Huntington's disease: a generalized membrane defect

L-Homocysteic acid as an alternative cytotoxin for studying glutamate-induced cellular degeneration of Huntington's disease and normal skin fibroblasts

Huntington's Disease (HD) and normal skin fibroblasts in culture were exposed to several acidic amino acids structurally related to L-glutamate which have excitotoxic properties in the nervous system. L-Homocysteic acid, a sulfonic acid analogue of glutamate, was the only other acidic amino acid causing fibroblast degeneration similar to that induced by glutamate. None of the other compounds tested, including the D isomer of homocysteic acid, were as toxic as 30 mM glutamate. As previously noted with glutamate treatment, HD fibroblasts demonstrated an increased sensitivity to L-homocysteic acid compared to controls. In contrast to glutamate, no cellular metabolism of L-homocysteic acid could be detected; a property which may account for the increased cytotoxicity of L-homocysteic acid compared to glutamate. The identification of L-homocysteic acid, a glutamate analogue which undergoes limited metabolism, should enable the elucidation of the toxic mechanism of glutamate and facilitate the determination of the site conferring increased sensitivity of cultured HD fibroblasts to glutamate.

The mechanism of glutamate-induced degeneration of cultured Huntington's disease and control fibroblasts

Human cultured skin fibroblasts undergo rapid cellular degeneration and cell death when exposed to moderate levels of glutamate (10-30 mM). Huntington's disease (HD) skin fibroblast cultures are more sensitive to the toxic effects of glutamate (Gray et al. 1980) and elucidation of the toxic mechanism may bear on degenerative processes occurring in the HD brain. Glutamate toxicity was found to be inversely related to cystine content in the culture medium. Supplementing culture media with cystine effectively suppressed toxicity of L-Glu and that of a closely related but more potent analogue, L-homocysteic acid (L-HCA). Glutamate and L-HCA treatment provoked a rapid and marked decrease in cellular glutathione levels prior to cell death. Depletion of cellular glutathione with buthionine sulfoximine potentiated the toxicity of glutamate and L-HCA. Glutamate- or L-HCA-induced cell death also could be reduced or inhibited by co-treatment with antioxidants, thereby suggesting that free radical-induced peroxidative damage may ultimately be responsible for the loss of cell viability. While no site for the differential sensitivity of HD and normal fibroblasts to glutamate was clearly discerned, our results suggest that HD fibroblasts are more susceptible to peroxidative damage. The increased deposition of lipofuscin (Tellez-Nagel et al. 1974) and abnormal neuronal membrane ultrastructure seen in HD (Roizin et al. 1979) suggest similar degenerative processes may occur in HD brain.

Diabetes mellitus in Huntington disease

There have been conflicting reports that individuals with Huntington disease (HD) are prone to abnormalities of carbohydrate metabolism. In this study information about the incidence and control of diabetes mellitus in 620 probands (278 living, 332 deceased) with HD and in their first and second degree relatives was obtained by questionnaire method from participants of the National HD Research Roster. Among the probands, 65 individuals (10.5%) were identified by the informant or verified by examination of Roster family records as diabetic. The prevalence of diabetes, particularly among those les than 50 years of age, is significantly greater than corresponding figures among the general U.S. Caucasian population (Scott 1977, Krolewski & Warram 1985). Incidence rates were not calculated because of ascertainment and other biases in the data. Results from the analysis of family data indicate that HD affected relatives of an HD proband with diabetes are 7 times as likely to have diabetes over the proband's non-HD relatives. A non-diabetic HD proband is equally likely to have an HD or non-HD relative with diabetes.

The alteration of membrane proteins in human erythrocyte membranes induced by quinolinic acid, an endogenous neurotoxin. Correlation of effect with structure

Quinolinic acid (2,3-pyridinedicarboxylic acid), an endogenous metabolite of L-tryptophan, reportedly via the kynurenine pathway, has been previously shown to possess neurotoxic properties when injected into rat striatum (Schwarcz, R., Whetsell, W.O., Jr. and Mangano, R.M. (1983) Science 219, 316-318) and to alter the physical state of human erythrocyte membrane proteins, as judged by ESR spectroscopy (Farmer, B.T., II and Butterfield, D.A. (1984) Life Sci. 35, 501-509). Both the morphologic and ESR studies employed nicotinic acid as one comparative control and found that the effect of quinolinic acid is significantly different from that of nicotinic acid. In the present study, we report that the effects of several structural analogues and positional isomers of quinolinic acid on the ESR parameter associated with the physical state of membrane proteins in human erythrocyte membranes suggest the following conclusions concerning the structure-effect relationship of quinolinic acid: The alteration in the conformation of membrane proteins: (1) requires the presence of two carboxylic acid groups; (2) is independent of their relationship to one another on the pyridine ring; (3) is slightly dependent on the presence of the pyridine nitrogen atom but is independent of the positional relationship of the two carboxylic acid moieties to the heteroatom; and (4) seems to depend upon the presence of restricted internal motion derived from the aromaticity in these compounds.

Membrane anomalies in Huntington's disease fibroblasts

Plasma membranes, microsomes, and mitochondria were isolated from paired, passage number matched, cultured human fibroblasts. The cells were obtained from skin biopsies of Huntington's disease (HD) subjects and from sex and age matched controls. All fibroblasts were cultured in identical media for three to seven passages. Enrichment of surface marker enzymes such as Na+,K+-ATPase indicated a 10-fold purification of the isolated plasma membrane. The specific activity of Na+,K+-ATPase was 62 and 82% greater in the crude homogenate and isolated plasma membrane, respectively, of HD fibroblasts than in control fibroblasts. The specific activity of plasma membrane Na+,K+-ATPase was correlated with lipid composition and with membrane structure as determined by measurement of the rotational relaxation time and limiting anisotropy of fluorescence probe molecules. Major alterations in the structure of the plasma membranes in HD fibroblasts were not noted. The rotational relaxation time and limiting anisotropy of 1,6-diphenyl-1,3,5-hexatriene and of trans-parinaric acid were not significantly different between the plasma membrane, microsomes, or mitochondria of HD versus those of control fibroblasts. trans-Parinaric acid demonstrated the coexistence of fluid and solid domains in all three subcellular membrane fractions of the normal and HD skin fibroblasts. Lastly, both trans-parinaric acid and 1,6-diphenyl-1,3,5-hexatriene displayed characteristic breakpoints in Arrhenius plots of absorbance corrected fluorescence in plasma membranes, microsomes, and mitochondria. In all cases, similar breakpoint temperatures, indicative of phase alterations, were noted near 20 degrees and 30 degrees C. These breakpoints were unaltered in HD. In summary, the data do not support the concept of major membrane structural defects in HD.

Cholesterol synthesis in cultured skin fibroblasts from patients with Huntington's disease

In view of the proposed membrane defect in Huntington's disease, cultured skin fibroblasts from healthy volunteers and patients with Huntington's disease were compared with respect to their ability to carry out de novo synthesis of cholesterol. At confluency, values for incorporation of [14C]acetate and 3H2O into cholesterol, and activities of HMG-CoA reductase (the rate-limiting enzyme in the cholesterol biosynthetic pathway), did not differ significantly in the Huntington's disease cells compared to the controls. Determinations of total cellular cholesterol gave similar ratios of cholesterol/protein and cholesterol/phospholipid in the Huntington's disease and control fibroblasts. The data suggest that the proposed generalized cell membrane abnormality in Huntington's disease cannot be attributed to a defect in the cholesterol biosynthetic pathway.

Electron spin resonance of erythrocytes in Huntington's disease

Electron spin resonance (ESR) studies were carried out with MAL-6 as spin label on erythrocytes from 21 patients with Huntington's disease (HD) and 18 controls. No significant difference was found between the ESR spectra from HD patients and controls. These findings do not support the theory that HD is a generalized membrane defect.

Evidence for normal fibroblast cell membranes from individuals with Huntington's disease. A fluorescence probe study

We used fluorescence spectroscopy in an attempt to identify differences in the fibroblast cell membranes from individuals with Huntington's disease (HD), compared to those from normal individuals. Eight pairs of age- and sex-matched fibroblast cultures were examined, each using four different fluorescent probes. To maximize the possibility of finding a difference between the normal and HD cells fluorescence probes were selected which localize in different regions of the membrane. Also, these probes differed in their sensitivity to the surrounding environment. Procedures were developed for labeling of the fibroblasts while they were still attached to glass coverslips. All fluorescence measurements were done on attached cells, which were all found to be viable by staining with trypan blue. Both low (5-7) and high (11-13) passage cultures were examined since earlier reports indicated that normal and HD fibroblasts could be distinguished at low passage using a fluorescence method. Using these 16 cultures, and multiple determinations of the fluorescence emission spectra, anisotropies, and lifetimes, we found no significant difference between normal and HD fibroblasts. If such a difference exists, it appears to be too small for use as a diagnostic indicator for Huntington's disease.

Studies of the mobility of maleimide spin labels within the erythrocyte membrane

We have confirmed a method yielding reproducible and reliable spectrometric parameters derived from spin-labeled erythrocyte ghosts using nitroxide derivatives of maleimide compounds. The disorder parameter, W/S, was shown to vary with changes in the structure of the label, the conditions utilized for labeling such as ionic strength and erythrocyte age and the presence of drugs such as alcohol and acetaminophen. The nitroxide spectrum was also found to change with increasing and decreasing temperature in an irreversible manner. These findings should permit increased reliance to be placed on the spin-labeling technique when used to monitor changes in membrane lipid or protein assembly.

Spin labeling studies of membrane proteins in erythrocyte ghosts from patients with Huntington's disease

Previous electron spin resonance (ESR) studies employing a protein-specific spin label have suggested an alteration in membrane proteins in erythrocytes in Huntington's disease (HD). Recently, Beverstock and Pearson [4] have published the results of an ESR study not confirming our findings. In the present communication, we show that with correct use of the statistical technique of the Dixon test to determine if extreme values can be eliminated, one of the data points of these authors can be rejected. Recalculation of their results suggests a significant difference in the physical state of membrane proteins in HD erythrocytes, confirming our findings.

Abnormal platelet aggregation response in Huntington's disease

Platelet aggregation response to epinephrine, dopamine, serotonin, adenosine diphosphate, arachidonic acid, and collagen was examined in seven patients with Huntington's disease and nine of their relatives. All patients, except for two cases that were in terminal states, showed enhanced response to all the stimulants, especially to dopamine and epinephrine. The platelet aggregation response in many relatives also deviated from the normal limit. The relationship between platelet aggregation abnormality in Huntington's disease and the pathophysiology of the disease was discussed from the view of a generalized membrane defect hypothesis in Huntington's disease, and of disturbed cathecholamine metabolism, both in the CNS and periphery. A possibility that platelet aggregation response examination will be a useful screening test of offspring at risk was proposed.

Time-dependence of, and effects of inhibition and cellular aging on, chloride efflux across erythrocyte membranes in Huntington's disease

Previous studies have demonstrated an increased rate of chloride transport across erythrocyte membranes in Huntington's disease, a process regulated at the external side of the major transmembrane protein Band 3. A marked effect of time was noted in 3 Huntington's disease samples that were studied more than 4 h after obtaining the blood. In order to study anion transport more closely in Huntington's disease and the apparent time-dependence of chloride efflux in this disorder, we have performed several sets of experiments. Chloride transport in Huntington's disease erythrocytes was found to be extremely sensitive to time with the efflux rate constant decreasing by approximately 30% over a 24 h period. Chloride transport in control cells was unaffected by time. Inhibition studies with the specific anion transport blocker 4,4'-diisothiocyanto-2,2'-disulfonic acid stilbene (DIDS) demonstrated that the same degree of inhibition of chloride transport could be achieved at a much lower concentration of DIDS in Huntington's disease than in controls. Comparison of chloride efflux in fractions enriched in young and old erythrocytes, respectively, obtained by density centrifugation of fresh blood, demonstrated that only in the young fraction of cells was chloride efflux diminished with time in Huntington's disease. Chloride transport in in vivo aged Huntington's cells or both young and old control cells was essentially not dependent on time. These results are discussed in terms of proposed molecular mechanisms for neuronal loss in this disorder. The alterations in chloride efflux in extraneural erythrocytes are consistent with a proposed cell-surface membrane defect involving a protein in Huntington's disease.

Lysophosphatidylcholine-induced lysis of erythrocytes in Duchenne and myotonic dystrophies and in Huntington's disease

Erythrocytes from Duchenne dystrophy patients lysed more readily than red cells from age-matched normal boys when lysophosphatidylcholine (LPC) concentrations that caused 50% lysis were compared. Erythrocytes from myotonic dystrophy patients appeared to be more resistant than cells from age-matched normal adults at certain medium LPC concentrations. Erythrocytes from patients with Huntington's disease showed no significant differences from erythrocytes of normal adults. Thus, the manner in which erythrocytes respond to LPC may reflect the putative membrane alterations in these diseases. Inhibition of LPC-induced lysis by 0.1 mM dipyridamole was observed in all groups. Since this agent did not inhibit LPC lysis at 0 degrees C, its action at 37 degrees C could be related to activation of a membrane enzyme. On the other hand, dipyridamole decreased osmotic fragility at 0 degrees C and 37 degrees C indicating that a physical change in membrane structure may be the primary alteration produced by this agent.

Erythrocyte phosphate content in Huntington's disease

Total phosphate was higher in erythrocytes of patients with Huntington's disease (HD) than in erythrocytes of age- and sex-matched normal adults. A more rapid rate of organic phosphate degradation was also noted in HD erythrocytes during metabolic depletion. These variations may be associated with alterations of membrane permeabilities of HD erythrocytes to inorganic phosphate and to cations.

Fluorescence studies of red blood cell membranes from individuals with Huntington's disease

Fluorescence spectroscopic methods were used to investigate and compare the properties of erythrocyte membranes from individuals with Huntington's Disease (HD) and from normal individuals. Erythrocyte ghosts were labeled with four different fluorescent probes: 1,6-diphenylhexatriene (DPH); 6-lauroyl-2-(dimethylamino)-naphthalene (Laurdan); 2-(4-maleimide anilino)-naphthalene-6-sulfonic acid (MIANS) and 5-(iodoacetamidoethyl)aminoaphthalene-1-sulfonic acid (IAEDANS). DPH is sensitive to the microviscosity of the membranes. Laurdan is highly sensitive to the polarity and relaxation time of its environment. MIANS and IAEDANS both react covalently with sulfhydryl groups in membrane proteins. Erythrocyte membranes were labeled separately with each of these four probes, and we measured the centers of gravity of the fluorescence emission, the fluorescence anisotropies, and the fluorescence lifetimes. In 10 separate experiments, including a total of 24 patients and 14 control individuals, we found no significant differences between the two groups for any of the probes or spectral parameters. These results do not support the existence of a "generalized membrane defect" in individuals with HD.

Elevation of erythrocyte glycine levels during lithium treatment of affective disorders

An elevation of erythrocyte (RBC) glycine was observed in a group of patients with bipolar affective disorder who were being treated with lithium (Li) carbonate. A maximal increase of 90% or more was generally attained after about 100 days of Li intake and was maintained for 2 years or longer. After 3 or more years of Li therapy, the RBC glycine had often returned to the normal range. Abrupt withdrawal of Li in such a case produced an immediate, dramatic, and paradoxical increase in RBC glycine which lasted for 100 days. Plasma glycine was not affected by Li.

Myotonic muscular dystrophy. Time-dependent alterations in erythrocyte membrane fluidity

The muscle cell membrane may be the site of the basic molecular defect in myotonic muscular dystrophy. Many laboratories, including our own, have suggested that this defect may also be manifested in membrane of extraneural tissue. In previous studies, we found that electron spin resonance results suggested an increased membrane fluidity in erythrocyte membranes that had aged two days in buffer, but we and others could find no such changes in fresh erythrocyte membranes. To investigate these findings further, the results of an initial study of the time course of the membrane fluidity changes in erythrocytes in myotonic muscular dystrophy are given in the present report. They suggested that increased membrane fluidity changes in erythrocytes in myotonic muscular dystrophy are given in the present report. They suggested that increased membrane fluidity in myotonic dystrophy is manifested after two days of in vivo ageing and confirm our original findings. These results are discussed in relation to possible effects of metabolic deprivation or of protein-lipid alterations in erythrocytes.