Induction of neonatal lupus in pups of mice immunized with synthetic peptides derived from amino acid sequences of the serotoninergic 5-HT4 receptor

Cardiac Roles of Serotonin (5-HT) and 5-HT-Receptors in Health and Disease

Serotonin acts solely via 5-HT4-receptors to control human cardiac contractile function. The effects of serotonin via 5-HT4-receptors lead to positive inotropic and chronotropic effects, as well as arrhythmias, in the human heart. In addition, 5-HT4-receptors may play a role in sepsis, ischaemia, and reperfusion. These presumptive effects of 5-HT4-receptors are the focus of the present review. We also discuss the formation and inactivation of serotonin in the body, namely, in the heart. We identify cardiovascular diseases where serotonin might play a causative or additional role. We address the mechanisms which 5-HT4-receptors can use for cardiac signal transduction and their possible roles in cardiac diseases. We define areas where further research in this regard should be directed in the future, and identify animal models that might be generated to this end. Finally, we discuss in what regard 5-HT4-receptor agonists or antagonists might be useful drugs that could enter clinical practice. Serotonin has been the target of many studies for decades; thus, we found it timely to summarise our current knowledge here.

Maternal antibodies and developing blood-brain barrier

We briefly review the protective role of maternal antibodies during fetal development and at early postnatal stages. We describe antibody delivery to fetuses, particularly in the context of the developing blood-brain barrier (BBB), and present the essential concepts regarding the adult BBB, together with existing information on the prenatal developing BBB. We focus on maternal antibody transfer to the developing brain and the consequences of the presence of pathogenic antibodies at early stages of brain development on subsequent brain dysfunction.

Autoantibodies and their Judicious Use in Pediatric Rheumatology Practice

Autoantibody testing forms an important part of diagnostic workup of patients in Pediatric rheumatology practice. However it is important to understand that the mere presence of autoantibodies does not necessarily mean the presence of an underlying autoimmune disease. Autoantibodies may be present decades before the development of clinical manifestations of an autoimmune disease and may be viewed as harbingers of Autoimmune disease. On the other hand, low-affinity autoantibodies may be present in normal healthy individuals; these natural autoantibodies serve an important function in immune regulation and tolerance. Autoantibody testing in pediatric practice mainly includes testing for anti-nuclear antibodies, anti-dsDNA antibodies, anti-neutrophil cytoplasmic autoantibodies and antiphospholipid antibodies. Rheumatoid factor and anti-CCP do not have much significance in the diagnostic schema in pediatric rheumatology, except perhaps for classification of juvenile idiopathic arthritis (JIA) and prognostication in late-onset polyarticular JIA. The positive predictive value (PPV) of any laboratory test depends on the prevalence of the disease in the population being tested. Hence, test ordering practices greatly impact the performance characteristics and positive predictive value of any laboratory test. A restricted test ordering only in patients with clinical signs and symptoms suggestive of autoimmune disease would thus greatly increase the PPV of tests such as antinuclear antibody used for diagnosing autoimmunity.

Congenital heart block maternal sera autoantibodies target an extracellular epitope on the α1G T-type calcium channel in human fetal hearts

BACKGROUND

Congenital heart block (CHB) is a transplacentally acquired autoimmune disease associated with anti-Ro/SSA and anti-La/SSB maternal autoantibodies and is characterized primarily by atrioventricular (AV) block of the fetal heart. This study aims to investigate whether the T-type calcium channel subunit α1G may be a fetal target of maternal sera autoantibodies in CHB.

METHODOLOGY/PRINCIPAL FINDINGS

We demonstrate differential mRNA expression of the T-type calcium channel CACNA1G (α1G gene) in the AV junction of human fetal hearts compared to the apex (18-22.6 weeks gestation). Using human fetal hearts (20-22 wks gestation), our immunoprecipitation (IP), Western blot analysis and immunofluorescence (IF) staining results, taken together, demonstrate accessibility of the α1G epitope on the surfaces of cardiomyocytes as well as reactivity of maternal serum from CHB affected pregnancies to the α1G protein. By ELISA we demonstrated maternal sera reactivity to α1G was significantly higher in CHB maternal sera compared to controls, and reactivity was epitope mapped to a peptide designated as p305 (corresponding to aa305-319 of the extracellular loop linking transmembrane segments S5-S6 in α1G repeat I). Maternal sera from CHB affected pregnancies also reacted more weakly to the homologous region (7/15 amino acids conserved) of the α1H channel. Electrophysiology experiments with single-cell patch-clamp also demonstrated effects of CHB maternal sera on T-type current in mouse sinoatrial node (SAN) cells.

CONCLUSIONS/SIGNIFICANCE

Taken together, these results indicate that CHB maternal sera antibodies readily target an extracellular epitope of α1G T-type calcium channels in human fetal cardiomyocytes. CHB maternal sera also show reactivity for α1H suggesting that autoantibodies can target multiple fetal targets.

Congenital heart block: evidence for a pathogenic role of maternal autoantibodies

During pregnancy in autoimmune conditions, maternal autoantibodies are transported across the placenta and may affect the developing fetus. Congenital heart block (CHB) is known to associate with the presence of anti-Ro/SSA and anti-La/SSB antibodies in the mother and is characterized by a block in signal conduction at the atrioventricular (AV) node. The mortality rate of affected infants is 15% to 30%, and most live-born children require lifelong pacemaker implantation. Despite a well-recognized association with maternal anti-Ro/La antibodies, CHB develops in only 1% to 2% of anti-Ro-positive pregnancies, indicating that other factors are important for establishment of the block. The molecular mechanisms leading to complete AV block are still unclear, and the existing hypotheses fail to explain all aspects of CHB in one comprehensive model. In this review, we discuss the different specificities of maternal autoantibodies that have been implicated in CHB as well as the molecular mechanisms that have been suggested to operate, focusing on the evidence supporting a direct pathogenic role of maternal antibodies. Autoantibodies targeting the 52-kDa component of the Ro antigen remain the antibodies most closely associated with CHB. In vitro experiments and animal models of CHB also point to a major role for anti-Ro52 antibodies in CHB pathogenesis and suggest that these antibodies may directly affect calcium regulation in the fetal heart, leading to disturbances in signal conduction or electrogenesis or both. In addition, maternal antibody deposits are found in the heart of fetuses dying of CHB and are thought to contribute to an inflammatory reaction that eventually induces fibrosis and calcification of the AV node, leading to a complete block. Considering that CHB has a recurrence rate of 12% to 20% despite persisting maternal autoantibodies, it has long been clear that maternal autoantibodies are not sufficient for the establishment of a complete CHB, and efforts have been made to identify additional risk factors for this disorder. Therefore, recent studies looking at the influence of genetic and environmental factors will also be discussed.

Cutaneous lupus in infancy and childhood

Cutaneous lupus may occur in infancy as transient lesions associated with and probably caused by maternal autoantibodies, or later in childhood, associated with the endogenous development of autoimmunity. In this review, clinical findings, diagnosis, management, and pathogenesis of neonatal lupus are discussed, and the management of cutaneous lupus in children is detailed.

Role of calcium channels in congenital heart block

Congenital heart block (CHB) is a conduction abnormality that affects hearts of foetuses and/or newborn to mothers with autoantibodies reactive with the intracellular soluble ribonucleoproteins 48-kD La, 52-kD Ro and 60-kD Ro. CHB carries substantial mortality and morbidity, with more than 60% of affected children requiring lifelong pacemakers. Several hypotheses have been proposed to explain the pathogenesis of CHB. These can be grouped under three main hypotheses: Apoptosis, Serotoninergic and Ca channel hypothesis. Here, we discuss these hypotheses and provide recent scientific thinking that will most likely dominate the future of this field of research.

Maternal MHC regulates generation of pathogenic antibodies and fetal MHC-encoded genes determine susceptibility in congenital heart block

Congenital heart block develops in fetuses of anti-Ro52 Ab-positive women. A recurrence rate of 20%, despite the persistence of maternal autoantibodies, indicates that there are additional, yet unidentified, factors critical for development of congenital heart block. In this study, we demonstrate that besides the maternal MHC controlling Ab specificity, fetal MHC-encoded genes influence fetal susceptibility to congenital heart block. Using MHC congenic rat strains, we show that heart block develops in rat pups of three strains carrying MHC haplotype RT1(av1) (DA, PVG.AV1, and LEW.AV1) after maternal Ro52 immunization, but not in LEW rats (RT1(l)). Different anti-Ro52 Ab fine specificities were generated in RT1(av1) versus RT1(l) animals. Maternal and fetal influence was determined in an F(2) cross between LEW.AV1 and LEW strains, which revealed higher susceptibility in RT1(l) than RT1(av1) pups once pathogenic Ro52 Abs were present. This was further confirmed in that RT1(l) pups more frequently developed heart block than RT1(av1) pups after passive transfer of RT1(av1) anti-Ro52 sera. Our findings show that generation of pathogenic Ro52 Abs is restricted by maternal MHC, whereas the fetal MHC locus regulates susceptibility and determines the fetal disease outcome in anti-Ro52-positive pregnancies.

Nucleic acid-associated autoantigens: pathogenic involvement and therapeutic potential

Autoimmunity to ubiquitously expressed macromolecular nucleic acid-protein complexes such as the nucleosome or the spliceosome is a characteristic feature of systemic autoimmune diseases. Disease-specificity and/or association with clinical features of some of these autoimmune responses suggest pathogenic involvement which, however, has been proven in only a few cases so far. Although the mechanisms leading to autoimmunity against nucleic acid-containing complexes are still far from being fully understood, there is increasing experimental evidence that the nucleic acid component may act as a co-stimulator or adjuvans via activation of nucleic acid-binding receptor systems such as Toll-like receptors in antigen-presenting cells. Dysregulated apoptosis and inappropriate stimulation of nucleic acid-sensing receptors may lead to loss of tolerance against the protein components of such complexes, activation of autoreactive T cells and formation of autoantibodies. This has been demonstrated to occur in systemic lupus erythematosus and seems to represent a general mechanism that may be crucial for the development of systemic autoimmune diseases. This review provides a comprehensive overview of the most thoroughly-characterized nucleic acid-associated autoantigens, describing their structure and biological function, as well as the nature and pathogenic importance of the reactivities directed against them. Furthermore, recent advances in immunotherapy such as antigen-specific approaches targeted at nucleic acid-binding antigens are discussed.

Losing your nerves? Maybe it's the antibodies

We propose that the normal immunocompetent B cell repertoire is replete with B cells making antibodies that recognize brain antigens. Although B cells that are reactive with self antigen are normally silenced during B cell maturation, the blood-brain barrier (BBB) prevents many brain antigens from participating in this process. This enables the generation of a B cell repertoire that is sufficiently diverse to cope with numerous environmental challenges. It requires, however, that the integrity of the BBBs is uninterrupted throughout life to protect the brain from antibodies that crossreact with microorganisms and brain antigens. Under conditions of BBB compromise, and during fetal development, we think that these antibodies can alter brain function in otherwise healthy individuals.

Antibody reactivity to alpha-enolase in mothers of children with congenital heart block

OBJECTIVE

To evaluate the frequency of anti-alpha-enolase antibodies in the sera of mothers whose children have congenital heart block (CHB), given provocative results in which alpha-enolase, a membrane protein, was recognized by monoclonal antibodies reactive with the peptide p200 of 52 kDa Ro/SSA in a neonatal rat heart library.

METHODS

An ELISA using a recombinant alpha-enolase protein was developed. Sera from 100 anti-Ro52+ CHB mothers in the Research Registry for Neonatal Lupus, 50 patients with systemic lupus erythematosus (SLE; 7 anti-Ro52+), and 48 healthy controls were tested for anti-alpha-enolase reactivity.

RESULTS

There were no significant differences in the median values obtained from CHB mothers, patients with SLE, or controls at each of the dilutions tested. Only 7 (7%) at 1:100 dilution and 2 (2%) at 1:1000 dilution of 100 CHB sera were 3 standard deviations above the mean value obtained for controls. Preincubation with recombinant Ro52 did not inhibit anti-alpha-enolase reactivity.

CONCLUSION

The low frequency of anti-alpha-enolase antibodies in the sera of CHB mothers and the absence of apparent cross-reactivity with Ro52 suggest that antibodies to Ro52 are not likely to mediate CHB via binding to alpha-enolase.

Autoantibody-mediated cardiac arrhythmias: mechanisms and clinical implications

Cardiac arrhythmias, including conduction defects and tach- yarrhythmias, represent an important source of morbidity and mortality in industrialized countries. Among the different pathophysiological mechanisms involved in the arrhythmogenesis, an inappropriate activation of the immune system represents a field of recent increasing interest. In fact, a large amount of studies suggest that specific autoantibody may be significantly involved in the pathogenesis of cardiac arrhythmias not only in the course of systemic autoimmune disease, but also in a number of rhythm disorders currently classified as "idiopathic." Although the strongest evidence concerns the relationship between anti-Ro/SSA antibodies and the development of congenital heart block in foetus and newborn, other specific autoantibodies demonstrated the aptitude to affect directly the myocardial tissue, thus producing interference in its bioelectric activity thereby leading to rhythm disorders, also life-threatening. The identification of an immunological autoantibody-mediated mechanism opens new perspectives in the treatment and prevention of cardiac arrhythmias in such patients, including the use of immunosuppressive agents and/or the removal of autoantibodies by immuno-adsorption technique.

Epidemiology, etiology, detection, and treatment of autoantibody-associated congenital heart block in neonatal lupus

Neonatal lupus syndrome is a model of passively acquired autoimmunity in which the pregnant woman's serum contains specific antibodies to 52 or 60 kd SSA/Ro and/or 48 kd SSB/La, which cross the placenta and are associated with the development of congenital heart block in the fetus and/or a transient rash or various liver and blood cell abnormalities in the newborn. To date, congenital heart block is a permanent condition that entails significant morbidity and mortality, with nearly all affected infants requiring pacemakers and with an 80% cumulative probability of survival at 3 years of age. An intensive search is on for the specific etiopathophysiology and for new clinical tools to approach and treat this disease.

Immunomodulation by maternal autoantibodies of the fetal serotoninergic 5-HT4 receptor and its consequences in early BALB/c mouse embryonic development

Background

The presence of functional 5-HT₄ receptors in human and its involvement in neonatal lupus erythematosus (NLE) have prompted us to study the receptor expression and role during embryogenesis. Earlier we managed to demonstrate that female BALB/c mice immunized against the second extracellular loop (SEL) of the 5-HT₄ receptor gave birth to pups with heart block. To explain this phenomenon we investigated the expression of 5-HT₄ receptors during mouse embryogenesis. At the same time we looked whether the consequence of 5-HT₄ receptor immunomodulation observed earlier is in relation to receptor expression.

We studied the expression of 5-HT₄ receptor at the mRNA level and its two isoforms 5-HT_4(a) and 5-HT_4(d) at the protein level in embryos from BALB/c mice, at 8^th, 12^th, 18^th gestation days (GD) and 1 day post natal (DPN). Simultaneously the receptor activity was inhibited by rising antibodies, in female mice against SEL of the receptor. The mice were mated and embryos were collected at 8^th, 12^th, 18^th GD and 1 DPN.

Results

5-HT₄ receptor mRNA increased in brain from 12^th GD to 1 DPN. Its expression gradually decreased in heart and disappeared at birth. This was consistent with expression of the receptor isoforms 5-HT_{4(a) and (d)}. Abnormalities like decreased number of embryos, growth delay, spina bifida and sinus arrhythmia from 12^th GD were documented in pups of mice showing anti-5-HT₄ receptor antibodies.

Conclusion

serotoninergic 5-HT₄ receptor plays an important role in mouse foetal development. In BALB/c mice there is a direct relation between the expression of receptor and the deleterious effect of maternal anti-5-HT₄ receptor autoantibodies in early embryogenesis.

52-kDa Ro/SSA epitopes preferentially recognized by antibodies from mothers of children with neonatal lupus and congenital heart block

Neonatal lupus erythematosus is a rare disorder caused by the transplacental passage of maternal autoantibodies. The 52-kDa Ro/SSA antigen (Ro52) ribonucleoprotein represents an antigenic target strongly associated with the autoimmune response in mothers whose children have neonatal lupus and cardiac conduction disturbances, mainly congenital heart block. The objective of this study was to identify putative Ro52/60-kDa Ro/SSA antigen (Ro60) epitopes associated with neonatal lupus and congenital heart block. The reactivity of IgG antibodies present in the sera from mothers with systemic lupus erythematosus and Sjögren's syndrome and in the sera from asymptomatic mothers (a longitudinal study of 192 samples from 66 subjects) was investigated by ELISA using Ro52, Ro60 and 48-kDa La/SSB antigen proteins, as well as 45 synthetic peptides, 13–24 residues long, of Ro52/Ro60 proteins. One to 19 samples collected before, during and after pregnancy were available for each mother. Forty-three disease controls selected randomly and normal sera were tested in parallel. Although no differences were found between Sjögren's syndrome and asymptomatic mothers of group I, who had at least one infant with neonatal lupus, and of group II, who had healthy babies only, significant differences were observed between lupus mothers from both groups. In the former group of lupus mothers, a significantly higher frequency of antibodies to Ro52 peptides 107–122 and 277–292 was observed. Between 18 and 30 weeks of gestation, the period of risk, there was clearly an elevated level of antibodies reacting with Ro52 peptides 1–13, 277–292 and 365–382. Antibodies to Ro52 peptide 365–382 have been shown previously to cross-react with residues 165–185 of the heart 5-HT₄ serotoninergic receptor, and might be pathologically important. The level of these Ro52 antibody subsets decreased at the end of pregnancy and after delivery. IgG antibodies to Ro52 peptides 1–13, 107–122, 277–292 and 365–382 may therefore represent important biomarkers to predict a complication in pregnant lupus women with Ro52 antibodies.

Specificity and effector mechanisms of autoantibodies in congenital heart block

Complete congenital atrio-ventricular (AV) heart block develops in 2-5% of fetuses of Ro/SSA and La/SSB autoantibody-positive pregnant women. During pregnancy, the Ro/SSA and La/SSB antibodies are transported across the placenta and affect the fetus. Emerging data suggest that this happens by a two-stage process. In the first step, maternal autoantibodies bind fetal cardiomyocytes, dysregulate calcium homestasis and induce apoptosis in affected cells. This step might clinically correspond to a first-degree heart block, and be reversible. La/SSB antibodies can bind apoptotic cardiomyocytes and thus increase Ig deposition in the heart. The tissue damage could, as a second step, lead to spread of inflammation in genetically pre-disposed fetuses, progressing to fibrosis and calcification of the AV-node and subsequent complete congenital heart block. Early intrauterine treatment of an incomplete AV-block with fluorinated steroids has been shown to prevent progression of the heart block, making it clinically important to find specific markers to identify the high-risk pregnancies.

5-Hydroxytryptamine receptors in the human cardiovascular system

The human cardiovascular system is exposed to plasma 5-hydroxytryptamine (5-HT, serotonin), usually released from platelets. 5-HT can produce harmful acute and chronic effects. The acute cardiac effects of 5-HT consist of tachycardia (preceded on occasion by a brief reflex bradycardia), increased atrial contractility and production of atrial arrhythmias. Acute inotropic, lusitropic and arrhythmic effects of 5-HT on human ventricle become conspicuous after inhibition of phosphodiesterase (PDE) activity. Human cardiostimulation is mediated through 5-HT4 receptors. Atrial and ventricular PDE3 activity exerts a protective role against potentially harmful cardiostimulation. Chronic exposure to high levels of 5-HT (from metastatic carcinoid tumours), the anorectic drug fenfluramine and its metabolites, as well as the ecstasy drug 3,4-methylenedioxymethamphetamine (MDMA) and its metabolite 3,4-methylenedioxyamphetamine (MDA) are associated with proliferative disease and thickening of cardiac valves, mediated through 5-HT2B receptors. 5-HT2B receptors have an obligatory physiological role in murine cardiac embryology but whether this happens in humans requires research. Congenital heart block (CHB) is, on occasion, associated with autoantibodies against 5-HT4 receptors. Acute vascular constriction by 5-HT is usually shared by 5-HT1B and 5-HT2A receptors, except in intracranial arteries which constrict only through 5-HT1B receptors. Both 5-HT1B and 5-HT2A receptors can mediate coronary artery spasm but only 5-HT1B receptors appear involved in coronary spasm of patients treated with triptans or with Prinzmetal angina. 5-HT2A receptors constrict the portal venous system including oesophageal collaterals in cirrhosis. Chronic exposure to 5-HT can contribute to pulmonary hypertension through activation of constrictor 5-HT1B receptors and proliferative 5-HT2B receptors, and possibly through direct intracellular effects.

A high-affinity monoclonal antibody with functional activity against the 5-hydroxytryptaminergic (5-HT4) receptor

Splenocytes from a BALB/c mouse immunised with a synthetic peptide corresponding to the second extracellular loop of the 5-HT4 receptor were fused with SP2/O myeloma cells to produce a monoclonal antibody. The monoclonal antibody was of the IgG2b isotype. The antibody recognised the human 5-HT4(g) (h5-HT4(g)) receptor by immunoblots and by immunofluorescence on chinese hamster ovary (CHO) cells expressing this 5-HT4 receptor isoform. Epitope mapping of the antibody suggested the recognition of a conformational epitope, encompassing the N- and C-terminal fragments of the second extracellular loop. Kinetic experiments using surface plasmon resonance showed that the antibody had a picomolar affinity for its cognate peptide. Inhibition experiments using the same methodology confirmed the specificity of the interaction. The antibody at a concentration of 500 pM competitively inhibited inverse agonist GR113808 binding and showed an inverse agonist effect on the basal activity of CHO cells expressing the 5-HT4(g) receptor. The antibody decreased the effect of 5-HT at 500 and 50 pM concentrations but it increased 5-HT-induced cAMP levels at 5 pM. The dual effect of the monoclonal antibody could be ascribed to mono- or bivalent recognition of the receptor. The antibody described here is the first example of a high-affinity modulator of the 5-HT4 receptor.

Autoantibodies against the serotoninergic 5-HT4 receptor and congenital heart block: a reassessment

The specificity of autoantibodies against the serotoninergic 5-HT4 receptor in congenital heart block has led to conflicting observations. In order to clarify the situation, a collaborative effort was undertaken to discover the reasons for these discrepancies and to reassess the importance of such autoantibodies by making use of the Research Registry for Neonatal Lupus. Sera from 128 patients (101 anti-SSA/Ro52 positive mothers among which 74 have children with congenital heart block (CHB), 9 anti-SSA/Ro52 negative patients of which 1 had a child with heart block and 18 healthy donors) were assessed in a single blind test using an ELISA coated with a 5-HT4 receptor-derived peptide. Discrepancies between previous observations in our two groups could be ascribed to small differences in the set up of the assay. Of the 75 sera from mothers of children with CHB, 12 were reactive with the 5-HT4 peptide. Four sera among which three were from 35 Ro52 negative mothers without affected children as well as 2 in the 18 controls were positive. Interestingly, in 1 mother with an isolated child with CHB but who had no detectable anti-SSA/Ro52 antibodies and 1 mother with a child with a structural heart block and no detectable antibodies to any component of SSA/Ro, reactivity with the 5-HT4 receptor was noted. While 5-HT4 receptor autoantibodies do not have the predictive value of anti-Ro52 autoantibodies, the presence of these antibodies in a minor subset of mothers whose children have CHB suggests an additional risk factor which may contribute to the pathogenesis of disease.

Transient autoimmunity related to maternal autoantibodies: neonatal lupus

Neonatal lupus (NLE) is an autoimmune disease associated with maternal antibodies to Ro/La and characterized by cutaneous lesions, heart block, cardiomyopathy, hepatobiliary disease, and hematologic cytopenias. In most cases, only one organ is affected, although multiple organ involvement is not unusual. Since NLE is presumably caused by maternal autoantibodies, the disease process is transient. However, cardiac NLE, in particular, may be fatal or persistently disabling. Optimal therapy has not yet been determined. Mothers of babies with NLE are often initially asymptomatic, but eventually most develop symptoms of autoimmune disease, particularly diseases associated with anti-Ro/La autoantibodies, such as Sjogren's syndrome and systemic lupus erythematosus. Children who have had NLE are probably at increased risk for autoimmunity later in life, sometimes as early as pre-adolescence, but the magnitude of the risk for the children is not known. Only a small percentage of babies exposed to maternal autoantibodies to Ro and/or La develop NLE. The factors governing which babies develop disease and, if disease develops, which organs will be affected have yet to be fully elucidated. In this review the clinical features, diagnosis, therapy, and prognosis of NLE are discussed, and a summary of experimental data relating to pathogenesis is presented.

Ro/SSA autoantibodies directly bind cardiomyocytes, disturb calcium homeostasis, and mediate congenital heart block

Congenital heart block develops in fetuses after placental transfer of Ro/SSA autoantibodies from rheumatic mothers. The condition is often fatal and the majority of live-born children require a pacemaker at an early age. The specific antibody that induces the heart block and the mechanism by which it mediates the pathogenic effect have not been elucidated. In this study, we define the cellular mechanism leading to the disease and show that maternal autoantibodies directed to a specific epitope within the leucine zipper amino acid sequence 200–239 (p200) of the Ro52 protein correlate with prolongation of fetal atrioventricular (AV) time and heart block. This finding was further confirmed experimentally in that pups born to rats immunized with p200 peptide developed AV block. p200-specific autoantibodies cloned from patients bound cultured cardiomyocytes and severely affected Ca²⁺ oscillations, leading to accumulating levels and overload of intracellular Ca²⁺ levels with subsequent loss of contractility and ultimately apoptosis. These findings suggest that passive transfer of maternal p200 autoantibodies causes congenital heart block by dysregulating Ca²⁺ homeostasis and inducing death in affected cells.

Ca2+ currents in cardiac myocytes: Old story, new insights

Calcium is a ubiquitous second messenger which plays key roles in numerous physiological functions. In cardiac myocytes, Ca2+ crosses the plasma membrane via specialized voltage-gated Ca2+ channels which have two main functions: (i) carrying depolarizing current by allowing positively charged Ca2+ ions to move into the cell; (ii) triggering Ca2+ release from the sarcoplasmic reticulum. Recently, it has been suggested than Ca2+ channels also participate in excitation-transcription coupling. The purpose of this review is to discuss the physiological roles of Ca2+ currents in cardiac myocytes. Next, we describe local regulation of Ca2+ channels by cyclic nucleotides. We also provide an overview of recent studies investigating the structure-function relationship of Ca2+ channels in cardiac myocytes using heterologous system expression and transgenic mice, with descriptions of the recently discovered Ca2+ channels alpha(1D) and alpha(1E). We finally discuss the potential involvement of Ca2+ currents in cardiac pathologies, such as diseases with autoimmune components, and cardiac remodeling.

Modulation of the M2 muscarinic acetylcholine receptor activity with monoclonal anti-M2 receptor antibody fragments

Antibodies directed against the second extracellular loop of G protein-coupled receptors are known to have functional activities. From a partial agonist monoclonal antibody directed against the M2 muscarinic receptor, we constructed and produced a single chain variable fragment with high affinity for its target epitope. The fragment is able to recognize its receptor on Chinese hamster ovary cells transfected with the M2 muscarinic acetylcholine receptor to block the effect of carbachol on this receptor and to exert an inverse agonist activity on the basal activity of the receptor. The antibody fragment is also able to increase the basal rhythm of cultured neonatal rat cardiomyocytes and to inhibit in a non-competitive manner the negative chronotropic effect of carbachol. This antibody fragment is able to exert its inverse agonist activity in vivo on mouse heart activity. The immunological strategy presented here could be useful to develop specific allosteric inverse agonist reagents for G protein-coupled receptors.

Neonatal lupus erythematosus: clinical findings and pathogenesis

Neonatal lupus erythematosus is an uncommon disease associated with maternal autoantibodies to proteins of the Ro/La (SSA/SSB) family. The clinical findings most often reported are third-degree heart block and cutaneous lupus lesions, but a significant number of children have cardiomyopathy, hepatobiliary disease, or hematologic cytopenias. The consistent presence of maternal autoantibodies and the transient nature of the disease implicate maternal autoantibodies as the cause of the disease, and developing animal models support the concept that the autoantibodies are pathogenic. Only a minority of babies exposed to the autoantibodies develop disease, however, and mothers and their babies have different disease manifestations. Thus, additional factors are likely to be important in determining disease expression.

Neonatal lupus: clinical features and management

Neonatal lupus is an uncommon autoimmune disease manifested primarily by cutaneous lupus lesions and/or congenital heart block. Maternal autoantibodies of the Ro/La family are present in virtually every case, although only approximately 1% of women who have these autoantibodies will have a baby with neonatal lupus. The cutaneous lesions of neonatal lupus may be present at birth, but more often develop within the first few weeks of life. Lesions are most common on the face and scalp, often in a distinctive periorbital distribution. Lesions tend to resolve in a few weeks or months without scarring. The most common cardiac manifestation of neonatal lupus is complete heart block. Heart block typically begins in utero during the second or third trimester. In some cases, heart block begins as first- or second-degree block and then progresses to third-degree block. Complete heart block, once established, appears to be irreversible. In some cases, cardiomyopathy occurs together with complete heart block. Most cases have been noted at birth, but delayed dilated cardiomyopathy has been reported. There have been a few cases of endocardial fibroelastosis occurring in the absence of congenital heart block. Hepatobiliary disease occurs in about 10% of cases. Three types of hepatobiliary disease have been observed: liver failure occurring at birth or in utero, transient conjugated hyperbilirubinemia occurring in infants, or transient transaminase elevations occurring in infants. Hematologic disease, consisting of thrombocytopenia, neutropenia, or anemia, occurs in about 10% of cases. It is common for children with neonatal lupus not to have the full expression of disease, but rather to have only one or two organ systems involved. The diagnosis rests largely on the finding of compatible clinical manifestations plus maternal autoantibodies to Ro and/or La, or, in a few cases, to U1 ribonuclear protein. Although the pathogenesis has not been conclusively established, accumulating evidence, including evidence from animal models, implicates autoantibodies in the pathogenesis of the disease. Therapeutic interventions include attempts at prevention, early intervention, and treatment of well established disease, mainly through the use of systemic corticosteroids. Optimal therapy has yet to be determined. The long-term prognosis for children who have had neonatal lupus is still under investigation, but some children who had neonatal lupus have developed other autoimmune diseases later in childhood. About half of the mothers are asymptomatic at the time of presentation of the child, but some of these women eventually develop symptoms of autoimmune disease.

Attenuated response to stress and novelty and hypersensitivity to seizures in 5-HT4 receptor knock-out mice

To study the functions of 5-HT4 receptors, a null mutation was engineered in the corresponding gene. 5-HT4 receptor knock-out mice displayed normal feeding and motor behaviors in baseline conditions but abnormal feeding and locomotor behavior in response to stress and novelty. Specifically, stress-induced hypophagia and novelty-induced exploratory activity were attenuated in the knock-out mice. In addition, pentylenetetrazol-induced convulsive responses were enhanced in the knock-out mice, suggesting an increase in neuronal network excitability. These results provide the first example of a genetic deficit that disrupts the ability of stress to reduce feeding and body weight and suggest that 5-HT4 receptors may be involved in stress-induced anorexia and seizure susceptibility.

The Ro52/SS-A autoantigen has elevated expression at the brain microvasculature

An expression cloning technique was used to identify proteins selectively expressed at the blood-brain barrier that may mediate interactions between the brain microvasculature and IgG molecules. Ro52/SS-A is an IgG-binding protein and is an autoantigen implicated in Sjogren's syndrome and systemic lupus erythematosus. The present studies describe the expression cloning of this gene product from a bovine brain microvascular cDNA library. The Ro52 protein interacted with IgG molecules independent of specificity suggesting a potential role as a general antibody receptor. Northern blotting shows the Ro52/SS-A transcript is enriched in the microvascular compartment of brain. The selective expression of Ro52/SS-A in brain at the microvasculature may play a role in brain vascular involvement in autoimmune diseases.

scFv single chain antibody variable fragment as inverse agonist of the beta2-adrenergic receptor

Antibodies directed against the second extracellular loop of G protein-coupled receptors were shown to possess functional activities. Using a functional monoclonal antibody against the human beta2-adrenergic receptor, a scFv fragment with high affinity for the target epitope was constructed and produced. The fragment recognized the beta2-adrenergic receptors on A431 cells, blocked cAMP accumulation induced by the beta2-agonist salbutamol, and decreased basal cAMP accumulation in the same cells. Their in vitro activity was tested on neonatal rat cardiomyocytes. The antibody fragments blocked the chronotropic activity induced by the beta2-agonist clenbuterol. They also decreased the in vivo heart beating frequency of mice pretreated with bisoprolol (a beta1-adrenergic receptor antagonist) for 4 min after injection. The immunological approach presented here may serve as a strategy for the synthesis of a new class of allosteric modulators for G protein-coupled receptors.

How biotinylation can interfere with recognition: a surface plasmon resonance study of peptide-antibody interactions

Biotinylation is one of the most frequently used labelling procedures in biochemistry and molecular biology. To study the influence of biotinylation on peptide antigenicity, we selected a peptide derived from the second extracellular loop of the beta(2)-adrenergic receptor. Interactions between different biotinylated and nonbiotinylated analogs and a monoclonal antibody directed against an epitope present within the N-terminal end of this peptide were studied in detail. Taking advantage of the BIACORE 3000 surface plasmon resonance equipment, we were able to compare antibody interactions with the immobilised peptides and with the same peptides in solution. While the nonbiotinylated peptide, immobilised by its N-terminus, was not recognised by the antibody, it was recognised either after immobilisation by means of the thiol group of the C-terminal cysteine residue or as a free peptide tested as analyte with the monoclonal antibody immobilised on the chip. The N-terminal biotinylated forms were well recognised when immobilised on streptavidin but poorly (for the aminocaproyl-biotin derivative) or not at all (for the biotinylated derivative) when they were allowed to react with immobilised monoclonal antibody. These results indicate that the biotinyl moiety interacts with residues that are important for antibody recognition in solution but such interactions are abrogated when it is bound to the streptavidin. Molecular modeling confirmed that the N-terminus of the peptide mimicked to some extent the streptavidin binding site.

Cardiac 5-HT(4) serotoninergic receptors, 52kD SSA/Ro and autoimmune-associated congenital heart block

It was recently reported that sera from patients with systemic lupus erythematosus contain antibodies reactive with the second extracellular loop of the serotoninergic 5-HT(4) receptor expressed in the human heart. This antibody response was associated with antibodies to 52kD SSA/Ro, a reactivity prevalent in mothers of children with congenital heart block (CHB). The current study was undertaken to determine whether the 5-HT(4) receptor is a target of the immune response in these mothers. Initial experiments demonstrated mRNA expression of the 5-HT(4) receptor in the human foetal atrium. Electrophysiologic studies established that human foetal atrial cells express functional 5-HT(4) receptors. Sera from 116 mothers enrolled in the Research Registry for Neonatal Lupus, whose children have CHB, were evaluated. Ninety-nine (85%) of these maternal sera contained antibodies to SSA/Ro, 84% of which were reactive with the 52kD SSA/Ro component by immunoblot. None of the 116 sera were reactive with the peptide spanning aa165-185 of the serotoninergic receptor. Rabbit antisera which recognized this peptide did not react with 52kD SSA/Ro or peptide aa365-382 in the C terminus. Although 5-HT(4) receptors are present and functional in the human foetal heart, maternal antibodies to the 5-HT(4) receptor are not associated with the development of CHB.

SS-56, a novel cellular target of autoantibody responses in Sjögren syndrome and systemic lupus erythematosus

Certain autoimmune disorders, including Sjögren syndrome (SS) and systemic lupus erythematosus (SLE), are characterized by autoantibodies against the Ro/SSA and La/SSB cellular antigens. Although the implication of these autoantibodies in disease pathogenesis is still unclear, it is believed that the aberrant responses against autoantigens may extend to other proteins that are not yet well defined. In an attempt to analyze the regulated gene expression in lymphocytes by an HIV-suppressive immunomodulator, we have identified and cloned a novel gene encoding a 56-kDa protein, named SS-56, which is structurally related to the 52-kDa Ro/SSA antigen. The new protein showed primarily perinuclear cytoplasmic localization, and recombinant SS-56 was found to react in ELISA with sera from most patients with SS or SLE. Western blot analysis confirmed the autoantigenic nature of native SS-56 in extracts from HeLa cells. Interestingly, the incidence of antibodies to SS-56 was associated with visceral complications in SLE, and roughly half of the 17 SS or SLE patients with no detectable antibodies to SSA and SSB antigens presented measurable antibodies against recombinant SS-56. Thus, SS-56 represents a new member of the SS family of autoantigens and could become an additional and important diagnostic marker for SS and SLE.

Autoantibodies from mothers of children with congenital heart block downregulate cardiac L-type Ca channels

Congenital heart block (CHB) affects offspring of mothers with autoantibodies (positive IgG) to intracellular SSA/Ro and SSB/La ribonucleoproteins and is associated with high morbidity and mortality. Here, we show that maternal anti-Ro/La antibodies immunoreact with human fetal cardiomyocyte sarcolemma, recognize human L-type Ca channel alpha(1C)-protein and functionally inhibit expressed current in oocytes injected with alpha(1C) cRNA and Purkinje L-type Ca current. Furthermore, cardiac myocytes from pups born to SSA/Ro-immunized mice exhibited reduced L-type Ca current density. All together, the data establish that L-type calcium channel is a target for maternal antibodies and may provide a functional basis for the electrocardiographic abnormalities seen in infants with CHB.