Possible communication between murine macrophages oriented in linear chains in tissue culture

Regenerative potential of multinucleated cells: bone marrow adiponectin-positive multinucleated cells take the lead

BACKGROUND

Polyploid cells can be found in a wide evolutionary spectrum of organisms. These cells are assumed to be involved in tissue regeneration and resistance to stressors. Although the appearance of large multinucleated cells (LMCs) in long-term culture of bone marrow (BM) mesenchymal cells has been reported, the presence and characteristics of such cells in native BM and their putative role in BM reconstitution following injury have not been fully investigated.

METHODS

BM-derived LMCs were explored by time-lapse microscopy from the first hours post-isolation to assess their colony formation and plasticity. In addition, sub-lethally irradiated mice were killed every other day for four weeks to investigate the histopathological processes during BM regeneration. Moreover, LMCs from GFP transgenic mice were transplanted to BM-ablated recipients to evaluate their contribution to tissue reconstruction.

RESULTS

BM-isolated LMCs produced mononucleated cells with characteristics of mesenchymal stromal cells. Time-series inspections of BM sections following irradiation revealed that LMCs are highly resistant to injury and originate mononucleated cells which reconstitute the tissue. The regeneration process was synchronized with a transient augmentation of adipocytes suggesting their contribution to tissue repair. Additionally, LMCs were found to be adiponectin positive linking the observations on multinucleation and adipogenesis to BM regeneration. Notably, transplantation of LMCs to myeloablated recipients could reconstitute both the hematopoietic system and BM stroma.

CONCLUSIONS

A population of resistant multinucleated cells reside in the BM that serves as the common origin of stromal and hematopoietic lineages with a key role in tissue regeneration. Furthermore, this study underscores the contribution of adipocytes in BM reconstruction.

The contribution of ion channels to shaping macrophage behaviour

The expanding roles of macrophages in physiological and pathophysiological mechanisms now include normal tissue homeostasis, tissue repair and regeneration, including neuronal tissue; initiation, progression, and resolution of the inflammatory response and a diverse array of anti-microbial activities. Two hallmarks of macrophage activity which appear to be fundamental to their diverse cellular functionalities are cellular plasticity and phenotypic heterogeneity. Macrophage plasticity allows these cells to take on a broad spectrum of differing cellular phenotypes in response to local and possibly previous encountered environmental signals. Cellular plasticity also contributes to tissue- and stimulus-dependent macrophage heterogeneity, which manifests itself as different macrophage phenotypes being found at different tissue locations and/or after different cell stimuli. Together, plasticity and heterogeneity align macrophage phenotypes to their required local cellular functions and prevent inappropriate activation of the cell, which could lead to pathology. To execute the appropriate function, which must be regulated at the qualitative, quantitative, spatial and temporal levels, macrophages constantly monitor intracellular and extracellular parameters to initiate and control the appropriate cell signaling cascades. The sensors and signaling mechanisms which control macrophages are the focus of a considerable amount of research. Ion channels regulate the flow of ions between cellular membranes and are critical to cell signaling mechanisms in a variety of cellular functions. It is therefore surprising that the role of ion channels in the macrophage biology has been relatively overlooked. In this review we provide a summary of ion channel research in macrophages. We begin by giving a narrative-based explanation of the membrane potential and its importance in cell biology. We then report on research implicating different ion channel families in macrophage functions. Finally, we highlight some areas of ion channel research in macrophages which need to be addressed, future possible developments in this field and therapeutic potential.

Gap Junctions in the Bone Marrow Lympho-Hematopoietic Stem Cell Niche, Leukemia Progression, and Chemoresistance

Abstract: The crosstalk between hematopoietic stem cells (HSC) and bone marrow (BM) microenvironment is critical for homeostasis and hematopoietic regeneration in response to blood formation emergencies after injury, and has been associated with leukemia transformation and progression. Intercellular signals by the BM stromal cells in the form of cell-bound or secreted factors, or by physical interaction, regulate HSC localization, maintenance, and differentiation within increasingly defined BM HSC niches. Gap junctions (GJ) are comprised of arrays of membrane embedded channels formed by connexin proteins, and control crucial signaling functions, including the transfer of ions, small metabolites, and organelles to adjacent cells which affect intracellular mechanisms of signaling and autophagy. This review will discuss the role of GJ in both normal and leukemic hematopoiesis, and highlight some of the most novel approaches that may improve the efficacy of cytotoxic drugs. Connexin GJ channels exert both cell-intrinsic and cell-extrinsic effects on HSC and BM stromal cells, involved in regenerative hematopoiesis after myelosuppression, and represent an alternative system of cell communication through a combination of electrical and metabolic coupling as well as organelle transfer in the HSC niche. GJ intercellular communication (GJIC) in the HSC niche improves cellular bioenergetics, and rejuvenates damaged recipient cells. Unfortunately, they can also support leukemia proliferation and survival by creating leukemic niches that provide GJIC dependent energy sources and facilitate chemoresistance and relapse. The emergence of new strategies to disrupt self-reinforcing malignant niches and intercellular organelle exchange in leukemic niches, while at the same time conserving normal hematopoietic GJIC function, could synergize the effect of chemotherapy drugs in eradicating minimal residual disease. An improved understanding of the molecular basis of connexin regulation in normal and leukemic hematopoiesis is warranted for the re-establishment of normal hematopoiesis after chemotherapy.

Connexins and pannexins in the immune system and lymphatic organs

Connexin43 and pannexin1 are found in immune cells. While gap junctional communication has been demonstrated between immune cells, hemichannels have been implicated in many cellular functions. Among the functions involved as being connexin dependent and pannexin dependent are cell migration, phagocytosis, antigen presentation, T-cell reactivity and B-cell responses. Surprisingly, many of these connexin-related and pannexin-related functions are not recapitulated in in vivo models. This is leading to a reevaluation of the role of these proteins in immune function.

Regulation of hemichannels and gap junction channels by cytokines in antigen-presenting cells

Autocrine and paracrine signals coordinate responses of several cell types of the immune system that provide efficient protection against different challenges. Antigen-presenting cells (APCs) coordinate activation of this system via homocellular and heterocellular interactions. Cytokines constitute chemical intercellular signals among immune cells and might promote pro- or anti-inflammatory effects. During the last two decades, two membrane pathways for intercellular communication have been demonstrated in cells of the immune system. They are called hemichannels (HCs) and gap junction channels (GJCs) and provide new insights into the mechanisms of the orchestrated response of immune cells. GJCs and HCs are permeable to ions and small molecules, including signaling molecules. The direct intercellular transfer between contacting cells can be mediated by GJCs, whereas the release to or uptake from the extracellular milieu can be mediated by HCs. GJCs and HCs can be constituted by two protein families: connexins (Cxs) or pannexins (Panxs), which are present in almost all APCs, being Cx43 and Panx1 the most ubiquitous members of each protein family. In this review, we focus on the effects of different cytokines on the intercellular communication mediated by HCs and GJCs in APCs and their impact on purinergic signaling.

Intercellular Communication in Atherosclerosis

Cell-to-cell communication is a process necessary for physiological tissue homeostasis and appears often altered during disease. Gap junction channels, formed by connexins, allow the direct intercellular communication between adjacent cells. After a brief review of the pathophysiology of atherosclerosis, we will discuss the role of connexins throughout the different stages of the disease.

Sphingosine-1-phosphate receptor expression in cardiac fibroblasts is modulated by in vitro culture conditions

The bioactive molecule sphingosine-1-phosphate (S1P) binds with high affinity to five recognized receptors (S1P(1-5)) to affect various tissues, including cellular responses of cardiac fibroblasts (CFbs) and myocytes. CFbs are essential components of myocardium, and detailed study of their cell signaling and physiology is required for a number of emerging disciplines. Meaningful studies on CFbs, however, necessitate methods for selective, reproducible cell isolations. Macrophages reside within normal cardiac tissues and often are isolated with CFbs. A protocol was therefore developed that significantly reduces macrophage levels and utilizes more CFb-specific markers (discoidin domain receptor-2) instead of, or in addition to, more commonly used cytoskeletal markers. Our results demonstrate that primary isolated, purified CFbs express predominantly S1P(1-3); however, the relative levels of these receptor subtypes are modulated with time and by culture conditions. In coculture experiments, macrophages altered CFb S1P receptor levels relative to controls. Further investigations using known macrophage-secreted factors showed that S1P and H(2)O(2) had minimal effects on CFb S1P(1-3) expression, whereas transforming growth factor-beta1, TNF-alpha, and PDGF-BB significantly altered all S1P receptor subtypes. Lowering FBS concentrations from 10% to 0.1% increased S1P(2), whereas supplementation with either PDGF-BB or Rho-associated protein kinase inhibitor Y-27632 significantly elevated S1P(3) levels. S1P(2) and S1P(3) receptor levels are known to regulate cell migration. Using cells isolated from either normal or S1P(3)-null mice, we demonstrate that S1P(3) is important and necessary for CFb migration. These results highlight the importance of demonstrating CFb culture purity in functional studies of S1P and also identify conditions that modulate S1P receptor expression in CFbs.

Connexin37: a potential modifier gene of inflammatory disease

There is an increasing appreciation of the importance of gap junction proteins (connexins) in modulating the severity of inflammatory diseases. Multiple epidemiological gene association studies have detected a link between a single nucleotide polymorphism in the human connexin37 (Cx37) gene and coronary artery disease or myocardial infarction in various populations. This C1019T polymorphism causes a proline-to-serine substitution (P319S) in the regulatory C terminal tail of Cx37, a protein that is expressed in the vascular endothelium as well as in monocytes and macrophages. Indeed, these three cell types are key players in atherogenesis. In the early phases of atherosclerosis, blood monocytes are recruited to the sites of injury in response to chemotactic factors. Monocytes adhere to the dysfunctional endothelium and transmigrate across endothelial cells to penetrate the arterial intima. In the intima, monocytes proliferate, mature, and accumulate lipids to progress into macrophage foam cells. This review focuses on Cx37 and its impact on the cellular and molecular events underlying tissue function, with particular emphasis of the contribution of the C1019T polymorphism in atherosclerosis. We will also discuss evidence for a potential mechanism by which allelic variants of Cx37 are differentially predictive of increased risk for inflammatory diseases.

Gap junctional communication in tissue inflammation and repair

Local injury induces a complex orchestrated response to stimulate healing of injured tissues, cellular regeneration and phagocytosis. Practically, inflammation is defined as a defense process whereby fluid and white blood cells accumulate at a site of injury. The balance of cytokines, chemokines, and growth factors is likely to play a key role in regulating important cell functions such as migration, proliferation, and matrix synthesis during the process of inflammation. Hence, the initiation, maintenance, and resolution of innate responses depend upon cellular communication. A process similar to tissue repair and subsequent scarring is found in a variety of fibrotic diseases. This may occur in a single organ such as liver, kidneys, pancreas, lung, skin, and heart, but fibrosis may also have a more generalized distribution such as in atherosclerosis. The purpose of this review is to summarize recent advances on the contribution of gap junction-mediated intercellular communication in the modulation of the inflammatory response and tissue repair.

Gap junctions and connexin-mediated communication in the immune system

Gap junctions and connexins are present in the immune system. In haematopoiesis, connexin 43, the most widely distributed gap junction protein, appears to be a key player in the development of progenitor cells and their communication with stromal cells. Connexin 43 is expressed by macrophages, neutrophils and mast cells. Lymphocytes also express connexin 43, and inhibition of gap junction channels in these cells by using highly specific connexin mimetic reagents has profound effects on immunoglobulin secretion and synthesis of cytokines. Lymphocytes and leukocytes also communicate directly in vitro with endothelial cells via gap junctions. Connexins are implicated in inflammatory reactions in a range of tissues. Their involvement in atherosclerotic plaque formation in the vascular system is also a current growth point in research, and could lead to the development of therapeutic interventions.

Microglia at brain stab wounds express connexin 43 and in vitro form functional gap junctions after treatment with interferon-gamma and tumor necrosis factor-alpha

Gap junctional communication between microglia was investigated at rat brain stab wounds and in primary cultures of rat and mouse cells. Under resting conditions, rat microglia (FITC-isolectin-B4-reactive cells) were sparsely distributed in the neocortex, and most (95%) were not immunoreactive for Cx43, a gap junction protein subunit. At brain stab wounds, microglia progressively accumulated over several days and formed aggregates that frequently showed Cx43 immunoreactivity at interfaces between cells. In primary culture, microglia showed low levels of Cx43 determined by Western blotting, diffuse intracellular Cx43 immunoreactivity, and a low incidence of dye coupling. Treatment with the immunostimulant bacterial lipopolysaccharide (LPS) or the cytokines interferon-gamma (INF-gamma) or tumor necrosis factor-alpha (TNF-alpha) one at a time did not increase the incidence of dye coupling. However, microglia treated with INF-gamma plus LPS showed a dramatic increase in dye coupling that was prevented by coapplication of an anti-TNF-alpha antibody, suggesting the release and autocrine action of TNF-alpha. Treatment with INF-gamma plus TNF-alpha also greatly increased the incidence of dye coupling and the Cx43 levels with translocation of Cx43 to cell-cell contacts. The cytokine-induced dye coupling was reversibly inhibited by 18 alpha-glycyrrhetinic acid, a gap junction blocker. Cultured mouse microglia also expressed Cx43 and developed dye coupling upon treatment with cytokines, but microglia from homozygous Cx43-deficient mice did not develop significant dye coupling after treatment with either INF-gamma plus LPS or INF-gamma plus TNF-alpha. This report demonstrates that microglia can communicate with each other through gap junctions that are induced by inflammatory cytokines, a process that may be important in the elaboration of the inflammatory response.

Intercellular communication in the immune system: differential expression of connexin40 and 43, and perturbation of gap junction channel functions in peripheral blood and tonsil human lymphocyte subpopulations

The distribution and function of connexins (integral membrane proteins assembled into gap junction intercellular communication channels) were studied in human lymphocyte subpopulations. The expression of mRNA encoding connexins in peripheral blood and tonsil-derived T, B and natural killer (NK) lymphocytes was examined. Connexin43 (Cx43) mRNA was expressed in peripheral blood and tonsil lymphocytes, but Cx40 mRNA expression was confined to tonsil-derived T and B lymphocytes; Cx26, Cx32, Cx37 and Cx45 were not detected by reverse transcription-polymerase chain reaction (RT-PCR). Western blot analysis also demonstrated the presence of Cx40 and Cx43 proteins in T and B lymphocytes in a manner coincidental to the mRNA detection. Stimulation in vitro of T and B lymphocytes with phytohaemagglutinin (PHA) and lipopolysaccharide (LPS), respectively, increased Cx40 and Cx43 protein expression. Flow cytometric analysis, using antibodies to extracellular loop amino acid sequences of connexins, confirmed the surface expression of connexins in all lymphocyte subpopulations. Assembly of connexins into gap junctions providing direct intercellular channels linking attached lymphocytes was demonstrated by using a dye transfer technique. The exchange of dye between lymphocytes was inhibited by a connexin extracellular loop mimetic peptide and alpha-glycyrrhetinic acid, two reagents that restrict intercellular communication across gap junctions. Dye coupling occurred between homologous and heterologous co-cultures of T and B lymphocytes, and was not influenced by their stimulation with PHA and LPS. The connexin mimetic peptide caused a significant decrease in the in vitro synthesis of immunoglobulin M (IgM) by T- and B-lymphocyte co-cultured populations in the presence or absence of stimulation by PHA. The results identify connexins as important cell surface components that modulate immune processes.

Gap junctions in cells of the immune system: structure, regulation and possible functional roles

Gap junction channels are sites of cytoplasmic communication between contacting cells. In vertebrates, they consist of protein subunits denoted connexins (Cxs) which are encoded by a gene family. According to their Cx composition, gap junction channels show different gating and permeability properties that define which ions and small molecules permeate them. Differences in Cx primary sequences suggest that channels composed of different Cxs are regulated differentially by intracellular pathways under specific physiological conditions. Functional roles of gap junction channels could be defined by the relative importance of permeant substances, resulting in coordination of electrical and/or metabolic cellular responses. Cells of the native and specific immune systems establish transient homo- and heterocellular contacts at various steps of the immune response. Morphological and functional studies reported during the last three decades have revealed that many intercellular contacts between cells in the immune response present gap junctions or "gap junction-like" structures. Partial characterization of the molecular composition of some of these plasma membrane structures and regulatory mechanisms that control them have been published recently. Studies designed to elucidate their physiological roles suggest that they might permit coordination of cellular events which favor the effective and timely response of the immune system.

Nerve injury and inflammatory cytokines modulate gap junctions in the peripheral nervous system

In the peripheral nervous system (PNS), myelinating Schwann cells express the gap junction protein connexin32 (Cx32) and lower levels of connexin43 (Cx43). Although the function of Cx43 in Schwann cells is not yet known, in adult mammals Cx32 is thought to form reflexive contacts within individual myelinating glial cells and provide direct pathways for intracellular ionic and metabolic exchange from the cell body to the innermost periaxonal cytoplasmic regions. In response to nerve injury, Schwann cells in the degenerating region down-regulate expression of Cx32 and there is increased expression of connexin46 (Cx46) mRNA and protein. The cascade of Schwann cell responses seen after the injury-induced decrease in Cx32, and the observation that dividing Schwann cells express Cx46, and possibly other connexins, and are coupled through gap junction channels, raise the intriguing possibility that there are coordinated changes in Schwann cell proliferation and connexin expression. Moreover, intercellular junctional coupling among cells in general may be important during injury responses. Consistent with this hypothesis, dividing Schwann cells are preferentially coupled through junctional channels as compared to non-dividing cells, which are generally uncoupled. Moreover, the strength of junctional coupling among cultured Schwann cells is modulated by a number of cytokines to which Schwann cells are exposed to in vivo after nerve injury, and Cx46 mRNA and protein levels correlate with the degree of coupling. Other injury-induced cellular changes in connexin expression that may be functionally important during injury responses include a transient increase in Cx43 in endoneurial fibroblasts. This paper reviews what is known about connexin expression and function in the adult mammalian PNS, and focuses on some of the changes that occur following nerve injury. Moreover, evidence that inflammatory cytokines released after injury modulate connexin expression and junctional coupling strength is presented.

Gap junctions in blood forming tissues

More than ten research groups have now reported the presence of gap junctions in blood-forming tissue or cultured cells. It is time to accept that these cell-coupling structures are present in this tissue. To find out what they are doing here we need to develop appropriate experimental techniques. This review covers the particular problems of investigating direct cell-cell communication by gap or other junctions in undisturbed haemopoietic tissue. It then describes and assesses the published reports of haemopoietic gap junctions. Recently, in the author's laboratory, three means of increasing the number of gap junctions 50- to 100-fold in mouse marrow have been described, as well as techniques for doing so in culture. There is a complete report of this work here. At present it is quite unclear what function gap junctions serve in blood-formation, perhaps it is some consolation that 30 years after their ultramicroscopic discovery it is also true for all other unexcitable tissues. Possibly the ability to up-regulate their expression in haemopoietic tissue will help us find out what their role is in blood formation.

Leukocytes express connexin 43 after activation with lipopolysaccharide and appear to form gap junctions with endothelial cells after ischemia-reperfusion

Levels and subcellular distribution of connexin 43 (Cx43), a gap junction protein, were studied in hamster leukocytes before and after activation with endotoxin (lipopolysaccharide, LPS) both in vitro and in vivo. Untreated leukocytes did not express Cx43. However, Cx43 was clearly detectable by indirect immunofluorescence in cells treated in vitro with LPS (1 micrograms/ml, 3 hr). Cx43 was also detected in leukocytes obtained from the peritoneal cavity 5-7 days after LPS-induced inflammation. In some leukocytes that formed clusters Cx43 immunoreactivity was present at appositional membranes, suggesting formation of homotypic gap junctions. In cell homogenates of activated peritoneal macrophages, Cx43, detected by Western blot analysis, was mostly unphosphorylated. A second in vivo inflammatory condition studied was that induced by ischemia-reperfusion of the hamster cheek pouch. In this system, leukocytes that adhered to venular endothelial cells after 1 hr of ischemia, followed by 1 hr of reperfusion, expressed Cx43. Electron microscope observations revealed small close appositions, putative gap junctions, at leukocyte-endothelial cell and leukocyte-leukocyte contacts. These results indicate that the expression of Cx43 can be induced in leukocytes during an inflammatory response which might allow for heterotypic or homotypic intercellular gap junctional communication. Gap junctions may play a role in leukocyte extravasation.

Up-regulation of the connexin43+ gap junction network in haemopoietic tissue before the growth of stem cells

The early developmental stages of haemopoiesis are thought to be regulated by paracrine growth factors and by the haemopoietic environment. Are gap junctions involved here? Gap junctions are structures in cell membranes allowing the direct transfer of ions and small molecules between adjacent cells and are known to be involved in development. We have found that although connexin43 gap junctions are rare (0.00016 +/- 0.0002/microns2 tissue) in normal adult mouse marrow their expression is 80-fold higher (0.0292 +/- 0.0147/microns2) in neonatal marrow. One difference between neonatal and adult haemopoietic tissue is that in the latter more haemopoietic cells are dividing. To test if more gap junctions were due to increased division we altered adult blood-formation by mobilizing or destroying end cells--granulocytes and red cells--or by forcing stem cells to divide by making them regenerate an ablated blood-forming system. Mobilizing end cells had no effect on the number or distribution of gap junctions in marrow but forced stem cell division caused a 100-fold increase in gap junction expression and did so before any recognizable haemopoietic cells formed. There were greater than normal numbers of gap junctions in radio-protected adult mouse marrow. The cells coupled by gap junctions are TE-7+ mesodermally derived fibroblasts, STRO-1+ stromal cells, and CD45+ and CD34+ haemopoietic cells. We propose that there is a latent network of Cx43+ gap junctions in normal quiescent marrow. In response to events that call for active division of stem cells this network is amplified and coupled to haemopoietic stem cells, perhaps enabling them to divide.

Direct cell-cell communication in the blood-forming system

In mammals, bone marrow is the principal tissue where blood is formed during adult life. Paracrine factors are generally considered to control this process but there is considerable evidence that gap junctions are present in haemopoietic tissues. Gap junctions have been implicated in developmental and patterning roles, and we set out to characterize the cells which are coupled, and to provide evidence for their role(s) in blood cell formation. Direct cell-cell communication, shown by dye-transfer, occurs between haemopoietic cells and certain stromal cells. In culture these stromal cells form a mat in which they retain their dye-coupling properties. Freeze-fracture electron microscopy confirms that this coupling is via gap junctions. When haemopoietic cells are cultured on top of these mats dye spreads upwards from the stromal cells into the haemopoietic cells above. Experiments in which haemopoietic cells were cultured alone, with stromal cell conditioned medium, or in direct contact with stromal cell underlays, were therefore carried out. The results of these experiments provide evidence that gap junctional communication may be playing a vital role in maintaining populations of precursor cells which would otherwise differentiate into end cells, leading to the ultimate demise of the system.

An ultrastructural study on the multinucleation process of mouse alveolar macrophages induced by 1 alpha,25-dihydroxyvitamin D3

The multinucleation process of isolated alveolar macrophages induced by 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] was examined using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). At the beginning of culture, most of the macrophages were spherical in shape. During incubation with 1.2 X 10(-8) M 1 alpha,25(OH)2D3, spreading macrophages appeared among the spherical macrophages, and they increased in number. Spreading macrophages extended many cytoplasmic processes toward adjacent macrophages, and interdigitations of these processes between those of neighboring cells were often seen. Two types of cell contact have been observed in the 1 alpha,25(OH)2D3-treated cells. In some, cytoplasmic processes were put into the cytoplasm of the adjacent cells, where clathrinlike structures were observed at the inner membrane of the concave portion. In others, spreading macrophages occasionally came in contact with adjacent cells by a peripheral rim of their cytoplasm with gap junctions. Cytoplasmic continuity was rarely observed at the boundaries between the closely associated cells. The two types of cell contact were also found, though not frequently, in the untreated cells. These results indicate that 1 alpha,25(OH)2D3 promotes multinucleation of alveolar macrophages through spreading forms with the formation of gap junctions and the coated membrane invagination.

Activation of anion channels by zymosan particles in membranes of peritoneal macrophages

Patch-clamp recordings were used to study the effect of zymosan adsorption on membranes of freshly isolated peritoneal macrophages of mouse. Superfusion of adherent macrophages by zymosan in the on-cell pipette configuration stimulated the appearance of anion channels after a varying time delay in the minute range. The channel is activated by passing through a stage of fluctuations of increasing amplitude. Once the full channel current has been reached, the fluctuations become transformed into the typical current pattern of three well-defined conducting channel states. The adoption of the two substates appeared to be dependent on zymosan. Up to nine simultaneously open anion channels could be observed with a single channel conductance of 220-400 pS. Absence of external Ca2+ had no inhibiting influence on the effect of zymosan. Anion channels could in some cases be observed under control conditions, after attachment of the pipette to the membrane. The channel activation could be mimicked by addition of A23187 to calcium-containing bath solutions. There is evidence that a zymosan-mediated rise of intracellular Ca2+ might be involved in the stimulus response coupling. The activation of calcium-dependent potassium channels was not observed.

The correlation of light and electron microscope observations

A technique is described to allow electron microscopic investigation of a specific feature of a section on a glass slide. A section on a glass slide (previously treated with a silicone release agent) is processed as required for light microscopy. The section is then impregnated with Araldite and cured with an epoxy resin block on top of it. The section and block are removed from the slide and viewed with a light microscope. The selected area for ultrastructural study remains under continuous observation while the block is trimmed. Semi-thin (1 micron) sections retain the original staining for light microscopy and ultra-thin sections are stained with heavy metals in the normal manner. We show how an inflammatory lesion in a large area of muscle in a case of polymyositis may be quickly located and studied at the ultrastructural level.

Leishmania mexicana amazonensis: attachment to the membrane of the phagocytic vacuole of macrophages in vivo

Intracellular forms of Leishmania mexicana amazonensis divide inside the phagocytic vacuole of macrophages. Some parasites attach to the membrane of the phagocytic vacuole while others remain free in the vacuole. Examination of thin sections of the attachment region by electron microscopy revealed a space of 2 nm between the membrane of the phagocytic vacuole and the plasma membrane of the parasite. Freeze-fracture replicas showed an array of intramembranous particles in some areas of the parasite's plasma membrane resembling a gap junction which, in other cells, is involved in the process of intracellular communication.

A study of metabolic cooperation with rat peritoneal macrophages

Rat peritoneal macrophages were studied for their ability to undergo metabolic cooperation with rat fibroblasts or with other macrophages. In contrast, rat reticular cells, mesothelial cells, and fibroblasts were able to cooperate with human fibroblasts.

Electrophysiology of phagocytic membranes. II. Membrane potential and induction of slow hyperpolarizations in activated macrophages

The potential differences measured on the cell surface and after penetration into the cytoplasm of activated macrophages are described. Linear regressions are made of the measured potential differences as functions of the tip potential of each microelectrode. The surface potential of the macrophage is not significantly different from zero. Mouse macrophages have a transmembrane potential of--26 mV, whereas in guinea-pig cells this value is--18 mV. The input resistances of guinea-pig cells are higher than those of mouse macrophages. The cytoplasmic location of the electrode was characterized both by fluorescent dye injection and by electric criteria. Slow membrane hyperpolarizations are directly elicited by mechanical stimulation. Electric responses evoked by current pulses were further characterized. Our results lead to the extablishment of objective criteria to validate intracellular recordings from macrophage.

Differentiation of macrophages from normal human bone marrow in liquid culture. Electron microscopy and cytochemistry

To study the various stages of human mononuclear phagocyte maturation, we cultivated bone marrow in an in vitro diffusion chamber with the cells growing in suspension and upon a dialysis membrane. At 2, 7, and 14 days, the cultured cells were examined by electron microscopy and cytochemical techniques for peroxidase and for more limited analysis of acid phosphatase and arylsulfatase. Peroxidase was being synthesized in promonocytes of 2- and 7-day cultures, as evidenced by reaction product in the rough-surfaced endoplasmic reticulum, Golgi complex, and storage granules. Peroxidase synthesis had ceased in monocytes and the enzyme appeared only in some granules. By 7 days, large macrophages predominated, containing numerous peroxidase-positive storage granules, and heterophagy of dying cells was evident. By 14 days, the most prevalent cell type was the large peroxidase-negative macrophage. Thus, peroxidase is present in high concentrations in immature cells but absent at later stages, presumably a result of degranulation of peroxidase-positive storage granules. Clusters of peroxidase-negative macrophages with indistinct borders (epithelioid cells), as well as obvious multinucleated giant cells, were noted. Frequently, the interdigitating plasma membranes of neighboring macrophages showed a modification resembling a septate junction--to our knowledge, representing the first documentation of this specialized cell contact between normal macrophages. We suggest that such junctions may serve as zones of adhesion between epithelioid cells.

Contact sites between lymphoid cells of the bursa of Fabricius, in vivo and in vitro

Contact sites between lymphocytes and between lymphocytes and macrophages were demonstrated by electron microscopy in the lymphoid follicles of the bursa of Fabricius. When compared with nonspecialized regions of the cell membranes, these contact sites were characterized by a decreased intercellular distance, subplasmalemmal densities and coated pits. Microfilaments, microtubules and coated vesicles of the subjacent cytoplasm were frequently associated with these contact sites. When the same cells were isolated and introduced into culture, they formed cluster-like assemblies in which cells were closely approximated along broadly contacting surfaces. The morphology of the sites appeared to involve primarily the plasma membrane (including coated pits) and the cell coat. These observations indicated that the same cells of a given lymphoid tissue can form one type of contact site in vivo and another, dissimilar type of contact site, in vitro.