Molecular pathogenesis of apolipoprotein E-mediated amyloidosis in late-onset Alzheimer’s disease

Apolipoprotein E (apoE) ɛ4 allele is a genetic risk factor for late-onset familial and sporadic Alzheimer’s disease (AD). In the central nervous system, apoE is secreted mainly by astrocytes as a constituent of high-density lipoproteins. A recent study using apoE knockout mice provided strong evidence that apoE promotes cerebral deposition of amyloid β protein (Aβ). However, no clear explanation of the pathogenesis of apoE-induced AD has been provided. Here we discuss two possible mechanisms by which apoE might enhance Aβ deposition. One is the intracellular pathway in which apoE is internalized by neurons and induces lysosomal accumulation of Aβ and amyloidogenic APP (amyloid precursor protein) fragments, leading to neuronal death. The other is the extracellular pathway in which apoE-containing lipoproteins are trapped by Aβ1–42 deposits mobilizing soluble Aβ peptides and consequently enlarge amyloid plaques. These two mechanisms may operate at different stages of AD pathogenesis and suggest a chaperone-like function for the apoE molecule. Received 4 February 1999; received after revision 9 April 1999; accepted 23 April 1999     

Synaptic dysfunction in Huntington’s disease: a new perspective

Huntington’s disease (HD) is caused by a polyglutamine expansion in the protein huntingtin and is characterized by intraneuronal inclusions and widespread neuronal death at the late stage of the disease. In research, most of the emphasis has been on understanding the cell death and its mechanisms. Until recently, it was believed that the vast majority, if not all, of the symptoms in HD are a direct consequence of neurodegeneration. However, increasing evidence shows that subtle alterations in synaptic function could underlie the early symptoms. It is of particular interest to understand the nature of this neuronal dysfunction. Normal huntingtin interacts with various cytoskeletal and synaptic vesicle proteins that are essential for exocytosis and endocytosis. Altered interactions of mutant huntingtin with its associated partners could contribute to abnormal synaptic transmission in HD. This review describes recent advances in understanding synaptic dysfunction in HD.Received 2 March 2005; received after revision 13 April 2005; accepted 19 April 2005     

Genome-wide association study to identify SNPs conferring risk of myocardial infarction and their functional analyses

Myocardial infarction might result from the interactions of multiple genetic and environmental factors, none of which can cause disease solely by each of themselves. Although molecular biological studies revealed that a number of proteins are possibly involved in its pathogenesis, little, if any genetic findings have been reported so far. To reveal genetic backgrounds of myocardial infarction, we performed a large-scale, case-control association study using 92,788 gene-based single-nucleotide polymorphism (SNP) markers. We have identified functional SNPs within the lymphotoxin-α gene (LTA) located on chromosome 6p21 that conferred susceptibility to myocardial infarction. Furthermore, we could identify galectin-2 protein as a binding partner of LTA protein. The association study further revealed that a functional SNP in LGALS2 encoding galectin-2, which led to altered secretion of LTA, also indicated a risk of myocardial infarction. A combined strategy of genetic and molecularcellular biological approaches may be useful in clarifying pathogenesis of common diseases.Received 7 March 2005; received after revision 22 April 2005; accepted 25 April 2005     

Spectrin and calpain: a ‘target’ and a ‘sniper’ in the pathology of neuronal cells

It is well documented that activation of calpain, a calcium-sensitive cysteine protease, marks the pathology of naturally and experimentally occuring neurodegenerative conditions. Calpain-mediated proteolysis of major membrane-skeletal protein, αII-spectrin, results in the appearance of two unique and highly stable breakdown products, which is an early event in neural cell pathology. This review focuses on spectrin degradation by calpain within neurons induced by diverse conditions, emphasizing a current picture of multi-pattern neuronal death and a recent success in the development of spectrin-based biomarkers. The issue is presented in the context of the major structural and functional properties of the two proteins.Received 7 March 2005; received after revision 22 April 2005; accepted 13 May 2005     

Aluminum-triggered structural modifications and aggregation of β-amyloids

We investigated the structural effects induced by Al³⁺ on different β-amyloid (Aβ) fragments at pH 7.4 and T= 25°C, with particular attention given to the sequences 1–40 and 1–42. Al³⁺ caused peptide enrichment in β sheet structure and formation of solvent-exposed hydrophobic clusters. These intermediates evolved to polymeric aggregates which organized in fibrillar forms in the case of the Al³⁺-Aβ_(1–42) complex. Comparative studies showed that Zn²⁺ and Cu²⁺ were much less efficient than Al³⁺ in stimulating the spontaneous aggregation/fibrillogenesis of Aβs. Studies with liposomes as membrane models showed dramatic changes in the structural properties of the lipid bilayer in the presence of Al³⁺-Aβ complexes, suggesting a major role of Al³⁺ in Aβ-induced cell dysfunction. Al³⁺ effects were abolished by desferrioxamine mesylate (DFO) only in solution. We concluded that, in vivo, DFO may act as a protective agent by preventing or reverting Aβ aggregation in the extracellular spaces.Received 29 March 2005; received after revision 10 May 2005; accepted 25 May 2005     

Transthyretin: the servant of many masters

Transthyretin (TTR) (formerly, thyroxine binding prealbumin) is an evolutionarily conserved serum and cerebrospinal fluid protein that transports holo-retinol-binding protein and thyroxine. Its serum concentration has been widely used to assess clinical nutritional status. It is also well known that wild-type transthyretin and approximately 100 different mutants give rise to a variety of forms of systemic amyloid deposition. It has been suspected and recently established that TTR can suppress the Alzheimer’s disease phenotype in transgenic animal models of cerebral Aβ deposition. Thus, while TTR is a systemic amyloid precursor, in the brain it seems to have an anti-amyloidogenic effect. TTR is found in other organs as a result of local synthesis or transport, suggesting that it may have other, as yet undiscovered, functions. It is possible that its capacity to bind many classes of compounds allows it to serve as an endogenous detoxifier of molecules with potential pathologic effects.     

The glycinergic control of spinal pain processing

Alterations in synaptic transmission within the spinal cord dorsal horn play a key role in the development of pathological pain. While N-methyl-D-aspartate (NMDA) receptors and activity-dependent synaptic plasticity have been the focus of research for many years, recent evidence attributes very specific functions to inhibitory glycinergic and γ-aminobutyric acid (GABA)-ergic neurotransmission in the generation of inflammatory and neuropathic pain. The central component of inflammatory pain originates from a disinhibition of dorsal horn neurons, which are relieved from glycinergic neurotransmission by the inflammatory mediator prostaglandin E₂ (PGE₂). PGE₂ activates prostaglandin E receptors of the EP2 subtype and leads to a protein kinase A-dependent phosphorylation and inhibition of glycine receptors containing the α3 subunit (GlyRα3). This GlyRα3 is distinctly expressed in the superficial dorsal horn, where nociceptive afferents terminate. Other but probably very similar disinhibitory mechanisms may well contribute to abnormal pain occurring after peripheral nerve injury.Received 11 March 2005; received after revision 1 April 2005; accepted 19 April 2005     

RNA and protein-dependent mechanisms in tauopathies: consequences for therapeutic strategies

Tauopathies are a group of neurodegenerative diseases characterised by intracellular deposits of the microtubule-associated protein tau. The most typical example of a tauopathy is Alzheimer’s disease. The importance of tau in neuronal dysfunction and degeneration has been demonstrated by the discovery of dominant mutations in the MAPT gene, encoding tau, in some rare dementias. Recent developments have shed light on the significance of tau phosphorylation and aggregation in pathogenesis. Furthermore, emerging evidence reveals the central role played by tau pre-mRNA processing in tauopathies. The present review focuses on the current understanding of tau-dependent pathogenic mechanisms and how realistic therapies for tauopathies can be developed. Received 3 December 2006; received after revision 23 February 2007; accepted 20 March 2007     

Glycogen synthase kinase 3β and Alzheimer’s disease: pathophysiological and therapeutic significance

Alzheimer’s disease (AD) is a neurodegenerative disorder associated with cognitive and behavioral dysfunction and is the leading cause of dementia in the elderly. Several studies have implicated molecular and cellular signaling cascades involving the serine-threonine kinase, glycogen synthase kinase β(GSK-3β) in the pathogenesis of AD. GSK-3β may play an important role in the formation of neurofibrillary tangles and senile plaques, the two classical pathological hallmarks of AD. In this review, we discuss the interaction between GSK-3β and several key molecules involved in AD, including the presenilins, amyloid precursor protein, tau, and β-amyloid. We identify the signal transduction pathways involved in the pathogenesis of AD, including Wnt, Notch, and the PI3 kinase/Akt pathway. These may be potential therapeutic targets in AD. Received 19 December 2005; received after revision 24 January 2006; accepted 6 February 2006     

Transthyretin: a review from a structural perspective

Transthyretin (formerly called prealbumin) plays important physiological roles as a transporter of thyroxine and retinol-binding protein. X-ray structural studies have provided information on the active conformation of the protein and the site of binding of both ligands. Transthyretin is also one of the precursor proteins commonly found in amyloid deposits. Both wild-type and single-amino-acid-substituted variants have been identified in amyloid deposits, the variants being more amyloidogenic. Sequencing of the gene and the resulting production of a transgenic mouse model have resulted in progress toward solving the mechanism of amyloid formation and detecting the variant gene in individuals at risk. Received 23 January 2001; received after revision 4 April 2001; accepted 30 April 2001     

Identification of tyrosine-phosphorylated proteins of the mitochondrial oxidative phosphorylation machinery

The role of some serine/threonine kinases in the regulation of mitochondrial physiology is now well established, but little is known about mitochondrial tyrosine kinases. We showed that tyrosine phosphorylation of rat brain mitochondrial proteins was increased by in vitro addition of ATP and H₂O₂, and also during in situ ATP production at state 3, and maximal reactive oxygen species production. The Src kinase inhibitor PP2 decreased tyrosine phosphorylation and respiratory rates at state 3. We found that the 39-kDa subunit of complex I was tyrosine phosphorylated, and we identified putative tyrosine-phosphorylated subunits for the other complexes. We also have strong evidence that the FoF1-ATP synthase α chain is probably tyrosine-phosphorylated, but demonstrated that the β chain is not. The tyrosine phosphatase PTP 1B was found in brain but not in muscle, heart or liver mitochondria. Our results suggest that tyrosine kinases and phosphatases are involved in the regulation of oxidative phosphorylation.Received 7 January 2005; received after revision 19 April 2005; accepted 22 April 2005     

Dogma and dreams: experimental lessons for epilepsy mechanism chasers

Epilepsy mechanism chasers face one major difficulty. Since we don’t know how the normal brain works, we can’t start to understand how the diseased brain fails. Most of today’s hypotheses are based on what we think about ‘normal’ brain function, which may lead to misconceptions, as will be developed here. Furthermore, since there are many different types of epilepsies, some mechanisms may only be relevant to some epilepsies. Here, I shall focus on temporal lobe epilepsy (TLE) the most common form of partial epilepsy in adults. TLE is often drug resistant, as are 30–40% of all forms of epilepsies. The failure of drug-treatments most likely reflects our lack of knowledge of the underlying mechanisms.Received 10 January 2005; received after revision 3 March 2005; accepted 23 March 2005     

Anti-amyloidogenic therapies: strategies for prevention and treatment of Alzheimer’s disease

Deposition of amyloid β-protein (Aβ) in the brain is an early and invariant neuropathological feature of Alzheimer’s disease (AD). The current search for anti-AD drugs is mainly focused on modification of the process of accumulation of Aβ in the brain. Here, we review four anti-amyloidogenic strategies: (i) reduction of Aβ production, which has mainly been approached with secretase inhibition, (ii) promotion of the Aβ degrading catabolic pathway, including an Aβ degrading enzyme, neprilysin, (iii) immunotherapy for Aβ and (iv) inhibition of Aβ aggregation. We have reported that AD patients have a favorable molecular environment for Aβ aggregation and that various compounds, such as polyphenols, interfere with Aβ aggregation and destabilize preformed Aβ fibrils. Received 21 December 2005; received after revision 14 February 2006; accepted 29 March 2006     

On the structural definition of amyloid fibrils and other polypeptide aggregates

Amyloid fibrils occur inside the human body, associated with ageing or a group of diseases that includes, amongst others, Alzheimer’s disease, atherosclerosis and type II diabetes. Many natural polypeptide chains are able to form amyloid fibrils in vivo or in vitro, and this ability has been suggested to represent an inherent consequence of the chemical structure of the polypeptide chain. Recent literature has provided a wealth of information about the structure of aggregates, precipitates, amyloid fibrils and other types of fibrillar polypeptide assemblies. However, the biophysical meaning associated with these terms can differ considerably depending on the context of their usage. This overview presents a structural comparison of amyloid fibrils and other types of polypeptide assemblies and defines amyloid fibrils, based on structural considerations, as fibrillar polypeptide aggregates with a cross-β conformation. Received 1 March 2007; received after revision 15 March 2007; accepted 25 April 2007     

Cerebral amyloidosis: amyloid subunits, mutants and phenotypes

Cerebral amyloid diseases are part of a complex group of chronic and progressive entities bracketed together under the common denomination of protein folding disorders and characterized by the intra- and extracellular accumulation of fibrillar aggregates. Of the more than 25 unrelated proteins known to produce amyloidosis in humans only about a third of them are associated with cerebral deposits translating in cognitive deficits, dementia, stroke, cerebellar and extrapyramidal signs, or a combination thereof. The familial forms reviewed herein, although infrequent, provide unique paradigms to examine the role of amyloid in the mechanism of disease pathogenesis and to dissect the link between vascular and parenchymal amyloid deposition and their differential contribution to neurodegeneration.     

Structural and dynamical features contributing to thermostability in α-amylases

In recent years an increasing number of studies on thermophilic and hyperthermophilic proteins aiming to elucidate determinants of protein thermostability have yielded valuable insights about the relevant mechanisms. In particular, comparison of homologous enzymes with different thermostabilities (isolated from psychrophilic, mesophilic, thermophilic and hyperthermophilic organsims) offers a unique opportunity to determine the strategies of thermal adaptation. In this respect, the medium-sized amylolytic enzyme α-amylase is a well-established representative. Various studies on α-amylases with very different thermostabilities (melting temperature T_m = 40–110°C) report structural and dynamical features as well as thermodynamical properties which are supposed to play key roles in thermal adaptation. Here, results from selected homologous α-amylases are presented and discussed with respect to some new and recently proposed strategies to achieve thermostability.Received 28 February 2005; received after revision 29 April 2005; accepted 19 May 2005     

Detailed molecular comparison between the inhibition mode of A/B-type carboxypeptidases in the zymogen state and by the endogenous inhibitor latexin

Treatment of advanced stages of prostate carcinoma with histone-deacetylase inhibitors entails expression of human procarboxypeptidase-A4 (hPCPA4). The three-dimensional structure of hPCPA4 has been solved and shows the features of related metallocarboxypeptidase zymogens, with a preformed α/β/-hydrolase active-enzyme moiety (hCPA4) and an inhibiting pro-domain (PD). The protease moiety recalls a sphere, out of which a spherical cone has been cut. This results in a funnel-like structure, at the bottom of which the active-site cleft resides. The border of this funnel is shaped by loops, which are responsible for the interaction with the PD, characterised by a large interface area and relatively few contacts. Such an inhibitory mode is evocative of the recently reported structure of the human inhibitor latexin in its complex with hCPA4. The main contacting structure of latexin is similar to the one employed for PD inhibition. In both cases, active-site blocking relies mainly on a loop provided by the central part of a β sheet.R. Garcia-Castellanos and R. Bonet-Figueredo contributed equally to this study and share first authorship.Received 28 April 2005; received after revision 1 June 2005; accepted 29 June 2005     

Familial hypobetalipoproteinemia: genetics and metabolism

Familial hypobetalipoproteinemia (FHBL), an autosomal dominant disorder, is defined as <5^th percentile LDL-cholesterol or apolipoprotein (apo) B in the plasma. FHBL subjects are generally heterozygous and asymptomatic. Three genetic forms exist: (i) premature stop codon specifying mutations of APOB; (ii) FHBL linked to a susceptibility locus on the chromosome 3p21; and (iii) FHBL linked neither to APOB nor to the chromosome 3p21. In heterozygous apoB-defective FHBL, the hepatic VLDL export system is defective because apoB 100, the product of the normal allele, is produced at ∼25% of normal rate, and truncated apoB is cleared too rapidly. The reduced capacity for hepatic triglyceride export increases hepatic fat three-fold. Indexes of adiposity and insulin action are similar to controls. ‘Knock-in’ mouse models of apoB truncations resemble human FHBL phenotypes. Liver fat in the chromosome 3p21-linked FHBL is normal. Elucidation of the genetic basis of the non-apoB FHBL could uncover attractive targets for lipid-lowering therapy. (See note added in proof.)Received 27 October 2004; received after revision 1 February 2005; accepted 22 February 2005     

Chromosome 17-linked dementias

Chromosome 17-linked dementias have been defined by linkage analysis. The most common of these syndromes has been estimated to be the cause of between 2 and 20% of all dementia and has alternately been called frontotemporal dementia, Pick's disease (without Pick bodies) and dementia lacking distinctive features [1 – 3]. The identification of the mutation responsible for these conditions in a group of clinically and pathologically heterogeneous disorders may allow us to gain broad insight into the processes of neurodegeneration.     

The molecular link between β- and γ-secretase activity on the amyloid β precursor protein

Alzheimer’s disease (AD) is characterized by an accumulation in the brain of amyloid β peptides (Aβ). The production of Aβ requires two sequential cleavages induced by β- and γ-secretases on the β-amyloid precursor protein (APP). Altered activity of these secretases is involved in the pathogenesis of AD. The expression and activity of β-secretase (BACE1) is augmented in the brain in late-onset sporadic AD. Mutant presenilin 1 (PS1), the major genetic defect of early-onset familial AD (FAD), alters the activity of γ-secretase, leading to increased production of Aβ42. Here we review the role of oxidative stress as a molecular link between the β- and the γ-secretase activities, and provide a mechanistic explanation of the pathogenesis of sporadic late-onset AD. We also discuss evidence for a role of the same mechanism in the pathogenesis of familial AD carrying PS1 mutations.