Copine-6 is a Ca2+ sensor for activity-induced AMPA receptor exocytosis

The recruitment of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors underlies the strengthening of neuronal connectivity during learning and memory. This process is triggered by N-methyl-D-aspartate (NMDA) receptor-dependent postsynaptic Ca2+ influx. Synaptotagmin (Syt)-1 and -7 have been proposed as Ca2+ sensors for AMPA receptor exocytosis but are functionally redundant. Here, we identify a cytosolic C2 domain-containing Ca2+-binding protein, Copine-6, that forms a complex with AMPA receptors. Loss of Copine-6 expression impairs activity-induced exocytosis of AMPA receptors in primary neurons, which is rescued by wild-type Copine-6 but not Ca2+-binding mutants. In contrast, Copine-6 loss of function does not affect steady-state expression or tetrodotoxin-induced synaptic upscaling of surface AMPA receptors. Loss of Syt-1/Syt-7 significantly reduces Copine-6 protein expression. Interestingly, overexpression of wild-type Copine-6, but not the Ca2+-binding mutants, restores activity-dependent exocytosis of AMPA receptors in Syt-1/Syt-7 double-knockdown neurons. We conclude that Copine-6 is a postsynaptic Ca2+ sensor that mediates AMPA receptor exocytosis during synaptic potentiation.

Multi-Institutional Clinical Outcomes of Biopsy Gleason Grade Group 5 Prostate Cancers Treated With Contemporary High-Dose Radiation and Long-Term Androgen Deprivation Therapy

AIMS

This multicentric retrospective study reports long-term clinical outcomes of non-metastatic grade group 5 prostate cancers treated with external beam radiotherapy (EBRT) alone with long-term androgen deprivation therapy (ADT).

MATERIALS AND METHODS

Patients treated across 19 institutions were studied. The key endpoints that were evaluated were 5-year biochemical recurrence-free survival (bRFS), metastases-free survival (MFS), overall survival, together with EBRT-related acute and late toxicities. The impact of various prognostic factors on the studied endpoints was analysed using univariate and multivariate analyses.

RESULTS

Among the 462 patients, 88% (405) had Gleason 9 disease and 31% (142) had primary Gleason pattern 5. A prostate-specific membrane antigen positron emission tomography-computed tomography scan was used for staging in 33% (153), 80% (371) were staged as T3/T4 and 30% (142) with pelvic nodal disease. The median ADT duration was 24 months; 66% received hypofractionated EBRT and 71.4% (330) received pelvic nodal irradiation. With a median follow-up of 56 months, the 5-year bRFS, MFS and overall survival were 73.1%, 77.4% and 90.5%, respectively. Primary Gleason pattern 5 was associated with worse bRFS, MFS and overall survival with hazard ratios of 0.51 (95% confidence interval 0.35 to 0.73, P < 0.001), 0.64 (95% confidence interval 0.43 to 0.96, P = 0.031) and 0.52 (95% confidence interval 0.28 to 0.97, P = 0.040), respectively, whereas pelvic nodal disease was associated with worse bRFS (hazard ratio 0.67, 95% confidence interval 0.46 to 0.98, P = 0.039) and MFS (hazard ratio 0.56, 95% confidence interval 0.37 to 0.85, P = 0.006). The acute and late radiation-related toxicities were low overall and pelvic nodal irradiation was associated with higher toxicities.

CONCLUSION

Contemporary EBRT and long-term ADT led to excellent 5-year clinical outcomes and low rates of toxicity in this cohort of non-metastatic grade group 5 prostate cancers. Primary Gleason pattern 5 and pelvic node disease portends inferior clinical outcomes.

Improved mammalian retromer cryo-EM structures reveal a new assembly interface

Retromer (VPS26/VPS35/VPS29 subunits) assembles with multiple sorting nexin proteins on membranes to mediate endosomal recycling of transmembrane protein cargoes. Retromer has been implicated in other cellular processes, including mitochondrial homeostasis, nutrient sensing, autophagy, and fission events. Mechanisms for mammalian retromer assembly remain undefined, and retromer engages multiple sorting nexin proteins to sort cargoes to different destinations. Published structures demonstrate mammalian retromer forms oligomers in vitro, but several structures were poorly resolved. We report here improved retromer oligomer structures using single-particle cryo-EM by combining data collected from tilted specimens with multiple advancements in data processing, including using a 3D starting model for enhanced automated particle picking in RELION. We used a retromer mutant (3KE retromer) that breaks VPS35-mediated interfaces to determine a structure of a new assembly interface formed by the VPS26A and VPS35 N-termini. The interface reveals how an N-terminal VPS26A arrestin saddle can link retromer chains by engaging a neighboring VPS35 N- terminus, on the opposite side from the well-characterized C-VPS26/N-VPS35 interaction observed within heterotrimers. The new interaction interface exhibits substantial buried surface area (∼7000 Å²) and further suggests that metazoan retromer may serve as an adaptable scaffold.

SNX27-Retromer directly binds ESCPE-1 to transfer cargo proteins during endosomal recycling

Coat complexes coordinate cargo recognition through cargo adaptors with biogenesis of transport carriers during integral membrane protein trafficking. Here, we combine biochemical, structural, and cellular analyses to establish the mechanistic basis through which SNX27-Retromer, a major endosomal cargo adaptor, couples to the membrane remodeling endosomal SNX-BAR sorting complex for promoting exit 1 (ESCPE-1). In showing that the SNX27 FERM (4.1/ezrin/radixin/moesin) domain directly binds acidic-Asp-Leu-Phe (aDLF) motifs in the SNX1/SNX2 subunits of ESCPE-1, we propose a handover model where SNX27-Retromer captured cargo proteins are transferred into ESCPE-1 transport carriers to promote endosome-to-plasma membrane recycling. By revealing that assembly of the SNX27:Retromer:ESCPE-1 coat evolved in a stepwise manner during early metazoan evolution, likely reflecting the increasing complexity of endosome-to-plasma membrane recycling from the ancestral opisthokont to modern animals, we provide further evidence of the functional diversification of yeast pentameric Retromer in the recycling of hundreds of integral membrane proteins in metazoans.

Biochemical basis for an interaction between SNX27 and the flexible SNX1 N-terminus

Metazoans require the sorting nexin (SNX) protein, SNX27, to recycle hundreds of important transmembrane protein receptors from endosomes to the plasma membrane. Cargo recycling by SNX27 requires its interaction with retromer, a heterotrimer known to assemble on membranes with multiple sorting nexins, including SNX-BAR proteins and SNX3. SNX27 has also been functionally linked to SNX-BARs, but the molecular basis of this interaction has been unknown. We identify a direct biochemical interaction between the conserved and flexible SNX1/SNX2 N-terminus and full-length SNX27 using purified proteins in pulldown experiments. Sequence alignments indicate both SNX1 and SNX2 contain two short and conserved stretches of acidic residues bearing a DxF motif in their flexible N-terminal regions. Biochemical pulldown and mapping experiments reveal forty residues in the N-terminus of either SNX1 or SNX2 can mediate binding to SNX27. SNX27 truncation analysis demonstrates the SNX27 FERM domain binds the SNX1 N-terminus. Calorimetry experiments quantified binding between the SNX1 N-terminus and SNX27 in the low micromolar affinity range (K_D ∼10 μM) and suggest the second DxF motif may play a more prominent role in binding. Mutation of either DxF sequence in SNX1 abrogates measurable binding to SNX27 in the calorimeter. Modelling from both predicted and experimentally determined structures suggests the SNX27 FERM domain could accommodate both DxF motifs simultaneously. Together, these data suggest SNX27 is directly linked to specific SNX-BAR proteins through binding acidic motifs in the SNX1 or SNX2 N-terminus.

A fluorescence polarization assay using recombinant protein ESAT-6 for the detection of antibodies against pathogenic Mycobacterium bovis in bovine

BACKGROUND

Bovine tuberculosis (bTB) is a major bacterial disease that causes significant economic disruption across the globe.

AIMS

Our study was based on using a fluorescence polarization assay (FPA) that used fluorescein-labeled ESAT-6 protein to detect Mycobacterium bovis antibodies in bovine serum.

METHODS

The ESAT-6 protein was used in a FPA. Positive TB reactors were determined by the comparative intradermal test (CID) and interferon gamma test (IFN-γ). Antibodies against M. bovis were detected using a fluorescein isothiocyanate (FITC) labeled tracer and a whole culture FITC labeled tracer in the positive cattle.

RESULTS

Of the 192 animals tested for bTB, 37 were found to be positive by either the CID or IFN-γ assays. Using the mP values from five culture-positive serum samples, a cutoff value of more than >127 mp provided the best discrimination between positive reactors and negative bTB animals. The ESAT-6 results of FPA in comparison with CID results revealed sensitivity of 92.9% and specificity of 64.6%, and in comparison with results IFN-γ, showed sensitivity of 95.7% and specificity of 49%. FPA using FITC labelled ESAT-6 as a tracer has better sensitivity (95.7%) and specificity (49.1%) than IFN-γ test in humoral immune response in animals.

CONCLUSION

This work revealed that the ESAT-6 protein as an antigen can be used in diagnosing bTB using a practical and sensitive humoral test.

Knowledge assessment and outcomes of otolaryngology practice amidst the COVID-19 pandemic in a busy ENT department

INTRODUCTION

Otolaryngology health personnel are at high risk of acquiring COVID-19 disease and, hence, are likely to have high stress levels. This study was designed to evaluate the feedback of otolaryngology healthcare workers in ENT departments who are managing patients in the coronavirus pandemic.

METHODS

A questionnaire focused on all aspects of healthcare delivery was completed by otolaryngology healthcare workers.

RESULTS

The findings, based on statistical analyses, included high stress levels and inadequate disease-related information in these workers.

CONCLUSIONS

Healthcare authorities need to take care of issues related to mental health in healthcare professionals in addition to spreading awareness about safe practices. Further studies are needed to continuously monitor feedback from personnel as the coronavirus pandemic unravels in the future.

Regulation of NMDA receptor trafficking and gating by activity-dependent CaMKIIα phosphorylation of the GluN2A subunit

NMDA receptor (NMDAR)-dependent Ca2+ influx underpins multiple forms of synaptic plasticity. Most synaptic NMDAR currents in the adult forebrain are mediated by GluN2A-containing receptors, which are rapidly inserted into synapses during long-term potentiation (LTP); however, the underlying molecular mechanisms remain poorly understood. In this study, we show that GluN2A is phosphorylated at Ser-1459 by Ca2+/calmodulin-dependent kinase IIα (CaMKIIα) in response to glycine stimulation that mimics LTP in primary neurons. Phosphorylation of Ser-1459 promotes GluN2A interaction with the sorting nexin 27 (SNX27)-retromer complex, thereby enhancing the endosomal recycling of NMDARs. Loss of SNX27 or CaMKIIα function blocks the glycine-induced increase in GluN2A-NMDARs on the neuronal membrane. Interestingly, mutations of Ser-1459, including the rare S1459G human epilepsy variant, prolong the decay times of NMDAR-mediated synaptic currents in heterosynapses by increasing the duration of channel opening. These findings not only identify a critical role of Ser-1459 phosphorylation in regulating the function of NMDARs, but they also explain how the S1459G variant dysregulates NMDAR function.

Toward Understanding the Molecular Role of SNX27/Retromer in Human Health and Disease

Aberrations in membrane trafficking pathways have profound effects in cellular dynamics of cellular sorting processes and can drive severe physiological outcomes. Sorting nexin 27 (SNX27) is a metazoan-specific sorting nexin protein from the PX-FERM domain family and is required for endosomal recycling of many important transmembrane receptors. Multiple studies have shown SNX27-mediated recycling requires association with retromer, one of the best-known regulators of endosomal trafficking. SNX27/retromer downregulation is strongly linked to Down's Syndrome (DS) via glutamate receptor dysfunction and to Alzheimer's Disease (AD) through increased intracellular production of amyloid peptides from amyloid precursor protein (APP) breakdown. SNX27 is further linked to addiction via its role in potassium channel trafficking, and its over-expression is linked to tumorigenesis, cancer progression, and metastasis. Thus, the correct sorting of multiple receptors by SNX27/retromer is vital for normal cellular function to prevent human diseases. The role of SNX27 in regulating cargo recycling from endosomes to the cell surface is firmly established, but how SNX27 assembles with retromer to generate tubulovesicular carriers remains elusive. Whether SNX27/retromer may be a putative therapeutic target to prevent neurodegenerative disease is now an emerging area of study. This review will provide an update on our molecular understanding of endosomal trafficking events mediated by the SNX27/retromer complex on endosomes.

The Glo3 GAP crystal structure supports the molecular niche model for ArfGAPs in COPI coats

Arf GTPase activating (ArfGAP) proteins are critical regulatory and effector proteins in membrane trafficking pathways. Budding yeast contain two ArfGAP proteins (Gcs1 and Glo3) implicated in COPI coat function at the Golgi, and yeast require Glo3 catalytic function for viability. A new X-ray crystal structure of the Glo3 GAP domain was determined at 2.1 Å resolution using molecular replacement methods. The structure reveals a Cys4-family zinc finger motif with an invariant residue (R59) positioned to act as an "arginine finger" during catalysis. Comparisons among eukaryotic GAP domains show a key difference between ArfGAP1 and ArfGAP2/3 family members in the final helix located within the domain. Conservation at both the sequence and structural levels suggest the Glo3 GAP domain interacts with yeast Arf1 switch I and II regions to promote catalysis. Together, the structural data presented here provide additional evidence for placing Glo3 near Arf1 triads within membrane-assembled COPI coats and further support the molecular niche model for COPI coat regulation by ArfGAPs.

Opposite Surfaces of the Cdc15 F-BAR Domain Create a Membrane Platform That Coordinates Cytoskeletal and Signaling Components for Cytokinesis

Many eukaryotes assemble an actin- and myosin-based cytokinetic ring (CR) on the plasma membrane (PM) for cell division, but how it is anchored there remains unclear. In Schizosaccharomyces pombe, the F-BAR protein Cdc15 links the PM via its F-BAR domain to proteins in the CR’s interior via its SH3 domain. However, Cdc15’s F-BAR domain also directly binds formin Cdc12, suggesting that Cdc15 may polymerize a protein network directly adjacent to the membrane. Here, we determine that the F-BAR domain binds Cdc12 using residues on the face opposite its membrane-binding surface. These residues also bind paxillin-like Pxl1, promoting its recruitment with calcineurin to the CR. Mutation of these F-BAR domain residues results in a shallower CR, with components localizing ~35% closer to the PM than in wild type, and aberrant CR constriction. Thus, F-BAR domains serve as oligomeric membrane-bound platforms that can modulate the architecture of an entire actin structure.

Multiple F-BAR domains link actin structures to membrane. Snider et al. show that the flat Cdc15 F-BAR domain utilizes opposite surfaces to bind the plasma membrane and cytokinetic ring proteins simultaneously. Disrupting Cdc15 F-BAR domain’s interaction with proteins results in an overall compression of the entire cytokinetic ring architecture.

EhC2B, a C2 domain-containing protein, promotes erythrophagocytosis in Entamoeba histolytica via actin nucleation

Remodelling of the actin cytoskeleton in response to external stimuli is obligatory for many cellular processes in the amoebic cell. A rapid and local rearrangement of the actin cytoskeleton is required for the development of the cellular protrusions during phagocytosis, trogocytosis, migration, and invasion. Here, we demonstrated that EhC2B, a C2 domain-containing protein, is an actin modulator. EhC2B was first identified as an effector of EhRab21 from E. histolytica. In vitro interaction studies including GST pull-down, fluorescence-based assay and ITC also corroborated with our observation. In the amoebic trophozoites, EhC2B accumulates at the pseudopods and the tips of phagocytic cups. FRAP based studies confirmed the recruitment and dynamics of EhC2B at the phagocytic cup. Moreover, we have shown the role of EhC2B in erythrophagocytosis. It is well known that calcium-dependent signal transduction is essential for the cytoskeletal dynamics during phagocytosis in the amoebic parasite. Using liposome pelleting assay, we demonstrated that EhC2B preferentially binds to the phosphatidylserine in the presence of calcium. The EhC2B mutants defective in calcium or lipid-binding failed to localise beneath the plasma membrane. The cells overexpressing these mutants have also shown a significant reduction in erythrophagocytosis. The role of EhC2B in erythrophagocytosis and pseudopod formation was also validated by siRNA-based gene knockdown approach. Finally, with the help of in vitro nucleation assay using fluorescence spectroscopy and total internal reflection fluorescence microscopy, we have established that EhC2B is an actin nucleator. Collectively, based on the results from the study, we propose that EhC2B acts like a molecular bridge which promotes membrane deformation via its actin nucleation activity during the progression of the phagocytic cup in a calcium-dependent manner.

Detection of non-tuberculous mycobacterial species using PCR-RFLP analysis in trans-tracheal washes in cattle and buffaloes with respiratory distress

BACKGROUND

Bovine tuberculosis (bTB) is a chronic disease of cattle with high economic importance in livestock farming caused by Mycobacterium bovis and bears a zoonotic potential. There are some non-tuberculous mycobacteria (NTM) which cause disease similar to bTB and interfere with diagnosis of bTB. Non-tuberculous mycobacteria are saprophytic in nature but some of them may cause pulmonary infections, mastitis, lesions in respiratory tract and lymph nodes of cattle, due to which they are being recognized worldwide and interfere with the diagnosis of bTB.

AIMS

The aim of the study was to detect NTM species from cattle and buffaloes with respiratory distress using biochemical test and polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis (PRA).

METHODS

A total of 50 trans-tracheal washes were collected from cattle (n=41) and buffaloes (n=9) with respiratory distress. The samples were inoculated on Middlebrook 7H10 media after proper decontamination with 4% NaOH. The isolate obtained was identified by biochemical testing. Extracted DNA from samples and isolate was subjected to PRA which involved hsp65 gene amplification (439 bp) and RFLP analysis of amplified product.

RESULTS

Out of 50 trans-tracheal washes only one isolate of Mycobacterium kansasii (n=1) (2%) was obtained which was confirmed by biochemical testing and PRA. Mycobacterium kansasii (n=4) (8%), Mycobacterium intracellulare (n=1) (2%), and Mycobacterium vaccae (n=1) (2%) were identified by PRA.

CONCLUSION

The study emphasizes the importance of NTM in animals. Polymerase chain reaction-restriction fragment length polymorphism analysis is a more reliable and rapid method for identification of NTM than conventional methods.

Comparison of multiplex and ordinary PCR for diagnosis of paratuberculosis and tuberculosis in blood samples (buffy coat) of cattle and buffaloes

BACKGROUND

Paratuberculosis and tuberculosis (TB) caused by Mycobacterium avium paratuberculosis (MAP) and Mycobacterium tuberculosis complex (MTC), respectively are economically important, chronic debilitating diseases affecting the dairy herds and are also potential zoonotic threats.

AIMS

Differential diagnosis of paratuberculosis and TB in blood samples of cattle and buffaloes.

METHODS

In this study, an in-house developed multiplex polymerase chain reaction (PCR) targeting MAP, Mycobacterium bovis and Mycobacterium smegmatis was used in blood samples (buffy coat) parallel with IS900 PCR and esxB PCR for diagnosis of paratuberculosis and TB, respectively; in a total of 202 cattle and buffaloes.

RESULTS

Out of 202 animals, 12 (5.9%) and 17 (8.4%) animals were positive for MAP by multiplex PCR and IS900 PCR, respectively; from which only 8 (4%) animals were positive by both tests; whereas 4 and 9 animals were exclusively positive by multiplex PCR and IS900 PCR, respectively. None of the animals were found to be positive for M. bovis and M. smegmatis by the multiplex PCR. However, the esxB PCR detected 13 (6.4%) animals positive for TB. In fact, 3 (1.5%) animals were found to be co-infected by both paratuberculosis and TB.

CONCLUSION

The in-house multiplex PCR detected MAP in buffy coat and there was a fair degree of agreement between the multiplex PCR and IS900 PCR in detection of MAP DNA though the latter detected more number of animals to be positive for MAP. Besides, esxB PCR showed a high diagnostic potential and can be used for diagnosis of TB from blood.

Atypical femur fracture incidence in women increases with duration of bisphosphonate exposure

In a northern California population of older women who were treated with oral bisphosphonate drugs, the incidence of atypical femur fracture, a rare complication of treatment, increased with longer duration of bisphosphonate exposure. These findings align with those previously reported in an independent southern California population.

INTRODUCTION

The age-adjusted incidence of atypical femur fracture (AFF) reported in southern California increased with bisphosphonate (BP) exposure, ranging up to 113 per 100,000 person-years for 8-10-year exposure. This study examines the incidence of AFF in a northern California population.

METHODS

Women age 45-89 years who initiated oral BP during 2002-2014 in Kaiser Permanente Northern California were followed for AFF outcome, defined by a primarily transverse diaphyseal femur fracture through both cortices, with focal periosteal/endosteal hypertrophy, minimal trauma, and minimal/no comminution. Total BP exposure was determined from dispensed prescriptions. The incidence of AFF, calculated for 2-year BP categories ranging from < 2 to > 10 years, was age-adjusted using the 2000 US Census.

RESULTS

Among 94,542 women, 107 experienced an AFF during or < 1 year after BP cessation (mean exposure 6.6 ± 3.0 years and total days' supply 5.7 ± 2.8 years at AFF). A strong relationship between AFF incidence and increasing BP exposure was seen, more than doubling for each 2-year category until 8-10 years. Among women with 2- to < 4-year BP, the crude and age-adjusted incidence was 18 and 9 per 100,000 person-years but increased over 2- and 5-fold for women with 4- to < 6- and 6- to < 8-year BP, respectively. For those receiving ≥ 8-year BP, the crude and age-adjusted incidence peaked at 196 and 112 per 100,000 person-years exposure.

CONCLUSION

Incidence of AFF increases markedly after 4-6 years of BP. These trends align with southern California and confirm a strong BP duration-related risk of this rare but serious event.

Drosophila Snazarus Regulates a Lipid Droplet Population at Plasma Membrane-Droplet Contacts in Adipocytes

Adipocytes store nutrients as lipid droplets (LDs), but how they organize their LD stores to balance lipid uptake, storage, and mobilization remains poorly understood. Here, using Drosophila fat body (FB) adipocytes, we characterize spatially distinct LD populations that are maintained by different lipid pools. We identify peripheral LDs (pLDs) that make close contact with the plasma membrane (PM) and are maintained by lipophorin-dependent lipid trafficking. pLDs are distinct from larger cytoplasmic medial LDs (mLDs), which are maintained by FASN1-dependent de novo lipogenesis. We find that sorting nexin CG1514 or Snazarus (Snz) associates with pLDs and regulates LD homeostasis at ER-PM contact sites. Loss of SNZ perturbs pLD organization, whereas Snz over-expression drives LD expansion, triacylglyceride production, starvation resistance, and lifespan extension through a DESAT1-dependent pathway. We propose that Drosophila adipocytes maintain spatially distinct LD populations and identify Snz as a regulator of LD organization and inter-organelle crosstalk.

Classification of the human phox homology (PX) domains based on their phosphoinositide binding specificities

Phox homology (PX) domains are membrane interacting domains that bind to phosphatidylinositol phospholipids or phosphoinositides, markers of organelle identity in the endocytic system. Although many PX domains bind the canonical endosome-enriched lipid PtdIns3P, others interact with alternative phosphoinositides, and a precise understanding of how these specificities arise has remained elusive. Here we systematically screen all human PX domains for their phospholipid preferences using liposome binding assays, biolayer interferometry and isothermal titration calorimetry. These analyses define four distinct classes of human PX domains that either bind specifically to PtdIns3P, non-specifically to various di- and tri-phosphorylated phosphoinositides, bind both PtdIns3P and other phosphoinositides, or associate with none of the lipids tested. A comprehensive evaluation of PX domain structures reveals two distinct binding sites that explain these specificities, providing a basis for defining and predicting the functional membrane interactions of the entire PX domain protein family.

Atypical Switch-I Arginine plays a catalytic role in GTP hydrolysis by Rab21 from Entamoeba histolytica

Entamoeba histolytica, the causative agent of amoebic dysentery, liver abscess and colitis, exploits its vesicular trafficking machinery for survival and virulence. Rab family of small GTPases play a key role in the vesicular transport by undergoing the GTP/GDP cycle which is central to the biological processes. Amoebic genome encodes several atypical Rab GTPases which are unique due to absence of conserved sequence motif(s) or atypical residues in their catalytic site [Saito-Nakano et al., 2005 ]. Previously, EhRab21 has been reported to involve in amoebic invasion and migration [Emmanuel et al., 2015 ]. The conserved Glutamine of switch-II region is universally accepted to be crucial for GTP hydrolysis. Mutations that reduce the sidechain polarity of Glutamine render the protein GTPase activity deficient [Krengel et al., 1990]. Here, we report a catalytic role of atypical switch-I Arginine (R36) in intrinsic GTP hydrolysis catalysed by EhRab21. Unlike the GTPase activity deficient QL mutants, the GTPase activity of EhRab21Q64L was found to be marginally enhanced compared to the wild-type protein. Although EhRab21R36L mutant showed normal GTPase activity, the double mutant (R36L/Q64L) was found to be GTPase deficient. Thus, EhRab21 is a unique member of small GTPase family in which an atypical switch-I Arginine is capable of driving GTP hydrolysis independent of the conserved switch-II Glutamine.

Structural insights into the architecture and membrane interactions of the conserved COMMD proteins

The COMMD proteins are a conserved family of proteins with central roles in intracellular membrane trafficking and transcription. They form oligomeric complexes with each other and act as components of a larger assembly called the CCC complex, which is localized to endosomal compartments and mediates the transport of several transmembrane cargos. How these complexes are formed however is completely unknown. Here, we have systematically characterised the interactions between human COMMD proteins, and determined structures of COMMD proteins using X-ray crystallography and X-ray scattering to provide insights into the underlying mechanisms of homo- and heteromeric assembly. All COMMD proteins possess an α-helical N-terminal domain, and a highly conserved C-terminal domain that forms a tightly interlocked dimeric structure responsible for COMMD-COMMD interactions. The COMM domains also bind directly to components of CCC and mediate non-specific membrane association. Overall these studies show that COMMD proteins function as obligatory dimers with conserved domain architectures.

Naturally Produced Extracellular Matrix Is an Excellent Substrate for Canine Endothelial Cell Proliferation and Resistance to Shear Stress on PTFE Vascular Grafts

Purpose: Successful development of a vascular prosthesis lined with endothelial cells (EC) may depend on the ability of the attached cells to resist shear forces after implantation. The present study was designed to investigate EC detachment from extracellular matrix (ECM) precoated vascular prostheses, caused by shear stress in vitro and to test the performance of these grafts in vivo. Methods: Bovine aortic endothelial cells were seeded inside untreated polytetrafluoro-ethylene (PTFE) vascular graft (10 X 0.6 cm), PTFE graft precoated with fibronectin (FN), or PTFE precoated with FN and a naturally produced ECM (106 cells/graft). Sixteen hours after seeding the medium was replaced and unattached cells counted. The strength of endothelial cell attachment was evaluated by subjecting the grafts to a physiologic shear stress of 15 dynes/cm2 for 1 h. The detached cells were collected and quantitated. PTFE or EC preseeded ECM coated grafts were implanted in the common carotid arteries of dogs. Results: While little or no differences were found in the extent of endothelial cell attachment to the various grafts (79%, 87% and 94% of the cells attached to PTFE, FN precoated PTFE, or FN+ECM precoated PTFE, respectively), the number of cells retained after a shear stress was significanly increased on ECM coated PTFE (20%, 54% and 85% on PTFE, FN coated PTFE, and FN+ECM coated PTFE, respectively, p <0.01). Implantation experiments in dogs revealed a significant increase in EC coverage and a reduced incidence of thrombus formation on ECM coated grafts that were seeded with autologous saphenous vein endothelial cells prior to implantation. Conclusion: ECM coating significantly increased the strength of endothelial cell attachment to vascular prostheses subjected to shear stress. The presence of adhesive macromolecules and potent endothelial cell growth promoting factors may render the ECM a promising substrate for vascular prostheses.

Sorting nexin 27 (SNX27) regulates the trafficking and activity of the glutamine transporter ASCT2

Alanine-, serine-, cysteine-preferring transporter 2 (ASCT2, SLC1A5) is responsible for the uptake of glutamine into cells, a major source of cellular energy and a key regulator of mammalian target of rapamycin (mTOR) activation. Furthermore, ASCT2 expression has been reported in several human cancers, making it a potential target for both diagnostic and therapeutic purposes. Here we identify ASCT2 as a membrane-trafficked cargo molecule, sorted through a direct interaction with the PDZ domain of sorting nexin 27 (SNX27). Using both membrane fractionation and subcellular localization approaches, we demonstrate that the majority of ASCT2 resides at the plasma membrane. This is significantly reduced within CrispR-mediated SNX27 knockout (KO) cell lines, as it is missorted into the lysosomal degradation pathway. The reduction of ASCT2 levels in SNX27 KO cells leads to decreased glutamine uptake, which, in turn, inhibits cellular proliferation. SNX27 KO cells also present impaired activation of the mTOR complex 1 (mTORC1) pathway and enhanced autophagy. Taken together, our data reveal a role for SNX27 in glutamine uptake and amino acid-stimulated mTORC1 activation via modulation of ASCT2 intracellular trafficking.