The role of SPIRE actin nucleators in cellular transport processes

Looking back at two decades of research on SPIRE actin nucleator proteins, the first decade was clearly dominated by the discovery of SPIRE proteins as founding members of the novel WH2-domain-based actin nucleators, which initiate actin filament assembly through multiple WH2 actin-binding domains. Through complex formation with formins and class 5 myosins, SPIRE proteins coordinate actin filament assembly and myosin motor-dependent force generation. The discovery of SPIRE-regulated cytoplasmic actin filament meshworks in oocytes initiated the next phase of SPIRE research, which has found that SPIRE proteins are integrated in a diverse range of cell biological processes. In addition to regulating vesicle-based actin filament meshworks, SPIRE proteins function in the organisation of actin structures driving the inward movement of pronuclei of the mouse zygote. Localisation at cortical ring structures and the results of knockdown experiments indicate that SPIRE proteins function in the formation of meiotic cleavage sites in mammalian oocytes and the externalisation of von Willebrand factor from endothelial cells. Alternative splicing targets mammalian SPIRE1 towards mitochondria, where it has a role in fission. In this Review, we summarise the past two decades of SPIRE research by addressing the biochemical and cell biological functions of SPIRE proteins in mammalian reproduction, skin pigmentation and wound healing, as well as in mitochondrial dynamics and host-pathogen interactions.

Oligodendrocytes regulate the adhesion molecule ICAM-1 in neuroinflammation

Recently, oligodendrocytes (Ol) have been attributed potential immunomodulatory effects. Yet, the exact mode of interaction with pathogenic CNS infiltrating lymphocytes remains unclear. Here, we attempt to dissect mechanisms of Ol modulation during neuroinflammation and characterize the interaction of Ol with pathogenic T cells. RNA expression analysis revealed an upregulation of immune-modulatory genes and adhesion molecules (AMs), ICAM-1 and VCAM-1, in Ol when isolated from mice undergoing experimental autoimmune encephalomyelitis (EAE). To explore whether AMs are involved in the interaction of Ol with infiltrating T cells, we performed co-culture studies on mature Ol and Th1 cells. Live cell imaging analysis showed direct interaction between both cell types. Eighty percentage of Th1 cells created contacts with Ol that lasted longer than 15 min, which may be regarded as physiologically relevant. Exposure of Ol to Th1 cells or their supernatant resulted in a significant extension of Ol processes, and upregulation of AMs as well as other immunomodulatory genes. Our observations indicate that blocking of oligodendroglial ICAM-1 can reduce the number of Th1 cells initially contacting the Ol. These results suggest that AMs may play a role in the interaction between Ol and Th1 cells. We identified Ol interacting with CD4⁺ cells in vivo in spinal cord tissue of EAE diseased mice indicating that our in vitro findings are of interest to further scientific research in this field. Further characterization and understanding of Ol interaction with infiltrating cells may lead to new therapeutic strategies enhancing Ol protection and remyelination potential. Oligodendrocytes regulate immune modulatory genes and adhesion molecules during autoimmune neuroinflammation Oligodendrocytes interact with Th1 cells in vitro in a physiologically relevant manner Adhesion molecules may be involved in Ol-Th1 cell interaction.

Rab27a co-ordinates actin-dependent transport by controlling organelle-associated motors and track assembly proteins

Cell biologists generally consider that microtubules and actin play complementary roles in long- and short-distance transport in animal cells. On the contrary, using melanosomes of melanocytes as a model, we recently discovered that the motor protein myosin-Va works with dynamic actin tracks to drive long-range organelle dispersion in opposition to microtubules. This suggests that in animals, as in yeast and plants, myosin/actin can drive long-range transport. Here, we show that the SPIRE-type actin nucleators (predominantly SPIRE1) are Rab27a effectors that co-operate with formin-1 to generate actin tracks required for myosin-Va-dependent transport in melanocytes. Thus, in addition to melanophilin/myosin-Va, Rab27a can recruit SPIREs to melanosomes, thereby integrating motor and track assembly activity at the organelle membrane. Based on this, we suggest a model in which organelles and force generators (motors and track assemblers) are linked, forming an organelle-based, cell-wide network that allows their collective activity to rapidly disperse the population of organelles long-distance throughout the cytoplasm.

RELN signaling modulates glioblastoma growth and substrate-dependent migration

Glioblastoma (GBM) represents the most common and most malignant type of primary brain tumor and significantly contributes to cancer morbidity and mortality. Invasion into the healthy brain parenchyma is a major feature of glioblastoma aggressiveness. Reelin (RELN) is a large secreted extracellular matrix glycoprotein that regulates neuronal migration and positioning in the developing brain and sustains functionality in the adult brain. We here show that both RELN and its main downstream effector DAB1 are silenced in glioblastoma as compared to non-neoplastic tissue and mRNA expression is inversely correlated with malignancy grade. Furthermore, RELN expression is positively correlated with patient survival in two large, independent clinically annotated datasets. RELN silencing occurs via promoter hypermethylation as shown by both database mining and bisulfite sequencing of the RELN promoter. Consequently, treatment with 5'-Azacytidine and trichostatin A induced RELN expression in vitro. On the functional level, we found RELN to regulate glioblastoma cell migration both in a DAB1 (tyrosine phosphorylation)-dependent and -independent fashion, depending on the substrate provided. Moreover, stimulation of RELN signaling strongly reduced proliferation in glioblastoma cells. This phenotype depends on DAB1 stimulation by RELN, as a mutant that lacks all RELN induced tyrosine phosphorylation sites (DAB1-5F) failed to induce a growth arrest. Proteomic analyzes revealed that these effects are mediated by a reduction in E2F targets and dephosphorylation of ERK1/2. Taken together, our data establish a relevance of RELN signaling in glioblastoma pathology and thereby might unearth novel, yet unrecognized treatment options.

Exploring the iceberg: Prospects of coordinated myosin V and actin assembly functions in transport processes

Spir actin nucleators and myosin V motor proteins were recently discovered to coexist in a protein complex. The direct interaction allows the coordinated activation of actin motor proteins and actin filament track generation at vesicle membranes. By now the cooperation of myosin V (MyoV) motors and Spir actin nucleation function has only been shown in the exocytic transport of Rab11 vesicles in metaphase mouse oocytes. Next to Rab11, myosin V motors however interact with a variety of Rab GTPases including Rab3, Rab8 and Rab10. As a common theme most of the MyoV interacting Rab GTPases function at different steps along the exocytic transport routes. We here summarize the different transport functions of class V myosins and provide as proof of principle data showing a colocalization of Spir actin nucleators and MyoVa at Rab8a vesicles. This suggests that besides Rab11/MyoV transport also the Rab8/MyoV and possibly other MyoV transport processes recruit Spir actin filament nucleation function.

Higher rates of neuropsychiatric adverse events leading to dolutegravir discontinuation in women and older patients

OBJECTIVES

Dolutegravir (DTG), a second-generation integrase strand transfer inhibitor (INSTI), is now among the most frequently used antiretroviral agents. However, recent reports have raised concerns about potential neurotoxicity.

METHODS

We performed a retrospective analysis of a cohort of HIV-infected patients who had initiated an INSTI in two large German out-patient clinics between 2007 and 2016. We compared discontinuation rates because of adverse events (AEs) within 2 years of starting treatment with dolutegravir, raltegravir or elvitegravir/cobicistat. We also evaluated factors associated with dolutegravir discontinuation.

RESULTS

A total of 1950 INSTI-based therapies were initiated in 1704 patients eligible for analysis within the observation period. The estimated rates of any AE and of neuropsychiatric AEs leading to discontinuation within 12 months were 7.6% and 5.6%, respectively, for dolutegravir (n = 985), 7.6% and 0.7%, respectively, for elvitegravir (n = 287), and 3.3% and 1.9%, respectively, for raltegravir (n = 678). Neuropsychiatric AEs leading to dolutegravir discontinuation were observed more frequently in women [hazard ratio (HR) 2.64; 95% confidence interval (CI) 1.23-5.65; P = 0.012], in patients older than 60 years (HR: 2.86; 95% CI: 1.42-5.77; P = 0.003) and in human leucocyte antigen (HLA)-B*5701-negative patients who initiated abacavir at the same time (HR: 2.42; 95% CI: 1.38-4.24; P = 0.002).

CONCLUSIONS

In this large cohort, the rate of discontinuation of dolutegravir because of neuropsychiatric adverse events was significantly higher than for other INSTIs, at almost 6% within 12 months. Despite the limitations of this retrospective study, the almost three-fold higher discontinuation rates observed amongst women and older patients underscore the need for further investigation, especially in patient populations usually underrepresented in clinical trials.

A Genome-Wide siRNA Screen Implicates Spire1/2 in SipA-Driven Salmonella Typhimurium Host Cell Invasion

Salmonella Typhimurium (S. Tm) is a leading cause of diarrhea. The disease is triggered by pathogen invasion into the gut epithelium. Invasion is attributed to the SPI-1 type 3 secretion system (T1). T1 injects effector proteins into epithelial cells and thereby elicits rearrangements of the host cellular actin cytoskeleton and pathogen invasion. The T1 effector proteins SopE, SopB, SopE2 and SipA are contributing to this. However, the host cell factors contributing to invasion are still not completely understood. To address this question comprehensively, we used Hela tissue culture cells, a genome-wide siRNA library, a modified gentamicin protection assay and S. TmSipA, a sopBsopE2sopE mutant which strongly relies on the T1 effector protein SipA to invade host cells. We found that S. TmSipA invasion does not elicit membrane ruffles, nor promote the entry of non-invasive bacteria "in trans". However, SipA-mediated infection involved the SPIRE family of actin nucleators, besides well-established host cell factors (WRC, ARP2/3, RhoGTPases, COPI). Stage-specific follow-up assays and knockout fibroblasts indicated that SPIRE1 and SPIRE2 are involved in different steps of the S. Tm infection process. Whereas SPIRE1 interferes with bacterial binding, SPIRE2 influences intracellular replication of S. Tm. Hence, these two proteins might fulfill non-redundant functions in the pathogen-host interaction. The lack of co-localization hints to a short, direct interaction between S. Tm and SPIRE proteins or to an indirect effect.

Coordinated recruitment of Spir actin nucleators and myosin V motors to Rab11 vesicle membranes

There is growing evidence for a coupling of actin assembly and myosin motor activity in cells. However, mechanisms for recruitment of actin nucleators and motors on specific membrane compartments remain unclear. Here we report how Spir actin nucleators and myosin V motors coordinate their specific membrane recruitment. The myosin V globular tail domain (MyoV-GTD) interacts directly with an evolutionarily conserved Spir sequence motif. We determined crystal structures of MyoVa-GTD bound either to the Spir-2 motif or to Rab11 and show that a Spir-2:MyoVa:Rab11 complex can form. The ternary complex architecture explains how Rab11 vesicles support coordinated F-actin nucleation and myosin force generation for vesicle transport and tethering. New insights are also provided into how myosin activation can be coupled with the generation of actin tracks. Since MyoV binds several Rab GTPases, synchronized nucleator and motor targeting could provide a common mechanism to control force generation and motility in different cellular processes.

Ibuprofen and Diclofenac Restrict Migration and Proliferation of Human Glioma Cells by Distinct Molecular Mechanisms

Background

Non-steroidal anti-inflammatory drugs (NSAIDs) have been associated with anti-tumorigenic effects in different tumor entities. For glioma, research has generally focused on diclofenac; however data on other NSAIDs, such as ibuprofen, is limited. Therefore, we performed a comprehensive investigation of the cellular, molecular, and metabolic effects of ibuprofen and diclofenac on human glioblastoma cells.

Methods

Glioma cell lines were treated with ibuprofen or diclofenac to investigate functional effects on proliferation and cell motility. Cell cycle, extracellular lactate levels, lactate dehydrogenase-A (LDH-A) expression and activity, as well as inhibition of the Signal Transducer and Activator of Transcription 3 (STAT-3) signaling pathway, were determined. Specific effects of diclofenac and ibuprofen on STAT-3 were investigated by comparing their effects with those of the specific STAT-3 inhibitor STATTIC.

Results

Ibuprofen treatment led to a stronger inhibition of cell growth and migration than treatment with diclofenac. Proliferation was affected by cell cycle arrest at different checkpoints by both agents. In addition, diclofenac, but not ibuprofen, decreased lactate levels in all concentrations used. Both decreased STAT-3 phosphorylation; however, diclofenac led to decreased c-myc expression and subsequent reduction in LDH-A activity, whereas treatment with ibuprofen in higher doses induced c-myc expression and less LDH-A alteration.

Conclusions

This study indicates that both ibuprofen and diclofenac strongly inhibit glioma cells, but the subsequent metabolic responses of both agents are distinct. We postulate that ibuprofen may inhibit tumor cells also by COX- and lactate-independent mechanisms after long-term treatment in physiological dosages, whereas diclofenac mainly acts by inhibition of STAT-3 signaling and downstream modulation of glycolysis.

Membrane targeting of the Spir·formin actin nucleator complex requires a sequential handshake of polar interactions

Spir and formin (FMN)-type actin nucleators initiate actin polymerization at vesicular membranes necessary for long range vesicular transport processes. Here we studied in detail the membrane binding properties and protein/protein interactions that govern the assembly of the membrane-associated Spir·FMN complex. Using biomimetic membrane models we show that binding of the C-terminal Spir-2 FYVE-type zinc finger involves both the presence of negatively charged lipids and hydrophobic contributions from the turret loop that intrudes the lipid bilayer. In solution, we uncovered a yet unknown intramolecular interaction between the Spir-2 FYVE-type domain and the N-terminal kinase non-catalytic C-lobe domain (KIND) that could not be detected in the membrane-bound state. Interestingly, we found that the intramolecular Spir-2 FYVE/KIND and the trans-regulatory Fmn-2-FSI/Spir-2-KIND interactions are competitive. We therefore characterized co-expressed Spir-2 and Fmn-2 fluorescent protein fusions in living cells by fluorescence cross-correlation spectroscopy. The data corroborate a model according to which Spir-2 exists in two different states, a cytosolic monomeric conformation and a membrane-bound state in which the KIND domain is released and accessible for subsequent Fmn-2 recruitment. This sequence of interactions mechanistically couples membrane binding of Spir to the recruitment of FMN, a pivotal step for initiating actin nucleation at vesicular membranes.

Orchestration of cell surface proteins by Rab11

The organization of cells into interconnected structures such as animal tissues requires a sophisticated system directing receptors and adhesion proteins to the cell surface. The Rab11 small G proteins (Rab11a, b, and Rab25) of the Ras superfamily are master regulators of the surface expression of receptors and adhesion proteins. Acting as a molecular switch, Rab11 builds distinct molecular machinery such as motor protein complexes and the exocyst to transport proteins to the cell surface. Recent evidence reveals Rab11 localization at the trans-Golgi network (TGN), post-Golgi vesicles, and the recycling endosome, placing it at the intersection between the endocytic and exocytic trafficking pathways. We review Rab11 in various cellular contexts, and discuss its regulation and mechanisms by which Rab11 couples with effector proteins.

Enhanced fear expression in Spir-1 actin organizer mutant mice

Spir proteins nucleate actin filaments at vesicle membranes and facilitate intracellular transport processes. The mammalian genome encodes two Spir proteins, namely Spir-1 and Spir-2. While the mouse spir-2 gene has a rather broad expression pattern, high levels of spir-1 expression are restricted to the nervous system, oocytes, and testis. Spir-1 mutant mice generated by a gene trap method have been employed to address Spir-1 function during mouse development and in adult mouse tissues, with a specific emphasis on viability, reproduction, and the nervous system. The gene trap cassette disrupts Spir-1 expression between the N-terminal KIND domain and the WH2 domain cluster. Spir-1 mutant mice are viable and were born in a Mendelian ratio. In accordance with the redundant function of Spir-1 and Spir-2 in oocyte maturation, spir-1 mutant mice are fertile. The overall brain anatomy of spir-1 mutant mice is not altered and visual and motor functions of the mice remain normal. Microscopic analysis shows a slight reduction in the number of dendritic spines on cortical neurons. Detailed behavioral studies of the spir-1 mutant mice, however, unveiled a very specific and highly significant phenotype in terms of fear learning in male mice. In contextual and cued fear conditioning experiments the male spir-1 mutant mice display increased fear memory when compared to their control littermates. Our data point toward a particular function of the vesicle associated Spir-1 actin organizer in neuronal circuits determining fear behavior.

Sexual behaviour, condom use and rates of sexually transmitted infections in HIV clinic attendees in South East London

The aim of this study was to examine sexual behaviour, condom use and rates of sexually transmitted infections (STIs) among attendees at a dedicated on-site STI clinic at a South London HIV centre. Data were prospectively collected by using a nurse-completed questionnaire. Ninety-eight percent of women reported one or no sexual partners in the preceding three months, whereas 57% of men who have sex with men (MSM) reported two or more partners. Only 28% of women, 53% of heterosexual men and 29% of MSM always used a condom for vaginal or anal intercourse. Positive STI diagnoses were found in 17.5% of women, 20% of heterosexual men and 49% of MSM. Twenty percent of patients who reported always using a condom and 38% of MSM reporting no sexual activity in the preceding three months had an STI. These results highlight the need for safe sex promotion and STI screening in HIV-infected patients regardless of self-reported sexual activity.

Improvement in screening for sexually transmitted infections in HIV-positive patients following implementation of a nurse-led clinic

The objective of this study was to assess changes in the uptake of sexually transmitted infection (STI) screening resulting from implementation of a specific STI clinic for HIV-positive patients. An audit of STI screening in HIV-positive patients was undertaken before and after the establishment of the clinic. Significant increases in the number of patients having a sexual health discussion and annual health screen were seen. High rates of positive results were found in all patient groups. National guidelines recommend regular STI screening for all HIV-positive patients. A specific clinic for HIV-positive patients is feasible and acceptable and may result in an increase in uptake of STI screening.

Why are children still being infected with HIV? Experiences in the prevention of mother-to-child transmission of HIV in south London

OBJECTIVES

To evaluate the effectiveness of interventions to prevent mother-to-child transmission of HIV at a large teaching hospital in South East London, and to assess reasons for the small numbers of transmissions that continue to occur.

DESIGN

A database of all pregnant women diagnosed as HIV positive between 1993 and 2005 was reviewed, with detailed (retrospective) case-note review of all mother-infant pairs where HIV transmission occurred.

SETTING

King's College Hospital, London, UK, a teaching hospital serving an ethnically diverse and socially deprived population.

RESULTS

296 pregnancies to 274 women were recorded. 9 of 296 (3.0%) women were lost to follow-up before the end of the pregnancy. Of 287 pregnancies followed up until after delivery, 6 (2.1%) resulted in HIV infection in the infant. More recently, between 2000 and 2004, this transmission rate was even lower, at 3 in 231 (1.3%). Each of these six women had complications, including late presentation to services and defaulting follow-up appointments, which were likely to increase the risk of HIV transmission. Four of the six transmissions occurred in utero.

CONCLUSION

The overall transmission rate of 2% attests to the efforts of the multidisciplinary care team in managing this population which is often hard to reach. Clearly, good systems are needed to trace those women who default. Further data are needed regarding in utero transmissions.

Severe acute maternal morbidity in rural South Africa

OBJECTIVE

Our objective was to identify the frequency, causes, and avoidable factors of severe acute maternal morbidity (SAMM) in four non-specialist hospitals in rural South Africa.

METHOD

We conducted a prospective audit using criteria for SAMM suited to the diagnostic and treatment facilities available in the primary hospital setting. For each case of SAMM, a local audit team assessed the standard of care against local management guidelines and examined avoidable factors. An external specialist also retrospectively examined avoidable factors.

RESULT

The facility-based incidence of SAMM was 541 cases per 100,000 births (95% CI 368-767). The commonest organ systems involved were cerebral (42%), coagulation (19%), and vascular dysfunctions (16%). The commonest obstetric diagnoses were eclampsia (39%) and obstetric haemorrhage (32%). Approximately 65% of cases were avoidable.

CONCLUSION

A qualitative case review audit of SAMM in a non-specialist rural setting appears feasible and sustainable, and provides valuable information towards improving deficiencies in maternal care.