Soluble amyloid precursor protein α reduces neuronal injury and improves functional outcome following diffuse traumatic brain injury in rats

Biology and pathophysiology of the amyloid precursor protein

The amyloid precursor protein (APP) plays a central role in the pathophysiology of Alzheimer's disease in large part due to the sequential proteolytic cleavages that result in the generation of β-amyloid peptides (Aβ). Not surprisingly, the biological properties of APP have also been the subject of great interest and intense investigations. Since our 2006 review, the body of literature on APP continues to expand, thereby offering further insights into the biochemical, cellular and functional properties of this interesting molecule. Sophisticated mouse models have been created to allow in vivo examination of cell type-specific functions of APP together with the many functional domains. This review provides an overview and update on our current understanding of the pathobiology of APP.

Circulating levels of brain-derived neurotrophic factor: correlation with mood, cognition and motor function

Brain-derived neurotrophic factor (BDNF) is the most widely distributed neurotrophin in the CNS, where it plays several pivotal roles in synaptic plasticity and neuronal survival. As a consequence, BDNF has become a key target in the physiopathology of several neurological and psychiatric diseases. Recent studies have consistently reported altered levels of BDNF in the circulation (i.e., serum or plasma) of patients with major depression, bipolar disorder, Alzheimer's disease, Huntington's disease and Parkinson's disease. Correlations between serum BDNF levels and affective, cognitive and motor symptoms have also been described. BDNF appears to be an unspecific biomarker of neuropsychiatric disorders characterized by neurodegenerative changes.

Impacts of membrane biophysics in Alzheimer's disease: from amyloid precursor protein processing to aβ Peptide-induced membrane changes

An increasing amount of evidence supports the notion that cytotoxic effects of amyloid-β peptide (Aβ), the main constituent of senile plaques in Alzheimer's disease (AD), are strongly associated with its ability to interact with membranes of neurons and other cerebral cells. Aβ is derived from amyloidogenic cleavage of amyloid precursor protein (AβPP) by β- and γ-secretase. In the nonamyloidogenic pathway, AβPP is cleaved by α-secretases. These two pathways compete with each other, and enhancing the non-amyloidogenic pathway has been suggested as a potential pharmacological approach for the treatment of AD. Since AβPP, α-, β-, and γ-secretases are membrane-associated proteins, AβPP processing and Aβ production can be affected by the membrane composition and properties. There is evidence that membrane composition and properties, in turn, play a critical role in Aβ cytotoxicity associated with its conformational changes and aggregation into oligomers and fibrils. Understanding the mechanisms leading to changes in a membrane's biophysical properties and how they affect AβPP processing and Aβ toxicity should prove to provide new therapeutic strategies for prevention and treatment of AD.

A substance P antagonist reduces axonal injury and improves neurologic outcome when administered up to 12 hours after traumatic brain injury

Previous studies have demonstrated that the compound N-acetyl-L-tryptophan (NAT) reduces brain edema and improves functional outcome following traumatic brain injury (TBI). In this study we examined whether this effect was mediated via the neurokinin-1 receptor, and whether there was an effect on axonal injury. We also explored whether the compound was effective, even when administered at delayed time points. Male Sprague-Dawley rats were subject to acceleration-induced, diffuse TBI and administered NAT, its inactive D-enantiomer, or saline vehicle. In contrast to NAT (2.5 mg/kg), the inactive D-enantiomer was ineffective at improving rotarod motor performance after TBI. NAT also improved cognitive outcome as assessed by the Morris water maze and novel object recognition tests, and reduced axonal injury at 5 and 24 h after TBI as assessed by amyloid precursor protein immunohistochemistry. However, efficacy of the membrane-impermeable NAT was limited to administration within 5 h, whereas administration of a form of NAT, L-732,138 (47 mg/kg), in which a trifluoromethyl benzyl ester group has been added, making it highly lipid soluble and able to cross the intact blood-brain barrier, significantly improved motor outcome, even when administration was delayed by as much as 12 h. We conclude that the neuroprotective effects of NAT are receptor-mediated, and that administration of the membrane-permeable form of the compound can be effective even up to 12 h after TBI.

The neuroprotective domains of the amyloid precursor protein, in traumatic brain injury, are located in the two growth factor domains

The amyloid precursor protein (APP) is known to increase following traumatic brain injury (TBI). This increase in levels of APP may be deleterious to outcome due to the production of neurotoxic Aβ. Conversely, this upregulation may be beneficial as cleavage of APP via the alternative non-amyloidogenic pathway produces the soluble α form of APP (sAPPα), which is known to have many neuroprotective and neurotrophic functions. Indeed it has previously been shown that treatment with sAPPα following a diffuse injury in rats improves outcome. However, the exact location within the sAPPα molecule which contains this neuroprotective activity has yet to be determined. The sAPPα peptide can consist of up to 6 domains, with the main isoform in the brain missing the 4th and 5th. Of the remaining domains, the D1 and D6a domains seem the most likely as they have been shown to have beneficial actions in vitro. This present study examined the effects of in vivo posttraumatic administration via an intracerebroventricular injection of the D1, D2 and D6a domains of sAPPα on outcome following moderate-impact acceleration TBI in rats. While treatment with either the D1 or D6a domains was found to significantly improve motor and cognitive outcome, as assessed on the rotarod and Y maze, treatment with the D2 domain had no effect. Furthermore axonal injury was reduced in D1 and D6a domain treated animals, but not those that received the D2 domain. As the D1 and D6a domains contain a heparin binding region while the D2 domain does not, this suggests that sAPPα mediates its neuroprotective response through its ability to bind to heparin sulfate proteoglycans.

Effects of fatty acid unsaturation numbers on membrane fluidity and α-secretase-dependent amyloid precursor protein processing

Fatty acids may integrate into cell membranes to change physical properties of cell membranes, and subsequently alter cell functions in an unsaturation number-dependent manner. To address the roles of fatty acid unsaturation numbers in cellular pathways of Alzheimer's disease (AD), we systematically investigated the effects of fatty acids on cell membrane fluidity and α-secretase-cleaved soluble amyloid precursor protein (sAPP(α)) secretion in relation to unsaturation numbers using stearic acid (SA, 18:0), oleic acid (OA, 18:1), linoleic acid (LA, 18:2), α-linolenic acid (ALA, 18:3), arachidonic acid (AA, 20:4), eicosapentaenoic acid (EPA, 20:5), and docosahexaenoic acid (DHA, 22:6). Treatments of differentiated human neuroblastoma (SH-SY5Y cells) with AA, EPA and DHA for 24h increased sAPP(α) secretion and membrane fluidity, whereas those treatments with SA, OA, LA and ALA did not. Treatments with AA and DHA did not alter the total expressions of amyloid precursor protein (APP) and α-secretases in SH-SY5Y cells. These results suggested that not all unsaturated fatty acids but only those with 4 or more double bonds, such as AA, EPA and DHA, are able to increase membrane fluidity and lead to increase in sAPP(α) secretion. This study provides insights into dietary strategies for the prevention of AD.

Zinc metalloproteinases and amyloid Beta-Peptide metabolism: the positive side of proteolysis in Alzheimer's disease

Alzheimer's disease is a neurodegenerative condition characterized by an accumulation of toxic amyloid beta- (Aβ-)peptides in the brain causing progressive neuronal death. Aβ-peptides are produced by aspartyl proteinase-mediated cleavage of the larger amyloid precursor protein (APP). In contrast to this detrimental "amyloidogenic" form of proteolysis, a range of zinc metalloproteinases can process APP via an alternative "nonamyloidogenic" pathway in which the protein is cleaved within its Aβ region thereby precluding the formation of intact Aβ-peptides. In addition, other members of the zinc metalloproteinase family can degrade preformed Aβ-peptides. As such, the zinc metalloproteinases, collectively, are key to downregulating Aβ generation and enhancing its degradation. It is the role of zinc metalloproteinases in this "positive side of proteolysis in Alzheimer's disease" that is discussed in the current paper.

Aspects of beta-amyloid as a biomarker for Alzheimer's disease

Alzheimer's disease is an age-related neurodegenerative disorder that results in progressive cognitive impairment and death. The accumulation of beta-amyloid (Abeta) in specific brain regions is believed by many to represent the earliest event in the pathogenesis of the disease. Here, we review the key aspects of Abeta as a biomarker for Alzheimer's disease, including the pathogenicity of Abeta, the possible biological functions of its precursor protein, the Abeta metabolism and homeostasis, the diagnostic performance of different Abeta assays in different settings and the potential usefulness of Abeta as a surrogate marker for treatment efficacy in clinical trials of novel Abeta-targeting drugs against Alzheimer's disease.

Membrane biophysics and mechanics in Alzheimer's disease

Alzheimer's disease is a chronic neurodegenerative disorder characterized by neuronal loss, cerebrovascular inflammation, and accumulation of senile plaques in the brain parenchyma and cerebral blood vessels. Amyloid-beta peptide (Abeta), a major component of senile plaques, has been shown to exert multiple toxic effects to neurons, astrocytes, glial cells, and brain endothelium. Oligomeric Abeta can disturb the structure and function of cell membranes and alter membrane mechanical properties, such as membrane fluidity and molecular order. Much of these effects are attributed to their capability to trigger oxidative stress and inflammation. In this review, we discuss the effects of Abeta on neuronal cells, astrocytes, and cerebral endothelial cells with special emphasis on cell membrane properties and cell functions.

Abeta promotes Alzheimer's disease-like cytoskeleton abnormalities with consequences to APP processing in neurons

Abeta is proteolytically produced from the Alzheimer's amyloid precursor protein (APP). Major properties attributed to Abeta include neurotoxic effects that contribute to Alzheimer's disease neurodegeneration. However, Abeta can also affect APP processing and trafficking that, in neurons, is anterogradelly transported via microtubules in a kinesin-associated manner. Herein we show that Abeta can induce accumulation of intracellular sAPP in primary neuronal cultures. Subcellular fractionation studies and immunofluorescence analysis revealed that upon Abeta exposure sAPP retention was localized to cytoskeleton associated vesicular structures along the neurite processes, positive for an APP N-terminal antibody and negative for an APP C-terminal antibody. These vesicular structures were also positive for kinesin light chain 1 (KLC). We confirm that Abeta alters both actin and microtubule networks. It increases F-actin polymerization and we report for the first time that Abeta decreases alpha-tubulin acetylation. The use of cytoskeleton associated drugs partially reversed the Abeta-induced effects on sAPP secretion. The data here presented show that Abeta causes intracellular sAPP retention by inducing alterations in the cytoskeleton network, thus contributing to impaired APP/sAPP vesicular transport. Moreover, the data strengthens the hypothesis that Abeta-induces neurodegeneration and provides a potential mechanism of action, as impaired vesicular and axonal transport have been linked to Alzheimer's disease pathology.

Experimental models of traumatic axonal injury

Traumatic brain injury (TBI) is one of the leading causes of death in people under 45 years of age worldwide. Such injury is characterized by a wide spectrum of mechanisms of injury and pathologies. Traumatic axonal injury (TAI), originally described as diffuse axonal injury, is one of the most common pathological features of TBI and is thought to be responsible for the long-lasting neurological impairments following TBI. Since the late 1980s a series of in vivo and in vitro experimental models of TAI have been developed to better understand the complex mechanisms of axonal injury and to define the relationship between mechanical forces and the structural and functional changes of injured axons. These models are designed to mimic as closely as possible the clinical condition of human TAI and have greatly improved our understanding of different aspects of TAI. The present review summarizes the most widely used experimental models of TAI. Focusing in particular on in vivo models, this survey aims to provide a broad overview of current knowledge and controversies in the development and use of the experimental models of TAI.

Secretory phospholipase A2 type III enhances alpha-secretase-dependent amyloid precursor protein processing through alterations in membrane fluidity

In the non-amyloidogenic pathway, amyloid precursor protein (APP) is cleaved by alpha-secretases to produce alpha-secretase-cleaved soluble APP (sAPP(alpha)) with neuroprotective and neurotrophic properties; therefore, enhancing the non-amyloidogenic pathway has been suggested as a potential pharmacological approach for the treatment of Alzheimer's disease. Here, we demonstrate the effects of type III secretory phospholipase A(2) (sPLA(2)-III) on sAPP(alpha) secretion. Exposing differentiated neuronal cells (SH-SY5Y cells and primary rat neurons) to sPLA(2)-III for 24 h increased sAPP(alpha) secretion and decreased levels of Abeta(1-42) in SH-SY5Y cells, and these changes were accompanied by increased membrane fluidity. We further tested whether sPLA(2)-III-enhanced sAPP(alpha) release is due in part to the production of its hydrolyzed products, including arachidonic acid (AA), palmitic acid (PA), and lysophosphatidylcholine (LPC). Addition of AA but neither PA nor LPC mimicked sPLA(2)-III-induced increases in sAPP(alpha) secretion and membrane fluidity. Treatment with sPLA(2)-III and AA increased accumulation of APP at the cell surface but did not alter total expressions of APP, alpha-secretases, and beta-site APP cleaving enzyme. Taken together, these results support the hypothesis that sPLA(2)-III enhances sAPP(alpha) secretion through its action to increase membrane fluidity and recruitment of APP at the cell surface.

Synaptic degeneration in Alzheimer's disease

Synaptic loss is the major neurobiological substrate of cognitive dysfunction in Alzheimer's disease (AD). Synaptic failure is an early event in the pathogenesis that is clearly detectable already in patients with mild cognitive impairment (MCI), a prodromal state of AD. It progresses during the course of AD and in most early stages involves mechanisms of compensation before reaching a stage of decompensated function. This dynamic process from an initially reversible functionally responsive stage of down-regulation of synaptic function to stages irreversibly associated with degeneration might be related to a disturbance of structural brain self-organization and involves morpho-regulatory molecules such as the amyloid precursor protein. Further, recent evidence suggests a role for diffusible oligomers of amyloid beta in synaptic dysfunction. To form synaptic connections and to continuously re-shape them in a process of ongoing structural adaptation, neurons must permanently withdraw from the cell cycle. Previously, we formulated the hypothesis that differentiated neurons after having withdrawn from the cell cycle are able to use molecular mechanisms primarily developed to control proliferation alternatively to control synaptic plasticity. The existence of these alternative effector pathways within neurons might put them at risk of erroneously converting signals derived from plastic synaptic changes into the program of cell cycle activation, which subsequently leads to cell death. The molecular mechanisms involved in cell cycle activation might, thus, link aberrant synaptic changes to cell death.

Amyloid precursor protein secretases as therapeutic targets for traumatic brain injury

Amyloid-beta (Abeta) peptides, found in Alzheimer's disease brain, accumulate rapidly after traumatic brain injury (TBI) in both humans and animals. Here we show that blocking either beta- or gamma-secretase, enzymes required for production of Abeta from amyloid precursor protein (APP), can ameliorate motor and cognitive deficits and reduce cell loss after experimental TBI in mice. Thus, APP secretases are promising targets for treatment of TBI.

Cytosolic prostaglandin E synthase: expression patterns in control and Alzheimer's disease brains

Anti-inflammatory drugs reduce the risk of Alzheimer's disease but fail to slow its progression. Studying the expression of prostaglandin E(2) synthases downstream of cyclooxygenase-2 is important. Here, the expression patterns of cytosolic prostaglandin E( 2) synthases, an immediate prostaglandin E(2) source was investigated. Sections taken from the middle frontal gyrus of brains of 10 patients with Alzheimer's and 5 age-matched controls were examined by immunostaining for the presence of the synthases. Immunofluorescence analysis of control brains showed that cytosolic prostaglandin E(2) synthases co-localize with microglia, neurons, and endothelium markers, but not with astrocytes or smooth muscle cells. Immunohistochemical staining for the synthases was positive in the pyramidal neurons of controls but barely detectable in the brain of Alzheimer's patients. These findings revealed that cytosolic prostaglandin E(2) synthases is found in microglia, neurons, and endothelium of control human middle frontal gyrus and that its levels decrease in pyramidal cells of Alzheimer's disease brains.

Effects of TNFalpha-converting enzyme inhibition on amyloid beta production and APP processing in vitro and in vivo

Tumor necrosis factor-alpha (TNFalpha) is a proinflammatory cytokine that is elevated in Alzheimer's disease (AD) brains. Because TNFalpha is released from cell membranes by the TNFalpha-converting enzyme (TACE), inhibition of TACE has the potential to mitigate TNFalpha effects in AD brain. TACE also cleaves amyloid precursor protein (APP) and generates sAPPalpha, precluding the formation of potentially harmful amyloid beta (Abeta) peptides by beta-site APP cleaving enzymes (BACE). Hence, the anti-inflammatory benefits of TACE inhibition might be offset by an increase in Abeta. We have examined the effects of the highly selective TACE inhibitor, BMS-561392, on APP processing in vitro and in vivo. In Chinese hamster ovary cells expressing APP, BMS-561392 significantly reduced secretion of sAPPalpha without a corresponding increase in Abeta production. Conversely, a BACE inhibitor decreased sAPPbeta and Abeta peptides with no change in the secretion of sAPPalpha. These data indicate an absence of TACE and BACE competition for the APP substrate. Despite this, we observed competition for APP when TACE activity was enhanced via phorbol ester treatment or if APP was modified such that it was retained within the trans-Golgi network (TGN). These results suggest that BACE and TACE share a common TGN localization, but under normal conditions do not compete for APP. To confirm this finding in vivo, BMS-561392 was infused into the brains of Tg2576 and wild-type mice. Although decreased brain sAPPalpha levels were observed, steady-state Abeta levels were not significantly changed. Accordingly, it is possible that TACE inhibitors could reduce TNFalpha levels without increasing Abeta levels within the AD brain.

Improvement of cerebral function by anti-amyloid precursor protein antibody infusion after traumatic brain injury in rats

We previously demonstrated the increased amyloid precursor protein (APP) immunoreactivity around the site of damage after traumatic brain injury (TBI). However, the function of APP after TBI has not been evaluated. In this study, we investigated the effects of direct infusion of an anti-APP antibody into the damaged brain region on cerebral function and morphological changes following TBI in rats. Three days after TBI, there were many TUNEL-positive neurons and astrocytes around the damaged region and a significantly greater number of TUNEL-positive cells in the PBS group compared with the anti-APP group found. Seven days after TBI, there were significantly a greater number of large glial fibrillary acidic protein-positive cells, long elongated projections, and microtubule-associated protein-2-positive cells around the damaged region in the anti-APP group compared with the PBS group found. Seven days after TBI, the region of brain damage was significantly smaller and the time to arrival at a platform was significantly shorter in the anti-APP group compared with the PBS group. Furthermore, after TBI in the anti-APP group, the time to arrival at the platform recovered to that observed in uninjured sham operation group rats. These data suggest that the overproduction of APP after TBI inhibits astrocyte activity and reduces neural cell survival around the damaged brain region, which speculatively may be related to the induction of Alzheimer disease-type dementia after TBI.

Relationships between expression of apolipoprotein E and beta-amyloid precursor protein are altered in proximity to Alzheimer beta-amyloid plaques: potential explanations from cell culture studies

Theories regarding the initiation and progression of Alzheimer disease (AD) often consider potential roles played by elevations of beta-amyloid precursor protein (betaAPP). Because it is the source of amyloid beta-peptide, betaAPP may simply contribute more pathogenic stimulus when elevated; some analyses have, however, reported a decline in betaAPP in AD. We found a progressive increase in neuronal betaAPP expression with increasing age in the brains of nondemented individuals, whereas in AD patient samples, betaAPP antigenicity decreased in neuronal somata in a manner that correlated with accumulation of mature amyloid beta-peptide plaques. In contrast, apolipoprotein E (ApoE) expression correlated with accumulation of plaques, and even greater amounts of ApoE were detected in plaques. Induction of betaAPP by glutamate in neuronal cell cultures was found to depend upon ApoE levels or activity. Thus, elevations in expression of ApoE and betaAPP by cellular stresses are likely normally linked in vivo, and uncoupling of this link, or other pathologic events in AD initiation, may leave neurons with diminished betaAPP expression, which might in turn reduce their resistance to stressors.

Emerging amyloid beta (Ab) peptide modulators for the treatment of Alzheimer's disease (AD)

BACKGROUND

According to the 'amyloid cascade hypothesis' of Alzheimer's disease (AD), abnormal processing of beta-amyloid precursor protein (betaAPP) into toxic amyloid beta (Abeta)-peptides is central to the etiopathology of this uniquely human brain disorder.

OBJECTIVE

To review current AD drugs, pharmacological approaches and strategies aimed at modulating Abeta-peptide generation and/or aggregation in the treatment of AD.

METHODS

Data searches at various websites: Alzheimer Research Forum; individual drug company databases; Medline; Pharmaprojects database; unpublished research; inter-University research communications.

RESULTS/CONCLUSION

Considerable research effort has focused on secretase-mediated mechanisms of betaAPP processing, and the latest pharmacological strategies have used selective Abeta-peptide-lowering agents (SALA) to provide therapeutic benefit against Abeta-initiated neurodegenerative pathology. Currently, dedicated anticholinesterase, glutamatergic agonist and Abeta-peptide immunization have had little impact in the clinical treatment of AD. One unexpected benefit of statins (HMG-CoA inhibitors), besides their cholesterol lowering abilities, has been their ancillary effects in potentiating the enzymatic mechanisms that generate Abeta-peptides. The long-term benefits or complications of statin-based therapies for use in the clinical management of AD are not known.

The effect of stress on the expression of the amyloid precursor protein in rat brain

The abnormal processing of the amyloid precursor protein (APP) is a pivotal event in the development of the unique pathology that defines Alzheimer's disease (AD). Stress, and the associated increase in corticosteroids, appear to accelerate brain ageing and may increase vulnerability to Alzheimer's disease via altered APP processing. In this study, rats were repeatedly exposed to an unavoidable stressor, an open elevated platform. Previous studies in this laboratory have shown that a single exposure produces a marked increase in plasma corticosterone levels but animals develop tolerance to this effect between 10 and 20 daily sessions. Twenty-four hours after stress, there was an increase in the ratio of the deglycosylated form of APP in the particulate fraction of the brain, which subsequently habituated after 20 days. The levels of soluble APP (APPs) tended to be lower in the stress groups compared to controls except for a significant increase in the hippocampus after 20 days of platform exposure. Since APPs is reported to have neurotrophic properties, this increased release may represent a neuroprotective response to repeated stress. It is possible that the ability to mount this response decreases with age thus increasing the vulnerability to stress-induced AD-related pathology.

The novel apolipoprotein E-based peptide COG1410 improves sensorimotor performance and reduces injury magnitude following cortical contusion injury

It has previously been shown that small peptide molecules derived from the apolipoprotein E (ApoE) receptor binding region are anti-inflammatory in nature and can improve outcome following head injury. The present study evaluated the preclinical efficacy of COG1410, a small molecule ApoE-mimetic peptide (1410 daltons), following cortical contusion injury (CCI). Animals were prepared with a unilateral CCI of the sensorimotor cortex (SMC) or sham procedure. Thirty mins post-CCI the animals received i.v. infusions of 0.8 mg/kg COG1410, 0.4 mg/kg COG1410, or vehicle. Starting on day 2, the animals were tested on a battery of behavioral measures to assess sensorimotor (vibrissae-forelimb placing and forelimb use-asymmetry), and motor (tapered balance beam) performance. Administration of the 0.8 mg/kg dose of COG1410 significantly improved recovery on the vibrissae-forelimb and limb asymmetry tests. However, no facilitation was observed on the tapered beam. The low dose (0.4 mg/kg) of COG1410 did not show any significant differences compared to vehicle. Lesion analysis revealed that the 0.8 mg/kg dose of COG1410 significantly reduced the size of the injury cavity compared to the 0.4 mg/kg dose and vehicle. The 0.8 mg/kg dose also reduced the number of glial fibrillary acid protein (GFAP+) reactive cells in the injured cortex. These results suggest that a single dose of COG1410 facilitates behavioral recovery and provides neuroprotection in a dose and task-dependent manner. Thus, the continued clinical development of ApoE based therapeutics is warranted and could represent a novel strategy for the treatment of traumatic brain injuries.

Extent of nerve cell injury in Marmarou's model compared to other brain trauma models

OBJECTIVES

We sought to determine the extent of nerve cell injury in the Marmarou's acceleration impact model of diffuse brain injury.

METHODS

Sensitive markers for cell injury including immunostaining for beta-amyloid precursor protein (beta-APP, a marker for diffuse axonal injury, DAI), Fluoro-Jade (FJ) histochemistry and electron microscopy (EM) were used in sham-operated and traumatized brains.

RESULTS

APP immunostaining confirmed and extended previous findings of DAI in association and subcortical fiber systems in the white matter after injury. Increasing FJ labeling of neurons in layers II-III of sensorimotor cortex (smCx) from 4 to 48 hours after trauma and scattered labeled cells were found in the lower cortical layers. EM confirmed the presence of dystrophic pyramidal neurons in layers II-III of smCx 24 and 48 hours post-trauma.

DISCUSSION

Taken together, the data revealed significant nerve cell injury without apparent cell death in this model.

Neuronal apoptosis in neurodegeneration

Apoptosis mediates the precise and programmed natural death of neurons and is a physiologically important process in neurogenesis during maturation of the central nervous system. However, premature apoptosis and/or an aberration in apoptosis regulation is implicated in the pathogenesis of neurodegeneration, a multifaceted process that leads to various chronic disease states, such as Alzheimer's (AD), Parkinson's (PD), Huntington's (HD) diseases, amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), and diabetic encephalopathy. The current review focuses on two major areas (a) the fundamentals of apoptosis, which includes elements of the apoptotic machinery, apoptosis inducers, and emerging concepts in apoptosis research, and (b) apoptotic involvement in neurodegenerative disorders, neuroprotective treatment strategies/modalities, and the mechanisms of, and signaling in, neuronal apoptosis. Current and new experimental models for apoptosis research in neurodegenerative diseases are also discussed.

Mechanisms of copper ion mediated Huntington's disease progression

Huntington's disease (HD) is caused by a dominant polyglutamine expansion within the N-terminus of huntingtin protein and results in oxidative stress, energetic insufficiency and striatal degeneration. Copper and iron are increased in the striata of HD patients, but the role of these metals in HD pathogenesis is unknown. We found, using inductively-coupled-plasma mass spectroscopy, that elevations of copper and iron found in human HD brain are reiterated in the brains of affected HD transgenic mice. Increased brain copper correlated with decreased levels of the copper export protein, amyloid precursor protein. We hypothesized that increased amounts of copper bound to low affinity sites could contribute to pro-oxidant activities and neurodegeneration. We focused on two proteins: huntingtin, because of its centrality to HD, and lactate dehydrogenase (LDH), because of its documented sensitivity to copper, necessity for normoxic brain energy metabolism and evidence for altered lactate metabolism in HD brain. The first 171 amino acids of wild-type huntingtin, and its glutamine expanded mutant form, interacted with copper, but not iron. N171 reduced Cu²⁺in vitro in a 1∶1 copper∶protein stoichiometry indicating that this fragment is very redox active. Further, copper promoted and metal chelation inhibited aggregation of cell-free huntingtin. We found decreased LDH activity, but not protein, and increased lactate levels in HD transgenic mouse brain. The LDH inhibitor oxamate resulted in neurodegeneration when delivered intra-striatially to healthy mice, indicating that LDH inhibition is relevant to neurodegeneration in HD. Our findings support a role of pro-oxidant copper-protein interactions in HD progression and offer a novel target for pharmacotherapeutics.

Survival signalling in Alzheimer's disease

Significant advancements in our understanding of cell-survival signalling in AD (Alzheimer's disease) stem from recent investigations into the metabolism, trafficking and fate of the essential omega-3 fatty acid DHA (docosahexaenoic acid) (C(22:6), n=3). Brain synaptic terminals and neuronal plasma membranes are highly enriched in DHA, and deficiencies in this polyunsaturated fatty acid are characteristic of AD-affected brain. Oxidative stress, targeting phospholipids containing DHA, and age-related DHA depletion are associated with the progressive erosion of normal cognitive function in AD. Current studies support the idea that DHA itself and novel DHA-derived neural synapse- and membrane-derived lipid messengers have considerable potential to modulate cell survival signalling in stressed cultured neural cell models in vitro and in mammalian models of learning, memory and AD in vivo. Key players in this intrinsic rescue system include the alpha-secretase-processed neurotrophin sAPPalpha [soluble APPalpha (amyloid precursor protein alpha)] peptide, the DHA-derived 10,17S-docosatriene NPD1 (neuroprotectin D1), a tandem brain cytosolic phospholipase A(2) and 15-lipoxygenase enzymatic system that biosynthesizes NPD1, and a small family of anti-apoptotic neuroprotective genes that encode Bcl-2, Bcl-X(L) and Bfl-1 (A1). This paper reviews current ideas regarding DHA and the oxygenated DHA derivative NPD1, intrinsically triggered biolipid neuroprotectants that along with their associated rescue pathways, contribute to life-or-death decisions of brain cells during homoeostasis, aging and neurodegenerative disease.

New insights into the roles of metalloproteinases in neurodegeneration and neuroprotection

Proteolytic enzymes constitute around 2% of the human genome and are involved in many stages of cell development from fertilization to death (apoptosis). The identification of many novel proteases from genome-sequencing programs has suggested them as potential new therapeutic targets. In addition, several well-characterized metallopeptidases were recently shown to possess new biological roles in neuroinflammation and neurodegeneration. As a result of these studies, metabolism of the neurotoxic and inflammatory amyloid peptide (Abeta) is considered as a physiologically relevant process with several metallopeptidases being suggested for the role of amyloid-degrading enzymes. These include the neprilysin (NEP) family of metalloproteinases (including its homologue endothelin-converting enzyme), insulin-degrading enzyme, angiotensin-converting enzyme, plasmin, and, possibly, some other enzymes. NEP also has a role in metabolism of sensory and inflammatory neuropeptides such as tachykinins and neurokinins. The existence of natural enzymatic mechanisms for removal of amyloid peptides has extended the therapeutic avenues in Alzheimer's disease (AD) and neurodegeneration. The proteolytic events underlying AD are highly compartmentalized in the cell and formation of amyloid peptide from its precursor molecule APP (amyloid precursor protein) takes place both within intracellular compartments and in the plasma membrane, especially in lipid raft domains. Degradation of amyloid peptide by metallopeptidases can also be both intra- and extracellular depending on the activity of membrane-bound enzymes and their soluble partners. Soluble forms of proteases can be secreted or released from the cell surface through the activity of "sheddases"-another group of proteolytic enzymes involved in key cellular regulatory functions. The activity of proteases involved in amyloid metabolism depends on numerous factors (e.g., genetic, environmental, age), and some conditions (e.g., hypoxia and ischemia) shift the balance of amyloid metabolism toward accumulation of higher concentrations of Abeta. In this regard, regulation of the activity of amyloid-degrading enzymes should be considered as a viable strategy in neuroprotection.

Kinetics of Inhibition of β-Amyloid Aggregation by Transthyretin†

Deposition of beta-amyloid (Abeta) fibrils is an early event in the neurodegenerative processes associated with Alzheimer's disease. According to the "amyloid cascade" hypothesis, Abeta aggregation, and its subsequent deposition as fibrils, is the underlying cause of disease. Abeta is a proteolytic product of amyloid precursor protein (APP); several mutations in APP have been identified that are associated with early onset of disease. Transgenic mice overexpressing APP with the Swedish mutation develop numerous plaques but, surprisingly, lack the neurofibrillary tangles and neuronal loss characteristic of Alzheimer's disease, in apparent contradiction of the amyloid cascade hypothesis. However, recent studies suggest that coproduction of sAPPalpha, an alternative proteolytic product of APP, increases synthesis of transthyretin that, in turn, interacts directly with Abeta to inhibit its toxicity. Here we report results from biophysical analysis of Abeta aggregation kinetics in the presence of transthryetin. At substoichiometric ratios, transthyretin drastically decreased the rate of aggregation without affecting the fraction of Abeta in the aggregate pool. Detailed analysis of the data using a mathematical model demonstrated that the decrease in aggregation rate was due to both a decrease in the rate of elongation relative to the rate of initiation of filaments and a decrease in lateral association of filaments to fibrils. Tryptophan quenching data indicated that transthyretin binds weakly to Abeta, with an estimated apparent KS of 2300 M-1. Taken together, the data support a hypothesis wherein transthyretin preferentially binds to aggregated rather than monomeric Abeta and arrests further growth of the aggregates.