Engineered red blood cells (activating antigen carriers) drive potent T cell responses and tumor regression in mice

Activation of T cell responses is essential for effective tumor clearance; however, inducing targeted, potent antigen presentation to stimulate T cell responses remains challenging. We generated Activating Antigen Carriers (AACs) by engineering red blood cells (RBCs) to encapsulate relevant tumor antigens and the adjuvant polyinosinic-polycytidylic acid (poly I:C), for use as a tumor-specific cancer vaccine. The processing method and conditions used to create the AACs promote phosphatidylserine exposure on RBCs and thus harness the natural process of aged RBC clearance to enable targeting of the AACs to endogenous professional antigen presenting cells (APCs) without the use of chemicals or viral vectors. AAC uptake, antigen processing, and presentation by APCs drive antigen-specific activation of T cells, both in mouse in vivo and human in vitro systems, promoting polyfunctionality of CD8+ T cells and, in a tumor model, driving high levels of antigen-specific CD8+ T cell infiltration and tumor killing. The efficacy of AAC therapy was further enhanced by combination with the chemotherapeutic agent Cisplatin. In summary, these findings support AACs as a potential vector-free immunotherapy strategy to enable potent antigen presentation and T cell stimulation by endogenous APCs with broad therapeutic potential.

Microfluidic Squeezing Enables MHC Class I Antigen Presentation by Diverse Immune Cells to Elicit CD8+ T Cell Responses with Antitumor Activity

CD8⁺ T cell responses are the foundation of the recent clinical success of immunotherapy in oncologic indications. Although checkpoint inhibitors have enhanced the activity of existing CD8⁺ T cell responses, therapeutic approaches to generate Ag-specific CD8⁺ T cell responses have had limited success. Here, we demonstrate that cytosolic delivery of Ag through microfluidic squeezing enables MHC class I presentation to CD8⁺ T cells by diverse cell types. In murine dendritic cells (DCs), squeezed DCs were ∼1000-fold more potent at eliciting CD8⁺ T cell responses than DCs cross-presenting the same amount of protein Ag. The approach also enabled engineering of less conventional APCs, such as T cells, for effective priming of CD8⁺ T cells in vitro and in vivo. Mixtures of immune cells, such as murine splenocytes, also elicited CD8⁺ T cell responses in vivo when squeezed with Ag. We demonstrate that squeezing enables effective MHC class I presentation by human DCs, T cells, B cells, and PBMCs and that, in clinical scale formats, the system can squeeze up to 2 billion cells per minute. Using the human papillomavirus 16 (HPV16) murine model, TC-1, we demonstrate that squeezed B cells, T cells, and unfractionated splenocytes elicit antitumor immunity and correlate with an influx of HPV-specific CD8⁺ T cells such that >80% of CD8s in the tumor were HPV specific. Together, these findings demonstrate the potential of cytosolic Ag delivery to drive robust CD8⁺ T cell responses and illustrate the potential for an autologous cell-based vaccine with minimal turnaround time for patients.

Abstract 3187: Engineering a new generation of cell therapies for solid tumor oncology using the SQZ platform

Effective T cell priming is crucial to induce anti-tumor CD8+ T cell responses and requires the efficient presentation of antigen on major histocompatibility complex class I (MHC-I) by antigen presenting cells (APCs). Previous efforts using dendritic cells to prime CD8+ T cell responses have proven difficult due to limited cell availability in the blood and challenges delivering antigen to the APC cytosol, a necessary step for MHC-I presentation and CD8+ T cell activation. To overcome this limitation, we deliver antigen directly to the cytosol of target APCs using the microfluidics-based SQZ platform. SQZ uniquely facilitates antigen loading into both professional and unconventional APCs, including B cells, T cells, and heterogenous populations of cells, which can be easily obtained directly from the blood. Protein and peptide antigens are delivered using SQZ to each of these APCs effectively, leading to efficient presentation of immunogenic epitopes on MHC-I. Here, we demonstrate that murine SQZ-APCs can stimulate antigen-specific CD8+ T cell responses in vitro and in vivo as measured by expansion of antigen-specific T cells and production of IFNγ. In the TC-1 tumor model for HPV-associated cancers, antigen-loaded SQZ-APCs have strong anti-tumor effects both prophylactically and therapeutically. Following therapeutic immunization, the anti-tumor responses correlate with an increase in antigen-specific CD8+ tumor infiltrating lymphocytes compared to untreated mice. In addition, compared to a traditional subcutaneous peptide vaccine, SQZ-APCs elicit a five-fold greater intratumoral CD8+ T cell response and drive significantly more tumor growth inhibition. Importantly, this SQZ-enabled cancer cell therapy translates to human B cells, T cells, and heterogenous populations of cells engineered to function as APCs. When a peptide is delivered to the cytosol using SQZ, all of these primary human cells activate antigen-specific CD8+ T cell responses in vitro by stimulation of IFNγ from antigen-specific CD8+ T cell responders. In comparison to cells incubated in the presence of peptide antigen, SQZ-APCs stimulate a 10-fold increase in IFNγ production from antigen-specific CD8+ responder T cells (n=13 donors). Finally, the SQZ process has been scaled to engineer human SQZ-APCs in preparation for clinical trials with a throughput of greater than 4 billion cells SQZ’d per minute. Collectively, these findings highlight the significant clinical potential of the SQZ platform to engineer potent APCs for a new generation of cancer cell therapies.

Citation Format: Kelan A. Hlavaty, Matthew G. Booty, Scott Loughhead, Katarina Blagovic, Alfonso Vicente-Suarez, Defne Yarar, Howard Bernstein, Armon Sharei. Engineering a new generation of cell therapies for solid tumor oncology using the SQZ platform [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3187.

Heregulin-ErbB3-Driven Tumor Growth Persists in PI3 Kinase Mutant Cancer Cells

PI3K is frequently mutated in cancer and plays an important role in cell growth and survival. Heregulin (HRG)-mediated autocrine or paracrine signaling through the receptor tyrosine kinase ErbB3 potently activates the PI3K/AKT pathway and has been shown to mediate resistance to a wide variety of anticancer agents. Although PI3K functions downstream of HRG-ErbB3, it is unknown whether activating mutations in PI3K render HRG ineffective. If so, patients with PI3K mutations would not be expected to benefit from ErbB3-directed therapies. Here, we find that a subset of cell lines harboring activating PI3K mutations can be further growth-stimulated by HRG, and this effect is blocked by incubation with seribantumab (MM-121), a monoclonal anti-ErbB3 antibody. Although expression of mutant PI3K in wild-type PI3K cells frequently results in loss of HRG-stimulated growth, some cell lines continue to respond to HRG. In cell lines where HRG-stimulated growth is lost, this loss is invariably accompanied by a reduction in ErbB3 levels, a corresponding increase in basal phosphorylation levels of FOXO-family transcription factors, and a reduction in HRG-induced downstream signaling. Importantly, HRG-stimulated growth is partially rescued by re-expressing ErbB3. This response is blocked by seribantumab, indicating that ErbB3 levels rather than downstream signaling proteins limit HRG-stimulated growth in PI3K mutant cells. Overall, these results suggest that activating mutations in PI3K do not preclude potential benefit from ErbB3-directed therapy, but that it may be important to measure ErbB3 levels in patients with PI3K mutant cancers to determine if they would benefit.

Model-Based Design of a Decision Tree for Treating HER2+ Cancers Based on Genetic and Protein Biomarkers

Human cancers are incredibly diverse with regard to molecular aberrations, dependence on oncogenic signaling pathways, and responses to pharmacological intervention. We wished to assess how cellular dependence on the canonical PI3K vs. MAPK pathways within HER2+ cancers affects responses to combinations of targeted therapies, and biomarkers predictive of their activity. Through an integrative analysis of mechanistic model simulations and in vitro cell line profiling, we designed a six-arm decision tree to stratify treatment of HER2+ cancers using combinations of targeted agents. Activating mutations in the PI3K and MAPK pathways (PIK3CA and KRAS), and expression of the HER3 ligand heregulin determined sensitivity to combinations of inhibitors against HER2 (lapatinib), HER3 (MM-111), AKT (MK-2206), and MEK (GSK-1120212; trametinib), in addition to the standard of care trastuzumab (Herceptin). The strategy used to identify effective combinations and predictive biomarkers in HER2-expressing tumors may be more broadly extendable to other human cancers.

Kinetochore genes are coordinately up-regulated in human tumors as part of a FoxM1-related cell division program

The key player in directing proper chromosome segregation is the macromolecular kinetochore complex, which mediates DNA-microtubule interactions. Previous studies testing individual kinetochore genes documented examples of their overexpression in tumors relative to normal tissue, leading to proposals that up-regulation of specific kinetochore genes may promote tumor progression. However, kinetochore components do not function in isolation, and previous studies did not comprehensively compare the expression behavior of kinetochore components. Here we analyze the expression behavior of the full range of human kinetochore components in diverse published expression compendia, including normal tissues and tumor samples. Our results demonstrate that kinetochore genes are rarely overexpressed individually. Instead, we find that core kinetochore genes are coordinately regulated with other cell division genes under virtually all conditions. This expression pattern is strongly correlated with the expression of the forkhead transcription factor FoxM1, which binds to the majority of cell division promoters. These observations suggest that kinetochore gene up-regulation in cancer reflects a general activation of the cell division program and that altered expression of individual kinetochore genes is unlikely to play a causal role in tumorigenesis.

Computational modeling of ERBB2-amplified breast cancer identifies combined ErbB2/3 blockade as superior to the combination of MEK and AKT inhibitors

Crosstalk and compensatory circuits within cancer signaling networks limit the activity of most targeted therapies. For example, altered signaling in the networks activated by the ErbB family of receptors, particularly in ERBB2-amplified cancers, contributes to drug resistance. We developed a multiscale systems model of signaling networks in ERBB2-amplified breast cancer to quantitatively investigate relationships between biomarkers (markers of network activity) and combination drug efficacy. This model linked ErbB receptor family signaling to breast tumor growth through two kinase cascades: the PI3K/AKT survival pathway and the Ras/MEK/ERK growth and proliferation pathway. The model predicted molecular mechanisms of resistance to individual therapeutics. In particular, ERBB2-amplified breast cancer cells stimulated with the ErbB3 ligand heregulin were resistant to growth arrest induced by inhibitors of AKT and MEK or coapplication of two inhibitors of the receptor ErbB2 [Herceptin (trastuzumab) and Tykerb (lapatinib)]. We used model simulations to predict the response of ErbB2-positive breast cancer xenografts to combination therapies and verified these predictions in mice. Treatment with trastuzumab, lapatinib, and the ErbB3 inhibitor MM-111 was more effective in inhibiting tumor growth than the combination of AKT and MEK inhibitors and even induced tumor regression, indicating that targeting both ErbB3 and ErbB2 may be an improved therapeutic approach for ErbB2-positive breast cancer patients.

Abstract C118: Optimization of MM-111 and lapatinib dosing regimens using mathematical modeling and quantitative biology

Designing therapeutically optimal dosing regimens for targeted biologic therapies is an open problem in translational oncology. There are at least two underlying reasons for this problem: First, biologics are generally well tolerated so it is not guaranteed that the maximum tolerated dose (MTD) will be observed in a Phase 1 dose escalation; and second, since targeted therapies work in only a subset of patients, there will be limited efficacy data available to determine a Phase 2 dose. This problem is further compounded by the use of combination drug treatments. MM-111 is a novel bispecific antibody against ErbB3 (HER3), using an ErbB2 (HER2) targeting arm to enhance avidity and inhibitor potency. Pre-clinical data shows that combining MM-111 with the ErbB2 inhibitor lapatinib (Tykerb®) results in synergistic tumor growth inhibition. We desired to leverage our pre-clinical modeling, signaling, pharmacokinetic (PK), pharmacodynamic (PD), and efficacy data to optimize dosing regimens of the combination therapy. While this study design is focused on MM-111 combined with lapatinib, this strategy is generally applicable to molecular targeted therapeutics given alone or in combination.

Our approach was to integrate a mathematical model of the MM-111 and lapatinib's mechanism of action, incorporating ErbB-family signal transduction, transcriptional regulation, and resulting tumor proliferation, with pre-clinical PK models of the two drugs. This integrated model was trained using in vitro dose response data from BT-474-M3 cells treated with the ErbB3 ligand heregulin plus combinations of the two drugs. Levels of phospho- and total ErbB3, ERbB2, AKT, ERK, and p27 were quantified using ELISA, and cell growth effects quantified using the Cell Titer Glow assay. This was then integrated with a 2-compartment PK-model, to predict PD profiles and tumor growth effects in murine xenografts treated with the drug combinations. After validation of our model with these initial in vivo studies, we designed therapeutically optimal dosing schedules in silico. The novel dosing strategies are now being validated in xenografts, and will help inform future clinical studies. By integrating PK/PD with known molecular mechanisms of action, we are able to rationally design combination treatment strategies to maximize therapeutic synergy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr C118.

Event ordering in live-cell imaging determined from temporal cross-correlation asymmetry

We use the temporal asymmetry of the cross-correlation function to determine the temporal ordering of spatially localized cellular events in live-cell multichannel fluorescence imaging. The analysis is well suited to noisy, stochastic systems where the temporal order may not be apparent in the raw data. The approach is applicable to any biochemical reaction not in chemical equilibrium, including protein complex assembly, sequential enzymatic processes, gene regulation, and other cellular signaling events. As an automated quantitative measure, this approach allows the data to be readily interpreted statistically with minimal subjective biases. We first test the technique using simulations of simple biophysical models with a definite temporal ordering. We then demonstrate the approach by extracting the temporal ordering of three proteins-actin, sorting nexin 9, and clathrin-in the endocytic pathway.

Differential requirements for clathrin-dependent endocytosis at sites of cell-substrate adhesion

Little is known about the influences of cell–substrate attachment in clathrin-mediated endocytosis. We find that cell–substrate adhesion reduces the rate of endocytosis. In addition, we demonstrate that actin assembly is differentially required for efficient endocytosis, with a stronger requirement for actin dynamics at sites of adhesion.

Clathrin-dependent endocytosis is a major route for the cellular import of macromolecules and occurs at the interface between the cell and its surroundings. However, little is known about the influences of cell–substrate attachment in clathrin-coated vesicle formation. Using biochemical and imaging-based methods, we find that cell–substrate adhesion reduces the rate of endocytosis. Clathrin-coated pits (CCPs) in proximity to substrate contacts exhibit slower dynamics in comparison to CCPs found more distant from adhesions. Direct manipulation of the extracellular matrix (ECM) to modulate adhesion demonstrates that tight adhesion dramatically reduces clathrin-dependent endocytosis and extends the lifetimes of clathrin structures. This reduction is in part mediated by integrin-matrix engagement. In addition, we demonstrate that actin cytoskeletal dynamics are differentially required for efficient endocytosis, with a stronger requirement for actin polymerization in areas of adhesion. Together, these results reveal that cell–substrate adhesion regulates clathrin-dependent endocytosis and suggests that actin assembly facilitates vesicle formation at sites of adhesion.

Cargo- and adaptor-specific mechanisms regulate clathrin-mediated endocytosis

Clathrin-coated pit size and dynamic behavior varies with low density lipoprotein receptor (LDLR) expression levels in a manner dependent on the LDLR-specific adaptors, Dab2 and ARH.

Clathrin-mediated endocytosis of surface receptors and their bound ligands (i.e., cargo) is highly regulated, including by the cargo itself. One of the possible sources of the observed heterogeneous dynamics of clathrin-coated pits (CCPs) might be the different cargo content. Consistent with this, we show that CCP size and dynamic behavior varies with low density lipoprotein receptor (LDLR) expression levels in a manner dependent on the LDLR-specific adaptors, Dab2 and ARH. In Dab2-mCherry–expressing cells, varying LDLR expression leads to a progressive increase in CCP size and to the appearance of nonterminal endocytic events. In LDLR and ARH-mCherry–expressing cells in addition to an increase in CCP size, turnover of abortive CCPs increases, and the rate of CCP maturation decreases. Altogether, our results underscore the highly dynamic and cargo-responsive nature of CCP assembly and suggest that the observed heterogeneity is, in part, related to compositional differences (e.g., cargo and adaptors) between CCPs.

Cargo and dynamin regulate clathrin-coated pit maturation

Total internal reflection fluorescence microscopy (TIR-FM) has become a powerful tool for studying clathrin-mediated endocytosis. However, due to difficulties in tracking and quantifying their heterogeneous dynamic behavior, detailed analyses have been restricted to a limited number of selected clathrin-coated pits (CCPs). To identify intermediates in the formation of clathrin-coated vesicles and factors that regulate progression through these stages, we used particle-tracking software and statistical methods to establish an unbiased and complete inventory of all visible CCP trajectories. We identified three dynamically distinct CCP subpopulations: two short-lived subpopulations corresponding to aborted intermediates, and one longer-lived productive subpopulation. In a manner dependent on AP2 adaptor complexes, increasing cargo concentration significantly enhances the maturation efficiency of productive CCPs, but has only minor effects on their lifetimes. In contrast, small interfering RNA (siRNA) depletion of dynamin-2 GTPase and reintroduction of wild-type or mutant dynamin-1 revealed dynamin's role in controlling the turnover of abortive intermediates and the rate of CCP maturation. From these data, we infer the existence of an endocytic restriction or checkpoint, responsive to cargo and regulated by dynamin.

Clathrin-mediated endocytosis is the major pathway for the uptake of molecules into eukaryotic cells and is regulated by the GTPase dynamin. Adaptor proteins recruit clathrin to the plasma membrane, where clathrin-coated pits capture transmembrane cargo molecules, again via adaptors. The pits invaginate and pinch off to form clathrin-coated vesicles that carry the cargo into the cell. Live cell imaging has revealed striking heterogeneity in the dynamic behavior of clathrin-coated pits associated with the plasma membrane, yet the nature of this heterogeneity and its functional implications are unknown. We used particle-tracking software to establish an unbiased and complete inventory of the trajectories of clathrin-coated pits visible by total internal reflection fluorescence microscopy. Through statistical analyses, we identified three dynamically distinct subpopulations of coated pits: two short-lived subpopulations corresponding to aborted intermediates, and one longer-lived productive subpopulation. The proportion of each subpopulation and their lifetimes respond independently to molecular perturbations. As a result of systematic modulation of cargo concentration, adaptor levels, and analysis of dynamin mutants, we postulate the existence of an endocytic restriction or checkpoint that governs the rate of clathrin-mediated endocytosis by gating the maturation of clathrin-coated pits.

Analysis of live-cell images reveals that clathrin-mediated endocytosis proceeds via distinct subpopulations of clathrin-coated pits, and suggests that an endocytosis check-point is regulated by dynamin and responsive to cargo load.

Diagnostic and prognostic use of FDG-PET scans in Waldenstrom's macroglobulinemia

e19518

Background: Waldenstrom's macroglobulinemia (WM) is a rare lymphoplasmacytic lymphoma. More sensitive tools of tumor burden and prognosis are needed in these patients (pts). The use of FDG-PET has not been previously studied in WM but has proved an effective diagnostic and prognostic tool effective in low-grade lymphomas. Therefore the objective of this study was to determine whether FDG-PET was an effective tool in evaluating pts with WM. Methods: We included all pts enrolled on a prospective phase II clinical trial of bortezomib and rituximab for pts with upfront or relapsed/refractory (RR) WM. All pts were CD20 positive with histologically confirmed disease and underwent staging evaluation by FDG-PET in combination with CT scan before and after therapy. Patients received 6 cycles of bortezomib IV weekly at 1.6mg/m2 on days 1, 8, 15q28 and rituximab at 375 mg/m2 on days 1, 8, 15 and 22 in cycles 1 and 4. Correlation was made with the international prognostic scoring system for WM and monoclonal protein response rate. Results: 53 pts were included, of which 35 were RR and 18 pts were upfront. Median age was 63 years (range 42–81), 64% were male. Before therapy, 34 pts (64.2%) had positive FDG-PET findings with a median IgM of 2825, Beta 2 microglobulin of 3.6, and Age of 61. 36 pts showed a minor response or greater with a correlation to negative PETs after therapy. Conclusions: Over 60% of WM pts demonstrated FDG-avid disease when using FDG-PET scans, with the majority showing negative disease after therapy. This correlates with the 67.9% response rate. Positive PET images were inversely correlated to IgM level and directly related to elevated Beta-2 microglobulin and age. Significantly PET positive individuals may demonstrate a poorer prognosis. FDG-PET scans may prove an effective tool in the diagnosis and prognosis in WM.

[Table: see text]

SNX9 activities are regulated by multiple phosphoinositides through both PX and BAR domains

Sorting nexin 9 (SNX9) functions at the interface between membrane remodeling and the actin cytoskeleton. In particular, SNX9 links membrane binding to potentiation of N-WASP and dynamin GTPase activities. SNX9 is one of a growing number of proteins that contain two lipid-binding domains, a phox homology (PX) and a Bin1/Amphiphysin/RVS167 (BAR) domain, and localizes to diverse membranes that are enriched in different phosphoinositides. Here, we investigate the mechanism by which SNX9 functions at these varied membrane environments. We show that SNX9 has low-lipid-binding affinity and harnesses a broad range of phosphoinositides to synergistically enhance both dynamin and N-WASP activities. We introduced point mutations in either the PX domain, BAR domain or both that are predicted to disrupt their functions and examined their respective roles in lipid-binding, and dynamin and N-WASP activation. We show that the broad lipid specificity of SNX9 is not because of independent and additive contributions by individual domains. Rather, the two domains appear to function in concert to confer lipid-binding and SNX9's membrane active properties. We also demonstrate that the two domains are differentially required for full SNX9 activity in N-WASP and dynamin regulation, and for localization of SNX9 to clathrin-coated pits and dorsal ruffles. In total, our results suggest that SNX9 can integrate signals from varied lipids through two domains to direct membrane remodeling events at multiple cellular locations.

The type III effector EspF coordinates membrane trafficking by the spatiotemporal activation of two eukaryotic signaling pathways

Bacterial toxins and effector proteins hijack eukaryotic enzymes that are spatially localized and display rapid signaling kinetics. However, the molecular mechanisms by which virulence factors engage highly dynamic substrates in the host cell environment are poorly understood. Here, we demonstrate that the enteropathogenic Escherichia coli (EPEC) type III effector protein EspF nucleates a multiprotein signaling complex composed of eukaryotic sorting nexin 9 (SNX9) and neuronal Wiskott-Aldrich syndrome protein (N-WASP). We demonstrate that a specific and high affinity association between EspF and SNX9 induces membrane remodeling in host cells. These membrane-remodeling events are directly coupled to N-WASP/Arp2/3-mediated actin nucleation. In addition to providing a biochemical mechanism of EspF function, we find that EspF dynamically localizes to membrane-trafficking organelles in a spatiotemporal pattern that correlates with SNX9 and N-WASP activity in living cells. Thus, our findings suggest that the EspF-dependent assembly of SNX9 and N-WASP represents a novel form of signaling mimicry used to promote EPEC pathogenesis and gastrointestinal disease.

SNX9 couples actin assembly to phosphoinositide signals and is required for membrane remodeling during endocytosis

Multiple modes of endocytosis require actin-dependent remodeling of the plasma membrane; however, neither the factors linking these processes nor their mechanisms of action are understood. The sorting nexin, SNX9, localizes to clathrin-coated pits where it interacts with dynamin and functions in clathrin-mediated endocytosis. Here, we demonstrate that SNX9 also localizes to actin-rich structures implicated in fluid-phase uptake, including tubular membranes containing GPI-anchored proteins and dorsal membrane ruffles. Moreover, we show that SNX9 is critical for dorsal ruffle formation and for clathrin-independent, actin-dependent fluid-phase endocytosis. In vitro, SNX9 directly associates with N-WASP, an Arp2/3 complex activator, and stimulates N-WASP/Arp2/3-mediated actin assembly. SNX9-stimulated actin polymerization is greatly enhanced by PI(4,5)P(2)-containing liposomes, due in part to PI(4,5)P(2)-induced SNX9 oligomerization. These results suggest a mechanism for the spatial and temporal regulation of N-WASP-dependent actin assembly and implicate SNX9 in directly coupling actin dynamics to membrane remodeling during multiple modes of endocytosis.

SNX9 regulates dynamin assembly and is required for efficient clathrin-mediated endocytosis

Dynamin, a central player in clathrin-mediated endocytosis, interacts with several functionally diverse SH3 domain-containing proteins. However, the role of these interactions with regard to dynamin function is poorly defined. We have investigated a recently identified protein partner of dynamin, SNX9, sorting nexin 9. SNX9 binds directly to both dynamin-1 and dynamin-2. Moreover by stimulating dynamin assembly, SNX9 stimulates dynamin's basal GTPase activity and potentiates assembly-stimulated GTPase activity on liposomes. In fixed cells, we observe that SNX9 partially localizes to clathrin-coated pits. Using total internal reflection fluorescence microscopy in living cells, we detect a transient burst of EGFP-SNX9 recruitment to clathrin-coated pits that occurs during the late stages of vesicle formation and coincides spatially and temporally with a burst of dynamin-mRFP fluorescence. Transferrin internalization is inhibited in HeLa cells after siRNA-mediated knockdown of SNX9. Thus, our results establish that SNX9 is required for efficient clathrin-mediated endocytosis and suggest that it functions to regulate dynamin activity.

A dynamic actin cytoskeleton functions at multiple stages of clathrin-mediated endocytosis

Clathrin-mediated endocytosis in mammalian cells is critical for a variety of cellular processes including nutrient uptake and cell surface receptor down-regulation. Despite the findings that numerous endocytic accessory proteins directly or indirectly regulate actin dynamics and that actin assembly is spatially and temporally coordinated with endocytosis, direct functional evidence for a role of actin during clathrin-coated vesicle formation is lacking. Here, we take parallel biochemical and microscopic approaches to address the contribution of actin polymerization/depolymerization dynamics to clathrin-mediated endocytosis. When measured using live-cell fluorescence microscopy, disruption of the F-actin assembly and disassembly cycle with latrunculin A or jasplakinolide results in near complete cessation of all aspects of clathrin-coated structure (CCS) dynamics. Stage-specific biochemical assays and quantitative fluorescence and electron microscopic analyses establish that F-actin dynamics are required for multiple distinct stages of clathrin-coated vesicle formation, including coated pit formation, constriction, and internalization. In addition, F-actin dynamics are required for observed diverse CCS behaviors, including splitting of CCSs from larger CCSs, merging of CCSs, and lateral mobility on the cell surface. Our results demonstrate a key role for actin during clathrin-mediated endocytosis in mammalian cells.

Signal analysis of total internal reflection fluorescent speckle microscopy (TIR-FSM) and wide-field epi-fluorescence FSM of the actin cytoskeleton and focal adhesions in living cells

Fluorescent speckle microscopy (FSM) uses low levels of fluorescent proteins to create fluorescent speckles on cytoskeletal polymers in high-resolution fluorescence images of living cells. The dynamics of speckles over time encode subunit turnover and motion of the cytoskeletal polymers. We sought to improve on current FSM technology by first expanding it to study the dynamics of a non-polymeric macromolecular assembly, using focal adhesions as a test case, and second, to exploit for FSM the high contrast afforded by total internal reflection fluorescence microscopy (TIR-FM). Here, we first demonstrate that low levels of expression of a green fluorescent protein (GFP) conjugate of the focal adhesion protein, vinculin, results in clusters of fluorescent vinculin speckles on the ventral cell surface, which by immunofluorescence labelling of total vinculin correspond to sparse labelling of dense focal adhesion structures. This demonstrates that the FSM principle can be applied to study focal adhesions. We then use both GFP-vinculin expression and microinjected fluorescently labelled purified actin to compare quantitatively the speckle signal in FSM images of focal adhesions and the actin cytoskeleton in living cells by TIR-FM and wide-field epifluorescence microscopy. We use quantitative FSM image analysis software to define two new parameters for analysing FSM signal features that we can extract automatically: speckle modulation and speckle detectability. Our analysis shows that TIR-FSM affords major improvements in these parameters compared with wide-field epifluorescence FSM. Finally, we find that use of a crippled eukaryotic expression promoter for driving low-level GFP-fusion protein expression is a useful tool for FSM imaging. When used in time-lapse mode, TIR-FSM of actin and GFP-conjugated focal adhesion proteins will allow quantification of molecular dynamics within interesting macromolecular assemblies at the ventral surface of living cells.

An assembly-incompetent mutant establishes a requirement for dynamin self-assembly in clathrin-mediated endocytosis in vivo

Dynamin GTPase activity is required for its biological function in clathrin-mediated endocytosis; however, the role of self-assembly has not been unambiguously established. Indeed, overexpression of a dynamin mutant, Dyn1-K694A, with impaired ability to self-assemble has been shown to stimulate endocytosis in HeLa cells (Sever et al., Nature 1999, 398, 481). To identify new, assembly-incompetent mutants of dynamin 1, we made point mutations in the GTPase effector/assembly domain (GED) and tested for their effects on self-assembly and clathrin-mediated endocytosis. Mutation of three residues, I690, K694, and I697, suggests that interactions with an amphipathic helix in GED are required for self-assembly. In particular, Dyn1-I690K failed to exhibit detectable assembly-stimulated GTPase activity under all assay conditions. Overexpression of this assembly-incompetent mutant inhibited transferrin endocytosis as potently as the GTPase-defective dominant-negative mutant, Dyn1-K44A. However, worm-like endocytic intermediates accumulated in cells expressing Dyn1-I690K that were structurally distinct from long tubules that accumulated in cells expressing Dyn1-K44A. Together these results provide new structural insight into the role of GED in self-assembly and assembly-stimulated GTPase activity and establish that dynamin self-assembly is essential for clathrin-mediated endocytosis.

Cortical patches on the move

Endocytosis is the primary means by which eukaryotic cells internalize materials from the environment. However, while many components of the endocytic machinery are known, the timing of molecular events leading to endocytosis remains undefined. In this issue of Cell, Kaksonen et al. use real-time microscopy to define the temporal assembly of components of the endocytic machinery in the yeast S. cerevisiae. They also provide new insight into how the actin cytoskeleton is coordinated with the endocytic machinery.

Formation of filopodia-like bundles in vitro from a dendritic network

We report the development and characterization of an in vitro system for the formation of filopodia-like bundles. Beads coated with actin-related protein 2/3 (Arp2/3)-activating proteins can induce two distinct types of actin organization in cytoplasmic extracts: (1) comet tails or clouds displaying a dendritic array of actin filaments and (2) stars with filament bundles radiating from the bead. Actin filaments in these bundles, like those in filopodia, are long, unbranched, aligned, uniformly polar, and grow at the barbed end. Like filopodia, star bundles are enriched in fascin and lack Arp2/3 complex and capping protein. Transition from dendritic to bundled organization was induced by depletion of capping protein, and add-back of this protein restored the dendritic mode. Depletion experiments demonstrated that star formation is dependent on Arp2/3 complex. This poses the paradox of how Arp2/3 complex can be involved in the formation of both branched (lamellipodia-like) and unbranched (filopodia-like) actin structures. Using purified proteins, we showed that a small number of components are sufficient for the assembly of filopodia-like bundles: Wiskott-Aldrich syndrome protein (WASP)-coated beads, actin, Arp2/3 complex, and fascin. We propose a model for filopodial formation in which actin filaments of a preexisting dendritic network are elongated by inhibition of capping and subsequently cross-linked into bundles by fascin.

Motility determinants in WASP family proteins

In response to upstream signals, proteins in the Wiskott-Aldrich Syndrome protein (WASP) family regulate actin nucleation via the Arp2/3 complex. Despite intensive study of the function of WASP family proteins in nucleation, it is not yet understood how their distinct structural organization contributes to actin-based motility. Herein, we analyzed the activities of WASP and Scar1 truncation derivatives by using a bead-based motility assay. The minimal region of WASP sufficient to direct movement was the C-terminal WCA fragment, whereas the corresponding region of Scar1 was insufficient. In addition, the proline-rich regions of WASP and Scar1 and the Ena/VASP homology 1 (EVH1) domain of WASP independently enhanced motility rates. The contributions of these regions to motility could not be accounted for by their direct effects on actin nucleation with the Arp2/3 complex, suggesting that they stimulate motility by recruiting additional factors. We have identified profilin as one such factor. WASP- and Scar1-coated bead motility rates were significantly reduced by depletion of profilin and VASP and could be more efficiently rescued by a combination of VASP and wild-type profilin than by VASP and a mutant profilin that cannot bind proline-rich sequences. Moreover, motility of WASP WCA beads was not affected by the depletion or addback of VASP and profilin. Our results suggest that recruitment of factors, including profilin, by the proline-rich regions of WASP and Scar1 and the EVH1 domain of WASP stimulates cellular actin-based motility.

Ultrafiltration method for measuring vascular access flow rates during hemodialysis

BACKGROUND

The vascular access blood flow rate (QA) has been shown to be an important predictor of vascular access failure; therefore, the routine measurement of QA may prove to be a useful clinical method of vascular access assessment.

METHODS

We have developed a new ultrafiltration (UF) method for determining QA during HD from changes in arterial hematocrit (H) after abrupt changes in the UF rate with the dialysis blood lines in the normal (DeltaHn) and reverse (DeltaHr) configurations. This method accounts for cardiopulmonary recirculation and requires neither intravenous saline injections nor accurate knowledge of the dialyzer blood flow rate. Clinical studies were conducted in 65 chronic HD patients from three different dialysis programs to compare QA determined by the UF method with that determined by saline dilution using an ultrasound flow sensor.

RESULTS

Arterial H increased (P<0.0001) after abrupt increases in the UF rate when the lines were in the normal and reverse configurations. An increase in the UF rate from the minimum setting to 1.8 liter/hr resulted in a DeltaHn of 0.3+/-0.2 (mean +/- SD) H units and a DeltaHr of 1.6+/-1.0 H units. Q(A) values determined by the UF method (1050+/-460 ml/min) were 16+/-25% higher (P<0.001) than those determined by saline dilution (950+/-440 ml/min); the calculated QA values by the UF and saline dilution methods correlated highly with each other (R = 0.92, P<0.0001). The average coefficient of variation for duplicate measurements of QA determined by the UF method in a subset of these patients (N = 21) was approximately 10% when assessed in either the same dialysis session or consecutive sessions.

CONCLUSIONS

The results from this study show that changes in arterial H after abrupt changes in the UF rate can be used to assess Q(A).

The Wiskott-Aldrich syndrome protein directs actin-based motility by stimulating actin nucleation with the Arp2/3 complex

Actin polymerization at the cell cortex is thought to provide the driving force for aspects of cell-shape change and locomotion. To coordinate cellular movements, the initiation of actin polymerization is tightly regulated, both spatially and temporally. The Wiskott-Aldrich syndrome protein (WASP), encoded by the gene that is mutated in the immunodeficiency disorder Wiskott-Aldrich syndrome [1], has been implicated in the control of actin polymerization in cells [2] [3] [4] [5]. The Arp2/3 complex, an actin-nucleating factor that consists of seven polypeptide subunits [6] [7] [8], was recently shown to physically interact with WASP [9]. We sought to determine whether WASP is a cellular activator of the Arp2/3 complex and found that WASP stimulates the actin nucleation activity of the Arp2/3 complex in vitro. Moreover, WASP-coated microspheres polymerized actin, formed actin tails and exhibited actin-based motility in cell extracts, similar to those behaviors displayed by the pathogenic bacterium Listeria monocytogenes. In extracts depleted of the Arp2/3 complex, WASP-coated microspheres and L. monocytogenes were non-motile and exhibited only residual actin polymerization. These results demonstrate that WASP is sufficient to direct actin-based motility in cell extracts and that this function is mediated by the Arp2/3 complex. WASP interacts with diverse signaling proteins and may therefore function to couple signal transduction pathways to Arp2/3-complex activation and actin polymerization.

Nuclear pore complex number and distribution throughout the Saccharomyces cerevisiae cell cycle by three-dimensional reconstruction from electron micrographs of nuclear envelopes

The number of nuclear pore complexes (NPCs) in individual nuclei of the yeast Saccharomyces cerevisiae was determined by computer-aided reconstruction of entire nuclei from electron micrographs of serially sectioned cells. Nuclei of 32 haploid cells at various points in the cell cycle were modeled and found to contain between 65 and 182 NPCs. Morphological markers, such as cell shape and nuclear shape, were used to determine the cell cycle stage of the cell being examined. NPC number was correlated with cell cycle stage to reveal that the number of NPCs increases steadily, beginning in G1-phase, suggesting that NPC assembly occurs continuously throughout the cell cycle. However, accumulation of nuclear envelope observed during the cell cycle, indicated by nuclear surface area, is not continuous at the same rate, such that the density of NPCs per unit area of nuclear envelope peaks in apparent S-phase cells. Analysis of the nuclear envelope reconstructions also revealed no preferred NPC-to-NPC distance. However, NPCs were found in large clusters over regions of the nuclear envelope. Interestingly, clusters of NPCs were most pronounced in early mitotic nuclei and were found to be associated with the spindle pole bodies, but the functional significance of this association is unknown.