Actin Dynamics as a Multiscale Integrator of Cellular Guidance Cues

Migrating cells must integrate multiple, competing external guidance cues. However, it is not well understood how cells prioritize among these cues. We investigate external cue integration by monitoring the response of wave-like, actin-polymerization dynamics, the driver of cell motility, to combinations of nanotopographies and electric fields in neutrophil-like cells. The electric fields provide a global guidance cue, and approximate conditions at wound sites in vivo. The nanotopographies have dimensions similar to those of collagen fibers, and act as a local esotactic guidance cue. We find that cells prioritize guidance cues, with electric fields dominating long-term motility by introducing a unidirectional bias in the locations at which actin waves nucleate. That bias competes successfully with the wave guidance provided by the bidirectional nanotopographies.

The social shape of sperm: using an integrative machine-learning approach to examine sperm ultrastructure and collective motility

Sperm is one of the most morphologically diverse cell types in nature, yet they also exhibit remarkable behavioural variation, including the formation of collective groups of cells that swim together for motility or transport through the female reproductive tract. Here, we take advantage of natural variation in sperm traits observed across Peromyscus mice to test the hypothesis that the morphology of the sperm head influences their sperm aggregation behaviour. Using both manual and automated morphometric approaches to quantify their complex shapes, and then statistical modelling and machine learning to analyse their features, we show that the aspect ratio of the sperm head is the most distinguishing morphological trait and statistically associates with collective sperm movements obtained from in vitro observations. We then successfully use neural network analysis to predict the size of sperm aggregates from sperm head morphology and show that species with relatively wider sperm heads form larger aggregates, which is consistent with the theoretical prediction that an adhesive region around the equatorial region of the sperm head mediates these unique gametic interactions. Together these findings advance our understanding of how even subtle variation in sperm design can drive differences in sperm function and performance.

Signaling through polymerization and degradation: Analysis and simulations of T cell activation mediated by Bcl10

The adaptive immune system serves as a potent and highly specific defense mechanism against pathogen infection. One component of this system, the effector T cell, facilitates pathogen clearance upon detection of specific antigens by the T cell receptor (TCR). A critical process in effector T cell activation is transmission of signals from the TCR to a key transcriptional regulator, NF-κB. The transmission of this signal involves a highly dynamic process in which helical filaments of Bcl10, a key protein constituent of the TCR signaling cascade, undergo competing processes of polymeric assembly and macroautophagy-dependent degradation. Through computational analysis of three-dimensional, super-resolution optical micrographs, we quantitatively characterize TCR-stimulated Bcl10 filament assembly and length dynamics, and demonstrate that filaments become shorter over time. Additionally, we develop an image-based, bootstrap-like resampling method that demonstrates the preferred association between autophagosomes and both Bcl10-filament ends and punctate-Bcl10 structures, implying that autophagosome-driven macroautophagy is directly responsible for Bcl10 filament shortening. We probe Bcl10 polymerization-depolymerization dynamics with a stochastic Monte-Carlo simulation of nucleation-limited filament assembly and degradation, and we show that high probabilities of filament nucleation in response to TCR engagement could provide the observed robust, homogeneous, and tunable response dynamic. Furthermore, we demonstrate that the speed of filament disassembly preferentially at filament ends provides effective regulatory control. Taken together, these data suggest that Bcl10 filament growth and degradation act as an excitable system that provides a digital response mechanism and the reliable timing critical for T cell activation and regulatory processes.

The immune system serves to protect organisms against pathogen-mediated disease. While a strong immune response is needed to eliminate pathogens in host organisms, immune responses that are too robust or too persistent can trigger autoimmune disorders, cancer, and a variety of additional serious human pathologies. Thus, a careful balance of activating and inhibitory mechanisms is necessary to prevent detrimental health outcomes of immune responses. For example, activated effector T cells marshal the immune response and direct killing of pathogen-infected cells; however, effector T cells that are chronically activated can damage and destroy healthy tissue. Here, we study an important internal activation pathway in effector T cells that involves the growth and counterbalancing disassembly (involving a process called macroautophagy) of filamentous cytoplasmic signaling structures. We utilize image analysis of 3-D super-resolution images and Monte Carlo simulations to study a key signal-transduction protein, Bcl10. We found that the speed of filament disassembly has the greatest effect on the magnitude and duration of the response, implying that pharmaceutical interventions aimed at macroautophagy may have substantial impact on effector T cell function. Given that filamentous structures are utilized in numerous immune signaling pathways, our analysis methods could have broad applicability in the signal transduction field.

Quantifying topography-guided actin dynamics across scales using optical flow

The dynamic rearrangement of the actin cytoskeleton is an essential component of many mechanotransduction and cellular force generation pathways. Here we use periodic surface topographies with feature sizes comparable to those of in vivo collagen fibers to measure and compare actin dynamics for two representative cell types that have markedly different migratory modes and physiological purposes: slowly migrating epithelial MCF10A cells and polarizing, fast-migrating, neutrophil-like HL60 cells. Both cell types exhibit reproducible guidance of actin waves (esotaxis) on these topographies, enabling quantitative comparisons of actin dynamics. We adapt a computer-vision algorithm, optical flow, to measure the directions of actin waves at the submicron scale. Clustering the optical flow into regions that move in similar directions enables micron-scale measurements of actin-wave speed and direction. Although the speed and morphology of actin waves differ between MCF10A and HL60 cells, the underlying actin guidance by nanotopography is similar in both cell types at the micron and submicron scales.

Actin Cytoskeleton and Focal Adhesions Regulate the Biased Migration of Breast Cancer Cells on Nanoscale Asymmetric Sawteeth

Physical guidance from the underlying matrix is a key regulator of cancer invasion and metastasis. We explore the effects of surface topography on the migration phenotype of multiple breast cancer cell lines using aligned nanoscale ridges and asymmetric sawtooth structures. Both benign and metastatic breast cancer cells preferentially move parallel to nanoridges, with enhanced speeds compared to flat surfaces. In contrast, asymmetric sawtooth structures unidirectionally bias the movement of breast cancer cells in a cell-type-dependent manner. Quantitative analysis shows that the level of bias in cell migration increases when cells move with higher speeds or with higher directional persistence. Live-cell imaging studies further reveal that actin polymerization waves are unidirectionally guided by the sawteeth in the same direction as the cell motion. High-resolution fluorescence imaging and scanning electron microscopy studies reveal that two breast cancer cell lines with opposite migrational profiles exhibit profoundly different cell cortical plasticity and focal adhesion patterns. These results suggest that the overall migration response of cancer cells to surface topography is directly related to the underlying cytoskeletal architectures and dynamics, which are regulated by both intrinsic and extrinsic factors.

Intravital microscopy of collective invasion plasticity in breast cancer

Cancer invasion programs are adaptive by switching between metastatic collective and single-cell dissemination; however, current intravital microscopy models for epithelial cancer in mice fail to reliably recreate such invasion plasticity. Using microimplantation of breast cancer spheroids into the murine mammary fat pad and live-cell monitoring, we show microenvironmental conditions and cytoskeletal adaptation during collective to single-cell transition in vivo E-cadherin-expressing 4T1 and E-cadherin-negative MMT tumors both initiated collective invasion along stromal structures, reflecting invasion patterns in 3D organotypic culture and human primary ductal and lobular carcinoma. Collectively invading cells developed weakly oscillatory actin dynamics, yet provided zones for single-cell transitions with accentuated, more chaotic actin fluctuations. This identifies collective invasion in vivo as a dynamic niche and efficient source for single-cell release.

A transient Malt1 aggresome sustains T cell receptor signaling to NF-κB

The T cell receptor (TCR) to NF-κB signaling pathway is crucial for T cell activation and differentiation. Upon engagement of the TCR with cognate antigen, a series of events leads to the formation of the Carma1-Bcl10-Malt1 (CBM) complex, increasingly recognized as vital to a properly functioning immune response. We have demonstrated that in effector T cells, the CBM complex gives rise to a polymeric, filamentous signalosome called POLKADOTS that directs terminal activation of NF-κB. POLKADOTS consist of the core protein Bcl10, its constitutive binding partner Malt1, and their recruited signaling partners. We have previously shown that Bcl10 is degraded via selective autophagy following T cell activation. Here, we show that Malt1, which serves both as an adaptor transmitting signals to NF-κB and as a protease which cleaves a variety of substrates, is not concurrently degraded. Instead, Malt1-containing POLKADOTS coalesce via microtubule transport to a canonical aggresome. Aggresomes are thought to be depots of misfolded protein destined for degradation via macroautophagy; however, the Malt1 aggresome promotes late stage NF-κB activation through prolonging pIKK-Malt1 interactions. Additionally, this aggresome enhances the proteolysis of a subset of Malt1 targets. These results establish a mechanism for sustaining cytoplasmic signals from the TCR. More broadly, our findings demonstrate that aggresomes can serve as a stable platform for signal transduction.

Lamin A and microtubules collaborate to maintain nuclear morphology

Lamin A (LA) is a critical structural component of the nuclear lamina. Mutations within the LA gene (LMNA) lead to several human disorders, most striking of which is Hutchinson-Gilford Progeria Syndrome (HGPS), a premature aging disorder. HGPS cells are best characterized by an abnormal nuclear morphology known as nuclear blebbing, which arises due to the accumulation of progerin, a dominant mutant form of LA. The microtubule (MT) network is known to mediate changes in nuclear morphology in the context of specific events such as mitosis, cell polarization, nucleus positioning and cellular migration. What is less understood is the role of the microtubule network in determining nuclear morphology during interphase. In this study, we elucidate the role of the cytoskeleton in regulation and misregulation of nuclear morphology through perturbations of both the lamina and the microtubule network. We found that LA knockout cells exhibit a crescent shape morphology associated with the microtubule-organizing center. Furthermore, this crescent shape ameliorates upon treatment with MT drugs, Nocodazole or Taxol. Expression of progerin, in LA knockout cells also rescues the crescent shape, although the response to Nocodazole or Taxol treatment is altered in comparison to cells expressing LA. Together these results describe a collaborative effort between LA and the MT network to maintain nuclear morphology.

Coalescence of Nanoscale Cytoplasmic Signalosomes Contributes to T Cell Receptor Signaling to NF-κB

T cell receptor (TCR) activation of the transcription factor NF-κB is a crucial determinant of effector T lymphocyte function. The complex regulatory network surrounding this pathway remains poorly understood, particularly at time periods further removed from initial TCR triggering. We have previously demonstrated that following activation of the TCR, the proteins p62, Bcl10, and Malt1 rapidly combine to form a cytoplasmic filamentous signalosome called POLKADOTS, which recruits further signaling proteins and initiates the terminal steps in activation of NF-κB. Here, we examine the fate of POLKADOTS signalosomes following initial activation of NF-κB. We demonstrate via super-resolution and confocal microscopy techniques that POLKADOTS filaments converge on the microtubule-organizing center via microtubule transport, aggregating in a peri-nuclear aggresomal structure. Aggresomes are poorly understood structures thought to be depots of misfolded protein destined for degradation; however, our data suggest that the aggresomal accumulations of POLKADOTS continue to promote T cell activation. We show that the formation and maintenance of this aggresome corresponds with a secondary increase in NF-κB translocation. Additionally, the aggresomal structure prolongs phosphorylation of IKK and promotes the proteolytic activity of Malt1. Together, these results demonstrate that TCR signaling to NF-κB directs the orchestrated assembly, transport, and stable coalescence of nanoscale cytoplasmic signaling complexes. More broadly, our findings provide evidence that aggresomes can serve as stable platforms of signal transduction, thereby proposing a new role for these little understood structures.

Abstract B046: Mechanisms of T lymphocyte activation revealed by super-resolution microscopy

Tight regulatory control of lymphocyte activation is necessary to avoid the deleterious consequences of an uncontrolled immune response. A complicated web of up-regulatory and down-regulatory processes governs a key activation pathway in lymphocytes, the antigen-receptor-to-NF-κB pathway; however, the complex interplay between positive and negative regulation of this pathway remains poorly understood. We have utilized cutting-edge super-resolution imaging technologies to examine the spatial and temporal organization of the POLKADOTS signalosome, which is inducibly formed in response to antigen stimulation of T lymphocytes. Our preliminary results suggest that this filamentous signalosome is divided into two distinct subregions: an activation domain which recruits signal transduction elements, and a downregulation domain which promotes macroautophagic degradation of the signalosome. Fixed and live cell imaging of the activation process indicates that POLKADOTS filaments converge on the microtubule-organizing center as the core filament protein Bcl10 is progressively degraded by autophagosomes. Furthermore, following Bcl10 degradation, the POLKADOTS components Malt1 and phospho-IKK are transferred to a peri-nuclear aggresomal structure. Surprisingly, our data suggest these aggresomal accumulations continue to promote NF-κB nuclear localization and T cell activation. Together, these results provide new insights into the complex regulatory processes which govern T lymphocyte activation, and for the first time suggest a signaling function for aggresomes.

Citation Format: Maria Traver, Leonard Campanello, Suman Paul, Wolfgang Losert, Hari Shroff, Brian Schaefer. Mechanisms of T lymphocyte activation revealed by super-resolution microscopy. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr B046.