Insights into the physiological function of cellular prion protein

Prions have been extensively studied since they represent a new class of infectious agents in which a protein, PrPsc (prion scrapie), appears to be the sole component of the infectious particle. They are responsible for transmissible spongiform encephalopathies, which affect both humans and animals. The mechanism of disease propagation is well understood and involves the interaction of PrPsc with its cellular isoform (PrPc) and subsequently abnormal structural conversion of the latter. PrPc is a glycoprotein anchored on the cell surface by a glycosylphosphatidylinositol moiety and expressed in most cell types but mainly in neurons. Prion diseases have been associated with the accumulation of the abnormally folded protein and its neurotoxic effects; however, it is not known if PrPc loss of function is an important component. New efforts are addressing this question and trying to characterize the physiological function of PrPc. At least four different mouse strains in which the PrP gene was ablated were generated and the results regarding their phenotype are controversial. Localization of PrPc on the cell membrane makes it a potential candidate for a ligand uptake, cell adhesion and recognition molecule or a membrane signaling molecule. Recent data have shown a potential role for PrPc in the metabolism of copper and moreover that this metal stimulates PrPc endocytosis. Our group has recently demonstrated that PrPc is a high affinity laminin ligand and that this interaction mediates neuronal cell adhesion and neurite extension and maintenance. Moreover, PrPc-caveolin-1 dependent coupling seems to trigger the tyrosine kinase Fyn activation. These data provide the first evidence for PrPc involvement in signal transduction

A receptor for infectious and cellular prion protein

Prions are an unconventional form of infectious agents composed only of protein and involved in transmissible spongiform encephalopathies in humans and animals. The infectious particle is composed by PrPsc which is an isoform of a normal cellular glycosyl-phosphatidylinositol (GPI) anchored protein, PrPc, of unknown function. The two proteins differ only in conformation, PrPc is composed of 40 percent helix while PrPsc has 60 percent ß-sheet and 20 percent helix structure. The infection mechanism is trigged by interaction of PrPsc with cellular prion protein causing conversion of the latter's conformation. Therefore, the infection spreads because new PrPsc molecules are generated exponentially from the normal PrPc. The accumulation of insoluble PrPsc is probably one of the events that lead to neuronal death. Conflicting data in the literature showed that PrPc internalization is mediated either by clathrin-coated pits or by caveolae-like membranous domains. However, both pathways seem to require a third protein (a receptor or a prion-binding protein) either to make the connection between the GPI-anchored molecule to clathrin or to convert PrPc into PrPsc. We have recently characterized a 66-kDa membrane receptor which binds PrPc in vitro and in vivo and mediates the neurotoxicity of a human prion peptide. Therefore, the receptor should have a role in the pathogenesis of prion-related diseases and in the normal cellular process. Further work is necessary to clarify the events triggered by the association of PrPc/PrPsc with the receptor

Regulaçäo dos receptores de glicocorticóide em leucócitos mononucleares humanos / Glucocorticoid receptores of human mononuclear leukocytes

O presente estudo foi realizado com a finalidade de analisar receptores de glicocorticóide em leucócitos mononucleares (MNL) de sangue periférico de indivíduos normais, näo tratados e de indivíduos normal aos quais foram administrados exogenamente os corticóides: prednisona e deflazacort. MNL de mulheres e homens normais apresentaram uma concentraçäo de receptores de glicocorticóide de 7,52 ñ 2,5fMol/10**6 cel e uma constante de dissociaçäo (Kd) de 9,5 ñ 2,3nM. Näo houve diferença em relaçäo ao sexo e as fases do ciclo menstrual. Em indivíduos tratados com predinisona, näo houve mudança nos parâmetros de ligaçäo. A administraçäo deflazacort determinou uma sensível reduçäo dos sítios de ligaçäo. Concluímos que certos glicocorticóides podem exercer um efeito regulatório sobre seus receptores

Steroid receptors in meningiomas

Dados epidemiológicos sugerem uma associaçäo entre meningiomas e hormônios sexuais. Há preponderância da incidência da incidência destes tumores em mulheres, sendo diagnosticados mais frequentemente entre 35 e 55 anos. Progressäo tumoral pode ocorrer durante a gravidez e a associaçäo entre meningiomas e câncer de mama é estatisticamente significativa. O efeito benéfico positivo dos glicocorticóides no tratamento clínico de tumores intracranianos está bem documentado, embora este efeito seja relacionado com o edema cerebral e näo com proliferaçäo tumoral. A presença de receptores intracelulares específicos parece ser condiçäo necessaria embora näo suficiente para a expressäo dos efeitos celulares destes esteróides. O objetivo do nosso trabalho foi a determinaçäo de receptores de estrógeno (ER), progesterona (PR), andrógeno (AR) e glicocorticóide (GR) em 10 meningiomas através da metodologia do carväo-dextrana. Neste estudo verificamos que somente 20% dos meningiomas contém receptores de estrógeno e as concentraçöes determinadas säo baixas. Receptores de progesterona estäo presentes em 90% dos tumores, em alta concentraçäo (média + ou - dp = 60 + ou - 38 fMol/mg prot). Cerca de 70% dos meningiomas contém níveis intermediários de AR e GR. Testes de competiçäo demonstram que todos os receptores säo específicos. Incidência e concentraçäo de todos os receptores säo maiores em pacientes do sexo feminino. A ocorrência de receptores de progesterona, andrógeno e glicocorticóide pode indicar a possível utilidade de manipulaçäo endócrinas em meningiomas näo operáveis

Afinidade da ligaçäo do deflazacort aos receptores de glicorticóide / Binding affinity of deflazacort to glucocorticoid receptors

O deflazacort (DF) é um glicocorticóide mais potente que prednisolona (P) como agente anti-inflamatório e timolítico. Utilizando como sonda radioativa (3H) dexametazona e concentraçöes crescentes de vários esteróides frios, determinamos a afinidade do DF e da P ao receptor de glicocorticóide presente em citosol de fígado, timo e hipófise de ratos adrenalectomizados. Verificamos que nos três tecidos examinados, há correlaçäo entre as afinidades determinadas e os efeitos timolítico e anti-inflamatório