Sci. Aging Knowl. Environ., 26 October 2005
Vol. 2005, Issue 43, p. pe32
[DOI: 10.1126/sageke.2005.43.pe32]

PERSPECTIVES

Neurochemical Insights

Carina Treiber

The author is at the Free University of Berlin, Thielallee 63, 14195 Berlin, Germany. E-mail: treiber{at}chemie.fu-berlin.de

http://sageke.sciencemag.org/cgi/content/full/2005/43/pe32

Key Words: amyloid • Alzheimer's disease • Parkinson's disease • prion • neuron • neurodegenerative disease

Introduction

Protein aggregation is associated with many different neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), and prion diseases (1) (see Berezovska Perspective). There is an ongoing debate whether these aggregates (called amyloids or amyloid fibrils) are themselves the toxic species or whether other species, such as the oligomers that form during the aggregation process, are in fact causing the disease (see Lashuel Perspective). In addition, these diseases have other features in common, such as a disturbed metal ion homeostasis, which might also play a role in disease progression (see Treiber Perspective).

The 20th biennial meeting of the International Society for Neurochemistry (ISN) and the European Society for Neurochemistry (ESN), held in Innsbruck, Austria from 21 to 26 August 2005, provided new insights into cellular mechanisms involved in such neurodegenerative diseases. The meeting gave researchers the opportunity to report on recent progress not only on disease development but also on the regulation of normal synaptic plasticity and memory building as well as the role of metal sequestering proteins in the injured brain.

Transmissible Spongiform Encephalopathies (TSEs)

These neurodegenerative disorders include Creutzfeld-Jakob disease (CJD) in humans, scrapie in sheep, and bovine spongiform encephalopathy in cattle. Although they, like the other neurodegenerative conditions listed above, are caused by the aggregation of an abnormal form of a constitutively expressed protein, these disorders are distinct in that they are transmissible. Understanding this aggregation process in these conditions, especially in the sporadic age-related form of CJD, is also expected to shed light on the generation of such protein aggregates in other neurodegenerative conditions. The pathological features of these diseases are the spongiform (or spongelike) changes that occur in the brain as the disease develops, astrocytosis (an increase in the number of astrocytes that occurs when neighboring neurons are destroyed), and the formation of amyloid deposits containing the prion protein (PrP) (2). The sole or major component of the infectious agent is an abnormal isoform of the normal cellular form of PrP (PrPC), which is referred to as PrPSc and is characterized by an increased {beta}-sheet content in comparison with PrPC (3).

The mechanisms underlying the conversion of PrPC into PrPSc are not well understood. Different organismal, cellular, and biochemical model systems are available for studying this enigmatic question in more detail. For example, transgenic mouse models of prion diseases have been developed. Additionally, many researchers employ in vitro conversion assays, in which PrPC and PrPSc are mixed together to generate new infectivity. Another approach is the use of cell cultures in which infectivity is propagated. A. F. Hill of the University of Melbourne, Australia, reported the use of a mouse hippocampal cell line (GT1) as well as a non-neuronal cell line RK13 (rabbit kidney epithelial cells) to generate prion-infected cells. He showed that infected cells display an increase in the concentration of PrPSc over time and that infectivity is associated with the culture medium in addition to cell lysates. Consistent with published data, this finding could be attributed to prion release in association with exosomes (small vesicles released by a variety of cell types). This model provides an excellent method to investigate the profile of expressed proteins in infected versus uninfected exosomes to identify cellular factors implicated in the propagation and spread of infectious prions.

V. A. Lawson, from the same university, presented her data about PrPC expression in peripheral tissues. It is known that PrPC expression is necessary for PrPSc propagation, because knockout mice that lack PrPC expression never become sick after infection with the pathogenic agent. Lawson's findings were quite interesting, because the possibility of transmission of variant CJD (one of four forms of CJD) through surgical organ donation has raised concerns. Using an in vitro conversion activity assay, she showed that the brain, tongue, eye, spleen, heart, and rectum are active conversion tissues (in which PrPC can be converted to PrPSc), whereas the liver, muscle, kidney, and lung are inactive conversion tissues. This former group of tissues might therefore act as reservoirs of infectivity and are potential sites for the peripheral transmission of TSE disease.

Parkinson's Disease

PD is associated with the loss of neurons that contain the neurotransmitter dopamine from the substantia nigra, a region of the brain that controls movement. The main symptoms of this disorder are tremor, slow movements, and rigidity (see Andersen Review). PD affects 1 to 2% of the population older than 65 years of age. The relatively rare familial forms of this disease have been linked to mutations in a number of genes, including those encoding {alpha}-synuclein, DJ-1, LRRK2/Dardardin, UCHL1, and Parkin (4) (see Giasson Perspective and Trojanowski Perspective). The main pathological hallmark of PD is the formation of intracytoplasmic inclusion bodies, called Lewy bodies, which consist largely of {alpha}-synuclein.

R. Cappai from the University of Melbourne, Australia, showed evidence that dopamine induces the formation of {alpha}-synuclein oligomers [which are resistant to disruption by the detergent sodium dodecyl sulfate (SDS)]. Cappai presented data showing that {alpha}-synuclein and dopamine interact, with an apparent dissociation constant of 16 µM. This interaction causes an alteration to the {alpha}-synuclein structure and influences oligomer formation. These results point to a physiological relevance of this interaction, because the normal intracellular dopamine concentration is 50 µM. Therefore, dopamine could act as a dominant modulator of {alpha}-synuclein aggregation in early stages of disease when dopamine is still present in the subtantia nigra. Whether a given structure represents an amyloid fibril can be determined by staining with Thioflavin T; these oligomers are not amyloidogenic by this criterion. Cappai's observations support the paradigm, emerging for other neurodegenerative diseases, that the toxic species involves a soluble oligomer and not the insoluble fibril (5).

Alzheimer's Disease

One of the primary characteristics of AD is the presence of extracellular amyloid plaques that consist chiefly of amyloid {beta} (A{beta}), which is derived from the amyloid precursor protein (APP) (see "Detangling Alzheimer's Disease"). A plenary lecture was given by C. Masters from the University of Melbourne, Australia, presenting A{beta}-based amyloid fibrils as the major molecular therapeutic and diagnostic target of this condition. In this view, the A{beta} that accumulates in extracellular plaques undergoes a toxic gain of function to induce neuronal damage. This finding was explained by either the generation of the reactive oxygen species H2O2 by A{beta} through its interaction with redox active copper ions or by toxic interactions of A{beta} with neuronal membranes (6).

Despite suggestions that amyloid plaques seem to play a central role in AD, the temporal progression of their formation does not correlate with disease progress. D. M. Walsh from the Harvard Medical School in Boston, USA, showed that in rats, SDS-resistant A{beta} oligomers produced by cells overproducing APP (7) blocked long-term potentiation (LTP; a phenomenon associated with learning in which repetitive stimulation of neurons intensifies the signals they send). Treatment of cells with an inhibitor of {gamma}-secretase (an intramembrane protease that is required, together with {beta}-secretase, for the production of A{beta} from APP) or inhibitors of amyloid fibrillogenesis like RS-0406 or RS-0466 was shown to prevent oligomer formation and rescue the block of LTP in rats. The in vivo administration of anti-A{beta} antibodies had the same beneficial effect, suggesting that soluble oligomers of A{beta} are the earliest effectors of synaptic compromise in AD.

K. J. Barnham of the University of Melbourne, Australia, asked whether oxidative modifications are the reason for the generation of neurotoxically active A{beta} peptides. He reported that two different residues in A{beta}, Tyr10 and Met35, are susceptible to metal-mediated oxidative modifications. Oxidation of Met35 alters A{beta} to a more soluble isoform, correlating with the increased concentrations of soluble A{beta} in diseased versus healthy brain. In the presence of H2O2, A{beta} associated with copper generates dityrosine-linked soluble A{beta} oligomers. Methylation of His residues (which prevents copper binding to A{beta} and thus related pathogenic events like oxidative stress) was shown to inhibit neurotoxicity, consistent with the hypothesis that inhibiting the metal-A{beta} interactions would have therapeutic benefit (8) (see "Mindful of Metal").

To study the binding of extracellular A{beta} to the membrane surface and to investigate the importance of this interaction to disease progression, plasmon resonance spectroscopy was used by M. I. Aguilar and colleagues of Monash University in Australia. They showed that A{beta}42 (a 42-residue N-terminal fragment of APP that is linked to amyloid-related pathologies, in contrast to A{beta}40) binds with higher affinity to a hydrophobic matrix as compared with A{beta}40. Furthermore, aged peptides (that is, peptides incubated at room temperature for a period of time) increased membrane binding (9). These data correlate with the effect of A{beta} peptides on smooth muscle cell viability in vitro. Cholesterol was necessary for a high level of binding of A{beta} to membranes; statins, which are inhibitors of cholesterol biosynthesis, decreased this binding as well as the toxicity of A{beta}. The results strongly suggest that A{beta} toxicity is a direct consequence of binding to lipids in the membrane.

Another hypothesis concerning A{beta} toxicity is that intracellular A{beta} peptides (in contrast to extracellular plaques or oligomers) cause neuronal dysfunction. G. K. Gouras of the Weill Medical College of Cornell University in New York, USA, presented his data about the subcellular accumulation of A{beta}42 in multivesicular bodies in late endosomes in cultured neurons. Some researchers think that this intracellular accumulation may be more relevant to AD than are extracellular amyloidogenic plaques (10). A marked accumulation of A{beta}42 in synapses of Tg2576 mice (a model of AD), but not in wild-type mice, was also observed. This accumulation was accompanied by a selective alteration in pre- and postsynaptic proteins as well as by down-regulation of the proteins GluR1, PSD-95, and synaptophysin, events that impair LTP, are associated with memory loss, and may promote early synaptic dysfunction in AD.

In agreement with these data, T. Hartmann of the University of Heidelberg, Germany, showed that intracellular A{beta}42 is produced in the endoplasmic reticulum (ER) in primary hippocampal neurons, whereas A{beta}40 is found in the trans-Golgi network. A{beta}40 is secreted, whereas A{beta}42 produced in the ER accumulates in that organelle. The amount of secreted A{beta}42 is below the limits of detection (11). The generation of A{beta}40 and A{beta}42 through the activity of {gamma}-secretase does not occur randomly, but seems to be regulated by membrane thickness, which differs in different cell compartments. Taken together, these findings suggest that intracellular A{beta} accumulation is a risk factor for developing AD.

In addition to A{beta}, proteolytically derived C-terminal fragments (CTs) of APP are assumed to be neurotoxic. A talk on this subject was given by Y. H. Suh of the Seoul National University of Korea. He pointed out that these peptides are translocated to the nucleus in a manner dependent on the phosphorylation of Thr668 in APP-CT. This event results in an up-regulation of glycogen synthase kinase-3beta (GSK3{beta}), hyperphosphorylation of the microtubule binding protein tau, which has been implicated in AD, and apoptosis. The same effects were observed for CTs arising from {gamma}-secretase cleavage of APLP2, a protein homologous to APP (12).

Huntington's Disease

HD is caused by CAG-repeat expansion in the gene encoding the Huntington protein (Htt), which forms aggregates in the disease state (13). To gain more information about the function of the protein, the yeast two-hybrid system was used to identify interaction partners of this protein. This work was presented by E. E. Wanker of the Max Delbr´┐Żck Center for Molecular Medicine in Berlin, Germany. This group identified 20 directly interacting proteins, including GIT1 and profilin-2a. GIT1, a G protein-coupled receptor kinase-interacting protein, enhances Htt aggregation, whereas profilin-2a, a neuron-specific actin-binding protein, reduces the formation of such aggregates. This result suggests that Htt is a multifunctional scaffold protein that forms protein complexes. These findings may provide the basis for new therapeutic approaches to prevent the accumulation of Htt aggregates.

Treatment of Neurodegenerative Diseases

One approach for treatment of neurodegenerative diseases in general is the use of RNA interference (RNAi) (see Lee Perspective), a tool to lower the expression of targeted gene products (14). Studies using this approach have been performed by C. Raoul of the Swiss Federal Institute of Technology, Lausanne, Switzerland, using a mouse model of ALS, in which mutated SOD1 leads to progressive death of motoneurons through a gain-of-function mechanism under disease-causing conditions. Short hairpin RNAs, which can cause RNAi, were directly introduced by intraspinal injection of lentiviral-based constructs, which can infect nondividing cells. Raoul presented evidence that lentiviral silencing of mutated SOD1 delays both the onset and the rate of progression of the disease. This approach promises to be an attractive tool in other neurodegenerative diseases with a genetic origin (15).

Another approach in AD treatment was presented by T. C. Saido of the RIKEN Brain Science Institute in Wako, Japan. A{beta} is degraded by neprilysin, a membrane-bound metallopeptidase, which is normally activated by the neuropeptide somatostatin (SST) (see Saito Perspective). The level of SST is reduced during aging and in AD. Saido discussed the inducement of A{beta} aggregation by loss of SST (16). His work suggests that SST receptor (SSTR) could be used as a new therapeutic target that could be activated by the application of SSTR agonists. He also suggested the hypothesis that declining SST concentrations during aging could be a risk factor for developing sporadic AD.

A. R. White of the University of Melbourne, Australia, presented data on the modulation of APP and A{beta} metabolism by metal chelators. He showed that clioquinol (CQ; a cell-permeable metal chelator) in complex with copper induces both extracellular loss of A{beta}40 and A{beta}42, although {beta}- or {gamma}-secretase activity was unchanged. This observation was attributed to the activation of mitogen-activated protein kinase and the phosphatidylinositol 3-kinase (PI3K)-protein kinase B/Akt-GSK3 pathways (see figure 1 in Longo Perspective) by employing several inhibitors that affect these pathways. In addition, it was shown that CQ copper complexes induce PI3K and JNK -dependent activation of the extracellular matrix metalloproteases MMP2 and MMP3, leading to A{beta} degradation. These results may have important implications in the beneficial modulating of metal bioavailability (17).

Metallothioneins

Metallothioneins (MTs) are zinc-binding proteins of about 61 to 68 amino acids in length that are found in most cells and all animals. Three isoforms are found in the mammalian brain: MTI, MTII, and MTIII. These proteins are produced primarily in astrocytes. It is known that wound healing is impaired in knockout mice lacking either MTI or MTII. R. S. Chung of the University of Tasmania, Australia, reported that MTI and MTII are specifically up-regulated in response to injury to neurons but not to astrocytes (18). The MT released by astrocytes is taken up by neurons in vivo by an unknown mechanism and promotes reactive sprouting (a process during which neurons produce new axons in response to injury) in vitro. This model represents a specific example of the general principle of neuron-astrocyte interactions within the injured central nervous system (CNS). Also in this context, M. Penkowa of the University of Copenhagen, Denmark, showed that MTI and MTII expression and exogenous administration have neuroprotective effects by decreasing the concentrations of proinflammatory cytokines and tumor necrosis factor-{alpha}. She also has shown that MTs function as antioxidants and reduce the level of apoptosis by inhibiting the activation of the effector caspase 3. Furthermore, she has shown that these proteins are involved in repair and regeneration processes after brain injury. For example, MTI and II induce neuronal survival, sprouting, and functional recovery and increase the concentration of growth factors. Moreover, MTI and II activate neural stem cells to proliferate and migrate into damaged CNS tissue (19).

Conclusion

A variety of insights about the causes of, and possible treatments for, different neurodegenerative diseases were presented at this meeting. Because of the parallels between these diseases, insights gained about one disease may be useful for thinking about others. It is hoped that new therapeutic strategies, potentially involving RNAi, metal chelators, or metallopeptidases, will soon be developed for treating a variety of neurodegenerative diseases.


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Citation: C. Treiber, Neurochemical Insights. Sci. Aging Knowl. Environ. 2005 (43), pe32 (2005).








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