Inflammation and therapeutic vaccination in CNS diseases

The spectrum of inflammatory diseases of the central nervous system has been steadily expanding from classical autoimmune disorders such as multiple sclerosis to far more diverse diseases. Evidence now suggests that syndromes such as Alzheimer's disease and stroke have important inflammatory and immune components and may be amenable to treatment by anti-inflammatory and immunotherapeutic approaches. The notion of 'vaccinating' individuals against a neurodegenerative disorder such as Alzheimer's disease is a marked departure from classical thinking about mechanism and treatment, and yet therapeutic vaccines for both Alzheimer's disease and multiple sclerosis have been validated in animal models and are in the clinic. Such approaches, however, have the potential to induce unwanted inflammatory responses as well as to provide benefit.     

Mutation of the beta-amyloid precursor protein in familial Alzheimer's disease increases beta-protein production.

Progressive cerebral deposition of the 39-43-amino-acid amyloid beta-protein (A beta) is an invariant feature of Alzheimer's disease which precedes symptoms of dementia by years or decades. The only specific molecular defects that cause Alzheimer's disease which have been identified so far are missense mutations in the gene encoding the beta-amyloid precursor protein (beta-APP) in certain families with an autosomal dominant form of the disease (familial Alzheimer's disease, or FAD). These mutations are located within or immediately flanking the A beta region of beta-APP, but the mechanism by which they cause the pathological phenotype of early and accelerated A beta deposition is unknown. Here we report that cultured cells which express a beta-APP complementary DNA bearing a double mutation (Lys to Asn at residue 595 plus Met to Leu at position 596) found in a Swedish FAD family produce approximately 6-8-fold more A beta than cells expressing normal beta-APP. The Met 596 to Leu mutation is principally responsible for the increase. These data establish a direct link between a FAD genotype and the clinicopathological phenotype. Further, they confirm the relevance of the continuous A beta production by cultured cells for elucidating the fundamental mechanism of Alzheimer's disease.     

Antibodies to paired helical filaments in Alzheimer's disease do not recognize normal brain proteins

During ageing of the human brain, and particularly in senile dementia of the Alzheimer type (AD), many neurones progressively accumulate abnormal cytoplasmic fibres, called paired helical filaments (PHF). Each such fibre consists of a pair of intermediate (10 nm) filaments twisted into a double helix with a periodicity of 160 nm. PHF accumulate in large perikaryal masses, called neurofibrillary tangles, and are also found in degenerating cortical neurites that form neurite plaques. The density of PHF-containing neurites and cell bodies correlates with the degree of dementia and the extent of loss of cholinergic neurotransmitter function in AD. Recently, we demonstrated that PHF from human cerebral cortex are large, rigid polymers with unusual molecular properties, including insolubility in SDS, urea and other denaturing solvents and apparent resistance to protease digestion. These properties have so far prevented complete purification and analysis of the constituents of PHF. Based on their insolubility, we have developed a new method of preparing highly enriched PHF fractions and have raised an antiserum that is highly specific for PHF. We report here that this antiserum specifically labels PHF, free of any associated normal fibrous proteins and, unexpectedly, it reacts with neither neurofilaments nor any other normal cytoskeletal protein in brain sections or on immunoblotted gels. These anti-PHF antibodies have been used for the specific detection of Alzheimer-type PHF and in the search for cross-reacting antigens in various tissues, and are suitable for immunoaffinity purification of PHF. Our results indicate that PHF contain determinants that are not shared with normal neuronal fibrous proteins.     

Targeting of cell-surface beta-amyloid precursor protein to lysosomes: alternative processing into amyloid-bearing fragments.

Progressive cerebral deposition of the amyloid beta-peptide is an early and invariant feature of Alzheimer's disease. The beta-peptide is released by proteolytic cleavages from the beta-amyloid precursor protein (beta APP), a membrane-spanning glycoprotein expressed in most mammalian cells. Normal secretion of beta APP involves a cleavage in the beta-peptide region, releasing the soluble extramembranous portion and retaining a 10K C-terminal fragment in the membrane. Because this secretory pathway precludes beta-amyloid formation, we searched for an alternative proteolytic processing pathway that can generate beta-peptide-bearing fragments from full-length beta APP. Incubation of living human endothelial cells with a beta APP antibody revealed reinternalization of mature beta APP from the cell surface and its targeting to endosomes/lysosomes. After cell-surface biotinylation, full-length biotinylated beta APP was recovered inside the cells. Purification of lysosomes directly demonstrated the presence of mature beta APP and an extensive array of beta-peptide-containing proteolytic products. Our results define a second processing pathway for beta APP and suggest that it may be responsible for generating amyloid-bearing fragments in Alzheimer's disease.     

Folding proteins in fatal ways

Human diseases characterized by insoluble extracellular deposits of proteins have been recognized for almost two centuries. Such amyloidoses were once thought to represent arcane secondary phenomena of questionable pathogenic significance. But it is has now become clear that many different proteins can misfold and form extracellular or intracellular aggregates that initiate profound cellular dysfunction. Particularly challenging examples of such disorders occur in the post-mitotic environment of the neuron and include Alzheimer's and Parkinson's diseases. Understanding some of the principles of protein folding has helped to explain how such diseases arise, with attendant therapeutic insights.     

α-Synuclein occurs physiologically as a helically folded tetramer that resists aggregation

Parkinson's disease is the second most common neurodegenerative disorder. Growing evidence indicates a causative role of misfolded forms of the protein α-synuclein in the pathogenesis of Parkinson's disease. Intraneuronal aggregates of α-synuclein occur in Lewy bodies and Lewy neurites, the cytopathological hallmarks of Parkinson's disease and related disorders called synucleinopathies. α-Synuclein has long been defined as a 'natively unfolded' monomer of about 14?kDa (ref. 6) that is believed to acquire α-helical secondary structure only upon binding to lipid vesicles. This concept derives from the widespread use of recombinant bacterial expression protocols for in vitro studies, and of overexpression, sample heating and/or denaturing gels for cell culture and tissue studies. In contrast, we report that endogenous α-synuclein isolated and analysed under non-denaturing conditions from neuronal and non-neuronal cell lines, brain tissue and living human cells occurs in large part as a folded tetramer of about 58?kDa. Several methods, including analytical ultracentrifugation, scanning transmission electron microscopy and in vitro cell crosslinking confirmed the occurrence of the tetramer. Native, cell-derived α-synuclein showed α-helical structure without lipid addition and had much greater lipid-binding capacity than the recombinant α-synuclein studied heretofore. Whereas recombinantly expressed monomers readily aggregated into amyloid-like fibrils in vitro, native human tetramers underwent little or no amyloid-like aggregation. On the basis of these findings, we propose that destabilization of the helically folded tetramer precedes α-synuclein misfolding and aggregation in Parkinson's disease and other human synucleinopathies, and that small molecules that stabilize the physiological tetramer could reduce α-synuclein pathogenicity.     

Naturally secreted oligomers of amyloid beta protein potently inhibit hippocampal long-term potentiation in vivo

Although extensive data support a central pathogenic role for amyloid beta protein (Abeta) in Alzheimer's disease, the amyloid hypothesis remains controversial, in part because a specific neurotoxic species of Abeta and the nature of its effects on synaptic function have not been defined in vivo. Here we report that natural oligomers of human Abeta are formed soon after generation of the peptide within specific intracellular vesicles and are subsequently secreted from the cell. Cerebral microinjection of cell medium containing these oligomers and abundant Abeta monomers but no amyloid fibrils markedly inhibited hippocampal long-term potentiation (LTP) in rats in vivo. Immunodepletion from the medium of all Abeta species completely abrogated this effect. Pretreatment of the medium with insulin-degrading enzyme, which degrades Abeta monomers but not oligomers, did not prevent the inhibition of LTP. Therefore, Abeta oligomers, in the absence of monomers and amyloid fibrils, disrupted synaptic plasticity in vivo at concentrations found in human brain and cerebrospinal fluid. Finally, treatment of cells with gamma-secretase inhibitors prevented oligomer formation at doses that allowed appreciable monomer production, and such medium no longer disrupted LTP, indicating that synaptotoxic Abeta oligomers can be targeted therapeutically.     

Two transmembrane aspartates in presenilin-1 required for presenilin endoproteolysis and gamma-secretase activity

Accumulation of the amyloid-beta protein (Abeta) in the cerebral cortex is an early and invariant event in the pathogenesis of Alzheimer's disease. The final step in the generation of Abeta from the beta-amyloid precursor protein is an apparently intramembranous proteolysis by the elusive gamma-secretase(s). The most common cause of familial Alzheimer's disease is mutation of the genes encoding presenilins 1 and 2, which alters gamma-secretase activity to increase the production of the highly amyloidogenic Abeta42 isoform. Moreover, deletion of presenilin-1 in mice greatly reduces gamma-secretase activity, indicating that presenilin-1 mediates most of this proteolytic event. Here we report that mutation of either of two conserved transmembrane (TM) aspartate residues in presenilin-1, Asp 257 (in TM6) and Asp 385 (in TM7), substantially reduces Abeta production and increases the amounts of the carboxy-terminal fragments of beta-amyloid precursor protein that are the substrates of gamma-secretase. We observed these effects in three different cell lines as well as in cell-free microsomes. Either of the Asp --> Ala mutations also prevented the normal endoproteolysis of presenilin-1 in the TM6 --> TM7 cytoplasmic loop. In a functional presenilin-1 variant (carrying a deletion in exon 9) that is associated with familial Alzheimer's disease and which does not require this cleavage, the Asp 385 --> Ala mutation still inhibited gamma-secretase activity. Our results indicate that the two transmembrane aspartate residues are critical for both presenilin-1 endoproteolysis and gamma-secretase activity, and suggest that presenilin 1 is either a unique diaspartyl cofactor for gamma-secretase or is itself gamma-secretase, an autoactivated intramembranous aspartyl protease.