Mitochondrial DNA depletion: mutations in thymidine kinase gene with myopathy and SMA

BACKGROUND

The mitochondrial DNA (mtDNA) depletion syndrome (MDS) is an autosomal recessive disorder of early childhood characterized by decreased mtDNA copy number in affected tissues. Recently, MDS has been linked to mutations in two genes involved in deoxyribonucleotide (dNTP) metabolism: thymidine kinase 2 (TK2) and deoxy-guanosine kinase (dGK). Mutations in TK2 have been associated with the myopathic form of MDS, and mutations in dGK with the hepatoencephalopathic form.

OBJECTIVES

To further characterize the frequency and clinical spectrum of these mutations, the authors screened 20 patients with myopathic MDS.

RESULTS

No patient had dGK gene mutations, but four patients from two families had TK2 mutations. Two siblings were compound heterozygous for a previously reported H90N mutation and a novel T77M mutation. The other siblings harbored a homozygous I22M mutation, and one of them had evidence of lower motor neuron disease. The pathogenicity of these mutations was confirmed by reduced TK2 activity in muscle (28% to 37% of controls).

CONCLUSIONS

These results show that the clinical expression of TK2 mutations is not limited to myopathy and that the myopathic form of MDS is genetically heterogeneous.

Barth's syndrome-like disorder: a new phenotype with a maternally inherited A3243G substitution of mitochondrial DNA (MELAS mutation)

An Argentine male child died at 4.5 years of age of a lethal mitochondrial disease associated with a MELAS mutation and a Barth syndrome-like presentation. The child had severe failure to thrive from the early months and for approximately two years thereafter. In addition, the patient had severely delayed gross motor milestones, marked muscle weakness, and dilated cardiomyopathy that progressed to congestive heart failure. He also had persistently elevated urinary levels of 3-methylglutaconic and 2-ethylhydracrylic acids and low blood levels of cholesterol. Detailed histopathologic evaluation of the skeletal muscle biopsy showed high activity of succinate dehydrogenase, a generalized decrease of COX activity, and abundant ragged-red fibers. Electron microscopic studies revealed multiple mitochondrial abnormalities in lymphocytes and monocytes, in the striated muscle, and in the postmortem samples (muscle, heart, liver, and brain). Biochemical analysis showed a pronounced and constant lactic acidosis, and abnormal urinary organic acid excretion (unchanged in the fasting and postprandial states). In addition, in CSF there was a marked increase of lactate and beta-hydroxybutyrate (beta-HOB) and also a high systemic ratio beta-HOB/acetoacetate. Enzymatic assay of the respiratory chain in biopsied muscle showed 10% of complex I activity and 24% of complex IV activity compared with controls. Molecular studies of the mitochondrial genome revealed an A to G mutation at nucleotide pair 3243 in mitochondrial DNA, a well-known pathogenetic mutation (MELAS mutation) in all the patient's tissues and also in the blood specimens of the probands mother and sibs (4 of 5). The diagnosis of MELAS mutation was reinforced by the absence of an identifiable mutation in the X-linked G4.5 gene of the propositus. The present observation gives additional evidence of the variable clinical expression of mtDNA mutations in humans and demonstrates that all clinical variants deserve adequate investigation to establish a primary defect. It also suggests adding Barth-like syndrome to the list of phenotypes with the MELAS mutation.

Muscarinic acetylcholine receptor antibodies as a new marker of dry eye Sjögren syndrome

PURPOSE

The authors investigated whether circulating autoantibodies against M(3) muscarinic acetylcholine receptors (mAChRs) could be a new marker for diagnosis for primary and secondary Sjögren syndrome (SS) dry eye.

METHODS

Enzyme-linked immunosorbent assay (ELISA) using both rat exorbital lacrimal gland acinar cell membranes and synthetic 25-mer peptide as antigens was used to determine autoantibodies against acinar cells and M(3) mAChRs. Also, nitric oxide synthase (NOS) activity was assessed to determine the biological effect of these autoantibodies in relation to the M(3) mAChR.

RESULTS

Sera from dry eye primary SS (pSS) or secondary SS (sSS) patients tested by ELISA recognized membrane lacrimal gland acinar cells antigens and the synthetic 25-mer peptide, corresponding to the second extracellular loop of human M(3) mAChRs. Moreover, the IgG fraction and the corresponding affinity-purified anti-M(3) peptide autoantibodies from the same patients were able to activate NOS coupled to lacrimal gland M(3) mAChRs. As controls, IgG and sera from women without dry eye with or without rheumatoid arthritis and from normal control subjects gave negative results on ELISA and biological assay; thus demonstrating the specificity of the reaction.

CONCLUSIONS

Autoantibodies against mAChR may be considered among the serum factors implicated in the pathophysiology of the development of pSS dry eyes and could be a new marker to differentiate SS dry eyes from non-SS dry eyes.

Sjögren autoantibodies modify neonatal cardiac function via M1 muscarinic acetylcholine receptor activation

Isolated congenital heart block may be associated with primary Sjögren syndrome. In this work we describe circulating antibodies in the sera of primary Sjögren syndrome patients that are able to interact with neonatal myocardium by activating muscarinic acetylcholine receptors of M1 subtype. We report on the presence of autoantibodies against the second extracellular loop of human M1 muscarinic acetylcholine receptors in primary Sjögren syndrome mothers whose children have congenital heart block using a synthetic peptide in indirect immunofluorescence technique. Autoantibodies from primary Sjögren syndrome patients gave positive image on neonatal atria but not on adult atria slices. The synthetic M1 peptide selectively abrogated indirect immunofluorescence recognition. The primary Sjögren syndrome-immunoglobulin G also displayed an 'agonist like' activity modifying the intracellular events associated with muscarinic acetylcholine receptor activation. The mechanism appears to occur secondarily to stimulation of phosphoinositides turnover via phospholipase C activation. This, in turn, triggers cascade reactions involving calcium/calmodulin and leads to activation of nitric oxide synthase and soluble guanylate cyclase. All of these effects were selectively blunted by pirenzepine and neutralized by M1 synthetic peptide. These biological effects were not obtained using adult instead of neonatal rat atria and neither occurred with the sera of normal healthy women of childbearing age. It could be concluded that antibodies against neonatal M1 muscarinic acetylcholine receptor may be another serum factor to be considered in the pathophysiology of the development of congenital heart block associated with primary Sjögren syndrome mothers.

Autoantibodies against lacrimal gland M3 muscarinic acetylcholine receptors in patients with primary Sjögren's syndrome

PURPOSE

The authors demonstrated that immunoglobulin G, present in the sera of patients with primary Sjögren syndrome (pSS), could recognize and activate muscarinic acetylcholine receptors (mAChRs) of rat exorbital acrimal gland.

METHODS

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), immunoblotting, and radioligand binding and biologic assays were used to demonstrate autoantibodies against mAChRs.

RESULTS

These autoantibodies recognized by means of SDS-PAGE and immunoblotting assay a band of approximately 70 kDa expressed on lacrimal gland membranes that comigrated with the peak of labeled mAChRs. Moreover, pSS IgG were able to inhibit, in an irreversible manner, the binding of [3H]quinuclidinyl benzilate to mAChRs of rat exorbital lacrimal glands and to simulate the biologic effect of mAChR agonists, because they trigger the activation of phosphoinositide turnover. Atropine and 4-diphenylacetoxy-N-methylpiperidine methiodide blocked the effect and carbachol mimicked it, confirming that the M3 subtype mAChRs mediated pSS IgG action. As control, IgG from sera of women without pSS gave negative results on immunoblotting, binding, and biologic assays, thus demonstrating the specificity of the reaction.

CONCLUSIONS

Autoantibodies against mAChRs may be considered among the serum factors implicated in the pathophysiology of the development of pSS dry eyes.

Differential cholinoceptor subtype-dependent activation of signal transduction pathways in neonatal versus adult rat atria

In this study, we investigated the expression and distribution of muscarinic cholinergic receptors (mAChRs) and the different signaling pathways associated with mAChR activation in atria isolated from adult and neonatal rats. Carbachol stimulation of mAChRs in both neonatal and adult rat atria led to a negative inotropic response with activation of phosphoinositide hydrolysis, an increase in cyclic GMP levels, and a decrease in cyclic AMP production. However, compared with adult atria, neonatal atria showed hypersensitivity in the contractile effect induced by carbachol. Pharmacological analysis with mAChR antagonists indicated that M1 and M2 mAChR subtypes are important mediators of the response to carbachol in neonatal atria. In contrast, in adult atria the effect of the agonist was coupled only to the M2 mAChR subtype. Moreover, an increased number of total mAChRs was labeled in neonatal atrial membranes compared with those of adults. Although a predominant M2 mAChR population is expressed in atria at both stages of development studied, competition binding parameters calculated for carbachol indicated the presence of high-affinity binding sites, with higher affinity in neonates than in adults. These results suggest that the differences observed between neonatal and adult atria in their response to a cholinergic agonist may be related to differential expression of mAChR subtypes and/or changes in functional coupling of mAChR subtypes during development.

Circulating antibodies against neonatal cardiac muscarinic acetylcholine receptor in patients with Sjögren's syndrome

Isolated congenital heart block may be associated with Primary Sjögren's Syndrome. In this work we demonstrated that IgG present in the sera of patients with Primary Sjögren's Syndrome (PSS) could bind and activate muscarinic acetylcholine receptors of rat neonatal atria. These antibodies were able to inhibit in a irreversible manner the binding of 3H-QNB to muscarinic cholinergic receptors of purified rat atria membranes. Moreover, IgG from PSS individuals could modify biological effects mediated by muscarinic cholinoceptors activation, i.e. decrease contractility and cAMP and increase phosphoinositide turnover and cGMP. Atropine blocked all of these effects and carbachol mimicked them; confirming muscarinic cholinergic receptors-mediated PSS IgG action. Neither binding nor biological effect were obtained using adult instead of neonatal rat atria. IgG from sera of normal women were not effective in the studied system. The prevalence of cholinergic antibody was 100% in PSS and was independent of Ro/SS-A and La/SS-B antibodies. It could be concluded that antibody against muscarinic cholinergic receptors may be another serum factor to be considered in the pathophysiology of the development of congenital heart block.

Circulating antibodies against rat parotid gland M3 muscarinic receptors in primary Sjögren's syndrome

In this study we demonstrate that IgG present in the sera of patients with primary Sjögren's syndrome (PSS) could bind and activate muscarinic acetylcholine receptors (mAChRs) of rat parotid gland. These antibodies were able to inhibit in a non-competitive manner the binding of 3H-quinuclidinyl benzilate (QNB) to mAChRs of purified rat parotid gland membranes. Moreover, IgG from PSS could modify biological effects mediated by mAChR activation; i.e. decrease cAMP, increase phosphoinositide turnover without affecting cGMP. Atropine and 4-DAMP blocked all of these effects, and carbachol mimicked them, confirming the M3 subtype mAChRs mediated PSS IgG action. Neither binding nor biological effect were obtained with IgG from sera of normal women. The prevalence of cholinergic antibody was 100% in PSS, and was independent of Ro/SS-A and La/SS-B antibodies. It could be concluded that antibody against mAChRs may be another serum factor to be considered in the pathophysiology of the development of PSS.

Pharmacological evidence for the existence of different subtypes of muscarinic acetylcholine receptors for phosphoinositide hydrolysis in neonatal versus adult rat atria

The negative inotropic effect of carbachol, as well as phosphoinositide hydrolysis, was measured in atria from neonatal and adult rats. Carbachol increased phosphoinositide hydrolysis and decreased dF/dt of both neonatal and adult atria; however, the neonatal atria showed hypereactivity to carbachol as compared with adult atria. Inhibition of phospholipase C reduced the supersensitivity to carbachol upon contractility in neonatal atria producing values similar to those of the adult atria, suggesting that muscarinic acetylcholine receptor (mAchR) stimulation is secondary to receptor-mediated hydrolysis of phosphoinositides. Pharmacological analysis with mAchR antagonists tends to support the idea that m1 and m2 subtypes are the most important mediators of the response to carbachol in neonatal atria. In adult atria, the effect of carbachol is coupled only to mAchR m2 subtypes.

Circulating antibodies against neurotransmitter receptor activities in children with congenital heart block and their mothers

In this work we demonstrated that IgG present in the sera of patients with congenital heart block (CHB) and their mothers could bind and activate beta adrenoceptors and muscarinic cholinergic receptors of neonatal heart. These antibodies were able to inhibit in a noncompetitive manner the binding of [3H]QNB and [3H]CGP to muscarinic cholinergic and beta adrenoceptors of purified neonatal rat myocardial membranes, respectively. Moreover, IgG from children with CHB and their mothers could modify biological effects mediated by these neurotransmitter receptors, i.e., heart contractility and cAMP production. Neither binding nor biological effects were obtained using adult instead of neonatal rat atria. Both reactivities (adrenergic and cholinergic) were independent of Ro/SS-A and La/SS-B antibodies and were absent in the sera of normal women of childbearing age and of normal children. It could be concluded that antibodies against cardiac neurotransmitter receptors may be another serum factor (or factors) to be considered in the pathophysiology of the development of CHB.

Antibodies against beta adrenoceptors in mothers of children with congenital heart block

To determine whether antibodies against beta adrenergic activity interact with neonatal cardiac cell receptors and alter their physiological behaviour. An "in vitro" experimental model measuring the frequency of contraction, the production of cAMP and binding assay on neonatal and adult rat atria was employed. Sera and IgG fraction from mothers of infants with congenital heart block (CHB) interact with neonatal rat atria increasing the frequency of contraction and cAMP production. These effects were abolished by propranolol, pointing to a beta adrenergic participation. IgG also competed with 3H-CGP to beta adrenergic receptors on neonatal cardiac membranes. Neither the contractile nor the cAMP effects or binding assay were obtained using adult instead of neonatal rat atria. Reactivity against cardiac neurotransmitter receptors may be another serum factor(s) to be considered in the pathophysiology of the development of CHB.

Participation of cytoskeleton in the effect of antilaminin IgG on cardiac cholinoceptors

1. We have previously demonstrated a molecular relationship between laminin and cardiac cholinoceptors. 2. We have now explored the participation of cytoskeletal proteins in the interaction between an antilaminin IgG with cardiac cholinoceptors. 3. Antilaminin IgG, whilst it specifically reacts with laminin molecules was able to induce cardiac cholinoceptor activation; acting like an agonist, decreasing cyclic AMP concentrations, reducing heart contractility and increasing phosphoinositide turnover. 4. Antilaminin IgG also interfered with the binding of a radiolabelled muscarinic antagonist, [3H]-quinuclidinyl benzilate. Colchicine and cytochalasin B, drugs that are able to prevent microfilament and microtubule polimerization, impaired the binding of antilaminin IgG to muscarinic cholinoceptors. 5. Cytochalasin B but not colchicine modified the muscarinic cholinoceptor effects mediated by regulatory G proteins (cyclic AMP and contractility) induced by antilaminin IgG. 6. It was demonstrated, by immunofluorescence, that none of these disrupting drugs altered the specific recognition of the antibody by its antigen. 7. These data indirectly suggest the participation of the cytoskeleton in the laminin and cholinergic receptor association.

Antilaminin IgG triggers the murine atria phosphoinositide hydrolysis through muscarinic receptor stimulation

Induction of polyphosphoinositide hydrolysis in cardiac tissue by specific recognition of laminin by antilaminin IgG was assayed. BALB/c mice atria were labelled with the myo-[3H]-inositol precursor and inositol phosphate production was measured in the presence and absence of antilaminin and normal IgG. Antilaminin IgG but not normal IgG specifically increased phosphoinositide (PI) turnover. This increment was blocked by the muscarinic cholinergic antagonist atropine and mimicked by the cholinergic agonist carbachol. The phospholipase C inhibitor diphenylcarbamate (NCDC) also antagonized the stimulatory action of antilaminin IgG on PI turnover. By using an immunofluorescence technique, antilaminin IgG reacted with myocardial cell basement membranes. This antibody fixation was not blocked by atropine. These data suggest that antilaminin IgG specifically recognized myocardial laminin molecules and activated PI turnover through cholinergic stimulation. Even though laminin and cholinergic receptors are different, they probably share common signal transduction systems.

Chagasic IgG stimulates phosphoinositide hydrolysis via neurotransmitter receptor activation: role of calcium

Induction of polyphosphoinositide hydrolysis in cardiac tissue by IgG from chagasic mice was assayed. BALB/c mice auricles were labelled with myo-[3H]inositol precursor and inositol phosphate production in the presence or absence of chagasic IgG and the corresponding F(ab')2 was measured. Both chagasic IgG and F(ab')2 but not the normal forms specifically increased phosphoinositide turnover. This increment was blocked by muscarinic cholinergic antagonists and to an even greater extent by the phospholipase C inhibitor NCDC. Moreover, calcium channel blocking agents such as diltiazem, verapamil and D-600 also exerted an inhibitory action. A muscarinic cholinergic agonist, carbachol, and the ionophore A-23187, mimicked the action of the chagasic IgG upon phosphoinositide turnover. It is concluded that murine chagasic IgG and its F(ab')2 fragments result in stimulation of phospholipase C-mediated phosphoinositide hydrolysis through the interaction with muscarinic cholinergic receptors requiring the cytosolic calcium concentration to be raised.

Antilaminin IgG releases TXB2 through activation of the cholinergic system

Antilaminin IgG was bound to cholinergic muscarinic receptors of normal mice heart and released TXB2, simulating the biological effect of a cholinergic agonist. Antilaminin IgG interfered with the binding of the radiolabelled muscarinis antagonist (-)3H-QNB in a noncompetitive fashion. Following the interaction of the antibody with the cholinergic receptor, an increased production of TXB2 occurred. This effect required the activation of the muscarinic cholinergic system, because it was blunted by atropine and mimicked by acetylcholine.

Antilaminin IgG binds and interacts with cardiac cholinergic receptors

Antilaminin IgG bound to cholinergic muscarinic receptors of normal mice heart and simulated the biological effect of a cholinergic agonist. Antilaminin IgG interfered with the binding of the radiolabelled muscarinic antagonist, (-)-[3H]quinuclidinyl benzilate, in a noncompetitive fashion. The interaction of antilaminin IgG with the muscarinic cholinergic receptor increased production of cGMP and decreased production of cAMP. Antilaminin IgG also decreased the contractile tension of mouse atria. Both the mechanical and enzymatic effect of antilaminin IgG required the activation of the muscarinic cholinergic system because they were blunted by atropine and mimicked by acetylcholine.

Negative inotropic activity of antilaminin IgG: participation of cholinergic mechanisms

1. Antilaminin IgG decreased the dF/dt of mouse isolated atria and inhibited the mechanical effect of acetylcholine in a non-competitive fashion. 2. Inhibitors of nicotinic and muscarinic cholinoceptors impaired the negative inotropic action of antilaminin IgG in mouse isolated atria. Hemicholinium and tetrodotoxin also reduced the response while the antihistamine, pyrilamine was without effect. 3. These results suggest that antilaminin IgG modulates cholinergic function in mouse isolated atria. Possible mechanisms are discussed.