The formins FMNL1 and mDia1 regulate coiling phagocytosis of Borrelia burgdorferi by primary human macrophages

Light-guided actin polymerization drives directed motility in protocells

Motility is a hallmark of life's dynamic processes, enabling cells to actively chase prey, repair wounds, and shape organs. Recreating these intricate behaviors using well-defined molecules remains a major challenge at the intersection of biology, physics, and molecular engineering. Although the polymerization force of the actin cytoskeleton is characterized as a primary driver of cell motility, recapitulating this process in protocellular systems has proven elusive. The difficulty lies in the daunting task of distilling key components from motile cells and integrating them into model membranes in a physiologically relevant manner. To address this, we developed a method to optically control actin polymerization with high spatiotemporal precision within cell-mimetic lipid vesicles known as giant unilamellar vesicles (GUVs). Within these active protocells, the reorganization of actin networks triggered outward membrane extensions as well as the unidirectional movement of GUVs at speeds of up to 0.43 μm/min, comparable to typical adherent mammalian cells. Notably, our findings reveal a synergistic interplay between branched and linear actin forms in promoting membrane protrusions, highlighting the cooperative nature of these cytoskeletal elements. This approach offers a powerful platform for unraveling the intricacies of cell migration, designing synthetic cells with active morphodynamics, and advancing bioengineering applications, such as self-propelled delivery systems and autonomous tissue-like materials.

Transendothelial migration of the Lyme disease spirochete involves spirochete internalization as an intermediate step through a transcellular pathway that involves Cdc42 and Rac1

Despite its importance in pathogenesis, the hematogenous dissemination pathway of B. burgdorferi is still largely uncharacterized. To probe the molecular details of transendothelial migration more easily, we studied this process using cultured primary or telomerase-immortalized human microvascular endothelial cells in a medium that maintains both the human cells and the spirochetes. In B. burgdorferi infected monolayers we observed ∼55% of wild-type spirochetes crossing the monolayer. Microscopic characterization revealed entrance points across the cellular surface rather than at cellular junctions, supporting a transcellular route. In support of this pathway, locking the endothelial junctions using a VE-PTP inhibitor did not reduce transendothelial migration. We also used inhibitors to block the most common endocytic pathways to elucidate effectors that might be involved in B. burgdorferi uptake and/or transmigration. Directly inhibiting Cdc42 reduced spirochete transmigration by impeding internalization. However, blocking Rac1 alone dramatically reduced transmigration and resulted in a concomitant increase in spirochete accumulation in the cell. Our combined results support that B. burgdorferi internalization is an intermediate step in the transendothelial migration process which requires both Cdc42 and Rac1; Cdc42 is needed for spirochete internalization while Rac1 is required for cellular egress. These are the first two host proteins implicated in B. burgdorferi transmigration across endothelial cells.

IMPORTANCE

Lyme borreliosis is caused by Borrelia burgdorferi and related bacteria. It is the most common tick-transmitted illness in the Northern Hemisphere. The ability of this pathogen to spread to a wide variety of locations results in a diverse set of clinical manisfestations, yet little is known regarding vascular escape of the spirochete, an important pathway for dissemination. Our current work has studied the traversal of B. burgdorferi across a monolayer of microvascular endothelial cells grown in culture. We show that this occurs by passage of the spirochetes directly through these cells rather than at cellular junctions and that internalization of B. burgdorferi is an intermediate step in the transmigration process. We also identify the first two host proteins, Cdc42 and Rac1, that are used by the spirochetes to promote traversal of the cellular monolayer. Our new experimental system also provides a new avenue for further studies of this important process.

Transmembrane formins as active cargoes of membrane trafficking

Formins are a large, evolutionarily old family of cytoskeletal regulators whose roles include actin capping and nucleation, as well as modulation of microtubule dynamics. The plant class I formin clade is characterized by a unique domain organization, as most of its members are transmembrane proteins with possible cell wall-binding motifs exposed to the extracytoplasmic space-a structure that appears to be a synapomorphy of the plant kingdom. While such transmembrane formins are traditionally considered mainly as plasmalemma-localized proteins contributing to the organization of the cell cortex, we review, from a cell biology perspective, the growing evidence that they can also, at least temporarily, reside (and in some cases also function) in endomembranes including secretory and endocytotic pathway compartments, the endoplasmic reticulum, the nuclear envelope, and the tonoplast. Based on this evidence, we propose that class I formins may thus serve as 'active cargoes' of membrane trafficking-membrane-embedded proteins that modulate the fate of endo- or exocytotic compartments while being transported by them.

Borrelia burgdorferi Engages Mammalian Type I IFN Responses via the cGAS-STING Pathway

Borrelia burgdorferi, the etiologic agent of Lyme disease, is a spirochete that modulates numerous host pathways to cause a chronic, multisystem inflammatory disease in humans. B. burgdorferi infection can lead to Lyme carditis, neurologic complications, and arthritis because of the ability of specific borrelial strains to disseminate, invade, and drive inflammation. B. burgdorferi elicits type I IFN (IFN-I) responses in mammalian cells and tissues that are associated with the development of severe arthritis or other Lyme-related complications. However, the innate immune sensors and signaling pathways controlling IFN-I induction remain unclear. In this study, we examined whether intracellular nucleic acid sensing is required for the induction of IFN-I to B. burgdorferi. Using fluorescence microscopy, we show that B. burgdorferi associates with mouse and human cells in culture, and we document that internalized spirochetes colocalize with the pattern recognition receptor cyclic GMP-AMP synthase (cGAS). Moreover, we report that IFN-I responses in mouse macrophages and murine embryonic fibroblasts are significantly attenuated in the absence of cGAS or its adaptor stimulator of IFN genes (STING), which function to sense and respond to intracellular DNA. Longitudinal in vivo tracking of bioluminescent B. burgdorferi revealed similar dissemination kinetics and borrelial load in C57BL/6J wild-type, cGAS-deficient, or STING-deficient mice. However, infection-associated tibiotarsal joint pathology and inflammation were modestly reduced in cGAS-deficient compared with wild-type mice. Collectively, these results indicate that the cGAS-STING pathway is a critical mediator of mammalian IFN-I signaling and innate immune responses to B. burgdorferi.

Stiff matrix induced srGAP2 tension gradients control migration direction in triple-negative breast cancer

Rationale: Cells migrating through interstitial matrix enables stiffening of the tumor micro-environment. To overcome the stiff resistance of extracellular matrix, aggressive cells require the extracellular mechanosensory activation and intracellular tension response. Mechanotransduction linker srGAP2 can synergistically control the mechanical-biochemical process of malignant cell migration. Methods: To mimic the tumor micro-environment containing abundant collagen fibers and moving durotaxis of triple-negative breast cancer cells, the stiff-directed matrix was established. The newly designed srGAP2 tension probe was used to real-time supervise srGAP2 tension in living cells. The phosphorylation sites responsible for srGAP2 tension were identified by phosphorylated mutagenesis. Transwell assays and Xenograft mouse model were performed to evaluate TNBC cells invasiveness in vitro and in vivo. Fluorescence staining and membrane protein isolation were used to detect protein localization. Results: The present study shows srGAP2 serves as a linker to transmit the mechanical signals among cytoskeleton and membrane. SrGAP2 exhibits tension gradients among different parts in the stiff-directionally migrating triple-negative breast cancer cells. Cells showing the polarized tension that increased in the leading edge move faster, particularly guided by the stiff interstitial matrix. The srGAP2 tension-directed cell migration results from the upstream events of PKCα-mediated phosphorylation at Ser²⁰⁶ in the F-bar domain of srGAP2. In addition, Syndecan-4 (SDC4), a transmembrane mechanoreceptor protein, drives PKCα regional recruit on the area of membrane trending deformation, which requires the distinct extent of extracellular mechanics. Conclusion: SDC4-PKCα polarized distribution leads to the intracellular tension gradient of srGAP2, presenting the extra- and intracellular physiochemical integration and essential for persistent cell migration in stiff matrix and caner progression. Targeting the srGAP2-related physicochemical signaling could be developed into the therapeutic strategies of inhibiting breast cancer cell invasion and durotaxis.

Distinct roles of the Chlamydia trachomatis effectors TarP and TmeA in the regulation of formin and Arp2/3 during entry

The obligate intracellular pathogen Chlamydia trachomatis manipulates the host actin cytoskeleton to assemble actin-rich structures that drive pathogen entry. The recent discovery of TmeA, which, like TarP, is an invasion-associated type III effector implicated in actin remodeling, raised questions regarding the nature of their functional interaction. Quantitative live-cell imaging of actin remodeling at invasion sites revealed differences in recruitment and turnover kinetics associated with the TarP and TmeA pathways, with the former accounting for most of the robust actin dynamics at invasion sites. TarP-mediated recruitment of actin nucleators, i.e. formins and the Arp2/3 complex, was crucial for rapid actin kinetics, generating a collaborative positive feedback loop that enhanced their respective actin-nucleating activities within invasion sites. In contrast, the formin Fmn1 was not recruited to invasion sites and did not collaborate with Arp2/3 within the context of TmeA-associated actin recruitment. Although the TarP-Fmn1-Arp2/3 signaling axis is responsible for the majority of actin dynamics, its inhibition had similar effects as the deletion of TmeA on invasion efficiency, consistent with the proposed model that TarP and TmeA act on different stages of the same invasion pathway.

Reforming the Barrier: The Role of Formins in Wound Repair

The restoration of an intact epidermal barrier after wound injury is the culmination of a highly complex and exquisitely regulated physiological process involving multiple cells and tissues, overlapping dynamic events and protein synthesis and regulation. Central to this process is the cytoskeleton, a system of intracellular proteins that are instrumental in regulating important processes involved in wound repair including chemotaxis, cytokinesis, proliferation, migration, and phagocytosis. One highly conserved family of cytoskeletal proteins that are emerging as major regulators of actin and microtubule nucleation, polymerization, and stabilization are the formins. The formin family includes 15 different proteins categorized into seven subfamilies based on three formin homology domains (FH1, FH2, and FH3). The formins themselves are regulated in different ways including autoinhibition, activation, and localization by a range of proteins, including Rho GTPases. Herein, we describe the roles and effects of the formin family of cytoskeletal proteins on the fundamental process of wound healing and highlight recent advances relating to their important functions, mechanisms, and regulation at the molecular and cellular levels.

Toxoplasma gondii infection and peripheral-blood gene expression profiling of older people reveals dysregulation of cytokines and identifies hub genes as potential therapeutic targets

Infections of humans with the protozoan parasite Toxoplasma gondii (T. gondii) can lead to the disease's development, even in an asymptomatic status. However, the mechanisms that result in these clinical outcomes after infection are poorly understood. This study aimed to explore the molecular pathogenesis of toxoplasmosis-related inflammation through next-generation sequencing, to assess RNA expression profiles in peripheral blood from 5 female patients with chronic toxoplasmosis and 5 healthy female controls. All plasma samples were analyzed for anti-Toxoplasma IgG and IgM antibody titers by using electrochemiluminescence. Detection of acute and chronic toxoplasmosis was carried out using the ELISA IgG avidity. We evaluated the levels of INF-γ, IL-2, IL-12, TNF-α, IL-10, and IL-1β in culture supernatants of Peripheral Blood Mononuclear Cells infected with Toxoplasma lysate antigen (TLA) prepared with tachyzoites of strain T. gondii RH. Differential expression analysis was performed using DESeq2, pathway and enrichment analysis of DEGs was done on WEB-based Gene SeT AnaLysis Toolkit (WebGestalt) and Protein-protein interaction was carried out using NetworkAnalyst with STRING. In older people with chronic asymptomatic infection, a significant difference in the levels of inflammatory cytokines INF-γ and IL-2 was observed compared to seronegative individuals. Our results revealed differences in the regulation of critical biological processes involved in host responses to chronic T. gondii infection. Gene ontology analysis revealed several biologically relevant inflammatory and immune-related pathways.

Neisseria gonorrhoeae subverts formin-dependent actin polymerization to colonize human macrophages

Dynamic reorganization of the actin cytoskeleton dictates plasma membrane morphogenesis and is frequently subverted by bacterial pathogens for entry and colonization of host cells. The human-adapted bacterial pathogen Neisseria gonorrhoeae can colonize and replicate when cultured with human macrophages, however the basic understanding of how this process occurs is incomplete. N. gonorrhoeae is the etiological agent of the sexually transmitted disease gonorrhea and tissue resident macrophages are present in the urogenital mucosa, which is colonized by the bacteria. We uncovered that when gonococci colonize macrophages, they can establish an intracellular or a cell surface-associated niche that support bacterial replication independently. Unlike other intracellular bacterial pathogens, which enter host cells as single bacterium, establish an intracellular niche and then replicate, gonococci invade human macrophages as a colony. Individual diplococci are rapidly phagocytosed by macrophages and transported to lysosomes for degradation. However, we found that surface-associated gonococcal colonies of various sizes can invade macrophages by triggering actin skeleton rearrangement resulting in plasma membrane invaginations that slowly engulf the colony. The resulting intracellular membrane-bound organelle supports robust bacterial replication. The gonococci-occupied vacuoles evaded fusion with the endosomal compartment and were enveloped by a network of actin filaments. We demonstrate that gonococcal colonies invade macrophages via a process mechanistically distinct from phagocytosis that is regulated by the actin nucleating factor FMNL3 and is independent of the Arp2/3 complex. Our work provides insights into the gonococci life-cycle in association with human macrophages and defines key host determinants for macrophage colonization.

During infection, the human-adapted bacterial pathogen Neisseria gonorrhoeae and causative agent of gonorrhea can invade the submucosa of the urogenital tract where it encounters tissue-resident innate immune sentinels, such as macrophages and neutrophils. Instead of eliminating gonococci, macrophages support robust bacterial replication. Here, we detail the life cycle of N. gonorrhoeae in association with macrophages and define key regulators that govern the colonization processes. We uncovered that N. gonorrhoeae establishes two distinct subcellular niches that support bacterial replication autonomously–one niche was on the macrophage surface and another one was intracellular. Gonococci subverted the host actin cytoskeleton through the actin nucleating factor FMNL3 to invade colonized macrophages and occupy a membrane-bound intracellular organelle. We propose that N. gonorrhoeae ability to occupy distinct subcellular niches when colonizing macrophages likely confers broad protection against multiple host defense responses.

mDia1 Assembles a Linear F-Actin Coat at Membrane Invaginations To Drive Listeria monocytogenes Cell-to-Cell Spreading

Direct cell-to-cell spreading of Listeria monocytogenes requires the bacteria to induce actin-based finger-like membrane protrusions in donor host cells that are endocytosed through caveolin-rich membrane invaginations by adjacent receiving cells. An actin shell surrounds these endocytic sites; however, its structure, composition, and functional significance remain elusive. Here, we show that the formin mDia1, but surprisingly not the Arp2/3 complex, is enriched at the membrane invaginations generated by L. monocytogenes during HeLa and Jeg-3 cell infections. Electron microscopy reveals a band of linear actin filaments that run along the longitudinal axis of the invagination membrane. Mechanistically, mDia1 expression is vital for the assembly of this F-actin shell. mDia1 is also required for the recruitment of Filamin A, a caveola-associated F-actin cross-linking protein, and caveolin-1 to the invaginations. Importantly, mixed-cell infection assays show that optimal caveolin-based L. monocytogenes cell-to-cell spreading correlates with the formation of the linear actin filament-containing shell by mDia1. IMPORTANCE Listeria monocytogenes spreads from one cell to another to colonize tissues. This cell-to-cell movement requires the propulsive force of an actin-rich comet tail behind the advancing bacterium, which ultimately distends the host plasma membrane into a slender bacterium-containing membrane protrusion. These membrane protrusions induce a corresponding invagination in the membrane of the adjacent host cell. The host cell that receives the protrusion utilizes caveolin-based endocytosis to internalize the structures, and filamentous actin lines these membrane invaginations. Here, we set out to determine the structure and function of this filamentous actin "shell." We demonstrate that the formin mDia1, but not the Arp2/3 complex, localizes to the invaginations. Morphologically, we show that this actin is organized into linear arrays and not branched dendritic networks. Mechanistically, we show that the actin shell is assembled by mDia1 and that mDia1 is required for efficient cell-to-cell transfer of L. monocytogenes.

Coordinated activity of amoebic formin and profilin are essential for phagocytosis

For the protist parasite Entamoeba histolytica, endocytic processes, such as phagocytosis, are essential for its survival in the human gut. The actin cytoskeleton is involved in the formation of pseudopods and phagosomal vesicles by incorporating a number of actin-binding and modulating proteins along with actin in a temporal manner. The actin dynamics, which comprises polymerization, branching, and depolymerization is very tightly regulated and takes place directionally at the sites of initiation of phagocytosis. Formin and profilin are two actin-binding proteins that are known to regulate actin cytoskeleton dynamics and thereby, endocytic processes. In this article, we report the participation of formin and profilin in E. histolytica phagocytosis and propose that these two proteins interact with each other and their sequential recruitment at the site is required for the successful completion of phagocytosis. The evidence is based on detailed microscopic, live imaging, interaction studies, and expression downregulation. The cells downregulated for expression of formin show absence of profilin at the site of phagocytosis, whereas downregulation of profilin does not affect formin localization.

Molecular Mechanisms of Borrelia burgdorferi Phagocytosis and Intracellular Processing by Human Macrophages

Lyme disease is the most common vector-borne illness in North America and Europe. Its causative agents are spirochetes of the Borrelia burgdorferi sensu latu complex. Infection with borreliae can manifest in different tissues, most commonly in the skin and joints, but in severe cases also in the nervous systems and the heart. The immune response of the host is a crucial factor for preventing the development or progression of Lyme disease. Macrophages are part of the innate immune system and thus one of the first cells to encounter infecting borreliae. As professional phagocytes, they are capable of recognition, uptake, intracellular processing and final elimination of borreliae. This sequence of events involves the initial capture and internalization by actin-rich cellular protrusions, filopodia and coiling pseudopods. Uptake into phagosomes is followed by compaction of the elongated spirochetes and degradation in mature phagolysosomes. In this review, we discuss the current knowledge about the processes and molecular mechanisms involved in recognition, capturing, uptake and intracellular processing of Borrelia by human macrophages. Moreover, we highlight interactions between macrophages and other cells of the immune system during these processes and point out open questions in the intracellular processing of borreliae, which include potential escape strategies of Borrelia.

Macrophage mediated recognition and clearance of Borrelia burgdorferi elicits MyD88-dependent and -independent phagosomal signals that contribute to phagocytosis and inflammation

BACKGROUND

Macrophages play prominent roles in bacteria recognition and clearance, including Borrelia burgdorferi (Bb), the Lyme disease spirochete. To elucidate mechanisms by which MyD88/TLR signaling enhances clearance of Bb by macrophages, we studied wildtype (WT) and MyD88-/- Bb-stimulated bone marrow-derived macrophages (BMDMs).

RESULTS

MyD88-/- BMDMs exhibit impaired uptake of spirochetes but comparable maturation of phagosomes following internalization of spirochetes. RNA-sequencing of infected WT and MyD88-/- BMDMs identified a large cohort of differentially expressed MyD88-dependent genes associated with re-organization of actin and cytoskeleton during phagocytosis along with several MyD88-independent chemokines involved in inflammatory cell recruitment. We computationally generated networks which identified several MyD88-dependent intermediate proteins (Rhoq and Cyfip1) that are known to mediate inflammation and phagocytosis respectively.

CONCLUSION

Our findings show that MyD88 signaling enhances, but is not required, for bacterial uptake or phagosomal maturation and provide mechanistic insights into how MyD88-mediated phagosomal signaling enhances Bb uptake and clearance.

Borrelia burgdorferi infection modifies protein content in saliva of Ixodes scapularis nymphs

BACKGROUND

Lyme disease (LD) caused by Borrelia burgdorferi is the most prevalent tick-borne disease. There is evidence that vaccines based on tick proteins that promote tick transmission of B. burgdorferi could prevent LD. As Ixodes scapularis nymph tick bites are responsible for most LD cases, this study sought to identify nymph tick saliva proteins associated with B. burgdorferi transmission using LC-MS/MS. Tick saliva was collected using a non-invasive method of stimulating ticks (uninfected and infected: unfed, and every 12 h during feeding through 72 h, and fully-fed) to salivate into 2% pilocarpine-PBS for protein identification using LC-MS/MS.

RESULTS

We identified a combined 747 tick saliva proteins of uninfected and B. burgdorferi infected ticks that were classified into 25 functional categories: housekeeping-like (48%), unknown function (18%), protease inhibitors (9%), immune-related (6%), proteases (8%), extracellular matrix (7%), and small categories that account for <5% each. Notably, B. burgdorferi infected ticks secreted high number of saliva proteins (n=645) than uninfected ticks (n=376). Counter-intuitively, antimicrobial peptides, which function to block bacterial infection at tick feeding site were suppressed 23-85 folds in B. burgdorferi infected ticks. Similar to glycolysis enzymes being enhanced in mammalian cells exposed to B. burgdorferi : eight of the 10-glycolysis pathway enzymes were secreted at high abundance by B. burgdorferi infected ticks. Of significance, rabbits exposed to B. burgdorferi infected ticks acquired potent immunity that caused 40-60% mortality of B. burgdorferi infected ticks during the second infestation compared to 15-28% for the uninfected. This might be explained by ELISA data that show that high expression levels of immunogenic proteins in B. burgdorferi infected ticks.

CONCLUSION

Data here suggest that B. burgdorferi infection modified protein content in tick saliva to promote its survival at the tick feeding site. For instance, enzymes; copper/zinc superoxide dismutase that led to production of H2O2 that is toxic to B. burgdorferi were suppressed, while, catalase and thioredoxin that neutralize H2O2, and pyruvate kinase which yields pyruvate that protects Bb from H2O2 killing were enhanced. We conclude data here is an important resource for discovery of effective antigens for a vaccine to prevent LD.

FIB-SEM-based analysis of Borrelia intracellular processing by human macrophages

Borrelia burgdorferi is the causative agent of Lyme disease, a multisystemic disorder affecting primarily skin, joints and nervous system. Successful internalization and intracellular processing of borreliae by immune cells, like macrophages, is decisive for the outcome of a respective infection. Here, we use, for the first time, focused ion beam scanning electron microscopy tomography (FIB-SEM tomography) to visualize the interaction of borreliae with primary human macrophages with high resolution. We report that interaction between macrophages and the elongated and highly motile borreliae can lead to formation of membrane tunnels that extend deeper into the host cytoplasm than the actual phagosome, most probably as a result of partial extrication of captured borreliae. We also show that membrane tubulation at borreliae-containing phagosomes, a process suggested earlier as a mechanism leading to phagosome compaction but hard to visualize in live-cell imaging, is apparently a frequent phenomenon. Finally, we demonstrate that the endoplasmic reticulum (ER) forms multiple STIM1-positive contact sites with both membrane tunnels and phagosome tubulations, confirming the important role of the ER during uptake and intracellular processing of borreliae.

Immune Response to Borrelia: Lessons from Lyme Disease Spirochetes

The mammalian host responds to infection with Borrelia spirochetes through a highly orchestrated immune defense involving innate and adaptive effector functions aimed toward limiting pathogen burdens, minimizing tissue injury, and preventing subsequent reinfection. The evolutionary adaptation of Borrelia spirochetes to their reservoir mammalian hosts may allow for its persistence despite this immune defense. This review summarizes our current understanding of the host immune response to B. burgdorferi sensu lato, the most widely studied Borrelia spp. and etiologic agent of Lyme borreliosis. Pertinent literature will be reviewed with emphasis on in vitro, ex vivo and animal studies that influenced our understanding of both the earliest responses to B. burgdorferi as it enters the mammalian host and those that evolve as spirochetes disseminate and establish infection in multiple tissues. Our focus is on the immune response of inbred mice, the most commonly studied animal model of B. burgdorferi infection and surrogate for one of this pathogen's principle natural reservoir hosts, the white-footed deer mouse. Comparison will be made to the immune responses of humans with Lyme borreliosis. Our goal is to provide an understanding of the dynamics of the mammalian immune response during infection with B. burgdorferi and its relation to the outcomes in reservoir (mouse) and non-reservoir (human) hosts.

Formin-like 1 mediates effector T cell trafficking to inflammatory sites to enable T cell-mediated autoimmunity

Lymphocyte migration is essential for the function of the adaptive immune system, and regulation of T cell entry into tissues is an effective therapy in autoimmune diseases. Little is known about the specific role of cytoskeletal effectors that mediate mechanical forces and morphological changes essential for migration in complex environments. We developed a new Formin-like-1 (FMNL1) knock-out mouse model and determined that the cytoskeletal effector FMNL1 is selectively required for effector T cell trafficking to inflamed tissues, without affecting naïve T cell entry into secondary lymphoid organs. Here, we identify a FMNL1-dependent mechanism of actin polymerization at the back of the cell that enables migration of the rigid lymphocyte nucleus through restrictive barriers. Furthermore, FMNL1-deficiency impairs the ability of self-reactive effector T cells to induce autoimmune disease. Overall, our data suggest that FMNL1 may be a potential therapeutic target to specifically modulate T cell trafficking to inflammatory sites.

SNX3 drives maturation of Borrelia phagosomes by forming a hub for PI(3)P, Rab5a, and galectin-9

Borrelia burgdorferi is the causative agent of Lyme disease. Klose et al. show that SNX3 drives processing of internalized B. burgdorferi by binding PI(3)P on the phagosome surface and recruiting galectin-9 vesicles, thus forming a convergence point for the endosomal recycling machinery during processing of spirochetes.

The spirochete Borrelia burgdorferi, the causative agent of Lyme disease, is internalized by macrophages and processed in phagolysosomes. Phagosomal compaction, a crucial step in phagolysosome maturation, is driven by contact of Rab5a-positive vesicles with the phagosomal coat. We show that the sorting nexin SNX3 is transported with Rab5a vesicles and that its PX domain enables vesicle–phagosome contact by binding to PI(3)P in the phagosomal coat. Moreover, the C-terminal region of SNX3 recruits galectin-9, a lectin implicated in protein and membrane recycling, which we identify as a further regulator of phagosome compaction. SNX3 thus forms a hub for two distinct vesicle populations, constituting a convergence point for the endosomal recycling machinery, to contribute to phagosome maturation and intracellular processing of borreliae. These data also suggest that the helical shape of B. burgdorferi itself, providing sites of high curvature and thus local PI(3)P enrichment at phagosomes, may be one of the driving elements underlying the efficient elimination of spirochetes by immune cells.

FMNL1 down-regulation suppresses bone metastasis through reducing TGF-β1 expression in non-small cell lung cancer (NSCLC)

Approximately 40% of patients with non-small cell lung cancer (NSCLC) develop bone metastasis. The formin protein formin-like 1 (FMNL1) plays a key role in the pathogenic processes of hematopoietic malignancies, and has been reported to be associated with the progression of multiple types of cancer. In the study, we found that FMNL1 expression was markedly up-regulated in primary NSCLC samples, and stronger expression of FNML1 was detected in bone metastasis. Reducing FMNL1 expression significantly suppressed cell proliferation in NSCLC cells. We also investigated the functional effects of FMNL1 knockdown on the inhibition of migration and invasion by meditating the expression of epithelial to mesenchymal transition (EMT)-associated signals in NSCLC cells. The transforming growth factor-β1 (TGF-β1)/SMADs signaling pathway was repressed in FMNL1-knockdown NSCLC cells. Further studies indicated that additional treatment with TGF-β1 could markedly abrogate FMNL1 knockdown-induced suppression of migration and invasion in NSCLC cells. In addition, NSCLC cell-induced osteoclastogenesis was also inhibited by FMNL1 deletion, as evidenced by the down-regulated expression of tartrate-resistant acid phosphatase (TRAP) and NFATc1. In vivo studies confirmed the results that FMNL1 knockdown markedly limited tumor growth. Importantly, decreasing FMNL1 reduced bone metastasis ability in vivo. Therefore, our results demonstrated that suppressing FMNL1 expression could inhibit bone metastasis in NSCLC through blocking TGF-β1 signaling, and FMNL1 might be a novel target for developing effective therapeutic strategy to limit the bone metastasis of NSCLC.

Roles for Ena/VASP proteins in FMNL3-mediated filopodial assembly

Filopodia are actin-dependent finger-like structures that protrude from the plasma membrane. Actin filament barbed-end-binding proteins localized to filopodial tips are key to filopodial assembly. Two classes of barbed-end-binding proteins are formins and Ena/VASP proteins, and both classes have been localized to filopodial tips in specific cellular contexts. Here, we examine the filopodial roles of the FMNL formins and Ena/VASP proteins in U2OS cells. FMNL3 suppression reduces filopodial assembly by 90%, and FMNL3 is enriched at >95% of filopodial tips. Suppression of VASP or Mena (also known as ENAH) reduces filopodial assembly by >75%. However, VASP and Mena do not display consistent filopodial tip localization, but are enriched in focal adhesions (FAs). Interestingly, >85% of FMNL3-containing filopodia are associated with FAs. Two situations increase Ena/VASP filopodial localization: (1) expression of myosin-X, and (2) actively spreading cells. In spreading cells, filopodia often mark sites of nascent adhesions. Interestingly, VASP suppression in spreading cells causes a significant increase in adhesion assembly at filopodial tips. This work demonstrates that, in U2OS cells, Ena/VASP proteins play roles in filopodia beyond those at filopodial tips.This article has an associated First Person interview with the first author of the paper.

The Formin mDia1 Regulates Acute Lymphoblastic Leukemia Engraftment, Migration, and Progression in vivo

Leukemias typically arise in the bone marrow and then spread to the blood and into other tissues. To disseminate into tissues, leukemia cells migrate into the blood stream and then exit the circulation by migrating across vascular endothelial barriers. Formin proteins regulate cytoskeletal remodeling and cell migration of normal and malignant cells. The Formin mDia1 is highly expressed in transformed lymphocytes and regulates lymphocyte migration. However, the role of mDia1 in regulating leukemia progression in vivo is unknown. Here, we investigated how mDia1 mediates the ability of leukemia cells to migrate and disseminate in vivo. For these studies, we used a mouse model of Bcr-Abl pre-B cell acute lymphoblastic leukemia. Our data showed that mDia1-deficient leukemia cells have reduced chemotaxis and ability to complete transendothelial migration in vitro. In vivo, mDia1 deficiency reduced the ability of leukemia cells to engraft in recipient mice. Furthermore, leukemia dissemination to various tissues and leukemia progression were inhibited by mDia1 depletion. Finally, mDia1 depletion in leukemia cells resulted in prolonged survival of recipient mice in a leukemia transfer model. Overall, our data show that the Formin mDia1 mediates leukemia cell migration, and drives leukemia engraftment and progression in vivo, suggesting that targeting mDia1 could provide a new method for treatment of leukemia.

Selective Regulation of Cytoskeletal Dynamics and Filopodia Formation by Teleost Leukocyte Immune-Type Receptors Differentially Contributes to Target Capture During the Phagocytic Process

Phagocytosis evolved from a fundamental nutrient acquisition mechanism in primitive unicellular amoeboids, into a dynamic and complex component of innate immunity in multicellular organisms. To better understand the cellular mechanisms contributing to phagocytic processes across vertebrates, our research has focused on characterizing the involvement of innate immune proteins originally identified in channel catfish (Ictalurus punctatus) called leukocyte immune-type receptors (IpLITRs). These unique teleost proteins share basic structural as well as distant phylogenetic relationships with several immunoregulatory proteins within the mammalian immunoglobulin superfamily. In the present study, we use a combination of live-cell confocal imaging and high-resolution scanning electron microscopy to further examine the classical immunoreceptor tyrosine-based activation motif (ITAM)-dependent phagocytic pathway mediated by the chimeric construct IpLITR 2.6b/IpFcRγ-L and the functionally diverse immunoreceptor tyrosine-based inhibitory motif-containing receptor IpLITR 1.1b. Results demonstrate that IpLITR 1.1b-expressing cells can uniquely generate actin-dense filopodia-like protrusions during the early stages of extracellular target interactions. In addition, we observed that these structures retract after contacting extracellular targets to secure captured microspheres on the cell surface. This activity was often followed by the generation of robust secondary waves of actin polymerization leading to the formation of stabilized phagocytic cups. At depressed temperatures of 27°C, IpLITR 2.6b/IpFcRγ-L-mediated phagocytosis was completely blocked, whereas IpLITR 1.1b-expressing cells continued to generate dynamic actin-dense filopodia at this lower temperature. Overall, these results provide new support for the hypothesis that IpLITR 1.1b, but not IpLITR 2.6b/IpFcRγ-L, directly triggers filopodia formation when expressed in representative myeloid cells. This also offers new information regarding the directed ability of immunoregulatory receptor-types to initiate dynamic membrane structures and provides insights into an alternative ITAM-independent target capture pathway that is functionally distinct from the classical phagocytic pathways.

Human neuroglial cells internalize Borrelia burgdorferi by coiling phagocytosis mediated by Daam1

Borrelia burgdorferi, the agent of Lyme borreliosis, can elude hosts’ innate and adaptive immunity as part of the course of infection. The ability of B. burgdorferi to invade or be internalized by host cells in vitro has been proposed as a mechanism for the pathogen to evade immune responses or antimicrobials. We have previously shown that B. burgdorferi can be internalized by human neuroglial cells. In this study we demonstrate that these cells take up B. burgdorferi via coiling phagocytosis mediated by the formin, Daam1, a process similarly described for human macrophages. Following coincubation with glial cells, B. burgdorferi was enwrapped by Daam1-enriched coiling pseudopods. Coiling of B. burgdorferi was significantly reduced when neuroglial cells were pretreated with anti-Daam1 antibody indicating the requirement for Daam1 for borrelial phagocytosis. Confocal microscopy showed Daam1 colocalizing to the B. burgdorferi surface suggesting interaction with borrelial membrane protein(s). Using the yeast 2-hybrid system for identifying protein-protein binding, we found that the B. burgdorferi surface lipoprotein, BBA66, bound the FH2 subunit domain of Daam1. Recombinant proteins were used to validate binding by ELISA, pull-down, and co-immunoprecipitation. Evidence for native Daam1 and BBA66 interaction was suggested by colocalization of the proteins in the course of borrelial capture by the Daam1-enriched pseudopodia. Additionally, we found a striking reduction in coiling for a BBA66-deficient mutant strain compared to BBA66-expressing strains. These results show that coiling phagocytosis is a mechanism for borrelial internalization by neuroglial cells mediated by Daam1.

RhoC regulates the actin remodeling required for phagosome formation during FcγR-mediated phagocytosis

Phagosome formation is a complicated process that requires spatiotemporally regulated actin reorganization. We found that RhoC GTPase is a critical regulator of FcγR-mediated phagocytosis in macrophages. Our live-cell imaging revealed that RhoC, but not RhoA, is recruited to phagocytic cups engulfing IgG-opsonized erythrocytes (IgG-Es). RhoC silencing through RNAi, CRISPR/Cas-mediated RhoC knockout, and the expression of dominant-negative or constitutively active RhoC mutants suppressed the phagocytosis of IgG-Es. Moreover, RhoC-GTP pulldown experiments showed that endogenous RhoC is transiently activated during phagosome formation. Notably, actin-driven pseudopod extension, which is required for the formation of phagocytic cups, was severely impaired in cells expressing the constitutively active mutant RhoC-G14V, which induced abnormal F-actin accumulation underneath the plasma membrane. mDia1 (encoded by DIAPH1), a Rho-dependent actin nucleation factor, and RhoC were colocalized at the phagocytic cups. Similar to what was seen for RhoC, mDia1 silencing through RNAi inhibited phagosome formation. Additionally, the coexpression of mDia1 with constitutively active mutant RhoC-G14V or expression of active mutant mDia1-ΔN3 drastically inhibited the uptake of IgG-Es. These data suggest that RhoC modulates phagosome formation be modifying actin cytoskeletal remodeling via mDia1.

Fluorescent membrane markers elucidate the association of Borrelia burgdorferi with tick cell lines

This study aimed to describe the association of Borrelia burgdorferi s.s. with ixodid tick cell lines by flow cytometry and fluorescence and confocal microscopy. Spirochetes were stained with a fluorescent membrane marker (PKH67 or PKH26), inoculated into 8 different tick cell lines and incubated at 30°C for 24 h. PKH efficiently stained B. burgdorferi without affecting bacterial viability or motility. Among the tick cell lines tested, the Rhipicephalus appendiculatus cell line RA243 achieved the highest percentage of association/internalization, with both high (90%) and low (10%) concentrations of BSK-H medium in tick cell culture medium. Treatment with cytochalasin D dramatically reduced the average percentage of cells with internalized spirochetes, which passed through a dramatic morphological change during their internalization by the host cell as observed in time-lapse photography. Almost all of the fluorescent bacteria were seen to be inside the tick cells. PKH labeling of borreliae proved to be a reliable and valuable tool to analyze the association of spirochetes with host cells by flow cytometry, confocal and fluorescence microscopy.

Non-canonical activity of the podosomal formin FMNL1γ supports immune cell migration

Having previously located the formin FMNL1 in macrophage podosomes, we developed an in vivo model to assess the role of FMNL1 in the migration activities of primary macrophages. Deletion of FMNL1 in mice was genetically lethal; however, targeted deletion in macrophages was achieved by employing macrophage-specific Cre. Unchallenged FMNL1-deficient mice exhibited an unexpected reduction in tissue-resident macrophages despite normal blood monocyte numbers. Upon immune stimulus, the absence of FMNL1 resulted in reduced macrophage recruitment in vivo, decreased migration in two-dimensional in vitro culture and a decrease in the number of macrophages exhibiting podosomes. Of the three described isoforms of FMNL1 - α, β and γ - only FMNL1γ rescued macrophage migration when expressed exogenously in depleted macrophages. Surprisingly, mutation of residues in the FH2 domain of FMNL1γ that disrupt barbed-end actin binding did not limit rescue of macrophage migration and podosome numbers. These observations suggest that FMNL1 contributes to macrophage migration activity by stabilizing the lifespan of podosomes without interaction of fast-growing actin termini.

A Diaphanous-related formin links Ras signaling directly to actin assembly in macropinocytosis and phagocytosis

Phagocytosis and macropinocytosis are Ras-regulated and actin-driven processes that depend on the dynamic rearrangements of the plasma membrane that protrudes and internalizes extracellular material by cup-shaped structures. However, the regulatory mechanisms underlying actin assembly in large-scale endocytosis remain elusive. Here, we show that the Diaphanous-related formin G (ForG) from the professional phagocyte Dictyostelium discoideum localizes to endocytic cups. Biochemical analyses revealed that ForG is a rather weak nucleator but efficiently elongates actin filaments in the presence of profilin. Notably, genetic inactivation of ForG is associated with a strongly impaired endocytosis and a markedly diminished F-actin content at the base of the cups. By contrast, ablation of the Arp2/3 (actin-related protein-2/3) complex activator SCAR (suppressor of cAMP receptor) diminishes F-actin mainly at the cup rim, being consistent with its known localization. These data therefore suggest that ForG acts as an actin polymerase of Arp2/3-nucleated filaments to allow for efficient membrane expansion and engulfment of extracellular material. Finally, we show that ForG is directly regulated in large-scale endocytosis by RasB and RasG, which are highly related to the human proto-oncogene KRas.

Pleomorphic forms of Borrelia burgdorferi induce distinct immune responses

Borrelia burgdorferi is the causative agent of tick-borne Lyme disease. As a response to environmental stress B. burgdorferi can change its morphology to a round body form. The role of B. burgdorferi pleomorphic forms in Lyme disease pathogenesis has long been debated and unclear. Here, we demonstrated that round bodies were processed differently in differentiated macrophages, consequently inducing distinct immune responses compared to spirochetes in vitro. Colocalization analysis indicated that the F-actin participates in internalization of both forms. However, round bodies end up less in macrophage lysosomes than spirochetes suggesting that there are differences in processing of these forms in phagocytic cells. Furthermore, round bodies stimulated distinct cytokine and chemokine production in these cells. We confirmed that spirochetes and round bodies present different protein profiles and antigenicity. In a Western blot analysis Lyme disease patients had more intense responses to round bodies when compared to spirochetes. These results suggest that round bodies have a role in Lyme disease pathogenesis.

Cell type-dependent mechanisms for formin-mediated assembly of filopodia

Filopodia are finger-like protrusions from the plasma membrane and are of fundamental importance to cellular physiology, but the mechanisms governing their assembly are still in question. One model, called convergent elongation, proposes that filopodia arise from Arp2/3 complex-nucleated dendritic actin networks, with factors such as formins elongating these filaments into filopodia. We test this model using constitutively active constructs of two formins, FMNL3 and mDia2. Surprisingly, filopodial assembly requirements differ between suspension and adherent cells. In suspension cells, Arp2/3 complex is required for filopodial assembly through either formin. In contrast, a subset of filopodia remains after Arp2/3 complex inhibition in adherent cells. In adherent cells only, mDia1 and VASP also contribute to filopodial assembly, and filopodia are disproportionately associated with focal adhesions. We propose an extension of the existing models for filopodial assembly in which any cluster of actin filament barbed ends in proximity to the plasma membrane, either Arp2/3 complex dependent or independent, can initiate filopodial assembly by specific formins.

Formins: Actin nucleators that regulate cytoskeletal dynamics during spermatogenesis

Formins are a growing class of actin nucleation proteins that promote the polymerization of actin microfilaments, forming long stretches of actin microfilaments to confer actin filament bundling in mammalian cells. As such, microfilament bundles can be formed in specific cellular domains, in particular in motile mammalian cells, such as filopodia. Since ectoplasmic specialization (ES), a testis-specific adherens junction (AJ), at the Sertoli cell-cell and Sertoli-spermatid interface is constituted by arrays of actin microfilament bundles, it is likely that formins are playing a significant physiological role on the homeostasis of ES during the epithelial cycle of spermatogenesis. In this Commentary, we provide a timely discussion on formin 1 which was recently shown to be a crucial regulator of actin microfilaments at the ES in the rat testis (Li N et al. Endocrinology, 2015, in press; DOI: 10.1210/en.2015-1161, PMID:25901598). We also highlight research that is needed to unravel the functional significance of formins in spermatogenesis.

Human Macrophages Utilize the Podosome Formin FMNL1 for Adhesion and Migration

Macrophages play a crucial role in detecting, regulating, and resolving immune crises, requiring migration through complex extracellular matrices. Unwarranted macrophage inflammatory activity potentiates kidney disease, rheumatoid arthritis, and transplant rejection. Proper remodeling of the actin cytoskeleton, especially at adhesion structures, is essential to the translocation of macrophages. Macrophages form actin-rich adhesions termed "podosomes", giving them the capacity to make contacts with the substratum for traction through interstitial tissues. Macrophages express multiple formins, including FMNL1, Dia1, and Fhod1, with potential to impact actin remodeling involved in migration. Formins are a family of proteins that are best known for modifying the actin cytoskeleton via nucleation, elongation, bundling, and/or severing actin filaments. In this study we demonstrate that the formin FMNL1 is a key regulator of podosomes and is required for normal macrophage migration. Additionally, this is the first study to demonstrate defects in primary human cell migration resulting from specific formin silencing. Pharmacologic inhibition of all formin activity results in a significant decrease in podosome formation and normal macrophage migration. Furthermore, targeted suppression of FMNL1 results in decreases in macrophage migration similar to inhibition of all expressed macrophage formins. These novel findings suggest FMNL1 as a possible chemotherapeutic target to hinder macrophage migration, which could offer an innovative method for limiting unnecessary macrophage-mediated inflammation. We hypothesize that formins are required in podosome actin dynamics to support macrophage migration.

The formin FMNL3 assembles plasma membrane protrusions that participate in cell-cell adhesion

FMNL3 localizes broadly over the plasma membrane as discrete puncta, with particular enrichment in filopodia and ruffles and at cell–cell contacts. In addition, a population of FMNL3-containing vesicles of endocytic origin can fuse with the plasma membrane. FMNL3 suppression causes reductions in filopodia and cell–cell adhesion.

FMNL3 is a vertebrate-specific formin protein previously shown to play a role in angiogenesis and cell migration. Here we define the cellular localization of endogenous FMNL3, the dynamics of GFP-tagged FMNL3 during cell migration, and the effects of FMNL3 suppression in mammalian culture cells. The majority of FMNL3 localizes in a punctate pattern, with >95% of these puncta being indistinguishable from the plasma membrane by fluorescence microscopy. A small number of dynamic cytoplasmic FMNL3 patches also exist, which enrich near cell–cell contact sites and fuse with the plasma membrane at these sites. These cytoplasmic puncta appear to be part of larger membranes of endocytic origin. On the plasma membrane, FMNL3 enriches particularly in filopodia and membrane ruffles and at nascent cell–cell adhesions. FMNL3-containing filopodia occur both at the cell–substratum interface and at cell–cell contacts, with the latter being 10-fold more stable. FMNL3 suppression by siRNA has two major effects: decrease in filopodia and compromised cell–cell adhesion in cells migrating as a sheet. Overall our results suggest that FMNL3 functions in assembly of actin-based protrusions that are specialized for cell–cell adhesion.

Formin mDia1, a downstream molecule of FMNL1, regulates Profilin1 for actin assembly and spindle organization during mouse oocyte meiosis

Mammalian diaphanous1 (mDia1) is a homologue of Drosophila diaphanous and belongs to the Formin-homology family of proteins that catalyze actin nucleation and polymerization. Although Formin family proteins, such as Drosophila diaphanous, have been shown to be essential for cytokinesis, whether and how mDia1 functions during meiosis remain uncertain. In this study, we explored possible roles and the signaling pathway involved for mDia1 using a mouse oocyte model. mDia1 depletion reduced polar body extrusion, which may have been due to reduced cortical actin assembly. mDia1 and Profilin1 had similar localization patterns in mouse oocytes and mDia1 knockdown resulted in reduced Profilin1 expression. Depleting FMNL1, another Formin family member, resulted in reduced mDia1 expression, while RhoA inhibition did not alter mDia1 expression, which indicated that there was a FMNL1-mDia1-Profilin1 signaling pathway in mouse oocytes. Additionally, mDia1 knockdown resulted in disrupting oocyte spindle morphology, which was confirmed by aberrant p-MAPK localization. Thus, these results demonstrated indispensable roles for mDia1 in regulating mouse oocyte meiotic maturation through its effects on actin assembly and spindle organization.

Formins as effector proteins of Rho GTPases

Formin proteins were recognized as effectors of Rho GTPases some 15 years ago. They contribute to different cellular actin cytoskeleton structures by their ability to polymerize straight actin filaments at the barbed end. While not all formins necessarily interact with Rho GTPases, a subgroup of mammalian formins, termed Diaphanous-related formins or DRFs, were shown to be activated by small GTPases of the Rho superfamily. DRFs are autoinhibited in the resting state by an N- to C-terminal interaction that renders the central actin polymerization domain inactive. Upon the interaction with a GTP-bound Rho, Rac, or Cdc42 GTPase, the C-terminal autoregulation domain is displaced from its N-terminal recognition site and the formin becomes active to polymerize actin filaments. In this review we discuss the current knowledge on the structure, activation, and function of formin-GTPase interactions for the mammalian formin families Dia, Daam, FMNL, and FHOD. We describe both direct and indirect interactions of formins with GTPases, which lead to formin activation and cytoskeletal rearrangements. The multifaceted function of formins as effector proteins of Rho GTPases thus reflects the diversity of the actin cytoskeleton in cells.

Bacterial subversion of host cytoskeletal machinery: hijacking formins and the Arp2/3 complex

The host actin nucleation machinery is subverted by many bacterial pathogens to facilitate their entry, motility, replication, and survival. The majority of research conducted in the past primarily focused on exploitation of a host actin nucleator, the Arp2/3 complex, by bacterial pathogens. Recently, new studies have begun to explore the role of formins, another family of host actin nucleators, in bacterial pathogenesis. This review provides an overview of recent advances in the study of the exploitation of the Arp2/3 complex and formins by bacterial pathogens. Secreted bacterial effector proteins seem to manipulate the regulation of these actin nucleators or functionally mimic them to drive bacterial entry, motility and survival within host cells. An enhanced understanding of how formins are exploited will provide us with greater insight into how a fundamental eurkaryotic cellular process is utilized by bacteria and will also advance our knowledge of host-pathogen interactions.

Phagosomal TLR signaling upon Borrelia burgdorferi infection

Internalization and degradation of live Bb within phagosomal compartments of monocytes, macrophages and dendritic cells (DCs), allows for the release of lipoproteins, nucleic acids and other microbial products, triggering a broad and robust inflammatory response. Toll-like receptors (TLRs) are key players in the recognition of spirochetal ligands from whole viable organisms (i.e., vita-PAMPs). Herein we will review the role of endosomal TLRs in the response to the Lyme disease spirochete.

Characterization of Leukocyte Formin FMNL1 Expression in Human Tissues

Formins are cytoskeleton regulating proteins characterized by a common FH2 structural domain. As key players in the assembly of actin filaments, formins direct dynamic cytoskeletal processes that influence cell shape, movement and adhesion. The large number of formin genes, fifteen in the human, suggests distinct tasks and expression patterns for individual family members, in addition to overlapping functions. Several formins have been associated with invasive cell properties in experimental models, linking them to cancer biology. One example is FMNL1, which is considered to be a leukocyte formin and is known to be overexpressed in lymphomas. Studies on FMNL1 and many other formins have been hampered by a lack of research tools, especially antibodies suitable for staining paraffin-embedded formalin-fixed tissues. Here we characterize, using bioinformatics tools and a validated antibody, the expression pattern of FMNL1 in human tissues and study its subcellular distribution. Our results indicate that FMNL1 expression is not restricted to hematopoietic tissues and that neoexpression of FMNL1 can be seen in epithelial cancer.

Daam1 is a regulator of filopodia formation and phagocytic uptake of Borrelia burgdorferi by primary human macrophages

Borrelia burgdorferi is the causative agent of Lyme disease, an infectious disease that primarily affects the skin, nervous system, and joints. Uptake of borreliae by immune cells is decisive for the course of the infection, and remodelling of the host actin cytoskeleton is crucial in this process. In this study, we showed that the actin-regulatory formin Daam1 is important in Borrelia phagocytosis by primary human macrophages. Uptake of borreliae proceeds preferentially through capture by filopodia and formation of coiling pseudopods that enwrap the spirochetes. Using immunofluorescence, we localized endogenous and overexpressed Daam1 to filopodia and to F-actin-rich uptake structures. Live-cell imaging further showed that Daam1 is enriched at coiling pseudopods that arise from the macrophage surface. This filopodia-independent step was corroborated by control experiments of phagocytic cup formation with latex beads. Moreover, siRNA-mediated knockdown of Daam1 led to a 65% reduction of borreliae-induced filopodia, and, as shown by the outside-inside staining technique, to a 50% decrease in phagocytic uptake of borreliae, as well as a 37% reduction in coiling pseudopod formation. Collectively, we showed that Daam1 plays a dual role in the phagocytic uptake of borreliae: first, as a regulator of filopodia, which are used for capturing spirochetes, and second, in the formation of the coiling pseudopod that enwraps the bacterial cell. These data identify Daam1 as a novel regulator of B. burgdorferi phagocytosis. At the same time, this is the first demonstration of a role for Daam1 in phagocytic processes in general.-Hoffmann, A.-K., Naj, X., Linder, S. Daam1 is a regulator of filopodia formation and phagocytic uptake of Borrelia burgdorferi by primary human macrophages.

Formin' cellular structures: Physiological roles of Diaphanous (Dia) in actin dynamics

Members of the Diaphanous (Dia) protein family are key regulators of fundamental actin driven cellular processes, which are conserved from yeast to humans. Researchers have uncovered diverse physiological roles in cell morphology, cell motility, cell polarity, and cell division, which are involved in shaping cells into tissues and organs. The identification of numerous binding partners led to substantial progress in our understanding of the differential functions of Dia proteins. Genetic approaches and new microscopy techniques allow important new insights into their localization, activity, and molecular principles of regulation.