An analysis of the effects of Alzheimer's plaques on living neurons

Exogenous human beta amyloid peptide interferes osteogenesis through Sox9a in embryonic zebrafish

The two major hallmarks of Alzheimer's disease (AD) are beta-amyloid plaques and neurofibrillary tangles. Amyloid peptide aggregations in the brain cause loss of synaptic connections and subsequent neurotoxicity leading to neurodegeneration and memory deficits. However, the physiological effects of beta-amyloid on early embryonic development still remain unclear. Administration of human beta-amyloid peptide (1-42) through cerebrospinal ventricular injection was carried out at 24 hpf (hours post fertilization) and it was uptaken into the cellular layers of the early ventricular development without any plaque aggregation. Whole-mount Immunostaining of zebrafish embryos injected with the beta-amyloid at 60 hpf revealed the delay in Sox9a expression. Decreased level of cartilage to bone transformation rate in 15 dpf (days post fertilization) zebrafish was observed by differential staining. These results suggest the possible existence of a genetic relationship between extrinsic amyloid peptide and Sox9a expression. Thus, our results demonstrated that the human beta-amyloid influences bone development through Sox9a expression during osteogenesis in zebrafish.

3D compartmented model to study the neurite-related toxicity of Aβ aggregates included in collagen gels of adaptable porosity

Insoluble deposits of β-amyloid (Aβ) are associated to neurodegenerative pathologies, in particular Alzheimer's Disease (AD). The toxicity of synthetic amyloid-like peptides has been largely demonstrated and shown to depend upon their aggregation state. However, standard 2D cell culture conditions are not well suited to study the role of the close vicinity of Aβ aggregates and growing neurites in the degenerative process. Here, we have designed a compartmented set-up where model neural cells are differentiated on the surface of Aβ-containing collagen matrices. The average pore size can be modulated, from below 0.2μm to more than 0.5μm by simple treatment with collagenase, to respectively hamper or permit neurite outgrowth towards the depth of the matrix. Dense Aβ aggregates (Congo red and ThT-positive) were obtained inside the collagen matrix with a homogeneous distribution and dimensions similar to those observed in post-mortem brain slices from Alzheimer's patients. The aggregates are not toxic to cells when the pore size is small, in spite of relatively high concentrations of 0.05-0.62mg of peptide per gram of collagen (equivalent to 11.3-113μM). In contrast, on Aβ-containing matrices with large pores, massive neural death is observed when the cells are seeded in the same conditions. It is the first time to our knowledge that Aβ aggregates with a typical morphology of dense plaques are obtained within a porous biomimetic matrix, and are shown to be toxic only when accessible to differentiating cells.

STATEMENT OF SIGNIFICANCE

Insoluble deposits of β-amyloid (Aβ) are associated to neurodegenerative pathologies, in particular Alzheimer's Disease (AD). In this study, we have formed Aβ aggregates directly inside a biomimetic collagen matrix loaded with growth factors to induce the differentiation of PC12 or SH-SY6Y cells. For the first time, we show that when the contact between cells and Aβ aggregates is allowed by opening up the matrix porosity, the close vicinity with aggregates induces neurite dystrophy. The compartmented 3D culture model developed and used in this study is a valuable tool to study the cytotoxicity of preformed dense Aβ aggregates and proves that contact between the aggregates and neurons is required to induce neurodegenerative processes.

Amyloid-β in Alzheimer Disease: The Null versus the Alternate Hypotheses

For nearly 20 years, the primary focus for researchers studying Alzheimer disease has been centered on amyloid-beta, such that the amyloid cascade hypothesis has become the "null hypothesis." Indeed, amyloid-beta is, by the current definition of the disease, an obligate player in pathophysiology, is toxic to neurons in vitro, and, perhaps most compelling, is increased by all of the human genetic influences on the disease. Therefore, targeting amyloid-beta is the focus of considerable basic and therapeutic interest. However, an increasingly vocal group of investigators are arriving at an "alternate hypothesis" stating that amyloid-beta, while certainly involved in the disease, is not an initiating event but rather is secondary to other pathogenic events. Furthermore and perhaps most contrary to current thinking, the alternate hypothesis proposes that the role of amyloid-beta is not as a harbinger of death but rather a protective response to neuronal insult. To determine which hypothesis relates best to Alzheimer disease requires a broader view of disease pathogenesis and is discussed herein.

Alzheimer' s disease, oxidative stress and gammahydroxybutyrate

Although the cause of Alzheimer's disease is unknown, oxidative stress, energy depletion, excitotoxicity and vascular endothelial pathology are all considered to play a part in its pathogenesis. In reaction to these adverse events, the Alzheimer brain appears to deploy a highly conserved biological response to tissue stress. Oxidative metabolism is turned down, the expression of antioxidative enzymes is increased and intermediary metabolism is shifted in the direction of the pentose phosphate shunt to promote reductive detoxification, repair and biosynthesis. Gathering evidence suggests that the release of beta-amyloid and the formation of neurofibrillary tangles, the two hallmarks of Alzheimer's disease, are components of this protective response. Gammahydroxybutyrate (GHB), an endogenous short chain fatty acid, may be able to buttress this response. GHB can reduce glucose utilization, shift intermediary metabolism in the direction the pentose phosphate shunt and generate NADPH, a key cofactor in the activity of many antioxidative and reductive enzymes. GHB has been shown to spare cerebral energy utilization, block excitotoxicity and maintain vascular integrity in the face of impaired perfusion. Most important, GHB has repeatedly been shown to prevent the tissue damaging effects of oxidative stress. It may therefore be possible to utilize GHB to strengthen the brain's innate defences against the pathological processes operating in the Alzheimer brain and, in this way, stem the advance of Alzheimer's disease.

The state versus amyloid-beta: the trial of the most wanted criminal in Alzheimer disease

Investigators studying the primary culprit responsible for Alzheimer disease have, for the past two decades, primarily focused on amyloid-beta (Abeta). Here, we put Abeta on trial and review evidence amassed by the prosecution that implicate Abeta and also consider arguments and evidence gathered by the defense team who are convinced of the innocence of their client. As in all trials, the arguments provided by the prosecution and defense revolve around the same evidence, with opposing interpretations. Below, we present a brief synopsis of the trial for you, the jury, to decide the verdict. Amyloid-beta: guilty or not-guilty?

Beta-amyloid plaques: stages in life history or independent origin?

Several types of discrete beta-amyloid (Abeta) deposit or senile plaque have been identified in the brains of individuals with Alzheimer's disease and Down's syndrome. The majority of these plaques can be classified into four morphological types: diffuse, primitive, classic and compact. Two hypotheses have been proposed to account for these plaques. Firstly, that the diffuse, primitive, classic and compact plaques develop in sequence and represent stages in the life history of a single plaque type. Secondly, that the different Abeta plaques develop independently and therefore, unique factors are involved in the formation of each type. To attempt to distinguish between these hypotheses, the morphology, ultrastructure, composition, and spatial distribution in the brain of the four types of plaque were compared. Although some primitive plaques may develop from diffuse plaques, the evidence suggests that a unique combination of factors is involved in the pathogenesis of each plaque type and, therefore, supports the hypothesis that the major types of Abeta plaque develop independently.

Sympathetic neurite outgrowth is greater on plaque-poor vs. plaque-rich regions of Alzheimer's disease cryostat sections

Senile plaques are a characteristic histopathological feature of Alzheimer's disease (AD) and are associated with altered neuritic morphology. Numerous individual plaque components, most notably beta-amyloid, have been studied for their possible effects on neurite outgrowth in culture. However, the effect of senile plaques on neuronal morphology and function is difficult to assess. In the present study, the effect of senile plaques on neurite outgrowth was studied by culturing embryonic chick sympathetic neuronal explants on Alzheimer's tissue sections. Explants were cultured for 3 days on amygdala tissue sections from AD as well as non-AD patients in serum-free medium. Neurite outgrowth on plaque-rich regions was compared with outgrowth on plaque-poor regions of the same tissue section, and with outgrowth on non-AD tissue, through colocalization of the living explants and the underlying plaques. Explants growing on plaque-rich regions showed significantly less neurite outgrowth compared with those on plaque-poor regions in the same section or on control brain tissue. These results suggest that plaques are poor substrates for neurite outgrowth as compared with non-plaque areas of the same tissue sections, and support the hypothesis that components of the senile plaques may inhibit neurite outgrowth.

Inhibitory neuronal activity can compensate for adverse effects of beta-amyloid in hippocampal neurons

One of the most prominent effects of Alzheimer disease is the disruption of finely tuned neuronal circuitry of discrete brain regions associated with learning and memory. Results from the present study support a role for the intrinsic inhibitory component of neuronal circuitry in determining the magnitude of beta-amyloid peptide induced cell death in the highly vulnerable pyramidal neurons of the hippocampus. Previous efforts have mostly focused on direct effects on excitatory neurons. By contrast, less emphasis has been placed on addressing a role for the intrinsic inhibitory component of cell-cell interactions of neuronal networks in response to Abeta. The present study provides evidence demonstrating that blockage of the intrinsic inhibitory component between Abeta exposed neurons leads to destabilization of calcium homeostasis and exacerbated neuronal death compared to Abeta treated cultures. Neuronal electrical activity was first silenced by exposing cultures to tetrodotoxin (TTX; 100 nM) plus Abeta, followed by survival counts. Cell death, unexpectedly, did not significantly differ from Abeta-exposed neurons. The intrinsic inhibition in Abeta-exposed cultures was then pharmacologically removed with picrotoxin (40 microM) or bicuculline (25 microM) resulting in significantly greater death than Abeta-exposed neurons alone. From these observations, it is proposed that intrinsic functional inhibition in hippocampal circuits can reduce adverse effects of Abeta on the excitatory component. By considering not just the excitatory component of electrical activity, but the intrinsic balance between excitation and inhibition, new strategies for the treatment of Alzheimer disease may emerge.

Rat microglia exhibit increased density on Alzheimer's plaques in vitro

The relationship of microglia to senile plaques was investigated by culturing glial cells derived from neonatal rat brain on cryostat sections of Alzheimer's disease (AD) or control brain. Rat microglia were identified by their uptake of DiI-acetylated LDL. Plaques were colocalized using Thioflavin-S staining. Although the number of microglia attached to AD tissue sections did not differ significantly from the number on control brain tissue, the density of microglia on senile plaques was significantly greater than on nonsenile plaque areas of the same sections. These results suggest that microglia may have a higher affinity for senile plaques than for nonsenile plaque regions of AD brain tissue and are consistent with the hypothesis that microglia respond to plaques.

Alzheimer's disease: a re-examination of the amyloid hypothesis

Alzheimer's disease (AD) is a neurodegenerative disorder of the brain characterized by the presence of neuritic amyloid plaques and neurofibrillary tangles. Although it most frequently occurs in the elderly, this disorder also afflicts younger patients. The majority of AD cases are late in onset, lack an obvious genetic etiology and are characterized as sporadic, whereas a small percentage of cases are early in onset and segregate strongly within families (FAD), suggesting a genetic etiology. During the past decade it has become evident that the clinical and histopathological phenotypes of this disease are caused by heterogeneous genetic, and probably environmental, factors. Indeed, several genes have been identified that together appear to cause most of the familial forms of the disease, whereas the epsilon4 allele of the apolipoprotein E (apoE) gene has been shown to be a significant risk factor for the late onset forms of AD. Despite this evidence of heterogeneity, it has been suggested that all of these factors work through a common pathway by triggering the deposition of amyloid in the brain, which is ultimately responsible for the neuronal degeneration of AD. This is a controversial theory, however, primarily because there is a poor correlation between the concentrations and distribution of amyloid depositions in the brain and several parameters of AD pathology, including degree of dementia, loss of synapses, loss of neurons and abnormalities of the cytoskeleton.

A potent inhibitor of neurite outgrowth that predominates in the extracellular matrix of reactive astrocytes

In a model of astrogliosis in vitro, cultured cortical astrocytes were triggered into a functionally reactive state by an immobilized fragment of the beta-amyloid peptide. Induced astrocytes produced an extracellular matrix that inhibited the outgrowth of embryonic CNS axons. Within the extracellular matrix deposited by reactive astrocytes, we found an overall increase in the deposition of chondroitin sulphate that accounted for the inhibition. Specifically, we have detected an increased biosynthesis of a small chondroitin/heparan sulphate proteoglycan that is a potent inhibitor of axon outgrowth. We further suggest that this proteoglycan, or related molecules yet to be discovered, may play a role in gliosis-mediated regenerative failure of CNS axons.

Nerve growth factor and Alzheimer's disease

Nerve growth factor (NGF) is a well-characterized protein that exerts pharmacological effects on a group of cholinergic neurons known to atrophy in Alzheimer's disease (AD). Considerable evidence from animal studies suggests that NGF may be useful in reversing, halting, or at least slowing the progression of AD-related cholinergic basal forebrain atrophy, perhaps even attenuating the cognitive deficit associated with the disorder. However, many questions remain concerning the role of NGF in AD. Levels of the low-affinity receptor for NGF appear to be at least stable in AD basal forebrain, and the recent finding of AD-related increases in cortical NGF brings into question whether endogenous NGF levels are related to the observed cholinergic atrophy and whether additional NGF will be useful in treating this disorder. Evidence regarding the localization of NGF within the central nervous system and its presumed role in maintaining basal forebrain cholinergic neurons is summarized, followed by a synopsis of the relevant aspects of AD neuropathology. The available data regarding levels of NGF and its receptor in the AD brain, as well as potential roles for NGF in the pathogenesis and treatment of AD, are also reviewed. NGF and its low affinity receptor are abundantly present within the AD brain, although this does not rule out an NGF-related mechanism in the degeneration of basal forebrain neurons, nor does it eliminate the possibility that exogenous NGF may be successfully used to treat AD. Further studies of the degree and distribution of NGF within the human brain in normal aging and in AD, and of the possible relationship between target NGF levels and the status of basal forebrain neurons in vivo, are necessary before engaging in clinical trials.

Alzheimer A beta amyloid forms an inhibitory neuronal substrate

Alzheimer's disease (AD) is identified by the accumulation of amyloid plaques, neurofibrillary degeneration, and the accompanying neuronal loss. AD amyloid assembles into compact fibrous deposits from the amyloid beta (A beta) protein, which is a proteolytic fragment of the membrane-associated amyloid precursor protein. To examine the effects of amyloid on neuron growth, a hybrid mouse motoneuron cell line (NSC34) exhibiting spontaneous process formation was exposed to artificial "plaques" created from aggregated synthetic A beta peptides. These correspond to full-length A beta residues 1-40 (A beta 1-40), an internal beta-sheet region comprising residues 11-28 (A beta 11-28), and a proposed toxic fragment comprising residues 25-35 (A beta 25-35). Fibers were immobilized onto culture dishes, and addition of cells to these in vitro plaques revealed that A beta was not a permissive substrate for cell adhesion. Neurites in close contact with these deposits displayed abnormal swelling and a tendency to avoid contact with the A beta fibers. In contrast, A beta did not affect the adhesion or growth of rat astrocytes, implicating a specific A beta-neuron relationship. The inhibitory effects were also unique to A beta as no response was observed to deposits of pancreatic islet amyloid polypeptide fibers. Considering the importance of cell adhesion in neurite elongation and axonal guidance, the antiadhesive properties of A beta amyloid plaques found in vivo may contribute to the neuronal loss responsible for the clinical manifestations of AD.

CNS white matter can be altered to support neuronal outgrowth

Previous work has demonstrated that white matter in the adult mammalian CNS inhibits cell adhesion and neurite outgrowth. This phenomenon has been investigated most recently by culturing neurons on cryostat sections of the adult CNS. Employing this same technique, we have found, in accord with others, that neurons seldom adhere to or grow on central nervous system white matter but will attach and grow on gray matter. In the experiments presented here, embryonic rat hippocampal neurons were grown on cryostat sections from the adult rat CNS, in the presence of brain derived glial cocultures. It was found that the white matter in cryostat sections can be modified by interaction with medium conditioned by brain-derived glial cells. Neurons plated on sections pretreated by such media show significant increases in both attachment and neurite outgrowth. The activity contained in glial conditioned medium is likely complex in nature. While the majority of the activity can be eliminated by heat treatment and trypsinization, neural adhesion but not neurite initiation is affected by protease treatment. Therefore, cell attachment and neurite outgrowth may be regulated by different factors in the conditioned media.