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Biomedical subjects

S H Appel

Publications and source records attributed to S H Appel.

At least 163 records · Page 9Linked to original sources
PubMed1985

Endogenous inhibitor of ligand binding to the muscarinic acetylcholine receptor.

An endogenous inhibitor of L-[3H]quinuclinidinyl benzilate binding to the brain muscarinic acetylcholine receptor was identified. [3H]Quinuclinidinyl benzilate binding to rat brain synaptosomes was measured using a filtration assay. The inhibitor was prepared from several calf tissues and was found in highest specific activity in thymus. The loss of binding activity was slow, requiring a 30-40 min preincubation of the synaptosomes with the inhibitor, and reversed by removing the inhibitor by washing the membranes. Scatchard analysis of the binding data showed that the inhibition was noncompetitive resulting from both a decrease in affinity and a decrease in the number of binding sites. Zn2+ was required in low concentrations for this effect. Muscarinic acetylcholine receptor in synaptic membranes and in membranes free of most peripheral membrane proteins was still sensitive to inhibition. Preliminary characterization of the inhibitory molecule showed that it is of low molecular weight, moderately heat-stable, and acidic. The inhibitor was inactivated by reagents that are nonspecific for nucleophiles, but not by reagents specific for primary amine or thiol groups.

Animals↗
PubMed1985

Complement-dependent lysis of cultured rat myotubes by myasthenic immunoglobulins.

We used cultured myotubes to demonstrate complement-dependent lysis of muscle membranes by serum from patients with myasthenia gravis. Lysis was monitored by light microscopy and release of incorporated [86Rb]. In the presence of guinea pig complement (GPC), 18 of 37 heat-inactivated myasthenic sera (49%), but none of 16 controls, caused morphologically detectable myotube lysis. Ten of 19 myasthenic sera (53%) increased [86Rb]-release compared with 10 controls. Immunoglobulin fractions retained the complement-dependent lytic activity. Inactivation of GPC prevented the lysis. [86Rb]-release appeared to correlate with clinical severity. The complement-dependent lysis resulted in a decrease in the number of acetylcholine receptors (AChRs) in myotubes, and AChR-immunoglobulin complexes were found in the medium of lysed cultures. The data suggest that cultured myotubes can be used to document complement-dependent antibody reactions in the pathogenesis of myasthenia.

Adult↗
PubMed1985

Choice reaction time modifiability in dementia and depression.

The effects of cognitive impairment resulting from either dementia of the Alzheimer type (DAT) or major depression (pseudodementia) on choice reaction time were examined in two conditions hypothesized to influence group performance selectively. Elderly controls had shorter reaction times than depressed patients who, in turn, were faster than dementia patients in the standard choice reaction time test. Elderly control and depressed subjects responded more quickly under conditions designed to reduce task demands. However, no effect was detected for DAT patients, presumably due to the neural constraints imposed on cognitive functioning in dementia. These results highlight the different etiologies for the intellectual decline in dementia and pseudodementia.

Aged↗
PubMed1984

Neurotrophic effects of hippocampal extracts on medial septal nucleus in vitro.

Within peripheral sympathetic and sensory systems, target tissue provides diffusible factors such as nerve growth factor that influence neuronal survival, growth, and differentiation. To determine whether target tissue may exert retrograde effects within the central nervous system, we have used hippocampus as a source of neurotrophic factors and cultured medial septal nucleus explants as a source of neurons that may respond to such factors. Soluble extracts from rat hippocampus enhance cholinergic activity (choline acetyltransferase, choline uptake, acetylcholine synthesis) of the rat medial septal nucleus cultured in serum-free defined medium. The enhancement is dose dependent, relatively specific for hippocampus, and varies with age, reaching a peak in 2- to 3-week-old rat hippocampus. The enhancing activity of the hippocampal extract appears to be mediated by protease-sensitive polypeptide(s) that differ from nerve growth factor in biological and chemical characteristics.

Acetylcholine↗
PubMed1984

Organization of acetylcholine receptor clusters in cultured rat myotubes is calcium dependent.

The effect of extracellular Ca2+ concentration and myasthenic globulin on the distribution and appearance of acetylcholine receptor (AChR) clusters on rat myotubes was studied with tetramethyl-rhodamine-labeled alpha BTX. Low Ca2+ medium (2.5 X 10(-5) M) caused a time-dependent loss of AChR clusters, and a concomitant increase in small punctate areas of fluorescence. High Ca2+ concentrations (1.5 X 10(-2) M) increased the size of AChR clusters without altering AChR synthesis. These changes were not observed with other divalent ions. In the presence of myasthenic globulin, the rate of AChR turnover increases, and AChR clusters are rapidly dispersed. High Ca2+ concentration partially protects the AChR clusters from dispersal and decreases the rate of receptor turnover.

Animals↗
PubMed1984

Calcium-dependent hydrolyses of polyphosphoinositides in human erythrocyte membranes.

Incubation of erythrocytes with [32P]phosphate resulted in a linear incorporation of the label into PtdIns(4,5)P2 (phosphatidylinositol 4,5-bisphosphate), PtdIns4P (phosphatidylinositol 4-monophosphate), and PA (phosphatidic acid) over a period of 2 h at 37 degrees C. Exposure of 32P-labelled erythrocyte ghosts to calcium caused a loss of label from PtdIns(4,5)P2 and PtdIns4P, but not PA. The concentration of calcium required for half-maximal hydrolyses of both polyphosphoinositides was about 1 microM. Strontium, at higher concentrations, stimulated the hydrolyses of both polyphosphoinositides but barium, up to 1 mM, had little effect. Intact erythrocytes incubated in the presence of Ca-EGTA buffers and the ionophore A23187 did not show marked losses of [32P]PtdIns(4,5)P2 or [32P]PtdIns4P, but rather exhibited a dramatic increase in the level of [32P]PA. In contrast, cells which had been depleted of their ATP lost significant amounts of [32P]PtdIns(4,5)P2 and [32P]PtdIns4P and had less change in their levels of [32P]PA relative to intact cells. The calcium activation curve and the time course for [32P]PA synthesis in intact cells were similar to the calcium activation curve and the time course for the hydrolyses of [32P]PtdIns(4,5)P2 and [32P]PtdIns4P in ATP-depleted erythrocytes. These results strongly support a link between Ca2+-dependent polyphosphoinositide breakdown and PA synthesis in human erythrocytes.

Adenosine Triphosphate↗
PubMed1983

Extracts of skeletal muscle increase neurite outgrowth and cholinergic activity of fetal rat spinal motor neurons.

A soluble extract of rat skeletal muscle increased neurite outgrowth and cholinergic activity of dissociated ventral spinal neurons in culture. The effects were concentration-dependent, saturable, and labile in the presence of heat or trypsin. The morphological enhancement was produced only by skeletal muscle extract and decreased with developmental age, whereas the cholinergic enhancement was produced by extracts of cerebral cortex and cardiac and skeletal muscle and did not change with age. These changes were specific for ventral cord neurons, but no species specificity was observed with respect to the muscle source or the neuronal target.

Acetylcholine↗
PubMed1983

Myasthenic globulin enhances the loss of acetylcholine receptor clusters.

Acetylcholine receptors are present in the sarcolemma of cultured skeletal muscle myotubes either as large clusters or in a diffuse distribution. Both the clustered and diffuse acetylcholine receptors are potentially removable from the membrane. Treatment of myotubes with globulin from patients with myasthenia gravis causes the loss of acetylcholine receptor clusters and the concomitant appearance of acetylcholine receptor microaggregates. The rate of acetylcholine receptor cluster loss is greater than the rate of acetylcholine receptor degradation, indicating that acetylcholine receptors are disrupted from clusters to form microaggregates before being removed from the plasma membrane.

Autoantibodies↗
PubMed1982

Inhibitors of membrane depolarization regulate acetylcholine receptor synthesis by a calcium-dependent, cyclic nucleotide-independent mechanism.

The inhibition of membrane depolarization by tetrodotoxin or the local anesthetic benzocaine elevates the acetylcholine receptor levels in cultured myotubes. The elevated acetylcholine receptor levels are due to increased receptor synthesis rather than to decreased degradation. The effects of tetrodotoxin and benzocaine on acetylcholine receptor levels are not additive, and are not inhibited by exogenously added cyclic GMP analogues or by elevated intracellular levels of cyclic GMP. However, the stimulation of acetylcholine receptor levels by tetrodotoxin or benzocaine is reversed by the addition of the calcium ionophore A23187. In contrast, tetrodotoxin or benzocaine stimulated acetylcholine receptor synthesis beyond the maximal stimulation produced by cholera toxin. These results suggest that the inhibition of membrane depolarization elevates acetylcholine receptor synthesis by a calcium-dependent, cyclic nucleotide-independent mechanism.

Animals↗
PubMed1982

Interaction of myasthenic immunoglobulins and cholinergic agonists on acetylcholine receptors of rat myotubes.

Acetylcholine receptor (AChR) antibodies from patients with myasthenia gravis (MG) impair neuromuscular transmission by altering the number of AChRs at the skeletal muscle motor endplate. Cholinergic agonists similarly impair transmission by altering the number and affinity of AChRs. The frequent clinical resistance ot anticholinesterase medication in patients with MG has raised the question of possible synergistic effects of these agents on regulation of AChRs and neuromuscular transmission. To investigate this question, rat myotube cultures were incubated with MG globulin and carbamylcholine (CMC) for 20 hours. The number of AChRs was assayed by 125I-alpha-bungarotoxin binding. Incubation of cultures with both MG globulins and CMC consistently produced greater reductions of AChRs than incubation with either substance alone. Cholinergic antagonists blocked the CMC effect but not the MG globulin effect. A muscarinic antagonist had no effect. The effects of short-term incubation with these substances on the affinity of AChR were assessed by the rate of 125I-alpha-bungarotoxin-AChR complex formation. CMC desensitized AChRs, but MG globulins did not alter the affinity of AChRs. Results with this in vitro model suggest that chronic anticholinesterase therapy in the presence of MG AChR antibodies may aggravate failure of neuromuscular transmission in MG.

Animals↗
PubMed1982

Acetylcholine receptor turnover in myasthenia gravis.

Humoral antibodies directed against the acetylcholine receptor are present in most patients with myasthenia gravis. These antibodies accelerate receptor degradation and thus cause a decrease in receptor level and impaired neuromuscular transmission. The stimulus that triggers and sustains the formation of the antibodies has not been identified, but it is possible that the primary antigen may be located on the cell surface of thymic lymphocytes. In some situations the clinical state in myasthenia gravis does not correlate with the antibody titer, suggesting that other factors also influence receptor amount and/or function of other potential regulators that have been investigated. Calcium and agents that increase cyclic AMP appear to increase the formation of the receptor and thus might serve to mitigate the effects of the circulating antibodies. In contrast, cholinergic agonists may cause a further decrease in receptor number, providing an explanation of why myasthenic patients may become resistant to therapy.

Animals↗
PubMed1981

Myotonic dystrophy: calcium-dependent phosphatidic acid synthesis in erythrocytes.

Recently it was reported that calcium-dependent phosphatidic acid synthesis in erythrocyte of patients with myotonic muscular dystrophy (MyD) is markedly impaired when compared to that in control subjects. Using 32P-loaded erythrocytes, we found no significant difference in the levels of 32P-phosphatidic acid synthesized after exposure to calcium and its ionophore A23187 between patients with MyD and controls. In a batch experiment typical of the experiments with 32P, and a twofold increase of phosphatidic acid in both groups was determined by inorganic phosphate measurements. Thus, the specific activity of the 32P-phosphatidic acid increased four- to five-fold in response to calcium Analyses of 32P-polyphosphoinositide breakdown in ghosts and in adenosine triphosphate-depleted erythrocytes also appeared normal for patients with myotonic muscular dystrophy. Possible discrepancies between the results presented here and those reported previously are discussed.

Adenosine Triphosphate↗
PubMed1981

A unifying hypothesis for the cause of amyotrophic lateral sclerosis, parkinsonism, and Alzheimer disease.

The causes of amyotrophic lateral sclerosis, Parkinson disease, and Alzheimer disease are unknown. Furthermore, treatment for two of these conditions is almost totally lacking. The thesis is presented that each of these disorders is due to lack of a disorder-specific neurotrophic hormone. The hormone would be elaborated or stored in the target of the affected neurons. It would be released by the postsynaptic cell and then exert its effects in a retrograde fashion after being taken up by the presynaptic terminal. In the lower motor neuron syndromes of amyotrophic lateral sclerosis, failure of muscle cells to release the appropriate motor neurotrophic hormone would result in impaired function of anterior horn cells. In Parkinson disease, the neurotrophic failure would be characterized by inability of striatal cells to provide the required dopamine neurotrophic hormone with resulting impairment of substantia nigra cells. In Alzheimer disease, the abnormalities would lie in failure of the hippocampus and cortical cells to supply the relevant cholinergic neurotrophic hormone with resulting impairment of medial septal and nucleus basalis neurons. Central nervous system tissue culture provides a convenient system in which to assay these neurotrophic hormones and should permit a test of the hypothesis.

Alzheimer Disease↗
PubMed1981

The effect of calcium on acetylcholine receptor synthesis.

The level of cytoplasmic calcium has been proposed to act as a regulator of acetylcholine receptor synthesis (Betz, H., and J. P. Changeaux (1979) Nature 278: 749-751). However, there is little known about the effect of altered calcium levels on the metabolism of the acetylcholine receptor. We have investigated the effect of decreased extracellular calcium on the metabolism of acetylcholine receptors in cultured rat myotubes. Our results show that the acetylcholine receptor levels on the surface of myotubes were decreased 25 to 30% following overnight incubation in calcium-deficient medium. In contrast, creatine phosphokinase activity levels and total protein synthesis were unaffected. Calcium depletion did not change the rate of receptor degradation significantly (0.037 hr-1, compared to 0.033 hr-1 for control cells) but dramatically decreased the rate of incorporation of new acetylcholine receptors into the plasma membrane. The time course for incorporation of new acetylcholine receptors into the plasma membrane of calcium-depleted cells was similar to control cells treated with cycloheximide, suggesting that de novo receptor synthesis was inhibited. These results indicate that intracellular calcium levels and acetylcholine receptor synthesis are not related in a simple reciprocal fashion and suggest that the regulation of acetylcholine receptor levels involves more than one intracellular compartment of calcium.

Animals↗
PubMed1980

Regulation of acetylcholine receptor by cyclic AMP.

In primary cultures of chick 11-day embryonic tissue a number of phosphodiesterase inhibitors were found to elevate acetylcholine receptor levels. Of these agents, Ro20-1724 was the most effective, elevating surface receptor content by 2-fold after 48 h of treatment. 8-Br-cAMP and cholera toxin, a natural activator of adenylate cyclase, mimicked the effect of Ro20-1724, while 8-Br-cGMP and dibutyryl cGMP had no effect. Cholera toxin, 8-Br-cAMP, and Ro20-1724 all increased the insertion rate of new receptor into the surface membrane without altering degradation. The enhanced insertion appears related to an actual increase in synthesis since total acetylcholine receptor was elevated by exposure to cholera toxin. In contrast, no change in creatine phosphokinase activity, myosin heavy chain content, or [35S] methionine incorporation into total cellular protein was observed during cholera toxin treatment. These results suggest that cAMP plays a role in the regulation of acetylcholine receptor.

Acetylcholine↗
PubMed1979

Neonatal myasthenia gravis in the infant of a myasthenic mother in remission.

A 26-year-old woman was in spontaneous clinical remission from myasthenia gravis (MG) for six months, yet gave birth to a full-term infant with typical neonatal MG. It is believed that transplacental transfer of anti-acetylcholine (ACh) receptor antibodies is responsible for neonatal MG; therefore, neonatal MG represents an in vivo assay of the pathogenic potential of anti-ACh receptor antibodies in 2 human individuals. Anti-ACh receptor antibodies were present in both mother and infant (titers 12.3 X 10(-9) and 4.4 X 10(-9) moles per liter, respectively) at the time of birth, and both mother's and infant's sera accelerated the degradation of ACh receptors in myotube cultures. This case suggests that "host factors" unique to the individual appear to modify or even determine whether the presence of anti-ACh receptor antibodies will result in clinical myasthenia.

Adult↗