New insights into the regulation and cellular functions of the ARP2/3 complex.

The actin-related protein 2/3 (ARP2/3) complex nucleates branched actin filament networks, but requires nucleation promoting factors (NPFs) to stimulate this activity. NPFs include proteins such as Wiskott-Aldrich syndrome protein (WASP), neural WASP (NWASP), WASP family verprolin-homologous protein (WAVE; also known as SCAR) and the recently identified WASP and SCAR homologue (WASH) complex. The mechanisms underlying NPF-dependent regulation and the cellular functions of ARP2/3 are being unravelled using new chemical and genetic approaches. Of particular interest is the role of the ARP2/3 complex in vesicular trafficking and directional cell motility.

Survival motor neuron protein deficiency alters microglia reactivity.

Survival motor neuron (SMN) protein deficiency results in loss of alpha motor neurons and subsequent muscle atrophy in patients with spinal muscular atrophy (SMA). Reactive microglia have been reported in SMA mice and depleting microglia rescues the number of proprioceptive synapses, suggesting a role in SMA pathology. Here, we explore the contribution of lymphocytes on microglia reactivity in SMA mice and investigate how SMN deficiency alters the reactive profile of human induced pluripotent stem cell (iPSC)-derived microglia. We show that microglia adopt a reactive morphology in spinal cords of SMA mice. Ablating lymphocytes did not alter the reactive morphology of SMA microglia and did not improve the survival or motor function of SMA mice, indicating limited impact of peripheral immune cells on the SMA phenotype. We found iPSC-derived SMA microglia adopted an amoeboid morphology and displayed a reactive transcriptome profile, increased cell migration, and enhanced phagocytic activity. Importantly, cell morphology and electrophysiological properties of motor neurons were altered when they were incubated with conditioned media from SMA microglia. Together, these data reveal that SMN-deficient microglia adopt a reactive profile and exhibit an exaggerated inflammatory response with potential impact on SMA neuropathology.

Myeloid Slc2a1-Deficient Murine Model Revealed Macrophage Activation and Metabolic Phenotype Are Fueled by GLUT1.

Macrophages (MΦs) are heterogeneous and metabolically flexible, with metabolism strongly affecting immune activation. A classic response to proinflammatory activation is increased flux through glycolysis with a downregulation of oxidative metabolism, whereas alternative activation is primarily oxidative, which begs the question of whether targeting glucose metabolism is a viable approach to control MΦ activation. We created a murine model of myeloid-specific glucose transporter GLUT1 (Slc2a1) deletion. Bone marrow-derived MΦs (BMDM) from Slc2a1M-/- mice failed to uptake glucose and demonstrated reduced glycolysis and pentose phosphate pathway activity. Activated BMDMs displayed elevated metabolism of oleate and glutamine, yet maximal respiratory capacity was blunted in MΦ lacking GLUT1, demonstrating an incomplete metabolic reprogramming. Slc2a1M-/- BMDMs displayed a mixed inflammatory phenotype with reductions of the classically activated pro- and anti-inflammatory markers, yet less oxidative stress. Slc2a1M-/- BMDMs had reduced proinflammatory metabolites, whereas metabolites indicative of alternative activation-such as ornithine and polyamines-were greatly elevated in the absence of GLUT1. Adipose tissue MΦs of lean Slc2a1M-/- mice had increased alternative M2-like activation marker mannose receptor CD206, yet lack of GLUT1 was not a critical mediator in the development of obesity-associated metabolic dysregulation. However, Ldlr-/- mice lacking myeloid GLUT1 developed unstable atherosclerotic lesions. Defective phagocytic capacity in Slc2a1M-/- BMDMs may have contributed to unstable atheroma formation. Together, our findings suggest that although lack of GLUT1 blunted glycolysis and the pentose phosphate pathway, MΦ were metabolically flexible enough that inflammatory cytokine release was not dramatically regulated, yet phagocytic defects hindered MΦ function in chronic diseases.

Lamellipodia are crucial for haptotactic sensing and response.

Haptotaxis is the process by which cells respond to gradients of substrate-bound cues, such as extracellular matrix proteins (ECM); however, the cellular mechanism of this response remains poorly understood and has mainly been studied by comparing cell behavior on uniform ECMs with different concentrations of components. To study haptotaxis in response to gradients, we utilized microfluidic chambers to generate gradients of the ECM protein fibronectin, and imaged the cell migration response. Lamellipodia are fan-shaped protrusions that are common in migrating cells. Here, we define a new function for lamellipodia and the cellular mechanism required for haptotaxis - differential actin and lamellipodial protrusion dynamics lead to biased cell migration. Modest differences in lamellipodial dynamics occurring over time periods of seconds to minutes are summed over hours to produce differential whole cell movement towards higher concentrations of fibronectin. We identify a specific subset of lamellipodia regulators as being crucial for haptotaxis. Numerous studies have linked components of this pathway to cancer metastasis and, consistent with this, we find that expression of the oncogenic Rac1 P29S mutation abrogates haptotaxis. Finally, we show that haptotaxis also operates through this pathway in 3D environments.

Keratin 16 regulates innate immunity in response to epidermal barrier breach.

Mutations in the type I keratin 16 (Krt16) and its partner type II keratin 6 (Krt6a, Krt6b) cause pachyonychia congenita (PC), a disorder typified by dystrophic nails, painful hyperkeratotic calluses in glabrous skin, and lesions involving other epithelial appendages. The pathophysiology of these symptoms and its relationship to settings in which Krt16 and Krt6 are induced in response to epidermal barrier stress are poorly understood. We report that hyperkeratotic calluses arising in the glabrous skin of individuals with PC and Krt16 null mice share a gene expression signature enriched in genes involved in inflammation and innate immunity, in particular damage-associated molecular patterns. Transcriptional hyper-activation of damage-associated molecular pattern genes occurs following de novo chemical or mechanical irritation to ear skin and in spontaneously arising skin lesions in Krt16 null mice. Genome-wide expression analysis of normal mouse tail skin and benign proliferative lesions reveals a tight, context-dependent coregulation of Krt16 and Krt6 with genes involved in skin barrier maintenance and innate immunity. Our results uncover a role for Krt16 in regulating epithelial inflammation that is relevant to genodermatoses, psoriasis, and cancer and suggest a avenue for the therapeutic management of PC and related disorders.

Macrophage migration is differentially regulated by fibronectin and laminin through altered adhesion and myosin II localization.

Macrophages are indispensable for proper immune surveillance and inflammatory regulation. They also exhibit dramatic phenotypic plasticity and are highly responsive to their local microenvironment, which includes the extracellular matrix (ECM). This work demonstrates that two fibrous ECM glycoproteins, fibronectin (FN) and laminin (LAM), elicit distinct morphological and migratory responses from macrophages in two-dimensional environments. LAM 111 inhibits macrophage cell spreading, but drives them to migrate rapidly and less persistently compared with cells on FN. Differential integrin engagement and ROCK/myosin II organization helps explain why macrophages alter their morphology and migration character on these two ECM components. This study also demonstrates that LAM 111 exerts a suppressive effect toward FN, as macrophages plated on a LAM/FN mixture adopt a morphology and migratory character almost identical to LAM alone. This suggests that distinct responses can be initiated downstream of receptor-ECM engagement, and that one component of the microenvironment may affect the cell's ability to sense another. Overall, macrophages appear intrinsically poised to rapidly switch between distinct migratory characters based on their ECM environments. The role of ECM composition in dictating motile and inflammatory responses in three-dimensional and in vivo contexts warrants further study.

Local monomer levels and established filaments potentiate non-muscle myosin 2 assembly.

The ability to dynamically assemble contractile networks is required throughout cell physiology, yet direct biophysical mechanisms regulating non-muscle myosin 2 filament assembly in living cells are lacking. Here, we use a suite of dynamic, quantitative imaging approaches to identify deterministic factors that drive myosin filament appearance and amplification. We find that actin dynamics regulate myosin assembly, but that the static actin architecture plays a less clear role. Instead, remodeling of actin networks modulates the local myosin monomer levels and facilitates assembly through myosin:myosin-driven interactions. Using optogenetically controlled myosin, we demonstrate that locally concentrating myosin is sufficient to both form filaments and jump-start filament amplification and partitioning. By counting myosin monomers within filaments, we demonstrate a myosin-facilitated assembly process that establishes filament stacks prior to partitioning into clusters that feed higher-order networks. Together, these findings establish the biophysical mechanisms regulating the assembly of non-muscle contractile structures that are ubiquitous throughout cell biology.

Key features of the innate immune response is mediated by the immunoproteasome in microglia.

Microglia are the resident immune cells of the central nervous system (CNS). We and others have shown that the inflammatory response of microglia is partially regulated by the immunoproteasome, an inducible form of the proteasome responsible for the generation of major histocompatibility complex (MHC) class I epitopes. While the role of the proteasome in the adaptive immune system is well established, emerging evidence suggests the immunoproteasome may have discrete functions in the innate immune response. Here, we show that inhibiting the immunoproteasome reduces the IFNγ-dependent induction of complement activator C1q, suppresses phagocytosis, and alters the cytokine expression profile in a microglial cell line and microglia derived from human inducible pluripotent stem cells. Moreover, we show that the immunoproteasome regulates the degradation of IκBα, a modulator of NF-κB signaling. Finally, we demonstrate that NADH prevents induction of the immunoproteasome, representing a potential pathway to suppress immunoproteasome-dependent immune responses.

Macrophage migration is differentially regulated by distinct ECM components.

Macrophages are indispensable for proper immune surveillance and inflammatory regulation. They also exhibit dramatic phenotypic plasticity and are highly responsive to their local microenvironment, which includes the extracellular matrix (ECM). The present work demonstrates that two fibrous ECM glycoproteins, fibronectin (FN) and laminin (LAM), elicit distinct morphological and migratory responses to macrophages in 2D environments. Laminin 111 inhibits macrophage cell spreading, but drives them to migrate rapidly and less persistently compared to cells on fibronectin. Differential integrin engagement and ROCK/myosin II organization helps explain why macrophages alter their morphology and migration character on these two ECM components. The present study also demonstrates that laminin 111 exerts a suppressive effect toward fibronectin, as macrophages plated on a LAM/FN mixture adopt a morphology and migratory character almost identical to LAM alone. This suggests that distinct responses can be initiated downstream of receptor-ECM engagement, and that one component of the microenvironment may affect the cell's ability to sense another. Overall, macrophages appear intrinsically poised to rapidly switch between distinct migratory modes based on their ECM environments. The role of ECM composition in dictating motile and inflammatory responses in 3D and in vivo contexts warrants further study.

Local Monomer Levels and Established Filaments Potentiate Non-Muscle Myosin 2 Assembly.

The ability to dynamically assemble contractile networks is required throughout cell physiology, yet the biophysical mechanisms regulating non-muscle myosin 2 filament assembly in living cells are lacking. Here we use a suite of dynamic, quantitative imaging approaches to identify deterministic factors that drive myosin filament appearance and amplification. We find that actin dynamics regulate myosin assembly, but that the actin architecture plays a minimal direct role. Instead, remodeling of actin networks modulates the local myosin monomer levels and facilitates assembly through myosin:myosin driven interactions. Using optogenetically controlled myosin, we demonstrate that locally concentrating myosin is sufficient to both form filaments and jump-start filament amplification and partitioning. By counting myosin monomers within filaments, we demonstrate a myosin-facilitated assembly process that establishes sub-resolution filament stacks prior to partitioning into clusters that feed higher-order networks. Together these findings establish the biophysical mechanisms regulating the assembly of non-muscle contractile structures that are ubiquitous throughout cell biology.

The Actin Cytoskeleton Responds to Inflammatory Cues and Alters Macrophage Activation.

Much remains to be learned about the molecular mechanisms underlying a class of human disorders called actinopathies. These genetic disorders are characterized by loss-of-function mutations in actin-associated proteins that affect immune cells, leading to human immunopathology. However, much remains to be learned about how cytoskeletal dysregulation promotes immunological dysfunction. The current study reveals that the macrophage actin cytoskeleton responds to LPS/IFNγ stimulation in a biphasic manner that involves cellular contraction followed by cellular spreading. Myosin II inhibition by blebbistatin blocks the initial contraction phase and lowers iNOS protein levels and nitric oxide secretion. Conversely, conditional deletion of Arp2/3 complex in macrophages attenuates spreading and increases nitric oxide secretion. However, iNOS transcription is not altered by loss of myosin II or Arp2/3 function, suggesting post-transcriptional regulation of iNOS by the cytoskeleton. Consistent with this idea, proteasome inhibition reverses the effects of blebbistatin and rescues iNOS protein levels. Arp2/3-deficient macrophages demonstrate two additional phenotypes: defective MHCII surface localization, and depressed secretion of the T cell chemokine CCL22. These data suggest that interplay between myosin II and Arp2/3 influences macrophage activity, and potentially impacts adaptive-innate immune coordination. Disrupting this balance could have detrimental impacts, particularly in the context of Arp2/3-associated actinopathies.

Dynein activation in vivo is regulated by the nucleotide states of its AAA3 domain.

Cytoplasmic dynein is activated by the dynactin complex, cargo adapters and LIS1 (Lissencephaly 1). How this process is regulated in vivo remains unclear. The dynein motor ring contains six AAA+ (ATPases associated with diverse cellular activities) domains. Here, we used the filamentous fungus Aspergillus nidulans to examine whether ATP hydrolysis at AAA3 regulates dynein activation in the context of other regulators. In fungal hyphae, early endosomes undergo dynein-mediated movement away from the microtubule plus ends near the hyphal tip. Dynein normally accumulates at the microtubule plus ends. The early endosomal adaptor Hook protein, together with dynactin, drives dynein activation to cause its relocation to the microtubule minus ends. This activation process depends on LIS1, but LIS1 tends to dissociate from dynein after its activation. In this study, we found that dynein containing a mutation-blocking ATP hydrolysis at AAA3 can undergo LIS1-independent activation, consistent with our genetic data that the same mutation suppresses the growth defect of the A. nidulans LIS1-deletion mutant. Our data also suggest that blocking AAA3 ATP hydrolysis allows dynein activation by dynactin without the early endosomal adaptor. As a consequence, dynein accumulates at microtubule minus ends whereas early endosomes stay near the plus ends. Dynein containing a mutation-blocking ATP binding at AAA3 largely depends on LIS1 for activation, but this mutation abnormally prevents LIS1 dissociation upon dynein activation. Together, our data suggest that the AAA3 ATPase cycle regulates the coordination between dynein activation and cargo binding as well as the dynamic dynein-LIS1 interaction.

Actin Cytoskeleton: Profilin Gives Cells an Edge.

A new study reports that the actin-monomer-binding protein profilin 1 dictates protrusion character at the cell edge. These findings help explain how distinct, tunable actin polymerization pathways collaborate to form higher-order cellular structures.

Arp2/3 inactivation causes intervertebral disc and cartilage degeneration with dysregulated TonEBP-mediated osmoadaptation.

Extracellular matrix and osmolarity influence the development and homeostasis of skeletal tissues through Rho GTPase-mediated alteration of the actin cytoskeleton. This study investigated whether the actin-branching Arp2/3 complex, a downstream effector of the Rho GTPases Cdc42 and Rac1, plays a critical role in maintaining the health of matrix-rich and osmotically loaded intervertebral discs and cartilage. Mice with constitutive intervertebral disc- and cartilage-specific deletion of the critical Arp2/3 subunit Arpc2 (Col2-Cre; Arpc2fl/fl) developed chondrodysplasia and spinal defects. Since these mice did not survive to adulthood, we generated mice with inducible Arpc2 deletion in disc and cartilage (Acan-CreERT2; Arpc2fl/fl). Inactivation of Arp2/3 at skeletal maturity resulted in growth plate closure, loss of proteoglycan content in articular cartilage, and degenerative changes in the intervertebral disc at 1 year of age. Chondrocytes with Arpc2 deletion showed compromised cell spreading on both collagen and fibronectin. Pharmacological inhibition of Cdc42 and Arp2/3 prevented the osmoadaptive transcription factor TonEBP/NFAT5 from recruiting cofactors in response to a hyperosmolarity challenge. Together, these findings suggest that Arp2/3 plays a critical role in cartilaginous tissues through the regulation of cell-extracellular matrix interactions and modulation of TonEBP-mediated osmoadaptation.

Arp2/3 Complex Is Required for Macrophage Integrin Functions but Is Dispensable for FcR Phagocytosis and In Vivo Motility.

The Arp2/3 complex nucleates branched actin, forming networks involved in lamellipodial protrusion, phagocytosis, and cell adhesion. We derived primary bone marrow macrophages lacking Arp2/3 complex (Arpc2-/-) and directly tested its role in macrophage functions. Despite protrusion and actin assembly defects, Arpc2-/- macrophages competently phagocytose via FcR and chemotax toward CSF and CX3CL1. However, CR3 phagocytosis and fibronectin haptotaxis, both integrin-dependent processes, are disrupted. Integrin-responsive actin assembly and αM/ß2 integrin localization are compromised in Arpc2-/- cells. Using an in vivo system to observe endogenous monocytes migrating toward full-thickness ear wounds we found that Arpc2-/- monocytes maintain cell speeds and directionality similar to control. Our work reveals that the Arp2/3 complex is not a general requirement for phagocytosis or chemotaxis but is a critical driver of integrin-dependent processes. We demonstrate further that cells lacking Arp2/3 complex function in vivo remain capable of executing important physiological responses that require rapid directional motility.

Lack of myeloid Fatp1 increases atherosclerotic lesion size in Ldlr-/- mice.

BACKGROUND AND AIMS: Altered metabolism is an important regulator of macrophage (MΦ) phenotype, which contributes to inflammatory diseases such as atherosclerosis. Broadly, pro-inflammatory, classically-activated MΦs (CAM) are glycolytic while alternatively-activated MΦs (AAM) oxidize fatty acids, although overlap exists. We previously demonstrated that MΦ fatty acid transport protein 1 (FATP1, Slc27a1) was necessary to maintain the oxidative and anti-inflammatory AAM phenotype in vivo in a model of diet-induced obesity. The aim of this study was to examine how MΦ metabolic reprogramming through FATP1 ablation affects the process of atherogenesis. We hypothesized that FATP1 limits MΦ-mediated inflammation during atherogenesis. Thus, mice lacking MΦ Fatp1 would display elevated formation of atherosclerotic lesions in a mouse model lacking the low-density lipoprotein (LDL) receptor (Ldlr-/-). METHODS: We transplanted bone marrow collected from Fatp1+/+ or Fatp1-/- mice into Ldlr-/- mice and fed chimeric mice a Western diet for 12 weeks. Body weight, blood glucose, and plasma lipids were measured. Aortic sinus and aorta lesions were quantified. Atherosclerotic plaque composition, oxidative stress, and inflammation were analyzed histologically. RESULTS: Compared to Fatp1+/+Ldlr-/- mice, Fatp1-/-Ldlr-/- mice exhibited significantly larger lesion area and elevated oxidative stress and inflammation in the atherosclerotic plaque. Macrophage and smooth muscle cell content did not differ by Fatp1 genotype. There were no significant systemic alterations in LDL, high-density lipoprotein (HDL), total cholesterol, or triacylglyceride, suggesting that the effect was local to the cells of the vessel microenvironment in a Fatp1-dependent manner. CONCLUSIONS: MΦ Fatp1 limits atherogenesis and may be a viable target to metabolically reprogram MΦs.

Deletion of the Arp2/3 complex in megakaryocytes leads to microthrombocytopenia in mice.

Actin reorganization regulates key processes in platelet activation. Here we examined the role of the Arp2/3 complex, an essential component in actin filament branching, in platelet function. The Arpc2 gene, encoding the p34 subunit of the Arp2/3 complex, was deleted in the megakaryocyte lineage (Arpc2fl/flPF4-Cre). Deletion of the Arp2/3 complex resulted in marked microthrombocytopenia in mice, caused by premature platelet release into the bone marrow compartment and impaired platelet survival in circulation. Arpc2fl/flPF4-Cre platelets exhibited alterations in their actin cytoskeleton and their peripheral microtubule coil. Thrombocytopenia was alleviated following clodronate liposome-induced macrophage depletion in Arpc2fl/flPF4-Cre mice. Arpc2fl/flPF4-Cre platelets failed to spread and showed a mild defect in integrin activation and aggregation. However, no significant differences in hemostasis or thrombosis were observed between Arpc2fl/flPF4-Cre and control mice. Thus, Arp2/3 is critical for platelet homeostasis but plays only a minor role for vascular hemostasis.

Profilin-1 serves as a gatekeeper for actin assembly by Arp2/3-dependent and -independent pathways.

Cells contain multiple F-actin assembly pathways, including the Arp2/3 complex, formins, and Ena/VASP, which have largely been analyzed separately. They collectively generate the bulk of F-actin from a common pool of G-actin; however, the interplay and/or competition between these pathways remains poorly understood. Using fibroblast lines derived from an Arpc2 conditional knockout mouse, we established matched-pair cells with and without the Arp2/3 complex. Arpc2(-/-) cells lack lamellipodia and migrate more slowly than WT cells but have F-actin levels indistinguishable from controls. Actin assembly in Arpc2(-/-) cells was resistant to cytochalasin-D and was highly dependent on profilin-1 and Ena/VASP but not formins. Profilin-1 depletion in WT cells increased F-actin and Arp2/3 complex in lamellipodia. Conversely, addition of exogenous profilin-1 inhibited Arp2/3 complex actin nucleation in vitro and in vivo. Antagonism of the Arp2/3 complex by profilin-1 in cells appears to maintain actin homeostasis by balancing Arp2/3 complex-dependent and -independent actin assembly pathways.

F-actin bundles direct the initiation and orientation of lamellipodia through adhesion-based signaling.

Mesenchymal cells such as fibroblasts are weakly polarized and reorient directionality by a lamellipodial branching mechanism that is stabilized by phosphoinositide 3-kinase (PI3K) signaling. However, the mechanisms by which new lamellipodia are initiated and directed are unknown. Using total internal reflection fluorescence microscopy to monitor cytoskeletal and signaling dynamics in migrating cells, we show that peripheral F-actin bundles/filopodia containing fascin-1 serve as templates for formation and orientation of lamellipodia. Accordingly, modulation of fascin-1 expression tunes cell shape, quantified as the number of morphological extensions. Ratiometric imaging reveals that F-actin bundles/filopodia play both structural and signaling roles, as they prime the activation of PI3K signaling mediated by integrins and focal adhesion kinase. Depletion of fascin-1 ablated fibroblast haptotaxis on fibronectin but not platelet-derived growth factor chemotaxis. Based on these findings, we conceptualize haptotactic sensing as an exploration, with F-actin bundles directing and lamellipodia propagating the process and with signaling mediated by adhesions playing the role of integrator.

Competition and collaboration between different actin assembly pathways allows for homeostatic control of the actin cytoskeleton.

Tremendous insight into actin-associated proteins has come from careful biochemical and cell biological characterization of their activities and regulation. However, many studies of their cellular behavior have only considered each in isolation. Recent efforts reveal that assembly factors compete for polymerization-competent actin monomers, suggesting that actin is homeostatically regulated. It seems that a major regulatory component is competition between Arp2/3-activating nucleation promoting factors and profilin for actin monomers. The result is differential delivery of actin to different pathways, allowing for simultaneous assembly of competing F-actin structures and collaborative building of higher order cellular structures. Although there are likely to be additional factors that regulate actin homeostasis, especially in a cell type-dependent fashion, we advance the notion that competition between actin assembly factors results in a tunable system that can be adjusted according to extracellular and intracellular cues.