Metabolic cooperation between cells

Gap junction proteins are key drivers of endocrine function

It has long been known that the main secretory cells of exocrine and endocrine glands are connected by gap junctions, made by a variety of connexin species that ensure their electrical and metabolic coupling. Experiments in culture systems and animal models have since provided increasing evidence that connexin signaling contributes to control the biosynthesis and release of secretory products, as well as to the life and death of secretory cells. More recently, genetic studies have further provided the first lines of evidence that connexins also control the function of human glands, which are central to the pathogenesis of major endocrine diseases. Here, we summarize the recent information gathered on connexin signaling in these systems, since the last reviews on the topic, with particular regard to the pancreatic beta cells which produce insulin, and the renal cells which produce renin. These cells are keys to the development of various forms of diabetes and hypertension, respectively, and combine to account for the exploding, worldwide prevalence of the metabolic syndrome. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Neurochemical and electrical modulation of the locus coeruleus: contribution to CO2drive to breathe

The locus coeruleus (LC) is a dorsal pontine region, situated bilaterally on the floor of the fourth ventricle. It is considered to be the major source of noradrenergic innervation in the brain. These neurons are highly sensitive to CO₂/pH, and chemical lesions of LC neurons largely attenuate the hypercapnic ventilatory response in unanesthetized adult rats. Developmental dysfunctions in these neurons are linked to pathological conditions such as Rett and sudden infant death syndromes, which can impair the control of the cardio-respiratory system. LC is densely innervated by fibers that contain glutamate, serotonin, and adenosine triphosphate, and these neurotransmitters strongly affect LC activity, including central chemoreflexes. Aside from neurochemical modulation, LC neurons are also strongly electrically coupled, specifically through gap junctions, which play a role in the CO₂ ventilatory response. This article reviews the available data on the role of chemical and electrical neuromodulation of the LC in the control of ventilation.

Physiology of myometrial function: intercellular coupling and its role in uterine contractility

The mammalian uterus is composed of a preponderance of small smooth muscle cells usually aligned in two layers. The number of muscle cells in the human uterus at term is estimated at 200 billion, each minute fusiform cell measuring about 5–10μm in diameter and about 200μm in length. The main function of the uterus is to harbour the developing fetus during pregnancy and then to contract vigorously during labour to expel the products of conception. In order for the uterus to contract rhythmically and forcefully, a mechanism must exist to allow interaction between muscle cells in order to achieve synchronous activity. Phasic or cyclical patterns of contractile activity of the uterus cannot be accounted for by stimulation or inhibition from the nervous or endocrine systems. Since myometrial cells are dependent upon action potentials for their contractile processes, some system must be present between the muscle cells for the propagation of action potentials between them. The observation that gap junctions occur in large numbers between myometrial cells during parturition is thought to be significant in this regard and they are considered to play an essential role in parturition and in the control and co-ordination of uterine contractility. In this brief review, we will discuss the role of gap junctions in the modulation of myometrial contractility and the mechanisms that regulate their synthesis and permeability.

Origin of segmentation in the human structure

Crystallographic analysis of biological and non-biological minerals does not reveal any significant differences between the two, which is indicative of common crystallization processes. It can be supposed that the human organism is a biocrystalloid in a sense that it is regarded both at the level of the whole organism and individual cells as a composite entity consisting of a crystal-like structure and pericrystalline medium. A similarity can be found between the growing layer of a crystal in the crystal-forming medium and a cell structure with liquid washing it. A mineral organism therefore can be regarded as the active superficial part of a crystal taken together with pericrystalline crystal-forming medium which controls crystal growth and modifies the system depending on the structure of the growing system. Aggregation is one of the fundamental features of minerals as they are found primarily not only as separate objects but also as aggregates, i.e. regular cohesive masses or synmineralogical systems. Ability to aggregation in an orderly way is expressed as self-organization. This feature is inherent not only to compound molecules but also to associates of a higher order. The cell biology has shown that when similar cells touch each other they tend to cohere forming aggregates characteristic of the given cell population. Similar live systems and their components that perform the same function have an ability to integrate and form firstly a conglomerate (colony) and then an organism. Integration explains association of multi-segmented entities into a single organism and the resulting structure would consists of the two groups of segments, i.e. appearance of an organism consisting of two different but of the same type specimens, each of which had different number of segments. Phylogenetically, an early precursor of the man evolved from a simple cell into an integrated multi-segment organism through several stages--initially a simple cell, then a cell colony, then a single-segment organism, then an organism like a concave ball, then a colony of one-segment multicellular organisms and finally a multi-segment multicellular organism. Integration of five- and eight-segment organisms resulted in the formation of a 13-segment precursor organism of man. Segmentation is nothing else but traces of boundaries left following integration of separate multicellular non-segmented specimens with gradual fading of their differences and formation of a new entity in compliance with formation laws.

Modulation of Connexin Expression in Sheep Endometrium in Response to Pregnancy

The expression pattern of two typical gap junction channel proteins, connexin 43 and connexin 26 (Cx43 and Cx26), was identified in the endometrium of sheep, a species with epitheliochorial type of implantation, by indirect immunohistochemistry during the cyclic phases, early and late pregnancy, and immediately after birth. The extent of Cx43 immunoreaction bound to endometrial stromal cells of the early implantation stage (day 15 p.c.) was comparable to the situation observed in oestrus. The subsequent intensification of feto-maternal contact correlated with a striking increase of stromal Cx43 in the intercaruncular and caruncular regions of the uterus (days 18 and 21 p.c.) and the induction of Cx26 in the glandular epithelium of late implantation (day 21 p.c.). In contrast, both gap junction proteins, coexpressed in the stroma of placentomes and interplacentomal sections on days 131 and 145 p.c., decreased during late pregnancy, while an intense and augmenting staining for Cx26 was detected at the cell borders of the glandular and luminal epithelium. The spatial and temporal distribution of both connexins suggests that, under embryonal and hormonal influences, gap junctional communication is involved in the implantation process and the regulation of endometrial tissue functions during sheep pregnancy and indicates further, that this connexin expression path resembles more the invasive type of implantation.

The Caenorhabditis elegans ortholog of human PHF5a shows a muscle-specific expression domain and is essential for C. elegans morphogenetic development

We have recently described a novel human and murine multigene-family that is highly conserved during evolution and shows a PHD-finger-like domain present in the deduced protein sequences. Here, we describe the cloning and characterization of the Caenorhabditis elegans ortholog of human PHF5a. Transgenic phf-5::yfp-reporter techniques in C. elegans identified temporal C. elegans phf-5 expression being restricted to late C. elegans development. The phf-5::yfp expression starts within the morphogenetic phase of embryonic development and lasts to the stage of adult worms. Spatial phf-5 expression is muscle-specific with an expression in the developing pharynx, in body wall muscular structures, and in the anal muscles. By phf-5 RNAi we further demonstrated that PHF-5 is essential in the morphogenetic phase of C. elegans embryonic development as well as in young larvae. In contrast, phf-5 RNAi does not show an evident phenotype to adult worms. Taken together, this is the first report providing evidence for a tissue and stage-specific expression of a PHF5a ortholog, named phf-5, in C. elegans while our data further suggest an essential role of the encoded PHF-5 protein in morphogenetic development and muscle function.

Gap junctions, homeostasis, and injury

Gap junctions (Gj) play an important role in the communication between cells of many tissues. They are composed of channels that permit the passage of ions and low molecular weight metabolites between adjacent cells, without exposure to the extracellular environment. These pathways are formed by the interaction between two hemichannels on the surface of opposing cells. These hemichannels are formed by the association of six identical subunits, named connexins (Cx), which are integral membrane proteins. Cell coupling via Gj is dependent on the specific pattern of Cx gene expression. This pattern of gene expression is altered during several pathological conditions resulting in changes of cell coupling. The regulation of Cx gene expression is affected at different levels from transcription to post translational processes during injury. In addition, Gj cellular communication is regulated by gating mechanisms. The alteration of Gj communication during injury could be rationalized by two opposite theories. One hypothesis proposes that the alteration of Gj communication attenuates the spread of toxic metabolites from the injured area to healthy organ regions. The alternative proposition is that a reduction of cellular communication reduces the loss of important cellular metabolisms, such as ATP and glucose.

Gene therapy of established medullary thyroid carcinoma with herpes simplex viral thymidine kinase in a rat tumor model: relationship of bystander effect and antitumor efficacy

Bystander effect (BSE) refers to killing of cells adjacent to a cell engineered to express a killing gene segment. BSE is considered an important aspect of suicide gene therapy with thymidine kinase. We evaluated the BSE of adenovirus expressing herpes simplex thymidine kinase (AdCMVtk) in rat medullary thyroid carcinomas (rMTC) and three rat thyroid epithelial cancer cell lines using an in vitro BSE assay. In the assay, different proportions of infected and uninfected cells are mixed. Only the proportion of directly infected cells was inhibited in the proliferation assay using rMTC cells. This indicates that there is little BSE in this cell line. One rat thyroid epithelial cancer cell line (RTC-R2) has a high BSE, with BSE index (BSEi) of 7. In the proliferation assay a greater proportion of cells was inhibited than those directly infected. BSE was also evaluated during in vivo tumor growth by subcutaneous injection of mixtures of AdCMVtk infected and uninfected cells. Ganciclovir (GCV) treatment of tumors developing from a 1:1 mixture of infected to uninfected rMTC cells failed to inhibit their growth. In contrast, GCV treatment of a 2:8 mixture of infected to uninfected RTC-R2 cells completely inhibit tumor development, indicating a high BSE. BSE is related to in vivo antitumor efficacy when replication-defective adenovirus AdCMVtk is directly injected into rMTC tumors. After treatment with 100 mg/kg per day of GCV, a growth-retardation effect was observed in small tumors (<100 mm3), but there was little antitumor activity in large tumors (>100 mm3). Our results indicate that there is a good correlation between this in vitro BSE assay and in vivo treatment efficacy. Not all kinds of tumors are suitable for thymidine kinase (TK)/GCV gene therapy because some lack BSE. Methods to improve BSE and/or transduction efficiency are needed in order to obtain an effective therapeutic result. It will be appropriate to test the BSE in human tumor cells before performing clinical trials with current adenoviral vectors expressing TK.

Cx32 mRNA in rat liver: effects of inflammation on poly(A) tail distribution and mRNA degradation

Previous studies showed that the expression of connexin 32 (Cx32), the polypeptide subunit component of the major hepatic gap junction, is reduced in liver by changes in mRNA stability during bacterial lipopolysaccharide (LPS)-induced inflammation. In this study, we examined the distribution of Cx32 mRNA poly(A) tail lengths during LPS-induced inflammation, because this is considered the first step in the degradation of many mRNAs. During LPS treatment the first detectable change in Cx32 mRNA was a gradual shortening of its poly(A) tail, which reached a final size of approximately 20 nucleotides. However, the poly(A) tail did not disappear entirely before the bulk of Cx32 mRNA was degraded. Treatment with actinomycin D, which blocks the degradation of Cx32 mRNA after LPS administration, resulted in the appearance of a completely deadenylated mRNA, which otherwise could not be detected. On the contrary, treatment with cycloheximide resulted in a decrease in the stability of Cx32 mRNA without an apparent change of the poly(A) tail size. The effect of cycloheximide on Cx32 mRNA stability seems to be due indirectly to the induction of an inflammatory response by this drug. These results suggest that, similar for many mRNAs, shortening of the poly(A) tail is one of the first steps in the degradation of Cx32 mRNA during inflammation.

High efficiency site-specific modification of the chromosomal immunoglobulin locus by gene targeting

Site-specific modification of the chromosomal immunoglobulin locus by gene targeting is a powerful tool in studying the molecular requirements for immunoglobulin gene structure and function and in the production of engineered antibodies. Here, we describe a two step- integration then excision-gene targeting procedure for introducing planned genetic alterations into the chromosomal immunoglobulin locus. The efficiency of gene targeting with an enhancer-trap vector in which an enhancerless neo and HSV-tk gene were inserted into the vector backbone was compared to that of the corresponding enhancer-positive vector. Both insertion vectors also contained homology to the chromosomal immunoglobulin target locus along with the desired genetic alteration. The first step involved insertion of the transferred vector into the target locus by homologous recombination. An approximately 15-fold enrichment in the frequency of vector insertion was obtained with the enhancer-trap compared to the enhancer-positive vector. The majority of targeted cells (75%) contained a single copy of the vector integrated into the chromosomal immunoglobulin locus. The second step involved excision of the integrated vector by intrachromosomal homologous recombination between the duplicated region of homology that removed the integrated vector, neo and tk genes along with one copy of homologous DNA. Vector excision was very efficient generating G418S, FIAU(R) secondary recombinants at the high rate of approximately 10(-3)/cell generation. In the secondary recombinants, the overall structure of the chromosomal immunoglobulin locus was restored with the desired genetic alteration being present in an expected proportion of the cells.

Differential decrease in connexin 32 expression in ischemic and nonischemic regions of rat liver during ischemia/reperfusion

The effect of a localized hepatic injury, regional ischemia/reperfusion, on the expression of connexin 32 (Cx32) was studied. Cx32 is the component of the major hepatic gap junction. Two regions of the injured liver were analyzed: the area directly affected by the ischemic insult (ischemic liver), and the remainder of the organ (nonischemic liver). In the ischemic liver, there were simultaneous reductions in Cx32 mRNA steady-state levels and the encoding polypeptide from the plasma membrane within 1 h of reperfusion. In contrast, Cx32 mRNA steady-state levels were only reduced after 4 h of reperfusion in the nonischemic liver. This reduction of Cx32 mRNA levels was followed by the disappearance of Cx32 on the plasma membrane within 24 h of the insult. Administration of actinomycin D prior to the ischemic insult prevented the reduction in Cx32 mRNA in both ischemic and nonischemic liver regions. Protein synthesis was blocked during the first hour of reperfusion in the ischemic liver but not in the nonischemic liver. To mimic this effect, animals were treated with cycloheximide in absence of the ischemic insult. A reduction in Cx32 mRNA and polypeptide in the liver was observed in cycloheximide treated animals. This finding suggests that the decrease in Cx32 expression in the ischemic, but not in the nonischemic, liver may be due to the inhibition of protein synthesis during ischemia/reperfusion. These observations suggest that an ischemic insult produces a selective deteriorating effect on Cx32 expression in both ischemic and nonischemic liver regions probably through different mechanisms.

Three-dimensional structure of the gap junction connexon

The gap junction membrane channel is composed of macular aggregations of intercellular channels permitting the direct intercellular transfer of ions and small molecules. Each intercellular channel is formed by the apposition of two hexameric transmembrane channels (connexons), one from each cell. The interlocking of the two channels occurs extracellularly in a narrow 2.5-nm "gap" separating the junctional membranes. The channel-channel interaction is known to be selective between members of the family of proteins, called connexins, which oligomerize into the connexons. In addition to selectivity, the molecular interfaces involved in the extracellular interactions between connexons must be very congruent, since the intercellular channel must provide high resistances to the leakage of small ions between the channel lumen and the extracellular space. By using a recently developed biochemical procedure for obtaining ordered arrays of connexons from gap junctions split in the extracellular gap, (Ghoshroy, S., D. A. Goodenough, and G. E. Sosinsky. 1994. Preparation, characterization, and structure of half gap junctional layers spit with urea and EGTA. J. Membr. Biol. 146:15-28) a three-dimensional reconstruction of a connexon has been obtained by electron crystallographic methods. This reconstruction emphasizes the structural asymmetry between the extracellular and cytoplasmic domains and assigns lobed structural features to the extracellular domains of the connexon. The implication of our hemichannel structure is discussed in relation to the in vivo state of unpaired connexons, which have been shown to exist in the plasma membrane.

Mixing of connexins in gap junction membrane channels

Gap junctions are plaque-like clusters of intercellular channels that mediate intercellular communication. Each of two adjoining cells contains a connexon unit which makes up half of the whole channel. Gap junction channels are formed from a multigene family of proteins called connexins, and different connexins may be coexpressed by a single cell type and found within the same plaque. Rodent gap junctions contain two proteins, connexins 32 and 26. Use of a scanning transmission electron microscope for mass analysis of rodent gap junction plaques and split gap junctions prvided evidence consistent with a model in which the channels may be made from (i) solely connexin 26, (ii) solely connexin 32, or (iii) mixtures of connexin 26 and connexin 32 in which the two connexons are made entirely of connexin 26 and connexin 32. The different types of channels segregate into distinct domains, implying tha connexon channels self-associate to give a non-random distribution within tissues. Since each connexin confers distinct physiological properties on its membrane channels, these results imply that the physiological properties of channels can be tailored by mixing the constituent proteins within these macromolecular structures.

Mechanisms of resistance to combinations of vincristine, etoposide and doxorubicin in Chinese hamster ovary cells

We have isolated from Chinese hamster ovary cells, 30 sublines resistant to vincristine, doxorubicin or etoposide and 43 sublines evading treatment with a pair of these drugs. Isolated in one step and under low selective pressure, sublines were 3- to 25-fold more resistant to their selecting drug(s) than the parental cells. Possible P-glycoprotein-associated multidrug resistance was investigated through pgp gene copy number and mRNA expression level. DNA topoisomerase II alteration was evaluated from the ability of nuclear extracts to form cleavable complexes. Vincristine (all sublines) and doxorubicin (6/7 sublines) preferentially selected for pgp gene amplification and mRNA overexpression, whereas selection with etoposide resulted in a decrease of cleavable complex formation in 11 out of 13 sublines. A common pgp gene-mediated resistance was found in the 13 doxorubicin plus vincristine-selected sublines, whereas all but one of the 12 etoposide plus vincristine-resistant sublines displayed both pgp mRNA overexpression and decreased ability to form cleavable complexes. Among the 18 doxorubicin plus etoposide selected sublines, five exhibited a decreased ability to form cleavable complexes only, six exhibited pgp mRNA overexpression only and six exhibited both alterations. Overall, drug resistance could not be attributed to either mechanism in three of the 73 sublines. We conclude that under low selective pressure it is possible to find a combination of drugs which require simultaneous selection of more than one resistance mechanism; such cells emerge with very low frequency.

Connexin43 in rat pituitary: localization at pituicyte and stellate cell gap junctions and within gonadotrophs

Immunohistochemical methods were employed to investigate the cellular and ultrastructural localization of the gap junction protein connexin43 (Cx43) in rat pituitary. Western blots of pituitary homogenates probed with anti-Cx43 antibodies showed the presence of Cx43 in both anterior and posterior pituitary lobes. By light microscopy (LM), Cx43-immunoreactive (Cx43-IR) puncta were found in all areas of the posterior lobe, but at greater concentrations in peripheral regions of this structure. By electron microscopy (EM), immunogold labelling for Cx43 was seen at gap junctions between thin cytoplasmic processes of pituicytes. No immunoreactivity was detected in the intermediate lobe. The anterior lobe contained puncta similar to but more sparsely scattered than those in the posterior lobe, and by EM analysis these were demonstrated to correspond to labelled gap junctions between stellate cells. In addition, anti-Cx43 antibodies produced intracellular labelling in a small percentage of endocrine cells, which were distributed throughout the anterior lobe and determined by double immunostaining methods to be cells containing luteinizing hormone. By EM, labelling within these cells was associated with predominantly large secretory granules and other loosely organized organelles. The results indicate that gap junctions in the pituitary are composed of Cx43 and that this or a related protein may have a novel intracellular function within gonadotrophs.

Postnatal development of the rat portal vein: correlation with occurrence of peptidergic innervation

The portal vein of the rat is immature at birth, and is composed of an endothelium surrounded by undifferentiated cells of mesenchymal origin. Three days after birth, these cells have begun to differentiate and aggregate around the lumen to form two separate layers of perpendicularly oriented myoblasts, while a rich calcitonin gene-related peptide (CGRP) innervation is present around the vessel. In the internal circular muscle layer of the media myofibrils first develop on the endothelial side of the myoblasts, and then progressively reach the other side. In the longitudinal muscular layer of the media, which is separated from the circular layer by a connective lamina as early as 3 days after birth, myofibrils develop randomly in the cells. At the time of the enlargement of the longitudinal layer, long close contacts and intermediate junctions between external myoblasts and adventitial fibroblast-like cells were noted, suggesting that recruitment of this cell type is necessary for the maturation of the vessel wall. At about 28 days, the vein has reached its final structure and the smooth muscle cells are fully differentiated. The dense CGRP perivascular innervation already present at birth persists for the first 14 days of postnatal life when most of the cells have not yet acquired their complete contractile differentiation and are still capable of division. This innervation decreases transiently between 15-17 days, when the vessel acquires its spontaneous contractile activity, then rises to a peak between 20 and 25 days, and falls again. CGRP innervation, which is very scarce at 28 days, slowly increases during the peripubescent stage, by which time the adult structure of the vessel is established. Similar fluctuations in the density of peptidergic innervation were observed for substance P and neuropeptide Y, although these peptides were not yet present at birth and occurred only after 5 days. Vasoactive intestinal polypeptide- and bombesin-immunoreactive fibres were not found at any stage investigated. In addition to a description of the different cell-to-cell contacts which could play a role in the maturation of the vessel wall, we discuss the possible implication of the different peptides in the differentiation, maturation or maintenance of the vessel wall.

Intercellular communication in smooth muscle

The functioning of a group of cells as a tissue depends on intercellular communication; an example is the spread of action potentials through intestinal tissue resulting in synchronized contraction. Recent evidence for cell heterogeneity within smooth muscle tissues has renewed research into cell coupling. Electrical coupling is essential for propagation of action potentials in gastrointestinal smooth muscle. Metabolic coupling may be involved in generation of pacemaker activity. This review deals with the role of cell coupling in tissue function and some of the issues discussed are the relationship between electrical synchronization and gap junctions, metabolic coupling, and the role of interstitial cells of Cajal in coupling.

Image analysis of gap junction structures

Isolated gap junction plaques contain hexagonal crystalline arrays of membrane channels called connexons which are a suitable specimen for electron crystallography. Image analysis of gap junction lattices has shown that while there is sufficient lattice order for structural analysis to approximately 25 A, there is enough disorder in both the lattice and the connexon to create a family of related images. This review is focused on how these images can be interpreted in terms of what is known about both the connexon and its constituent protein, connexin.

Gap-junction quantification in biological tissues: freeze-fracture replicas versus thin sections

The relative efficiency of freeze-fracture replicas versus thin sections for the visualization and quantification of gap junctions in biological tissues has been evaluated. Both methods may underestimate gap-junction number--thin sections for reasons of tissue resolution and freeze-fracture replicas due to the mechanics of the fracturing process. Freeze-fracture misses gap junctions in regions of plasma membrane which are highly contoured, such as the overlapping basal cell processes of Drosophila imaginal wing discs and the interdigitating lateral membrane plications of intercalated discs in cardiac tissue. If the missed gap junctions are relatively large, as they are in both of these examples, freeze-fracture significantly underestimates the total gap-junctional area. Thin sections may miss small gap junctions, but in tissues which contain a range of gap-junction sizes the lost junctions constitute a relatively small fraction of the total junctional area. In neoplastic imaginal wing discs, thin sections were as efficient as freeze-fracture replicas in identifying even the smallest gap junctions. Although freeze-fracture may be the better technique for the qualitative and quantitative documentation of small gap junctions in tissues with relatively flat to gently contoured plasma membranes and thin sections may be the superior method for gap-junction quantification in tissues containing a range of gap-junctional sizes and highly contoured cellular processes, the data suggest that a combination of the two approaches should be utilized whenever possible.

Testing an "in-out" targeting procedure for making subtle genomic modifications in mouse embryonic stem cells

We have introduced a 4-bp insertion into the hypoxanthine phosphoribosyltransferase (HPRT) gene of a mouse embryonic stem (ES) cell line by using an "in-out" targeting procedure. During the in step, a homologous integration reaction, we targeted a correcting plasmid to a partially deleted hprt- locus by using an integrating vector that carried a 4-bp insertion in the region of DNA homologous to the target locus. HPRT+ recombinants were isolated by direct selection in hypoxanthine-aminopterin-thymidine (HAT) medium. The HATr cell lines were then grown in medium containing 6-thioguanine (6-TG) to select for hprt- revertants resulting from the excision of the integrated vector sequences. The revertants were examined by Southern blot hybridization to determine the accuracy of this out reaction and the frequency of retaining the 4-bp modification in the genome. Of the 6-TGr colonies examined, 88% had accurately excised the integrated vector sequences; 19 of 20 accurate revertants retained the 4-bp insertion in the resulting hprt- gene. We suggest a scheme for making the in-out targeting procedure generally useful to modify the mammalian genome.

Testing an "in-out" targeting procedure for making subtle genomic modifications in mouse embryonic stem cells

We have introduced a 4-bp insertion into the hypoxanthine phosphoribosyltransferase (HPRT) gene of a mouse embryonic stem (ES) cell line by using an "in-out" targeting procedure. During the in step, a homologous integration reaction, we targeted a correcting plasmid to a partially deleted hprt- locus by using an integrating vector that carried a 4-bp insertion in the region of DNA homologous to the target locus. HPRT+ recombinants were isolated by direct selection in hypoxanthine-aminopterin-thymidine (HAT) medium. The HATr cell lines were then grown in medium containing 6-thioguanine (6-TG) to select for hprt- revertants resulting from the excision of the integrated vector sequences. The revertants were examined by Southern blot hybridization to determine the accuracy of this out reaction and the frequency of retaining the 4-bp modification in the genome. Of the 6-TGr colonies examined, 88% had accurately excised the integrated vector sequences; 19 of 20 accurate revertants retained the 4-bp insertion in the resulting hprt- gene. We suggest a scheme for making the in-out targeting procedure generally useful to modify the mammalian genome.

Effect of hyperlipidemic serum and irradiation on wound healing in primary quiescent cultures of vascular cells

After 8 weeks in culture, outgrowths from explants of aortic media of rhesus monkeys and New Zealand rabbits result in circular colonies of mature smooth muscle cells, quiescent in 10% serum. Such cultures were wounded by cutting out a 1.5-mm-wide strip. Migration of cells into the wound area was measured daily, and proliferation was assessed by [3H]thymidine incorporation. Migration began within 24 hr and at 7 days the defect was filled by proliferates of migrated cells. The cumulative labeling index was highest in the cells in the wound gap but was also increased in the remaining part of the culture. Wounding thus stimulated the uninjured portion of these primary cultures to proliferate, while in subcultures of these cells increase in [3H]thymidine incorporation was confined to the wound area. While hyperlipidemic serum has been shown to induce proliferation in unwounded cultures, it did not enhance cell replication elicited by wounding but reduced cell density and labeling index in the wound gap. Irradiation prior to wounding reduced cell proliferation to control values, while migration of cells was not significantly affected. In irradiated cultures, the inhibitory action of hyperlipidemic serum on cell migration became evident. Such quiescent cultures thus allow us to separate the effects of a specific injury on the proliferative and migratory responses of vascular smooth muscle.

Modulation of gap junctions in the myometrium of pigs during early gestation, induced abortion and the oestrous cycle

Myometrial samples were biopsied from pigs at early pregnancy, within 12 h after initiated abortion and at first dioestrus post-abortion. Abortion was induced either with an isolated gram-negative bacterial endotoxin or the prostaglandin (PG) F2 alpha analog cloprostenol. The tissues were quantitatively examined for gap junctions in electron micrographs. Very small numbers of gap junctions were present between smooth muscle cells in myometria at 24-27 days of pregnancy. The cell-to-cell junctions increased both in number and size at abortion, irrespective of the agent used as abortifacient. A low number of gap junctions were recorded in samples obtained after reinitiation of the oestrous cycle. The plasma levels of oestradiol-17 beta were low at the three biopsy procedures while the increased numbers of gap junctions at abortion occurred when significantly high levels of PGF2 alpha concomitantly with withdrawal of progesterone were determined in circulating blood plasma. It remains therefore unclear, which role do oestrogens play in the synthesis of gap junctions at induced abortion.

Promoting effect of polybrominated biphenyls on tracheal papillomas in Syrian golden hamsters

Male Syrian golden hamsters were used for an initiation-promotion study of respiratory tract carcinogenesis. Hamsters were given a single subcutaneous dose of 0 or 80 mg N-nitrosodiethylamine (NDEA)/kg body weight and were fed diets containing 0 or 100 mg polybrominated biphenyls (PBB)/kg diet for 140 d. Basal diet was fed from d 140 until the end of the experiment on d 273. The number of tracheal papillomas was significantly increased in hamsters given NDEA and PBB as compared to those in hamsters given only NDEA. Tracheal papillomas were not seen in untreated hamsters or in those not given NDEA and fed diets containing PBB. Nasal tumors occurred at approximately the same incidence in hamsters given NDEA as in those given NDEA and PBB. Results indicate that PBB may promote the development of tracheal papillomas in the hamster.

Contact relationships between vascular smooth muscle cells: an in vivo and in vitro study

Cell/cell contacts in the intact media of the rat thoracic aorta and inferior vena cava, and in cultured smooth muscle cells from these vessels have been quantified in order to determine whether their density and their type can be correlated with the differences in load at these two sites. The type and number of contacts per 100 microns of cell perimeter and per 100 cells were determined with a semi-automatic image analysis system. In all cases there was a predominance of simple appositions; intermediate junctions, interdigitations, and nexus junctions were less frequently observed. There were more cell contacts of each type in the intact arteries than in the intact veins. In cell cultures, all types of cell contacts were increased but the proportion of load bearing types was reduced.

An ultrastructural study of human epithelial rests of Malassez maintained in a differentiated state in vitro

Healthy human periodontal ligaments (PDL), obtained from the extracted teeth (premolars and third molars), were cultivated for 1-35 days, using a multi-purposes culture chamber (MPCC) equipped with various transparent membranes. The resting state of the epithelial rests of Malassez (ERM), similar to their in vivo counterparts, appeared as small islands or strands with scant cytoplasm containing poorly developed organelles. This state was most effectively maintained in MPCC with a cellophane sheet. MPCC with a Sartorius membrane filter permitted proliferation and emigration of ERM. Proliferating ERM were characterized by more profiles of rough endoplasmic reticulum and free ribosomes, new formation of actin-containing microfilaments, less prominent tonofilaments and desmosomes and loss of gap junctions. Most of these ultrastructural changes are manifested in epithelial cells during wound healing. The emigrating ERM from PDL explants, as well as occasional proliferating ERM within explants, consisted of two cell types--outer basal-like cells, as described above, and inner tonofilament-rich prickle-like cells, suggesting a propensity for differentiation of ERM. The results show the possibility of controlling the growth and differentiation of ERM through the MPCC culture environment.

Intercellular junctions in the gill epithelium of the Atlantic hagfish, Myxine glutinosa

The intramembrane organization of the occluding junctions in the gill epithelium of the Atlantic hagfish, Myxine glutinosa, was studied by means of freeze-fracture electron microscopy. Mitochondria-rich cells, characterized by assemblies of rod-shaped particles in the luminal plasma membrane and by an extensive intracellular amplification of the basolateral plasma membrane, are singly distributed between the pavement cells in the gill epithelium of this marine and stenohaline cyclostome. The occluding junctions between mitochondria-rich cells and pavement cells do not differ from those between adjacent pavement cells, concerning the number of superimposed strands (median 6, range 4-9) and their geometrical organization. These observations suggest that, in contrast to marine teleosts, the paracellular pathway plays a minor role in transepithelial ion movements in the hagfish gill epithelium. The findings are in agreement with the absence of hypoosmoregulatory mechanisms in hagfish, as have been evolved in various marine vertebrates. In addition, small communicating junctions are demonstrated between pavement cells; they possibly serve for a coordinated synthesis and secretion of mucus by the pavement cells.

Roles of cell junctions in gametogenesis and in early embryonic development

Recent reviews of the role of cell junctions in development have focused primarily upon functions related to the relatively subtle physiological modulation of their subunits in relation to fundamental developmental processes in a wide variety of organisms. There is, however, considerable support from numerous laboratories that the more radical modulation of the presence and number of junctional subunits in many diverse tissues may play a pivotal role in a wide spectrum of developmental phenomena ranging from gametogenesis to organogenesis. Since a great deal of recent interest in this latter subject has concentrated upon vertebrate systems including mammals, this review will examine the functional significance of the modulation of gap junctions, tight junctions and desmosomes in a developing idealized mammalian system from gamete formation to tissue and organ differentiation during embryo-genesis.

Intercellular contacts in tumours of the vascular smooth muscle cells in man

Electron microscopy has been used to study the cell to cell and cell to stroma contacts between smooth muscle cells (SMC) in normal vessels, angiomyolipoma and well and poorly differentiated vascular leiomyosarcoma. Micrographs were examined with a semiautomatic image analysis system. The length of the cell borders was calculated and the type and number of contacts per 100 micron cell perimeter and per 100 cells were determined. In all cases there was a predominance of simple appositions. Intermediate junctions, nexus junctions, interdigitations, intermediate contacts and junctional interdigitations were less frequently observed. In general, as the SMC become malignant and less differentiated the number of cell to stroma attachments decreases markedly and the intercellular contacts increase slightly.

Sister chromatid exchange in vivo, chromosomal characterization and NORs activity of leukemia cells during 5FU-treatments

A transplantable mouse leukemia model, the leukemia cell of which has a marker chromosome and the XX genome type which differ obviously from their male host cells provides a possibility to precisely identify the leukemia cells among their male host cells cytogenetically. A sister chromatid exchange (SCE) plus chromosomal C-banding technique that we report here is very useful. The SCE frequencies in vivo of both leukemia cells and host cells were twice as high as the normal mouse cells. The higher SCE frequencies of the host cells in the leukemia mice may be due to some toxicities from the leukemia cells or some biological large molecule exchanges between the leukemia cells and the host cells. There was no significant difference in SCE frequencies between cells from the spleen and from the bone marrow of the leukemia mice. The percentages of leukemia cells in both spleen and bone marrow were more than 90% when the mice had been injected with the leukemia cells for five days. The host cells in the leukemia mice did not become leukemia cells. The 5FU-treated leukemia mice survived very well for more than twenty-three days. After the 5FU-treatments, most of the leukemia cells died, subsequently, SCE frequencies decreased to a normal level. Both the number of Ag-NORs per cell and the number of chromosomes bearing Ag-NORs per cell in the leukemia mice decreased to 60% and 40%, respectively, of the level found in normal mouse cells.

Metabolic cooperation in aggregates of embryonal carcinoma cells

Metabolic cooperation may be associated with the processes of compaction and subsequent differentiation in aggregates of embryonal carcinoma cells (ECC). To determine if the gap junctions present in loose and compacted aggregates of H6 ECC are active in metabolic cooperation, aggregates of each type containing a mixture of 5-bromodeoxyuridine- and 6-thioguanine-resistant H6 cells were exposed to HAT medium, 6-thioguanine, or [3H]thymidine. These three methods indicated that some crossfeeding occurred through the small clusters of gap junctions in loose aggregates and more crossfeeding occurred through the larger clusters of gap junctions in compacted aggregates.

Restoration of LDL receptor activity in mutant cells by intercellular junctional communication

Exchange of small molecules between cells through intercellular junctions is a widespread phenomenon implicated in many physiological and developmental processes. This type of intercellular communication can restore the activity of low-density lipoprotein (LDL) receptors in mammalian cells that are deficient in the enzyme UDP-Gal/UDP-GalNAc 4-epimerase. Pure cultures of the 4-epimerase mutant are unable to synthesize normal carbohydrate chains on LDL receptors and many other glycoproteins and therefore do not express LDL receptor activity. When these cells are cocultivated with cells expressing normal 4-epimerase activity, the structure and function of LDL receptors are restored to normal by the transfer of this enzyme's products through intercellular junctions. The formation of functional junctions does not require normal glycosylation of membrane proteins. Because many convenient assays and selections for LDL receptor activity are available, this mutant can provide a powerful new tool for biochemical and genetic studies of intercellular junctional communication.

Clones defective in metabolic cooperation selected from a pluripotent feeder-dependent mouse embryonal carcinoma cell line

Starting from embryonal carcinoma (e.c.) cells capable of extensive differentiation in culture, the technique of thioguanine 'kiss of death' has been used to select four independent metabolic cooperation-defective variants. The communication ability of these variant cell lines has been quantified by autoradiographic measurement of the transfer of uridine nucleotides, and also by an assay of the extent of junction-mediated rescue from ouabain toxicity by resistant fibroblasts. The cell lines which are defective in ability to transfer nucleotides, as measured by the uridine nucleotide transfer assay, are also defective in their ability to differentiate into endoderm and to form the cavitated 'embryoid bodies' which are produced by the parental cell line when grown in suspension culture. However, it is not clear whether this is related to the defects in metabolic cooperation, since clones which had been subjected to the same selective conditions but which cooperate normally have also lost some of the capacity to undergo this differentiation. Endoderm differentiation was classified into two categories, one being visceral endoderm and the other, primary plus parietal endoderm, on the basis of morphology, immunocytochemical staining for alpha-fetoprotein, and basement membrane formation. With the exception of correlations arising from variations between experiments and differences between cell lines, there is no statistical association between these two categories of differentiation. The formation of cavities was observed only in embryoid bodies with endoderm differentiation: the present of either category was a sufficient condition for cavitation to occur.

Selection of a mouse embryonal carcinoma clone resistant to the inhibition of metabolic cooperation by retinoic acid

A novel selection for cell lines resistant to the blocking of metabolic cooperation by chemical inhibitors is described. The selection has been used to isolate a pluripotent embryonal carcinoma clone which cooperates in retinoic acid (RA), an inhibitor of junctional communication in all other cell lines tested to date. The selected cells require higher levels of RA to induce differentiation, but show little or no increase in resistance to toxic effects of RA.

Influence of both coculture and BrdU on NOR activity of mouse, rat and human cells

The coculture of mouse PG19 cells with human MGC cells can significantly suppress nucleolar organizer region (NORs) activity of both PG19 and MGC cells. 5'-bormodeoxyuridine (BrdU) can also significantly suppress the NOR activity of rat RC cells, human MGC and Hela cells, and mouse PG19 cells: i.e. the average number of Ag-NORs and the number of chromosomes bearing Ag-NORs per cell decrease significantly. The degree of the suppression increases with increase in both BrdU concentration in the culture medium and BrdU treatment time. The suppressed NOR activity of the PG19 cells can gradually be restored when the BrdU-treated cells are transferred into BrdU-free medium for 50 h. In PG19 cells deoxycytidine (dC) can reverse the suppression of NOR activity caused by BrdU. Coculture plus BrdU treatment suppress the NOR activity of PG19 cells more severely than BrdU treatment alone. In coculture medium containing 30 μg BrdU/ml, dC can also reverse the suppression of the NOR activity of PG19 cells but not that of the MGC cells. The degree of the reversion in the coculture plus BrdU treatment is significantly lower than that found with BrdU-treatment alone.

Cooperation between epithelial cells demonstrated by potassium transfer

Junction-mediated communication can be measured in fibroblast cultures by determining the ability of mixed cultures of cells sensitive and resistant to ouabain to concentrate K+ in the presence of ouabain. We now report the extension of this assay procedure to cultured epithelial cells. Hamster kidney (HaK) cells maintain their ability to concentrate K+ in ouabain at levels inhibitory to dog kidney (MDCK) cells. When HaK and MDCK cells were cultured together in ouabain-containing medium, the K+ (measured as 86Rb+) in the mixed population was greater than expected if the cells were not interacting. The degree of enhancement, expressed as "index of cooperation," depended on the numbers of cells in the cultures, their opportunity for cell-to-cell contact, and (above a certain permissive level) the concentration of ouabain. As with other cell types, protein synthesis in MDCK cells depends on maintenance of cell K+. Autoradiography of cells incubated with [3H]leucine demonstrated that MDCK cells in ouabain-treated mixed cultures were able to synthesize proteins only when physically adjacent to HaK cells. The transmission of labeled nucleosides among the cells provides independent evidence of the phenomenon of cooperation, probably mediated by gap junctions. This system offers promise for investigation of stimuli modulating junctional communication.

Effects of phorbol myristate acetate, phorbol dibutyrate, ethanol, dimethylsulfoxide, phenol, and seven metabolites of phenol on metabolic cooperation between Chinese hamster V79 lung fibroblasts

The effect of phorbol myristate acetate, phorbol dibutyrate, ethanol, dimethylsulfoxide, phenol, and seven metabolites of phenol on metabolic cooperation were assessed as a function of mutant cell recovery from populations of cocultivated hypoxanthine-guanine phosphoribosyl transferase-deficient mutant (HGPRT-) and wild-type (HGPRT+) Chinese hamster V79 lung fibroblasts. Phorbol myristate acetate and phorbol dibutyrate, two established tumor promoters, were potent inhibitors of metabolic cooperation. Ethanol and dimethylsulfoxide, solvents commonly used to prepare chemicals for testing, weakly inhibited metabolic cooperation. Phenol and phenylglucuronide had no effect on metabolic cooperation. Four oxidative metabolites (1,4-benzoquinone, catechol, hydroxyquinol and quinol) inhibited metabolic cooperation. Phenylsulfate weakly inhibited metabolic cooperation. Conversely, 2-methoxyphenol, a methylated derivative of catechol, appeared to enhance metabolic cooperation. These results generally support the hypothesis that tumor promoters inhibit metabolic cooperation and illustrate the importance of considering metabolites when testing this hypothesis. The weak capacity of five metabolites of phenol to inhibit metabolic cooperation correlates with the weakness of phenol as a tumor promoter. Interpretation of these results is complicated because two metabolic cooperation-inhibiting metabolites (catechol and quinol) are nonpromoting when tested individually in the same assay where phenol shows promoting activity. Such metabolites may be incomplete (stage) promoters, and exposure to two or more may be required for a promoting effect. The significance of enhanced metabolic cooperation requires further investigation, particularly in relation to antipromoting effects.