Expression and purification of dynamin II domains and initial studies on structure and function

The Alzheimer susceptibility gene BIN1 induces isoform-dependent neurotoxicity through early endosome defects

The Bridging Integrator 1 (BIN1) gene is a major susceptibility gene for Alzheimer’s disease (AD). Deciphering its pathophysiological role is challenging due to its numerous isoforms. Here we observed in Drosophila that human BIN1 isoform1 (BIN1iso1) overexpression, contrary to human BIN1 isoform8 (BIN1iso8) and human BIN1 isoform9 (BIN1iso9), induced an accumulation of endosomal vesicles and neurodegeneration. Systematic search for endosome regulators able to prevent BIN1iso1-induced neurodegeneration indicated that a defect at the early endosome level is responsible for the neurodegeneration. In human induced neurons (hiNs) and cerebral organoids, BIN1 knock-out resulted in the narrowing of early endosomes. This phenotype was rescued by BIN1iso1 but not BIN1iso9 expression. Finally, BIN1iso1 overexpression also led to an increase in the size of early endosomes and neurodegeneration in hiNs. Altogether, our data demonstrate that the AD susceptibility gene BIN1, and especially BIN1iso1, contributes to early-endosome size deregulation, which is an early pathophysiological hallmark of AD pathology.

Abnormal Excitation-Contraction Coupling and Calcium Homeostasis in Myopathies and Cardiomyopathies

Muscle contraction requires specialized membrane structures with precise geometry and relies on the concerted interplay of electrical stimulation and Ca2+ release, known as excitation-contraction coupling (ECC). The membrane structure hosting ECC is called triad in skeletal muscle and dyad in cardiac muscle, and structural or functional defects of triads and dyads have been observed in a variety of myopathies and cardiomyopathies. Based on their function, the proteins localized at the triad/dyad can be classified into three molecular pathways: the Ca2+ release complex (CRC), store-operated Ca2+ entry (SOCE), and membrane remodeling. All three are mechanistically linked, and consequently, aberrations in any of these pathways cause similar disease entities. This review provides an overview of the clinical and genetic spectrum of triad and dyad defects with a main focus of attention on the underlying pathomechanisms.

Pathophysiological consequences of receptor mistraffic: Tales from the platelet P2Y12 receptor

Genetic variations in G protein-coupled receptor (GPCR) genes can disrupt receptor function in a wide variety of human genetic diseases, including platelet bleeding disorders. Platelets are critical for haemostasis with inappropriate platelet activation leading to the development of arterial thrombosis, which can result in heart attack and stroke whilst decreased platelet activity is associated with an increased risk of bleeding. GPCRs expressed on the surface of platelets play key roles in regulating platelet activity and therefore function. Receptors include purinergic receptors (P2Y₁ and P2Y₁₂), proteinase-activated receptor (PAR1 and PAR4) and thromboxane receptors (TPα), among others. Pharmacological blockade of these receptors forms a powerful therapeutic tool in the treatment and prevention of arterial thrombosis. With the advance of genomic technologies, there has been a substantial increase in the identification of naturally occurring rare and common GPCR variants. These variants include single-nucleotide polymorphisms (SNPs) and insertion or deletions that have the potential to alter GPCR expression or function. A number of defects in platelet GPCRs that disrupt receptor function have now been characterized in patients with mild bleeding disorders. This review will focus on rare, function-disrupting variants of platelet GPCRs with particular emphasis upon mutations in the P2Y₁₂ receptor gene that affect receptor traffic to modulate platelet function. Further this review will outline how the identification and characterization of function-disrupting GPCR mutations provides an essential link in translating our detailed understanding of receptor traffic and function in cell line studies into relevant human biological systems.

A novel mutation in the DNM2 gene impairs dynamin 2 localization in skeletal muscle of a patient with late onset centronuclear myopathy

Centronuclear myopathies constitute a group of heterogeneous congenital myopathies characterized by the presence of abnormal, centrally located nuclei within muscle fibers. Centronuclear myopathies can be caused by mutations of several different genes, including DNM2, encoding dynamin 2 (DNM2) a large GTPase involved in membrane trafficking and endocytosis. We report a 52-year-old female with slowly progressive muscle weakness, and a family history of the disease. Clinical, morphological, biochemical and genetic analyses of the proband and her family members were performed, including analyses of the proband's muscle biopsy. A novel D614N mutation, located in the C-terminal region pleckstrin-homology (PH) domain of DNM2 was identified in the proband and four family members, who exhibited similar symptoms. The mutation was associated with profound changes in the localization of DNM2 in muscle fibers without significant changes in protein expression. Mutated DNM2 and proteins involved in the membrane trafficking or membrane compartments maintenance were dislocalized within the myofiber, and concentrated at centrally located nuclei. This novel causative mutation (D614N) within the DNM2 gene in a large Polish centronuclear myopathy family with a late age of overt clinical manifestation caused profound changes in DNM2 localization and impaired proper organization of myofibers, and skeletal muscle functioning.

Molecular and structural characterization of the SH3 domain of AHI-1 in regulation of cellular resistance of BCR-ABL(+) chronic myeloid leukemia cells to tyrosine kinase inhibitors

ABL tyrosine kinase inhibitor (TKI) therapy induces clinical remission in chronic myeloid leukemia (CML) patients but early relapses and later emergence of TKI-resistant disease remain problematic. We recently demonstrated that the AHI-1 oncogene physically interacts with BCR-ABL and JAK2 and mediates cellular resistance to TKI in CML stem/progenitor cells. We now show that deletion of the SH3 domain of AHI-1 significantly enhances apoptotic response of BCR-ABL(+) cells to TKIs compared to cells expressing full-length AHI-1. We have also discovered a novel interaction between AHI-1 and Dynamin-2, a GTPase, through the AHI-1 SH3 domain. The crystal structure of the AHI-1 SH3 domain at 1.53-Å resolution reveals that it adopts canonical SH3 folding, with the exception of an unusual C-terminal α helix. PD1R peptide, known to interact with the PI3K SH3 domain, was used to model the binding pattern between the AHI-1 SH3 domain and its ligands. These studies showed that an "Arg-Arg-Trp" stack may form within the binding interface, providing a potential target site for designing specific drugs. The crystal structure of the AHI-1 SH3 domain thus provides a valuable tool for identification of key interaction sites in regulation of drug resistance and for the development of small molecule inhibitors for CML.

Mutation spectrum in the large GTPase dynamin 2, and genotype-phenotype correlation in autosomal dominant centronuclear myopathy

Centronuclear myopathy (CNM) is a genetically heterogeneous disorder associated with general skeletal muscle weakness, type I fiber predominance and atrophy, and abnormally centralized nuclei. Autosomal dominant CNM is due to mutations in the large GTPase dynamin 2 (DNM2), a mechanochemical enzyme regulating cytoskeleton and membrane trafficking in cells. To date, 40 families with CNM-related DNM2 mutations have been described, and here we report 60 additional families encompassing a broad genotypic and phenotypic spectrum. In total, 18 different mutations are reported in 100 families and our cohort harbors nine known and four new mutations, including the first splice-site mutation. Genotype-phenotype correlation hypotheses are drawn from the published and new data, and allow an efficient screening strategy for molecular diagnosis. In addition to CNM, dissimilar DNM2 mutations are associated with Charcot-Marie-Tooth (CMT) peripheral neuropathy (CMTD1B and CMT2M), suggesting a tissue-specific impact of the mutations. In this study, we discuss the possible clinical overlap of CNM and CMT, and the biological significance of the respective mutations based on the known functions of dynamin 2 and its protein structure. Defects in membrane trafficking due to DNM2 mutations potentially represent a common pathological mechanism in CNM and CMT.

Charcot-Marie-Tooth disease and intracellular traffic

Mutations of genes whose primary function is the regulation of membrane traffic are increasingly being identified as the underlying causes of various important human disorders. Intriguingly, mutations in ubiquitously expressed membrane traffic genes often lead to cell type- or organ-specific disorders. This is particularly true for neuronal diseases, identifying the nervous system as the most sensitive tissue to alterations of membrane traffic. Charcot-Marie-Tooth (CMT) disease is one of the most common inherited peripheral neuropathies. It is also known as hereditary motor and sensory neuropathy (HMSN), which comprises a group of disorders specifically affecting peripheral nerves. This peripheral neuropathy, highly heterogeneous both clinically and genetically, is characterized by a slowly progressive degeneration of the muscle of the foot, lower leg, hand and forearm, accompanied by sensory loss in the toes, fingers and limbs. More than 30 genes have been identified as targets of mutations that cause CMT neuropathy. A number of these genes encode proteins directly or indirectly involved in the regulation of intracellular traffic. Indeed, the list of genes linked to CMT disease includes genes important for vesicle formation, phosphoinositide metabolism, lysosomal degradation, mitochondrial fission and fusion, and also genes encoding endosomal and cytoskeletal proteins. This review focuses on the link between intracellular transport and CMT disease, highlighting the molecular mechanisms that underlie the different forms of this peripheral neuropathy and discussing the pathophysiological impact of membrane transport genetic defects as well as possible future ways to counteract these defects.

Dynamin 2 and human diseases

Dynamin 2 (DNM2) mutations cause autosomal dominant centronuclear myopathy, a rare form of congenital myopathy, and intermediate and axonal forms of Charcot-Marie-Tooth disease, a peripheral neuropathy. DNM2 is a large GTPase mainly involved in membrane trafficking through its function in the formation and release of nascent vesicles from biological membranes. DNM2 participates in clathrin-dependent and clathrin-independent endocytosis and intracellular membrane trafficking (from endosomes and Golgi apparatus). Recent studies have also implicated DNM2 in exocytosis. DNM2 belongs to the machinery responsible for the formation of vesicles and regulates the cytoskeleton providing intracellular vesicle transport. In addition, DNM2 tightly interacts with and is involved in the regulation of actin and microtubule networks, independent from membrane trafficking processes. We summarize here the molecular, biochemical, and functional data on DNM2 and discuss the possible pathophysiological mechanisms via which DNM2 mutations can lead to two distinct neuromuscular disorders.

Dynamin 2 mutations associated with human diseases impair clathrin-mediated receptor endocytosis

Dynamin 2 (DNM2) is a large GTPase involved in the release of nascent vesicles during endocytosis and intracellular membrane trafficking. Distinct DNM2 mutations, affecting the middle domain (MD) and the Pleckstrin homology domain (PH), have been identified in autosomal dominant centronuclear myopathy (CNM) and in the intermediate and axonal forms of the Charcot-Marie-Tooth peripheral neuropathy (CMT). We report here the first CNM mutation (c.1948G>A, p.E650 K) in the DNM2 GTPase effector domain (GED), leading to a slowly progressive moderate myopathy. COS7 cells transfected with DNM2 constructs harboring a disease-associated mutation in MD, PH, or GED show a reduced uptake of transferrin and low-density lipoprotein (LDL) complex, two markers of clathrin-mediated receptor endocytosis. A decrease in clathrin-mediated endocytosis was also identified in skin fibroblasts from one CNM patient. We studied the impact of DNM2 mutant overexpression on epidermal growth factor (EGF)-induced extracellular signal-regulated kinase 1 (ERK1) and ERK2 activation, known to be an endocytosis- and DNM2-dependent process. Activation of ERK1/2 was impaired for all the transfected mutants in COS7 cells, but not in CNM fibroblasts. Our results indicate that impairment of clathrin-mediated endocytosis may play a role in the pathophysiological mechanisms leading to DNM2-related diseases, but the tissue-specific impact of DNM2 mutations in both diseases remains unclear.

Complexes of syndapin II with dynamin II promote vesicle formation at the trans-Golgi network

The role of dynamin and so-called accessory proteins in endocytosis is well established. However, molecular details of the function(s) of dynamin II at the Golgi are largely unclear. We demonstrate that the ubiquitously expressed syndapin II isoform interacts with the proline-rich domain (PRD) of dynamin II through its Src-homology 3 (SH3) domain. Co-immunoprecipitation of endogenous syndapin II and dynamin II, and successful reconstitutions of such complexes at membranes in COS-7 cells, show the in vivo relevance of the interaction. Syndapin II can associate with Golgi membranes and this association increases upon Golgi exit block. Brefeldin A treatment clearly shows that the observed perinuclear localization of syndapin II co-localizing with syntaxin 6 reflects the Golgi complex and that it requires functional integrity of the Golgi. Syndapins are crucial for Golgi vesicle formation because anti-syndapin antibodies, used either in in vitro reconstitutions or in living cells, inhibited this process. Both types of assays additionally revealed the essential role of syndapin II SH3 interactions with the dynamin II PRD in vesicle formation. An excess of the syndapin SH3 domain strongly inhibited budding from Golgi membranes in vitro. Likewise, overexpression of the syndapin SH3 domain or of a dynamin II variant incapable of associating with syndapin II (dynamin IIΔPRD) impaired trafficking of vesicular stomatitis virus glycoprotein (VSVG)-GFP in vivo. By contrast, full-length syndapin II-l had no negative effect, and instead promoted VSVG-GFP export from the Golgi. Importantly, a cytosolic fraction containing endogenous syndapin-dynamin complexes was sufficient to promote vesicle formation from Golgi membranes in a syndapin-dependent manner. Thus, syndapin-dynamin complexes are crucial and sufficient to promote vesicle formation from the trans-Golgi network.

Kinetics of Src Homology 3 Domain Association with the Proline-rich Domain of Dynamins

Dynamin function is mediated in part through association of its proline-rich domain (PRD) with the Src homology 3 (SH3) domains of several putative binding proteins. To assess the specificity and kinetics of this process, we undertook surface plasmon resonance studies of the interaction between isolated PRDs of dynamin-1 and -2 and several purified SH3 domains. Glutathione S-transferase-linked SH3 domains bound with high affinity (K(D) approximately 10 nm to 1 microm) to both dynamin-1 and -2. The simplest interaction appeared to take place with the amphiphysin-SH3 domain; this bound to a single high affinity site (K(D) approximately 10 nm) in the C terminus of dynamin-1 PRD, as predicted by previous studies. Binding to the dynamin-2 PRD was also monophasic but with a slightly lower affinity (K(D) approximately 25 nm). Endophilin-SH3 binding to both dynamin-1 and -2 PRDs was biphasic, with one high affinity site (K(D) approximately 14 nm) in the N terminus of the PRD and another lower affinity site (K(D) approximately 60 nm) in the C terminus of dynamin-1. The N-terminal site in dynamin-2 PRD had a 10-fold lower affinity for endophilin-SH3. Preloading of dynamin-1 PRD with the amphiphysin-SH3 domain partially occluded binding of the endophilin-SH3 domain, indicating overlap between the binding sites in the C terminus, but endophilin was still able to interact with the high affinity N-terminal site. This shows that more than one SH3 domain can simultaneously bind to the PRD and suggests that competition probably occurs in vivo between different SH3-containing proteins for the limited number of PXXP motifs. Endophilin-SH3 binding to the high affinity site was disrupted when dynamin-1 PRD was phosphorylated with Cdk5, indicating that this site overlaps the phosphorylation sites, but amphiphysin-SH3 binding was unaffected. Other SH3 domains showed similarly complex binding characteristics, and substantial differences were noted between the PRDs from dynamin-1 and -2. For example, SH3 domains from c-Src, Grb2, and intersectin bound only to the C-terminal half of dynamin-2 PRD but to both the N- and C-terminal portions of dynamin-1 PRD. Thus, differential binding of SH3 domain-containing proteins to dynamin-1 and -2 may contribute to the distinct functions performed by these isoforms.

Sorting nexin 9 participates in clathrin-mediated endocytosis through interactions with the core components

Sorting nexin 9 (SNX9) belongs to a family of proteins, the sorting nexins, that are characterized by the presence of a subclass of the phosphoinositide-binding phox domain. SNX9 has in its amino terminus a Src homology 3 domain and a region with predicted low complexity followed by a carboxyl-terminal part containing the phox domain. We previously found that SNX9 is one of the major proteins in hematopoietic cells that binds to the alpha and beta2-appendages of adaptor protein complex 2 (AP-2), a protein with a critical role in the formation of clathrin-coated vesicles at the plasma membrane. In the present study we show that clathrin and dynamin-2, two other essential molecules in the endocytic process, also interact with SNX9. We found that both AP-2 and clathrin bind to the low complexity region in SNX9 in a cooperative manner, whereas dynamin-2 binds to the Src homology 3 domain. In the cytosol, SNX9 is present in a 14.5 S complex containing dynamin-2 and an unidentified 41-kDa protein. In HeLa cells, SNX9 co-localized with both AP-2 and dynamin-2 at the plasma membrane or on vesicular structures derived from it but not with the early endosomal marker EEA1 or with AP-1. The results suggest that SNX9 may be recruited together with dynamin-2 and become co-assembled with AP-2 and clathrin at the plasma membrane. Overexpression in both K562 and HeLa cells of truncated forms of SNX9 interfered with the uptake of transferrin, consistent with a role of SNX9 in endocytosis.