing excessive fragmentation of T-tubules in the muscle cells. Our work reveals how Bin1-Dyn2 interaction fine-tunes membrane remodeling at the molecular level, and lay the foundation for future exploration of endocytic regulation and hereditary muscle diseases.

Beta-Pix-dynamin 2 complex promotes colorectal cancer progression by facilitating membrane dynamics

Purpose Spatiotemporal regulation of cell membrane dynamics is a major process that promotes cancer cell invasion by acting as a driving force for cell migration. Beta-Pix (βPix), a guanine nucleotide exchange factor for Rac1, has been reported to be involved in actin-mediated cellular processes, such as cell migration, by interacting with various proteins. As yet, however, the molecular mechanisms underlying βPix-mediated cancer cell invasion remain unclear. Methods The clinical significance of βPix was analyzed in patients with colorectal cancer (CRC) using public clinical databases. Pull-down and immunoprecipitation assays were employed to identify novel binding partners for βPix. Additionally, various cell biological assays including immunocytochemistry and time-lapse video microscopy were performed to assess the effects of βPix on CRC progression. A βPix-SH3 antibody delivery system was used to determine the effects of the βPix-Dyn2 complex in CRC cells. Results We found that the Src homology 3 (SH3) domain of βPix interacts with the proline-rich domain of Dynamin 2 (Dyn2), a large GTPase. The βPix-Dyn2 interaction promoted lamellipodia formation, along with plasma membrane localization of membrane-type 1 matrix metalloproteinase (MT1-MMP). Furthermore, we found that Src kinase-mediated phosphorylation of the tyrosine residue at position 442 of βPix enhanced βPix-Dyn2 complex formation. Disruption of the βPix-Dyn2 complex by βPix-SH3 antibodies targeting intracellular βPix inhibited CRC cell invasion. Conclusions Our data indicate that spatiotemporal regulation of the Src-βPix-Dyn2 axis is crucial for CRC cell invasion by promoting membrane dynamics and MT1-MMP recruitment into the leading edge. The development of inhibitors that disrupt the βPix-Dyn2 complex may be a useful therapeutic strategy for CRC.

Insulin Signaling Attenuates GLUT4 Endocytosis in Muscle Cells via GSK3α-Dyn2-Bin1 Interplay

Insulin-induced translocation of glucose transporter 4 (GLUT4) to the plasma membrane of skeletal muscle is critical for postprandial glucose uptake; however, whether the internalization of GLUT4 into cells is also regulated by insulin signaling remains unclear. Here, we discover that the activity of dynamin-2 (Dyn2), pivotal GTPase catalyzing GLUT4 internalization, is regulated by insulin signaling in muscle cells. The membrane fission activity of Dyn2 is inhibited in muscle cells through binding with the SH3 domain-containing protein Bin1. Phosphorylation of Serine848 on Dyn2 by GSK3α or the mutations of Bin1-SH3 in patients with centronuclear myopathy, elevate the activity of Dyn2 due to reduced binding affinity toward Bin1. The augmented Dyn2 fission activity in muscle cells leads to GLUT4 internalization and Bin1-tubule vesiculation. Together, our findings reveal a new role of insulin signaling in glucose metabolism and muscle physiology via attenuating Dyn2 activity thus regulating GLUT4 endocytosis in muscle cell.

Salmonella effector SopD promotes plasma membrane scission by inhibiting Rab10

Salmonella utilizes translocated virulence proteins (termed effectors) to promote host cell invasion. The effector SopD contributes to invasion by promoting scission of the plasma membrane, generating Salmonella-containing vacuoles. SopD is expressed in all Salmonella lineages and plays important roles in animal models of infection, but its host cell targets are unknown. Here we show that SopD can bind to and inhibit the small GTPase Rab10, through a C-terminal GTPase activating protein (GAP) domain. During infection, Rab10 and its effectors MICAL-L1 and EHBP1 are recruited to invasion sites. By inhibiting Rab10, SopD promotes removal of Rab10 and recruitment of Dynamin-2 to drive scission of the plasma membrane. Together, our study uncovers an important role for Rab10 in regulating plasma membrane scission and identifies the mechanism used by a bacterial pathogen to manipulate this function during infection.