Progrès récents des neurosciences et maladies neurodégénératives/ Recent progress made in neurosciences and neurodegenerative diseases

Colloque organisé par François Gros et Émile Benassayag du 5 au 10 septembre 1998

Participants

Ruth Arnon (Institut Weizmann, Israël), Jean-François Bach (Hôpital Necker, Paris), Louis Benabid Alim (Université Joseph-Fourier, Grenoble), Émile Benassayag (Hôpital Cochin, Paris), Konrad Beyreuther (Université de Heidelberg), Dominique Dormont (Commissariat à l’énergie atomique (CEA), Fontenay-aux-Roses), Bruno Dubois (Hôpital de la Salpêtrière, Paris), Gilles Fenelon (Hôpital Tenon, Paris), Irith Ginzburg (Institut Weizmann), François Gros (Académie des sciences), Jacques Mallet (Hôpital de la Salpêtrière, Paris), Jean-Louis Mandel (Université Louis-Pasteur, Illkirch), Judith Melki (Université Louis-Pasteur), Kenneth Moya (CEA), Michael Sela (Institut Weizmann), Agnès Ullmann (Institut Pasteur, Paris).

Résumé

Le rapprochement, non habituel dans les réunions scienti­figues, entre cliniciens et fondamentalistes, l’originalité de certaines notions et l’excellente convivialité ont marqué le col­loque de la Fondation des Treilles sur les maladies neurodégé­nératives.

L’échange des points de vue a été ininterrompu, tant au cours des communications que pendant les temps libres et les repas pris en commun ; nul formalisme n’a entravé la simplicité, la profondeur des conversations et des débats.

Les tableaux cliniques présentés ont mis en exergue les parti­cularités intéressantes pour la compréhension de la maladie et de ses mécanismes biologiques et, en parallèle, les travaux de recherches fondamentales ont été originaux. La biologie fon­damentale connaît en effet d’importants progrès, et notamment dans les neurosciences, l’immunologie cellulaire, la biologie du développement et la génétique. La génétique moderne, dans sa nouvelle démarche baptisée “génomique”, a permis de faire un pas très important au cours de la décennie écoulée. En effet, la recherche de gènes “candidats” (sites mutationnels potentielle­ment responsables des maladies étudiées), leur localisation sur la carte chromosomique et, dans divers cas, leur clonage, suivi de l’établissement de leur séquence, ont permis de relier de nombreuses neuropathies centrales ou périphériques à des alté­rations génétiques précises.

Ces données contribuent à faciliter considérablement la clas­sification de maladies d’étiologie souvent complexes, telles que de nombreuses affections neuromusculaires, à en établir le dia­gnostic et devraient permettre d’ouvrir des pistes nouvelles à la thérapeutique.

Report

D’une manière générale, il apparaît que la classification entre maladies monogéniques et maladies polygéniques est sans doute moins tranchée qu’on ne le pensait au début, dans la mesure où la même mutation chez des individus distincts peut déclencher une pathologie définie ou non, ou donner lieu à des pathologies ayant des degrés de sévérité très variés. Cela suggère que le terrain génétique joue un rôle majeur (gènes modificateurs). Quant aux gènes dits de “risque”, leur impor­tance dans la prédisposition à certaines neuropathies centrales multifactorielles (type Alzheimer) a été plusieurs fois soulignée.

Une illustration particulièrement frappante des apports de la génomique à l’analyse des mécanismes de diverses neuropa­thies réside dans les observations récentes sur le rôle des expan­sions de triplets dans leur déclenchement (ex. : ataxies de Friedrich, myatonies, etc.). Là encore, des possibilités très importantes se dessinent en matière de diagnostic.

La biotechnologie moderne permet de créer de nouveaux procédés de diagnostic et de forger des outils thérapeutiques originaux. Et pourtant, des maladies à haute incidence sociale restent en partie inexpliquées : c’est le cas en particulier de plu­sieurs types de neuropathies. Il s’agit d’affections neurodégéné­ratives : encéphalopathies et prions, maladie d’Alzheimer, mala­die de Parkinson, sclérose en plaques et maladies neuromuscu­laires (amyotrophie spinale infantile, etc.).

Rapprocher cliniciens et fondamentalistes de divers pays sur des questions relatives à des pathologies de grande importance humaine et sociale contemporaine : voilà quel fut l’objet de ce colloque.

L’ensemble des conclusions a suscité préoccupations et espoirs. Préoccupations très grandes pour les maladies neuro­musculaires d’origine neurogène, telles la sclérose latérale amyo­trophique et la très voisine atrophie musculaire spinale infantile. Tous les espoirs et tous les efforts de recherches portent sur l’étu­de des désordres génétiques. Préoccupations très grandes aussi en ce qui concerne les prions, qui bouleversent les notions sur la pathogénicité, car ils pourraient être une entité infectieuse purement protéique. En tout cas, ils bouleversent déjà bien des aspects de la chaîne alimentaire et ébranlent fortement les notions et la pratique de la stérilisation en milieu chirurgical.

Pour la maladie d’Alzheimer, l’aspect clinique, mais surtout les modifications biomoléculaires et leur incidence sur la struc­ture même et la physiologie de la cellule nerveuse ont été débat­tus. La note originale est peut-être une analogie de perturbation biomoléculaire entre Alzheimer et prions, avec une étude toute particulière d’un produit antiamyloïdogénique, ayant peut-être des potentialités thérapeutiques dans les deux cas. En ce qui concerne la maladie de Parkinson, bien des aspects cliniques ont été précisés mais, surtout, un espoir chirurgical novateur semble marquer un véritable tournant thérapeutique. Espoir aussi pour la sclérose en plaques qui connaît maintenant — au moins en partie — un traitement original par l’interféron B d’ori­gine génique et un copolymère.

Emile Benassayag: Bladder and prostate problems in Parkinson’s disease patients

The search for the origin of bladder and prostate problems in Parkinson’s disease patients — whether they be neurological, urological or both — is as important as determining how to treat them. Parkinson’s disease provokes bladder instability in 70% of patients and dysuria in 40% of patients.

Bladder instability

There are 3 clinical symptoms:

– pollakiuria, i.e., a higher frequency of the need to void – the urgency to void, which is an imperious need

– urinary incontinence, caused by an imperious need; it is an anhibited vesical contraction, i.e., an active miction.

These bladder problems relate to filling and stockage. Those of urogenic origin are named hyperreflexia; hypertonia is the­refore non-urogenic. Both are apparent in mild forms of Parkinson’s.

Dysuria

Dysuria refers to the difficulty in emptying the bladder. Abdominal effort is needed to increase the pressure on the blad­der but bladder neck function is normal. Characterised by the same symptoms, pollakiuria, urgency and dysuria are the classic urolugical pathology and may occur in men or women with or without Parkinson’s disease.

The cause is bladder neck dysfunction, with loss of extensi­bility of the urethral sphincter and sclerosis of the prostatic tissue, seen in malignant or benign prostatic disease and also by menapausal disturbance. The deficiency of oestrogen in women changes the smooth urethra into a thick, rigid, dyspla­sic tissue giving the same symptoms. In both men and women, pollakiuria and urgency can be the result of bladder lithiasis, bladder tumors or above all a carcinome in situ or an intra epithelial cancer.

Diagnosis is based on interview, dinical examination, paradinic tests and on the urodynarnic test which shows if either hyper or hyporeflexivity, hyper or hypotonia of the bladder is present. It also shows the dosing uretral pressure which is generally normal in Parkinson’s disease and increased in prostatic disease.

Conclusion

Urinary troubles of neurogenic origin will be cured by a neu­rologist using the usual treatment of Parkinson’s. Those of uro­logic origin will be cured as usual by a urologist. But whose expertise should be sollicited? That is the question…

Bruno Dubois : Données récentes sur la maladie d’Alzheimer

La maladie d’Alzheimer (MA) est fréquente. Globalement, sa prévalence est de 3 % de la population âgée de plus de 65 ans et de 15% au-delà de 85 ans. Il y a 250 000 patients atteints de MA en France. Son coût pour la société est élevé. Le coût annuel moyen, évalué dans une étude anglaise réalisée au début des années quatre-vingt dix, est de l’ordre du milliard de livres. La maladie d’Alzheimer est une maladie dégénérative.

Au niveau neuronal, quatre étapes successives permettent d’envisager autant de cibles pharmacologiques possibles:

  1. Il y a, au tout début, un phénomène causal, qu’il soit géné­tiquement déterminé, comme dans les rares cas de la maladie d’Alzheimer familiale avec mutation identifiée sur le gène de la préséniline 1 (chromosome 14), de la préséniline 2 (chromo­some 1) ou du précurseur de l’APP (chromosome 21), ou qu’il soit pluri-factoriel, comme dans les cas de maladie d’Alzheimer sporadique, pouvant résulter de l’action d’agents xénobiotiques non identifiés, d’une prédisposition génétique telle que l’exis­tence d’un allèle E4 de l’apolipoprotéine E ou de l’activité anor­male de systèmes neuroexcitateurs.
  2. Cela entraîne des erreurs métaboliques, comme l’hyper­phosphorilation des protéines TAU, protéine du cytosquelette, et surtout la formation de peptide AS à partir d’un clivage anormal du précurseur de l’APP. Le peptide AS ainsi formé n’est pas métabolisé par l’organisme. Il se dépose alors dans le cerveau et s’agrège sous la forme de substance amyloïde au sein des plaques séniles. La neurotoxicité du peptide AS a été mise en évidence in vitro. Là encore, on peut envisager, à ce stade, la mise au point de médicaments spécifiques venant bloquer cette voie métabolique anormale et la formation de protéine S A4.
  1. L’ensemble de ces phénomènes retentit sur l’activité cellu­laire et conduit à la dégénérescence neuronale. Il est possible de freiner ces étapes terminales, non spécifiques, de la mort du neurone, mais à un stade probablement tardif de l’histoire natu­relle de l’affection.
  2. Enfin, pour un certain seuil de perte neuronale, l’affection devient symptomatique. Les symptômes résultent de la dégé­nérescence de systèmes neuronaux qui peuvent être caractéri­sés sur le plan neurochimique. A ce stade, le traitement est symptomatique, visant à compenser les déficits neurochi­miques liés à la mort neuronale. C’est dans ce cadre qu’inter­viennent les médicaments qui augmentent la transmission cho­linergique centrale, qu’il s’agisse des inhibiteurs de la cholines­térase ou d’agonistes des récepteurs muscariniques.

Sur le plan clinique, deux phases : une phase non sympto­matique, probablement longue. Puis, une phase symptoma­tique dont l’expression sémiologique est directement détermi­née par le siège des lésions neuronales.

Cette localisation a été récemment précisée par des études neuropathologiques. Les lésions histologiques débutent dans la région la plus interne du lobe temporal (cortex entorinal et hippocampique), impliquée dans la mémoire à long terme. La mise en évidence d’une atrophie des formations hippocam­piques au stade précoce de la maladie d’Alzheimer par l’étude volumétrique en IRM in vivo confirme cette hypothèse. Les lésions s’étendent secondairement vers les hémisphères céré­braux, et plus particulièrement le néocortex associatif des lobes temporaux, pariétaux, occipitaux et frontaux, impliqués dans les fonctions dites supérieures comme le langage, l’exécution de gestes intentionnels, l’identification visuelle des objets ou des visages, ou le raisonnement. Ainsi s’explique la chronolo­gie habituelle des troubles.

Les troubles mnésiques constituent l’élément central. Ils sont en effet précoces, ne se limitent pas au rappel des faits récents et concernent plusieurs systèmes de mémoire : la mémoire de travail, la mémoire épisodique et la mémoire sémantique.

Les troubles du langage, évidemment liés aux déficits de la mémoire sémantique, sont couramment observés. A ce stade, il n’y a généralement pas de perturbations phonémiques ou syn­taxiques, peu ou pas de troubles de la compréhension, et le débit élocutoire est normal. A un stade plus avancé, les para­phasies, surtout de type sémantique, sont nombreuses. Le discours devient dyssyntaxique et la compréhension orale est per­turbée, évoquant un tableau d’aphasie transcorticale sensorielle. Enfin, au stade de démence sévère, la production spontanée est fortement réduite, voire inexistante.

Les troubles des fonctions exécutives apparaissent précoce­ment et se manifestent souvent par une incapacité à gérer des situations qui requièrent un partage des ressources attention­nelles entre plusieurs stimuli ou tâches. Des déficits massifs en mémoire épisodique, par défaut de stratégie active de récupéra­tion en mémoire et des déficits d’abstraction, de raisonnement, de flexibilité et de planification, témoignent d’un dysfonction­nement frontal. C’est également ce dysfonctionnement qui expliquerait en partie l’anosognosie observée chez certains de ces patients.

Les troubles visuels complexes sont dus à des lésions du cor­tex associatif visuel et rendent compte des difficultés à s’orien­ter dans un environnement familier, ainsi que de la prosopa­gnosie et des troubles de l’attention spatiale pouvant aller jus­qu’à réaliser un syndrome de Balint. Des troubles d’identifica­tion d’objets, souvent interprétés comme le témoin d’une agno­sie visuelle, peuvent résulter d’une désorganisation sémantique, voire de perturbations plus en amont, lors des étapes précoces du traitement perceptif.

Les troubles praxiques sont généralement plus tardifs. L’apraxie idéatoire est fréquente, à rapprocher de la notion de l’apraxie conceptuelle.

Les troubles psycho-comportementaux, chez 75% des patients, se répartissent en comportements passifs (perte de l’initiative, émoussement affectif), agités (irritabilité, activité sans but) ou égocentriques (désintérêt vis-à-vis des siens). Ces modi­fications peuvent orienter à tort vers un diagnostic d’état dépres­sif. A un stade ultérieur, on peut observer des comportements d’agitation avec conduites stéréotypées, déambulation sans but, fugues, agressivité. Également tardives sont les hallucinations (dans 20% des cas) et les idées délirantes, dont les plus fré­quentes sont les thèmes de vol ou d’abandon.

Gianluigi Forloni: Pathogenetic rote of amyloid deposits in Alzheimer’s and Prion’s diseases

Deposition of amyloid-1 protein (M) in brain parenchyma and vessel walls is a major pathological feature of Alzheimer’s disease (AD). In prion-related encephalopathies (PRE) too, an altered form of prion protein (PrPsc) forms amyloid fibrils and accumulates in the brain. In both conditions the amyloid depo­sition is accompanied by nerve cell loss, whose pathogenesis and molecular basis are not understood. Neuropathological, genetic and biochemical studies indicate a central role of PA in the AD pathogenesis. Different genetic background associated with AD: mutated forms of presenilin 1 and 2 (PS-1 and 2); the allele E 4 of ApoE or the APP mutations, resulted in an increa­se of ßA production. Synthetic peptides homologous to ßA and its fragments contribute to investigate the mechanisms of ßA deposits formation and the role played by ßA in AD pathoge­nesis. The physicochemical studies on the S-sheet conforma­tion and self-aggregation properties of ßA peptides indicate the conditions and the factors influencing the formation of PA deposits. The neurotoxic activity of SA and its fragments sup­ported the causal relationship between i A deposits and the neuropathological events in AD. Numerous studies were per­formed to clarify the mechanism of neuronal death induced by exposure to ßA peptides and recently a direct connections between PS-1 and 2 an apoptosis induced by has been found.

Similar approach has been used to investigate the role of PrPsc in PRE. In these diseases the association between accu­mulation of PrPsc and the neuropathology is evident and numerous data indicate that PrPsc itself might participate to the infections agent responsible of disease transmission. Thus, PrP peptides were used to investigate the pathogenic role of PrPsc in PRE and the conformational change responsible for the conversion PrPc to PrPsc that makes the molecule pathological and capable to contribute to the infectivity. In particular, we synthetized the peptide homologous to consecutive segments PrP 106-126, an integral part of all abnormal PrP isoforms that accumulate in the brain of subjects PRE. This peptide is fibrillo­genic, has secondary structure largely composed of ß-sheet and proteinase-resistant properties, is neurotoxic and induces astro­gliosis. Similar features were found in a peptide homologous to the fragment PrP 82-146 corresponding to a peptide found in the PRE’s brain. We compared the data obtained with SA and. PrP peptides and analyzed the significance in terms of amyloido­genic proteins and neurodegeneration. This approach has been utilized to develop anti-amyloidogenic drugs potentially useful in AD and PRE.

Jean-Louis Mandel: Trinucleotide expansion diseases and mechanisms of polyglutamine expansion diseases

Unstable expansions of trinucleotide repeats have been found associated, since 1991, to 12 neurological diseases. These diseases are often characterised by some form of anticipation, linked to the tendency of pathological alleles to further expand, and by biases in the parental origin of mutations leading to the most severe forms. Triplet diseases can be divided in 4 classes. The first three correspond to large expansions in non coding sequences, leading to pleiotropic diseases:

  • Massive expansions of 5′ untranslated or flanking CGG/CCG repeats associated to abnormal DNA methylation cause the fragile X mental retardation syndrome, and mild mental retardation linked to FRAXE (through loss of function of the target genes).
  • Massive expansion of an untranslated CTG repeat causes myotonic dystrophy (not a simple loss of function).
  • Massive expansion of an intronic GAA repeat is found in Friedreich’s ataxia, and causes a loss of function of the frataxin gene. The fourth class corresponds to moderate expansions of CAG repeats coding for polyglutamines, found in 8 purely neu­rodegenerative diseases, that includes Huntington’s disease and 5 spinocerebellar ataxias. Polyglutamine expansions cause a gain of toxic property in the target proteins, with the possible exception of SCA6. Recently, expansion of a dodecamer was found in recessive progressive myoclonus epilepsy, the first break in the triplet ride.

Huntington’s disease (HD) is caused by an expansion of a CAG/polyglutamine repeat in huntingtin, a protein of unk­nown function. A similar mechanism was demonstrated in spino­bulbar muscular atrophy (SBMA), spinocerebellar ataxia (SCAs) types 1, 2, 3, 6 and 7, and in dentatorubro-pallidoluy­sian atrophy (DRPLA). These diseases are characterised by adult-onset neuronal death in selected but different regions of the nervous system, and by an inverse correlation between length of the polyglutamine expansion and age of onset. However, the age of onset for patients carrying the same expan­sion can vary by as much as 20 years, and it would be impor­tant to determine whether other genes may modulate the clinical severity.

The proteins implicated in these diseases show no common features apart from the polyglutamine tract, and their distribu­tion shows no obvions correlation with the site of neurodege­nerescence. Recent observations in patients and in mouse models suggest that aggregation of polyglutamine containing proteins within the nucleus may constitute a common patho­logical mechanism. We have characterized a monoclonal anti­body (1C2) that recognizes the expanded polyglutamine­containing proteins in HD and in SCAs 1, 2, 3 and 7, while the normal proteins are not detected under the same conditions. The efficiency of the detection is dependent on polyglutamine length, and thus parallels disease severity. This indicates that the pathological threshold observed in patients corresponds to a conformation change recognized specifically by this antibody, that occurs in the target proteins. The SCAT is the most recently cloned SCA locus, and shows both a high sensitivity to poly­glutamine length (similar to SCA2), and the greatest instability of all loci, accounting for an anticipation of 20 years/generation.

Cell lines and transgenic mice that carry and express a muta­ted protein have been constructed in various labs, and some show pathological effects. They allow an investigation of the disease mechanisms. We have recently obtained a cellular model where expression of full length mutated huntingtin pro­tein results in the formation of ubiquitinated nuclear inclusions, that contain however only the N-terminal region of huntingtin. This model appears thus to recapitulate major steps observed in patients brain.

The 1C2 antibody was also tested on cell lines from patients with early onset forms of schizophrenia, manic depressive ill­ness or rare familial forms of Parkinson’s disease, but no new clearly pathological protein was detected in this way up to now. Other approaches based on testing of CAG repeats have also failed to give unequivocal evidence for an implication of such repeats in psychiatric diseases.

Eva-Maria Mandelkow: Tau protein and Alzheimer paired helical filaments: phosphorylation, structure, and influence on microtubule dynamics

One of the hallmarks of Alzheimer’s disease is the neuro­fibrillary pathology which occurs in neuritic plaques, neuro­fibrillary tangles, or neuropil threads. The main component of these deposits is tau protein which is abnormal in at least two ways: it is hyperphosphorylated and aggregated into paired helical filaments’, PHFs. The question is therefore, how are these modifications related to one another, what causes them, and how do they affect the physiological functions of micro-tubules in the neuron (such as axonal transport1) We have identified a number of phosphorylation sites, including some that affect tau’s interactions with its natural partner, the axonal microtubules. Several kinases and phosphatages affect the phos­phorylation state of tau, including some that are activated during mitosis. Thus, during mitosis the phosphorylation of tau transfected into CHO cells resembles that of neuronal tau in Alzheimer’s disease; one of the effects is to dissociate tau from microtubules and make them more dynamic.2

Phosphorylation affects the tau-microtubule interaction to different extents. The most potent effect is observed with the kinase MARK which phosphorylates Ser 262 of tau and similar KXGS motifs in MAP2 or MAP4.3 The kinase has now been do­ned and characterized.4 MARK represents a family of kinases, two members of which are MARK1 (88 kDal) and MARK2 (81 kDal) ; they belong into the AMPK subfamily of the CaMKII group of kinases. MARK is activated by phosphorylation at its regulatory loop in the catalytic domain. Tissue expression is ubiquitous, it is pronounced in brain and in fetal tissue. Overexpression in CHO cells leads to hyperphosphorylation of MAPs on KXGS motifs and to massive disruption of microtu­bules, resulting in morphological changes and cell death. MARK-related kinases in yeast (KIN1) and C.elegans (par-1) are involved in the development of cell polarity. In Alzheimer’s disease, neuronal polarity is lost, and axonal tau accumulates in the somatodendritic compartment. Hence one might speculate that a defect in MARK function could be a common link.

1 E.-M. Mandelkow et al. (1995). Neumbiol. Aging 16, 355-362.

2 U. Preuss et al. (1995). Mol. Biol. Cell. 6, 1397.

3 S. Illenberger et al. (1996). J. Biol. Chem. 271, 10834.

4 G. Drewes et al. (1997). Cell 89, 297.

Judith Meiki: Molecular advances in spinal muscular atrophy syndromes

Spinal muscular atrophies (SMA) are a group of lower motor neuron diseases that are clinically and genetically heteroge­neous. Indeed, the age of onset of symptoms varies from birth to adulthood, muscle weakness can be either proximal or discal. Most pedigrees with proximal childhood SMA show a recessive autosomal pattern of inheritance. However, neonatal form of SMA associated with arthrogryposis has also been shown to be X-linked. In adult SMA, autosomal recessive and dominant forms have been described as well as a X-linked form known as Kennedy syndrome or spinal and bulbar muscular atrophy in which the androgen receptor gene is associated with an abnormal (CAG) triplet repeat expansion.

The groups belonging to the International SMA Consortium focused their efforts on localising and identifying the defective gene for the autosomal recessive form of proximal childhood SMA, the most frequent form. This represents the first steps towards understanding the biological bases of motor neuron degeneration that characterizes the disease. A positional cloning strategy lead to the identification of the SMN gene as the SMA determining gene. The high frequency of homozygous SMN gene deletion greatly improved genetic counselling in families and represents a useful tool for SMA diagnosis. Moreover, SMN gene deletion test also contributed to the nosology of closely related disorders.

SMN encodes a novel protein located both in the cytoplasm and in a nuclear structure named ‘gems’ for gemini of coiled bodies. SMN interacts with RNA binding proteins and with a novel protein named SIP1, the SMN-SIP1 complex having an essentiel role in spliceosomal snRNP biogenesis. In SMA patients, protein analysis showed a strong correlation of the amount of the protein encoded by the SMNc gene (a highly homologous copy of SMN) with the clinical severity of the disease. These results provided the first molecular basis of a severe or a mild form of the disease. In addition, these data favour the view that the protein encoded by the SMNc gene has a functional active role, its level directly determining the clini­cat severity of the disease. These data also showed that the expression and/or stability of SMN and SMNc gene products are different. Thus, the SMNc gene can be regarded as a modifying gene in SMA. Therefore, the fine characterization of SMN and SMNc promoters will be very helpful in order to identify trans­cription factors up-regulating their activities. The hyper-expres­sion of the SMNc gene could indeed represent an attractive stra­tegy for therapy in SMA.

In order to elucidate the pathophysiological mechanisms underlying SMA, a conditional mutagenesis of the murine SMN gene using the CRE-LoxP system has been undertaken in order to induce SMN gene deletion in a given tissue and at a given time. This strategy should avoid the early embryonic lethality such as described using a conventional knocking-out approach. Moreover, this strategy should enable to get inducible animal and cellular models in order to further characterize the SMN gene defect at the histological, cellular and molecular levels. The generation of such models as well as a better knowledge of the SMN function(s) should allow the understanding of the bio­logical basis of motor neuron degeneration which characterizes this devastating disorder and should assist in the development of therapy.

Although the SMN protein function is not fully understood, these recent advances favour the view that SMN plays a role in RNA processing. Interestingly, recent advances in amyotrophic lateral sclerosis (ALS), a closely related disorder, revealed a defect in the splicing machinery that results in aberrant mRNAs of the astroglial glutamate transporter EAAT2 in affected areas of ALS patients. The restricted affected areas in both SMA and ALS may imply a defect in splicing regulatory factor(s) that is (are) required for splicing few transcripts of critical importance for motor neuron survival. Abnormal RNA processing either as a primary defect as in SMA or secondary as in ALS could repre­sent new mechanisms for neurodegenerative diseases.

Gilles Fenelon : Les aspects cliniques de la maladie de Parkinson

Le diagnostic est porté par les neurologues avertis, avec 15 % d’erreur par excès. C’est une maladie neurodégénérative dont les critères sont cliniques ; il n’y a pas de marqueur spécifique. Il faut la distinguer des maladies voisines, telles que les “démences à corps de Leroy”.

Aspect moteur. La Dopa est toujours efficace, avec une “lune de miel” de quelques années, puis surviennent des dyskinésies avec mouvements anormaux, source d’instabilité motrice et de handicap. Ces complications sous traitement sont surtout le fait des malades jeunes. Il y aurait interaction entre la stimulation dopaminergique thérapeutique et la “dénervation” dopaminer­gique striatale.

Les troubles moteurs axiaux (marche, posture, parole) sont plus fréquents et en partie Dopa résistants chez les sujets âgés.

Les aspects non moteurs donnent la difficulté de prise en charge des malades par dysautonomie, troubles sensitifs, troubles du sommeil. Les troubles cognitifs sont parfois mineurs, parfois majeurs avec démence, différente de la mala­die d’Alzheimer.

Les troubles psychiatriques, enfin, avec anxiété, dépression, hallucinations sont fréquents (de 25 à 40 % des cas), surtout sous traitement et avec des troubles cognitifs.

Le fait essentiel reste, certes, la baisse de la Dopamine intras­triatable en liaison avec l’akinésie, tandis que d’autres symp­tômes, tels les tremblements, dépendent au moins en partie de mécanismes non dopaminergiques.

Dominique Dormont : Les formes moléculaires nou­velles d’information en microbiologie : les prions

Chez l’individu infecté, la protéine PrP devient résistante aux enzymes protéolytiques, au moins partiellement, et les encé­phalopathies subaiguës spongiformes transmissibles sont des maladies neurodégénératives lentes, toujours mortelles, trans­missibles à l’animal de laboratoire.

Des agents transmissibles non conventionnels, ou prions, en sont à l’origine. Les prions possèdent des propriétés proches de celles des virus conventionnels, mais leur résistance inhabituelle aux procédés d’inactivation et la composition exclusivement protéïque des fractions infectieuses spécifiques en font des enti­tés infectieuses purement protéiques.

A ce jour, il est impossible d’obtenir des fractions assez pures pour permettre une caractérisation biochimique précise. Le seul composant variant avec le titre infectieux est la quantité de pro­téine PrP, une protéine de l’hôte qui s’accumule sous une forme pathologique la PrP sc ou PrP res. C’est une syalo-glycoprotéine exprimée par les neurones chez le sujet normal, et par les cellules du système immunitaire et réticulo-endothélial — le gène codant est sur le bras court du chromosome 20. Cette protéine de 253 acides aminés s’exprime à la face externe de la membrane cellulaire ; sa demi-vie est courte ; elle échappe au catabolisme cellulaire. La pathogénicité du prion semble liée uniquement à l’acquisition d’une conformation pathologique par la PrP.

L’apparition de dix cas de maladie de Creutzfeldt-Jakob chez des sujets jeunes, et avec des caractéristiques atypiques, a fait penser à une nouvelle forme, “le nouveau variant de la maladie de Creutzfeldt-Jakob”, avec des lésions similaires à celles de la maladie bovine. Ainsi a été évoquée la contamination par l’agent bovin, avec la possibilité de franchir, par voie orale même, les barrières interspécifiques. L’attention doit être attirée sur la nécessité de sécurisation de tous les maillons de la chaîne alimentaire. Aucune transmission par acte transfusionnel n’est connue. Enfin, la présence de PrP res dans les amygdales et la rate a été récemment décrite.

Michael Sela: The concept of specific immune vaccines against autoimmune diseases

Vaccines against infections diseases are known to be highly specific. This concept is extended here to autoimmune diseases: whenever it is possible to identify the putative cause of the diseases, it should be possible to find a close molecular analog which will combat the disease. In one case, that of mul­tiple sclerosis, we developed a drug/vaccine, denoted Cop 1, which has been approved by the FDA in the USA, as well as several other countries. Cop 1 (Copaxone) was found in clini­cal trials to slow progression of disability and to reduce the relapse rate in exacerbating-remitting multiple sclerosis patients.

A similar approach is possible in myasthenia gravis. We used two myasthenogenic T cell epitopes of the human acetylcholi­ne receptor a-subunit and demonstrated that they are capable of triggering peripheral blood lymphocytes of the majority (approx. 80 %) of myasthenic patients tested. Both single amino acid analogs, and a dual analog composed of the tandemly arranged two single amino acid analogs were able to inhibit in vitro pro­liferative responses of T cell lines, and in vivo priming of lymph node cells. The dual analog inhibited experimental autoimmu­ne myasthenia gravis even when the mice were treated fourteen days alter the injection of the pathogenic T cell line.

Kenneth L Moya: Looking through the eye into the Alzheimer’s brain

We have used the primary visual pathway as a CNS model system with which to study the metabolism of neuronal proteins destined for brain synapses in vivo. Prominent among the proteins synthesized in retinal ganglion cells and transported down the axons to synaptic terminals is the amyloid precursor protein (APP) implicated in Alzheimer’s disease (AD). APP is developmentally regulated in the brain with some isoforms associated with the growth of axons while other APP isoforms are correlated with synaptogenesis. In vitro studies have shown that the inhibition of APP expression inhibits neurite outgrow­th and in the developing brain, APP is localized along growing axons. In vivo studies have suggested that the mechanism for the cleavage and secretion of APP is maturation dependent and is correlated with the formation of synaptic contacts.

APP from mature brain interacts with the detergent insoluble cytoskeleton, most likely via the C-terminal of the protein. The APP that arrives at the synaptic terminal in vivo is N- and 0-glyco­sylated, contains sialic acid and is sulfated. The full-length trans­membrane form of APP is cleaved and rapidly eliminated from the synapse in vivo. The half-life of APP at the synaptic termi­nal is 2-3 hours and the turnover of APP is not dependent on neuronal activity.

These data lead us to propose a model of Alzheimer’s disea­se in which a loss of synaptic efficacy due to an alteration of APP metabolism is the precipitating cellular event in the disea­se process. We hypothesize that APP plays a fundamental role in normal synaptic function, perhaps through interactions with other neuronal surface glycoproteins. In AD, subtle changes in APP metabolism could arise sporadically in cases of nonfamilial AD or from mutations in APP itself or proteins essential for APP processing such as the presenilins in familial forms of the disease. With a synaptic half-life of 2-3 hours, small perturba­tions in the rate of APP synthesis or its rate of elimination would result in rapid changes in the levels of APP at the nerve terminal with a subsequent loss of synaptic efficacy. Such a change in synaptic efficacy and associated loss of synapses readily explains the early cognitive change in the course of the disease. A continuing synaptic loss leads to neuronal dysfunction and eventually to the accumulation of amyloid plaque and neurofibrillary tangles resulting in frank neuronal loss and severe cognitive decline.

Ruth Amon: Therapeutic use of copolypeptides

The synthetic amino acid copolymer 1 (Cop 1, Copaxone) was demonstrated by us to suppress and prevent experimental autoimmune encephalomyelitis (EAE) in various animal species. Following several clinical trials, including multi-center studies, which demonstrated its safety and beneficial effect in multiple sclerosis (MS) patients, it was approved in 1996 by the FDA in the USA, as well as in Israel and other countries, for the treatment of relapsing-remitting MS. In more recent studies we determined the mechanism of activity of Cop 1.

Cop 1 binds promiscuously to many different cells regardless of their DR restriction. It binds avidly and fast, competes with the binding of many antigens and can also displace already bound antigens, including the myelin components MBP, PLP and MOG. Its specificity is manifested by the prevention of the APC presentation, only of MBP, to the T-cell receptor (TC-R). We have also shown that it induces specific suppressor T cells. Recent studies on their cytokine profile showed that unlike T cell fines induced by MBP, which secrete either Th 1 or both Th 1 and Th2 cytokines, the T cell lines/clones induced by Cop 1 were confined exclusively to the Th2 pathway and secre­ted high amounts of IL-4, IL-6 and IL-10 in response to Cop 1, with no secretion of IL-2 or interferon gamma. The Cop 1 indu­ced cells cross reacted with MBP on the level of Th2 cytokine secretion, but not with other myelin antigens, such as PLP pep­tides, although they can suppress EAE induced by these antigens. This bystander suppression demonstrated by Cop 1-spe­cific T cells may explain the therapeutic effect of Cop 1 in MS, in which several encephalitogens may be implicated.

Konrad Beyreuther: Alzheimer’s disease: does neuronal transport hold the key?

Recently, evidence has emerged supporting the concept of a disturbance of axonal transport as being the major molecular defect underlying both the synaptic loss and neurodegenera­tion in Alzheimer’s disease1. The evidence is as follows. The hie­rarchical appearance of newly synthesized APP first in the axon and later in the dendrites raises questions as to how this trans­cytosis is regulated and related to Alzheimer’s disease. When considering APP transcytosis as a sorting problem, it appears that neurons should utilize more than one sorting station in regulating polarized distribution of APP. The first sorting station could be sphingolipid rafts of the Golgi complex responsible for initial axonal delivery, and further sorting could occur sub­sequently during endocytoses and transcytosis. Since choleste­rol depletion reduces raft association of APP and inhibits the generation of S-amyloid in hippocampal neurons while the generation of APPsec is unperturbed, this implies a link bet­ween APP transport, cholesterol and Aß formation. Our dele­tion mapping experiments showed that the membrane-proxi­mal AS-part would be the major and dominant signal to axonal sorting and the cytoplasmic tail the recessive signal to somato­dendritic sorting of APP. This provides a physiological explana­tion to the role of Aß production in neurons: regulation of axo­nal transport of APP.2 We and other workers have provided evi­dence that this Aß production occurs in neurons within the relevant sorting stations: ER, TGN and endosomes/cell surface.’ An attractive hypothesis for the role of the Aß domain in intra­ neuronal sorting would be that newly synthesized APP inter­acts via this domain with a sorting receptor in the Golgi com­plex and enables axonal transport of APP. Such a receptor func­tion could be mediated by presenilins since we showed that presenilin 2 can be coimmunoprecipitated with the ER form of APP.4 The Aß domain should be accessible for interactions with such a receptor since it represents the region where the major proteolytic cleavages of APP occur. Free Aß is also expected to bind to this receptor and should thus be able to regulate and, at high concentrations, to inhibit the transport of APP and other proteins utilizing the same receptor and/or the same transport machinery. From these molecular insights, it seems very likely that effective therapeutic strategies will be derived by targeting APP expression, APP metabolism, regulation of APP transport, Aß production and Aß aggregation.

1Beyreuther, K. and Masters, CL (1997), `The ins and outs of amyloid-ß’, Nature 389: 677-678.

Tienari PJ., De Strooper B., Ikonen E., Ida N., Simons M., Masters C.L., Dotti C.G. and Beyreuther K. (1996), ‘Neuronal sorting and processing of amyloid precursor protein: Implications to Alzheimer’s disease’, Cold Spring Harbor Quant. Symp. 61: 575-585.

Hartmann T, Bieger S.C., Brühl B., Tienari Pj., Ida N., Roberts G.W., Allsop D., Masters C.L., Dotti C.G., Unsicker K., Beyreuther K. (1997), ‘Distinct sites of intracellular production for Alzheimer’s disease Aß40/42 amyloid peptides’, Nature Med. 3: 1016-1020.

Weideman A., Paliga K., Dürrwang U., Czech C., Evin G., Masters C.L. and Beyreuther K. (1997), ‘Formation of stable com­plexes between two Alzheimer’s disease gene products: preseni­lin-2 and ß-amyloid precursor protein’, Nature Med. 3:328-332.

Agnès Ullmann: Screeningfor molecular interactions by using a novel bacterial two-bybrid system

A bacterial two-hybrid system that allows a rapid and cosy in vivo screening and selection of functional interaction between two proteins will be described. This genetic test is based on the reconstitution, in an Escheria coli adenylate cyclase deficient strain, of a signal transduction pathway that takes advantage of the positive control exerted by cAMP. Two putative interacting proteins are genetically fused to two complementary fragments, T25 and T18, that constitute the catalytic domain of Bordetella pertussis adenylate cyclase. When the two fragments are coex­pressed as independent polypeptides in E. coli, they are unable to interact and no cAMP synthesis occurs. Association of the two interacting proteins fused to T25 and T18, results in functional complementation between the two adenylate cyclase fragments and leads to cAMP synthesis, a regulatory molecule in E. coli. Cyclic AMP then triggers transcriptional activation of several E. coli resident genes, that yields a characteristic phenotype which can be scored either on indicator plates or on selective media. Since this essay involves a signaling cascade, one can select spe­cific clones expressing a protein that interacts with a given tar­get, by a simple genetic test. In addition, it permits a versatile design of screening procedures either for ligands that bind to a given `hait’, or for molecules or mutations that block a given interaction between two proteins of interest. Using this genetic screen, it can be anticipated that this system could be particular­ly suitable to analyze colocalization of given proteins within multimolecular assemblies.

G. Karimova, J. Pidoux, A. Ullmann & D. Ladant, Proc. Natl. Acad. Sci. USA (1998), 95, 5752-5756.

Jean-François Bach : Immunopathogénie de la sclérose en plaques

La sclérose en plaques (SEP) est une maladie inflammatoire démyélinisante chronique. Un grand nombre d’arguments convergent pour laisser penser que la maladie fait intervenir une réaction auto-immune de type cellulaire (T), au moins au stade lésionnel. On trouve dans le sang des malades atteints de SEP les stigmates d’une réaction auto-immune, B et T, contre divers constituants de la myéline ou des oligodendrocytes. On observe, tant au niveau des immunoglobulines que des cellules T du système nerveux central (SNC), une restriction d’hétéro­généité suggestive d’une expansion préférentielle de certains clones. La maladie est associée au gène HLA DR2. Enfin, et surtout, il existe un modèle animal de la maladie, très proche de la maladie humaine, l’encéphalomyélite allergique expéri­mentale, qui est induite par l’immunisation contre des anti­gènes de la myéline. On pourrait dès lors penser qu’un anti­gène de la myéline, tel que la protéine basique de la myéline, est à l’origine de la maladie humaine. Deux questions non résolues, néanmoins, ne permettent pas d’accepter cette hypothèse en première instance. Tout d’abord, il s’est révélé impossible d’identifier un antigène de la myéline reconnu de façon préférentielle ou précoce par les lymphocytes B et T de malades atteints de SEP. Au moins sept antigènes candidats ont été proposés. En outre, on ne comprend pas comment la (ou les) molécule(s) responsable (s) deviennent immunogé­nique(s) et produisent la réaction auto-immune démyélini­sante pathogène. Sur la base de diverses données expérimen­tales récentes, un schéma relativement consensuel peut être proposé. Les oligodendrocytes pourraient présenter une aug­mentation d’expression des molécules impliquées dans la reconnaissance des antigènes par les cellules T. Peut-être une infection virale neurotrope en est-elle à l’origine par le biais de l’inflammation locale créée par les interférons. Peut-être une lésion neurodégénérative en est responsable puisque l’on sait qu’une lésion des neurones du SNC peut entraîner une aug­mentation de l’expression des molécules HLA. Cette anoma­lie de présentation pourrait entraîner une augmentation de l’immunogénicité des différentes molécules présentes dans la myéline et provoquer une réponse immunitaire lorsque les déterminants immunodominants de ces molécules se lient aux molécules HLA du malade. L’inflammation ainsi créée accroîtrait progressivement le nombre de spécificités, rendant difficile l’identification des cellules T pathogènes.

Alim Louis Benabid: High frequency stimulation of deep brain structure, application to the treatment of Parkinson’s disease symptoms

How can the electric stimulation of a neuron structure produce the same effects as its destruction? Could it be a new mechanism such as the jamming of a reasoning neuron net­work? Some experimental facts suggest so, but the demonstra­tion has yet to be done. On animals and humans, the high-frequency stimulation of the ventral intermediate thalamic nucleus (VIM) and the sub-thalamic nucleus (NST) respectively sup­presses the tremor and the Akeneto-rigid syndrome in Parkinson’s disease.

One hundred and twenty patients have undergone the chro­nic implantation of an electrode into their VIMs. The stimulation is only efficient above 100 Hz with an optimal effect bet­ween 130 and 2 000 Hz, where the tremor is suppressed with very low morbidity. Sixty patients have had bilateral electrode in the subthalamus nucleus area. The stimulation effects appear at lower level (60 Hz) than in VIM. The operating technique, stereotaxic in both, is based on radiological, ventriculographic and NMR data.

On rats, high frequency NST stimulation inducts an inhibi­tion in internal pallidum activity and an increase in external pallidum. It could be due to the loss of ail exciting glutamater­gic efference from the NST with direct G inhibition or by indi­rect G Pi inhibition by Gabaergic efferences from the Gpe retroactivated. Finally the hypothesis is that a Glutamate secre­tion decrease might alterate the pace in the process of Parkinson’s disease. Experimental evidence exists that STN neuro-inhibition is due to a shutdown of the glutamatergic out­put of STN cells and it may have potential effects on the deve­lopment of degenerative process of the dopaminergic cells.

High frequency stimulation of VIM, GPI or STN induces always a clear cut arrest of tremor and in GPI and STN a signi­ficant improvement of symptoms such as rigidity and brady­kinesia.

We now better understand the function of the basal ganglia.

Irith Ginsburg: Tau protein-regulation of expression in normal and Alzbeimer’s train

Alzheimer’s disease is characterized by two hallmarks: senile plaques composed of ß-amyloid proteins and neurofibrillary tangles, consisting of tall microtubule associated protein (MAP). Tau MAP is a neuronal-specific protein found primarily in axons and is developmentally regulated. Tau is encoded by a single gene located on chromosome 17 and its regulation of expres­sion lies in four levels: a) promoter activity, b) alternative spli­cing, c) subcellular localization and d) posttranslational modi­fications in particularly phosphorylations. The function of tau in stabilization of microtubules is important in establishing and maintaining neuronal polarity. The molecular mechanisms res­ponsible for the segregation of proteins and organelles into the axons and dendrites is not yet fully understood. Deregulation at any of the above mentioned points, may cause dysfunction of the protein thus affecting axonal transport and culminating in cell death. The typical lesions found in AD, i.e., neuro­fibrillary tangles are aggregated tau protein within the dead neuronal cells.

One mechanism that contributes to the generation and main­tenance of neuronal polarity and contributes to neuronal plas­ticit, is the subcellular localization of specific mRNAs leading to their local translation. Subcellular localization of tau mRNA into the proximal segment of the axon is a multistep process which involves the interaction between cis-acting signais and trans­acting protein factors. We have identified those signals and pro­tein factors. Mislocalization of tau in neuronal cells may affect its association with microtubules and lead to the disassembly of cytoskeletal systems culminating in neuronal cell death.

These studies contribute to the understanding of the regula­tion of tau expression and localization during development and aging of neuronal cells, as well as in neurodegenerative disorders such as Alzheimer’s disease. The involvement of various agents that interfere with the changes occurring during development of the disease may postpone its onset which may have important clinicat and socioeconomic effects on the modem society when life span has been increased and neuro­degenerative disorders become a major devastating problem.


OpenEdition vous propose de citer ce billet de la manière suivante :
ldiebold (21 septembre 1998). Progrès récents des neurosciences et maladies neurodégénératives/ Recent progress made in neurosciences and neurodegenerative diseases. Les carnets de la Fondation des Treilles. Consulté le 20 juillet 2024 à l’adresse https://doi.org/10.58079/quu6


Rechercher dans OpenEdition Search

Vous allez être redirigé vers OpenEdition Search