Mitochondria regulate proliferation in adult cardiac myocytes

Newborn mammalian cardiomyocytes quickly transition from a fetal to an adult phenotype that utilizes mitochondrial oxidative phosphorylation but loses mitotic capacity. We tested whether forced reversal of adult cardiomyocytes back to a fetal glycolytic phenotype would restore proliferative capacity. We deleted Uqcrfs1 (mitochondrial Rieske Iron-Sulfur protein, RISP) in hearts of adult mice. As RISP protein decreased, heart mitochondrial function declined, and glucose utilization increased. Simultaneously, they underwent hyperplastic remodeling during which cardiomyocyte number doubled without cellular hypertrophy. Cellular energy supply was preserved, AMPK activation was absent, and mTOR activation was evident. In ischemic hearts with RISP deletion, new cardiomyocytes migrated into the infarcted region, suggesting the potential for therapeutic cardiac regeneration. RNA-seq revealed upregulation of genes associated with cardiac development and proliferation. Metabolomic analysis revealed a decrease in alpha-ketoglutarate (required for TET-mediated demethylation) and an increase in S-adenosylmethionine (required for methyltransferase activity). Analysis revealed an increase in methylated CpGs near gene transcriptional start sites. Genes that were both differentially expressed and differentially methylated were linked to upregulated cardiac developmental pathways. We conclude that decreased mitochondrial function and increased glucose utilization can restore mitotic capacity in adult cardiomyocytes resulting in the generation of new heart cells, potentially through the modification of substrates that regulate epigenetic modification of genes required for proliferation.

Neutrophil Infiltration and Function in the Pathogenesis of Inflammatory Airspace Disease

Neutrophils are an important cell type often considered the body's first responders to inflammatory insult or damage. They are recruited to the tissue of the lungs in patients with inflammatory airspace diseases and have unique and complex functions that range from helpful to harmful. The uniqueness of these functions is due to the heterogeneity of the inflammatory cascade and retention in the vasculature. Neutrophils are known to marginate, or remain stagnant, in the lungs even in nondisease conditions. This review discusses the ways in which the recruitment, presence, and function of neutrophils in the airspace of the lungs are unique from those of other tissues, and the complex effects of neutrophils on pathogenesis. Inflammatory mediators produced by neutrophils, such as neutrophil elastase, proresolving mediators, and neutrophil extracellular traps, dramatically affect the outcomes of patients with disease of the lungs.

A Physiologic Roll for Cell Surface GlycoRNAs

Glycosylated RNA molecules (glycoRNAs) that can be bound by lectins have been demonstrated on the surfaces of leukocytes, but their physiologic function(s) was not known. A recent paper (PMID 38262409) demonstrates that at least one function is to promote capture and rolling of neutrophils in the vasculature. Of interest, the neutrophil glycoRNAs bind to P-selectin, but not E-selectin.

Age-related loss of Notch3 underlies brain vascular contractility deficiencies, glymphatic dysfunction, and neurodegeneration in mice

Vascular aging affects multiple organ systems, including the brain, where it can lead to vascular dementia. However, a concrete understanding of how aging specifically affects the brain vasculature, along with molecular readouts, remains vastly incomplete. Here, we demonstrate that aging is associated with a marked decline in Notch3 signaling in both murine and human brain vessels. To clarify the consequences of Notch3 loss in the brain vasculature, we used single-cell transcriptomics and found that Notch3 inactivation alters regulation of calcium and contractile function and promotes a notable increase in extracellular matrix. These alterations adversely impact vascular reactivity, manifesting as dilation, tortuosity, microaneurysms, and decreased cerebral blood flow, as observed by MRI. Combined, these vascular impairments hinder glymphatic flow and result in buildup of glycosaminoglycans within the brain parenchyma. Remarkably, this phenomenon mirrors a key pathological feature found in brains of patients with CADASIL, a hereditary vascular dementia associated with NOTCH3 missense mutations. Additionally, single-cell RNA sequencing of the neuronal compartment in aging Notch3-null mice unveiled patterns reminiscent of those observed in neurodegenerative diseases. These findings offer direct evidence that age-related NOTCH3 deficiencies trigger a progressive decline in vascular function, subsequently affecting glymphatic flow and culminating in neurodegeneration.

Mechanotransduction via endothelial adhesion molecule CD31 initiates transmigration and reveals a role for VEGFR2 in diapedesis

Engagement of platelet endothelial cell adhesion molecule 1 (PECAM, PECAM-1, CD31) on the leukocyte pseudopod with PECAM at the endothelial cell border initiates transendothelial migration (TEM, diapedesis). We show, using fluorescence lifetime imaging microscopy (FLIM), that physical traction on endothelial PECAM during TEM initiated the endothelial signaling pathway. In this role, endothelial PECAM acted as part of a mechanotransduction complex with VE-cadherin and vascular endothelial growth factor receptor 2 (VEGFR2), and this predicted that VEGFR2 was required for efficient TEM. We show that TEM required both VEGFR2 and the ability of its Y1175 to be phosphorylated, but not VEGF or VEGFR2 endogenous kinase activity. Using inducible endothelial-specific VEGFR2-deficient mice, we show in three mouse models of inflammation that the absence of endothelial VEGFR2 significantly (by ≥75%) reduced neutrophil extravasation by selectively blocking diapedesis. These findings provide a more complete understanding of the process of transmigration and identify several potential anti-inflammatory targets.

Dynamic Glycoprotein Hyposialylation Promotes Chemotherapy Evasion and Metastatic Seeding of Quiescent Circulating Tumor Cell Clusters in Breast Cancer

Most circulating tumor cells (CTC) are detected as single cells, whereas a small proportion of CTCs in multicellular clusters with stemness properties possess 20- to 100-times higher metastatic propensity than the single cells. Here we report that CTC dynamics in both singles and clusters in response to therapies predict overall survival for breast cancer. Chemotherapy-evasive CTC clusters are relatively quiescent with a specific loss of ST6GAL1-catalyzed α2,6-sialylation in glycoproteins. Dynamic hyposialylation in CTCs or deficiency of ST6GAL1 promotes cluster formation for metastatic seeding and enables cellular quiescence to evade paclitaxel treatment in breast cancer. Glycoproteomic analysis reveals newly identified protein substrates of ST6GAL1, such as adhesion or stemness markers PODXL, ICAM1, ECE1, ALCAM1, CD97, and CD44, contributing to CTC clustering (aggregation) and metastatic seeding. As a proof of concept, neutralizing antibodies against one newly identified contributor, PODXL, inhibit CTC cluster formation and lung metastasis associated with paclitaxel treatment for triple-negative breast cancer.

SIGNIFICANCE

This study discovers that dynamic loss of terminal sialylation in glycoproteins of CTC clusters contributes to the fate of cellular dormancy, advantageous evasion to chemotherapy, and enhanced metastatic seeding. It identifies PODXL as a glycoprotein substrate of ST6GAL1 and a candidate target to counter chemoevasion-associated metastasis of quiescent tumor cells. This article is featured in Selected Articles from This Issue, p. 1949.

Methods for Imaging Inflammation and Transendothelial Migration in Vivo and ex Vivo

Inflammation is the body's response to injury and harmful stimuli and contributes to a range of infectious and noninfectious diseases. Inflammation occurs through a series of well-defined leukocyte-endothelial cell interactions, including rolling, activation, adhesion, transmigration, and subsequent migration through the extracellular matrix. Being able to visualize the stages of inflammation is important for a better understanding of its role in diseases processes. Detailed in this article are protocols for imaging immune cell infiltration and transendothelial migration in vascular tissue beds, including those in the mouse ear, cremaster muscle, brain, lung, and retina. Also described are protocols for inducing inflammation and quantifying leukocytes with FIJI imaging software. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Induction of croton oil dermatitis Alternate Protocol 1: Induction of croton oil dermatitis using genetically fluorescent mice Basic Protocol 2: Intravital microscopy of the mouse cremaster muscle Support Protocol: Making a silicone stage Basic Protocol 3: Wide-field microscopy of the mouse brain Basic Protocol 4: Imaging the lungs (ex vivo) Alternate Protocol 2: Inflating the lungs without tracheostomy Basic Protocol 5: Inducing, imaging, and quantifying infiltration of leukocytes in mouse retina.

Circadian regulator CLOCK promotes tumor angiogenesis in glioblastoma

Glioblastoma (GBM) is one of the most aggressive tumors in the adult central nervous system. We previously revealed that circadian regulation of glioma stem cells (GSCs) affects GBM hallmarks of immunosuppression and GSC maintenance in a paracrine and autocrine manner. Here, we expand the mechanism involved in angiogenesis, another critical GBM hallmark, as a potential basis underlying CLOCK's pro-tumor effect in GBM. Mechanistically, CLOCK-directed olfactomedin like 3 (OLFML3) expression results in hypoxia-inducible factor 1-alpha (HIF1α)-mediated transcriptional upregulation of periostin (POSTN). As a result, secreted POSTN promotes tumor angiogenesis via activation of the TANK-binding kinase 1 (TBK1) signaling in endothelial cells. In GBM mouse and patient-derived xenograft models, blockade of the CLOCK-directed POSTN-TBK1 axis inhibits tumor progression and angiogenesis. Thus, the CLOCK-POSTN-TBK1 circuit coordinates a key tumor-endothelial cell interaction and represents an actionable therapeutic target for GBM.

Platelet Endothelial Cell Adhesion Molecule (PECAM/CD31) Blockade Modulates Neutrophil Recruitment Patterns and Reduces Infarct Size in Experimental Ischemic Stroke

The infiltration of polymorphonuclear leukocytes (PMNs) in ischemia-reperfusion injury (I/RI) has been implicated as a critical component of inflammatory damage following ischemic stroke. However, successful blockade of PMN transendothelial migration (TEM) in preclinical studies has not translated to meaningful clinical outcomes. To investigate this further, leukocyte infiltration patterns were quantified, and these patterns were modulated by blocking platelet endothelial cell adhesion molecule-1 (PECAM), a key regulator of TEM. LysM-eGFP mice and microscopy were used to visualize all myeloid leukocyte recruitment following ischemia/reperfusion. Visual examination showed heterogeneous leukocyte distribution across the infarct at both 24 and 72 hours after I/RI. A semiautomated process was designed to precisely map PMN position across brain sections. Treatment with PECAM function-blocking antibodies did not significantly affect total leukocyte recruitment but did alter their distribution, with more observed at the cortex at both early and later time points (24 hours: 89% PECAM blocked vs. 72% control; 72 hours: 69% PECAM blocked vs. 51% control). This correlated with a decrease in infarct volume. These findings suggest that TEM, in the setting of I/RI in the cerebrovasculature, occurs primarily at the cortical surface. The reduction of stroke size with PECAM blockade suggests that infiltrating PMNs may exacerbate I/RI and indicate the potential therapeutic benefit of regulating the timing and pattern of leukocyte infiltration after stroke.

Human CD99L2 Regulates a Unique Step in Leukocyte Transmigration

CD99-like 2 (CD99L2 [L2]) is a highly glycosylated 52-kDa type 1 membrane protein that is important for leukocyte transendothelial migration (TEM) in mice. Inhibiting L2 using function-blocking Ab significantly reduces the recruitment of leukocytes to sites of inflammation in vivo. Similarly, L2 knockout mice have an inherent defect in leukocyte transmigration into sites of inflammation. However, the role of L2 in inflammation has only been studied in mice. Furthermore, the mechanism by which it regulates TEM is not known. To study the relevance to human inflammation, we studied the role of L2 on primary human cells in vitro. Our data show that like PECAM and CD99, human L2 is constitutively expressed at the borders of endothelial cells and on the surface of leukocytes. Inhibiting L2 using Ab blockade or genetic knockdown significantly reduces transmigration of human neutrophils and monocytes across endothelial cells. Furthermore, our data also show that L2 regulates a specific, sequential step of TEM between PECAM and CD99, rather than operating in parallel or redundantly with these molecules. Similar to PECAM and CD99, L2 promotes transmigration by recruiting the lateral border recycling compartment to sites of TEM, specifically downstream of PECAM initiation. Collectively, our data identify a novel functional role for human L2 in TEM and elucidate a mechanism that is distinct from PECAM and CD99.

Abstract P3136: Notch3 Deficiency Impairs Vascular Smooth Muscle Cell Contractility And Glymphatic Function In The Brain

While most cases of vascular dementia represent complex interactions between host genetics and environmental factors, mendelian forms of vascular dementia also exist. Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), is a mendelian disease characterized by progressive vascular deterioration, cognitive deficits, and strokes. Mutations in the NOTCH3 receptor underlies the pathologies in CADASIL. NOTCH3 is primarily expressed in vascular smooth muscle cells (vSMCs) and its’ expression is critical for differentiation and functional integrity of arterial vSMCs, albeit through unclear mechanism(s). To elucidate the contribution of NOTCH3 in the maintenance of cerebral vascular architecture and function, we performed micro-computed tomography (micro-CT) on the brains of aged Notch3 -deficient animals. Micro-CT assessment of the cerebral vasculature architecture showed significant abnormalities including severe vessel dilation and tortuosity (dolicoectasia) of the middle cerebral artery and its branches in the Notch3 -/- compared to aged-match controls. To identify the molecular pathway from NOTCH3 dysregulation to the observed cerebral vascular dysfunction, we performed single-cell RNASeq on cerebral arteries isolated from young (4w) and old (104w) Notch3 -/- animals. Evaluation of the vSMC-specific transcriptomes indicated significant loss of proteins associated with muscle contraction and increased extracellular matrix production in animals that lack NOTCH3. Using a combination of immunofluorescence microscopy and in vitro functional assays, we confirmed that continued expression of Notch3 is a critical requirement for maintenance of vSMC contractile function. Impaired contractility also affected flow of cerebrospinal fluid in the parenchyma of Notch3 -/- . MRI and behavioral assessments were performed in the Notch3 -/- animals to elucidate the relationship between impaired vascular contractility to cognitive function. Taken together these findings link the molecular dysfunction of NOTCH3 through its regulation of vascular contractility and cerebral vessel architecture to altered neurological function and clarify the molecular pathways to cellular pathology of Notch3 driven dementias.

Abstract WP258: Post-stroke Microvascular No-reflow Demonstrates Spatiotemporal Heterogeneity Using Visible-Light Optical Coherence Tomography

Introduction: The spatiotemporal pattern of microvascular reperfusion in vivo after large vessel recanalization is poorly understood. Combining chronic cranial window with embedded microprism (CCW-MP) and visible-light optical coherence tomography angiography (Vis-OCTA) enables the study of mouse cortical cerebrovasculature up to 60 days post-stroke. Vis-OCTA/CCW-MP can identify the presence or absence of microvascular flow, without labelling, up to 1 mm in cortical depth of the mouse brain at 1.3 micron resolution. We sought to build on these findings to understand the pattern of microvascular no-reflow in a mouse model of stroke that simulates large vessel occlusion and reperfusion.

Hypothesis: Different regions of the cortex have striking variation in flow characteristics after macrovascular reperfusion.

Methods: All mice studied were 3-4 mo old, C57/Bl6 background, and subject to IMPROVE guideline. Control mice (n=3) were studied to confirm absence of cortical or vascular changes induced by CCW-MP surgery. Test mice (n =6) underwent CCW-MP as previously published. After 2 weeks to allow for healing post-surgery, mice were subject to transient middle cerebral artery occlusion (tMCAO) to model large vessel occlusion and reperfusion. Vis-OCTA measured cortical flow at baseline, 24 hours post-stroke, and 72 hours post-stroke. Cortical mouse vascular regions were divided into layers 1-3: layer 4 : layer 5-6 from top to bottom.

Results: Control mice showed no neuronal death, astrogliosis, microgliosis, or neutrophil recruitment induced by CCW-MP implantation. There was no significant difference between vascular density for prism-adjacent cortex and prism-remote cortex. We also determined that the CCW-MP images cortical vascular territory supplied by the middle cerebral artery. For the test mice, all six mice survived combined surgery. Vis-OCTA showed four of six mice had reduction of microvascular flow > 20% from baseline by 72h post-stroke at layer 5-6.

Conclusions: Microvascular reperfusion post large vessel recanalization can be incomplete up to 72 hours post-stroke, especially at deeper cortical levels. Further studies should define associated changes in inflammatory cells as well as oxygenation of vessels.

Experimental Colitis Enhances Temporal Variations in CX3CR1 Cell Colonization of the Gut and Brain Following Irradiation

Peripheral monocyte-derived CX3C chemokine receptor 1 positive (CX3CR1⁺) cells play important roles in tissue homeostasis and gut repopulation. Increasing evidence also supports their role in immune repopulation of the brain parenchyma in response to systemic inflammation. Adoptive bone marrow transfer from CX3CR1 fluorescence reporter mice and high-resolution confocal microscopy was used to assess the time course of CX3CR1⁺ cell repopulation of steady-state and dextran sodium sulfate (DSS)-inflamed small intestine/colon and the brain over 4 weeks after irradiation. CX3CR1⁺ cell colonization and morphologic polarization into fully ramified cells occurred more rapidly in the small intestine than in the colon. For both organs, the crypt/mucosa was more densely populated than the serosa/muscularis layer, indicating preferential temporal and spatial occupancy. Repopulation of the brain was delayed compared with that of gut tissue, consistent with the immune privilege of this organ. However, DSS-induced colon injury accelerated the repopulation. Expression analyses confirmed increased chemokine levels and macrophage colonization within the small intestine/colon and the brain by DSS-induced injury. Early increases of transmembrane protein 119 and ionized calcium binding adaptor molecule 1 expression within the brain after colon injury suggest immune-priming effect of brain resident microglia in response to systemic inflammation. These findings identify temporal differences in immune repopulation of the gut and brain in response to inflammation and show that gut inflammation can impact immune responses within the brain.

Spatiotemporal restriction of endothelial cell calcium signaling is required during leukocyte transmigration

Endothelial cell calcium flux is critical for leukocyte transendothelial migration (TEM), which in turn is essential for the inflammatory response. Intravital microscopy of endothelial cell calcium dynamics reveals that calcium increases locally and transiently around the transmigration pore during TEM. Endothelial calmodulin (CaM), a key calcium signaling protein, interacts with the IQ domain of IQGAP1, which is localized to endothelial junctions and is required for TEM. In the presence of calcium, CaM binds endothelial calcium/calmodulin kinase IIδ (CaMKIIδ). Disrupting the function of CaM or CaMKII with small-molecule inhibitors, expression of a CaMKII inhibitory peptide, or expression of dominant negative CaMKIIδ significantly reduces TEM by interfering with the delivery of the lateral border recycling compartment (LBRC) to the site of TEM. Endothelial CaMKII is also required for TEM in vivo as shown in two independent mouse models. These findings highlight novel roles for endothelial CaM and CaMKIIδ in transducing the spatiotemporally restricted calcium signaling required for TEM.

ICAM1 initiates CTC cluster formation and trans-endothelial migration in lung metastasis of breast cancer

Circulating tumor cell (CTC) clusters mediate metastasis at a higher efficiency and are associated with lower overall survival in breast cancer compared to single cells. Combining single-cell RNA sequencing and protein analyses, here we report the profiles of primary tumor cells and lung metastases of triple-negative breast cancer (TNBC). ICAM1 expression increases by 200-fold in the lung metastases of three TNBC patient-derived xenografts (PDXs). Depletion of ICAM1 abrogates lung colonization of TNBC cells by inhibiting homotypic tumor cell-tumor cell cluster formation. Machine learning-based algorithms and mutagenesis analyses identify ICAM1 regions responsible for homophilic ICAM1-ICAM1 interactions, thereby directing homotypic tumor cell clustering, as well as heterotypic tumor-endothelial adhesion for trans-endothelial migration. Moreover, ICAM1 promotes metastasis by activating cellular pathways related to cell cycle and stemness. Finally, blocking ICAM1 interactions significantly inhibits CTC cluster formation, tumor cell transendothelial migration, and lung metastasis. Therefore, ICAM1 can serve as a novel therapeutic target for metastasis initiation of TNBC.

Circulating tumor cell (CTC) clusters are more efficient at mediating metastasis as compared to single cells and are associated with poor prognosis in breast cancer. Here, the authors show that ICAM1 is enriched in CTC clusters and its loss suppresses cell-cell interaction and CTC cluster formation, and propose ICAM1 as a therapeutic target for treating breast cancer metastasis.

Autophagy modulates endothelial junctions to restrain neutrophil diapedesis during inflammation

The migration of neutrophils from the blood circulation to sites of infection or injury is a key immune response and requires the breaching of endothelial cells (ECs) that line the inner aspect of blood vessels. Unregulated neutrophil transendothelial cell migration (TEM) is pathogenic, but the molecular basis of its physiological termination remains unknown. Here, we demonstrated that ECs of venules in inflamed tissues exhibited a robust autophagic response that was aligned temporally with the peak of neutrophil trafficking and was strictly localized to EC contacts. Genetic ablation of EC autophagy led to excessive neutrophil TEM and uncontrolled leukocyte migration in murine inflammatory models, while pharmacological induction of autophagy suppressed neutrophil infiltration into tissues. Mechanistically, autophagy regulated the remodeling of EC junctions and expression of key EC adhesion molecules, facilitating their intracellular trafficking and degradation. Collectively, we have identified autophagy as a modulator of EC leukocyte trafficking machinery aimed at terminating physiological inflammation.

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Inflamed venular ECs exhibit an autophagic response that localizes to EC contacts

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EC ATG5 deficiency promotes excessive and faster neutrophil TEM

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Ablation of EC autophagy increases cell surface expression of adhesion molecules

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Non-canonical autophagy operates in inflamed ECs and controls neutrophil migration

Spinocerebellar Ataxia Type 3: A Case Report and Literature Review

Spinocerebellar ataxia type 3 (SCA3), also known by the eponym Machado-Joseph disease, is an autosomal dominant CAG trinucleotide (polyglutamine) repeat disease that presents in young- to middle-aged adults. SCA3 was first described in Azorean individuals and has interesting epidemiological patterns. It is characterized clinically by progressive ataxia and neuropathologically by progressive degenerative changes in the spinal cord and cerebellum, along with degeneration of the cortex and basal ganglia. Here, we describe the clinical and neuropathologic features in a case of SCA3 with unique findings, including involvement of the inferior olivary nucleus and cerebellar Purkinje cell layer, which are classically spared in the disease. We also discuss research into the disease mechanisms of SCA3 and the potential for therapeutic intervention.

Beyond genes and transcription factors: A potential mechanism for the pathogenesis of cerebral cavernous malformations

In this issue of JEM, Hong et al. (https://doi.org/10.1084/jem.20200140) identify a major step in the pathogenesis of cerebral cavernous malformations (CCMs), which at the same time offers insight into potential therapy for this disease.

Abstract 2603: Single cell RNA sequencing-based identification of molecular drivers in circulating tumor cell cluster formation and lung metastasis

Introduction: Circulating tumor cells (CTCs) mediate metastases, which account for 90% of solid tumor-related mortality. Compared to single cells, clustered CTCs mediate metastasis at a 20-100 times higher efficiency and are associated with lower overall survival in breast cancer (BC). We have recently identified a new mechanism of CTC cluster formation through cellular aggregation instead of cohesive shedding, and demonstrated that CTC clusters have enhanced stemness (Cancer Discovery, 2019). However, the cellular heterogeneity and molecular mechanisms underlying CTC cluster formation and polyclonal metastasis have yet to be fully elucidated. We hypothesize that molecular drivers of metastasis initiation enhance cancer stemness and CTC cluster formation, and serve as a novel therapeutic target for BC metastasis.

Methods: Using single cell RNA sequencing, we compared tumor cells from the primary breast tumor site and lung metastases of BC patient-derived xenografts. We identified genes specifically expressed in the lung metastases and determined their functional importance in CTC clustering, cancer stemness, and lung colonization. We performed proteomic and transcriptomic analyses as well as machine learning to elucidate the downstream signaling pathways and protein structural basis involved in CTC cluster formation and lung metastasis. Finally, we explored therapeutic intervention options in blocking CTC cluster formation and lung metastasis.

New results: Compared to the primary breast tumor cells, we identified a stemness gene signature enriched in a subpopulation of the CD44+ lung metastases, with 30-60 fold higher expression of CD34, CD36, ICAM1, VCAM1, ZEB1, ALDH1A1, TGFBR2 and TSPAN8. Analysis of patient blood samples (N=40) revealed that CD44 and many of these new candidate proteins were enriched in CTC clusters in comparison to single CTCs. We then examined the CSC-related properties of these tumor cells, such as tumorigenesis, sphere formation, and lung metastasis. Knockdown of selected surface molecules (e.g. ICAM1) significantly reduced the efficiency of lung metastasis of BC cells in vivo. A subset of ICAM1+/CD44+ tumor cells had increased stemness and tumor growth upon orthotopic implantation in vivo. Knockdown of ICAM1 dramatically reduced the self-renewal ability in mammosphere formation of breast tumor cells in vitro. In addition, our studies also revealed that these tumor cells cluster through CD44 and other surface protein-mediated homophilic binding between two neighboring tumor cells. Neutralizing antibodies significantly blocked tumor cluster formation and lung colonization.

Conclusions: We identified new molecular mediators of CTC aggregation and lung metastasis in BC. We anticipate that specific blockade of tumor clustering could decrease cancer progression and improve survival of BC patients.

Citation Format: Rokana Taftaf, Xia Liu, Salendra Singh, Yuzhi Jia, David Scholten, Youbin Zhang, Andrew Davis, Carolina Reduzzi, Yue Cao, Yang Shen, Massimo Cristofanilli, William A. Muller, Vinay Varadan, Huiping Liu. Single cell RNA sequencing-based identification of molecular drivers in circulating tumor cell cluster formation and lung metastasis [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2603.

Abstract B42: Identification of molecular drivers in circulating tumor cell cluster formation and lung metastasis

Introduction: Breast cancer (BC) is the leading type of cancer and the second leading cause of cancer-related deaths among women in the US. Metastasis accounts for 90% of solid tumor-related mortality and is mainly mediated by hematogenous spread of circulating tumor cells (CTCs). Compared to single cells, clustered CTCs mediate metastasis at a 20-100 times higher efficiency and are associated with lower overall survival. We have recently identified a new mechanism of CTC cluster formation through cellular aggregation instead of cohesive shedding and demonstrated that CTC clusters have enhanced stemness (Cancer Discovery 2019). However, the cellular heterogeneity and molecular mechanisms underlying CTC cluster formation and polyclonal metastasis have yet to be fully elucidated. We hypothesize that molecular drivers of metastasis initiation enhance cancer stemness and CTC cluster formation and serve as a novel therapeutic target for BC metastasis.

Methods: Using single-cell RNA sequencing, we compared tumor cells from the primary breast tumor site and lung metastases of breast cancer patient-derived xenograft. We identified genes with differential expression levels in the lung mets and determined their functional importance in CTC clustering, cancer stemness, and lung colonization. Upon the candidate gene modulation, we performed proteomic and transcriptome analyses to elucidate the downstream signaling pathways involved in CTC cluster formation and lung metastasis. Finally, we explored therapeutic intervention options in blocking CTC cluster formation and lung metastasis.

New Results: Compared to the primary breast tumor cells, we identified a stemness gene signature enriched in a subpopulation of the CD44+ lung metastases, with 30- to 60-fold higher expression of CD34, CD36, VCAM1, ZEB1, ALDH1A1, TGFBR2, TSPAN8, and ICAM1. Patient blood analyses (N=40) revealed that CD44 and many of these new candidate proteins are enriched in CTC clusters in comparison to single CTCs. We then examined the CSC-related properties of these tumor cells, such as tumorigenesis, sphere formation, and lung metastasis. Knockdown of selected surface molecules from the signature genes significantly reduced the efficiency of lung metastasis of BC cells in vivo. A subset of these tumor cells had increased stemness and highest tumor growth upon orthotopic implantation in vivo. Knockdown of the signature genes dramatically reduced the self-renewal ability in mammosphere formation of breast tumor cells in vitro. In addition, our studies also revealed that these tumor cells cluster through CD44 and other surface protein-mediated homophilic binding between two neighboring tumor cells. Neutralizing antibodies significantly blocked tumor cluster formation and lung colonization.

Conclusion: We identified new molecular mediators of CTC aggregation and lung metastasis in breast cancer. We anticipate that a specific blocking of tumor clustering could decrease cancer progression and improve survival of breast cancer patients.

Citation Format: Rokana Taftaf, Xia Liu, Salendra Singh, Yuzhi Jia, David Scholten, Massimo Cristofanilli, William A. Muller, Vinay Varadan, Huiping Liu. Identification of molecular drivers in circulating tumor cell cluster formation and lung metastasis [abstract]. In: Proceedings of the AACR Special Conference on Advances in Liquid Biopsies; Jan 13-16, 2020; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(11_Suppl):Abstract nr B42.

Return of Individual Research Results: A Guide for Biomedical Researchers Utilizing Human Biospecimens

The recent movement toward returning individual research results to study subjects/participants generates ethical and legal challenges for laboratories performing research on human biospecimens. The concept of an individual's interest in knowing the results of testing on their tissue is pitted against individual and systemic risks and an established legal framework regulating the performance of laboratory testing for medical care purposes. This article discusses the rationale for returning individual research results to subjects, the potential risks associated with returning these results, and the legal framework in the United States that governs testing of identifiable human biospecimens. On the basis of these considerations, this article provides recommendations for investigators to consider when planning and executing human biospecimen research, with the objective of appropriately balancing the interests of research subjects, the need for ensuring integrity of the research process, and compliance with US laws and regulations.

Human CD4+ T cell subsets differ in their abilities to cross endothelial and epithelial brain barriers in vitro

Background

The brain barriers establish compartments in the central nervous system (CNS) that significantly differ in their communication with the peripheral immune system. In this function they strictly control T-cell entry into the CNS. T cells can reach the CNS by either crossing the endothelial blood–brain barrier (BBB) or the epithelial blood-cerebrospinal fluid barrier (BCSFB) of the choroid plexus (ChP).

Objective

Analysis of the cellular and molecular mechanisms involved in the migration of different human CD4⁺ T-cell subsets across the BBB versus the BCSFB.

Methods

Human in vitro models of the BBB and BCSFB were employed to study the migration of circulating and CNS-entry experienced CD4⁺ T helper cell subsets (Th1, Th1*, Th2, Th17) across the BBB and BCSFB under inflammatory and non-inflammatory conditions in vitro.

Results

While under non-inflammatory conditions Th1* and Th1 cells preferentially crossed the BBB, under inflammatory conditions the migration rate of all Th subsets across the BBB was comparable. The migration of all Th subsets across the BCSFB from the same donor was 10- to 20-fold lower when compared to their migration across the BBB. Interestingly, Th17 cells preferentially crossed the BCSFB under both, non-inflamed and inflamed conditions. Barrier-crossing experienced Th cells sorted from CSF of MS patients showed migratory characteristics indistinguishable from those of circulating Th cells of healthy donors. All Th cell subsets could additionally cross the BCSFB from the CSF to ChP stroma side. T-cell migration across the BCSFB involved epithelial ICAM-1 irrespective of the direction of migration.

Conclusions

Our observations underscore that different Th subsets may use different anatomical routes to enter the CNS during immune surveillance versus neuroinflammation with the BCSFB establishing a tighter barrier for T-cell entry into the CNS compared to the BBB. In addition, CNS-entry experienced Th cell subsets isolated from the CSF of MS patients do not show an increased ability to cross the brain barriers when compared to circulating Th cell subsets from healthy donors underscoring the active role of the brain barriers in controlling T-cell entry into the CNS. Also we identify ICAM-1 to mediate T cell migration across the BCSFB.

Abstract WMP80: Live Animal Imaging Reveals Delayed But Sustained Neutrophil Recruitment to Both Ischemic and Non-Ischemic Cortical Cerebrovasculature in a Mouse Transient Stroke Model

Background: Treatment for acute ischemic stroke (AIS) relies on early restoration of blood flow; however, injury persists despite reperfusion. The innate immune response plays a pivotal role in reperfusion injury, but the mechanisms and timing of neutrophil recruitment to the cerebrovasculature (CBV) during AIS remain unclear. We sought to characterize the neutrophil response in the CBV following ischemia/reperfusion using a transient middle cerebral artery occlusion (tMCAO) AIS model.

Methods: Fluorescent reporter mice underwent 90-minute tMCAO confirmed with LSCI or surgery without occlusion (sham). Real-time confocal intravital microscopy of the CBV was performed at 24 and 72 hrs post-reperfusion through cranial windows in ischemic and non-ischemic hemispheres. Neutrophil recruitment (adhesion, rolling) and extravasation were quantified in vivo over both hemispheres. Neutrophil extravasation was further quantified using whole mount sections. Infarct volume was measured by 2,3,5-triphenyltetrazolium chloride staining.

Results: An increase in neutrophil rolling, adhesion and extravasation was observed within the ischemic hemisphere compared to sham (p < 0.01) at 24 and markedly 72 hrs. Non-ischemic hemispheres showed increased neutrophil rolling and adhesion, but not extravasation, compared to sham surgery (p < 0.05). Whole mount sections demonstrated increased cortical and subcortical neutrophil extravasation in the ischemic hemisphere only (p < 0.01).

Conclusions: Early neutrophil recruitment to the ischemic hemisphere following tMCAO was present at both the CBV and within the infarcted parenchyma at 72 hours. Acute neutrophil recruitment at the CBV in the non-ischemic hemisphere, but not the cortex, suggests that AIS triggers a global inflammatory response within the CBV. Targeted neutrophil-specific therapies during reperfusion may reduce global inflammation at the CBV, and potentially mitigate secondary neuronal injury.

Abstract TP266: Ischemic Stroke Induces Striking Heterogeneity in the Inflammatory Leukocyte Infiltrate Across the Core and Penumbra

Background: Current therapies for ischemic stroke focus on reperfusion but do not address the acute inflammatory response. Previous clinical trials aimed at modulating the inflammatory milieu by disrupting leukocyte infiltration failed to show clinical efficacy. One possible explanation for this unexpected shortcoming is an incomplete understanding of the precise spatio-temporal underpinnings of leukocyte extravasation and infiltration.

Methods: Here we describe the evolution of the inflammatory response in a mouse transient middle cerebral artery occlusion (tMCAO) stroke model at 0, 1, 2 and 3 days post reperfusion. We used wide field and confocal immunofluorescence microscopy to examine the exact nature and location of the invading myelomonocytic populations, with close examination of the leukocyte position with regard to the brain vasculature and the perivascular space.

Results: Our findings suggest that the vast majority of infiltrating myelomonocytic cells escape the perivascular compartment and enter the parenchyma. Interestingly, leukocyte extravasation and accumulation in the subcortex occurred over several days. Dramatic heterogeneity in the inflammatory infiltrate was observed across the infarcted tissue, but also in the surrounding penumbra and adjacent cortical surface. In addition, triphenyl tetrazolium chloride staining, a common indicator for infarcted tissue, did not correlate with the amount or location of leukocyte infiltration.

Conclusion: Taken together our findings demonstrate that the infiltration of leukocytes dynamically evolves over several days following reperfusion. Furthermore, leukocytes infiltrate in a heterogeneous pattern that does not correlate well with traditional markers of cellular dysfunction. A better understating of the precise spatio-temporal infiltration of inflammatory cells could help inform the next generation of therapeutic interventions.

Endothelial Cell Calcium Signaling during Barrier Function and Inflammation

Calcium is an essential second messenger in endothelial cells and plays a pivotal role in regulating a number of physiologic processes, including cell migration, angiogenesis, barrier function, and inflammation. An increase in intracellular Ca²⁺ concentration can trigger a number of diverse signaling pathways under both physiologic and pathologic conditions. In this review, we discuss how calcium signaling pathways in endothelial cells play an essential role in affecting barrier function and facilitating inflammation. Inflammatory mediators, such as thrombin and histamine, increase intracellular calcium levels. This calcium influx causes adherens junction disassembly and cytoskeletal rearrangements to facilitate endothelial cell retraction and increased permeability. During inflammation endothelial cell calcium entry and the calcium-related signaling events also help facilitate several leukocyte-endothelial cell interactions, such as leukocyte rolling, adhesion, and ultimately transendothelial migration.

Longitudinal deep-brain imaging in mouse using visible-light optical coherence tomography through chronic microprism cranial window

We longitudinally imaged both the superficial and deep cortical microvascular networks in brains of healthy mice and in a mouse model of stroke in vivo using visible-light optical coherence tomography (vis-OCT). We surgically implanted a microprism in mouse brains sealed by a chronic cranial window. The microprism enabled vis-OCT to image the entire depth of the mouse cortex. Following microprism implantation, we imaged the mice for 28 days and found that that it took around 15 days for both the superficial and deep cortical microvessels to recover from the implantation surgery. After the brains recovered, we introduced ischemic strokes by transient middle cerebral artery occlusion (tMCAO). We monitored the strokes for up to 60 days and observed different microvascular responses to tMCAO at different cortical depths in both the acute and chronic phases of the stroke. This work demonstrates that the combined microprism and cranial window is well-suited for longitudinal investigation of cortical microvascular disorders using vis-OCT.

Endothelial IQGAP1 regulates leukocyte transmigration by directing the LBRC to the site of diapedesis

The function of endothelial cell IQGAP1 during diapedesis requires its actin-binding domain and IQ motifs to recruit the lateral border recycling compartment. Genetic ablation of endothelial cell IQGAP1 expression in vivo causes significant disruption of diapedesis in two models of inflammation.

Transendothelial migration (TEM) of leukocytes across the endothelium is critical for inflammation. In the endothelium, TEM requires the coordination of membrane movements and cytoskeletal interactions, including, prominently, recruitment of the lateral border recycling compartment (LBRC). The scaffold protein IQGAP1 was recently identified in a screen for LBRC-interacting proteins. Knockdown of endothelial IQGAP1 disrupted the directed movement of the LBRC and substantially reduced leukocyte TEM. Expression of truncated IQGAP1 constructs demonstrated that the calponin homology domain is required for IQGAP1 localization to endothelial borders and that the IQ domain, on the same IQGAP1 polypeptide, is required for its function in TEM. This is the first reported function of IQGAP1 requiring two domains to be present on the same polypeptide. Additionally, we show for the first time that IQGAP1 in the endothelium is required for efficient TEM in vivo. These findings reveal a novel function for IQGAP1 and demonstrate that IQGAP1 in endothelial cells facilitates TEM by directing the LBRC to the site of TEM.

Time-Variant SRC Kinase Activation Determines Endothelial Permeability Response

In the current model of endothelial barrier regulation, the tyrosine kinase SRC is purported to induce disassembly of endothelial adherens junctions (AJs) via phosphorylation of VE cadherin, and thereby increase junctional permeability. Here, using a chemical biology approach to temporally control SRC activation, we show that SRC exerts distinct time-variant effects on the endothelial barrier. We discovered that the immediate effect of SRC activation was to transiently enhance endothelial barrier function as the result of accumulation of VE cadherin at AJs and formation of morphologically distinct reticular AJs. Endothelial barrier enhancement via SRC required phosphorylation of VE cadherin at Y731. In contrast, prolonged SRC activation induced VE cadherin phosphorylation at Y685, resulting in increased endothelial permeability. Thus, time-variant SRC activation differentially phosphorylates VE cadherin and shapes AJs to fine-tune endothelial barrier function. Our work demonstrates important advantages of synthetic biology tools in dissecting complex signaling systems.

PECAM-1 Stabilizes Blood-Brain Barrier Integrity and Favors Paracellular T-Cell Diapedesis Across the Blood-Brain Barrier During Neuroinflammation

Breakdown of the blood-brain barrier (BBB) and increased immune cell trafficking into the central nervous system (CNS) are hallmarks of the pathogenesis of multiple sclerosis (MS). Platelet endothelial cell adhesion molecule-1 (PECAM-1; CD31) is expressed on cells of the vascular compartment and regulates vascular integrity and immune cell trafficking. Involvement of PECAM-1 in MS pathogenesis has been suggested by the detection of increased levels of soluble PECAM-1 (sPECAM-1) in the serum and CSF of MS patients. Here, we report profound upregulation of cell-bound PECAM-1 in initial (pre-phagocytic) white matter as well as active cortical gray matter MS lesions. Using a human in vitro BBB model we observed that PECAM-1 is not essential for the transmigration of human CD4⁺ T-cell subsets (Th1, Th1^*, Th2, and Th17) across the BBB. Employing an additional in vitro BBB model based on primary mouse brain microvascular endothelial cells (pMBMECs) we show that the lack of endothelial PECAM-1 impairs BBB properties as shown by reduced transendothelial electrical resistance (TEER) and increases permeability for small molecular tracers. Investigating T-cell migration across the BBB under physiological flow by in vitro live cell imaging revealed that absence of PECAM-1 in pMBMECs did not influence arrest, polarization, and crawling of effector/memory CD4⁺ T cells on the pMBMECs. Absence of endothelial PECAM-1 also did not affect the number of T cells able to cross the pMBMEC monolayer under flow, but surprisingly favored transcellular over paracellular T-cell diapedesis. Taken together, our data demonstrate that PECAM-1 is critically involved in regulating BBB permeability and although not required for T-cell diapedesis itself, its presence or absence influences the cellular route of T-cell diapedesis across the BBB. Upregulated expression of cell-bound PECAM-1 in human MS lesions may thus reflect vascular repair mechanisms aiming to restore BBB integrity and paracellular T-cell migration across the BBB as it occurs during CNS immune surveillance.

MerTK Cleavage on Resident Cardiac Macrophages Compromises Repair After Myocardial Ischemia Reperfusion Injury

RATIONALE

Clinical benefits of reperfusion after myocardial infarction are offset by maladaptive innate immune cell function, and therapeutic interventions are lacking.

OBJECTIVE

We sought to test the significance of phagocytic clearance by resident and recruited phagocytes after myocardial ischemia reperfusion.

METHODS AND RESULTS

In humans, we discovered that clinical reperfusion after myocardial infarction led to significant elevation of the soluble form of MerTK (myeloid-epithelial-reproductive tyrosine kinase; ie, soluble MER), a critical biomarker of compromised phagocytosis by innate macrophages. In reperfused mice, macrophage Mertk deficiency led to decreased cardiac wound debridement, increased infarct size, and depressed cardiac function, newly implicating MerTK in cardiac repair after myocardial ischemia reperfusion. More notably, Mertk(CR) mice, which are resistant to cleavage, showed significantly reduced infarct sizes and improved systolic function. In contrast to other cardiac phagocyte subsets, resident cardiac MHCII^LOCCR2^- (major histocompatibility complex II/C-C motif chemokine receptor type 2) macrophages expressed higher levels of MerTK and, when exposed to apoptotic cells, secreted proreparative cytokines, including transforming growth factor-β. Mertk deficiency compromised the accumulation of MHCII^LO phagocytes, and this was rescued in Mertk(CR) mice. Interestingly, blockade of CCR2-dependent monocyte infiltration into the heart reduced soluble MER levels post-ischemia reperfusion.

CONCLUSIONS

Our data implicate monocyte-induced MerTK cleavage on proreparative MHCII^LO cardiac macrophages as a novel contributor and therapeutic target of reperfusion injury.

Transendothelial migration: unifying principles from the endothelial perspective

Transendothelial migration (TEM) of polymorphonuclear leukocytes (PMN) involves a carefully orchestrated dialog of adhesion and signaling events between leukocyte and endothelial cell. This article focuses on the contribution of endothelial cells to transmigration. The initiation of TEM itself generally requires interaction of PECAM on the leukocyte with PECAM at the endothelial cell border. This is responsible for the transient elevation of cytosolic-free calcium ions in endothelium that is required for TEM and for recruitment of membrane from the lateral border recycling compartment (LBRC). TEM requires LBRC to move to the site at which TEM will take place and for VE-cadherin to move away. Targeting of the LBRC to this site likely precedes movement of VE-cadherin and may play a role in clearing VE-cadherin from the site of TEM. The process of TEM can be dissected into steps mediated by distinct pairs of PMN/endothelial interacting molecules. CD99 regulates a step at or close to the end of TEM. CD99 signals through soluble adenylyl cyclase to activate PKA to trigger ongoing targeted recycling of the LBRC. Paracellular transmigration predominates (≥90% of events) in the cremaster muscle circulation, but transcellular migration may be more important at sites such as the blood-brain barrier. Both processes involve many of the same molecules and recruitment of the LBRC.

Differential effect of Platelet Endothelial Cell Adhesion Molecule-1 (PECAM-1) on leukocyte infiltration during contact hypersensitivity responses

Background

2′–4′ Dinitrofluorobenzene (DNFB) induced contact hypersensitivity is an established model of contact sensitivity and leukocyte migration. Platelet Endothelial Cell Adhesion Molecule-1 (PECAM-1) deficient mice were used to examine the role of PECAM-1 in the migration capacity of several different leukocyte populations after primary and secondary application.

Results

γδ T lymphocytes, granulocytes, and Natural Killer cells were most affected by PECAM-1 deficiency at the primary site of application. γδ T lymphocytes, granulocytes, DX5+ Natural Killer cells, and, interestingly, effector CD4+ T lymphocytes were most affected by the loss of PECAM-1 at the secondary site of application.

Conclusions

PECAM-1 is used by many leukocyte populations for migration, but there are clearly differential effects on the usage by each subset. Further, the overall kinetics of each population varied between primary and secondary application, with large relative increases in γδ T lymphocytes during the secondary response.

Roles of transient receptor potential channels in regulation of vascular and epithelial barriers

Transient receptor potential (TRP) channels are a ubiquitously expressed multi-family group of cation channels that are critical to signaling events in many tissues. Their roles have been documented in many physiologic and pathologic conditions. Nevertheless, direct studies of their roles in maintain barrier function in endothelial and epithelia are relatively infrequent. This seems somewhat surprising considering that calcium ion concentrations are known to regulate barrier function. This short review provides an introduction to TRP channels and reviews some of the work in which investigators directly studied the role of TRP channels in endothelial permeability to electric current, solute, or leukocytes during the inflammatory response.

4D intravital microscopy uncovers critical strain differences for the roles of PECAM and CD99 in leukocyte diapedesis

Leukocyte transendothelial migration (TEM) is an essential component of the inflammatory response. In vitro studies with human cells have demonstrated that platelet/endothelial cell adhesion molecule (PECAM) functions upstream of CD99 during TEM; however, results in vivo with mice have been apparently contradictory. In this study we use four-dimensional (4D) intravital microscopy to demonstrate that the site and order of function of PECAM and CD99 in vivo are dependent on the strain of mice. In FVB/n mice, PECAM functions upstream of CD99, as in human cells in vitro, and blocking antibodies against either molecule arrest neutrophils before they traverse the endothelium. However, in C57BL/6 mice, PECAM and CD99 appear to function at a different step, as the same antibodies arrest leukocyte migration through the endothelial basement membrane. These results are the first direct comparison of PECAM and CD99 function in different murine strains as well as the first demonstration of the sequential function of PECAM and CD99 in vivo.

Segregation of VE-cadherin from the LBRC depends on the ectodomain sequence required for homophilic adhesion

The lateral border recycling compartment (LBRC) is a reticulum ofperijunctional tubulovesicular membrane that is continuous with the plasmalemma of endothelial cells and is essential for efficient transendothelial migration (TEM) of leukocytes. The LBRC contains molecules involved in TEM, such as PECAM, PVR and CD99, but not VE-cadherin. Despite its importance, how membrane proteins are included in or excluded from the LBRC is not known. Immunoelectronmicroscopy and biochemical approaches demonstrate that inclusion into the LBRC is the default pathway for transmembrane molecules present at endothelial cell borders. A chimeric molecule composed of the extracellular domain of VE-cadherin and cytoplasmic tail of PECAM (VE-CAD/PECAM) did not enter the LBRC, suggesting that VE-cadherin was excluded by a mechanism involving its extracellular domain. Deletion of the homophilic interaction domain EC1 or the homophilic interaction motif RVDAE allowed VE-CAD/PECAM and even native VE-cadherin to enter the LBRC. Similarly, treatment with RVDAE peptide to block homophilic VE-cadherin interactions allowed endogenous VE-cadherin to enter the LBRC. This suggests that homophilic interactions of VE-cadherin stabilize it at cell borders and prevent entry into the LBRC.

A Unique Role for Endothelial Cell Kinesin Light Chain 1, Variant 1 in Leukocyte Transendothelial Migration

A reservoir of parajunctional membrane in endothelial cells, the lateral border recycling compartment (LBRC), is critical for transendothelial migration (TEM). We have previously shown that targeted recycling of the LBRC to the site of TEM requires microtubules and a kinesin molecular motor. However, the identity of the kinesin and mechanism of cargo binding were not known. We show that microinjection of endothelial cells with a monoclonal antibody specific for kinesin-1 significantly blocked LBRC-targeted recycling and TEM. In complementary experiments, knocking down KIF5B, a ubiquitous kinesin-1 isoform, in endothelial cells significantly decreased targeted recycling of the LBRC and leukocyte TEM. Kinesin heavy chains move cargo along microtubules by one of many kinesin light chains (KLCs), which directly bind the cargo. Knocking down KLC 1 isoform variant 1 (KLC1C) significantly decreased LBRC-targeted recycling and TEM, whereas knocking down other isoforms of KLC1 had no effect. Re-expression of KLC1C resistant to the knockdown shRNA restored targeted recycling and TEM. Thus kinesin-1 and KLC1C are specifically required for targeted recycling and TEM. These data suggest that of the many potential combinations of the 45 kinesin family members and multiple associated light chains, KLC1C links the LBRC to kinesin-1 (KIF5B) during targeted recycling and TEM. Thus, KLC1C can potentially be used as a target for anti-inflammatory therapy.

Targeted Recycling of the Lateral Border Recycling Compartment Precedes Adherens Junction Dissociation during Transendothelial Migration

Leukocyte transendothelial migration (TEM) requires two major events: local dissociation of adherens junctions manifested as gaps in vascular endothelial (VE)-cadherin staining at the site of TEM and targeted trafficking of the lateral border recycling compartment (LBRC) to the site of TEM. However, the association between LBRC recycling and VE-cadherin gaps remains unknown. We found that when targeting of the LBRC is selectively inhibited using established methods, such as a function blocking anti-platelet endothelial cell adhesion molecule 1 antibody, depolymerizing microtubules, or microinjection of an antibody that inhibits kinesin, VE-cadherin gaps do not form around the blocked leukocyte. This is the first time that the LBRC has been implicated in this process. We obtained similar results for neutrophils and monocytes and in studies using live cell imaging microscopy conducted under fluid shear conditions. Depolymerizing microtubules did not affect the ability of leukocytes to induce tyrosine phosphorylation of VE-cadherin. A VE-cadherin double mutant (Y658F, Y731F) expressed in endothelial cells acted as a dominant negative and inhibited VE-cadherin gap formation and TEM, yet targeting of the LBRC still occurred. These data suggest that targeting of the LBRC to the site of TEM precedes VE-cadherin clearance. Recruitment of the LBRC may play a role in clearing VE-cadherin from the site of TEM.

Abstract PR03: GLUT1 is required for induction of mammary tumorigenesis by activated ErbB2/HER2/Neu

Alterations in tumor cell metabolism have been investigated for decades, and include changes in glucose utilization, as well as changes in mitochondrial and fatty acid metabolism; however, it is unclear at what point during the process of tumorigenesis these metabolic changes occur. We hypothesized that elevated glucose uptake represents the first metabolic adaptation to transformation, and thus may be a target for the prevention of breast cancer. The GLUT1 transporter is expressed in breast cancer cells and is responsible for the majority of their glucose uptake. Not surprisingly, breast cancer patients with high levels of GLUT1 have a poorer clinical outcome than those with lower levels, likely due to the association between glucose uptake and high tumor grade. We previously showed that the reduction of GLUT1 in established tumor cells resulted in a 50% decrease in glucose consumption and lactate generation. There was a corresponding decrease in proliferation in both two and three-dimensional culture, and in tumor growth in immunodeficient mice. Mammary epithelial cells from GLUT1fl/fl mice were transformed using polyoma middle tumor antigen (PyMT), and GLUT1 excised using Cre recombinase. Cells expressing GLUT1 were tumorigenic in immunodeficient mice, while cells lacking GLUT1 were not tumorigenic. This suggests that in mammary epithelial cells transformed in vitro, loss of GLUT1 was sufficient to prevent tumor outgrowths when injected into the mammary fat pad of nude mice. This indicates that GLUT1 and by extension glucose, are critical at an early stage of mammary tumorigenesis.

To test the hypothesis that elevated glucose uptake is required very early in the process of tumor formation, we crossed the MMTV-NIC (NIC) mice, which contain activated NeuNT, an internal ribosome entry site (IRES), and Cre recombinase, to mice bearing floxed alleles of GLUT1, and monitored tumor development. All of the control MMTV-NIC mice developed tumors in less than 200 days. In contrast, none of the NIC-GLUT1fl/fl mice developed mammary tumors even after eighteen months. We anticipated that loss of one allele of GLUT1 would significantly delay mammary tumorigenesis; however, we did not observe any tumors in these mice, even at eighteen months of age. Tumors that formed in the NIC mice were typical Neu-dependent tumors observed in similar transgenic mice with a high proliferative index (38% by Ki67 staining), and there was evidence of preneoplastic lesions in the non tumor-bearing mammary glands. Small Cre-positive lesions were also positive for GLUT1 indicating GLUT1 expression increases very early in Neu-mediated tumorigenesis. The mammary ductal tree developed normally in NIC-Glut1fl/fl and NIC-Glut1fl/+ mice. Mammary glands from mice lacking one allele of GLUT1 had increased epithelial content and higher proliferative index, compared to mice lacking both Glut1 alleles; however, no palpable tumors ever formed in either group. These results suggest that loss of a single allele of GLUT1 is sufficient to suppress Neu-induced mammary tumorigenesis and that there is an absolute requirement for GLUT1 at the earliest stages of tumorigenesis.

Citation Format: Elizabeth A. Wellberg, Angelo D'Allessandro, Andrew S. Lewis, Kristina Terrell, E. Dale Abel, William A. Muller, Kirk A. Hansen, Steven M. Anderson. GLUT1 is required for induction of mammary tumorigenesis by activated ErbB2/HER2/Neu. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr PR03.

Cutting Edge: CD99 Is a Novel Therapeutic Target for Control of T Cell-Mediated Central Nervous System Autoimmune Disease

Leukocyte trafficking into the CNS is a prominent feature driving the immunopathogenesis of multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis. Blocking the recruitment of inflammatory leukocytes into the CNS represents an exploitable therapeutic target; however, the adhesion molecules that specifically regulate the step of leukocyte diapedesis into the CNS remain poorly understood. We report that CD99 is critical for lymphocyte transmigration without affecting adhesion in a human blood-brain barrier model. CD99 blockade in vivo ameliorated experimental autoimmune encephalomyelitis and decreased the accumulation of CNS inflammatory infiltrates, including dendritic cells, B cells, and CD4(+) and CD8(+) T cells. Anti-CD99 therapy was effective when administered after the onset of disease symptoms and blocked relapse when administered therapeutically after disease symptoms had recurred. These findings underscore an important role for CD99 in the pathogenesis of CNS autoimmunity and suggest that it may serve as a novel therapeutic target for controlling neuroinflammation.