Cytolytic CD8+ T cells infiltrate germinal centers to limit ongoing HIV replication in spontaneous controller lymph nodes

Follicular CD8⁺ T cells (fCD8) mediate surveillance in lymph node (LN) germinal centers against lymphotropic infections and cancers, but the precise mechanisms by which these cells mediate immune control remain incompletely resolved. To address this, we investigated functionality, clonotypic compartmentalization, spatial localization, phenotypic characteristics, and transcriptional profiles of LN-resident virus-specific CD8⁺ T cells in persons who control HIV without medications. Antigen-induced proliferative and cytolytic potential consistently distinguished spontaneous controllers from noncontrollers. T cell receptor analysis revealed complete clonotypic overlap between peripheral and LN-resident HIV-specific CD8⁺ T cells. Transcriptional analysis of LN CD8⁺ T cells revealed gene signatures of inflammatory chemotaxis and antigen-induced effector function. In HIV controllers, the cytotoxic effectors perforin and granzyme B were elevated among virus-specific CXCR5⁺ fCD8s proximate to foci of HIV RNA within germinal centers. These results provide evidence consistent with cytolytic control of lymphotropic infection supported by inflammatory recruitment, antigen-specific proliferation, and cytotoxicity of fCD8s.

Temporal changes in T cell subsets and expansion of cytotoxic CD4+ T cells in the lungs in severe COVID-19

Many studies have been performed in severe COVID-19 on immune cells in the circulation and on cells obtained by bronchoalveolar lavage. Most studies have tended to provide relative information rather than a quantitative view, and it is a combination of approaches by various groups that is helping the field build a picture of the mechanisms that drive severe lung disease. Approaches employed to date have not revealed information on lung parenchymal T cell subsets in severe COVID-19. Therefore, we sought to examine early and late T cell subset alterations in the lungs and draining lymph nodes in severe COVID-19 using a rapid autopsy protocol and quantitative imaging approaches. Here, we have established that cytotoxic CD4+ T cells (CD4 + CTLs) increase in the lungs, draining lymph nodes and blood as COVID-19 progresses. CD4 + CTLs are prominently expanded in the lung parenchyma in severe COVID-19. In contrast CD8+ T cells are not prominent, exhibit increased PD-1 expression, and no obvious increase is seen in the number of Granzyme B+ CD8+ T cells in the lung parenchyma in severe COVID-19. Based on quantitative evidence for re-activation in the lung milieu, CD4 + CTLs may be as likely to drive viral clearance as CD8+ T cells and may also be contributors to lung inflammation and eventually to fibrosis in severe COVID-19.

Myeloid Cells Are Enriched in Tonsillar Crypts, Providing Insight into the Host Tropism of Human Papillomavirus

Viruses are the second leading cause of cancer worldwide, and human papillomavirus (HPV)-associated head and neck cancers are increasing in incidence in the United States. HPV preferentially infects the crypts of the tonsils rather than the surface epithelium. The present study sought to characterize the unique microenvironment within the crypts to better understand the host tropism of HPV to a lymphoid-rich organ. Laser-capture microdissection of distinct anatomic areas (crypts, surface epithelium, and germinal centers) of the tonsil, coupled with transcriptional analysis and multiparameter immunofluorescence staining, was performed and demonstrated that the tonsillar crypts are enriched with myeloid populations that co-express multiple canonical and noncanonical immune checkpoints, including PD-L1, CTLA-4, HAVCR2 (TIM-3), ADORA2A, IDO1, BTLA, LGALS3, CDH1, CEACAM1, PVR, and C10orf54 (VISTA). The resident monocytes may foster a permissive microenvironment that facilitates HPV infection and persistence. Furthermore, the myeloid populations within HPV-associated tonsil cancers co-express the same immune checkpoints, providing insight into potential novel immunotherapeutic targets for HPV-associated head and neck cancers.

Fecal SARS-CoV-2 RNA is associated with decreased COVID-19 survival

The clinical significance of SARS CoV-2 RNA in stool remains uncertain. We found that extrapulmonary dissemination of infection to the gastrointestinal (GI) tract, assessed by the presence of SARS-CoV-2 RNA in stool, is associated with decreased COVID-19 survival. Measurement of SARS-CoV-2 RNA in stool may have utility for clinical risk assessment.

Expansion of Cytotoxic CD4+ T cells in the lungs in severe COVID-19

The contributions of T cells infiltrating the lungs to SARS-CoV-2 clearance and disease progression are poorly understood. Although studies of CD8+ T cells in bronchoalveolar lavage and blood have suggested that these cells are exhausted in severe COVID-19, CD4+ T cells have not been systematically interrogated within the lung parenchyma. We establish here that cytotoxic CD4+ T cells (CD4+CTLs) are prominently expanded in the COVID-19 lung infiltrate. CD4+CTL numbers in the lung increase with disease severity and progression is accompanied by widespread HLA-DR expression on lung epithelial and endothelial cells, increased apoptosis of epithelial cells and tissue remodeling. Based on quantitative evidence for re-activation in the lung milieu, CD4+ CTLs are as likely to drive viral clearance as CD8+ T cells and may also be contributors to lung inflammation and eventually to fibrosis in severe COVID-19.

In Brief

In severe COVID-19 cytotoxic CD4+ T cells accumulate in draining lymph nodes and in the lungs during the resolving phase of the disease. Re-activated cytotoxic CD4+ T cells and cytotoxic CD8+ T cells are present in roughly equivalent numbers in the lungs at this stage and these cells likely collaborate to eliminate virally infected cells and potentially induce fibrosis. A large fraction of epithelial and endothelial cells in the lung express HLA class II in COVID-19 and there is temporal convergence between CD4+CTL accumulation and apoptosis in the lung.

Highlights

In severe COVID-19, activated CD4+ CTLs accumulate in the lungs late in diseaseThese cells likely participate in SARS-CoV-2 clearance, collaborating with CD8+ T cells many of which exhibit an exhausted phenotypeT cells likely contribute to the late exacerbation of inflammationCD4+CTLs have been linked to fibrosis in many disorders and could also be responsible for the eventual induction of fibrosis in a subset of COVID-19 patients.

Summary

The contributions of T cells infiltrating the lungs to SARS-CoV-2 clearance and disease progression are poorly understood. Although studies of CD8+ T cells in bronchoalveolar lavage and blood have suggested that these cells are exhausted in severe COVID-19, CD4+ T cells have not been systematically interrogated within the lung parenchyma. We establish here that cytotoxic CD4+ T cells (CD4+CTLs) are prominently expanded in the COVID-19 lung infiltrate. CD4+CTL numbers in the lung increase with disease severity and progression is accompanied by widespread HLA-DR expression on lung epithelial and endothelial cells, increased apoptosis of epithelial cells and tissue remodeling. Based on quantitative evidence for re-activation in the lung milieu, CD4+ CTLs are as likely to drive viral clearance as CD8+ T cells and may also be contributors to lung inflammation and eventually to fibrosis in severe COVID-19.

T cell-tropic HIV efficiently infects alveolar macrophages through contact with infected CD4+ T cells

Alveolar macrophages (AMs) are critical for defense against airborne pathogens and AM dysfunction is thought to contribute to the increased burden of pulmonary infections observed in individuals living with HIV-1 (HIV). While HIV nucleic acids have been detected in AMs early in infection, circulating HIV during acute and chronic infection is usually CCR5 T cell-tropic (T-tropic) and enters macrophages inefficiently in vitro. The mechanism by which T-tropic viruses infect AMs remains unknown. We collected AMs by bronchoscopy performed in HIV-infected, antiretroviral therapy (ART)-naive and uninfected subjects. We found that viral constructs made with primary HIV envelope sequences isolated from both AMs and plasma were T-tropic and inefficiently infected macrophages. However, these isolates productively infected macrophages when co-cultured with HIV-infected CD4+ T cells. In addition, we provide evidence that T-tropic HIV is transmitted from infected CD4+ T cells to the AM cytosol. We conclude that AM-derived HIV isolates are T-tropic and can enter macrophages through contact with an infected CD4+ T cell, which results in productive infection of AMs. CD4+ T cell-dependent entry of HIV into AMs helps explain the presence of HIV in AMs despite inefficient cell-free infection, and may contribute to AM dysfunction in people living with HIV.

Multicohort Retrospective Validation of a Predictive Biomarker for Topoisomerase I Inhibitors

PURPOSE

The camptothecin (CPT) analogs topotecan and irinotecan specifically target topoisomerase I (topoI) and are used to treat colorectal, gastric, and pancreatic cancer. Response rate for this class of drug varies from 10% to 30%, and there is no predictive biomarker for patient stratification by response. On the basis of our understanding of CPT drug resistance mechanisms, we developed an immunohistochemistry-based predictive test, P-topoI-Dx, to stratify the patient population into those who did and did not experience a response.

PATIENTS AND METHODS

The retrospective validation studies included a training set (n = 79) and a validation cohort (n = 27) of gastric cancer (GC) patients, and 8 cohorts of colorectal cancer (CRC) patient tissue (n = 176). Progression-free survival for 6 months was considered a positive response to CPT-based therapy. Formalin-fixed, paraffin-embedded slides were immunohistochemically stained with anti-phospho-specific topoI-Serine10 (topoI-pS10), quantitated, and analyzed statistically.

RESULTS

We determined a threshold of 35% positive staining to offer optimal test characteristics in GC. The GC (n = 79) training set demonstrated 76.6% (95% confidence interval, 64-86) sensitivity; 68.8% (41-88) specificity; positive predictive value (PPV) 92.5% (81-98); and negative predictive value (NPV) 42.3% (24-62). The GC validation set (n = 27) demonstrated 82.4% (56-95) sensitivity and 70.0% (35-92) specificity. Estimated PPV and NPV were 82.4% (56-95) and 70.0% (35-92) respectively. In the CRC validation set (n = 176), the 40% threshold demonstrated 87.5% (78-94) sensitivity; 70.0% (59-79) specificity; PPV 70.7% (61-79); and NPV 87.0 % (77-93).

CONCLUSION

The analysis of retrospective data from patients (n = 282) provides clinical validity to our P-topoI-Dx immunohistochemical test to identify patients with disease that is most likely to respond to topoI inhibitors.

Striosomes Mediate Value-Based Learning Vulnerable in Age and a Huntington's Disease Model

Learning valence-based responses to favorable and unfavorable options requires judgments of the relative value of the options, a process necessary for species survival. We found, using engineered mice, that circuit connectivity and function of the striosome compartment of the striatum are critical for this type of learning. Calcium imaging during valence-based learning exhibited a selective correlation between learning and striosomal but not matrix signals. This striosomal activity encoded discrimination learning and was correlated with task engagement, which, in turn, could be regulated by chemogenetic excitation and inhibition. Striosomal function during discrimination learning was disturbed with aging and severely so in a mouse model of Huntington's disease. Anatomical and functional connectivity of parvalbumin-positive, putative fast-spiking interneurons (FSIs) to striatal projection neurons was enhanced in striosomes compared with matrix in mice that learned. Computational modeling of these findings suggests that FSIs can modulate the striosomal signal-to-noise ratio, crucial for discrimination and learning.

Loss of Bcl-6-Expressing T Follicular Helper Cells and Germinal Centers in COVID-19

Humoral responses in coronavirus disease 2019 (COVID-19) are often of limited durability, as seen with other human coronavirus epidemics. To address the underlying etiology, we examined post mortem thoracic lymph nodes and spleens in acute SARS-CoV-2 infection and observed the absence of germinal centers and a striking reduction in Bcl-6⁺ germinal center B cells but preservation of AID⁺ B cells. Absence of germinal centers correlated with an early specific block in Bcl-6⁺ T_FH cell differentiation together with an increase in T-bet⁺ T_H1 cells and aberrant extra-follicular TNF-α accumulation. Parallel peripheral blood studies revealed loss of transitional and follicular B cells in severe disease and accumulation of SARS-CoV-2-specific "disease-related" B cell populations. These data identify defective Bcl-6⁺ T_FH cell generation and dysregulated humoral immune induction early in COVID-19 disease, providing a mechanistic explanation for the limited durability of antibody responses in coronavirus infections, and suggest that achieving herd immunity through natural infection may be difficult.

The Loss of Bcl-6 Expressing T Follicular Helper Cells and the Absence of Germinal Centers in COVID-19

Humoral responses in COVID-19 disease are often of limited durability, as seen with other human coronavirus epidemics. To address the underlying etiology, we examined postmortem thoracic lymph nodes and spleens in acute SARS-CoV-2 infection and observed the absence of germinal centers, a striking reduction in Bcl-6+ germinal center B cells but preservation of AID+ B cells. Absence of germinal centers correlated with an early specific block in Bcl-6+TFH cell differentiation together with an increase in T-bet+TH1 cells and aberrant extra-follicular TNF-a accumulation.  Parallel peripheral blood studies revealed loss of transitional and follicular B cells in severe disease and accumulation of SARS-CoV-2-specific "disease-related" B cell populations. These data identify defective Bcl-6+TFH cell generation and dysregulated humoral immune induction early in COVID-19 disease, providing a mechanistic explanation for the limited durability of antibody responses in coronavirus infections and suggest that achieving herd immunity through natural infection may be difficult. Funding: This work was supported by NIH U19 AI110495 to SP, NIH R01 AI146779 to AGS, NIH R01AI137057 and DP2DA042422 to DL, BMH was supported by NIGMS T32 GM007753, TMC was supported by T32 AI007245. Funding for these studies from the Massachusetts Consortium of Pathogen Readiness, the Mark and Lisa Schwartz Foundation and Enid Schwartz is also acknowledged. Conflict of Interest: None. Ethical Approval: This study was performed with the approval of the Institutional Review Boards at the Massachusetts General Hospital and the Brigham and Women's Hospital.

A versatile high-throughput assay to characterize antibody-mediated neutrophil phagocytosis

Neutrophils, the most abundant white blood cell, play a critical role in anti-pathogen immunity via phagocytic clearance, secretion of enzymes and immunomodulators, and the release of extracellular traps. Neutrophils non-specifically sense infection through an array of innate immune receptors and inflammatory sensors, but are also able to respond in a pathogen/antigen-specific manner when leveraged by antibodies via Fc-receptors. Among neutrophil functions, antibody-dependent neutrophil phagocytosis (ADNP) results in antibody-mediated opsonization, enabling neutrophils to sense and respond to infection in a pathogen-appropriate manner. Here, we describe a high-throughput flow cytometric approach to effectively visualize and quantify ADNP and its downstream consequences. The assay is easily adaptable, supporting both the use of purified neutrophils or white blood cells, the use of purified Ig or serum, and the broad utility of any target antigen. Thus, this ADNP assay represents a high-throughput platform for the in-depth characterization of neutrophil function.

•

A high-throughput antibody-dependent neutrophil phagocytosis (ADNP) assay was developed.

•

This flow cytometry assay is flexible and can be easily adapted to any pathogen.

•

Analysis of sample sets by ADNP assay is fast, robust and cost-effective.

•

Additional neutrophil functions can be profiled in secondary analyses.

PD-1 Expression in Head and Neck Squamous Cell Carcinomas Derives Primarily from Functionally Anergic CD4+ TILs in the Presence of PD-L1+ TAMs

Oral tongue squamous cell carcinoma (OTSCC) is the most common oral cavity tumor. In this study, we examined the basis for the activity of programmed cell death protein (PD-1)-based immune checkpoint therapy that is being explored widely in head and neck cancers. Using multispectral imaging, we systematically investigated the OTSCC tumor microenvironment (TME) by evaluating the frequency of PD-1 expression in CD8⁺, CD4⁺, and FoxP3⁺ tumor-infiltrating lymphocytes (TIL). We also defined the cellular sources of PD-1 ligand (PD-L1) to evaluate the utility of PD-1:PD-L1 blocking antibody therapy in this patient population. PD-L1 was expressed in 79% of the OTSCC specimens examined within the TME. Expression of PD-L1 was associated with moderate to high levels of CD4⁺ and CD8⁺ TILs. We found that CD4⁺ TILs were present in equal or greater frequencies than CD8⁺ TILs in 94% of OTSCC and that CD4⁺FOXP3neg TILs were colocalized with PD-1/PD-L1/CD68 more frequently than CD8⁺ TILs. Both CD4⁺PD1⁺ and CD8⁺PD1⁺ TILs were anergic in the setting of PD-L1 expression. Overall, our results highlight the importance of CD4⁺ TILs as pivotal regulators of PD-L1 levels and in determining the responsiveness of OTSCC to PD1-based immune checkpoint therapy. Cancer Res; 77(22); 6365-74. ©2017 AACR.

Facial Layer-by-Layer Engineering of Upconversion Nanoparticles for Gene Delivery: Near-Infrared-Initiated Fluorescence Resonance Energy Transfer Tracking and Overcoming Drug Resistance in Ovarian Cancer

Development of multidrug resistance (MDR) contributes to the majority of treatment failures in clinical chemotherapy. We report facial layer-by-layer engineered upconversion nanoparticles (UCNPs) for near-infrared (NIR)-initiated tracking and delivery of small interfering RNA (siRNA) to enhance chemotherapy efficacy by silencing the MDR1 gene and resensitizing resistant ovarian cancer cells to drug. Layer-by-layer engineered UCNPs were loaded with MDR1 gene-silencing siRNA (MDR1-siRNA) by electrostatic interaction. The delivery vehicle enhances MDR1-siRNA cellular uptake, protects MDR1-siRNA from nuclease degradation, and promotes endosomal escape for silencing the MDR gene. The intrinsic photon upconversion of UCNPs provides an unprecedented opportunity for monitoring intracellular attachment and release of MDR1-siRNA by NIR-initiated fluorescence resonance energy transfer occurs between donor UCNPs and acceptor fluorescence dye-labeled MDR1-siRNA. Enhanced chemotherapeutic efficacy in vitro was demonstrated by cell viability assay. The developed delivery vehicle holds great potential in delivery and imaging-guided tracking of therapeutic gene targets for effective treatment of drug-resistant cancers.

Sex Differences in Plasmacytoid Dendritic Cell Levels of IRF5 Drive Higher IFN-α Production in Women

Increased IFN-α production contributes to the pathogenesis of infectious and autoimmune diseases. Plasmacytoid dendritic cells (pDCs) from females produce more IFN-α upon TLR7 stimulation than pDCs from males, yet the mechanisms underlying this difference remain unclear. In this article, we show that basal levels of IFN regulatory factor (IRF) 5 in pDCs were significantly higher in females compared with males and positively correlated with the percentage of IFN-α-secreting pDCs. Delivery of recombinant IRF5 protein into human primary pDCs increased TLR7-mediated IFN-α secretion. In mice, genetic ablation of the estrogen receptor 1 (Esr1) gene in the hematopoietic compartment or DC lineage reduced Irf5 mRNA expression in pDCs and IFN-α production. IRF5 mRNA levels furthermore correlated with ESR1 mRNA levels in human pDCs, consistent with IRF5 regulation at the transcriptional level by ESR1. Taken together, these data demonstrate a critical mechanism by which sex differences in basal pDC IRF5 expression lead to higher IFN-α production upon TLR7 stimulation in females and provide novel targets for the modulation of immune responses and inflammation.

Identification and evolutionary implications of neurotransmitter-ciliary interactions underlying the behavioral response to hypoxia in Lymnaea stagnalis embryos

Acceleration of embryonic rotation is a common response to hypoxia among pond snails. It was first characterized in Helisoma trivolvis embryos, which have a pair of sensorimotor neurons that detect hypoxia and release serotonin onto postsynaptic ciliary cells. The objective of the present study was to determine how the hypoxia response is mediated in Lymnaea stagnalis, which differ from H. trivolvis by having both serotonergic and dopaminergic neurons, and morphologically distinct ciliated structures at comparative stages of embryonic development. Time-lapse video recordings of the rotational behavior in L. stagnalis revealed similar rotational features to those previously observed in H. trivolvis, including rotational surges and rotational responses to hypoxia. Serotonin and dopamine increased the rate of rotation with similar potency. In contrast, serotonin was more potent than dopamine in stimulating the ciliary beat frequency of isolated pedal cilia. Isolated apical plate cilia displayed an irregular pattern of ciliary beating that precluded the measurement of ciliary beat frequency. A qualitative assessment of ciliary beating revealed that both serotonin and dopamine were able to stimulate apical plate cilia. The ciliary responses to dopamine were reversible in both pedal and apical plate cilia, whereas the responses to serotonin were only reversible at concentrations below 100 μmol l(-1). Mianserin, a serotonin receptor antagonist, and SKF83566, a dopamine receptor antagonist, effectively blocked the rotational responses to serotonin and dopamine, respectively. The rotational response to hypoxia was only partially blocked by mianserin, but was fully blocked by SKF83566. These data suggest that, despite the ability of serotonin to stimulate ciliary beating in L. stagnalis embryos, the rotational response to hypoxia is primarily mediated by the transient apical catecholaminergic neurons that innervate the ciliated apical plate.

Expression of PD-L1 and PD-L2 on human macrophages is up-regulated by HIV-1 and differentially modulated by IL-10

PD-1 plays an important role in T cell exhaustion during HIV infection. PD-1 has two ligands: PD-L1, expressed on hematopoietic and nonhematopoietic cells, and PD-L2, limited to DCs and macrophages. Little is known about PD-L1 expression and regulation in human macrophages. Previous reports have found few immediate effects of macrophage exposure to HIV, suggesting that macrophages lack PRRs for this virus. Using quantitative confocal microscopy and a multiplexed cytokine bead array, we measured induction of PD-L1, PD-L2, and innate response cytokines in human MDMs in response to chemically inactivated HIV virions. Consistent with previous reports, no cytokines were induced by HIV virion exposure. Whereas PD-L1 and PD-L2 had low baseline expression, TLR ligands (LPS and CL097) up-regulated PD-L1 but not PD-L2. Unlike what we found for cytokine expression, PD-L1 and PD-L2 were up-regulated in response to exposure with inactivated HIV virions or with replication-competent HIV. Expression of PD-L1 was differentially modulated by IL-10, which induced up-regulation of PD-L1 but not of PD-L2, and IL-10 blockade enhanced only PD-L2 expression. We discuss implications for innate recognition of HIV by macrophages and potential, different roles for PD-L1 and PD-L2 in immunity and pathogenesis.

Chromophore-assisted laser inactivation

The major challenge of the post-genome world is ascribing in situ function to the myriad of proteins expressed in the proteome. This challenge is met by an arsenal of inactivation strategies that include RNAi and genetic knockout. These are powerful approaches but are indirect with respect to protein function and are subject to time delays before onset and possible genetic compensation. This chapter describes two protein-based inactivation approaches called chromophore-assisted laser inactivation (CALI) and fluorophore-assisted light inactivation (FALI). For CALI and FALI, light inactivation is targeted via photosensitizers that are localized to proteins of interest through antibody binding or expressed domains that are fluorescent or bind fluorescent probes. Inactivation occurs when and where the cells or tissues are irradiated and thus CALI and FALI provide an unprecedented level of spatial and temporal resolution of protein inactivation. Here we provide methods for the labeling of antibodies and setup of light sources and discuss controls, advantages of the technology, and potential pitfalls. We conclude with a discussion on a number of new technologies derived from CALI that combine molecular genetic approaches with light-induced inactivation that provide new tools to address in situ protein function.

Modeling the role of myosin 1c in neuronal growth cone turning

We addressed the mechanical basis for how embryonic chick dorsal root ganglion growth cones turn on a uniform substrate of laminin-1. Turning is significantly correlated with lamellipodial area but not with filopodial length. We assessed the lamellipodial contribution to turning by asymmetric micro-CALI of myosin isoforms that causes localized lamellipodial expansion (myosin 1c) or filopodial retraction (myosin V). Episodes of asymmetric micro-CALI of myosin 1c (or myosin 1c and V together) caused significant turning of the growth cone. In contrast, repeated micro-CALI of myosin V or irradiation without added antibody did not turn growth cones. These findings argue that lamellipodia and not filopodia are necessary for growth cone turning. To model the role of myosin 1c on growth cone turning, we fitted the measured trajectories from asymmetric micro-CALI of myosin 1c-treated and untreated growth cones to the persistent random walk model. The first parameter in this equation, root-mean-square speed, is indistinguishable between the two data sets whereas the second parameter, the persistence of motion, is significantly increased (2.5-fold) as a result of asymmetric inactivation of myosin 1c by micro-CALI. This analysis demonstrates that growth cone turning results from an increase in the persistence of directional motion rather than a change in speed. Taken together, our results suggest that myosin 1c is a molecular correlate for directional persistence underlying growth cone motility.

Coordinated development of identified serotonergic neurons and their target ciliary cells in Helisoma trivolvis embryos

Embryonic neuron C1s (ENC1s) are bilateral serotonergic neurons that function as cilioexcitatory motor neurons in embryonic development of the pond snail, Helisoma trivolvis. Recent experiments demonstrated that these neurons stimulate cilia-driven embryo rotation in response to hypoxia. In the present study, a comprehensive anatomic analysis of these cells and their target ciliary structures was done to address the following questions: (1) Does ENC1 have a morphology consistent with an oxygen-sensitive sensory cell; (2) Is the development of ENC1's neurite outgrowth pathway coordinated with the development of its target effectors, the pedal and dorsolateral ciliary bands; and (3) What is the anatomic basis of ENC1-ciliary communication? By using an array of microscopic techniques on live and serotonin-immunostained embryos, we found that each ENC1 possessed an apical dendrite that was capped with an integral dendritic knob penetrating the embryo surface. The dendritic knobs contained both microvilli and nonmotile cilia that suggested a sensory transduction role. Each ENC1 also possessed a descending projection, whose development was characterized by the rapid formation of the primary neurite pathway between stages E13 and E15, with the primary neurite of the right ENC1 developing in advance of its contralateral homologue. Secondary neurite branches formed between stages E15 and E30 in a spatiotemporal pattern that closely matched the development of the dorsolateral and pedal bands of cilia. Both dorsolateral and pedal ciliated cells formed basal processes that contacted ENC1 neurites. Finally, gap junction profiles were observed at neurite-neurite, neurite-ciliary cell, and ciliary cell-ciliary cell apposition sites, whereas putative chemical synaptic profiles were observed at neurite-neurite and neurite-ciliary cell apposition sites.

Myosin 1c and myosin IIB serve opposing roles in lamellipodial dynamics of the neuronal growth cone

The myosin family of motor proteins is implicated in mediating actin-based growth cone motility, but the roles of many myosins remain unclear. We previously implicated myosin 1c (M1c; formerly myosin I beta) in the retention of lamellipodia (Wang et al., 1996). Here we address the role of myosin II (MII) in chick dorsal root ganglion neuronal growth cone motility and the contribution of M1c and MII to retrograde F-actin flow using chromophore-assisted laser inactivation (CALI). CALI of MII reduced neurite outgrowth and growth cone area by 25%, suggesting a role for MII in lamellipodial expansion. Micro-CALI of MII caused a rapid reduction in local lamellipodial protrusion in growth cones with no effects on filopodial dynamics. This is opposite to micro-CALI of M1c, which caused an increase in lamellipodial protrusion. We used fiduciary beads (Forscher et al., 1992) to observe retrograde F-actin flow during the acute loss of M1c or MII. Micro-CALI of M1c reduced retrograde bead flow by 76%, whereas micro-CALI of MII or the MIIB isoform did not. Thus, M1c and MIIB serve opposite and nonredundant roles in regulating lamellipodial dynamics, and M1c activity is specifically required for retrograde F-actin flow.

The TSC1 tumour suppressor hamartin regulates cell adhesion through ERM proteins and the GTPase Rho

Loss of the tumour-suppressor gene TSC1 is responsible for hamartoma development in tuberous sclerosis complex (TSC), which renders several organs susceptible to benign tumours. Hamartin, the protein encoded by TSC1, contains a coiled-coil domain and is expressed in most adult tissues, although its function is unknown. Here we show that hamartin interacts with the ezrin-radixin-moesin (ERM) family of actin-binding proteins. Inhibition of hamartin function in cells containing focal adhesions results in loss of adhesion to the cell substrate, whereas overexpression of hamartin in cells lacking focal adhesions results in activation of the small GTP-binding protein Rho, assembly of actin stress fibres and formation of focal adhesions. Interaction of endogenous hamartin with ERM-family proteins is required for activation of Rho by serum or by lysophosphatidic acid (LPA). Our data indicate that disruption of adhesion to the cell matrix through loss of hamartin may initiate the development of TSC hamartomas and that a Rho-mediated signalling pathway regulating cell adhesion may constitute a rate-limiting step in tumour formation.

Stimulus history alters behavioral responses of neuronal growth cones

Generally, it is assumed that growth cones respond to a specific guidance cue with a single, specific, and stereotyped behavior. However, there is evidence to suggest that previous exposure to a given cue might alter subsequent responses to that cue (Snow and Letourneau, 1992; Shirasaki et al., 1998). We therefore tested the hypothesis that growth cone responses to stimuli are dependent on the history of previous stimulation. Growth cones of chick dorsal root ganglion neurons were exposed to well characterized stimuli: (1) contact with a laminin-coated bead, which causes growth cone turning, or (2) electrical stimulation, which causes growth cone collapse. Although the expected behavioral responses were observed after the initial stimulation, strikingly different responses to a subsequent stimulation were observed. Growth cones that had recovered from electrical stimulation-induced collapse rapidly developed insensitivity to a second identical electrical stimulation. Growth cones that previously turned in response to contact with a laminin-coated bead responded to a second bead with a "stall" or cessation in outgrowth. This stimulus history dependence of growth cone behavior could be generalized across dissimilar stimuli: after contact with a laminin-coated bead, growth cones failed to collapse in response to electrical stimulation. The calcium/calmodulin-dependent protein kinase II (CaMKII) was implicated in this history dependence by pharmacological experiments. Together, these results demonstrate that growth cones can alter their behavioral response rapidly to a given stimulus in a manner dependent on previous history and that knowledge of past events in growth cone navigation may be required to predict future growth cone behavior.

Membrane recycling in the neuronal growth cone revealed by FM1-43 labeling

Membrane dynamics within the chick ciliary neuronal growth cone were investigated by using the membrane-impermeant dye FM1-43. A depolarization-evoked endocytosis was observed that shared many properties with the synaptic vesicle recycling previously described at the presynaptic terminal. In addition, in the absence of depolarization a basal level of constitutive endocytotic activity was observed at approximately 30% of the rate of evoked endocytosis. This constitutive endocytosis accounted for large amounts of membrane retrieval: the equivalent of the entire growth cone surface area could be internalized within a 30 min period. Endosomes generated via constitutive and evoked processes were highly mobile and could move considerable distances both within the growth cone and out to the neurite. In addition to their different requirements for formation, evoked and constitutive endosomes displayed a significant difference in release properties. After a subsequent depolarization of labeled growth cones, evoked endosomes were released although constitutive endosomes were not released. Furthermore, treatment with latrotoxin released evoked endosomes, but not constitutive endosomes. Although the properties of evoked endosomes are highly reminiscent of synaptic vesicles, constitutive endosomes appear to be a separate pool resulting from a distinct and highly active process within the neuronal growth cone.

Early development of an identified serotonergic neuron in Helisoma trivolvis embryos: serotonin expression, de-expression, and uptake

In early-stage embryos of Helisoma trivolvis, a bilateral pair of identified neurons (ENC1) express serotonin and project primary descending neurites that ramify in the pedal region of the embryo prior to the formation of central ganglia. Pharmacological studies suggest that serotonin released from ENC1 acts in an autoregulatory pathway to regulate its own neurite branching and in a paracrine or synaptic pathway to regulate the activity of pedal ciliary cells. In the present study, several key features of early ENC1 development were characterized as a necessary foundation for further experimental studies on the mechanisms underlying ENC1 development and its physiological role during embryogenesis. ENC1 morphology was determined by confocal microscopy of serotonin-immunostained embryos and by differential-interference contrast (DIC) microscopy of live embryos. The soma was located at an anteriolateral superficial position and contained several distinguishing features, including a large spherical nucleus with prominent central nucleolus, large granules in the apical cytoplasm, a broad apical dendrite ending in a sensory-like structure at the embryonic surface, and a ventral neurite. ENC1 first expressed serotonin immunoreactivity around stage E13, followed immediately by the appearance of an immunoreactive neurite (stage E14). Both the intensity of immunoreactivity and primary neurite length were consistently greater in the right ENC1 at early stages. Serotonin uptake, as indicated by 5,7-dihydroxytryptamine-induced fluorescence, first occurred between stages E18 and E25. At later stages of embryogenesis (after stage E65), serotonin immunoreactivity disappeared, whereas serotonin uptake and normal cell morphology were retained.

Neurite branch development of an identified serotonergic neuron from embryonic Helisoma: evidence for autoregulation by serotonin

Previous studies have shown that in select neurons, neurite outgrowth can be regulated by the same neurotransmitter that is synthesized and released by those neurons. However, it is not known whether such an autoregulatory mechanism is utilized during the normal course of nervous system development in either invertebrates or vertebrates. In the present study, we tested this hypothesis on the first pair of identified serotonergic neurons to be expressed in embryos of the pulmonate gastropod, Helisoma trivolvis. Embryonic neurons C1 (ENC1) elaborate a stereotyped pattern of neurite outgrowth prior to the differentiation of subsequent serotonergic neurons. Embryos were treated with either p-chlorophenylalanine (pCPA) or 5-hydroxytryptophan (5-HTP) to lower or raise embryonic serotonin content, respectively. High-performance liquid chromatography with electrochemical detection was used to measure the effects of these treatments on serotonin content, and serotonin immunohistochemistry was carried out to quantify the extent of neurite outgrowth of ENC1. Embryonic serotonin content was significantly reduced at both 24 and 48 hr after treatment with 0.02% pCPA, whereas dopamine levels were unchanged. Although the proximal neurite outgrowth of ENC1 appeared unaffected by the pCPA treatment at both of these time points, the distal outgrowth in the target cell region appeared more profuse. This effect on outgrowth was quantified by counting the number of neurite branch points, which was significantly increased both 24 and 48 hr after pCPA treatment. In contrast, 5-HTP treatment resulted in an increase in embryonic serotonin content and a significant decrease in the number of ENC1 branch points. Treatment with dopamine had no effect on the pattern of ENC1 neurite outgrowth. Together, these results support the hypothesis that a neuron may utilize its own transmitter in an autoregulatory fashion to regulate neurite formation during embryonic development.

Pharmacological characterization of a serotonin receptor involved in an early embryonic behavior of Helisoma trivolvis

In contrast to the abundance of information on the many physiological and developmental actions of serotonin in molluscan nervous systems, comparatively little is known about the serotonin receptors involved in these responses. Embryos of the pulmonate gastropod, Helisoma trivolvis, display a cilia-driven rotational behavior that is regulated by endogenous serotonin. In the present study, two functional assays were used to determine some of the pharmacological properties of the receptors that mediate the cilio-excitatory action of serotonin. Time-lapse video microscopy was used to measure whole embryo rotation rate and cilia beat frequency in isolated cells. In dose-response experiments, serotonin was approximately 10 times more potent in stimulating cilia beat frequency over embryo rotation. In rotation experiments, 5-carboxyamidotryptamine and methysergide had effective agonist activity in dose ranges similar to that of serotonin (1 to 100 microM). In contrast, 8-hydroxydipropylaminotetralin HBr (8-OH-DPAT) displayed agonist activity of lower potency and effectiveness. Several compounds displayed antagonist activity in the 1 to 100 microM dose range, including mianserin, spiperone, ritanserin, 1-(1-naphthyl)piperazine, and propranolol. alpha-Methylserotonin had mixed agonist-antagonist activity, and metoclopramide, MDL-72222, and ketanserin were inactive. Experiments on isolated cells suggested that the extremely effective antagonism displayed by mianserin in the embryo rotation assay was due to its specific activity at ciliary serotonin receptors. These results implicate the presence of a novel serotonin receptor on embryonic ciliated cells that is pharmacologically distinct from those previously characterized in vertebrate or invertebrate systems.

Characterization and development of rotational behavior in Helisoma embryos: role of endogenous serotonin

Cilia-driven rotational behavior displayed by embryos of the pond snail Helisoma trivolvis was characterized in terms of its behavioral subcomponents, developmental changes, and response to exogenous serotonin. Rotation was found to be a complex behavior characterized by four parameters; rotational direction, rotation rate, rotational surges, and periods of inactivity. These parameters all exhibited characteristic developmental changes from embryonic stage E15 through stage E30. Notably, both rotation rate and frequency of rotational surges increased from stage E15 to E25 and declined to an intermediate level by stage E30. It appeared that the developmental increase in overall rotation rate was caused primarily by an increase in surge frequency, rather than an increase in the rate of nonsurge rotation. Immersion of embryos inserotonin-containing pond water resulted in a dose-dependent, reversible increase in rotation rate as well as a dose-dependent, reversible decrease in surge frequency. The serotonin antagonist, mianserin, abolished the excitatory effect of exogenous serotonin. Furthermore, application of mianserin alone reduced rotation rate and virtually abolished rotational surges. Taken together, these pharmacological results suggest that endogenous serotonin is responsible for generating rotational surges. Given that early embryos contain only a single pair of serotonergic neurons (Goldberg and Kater, 1989) during the stages when rotational surges are expressed, these results also prompt the hypothesis that these neurons, embryonic neurons C1, act as cilioexcitatory motor neurons during embryonic development.

Postembryonic expression of the serotonin phenotype in Helisoma trivolvis: comparison between laboratory-reared and wild-type strains

In a laboratory-reared albino strain of the snail Helisoma trivolvis, the number of neurons expressing the serotonin phenotype increases markedly during postembryonic life. To address whether these latent changes occur selectively in the laboratory-reared strain, postembryonic expression of serotonin immunoreactivity was directly compared in laboratory-reared and wild-type specimens. The spatial pattern of serotonin-immunoreactive neurons was generally the same in the two strains. In contrast, the time course over which this pattern was generated was more prolonged in the wild types than in the laboratory-reared strain. The cerebral, left parietal, and visceral ganglia of laboratory-reared animals completed their postembryonic acquisition of serotonin-immunoreactive neurons by stage P10. Acquisition of serotonin-immunoreactive neurons after stage P10 occurred only in the pedal ganglia. In the wild types, addition of serotonin-immunoreactive neurons continued at least until stage P20 in all of the ganglia examined. Analysis of serotonergic clusters within the cerebral and pedal ganglia revealed distinct developmental patterns for individual clusters. Therefore, the acquisition of the serotonin phenotype during postembryonic life is a normal component of nervous system development in wild-type H. trivolvis.