Reduced cell-cell communication in experimentally induced autoimmune thyroid disease

Morphological and Functional Colonic Defects Caused by a Mutated Thyroid Hormone Receptor α

Background: Mutations of thyroid hormone receptor α (TRα1) result in resistance to thyroid hormone (RTHα), exhibiting symptoms of retarded growth, delayed bone maturation, anemia, and severe constipation. Using a mouse model of RTHα (Thra1PV/+ mouse), we aimed at understanding the molecular basis underlying the severe constipation observed in patients. Methods: The Thra1PV/+ mouse expresses a strong dominant negative mutant, PV, which has lost T3 binding and transcription activity. Thra1PV/+ mouse faithfully reproduces growth abnormalities and anemia as shown in RTHα patients and therefore is a valid model to examine causes of severe constipation in patients. We used histopathological analysis, confocal fluorescence imaging, transmission electron microscopy (TEM), and gene expression profiles to comprehensively analyze the colonic abnormalities of Thra1PV/+ mouse. Results: We found a significant increase in colonic transit time and decrease stool water content in Thra1PV/+ mouse, mimicking constipation as found in patients. Histopathological analysis showed expanded lamina propria filled with interstitium fluid between crypt columns, enlarged muscularis mucosa, and increased content of collagen in expanded submucosa. The TEM analysis revealed shorter muscle fibers with wider gap junctions between muscle cells, fewer caveolae, and hypoplastic interstitial cells of Cajal (ICC) in the rectal smooth muscles of Thra1PV/+ mice. These abnormal histological manifestations suggested defective intercellular transfer of small molecules, electrolytes, and signals for communication among muscles cells, validated by Lucifer Yellow transferring assays. Expression of key smooth muscle contractility regulators, such as calmodulin, myosin light-chain kinase, and phosphorylated myosin light chain, was markedly lower, and c-KIT signaling in ICC was attenuated, resulting in decreased contractility of the rectal smooth muscles of Thra1PV/+ mice. Collectively, these abnormal histopathological alterations and diminished contractility regulators led to the constipation exhibited in patients. Conclusions: This is the first demonstration that TRα1 mutants could act to cause abnormal rectum smooth muscle organization, defects in intercellular exchange of small molecules, and decreased expression of contractility regulators to weaken the contractility of rectal smooth muscles. These findings provide new insights into the molecular basis underlying constipation found in RTHα patients.

Alternative mechanisms of translation initiation: An emerging dynamic regulator of the proteome in health and disease

Eukaryotic mRNAs were historically thought to rely exclusively on recognition and binding of their 5' cap by initiation factors to effect protein translation. While internal ribosome entry sites (IRESs) are well accepted as necessary for the cap-independent translation of many viral genomes, there is now recognition that eukaryotic mRNAs also undergo non-canonical modes of translation initiation. Recently, high-throughput assays have identified thousands of mammalian transcripts with translation initiation occurring at non-canonical start codons, upstream of and within protein coding regions. In addition to IRES-mediated events, regulatory mechanisms of translation initiation have been described involving alternate 5' cap recognition, mRNA sequence elements, and ribosome selection. These mechanisms ensure translation of specific mRNAs under conditions where cap-dependent translation is shut down and contribute to pathological states including cardiac hypertrophy and cancer. Such global and gene-specific dynamic regulation of translation presents us with an increasing number of novel therapeutic targets. While these newly discovered modes of translation initiation have been largely studied in isolation, it is likely that several act on the same mRNA and exquisite coordination is necessary to maintain 'normal' translation. In this short review, we summarize the current state of knowledge of these alternative mechanisms of eukaryotic protein translation, their contribution to normal and pathological cell biology, and the potential of targeting translation initiation therapeutically in human disease.

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.

Methods for Testing Immunological Factors

Hypersensitivity reactions can be elicited by various factors: either immunologically induced, i.e., allergic reactions to natural or synthetic compounds mediated by IgE, or non-immunologically induced, i.e., activation of mediator release from cells through direct contact, without the induction of, or the mediation through immune responses. Mediators responsible for hypersensitivity reactions are released from mast cells. An important preformed mediator of allergic reactions found in these cells is histamine. Specific allergens or the calcium ionophore 48/80 induce release of histamine from mast cells. The histamine concentration can be determined with the o-phthalaldehyde reaction.

Altered expression of the gap junction protein connexin43 is associated with papillary thyroid carcinomas when compared with other noncancer pathologies of the thyroid

BACKGROUND

Gap junctions are membrane structures composed of connexins (Cx) that allow diffusion of small molecules between cells. They are involved in tissue homeostasis, and various organ dysfunctions have been associated with gap junction defects. To verify their possible involvement in thyroid pathologies, the expression of connexin43 (Cx43), the major Cx in the human thyroid, was evaluated in a variety of diseases including cancer.

METHODS

There were 122 samples from various thyroid pathologies that were collected to analyze the presence of Cx43 by immunofluorescence. Through confocal microscopy, different patterns of Cx43 localization were identified as normal (membrane) or abnormal (cytoplasmic or lack of detection). The analysis of Cx43 expression was further performed by quantitative reverse transcriptase-polymerase chain reaction and immunohistochemistry in a subset of 25 papillary carcinomas and compared with nontumoral thyroid tissues.

RESULTS

The presence of Cx43 was commonly altered in thyroid cancer, as abnormal Cx43 staining was detected in 94.1% of cancer, 47.4% of adenomas, 45.7% of multinodular goiter, 16.7% of Graves' disease, and 25% of thyroiditis. In papillary carcinoma samples, the deregulation of Cx43 expression was mostly the consequence of a decrease of Cx43 mRNA (68% of cases) when compared with normal tissue. When Cx43 mRNA was not downregulated (32% of cases), both loss of membrane staining and aberrant cytoplasmic distribution of the protein were observed.

CONCLUSIONS

These results show that aberrations of Cx43 expression are associated with thyroid papillary carcinoma.

Connexin-dependent signaling in neuro-hormonal systems

The advent of multicellular organisms was accompanied by the development of short- and long-range chemical signalling systems, including those provided by the nervous and endocrine systems. In turn, the cells of these two systems have developed mechanisms for interacting with both adjacent and distant cells. With evolution, such mechanisms have diversified to become integrated in a complex regulatory network, whereby individual endocrine and neuro-endocrine cells sense the state of activity of their neighbors and, accordingly, regulate their own level of functioning. A consistent feature of this network is the expression of connexin-made channels between the (neuro)hormone-producing cells of all endocrine glands and secretory regions of the central nervous system so far investigated in vertebrates. This review summarizes the distribution of connexins in the mammalian (neuro)endocrine systems, and what we know about the participation of these proteins on hormone secretion, the life of the producing cells, and the action of (neuro)hormones on specific targets. The data gathered since the last reviews on the topic are summarized, with particular emphasis on the roles of Cx36 in the function of the insulin-producing beta cells of the endocrine pancreas, and of Cx40 in that of the renin-producing juxta-glomerular epithelioid cells of the kidney cortex. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Involvement of gap junctional communication in secretion

Glands were the first type of tissues in which the permissive role of gap junctions in the cell-to-cell transfer of membrane-impermeant molecules was shown. During the 40 years that have followed this seminal finding, gap junctions have been documented in all types of multicellular secretory systems, whether of the exocrine, endocrine or pheromonal nature. Also, compelling evidence now indicates that gap junction-mediated coupling, and/or the connexin proteins per se, play significant regulatory roles in various aspects of gland functions, ranging from the biosynthesis, storage and release of a variety of secretory products, to the control of the growth and differentiation of secretory cells, and to the regulation of gland morphogenesis. This review summarizes this evidence in the light of recent reports.

Neonatal hypothyroidism alters the localization of gap junctional protein connexin 43 in the testis and messenger RNA levels in the epididymis of the rat

The objectives of this study were to determine the effects of propylthiouracil (PTU)-induced neonatal hypothyroidism on the gap junctional protein Cx43 in rat testis and epididymis. PTU (0.02%) was administered via lactation from birth to Day 30, and the rats were sampled at 14, 18, 22, 26, 30, and 91 days of age. Testicular Cx43 was localized along the plasma membranes and cytoplasm of Sertoli cells until Day 22. At Day 30, the immunostaining was localized exclusively along the plasma membrane of Sertoli cells. In PTU-treated rats, Cx43 did not localize to the plasma membrane and was still cytoplasmic at 30 days of age. Occludin was present in tubules of treated rats, but was not localized to the blood-testis barrier in 30-day-old rats, as in controls. There were no differences in Cx43 immunostaining in the adult testis. In the proximal epididymis (initial segment, caput, corpus), Cx43 mRNA levels were lower in PTU-treated rats at 14, 18, and 22 days of age, but no differences were observed in the distal (cauda) epididymis at these ages. In 22- and 30-day-old rats, Cx43 was localized along the plasma membrane between principal and basal cells throughout the epididymis. In PTU-treated rats, Cx43 was not detectable in initial segment, caput, or corpus epididymidis. In the cauda epididymidis, however, Cx43 immunostaining in PTU-treated rats was similar to controls. These data suggest that thyroid hormones regulate Cx43-dependent gap junctional communication in the testis and epididymis.

Age- and myopathy-dependent changes in connexins of normal and cardiomyopathic syrian hamster ventricular myocardium

The conduction of cardiac action potentials depends on the flow of excitation through gap junctions, which are hexameric protein associations of connexins (Cxs). The major Cx reported in the heart is Cx43, although some Cx40 and Cx45 are also present. There is some evidence for altered Cx content in heart failure. In heart failure, conduction is depressed and slowed conduction may contribute to arrhythmogenesis and (or) the maintenance of arrhythmia. Cx content and distribution were determined in ventricular tissues from normal and cardiomyopathic Syrian hamsters, an animal model of heart failure which has reproducible age-specific cardiomyopathy resulting in heart failure and age-matched controls in three groups: young (3-5 weeks), adult (13-18 weeks), and old (>45 weeks). Frozen, unfixed sections of ventricular tissues were immunofluorescently stained using antibodies against Cx43, Cx40, and Cx45. Cx43 was the predominant Cx detected in all samples. In normal hamsters, Cx43 was localized predominantly at the intercalated disc region, while in myopathic myocytes, it was scattered. In Western blots, Cx43 content of normal hamster hearts was highest in the adult hearts compared with young and old hamster hearts. In contrast, Cx43 content was significantly lower in adult cardiomyopathic hamster hearts compared with all other groups. The alterations of content and distribution of gap junction Cx43 may contribute to diminished conduction, pump function, and arrhythmogenesis in heart failure.Key words: connexin, gap junction, heart failure, arrhythmia, confocal microscopy.

Formation of three-dimensional thyroid follicle-like structures by polarized FRT cells made communication competent by transfection and stable expression of the connexin-32 gene

Pig thyrocytes, either in the intact gland or cultured under conditions leading to thyroid follicle reconstitution, coexpress two gap junction proteins, connexin-32 (Cx32) and connexin-43 (Cx43). As thyrocytes cultured in the form of a monolayer only express Cx43, we hypothesized that Cx32 could play a role in thyroid folliculogenesis. In the present work, we analyzed the ability of polarized FRT cells (that are gap junction deficient) to form follicle-like structures after stable transfection with either Cx32 or Cx43 genes. Wild-type and transfected FRT cells, while growing, showed the capacity to form three-dimensional structures corresponding to domes that result from the accumulation of fluid underneath limited areas of the cell layer. The number of domes formed by FRT cells expressing Cx32 (FRT-Cx32) was 2- to 3-fold higher than that obtained with either wild-type or Cx43-transfected FRT cells (FRT-Cx43). Domes generated by FRT-Cx32 cells were stable (beyond 3 weeks of culture), whereas those formed from wild-type or FRT-Cx43 cells were transient, disappearing when cells reached confluence. Inspection of the cell organization within domes formed from FRT-Cx32 cells by phase contrast and confocal microscopy revealed a progressive transition from domes toward closed structures with a lumen. The tightness of the lumen was demonstrated by the retention of a fluorescent probe, lucifer yellow, introduced by microinjection. Electron microscope examinations showed that the neoformed follicle-like structures had an inside-out polarity. Analyses of cell motion and division with time, by fluorescence video microscopy, indicated that the transformation of domes into inside-out follicles brings into play the migration of cells and, to a lesser extent, cell multiplication underneath the domes. In conclusion, FRT cells forced to express Cx32 give rise to domes that transform into closed inside-out follicles. This gain of function appears Cx specific, as FRT-Cx43 cells did not form similar structures. Our data suggest that the formation and/or functioning of Cx32 gap junctions might represent a key event in thyroid epithelium morphogenesis, i.e. formation of a lumen from a tight epithelial cell layer.

Elevation of protein kinase C in thyrocytes isolated from a Lewis rat model of autoimmune thyroiditis prevents assembly of immunodetectable connexin43 gap junctions and reduces intercellular communication

In the Lewis rat model of experimental autoimmune thyroiditis (EAT), decreased immunodetectable connexin assembly into gap junctions and diminished intercellular communication are associated with the loss of thyroid function (hypothyroidism) that occurs prior to significant tissue destruction. The current study explores the hypothesis that the loss of connexin 43 (Cx43)-mediated intercellular communication in these cells is caused by upregulation of protein kinase C (pKC) activity. Thyrocytes isolated from EAT rats exhibited a 78% increase in basal pKC activity; whereas, basal protein kinase A (pKA) activity was unchanged. Increased pKC activity was a result of increased isozyme protein levels. Thyroid cells expressed pKC isozymes gamma and lambda and had elevated levels of alpha (40%), beta (30%), delta (31%), and epsilon (25%) as quantified by western blot analyses. Furthermore, modulation of pKC activity inversely altered Cx43 assembly and function in monolayer thyrocytes. For example, octoacetyl glycerol (OAG) treatment of normal thyrocyte monolayers to increase pKC activity resulted in deficient Cx43 gap junction assembly and reduced intercellular communication indistinguishable from the deficits in EAT thyrocytes. Conversely, calphostin C inhibition of pKC activity in EAT thyrocyte monolayers restored these parameters to normal. Thus, pharmacological modulations of pKC activity in cultured thyrocytes support a causal relation between the changes in pKC activity and Cx43-mediated intercellular communication. Abnormalities in autoimmune diseased thyroid tissue (eg, increased pKC) appear to contribute to reduced intercellular coordination of thyroid follicles and thereby can affect subsequent thyroid function. The persistence of target cell abnormalities in the absence of infiltrating lymphocytes and their products supports an alternative mechanism by which thyroid function can be affected that does not depend on the loss of thyroid glandular epithelium.