alpha1 Integrin activation: a link between beta-amyloid deposition and neuronal death in aging hippocampal neurons

Modulation of Alzheimer's Disease Aβ40 Fibril Polymorphism by the Small Heat Shock Protein αB-Crystallin

Deposition of amyloid plaques in the brains of Alzheimer's disease (AD) patients is a hallmark of the disease. AD plaques consist primarily of the beta-amyloid (Aβ) peptide but can contain other factors such as lipids, proteoglycans, and chaperones. So far, it is unclear how the cellular environment modulates fibril polymorphism and how differences in fibril structure affect cell viability. The small heat-shock protein (sHSP) alpha-B-Crystallin (αBC) is abundant in brains of AD patients, and colocalizes with Aβ amyloid plaques. Using solid-state NMR spectroscopy, we show that the Aβ40 fibril seed structure is not replicated in the presence of the sHSP. αBC prevents the generation of a compact fibril structure and leads to the formation of a new polymorph with a dynamic N-terminus. We find that the N-terminal fuzzy coat and the stability of the C-terminal residues in the Aβ40 fibril core affect the chemical and thermodynamic stability of the fibrils and influence their seeding capacity. We believe that our results yield a better understanding of how sHSP, such as αBC, that are part of the cellular environment, can affect fibril structures related to cell degeneration in amyloid diseases.

The role of integrins in brain health and neurodegenerative diseases

Integrins are heterodimeric membrane proteins expressed on the surface of most cells. They mediate adhesion and signaling processes relevant for a wealth of physiological processes, including nervous system development and function. Interestingly, integrins are also recognized therapeutic targets for inflammatory diseases, such as multiple sclerosis. Here, we discuss the role of integrins in brain development and function, as well as in neurodegenerative diseases affecting the brain (Alzheimer's disease, multiple sclerosis, stroke). Furthermore, we discuss therapeutic targeting of these adhesion receptors in inflammatory diseases of the brain.

Irisin reduces amyloid-β by inducing the release of neprilysin from astrocytes following downregulation of ERK-STAT3 signaling

A pathological hallmark of Alzheimer's disease (AD) is the deposition of amyloid-β (Aβ) protein in the brain. Physical exercise has been shown to reduce Aβ burden in various AD mouse models, but the underlying mechanisms have not been elucidated. Irisin, an exercise-induced hormone, is the secreted form of fibronectin type-III-domain-containing 5 (FNDC5). Here, using a three-dimensional (3D) cell culture model of AD, we show that irisin significantly reduces Aβ pathology by increasing astrocytic release of the Aβ-degrading enzyme neprilysin (NEP). This is mediated by downregulation of ERK-STAT3 signaling. Finally, we show that integrin αV/β5 acts as the irisin receptor on astrocytes required for irisin-induced release of astrocytic NEP, leading to clearance of Aβ. Our findings reveal for the first time a cellular and molecular mechanism by which exercise-induced irisin attenuates Aβ pathology, suggesting a new target pathway for therapies aimed at the prevention and treatment of AD.

Amyloid precursor protein (APP) and amyloid β (Aβ) interact with cell adhesion molecules: Implications in Alzheimer’s disease and normal physiology

Alzheimer’s disease (AD) is an incurable neurodegenerative disorder in which dysfunction and loss of synapses and neurons lead to cognitive impairment and death. Accumulation and aggregation of neurotoxic amyloid-β (Aβ) peptides generated via amyloidogenic processing of amyloid precursor protein (APP) is considered to play a central role in the disease etiology. APP interacts with cell adhesion molecules, which influence the normal physiological functions of APP, its amyloidogenic and non-amyloidogenic processing, and formation of Aβ aggregates. These cell surface glycoproteins also mediate attachment of Aβ to the neuronal cell surface and induce intracellular signaling contributing to Aβ toxicity. In this review, we discuss the current knowledge surrounding the interactions of cell adhesion molecules with APP and Aβ and analyze the evidence of the critical role these proteins play in regulating the processing and physiological function of APP as well as Aβ toxicity. This is a necessary piece of the complex AD puzzle, which we should understand in order to develop safe and effective therapeutic interventions for AD.

Using machine intelligence to uncover Alzheimer’s disease progression heterogeneity

Aim: Research suggests that Alzheimer’s disease (AD) is heterogeneous with numerous subtypes. Through a proprietary interactive ML system, several underlying biological mechanisms associated with AD pathology were uncovered. This paper is an introduction to emerging analytic efforts that can more precisely elucidate the heterogeneity of AD. Methods: A public AD data set (GSE84422) consisting of transcriptomic data of postmortem brain samples from healthy controls (n = 121) and AD (n = 380) subjects was analyzed. Data were processed by an artificial intelligence platform designed to discover potential drug repurposing candidates, followed by an interactive augmented intelligence program. Results: Using perspective analytics, six perspective classes were identified: Class I is defined by TUBB1, ASB4, and PDE5A; Class II by NRG2 and ZNF3; Class III by IGF1, ASB4, and GTSE1; Class IV is defined by cDNA FLJ39269, ITGA1, and CPM; Class V is defined by PDE5A, PSEN1, and NDUFS8; and Class VI is defined by DCAF17, cDNA FLJ75819, and SLC33A1. It is hypothesized that these classes represent biological mechanisms that may act alone or in any combination to manifest an Alzheimer’s pathology. Conclusions: Using a limited transcriptomic public database, six different classes that drive AD were uncovered, supporting the premise that AD is a heterogeneously complex disorder. The perspective classes highlighted genetic pathways associated with vasculogenesis, cellular signaling and differentiation, metabolic function, mitochondrial function, nitric oxide, and metal ion metabolism. The interplay among these genetic factors reveals a more profound underlying complexity of AD that may be responsible for the confluence of several biological factors. These results are not exhaustive; instead, they demonstrate that even within a relatively small study sample, next-generation machine intelligence can uncover multiple genetically driven subtypes. The models and the underlying hypotheses generated using novel analytic methods may translate into potential treatment pathways.

Modelling Alzheimer's disease: Insights from in vivo to in vitro three-dimensional culture platforms

Alzheimer's disease (AD) is the most common form of dementia and is characterized by progressive memory loss, impairment of other cognitive functions, and inability to perform activities of daily life. The key to understanding AD aetiology lies in the development of effective disease models, which should ideally recapitulate all aspects pertaining to the disease. A plethora of techniques including in vivo, in vitro, and in silico platforms have been utilized in developing disease models of AD over the years. Each of these approaches has revealed certain essential characteristics of AD; however, none have managed to fully mimic the pathological hallmarks observed in the AD human brain. In this review, we will provide details into the genesis, evolution, and significance of the principal methods currently employed in modelling AD, the advantages and limitations faced in their application, including the headways made by each approach. This review will focus primarily on two-dimensional and three-dimensional in vitro modelling of AD, which during the last few years has made significant breakthroughs in the areas of AD pathology and therapeutic screening. In addition, a glimpse into state-of-the-art neural tissue engineering techniques incorporating biomaterials and microfluidics technologies is provided, which could pave the way for the development of more accurate and comprehensive AD models in the future.

Synaptic Cell Adhesion Molecules in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative brain disorder associated with the loss of synapses between neurons in the brain. Synaptic cell adhesion molecules are cell surface glycoproteins which are expressed at the synaptic plasma membranes of neurons. These proteins play key roles in formation and maintenance of synapses and regulation of synaptic plasticity. Genetic studies and biochemical analysis of the human brain tissue, cerebrospinal fluid, and sera from AD patients indicate that levels and function of synaptic cell adhesion molecules are affected in AD. Synaptic cell adhesion molecules interact with Aβ, a peptide accumulating in AD brains, which affects their expression and synaptic localization. Synaptic cell adhesion molecules also regulate the production of Aβ via interaction with the key enzymes involved in Aβ formation. Aβ-dependent changes in synaptic adhesion affect the function and integrity of synapses suggesting that alterations in synaptic adhesion play key roles in the disruption of neuronal networks in AD.

3D in vitro modeling of the central nervous system

There are currently more than 600 diseases characterized as affecting the central nervous system (CNS) which inflict neural damage. Unfortunately, few of these conditions have effective treatments available. Although significant efforts have been put into developing new therapeutics, drugs which were promising in the developmental phase have high attrition rates in late stage clinical trials. These failures could be circumvented if current 2D in vitro and in vivo models were improved. 3D, tissue-engineered in vitro systems can address this need and enhance clinical translation through two approaches: (1) bottom-up, and (2) top-down (developmental/regenerative) strategies to reproduce the structure and function of human tissues. Critical challenges remain including biomaterials capable of matching the mechanical properties and extracellular matrix (ECM) composition of neural tissues, compartmentalized scaffolds that support heterogeneous tissue architectures reflective of brain organization and structure, and robust functional assays for in vitro tissue validation. The unique design parameters defined by the complex physiology of the CNS for construction and validation of 3D in vitro neural systems are reviewed here.

Neuronal degeneration, synaptic defects, and behavioral abnormalities in tau₄₅₋₂₃₀ transgenic mice

The complement of mechanisms underlying tau pathology in neurodegenerative disorders has yet to be elucidated. Among these mechanisms, abnormal tau phosphorylation has received the most attention because neurofibrillary tangles present in Alzheimer's disease (AD) and related disorders known as tauopathies are composed of hyperphosphorylated forms of this microtubule-associated protein. More recently, we showed that calpain-mediated cleavage leading to the generation of the 17kDa tau₄₅₋₂₃₀ fragment is a conserved mechanism in these diseases. To obtain insights into the role of this fragment in neurodegeneration, we generated transgenic mice that express tau₄₅₋₂₃₀ and characterized their phenotype. Our results showed a significant increase in cell death in the hippocampal pyramidal cell layer of transgenic tau₄₅₋₂₃₀ mice when compared to wild-type controls. In addition, significant synapse loss was detected as early as six months after birth in transgenic hippocampal neurons. These synaptic changes were accompanied by alterations in the expression of the N-methyl-d-aspartate glutamate (NMDA) receptor subunits. Furthermore, functional abnormalities were detected in the transgenic mice using Morris Water Maze and fear conditioning tests. These results suggest that the accumulation of tau₄₅₋₂₃₀ is responsible, at least in part, for neuronal degeneration and some behavioral changes in AD and other tauopathies. Collectively, these data provide the first direct evidence of the toxic effects of a tau fragment biologically produced in the context of these diseases in vertebrate neurons that develop in situ.

The dendritic hypothesis for Alzheimer's disease pathophysiology

Converging evidence indicates that processes occurring in and around neuronal dendrites are central to the pathogenesis of Alzheimer's disease. These data support the concept of a "dendritic hypothesis" of AD, closely related to the existing synaptic hypothesis. Here we detail dendritic neuropathology in the disease and examine how Aβ, tau, and AD genetic risk factors affect dendritic structure and function. Finally, we consider potential mechanisms by which these key drivers could affect dendritic integrity and disease progression. These dendritic mechanisms serve as a framework for therapeutic target identification and for efforts to develop disease-modifying therapeutics for Alzheimer's disease.

Pivotal role of the RanBP9-cofilin pathway in Aβ-induced apoptosis and neurodegeneration

Neurodegeneration associated with amyloid β (Aβ) peptide accumulation, synaptic loss, neuroinflammation, tauopathy, and memory impairments encompass the pathophysiological features of Alzheimer's disease (AD). We previously reported that the scaffolding protein RanBP9, which is overall increased in brains of AD patients, simultaneously promotes Aβ generation and focal adhesion disruption by accelerating the endocytosis of amyloid precursor protein (APP) and β1-integrin, respectively. Here, we show that RanBP9 protein levels are increased by fourfold in FAD mutant APP transgenic mice. Accordingly, RanBP9 transgenic mice demonstrate significantly increased synapse loss, neurodegeneration, gliosis, and spatial memory deficits. RanBP9 overexpression promotes apoptosis and potentiates Aβ-induced neurotoxicity independent of its capacity to promote Aβ generation. Conversely, RanBP9 reduction by siRNA or gene dosage mitigates Aβ-induced neurotoxicity. Importantly, RanBP9 activates/dephosphorylates cofilin, a key regulator of actin dynamics and mitochondria-mediated apoptosis, and siRNA knockdown of cofilin abolishes both Aβ and RanBP9-induced apoptosis. These findings implicate the RanBP9-cofilin pathway as critical therapeutic targets not only for stemming Aβ generation but also antagonizing Aβ-induced neurotoxicity.

Pivotal role of RanBP9 in integrin-dependent focal adhesion signaling and assembly

Accumulation of the amyloid β (Aβ) peptide derived from the amyloid precursor protein (APP) plays a central role in the pathogenesis of Alzheimer's disease (AD). We previously reported that the scaffolding protein RanBP9 is markedly increased in AD brains and promotes Aβ generation by scaffolding APP/BACE1/LRP complexes together and accelerating APP endocytosis. Because APP, LRP, and RanBP9 all physically interact with β-integrins, we investigated whether RanBP9 alters integrin-dependent cell adhesion and focal adhesion signaling. Here, we show that RanBP9 overexpression dramatically disrupts integrin-dependent cell attachment and spreading in NIH3T3 and hippocampus-derived HT22 cells, concomitant with strongly decreased Pyk2/paxillin signaling and talin/vinculin localization in focal adhesion complexes. Conversely, RanBP9 knockdown robustly promotes cell attachment, spreading, and focal adhesion signaling and assembly. Cell surface biotinylation and endocytosis assays reveal that RanBP9 overexpression and RanBP9 siRNA potently reduces and increases surface β1-integrin and LRP by accelerating and inhibiting their endocytosis, respectively. Primary hippocampal neurons derived from RanBP9-transgenic mice also demonstrate severely reduced levels of surface β1-integrin, LRP, and APP, as well as neurite arborization. Therefore, these data indicate that RanBP9 simultaneously inhibits cell-adhesive processes and enhances Aβ generation by accelerating APP, LRP, and β1-integrin endocytosis.

Non-RGD-containing snake venom disintegrins, functional and structural relations

Snake venom disintegrins are present in a variety of species and are functionally divided into three families: RGD, MLD and R/KTS. The RGD family of disintegrins, which bind and inhibit the physiological functions of RGD-dependent integrins, constitute the largest and most investigated family. This review will be focused on characterization of two relatively new families of snake venom disintegrins, expressing in their active site MLD and R/KTS motifs. The MLD motif, present only in heterodimeric disintegrins, mediates binding of these disintegrins to α4β1, α4β7 and α9β1 integrins, whereas the presence of a KTS or RTS sequence in the active site selectively directs activity of disintegrins to the collagen receptor α1β1 integrin. Structurally, KTS-disintegrins are short, monomeric molecules containing 41 amino acids in its polypeptide chain. Biological activities of MLD and KTS-disintegrins were investigated in many systems in vitro and in vivo. Purified disintegrins are non-toxic in therapeutic doses in rodent and avian models. Their modulatory properties were observed in investigations of cancer angiogenesis and metastasis, immunosuppression of IDDM (insulin-dependent diabetes mellitus) and asthma, as well as in neurodegenerative assays and cell apoptosis.

The Interface between Cytoskeletal Aberrations and Mitochondrial Dysfunction in Alzheimer's Disease and Related Disorders

The major defining pathological hallmarks of Alzheimer's disease (AD) are the accumulations of Aβ in senile plaques and hyperphosphorylated tau in neurofibrillary tangles and neuropil threads. Recent studies indicate that rather than these insoluble lesions, the soluble Aβ oligomers and hyperphosphorylated tau are the toxic agents of AD pathology. Such pathological protein species are accompanied by cytoskeletal changes, mitochondrial dysfunction, Ca²⁺ dysregulation, and oxidative stress. In this review, we discuss how the binding of Aβ to various integrins, defects in downstream focal adhesion signaling, and activation of cofilin can impact mitochondrial dysfunction, cytoskeletal changes, and tau pathology induced by Aβ oligomers. Such pathological consequences can also feedback to further activate cofilin to promote cofilin pathology. We also suggest that the mechanism of Aβ generation by the endocytosis of APP is mechanistically linked with perturbations in integrin-based focal adhesion signaling, as APP, LRP, and β-integrins are physically associated with each other.

Aβ toxicity in primary cultured neurons

The aggregation of beta-amyloid (Aβ) into soluble oligomers is considered an early event in Alzheimer's disease. Furthermore, the presence of these aggregates seems to lead to neurodegeneration in the context of this disease. However, the mechanisms underlying Aβ-induced neurotoxicity are not completely understood. Primary cultures of pyramidal neurons have proven to be an excellent model system for the study of such mechanisms. These cultures provide a homogenous population of neurons that extend and differentiate axons and dendrites and that establish functional synapses among them. In addition, the neurotoxic effects of preaggregated Aβ can be easily analyzed both morphologically and biochemically. Here, we describe in detail the materials and methods used for the preparation and maintenance of primary cultures of hippocampal pyramidal neurons, as well as for the aggregation of and treatment with Aβ.

Examination of potential mechanisms of amyloid-induced defects in neuronal transport

Microtubule-based neuronal transport pathways are impaired during the progression of Alzheimer's disease and other neurodegenerative conditions. However, mechanisms leading to defects in transport remain to be determined. We quantified morphological changes in neuronal cells following treatment with fibrils and unaggregated peptides of beta-amyloid (Abeta). Abeta fibrils induce axonal and dendritic swellings indicative of impaired transport. In contrast, Abeta peptides induce a necrotic phenotype in both neurons and non-neuronal cells. We tested several popular hypotheses by which aggregated Abeta could disrupt transport. Using fluorescent polystyrene beads, we developed experimental models of physical blockage and localized release of reactive oxygen species (ROS) that reliably induce swellings. Like the beads, Abeta fibrils localize in close proximity to swellings; however, fibril internalization is not required for disrupting transport. ROS and membrane permeability are also unlikely to be responsible for fibril-mediated toxicity. Collectively, our results indicate that multiple initiating factors converge upon pathways of defective transport.

The protective effect of dietary eicosapentaenoic acid against impairment of spatial cognition learning ability in rats infused with amyloid beta(1-40)

BACKGROUND

Amyloid beta (Abeta) peptide (1-40) can cause cognitive impairment.

EXPERIMENTAL DESIGN

We investigated whether dietary preadministration of eicosapentaenoic acid (EPA) is conducive to cognition learning ability and whether it protects against the impairment of learning ability in rats infused with Abeta peptide (1-40) into the cerebral ventricle.

RESULTS

Dietary EPA administered to rats for 12 weeks before the infusion of Abeta into the rat brain significantly decreased the number of reference memory errors (RMEs) and working memory errors (WMEs), suggesting that chronic administration of EPA improves cognition learning ability in rats. EPA preadministered to the Abeta-infused rats significantly reduced the increase in the number of RMEs and WMEs, with concurrent proportional increases in the levels of corticohippocampal EPA and docosahexaenoic acid (DHA) and in the DHA/arachidonic acid molar ratio. Decrease in oxidative stress in these tissues was evaluated by determining the reactive oxygen species and lipid peroxide levels. cDNA microarray analysis revealed that altered genes included those that control synaptic signal transduction, cell communication, membrane-related vesicular transport functions, and enzymes and several other proteins.

CONCLUSION

The present study suggests that EPA, by acting as a precursor for DHA, ameliorates learning deficits associated with Alzheimer's disease and that these effects are modulated by the expression of proteins involved in neuronal plasticity.

Integrins mediate beta-amyloid-induced cell-cycle activation and neuronal death

Early intracellular events responsible for cell-cycle induction by beta-amyloid (A beta) in neurons have not been identified yet. Extracellular signal-regulated kinases 1/2 (ERK1/2) have been identified in this pathway, and inhibition of ERK activity prevents cell-cycle activation and reduces neuronal death induced by A beta. To identify upstream events responsible for ERK activation, attention has been focused on integrins. Treatment of SH-SY5Y cells, differentiated by long-term exposure to 10 microM retinoic acid with a neutralizing anti-alpha1-integrin antibody significantly reduced A beta-induced neuronal death. However, cell-cycle analysis showed that treatment with anti-alpha1-integrin per se produced changes in the distribution of cell populations, thus hampering any effect on A beta-induced cell-cycle activation. 4-Amino-5-(4-chlorophenyl)-7(t-butyl)pyrazol(3,4-D)pyramide, an inhibitor of src protein kinases that colocalizes with focal adhesion kinase (FAK) and is involved in integrin signaling, was effective in reducing activation of the cell cycle and preventing induction of neuronal death by A beta while inhibiting ERK1/2 phosphorylation. Similar results were obtained when FAK expression was down-regulated by siRNA silencing. The present study identifies a sequence of early events in the toxic effect of A beta in neuronal cultures that involves interaction with integrins, activation of FAK/src, enhanced phosphorylation of ERK1/2, and induction of the cell cycle, all leading to neuronal death.

Synaptic memory mechanisms: Alzheimer's disease amyloid beta-peptide-induced dysfunction

There is growing evidence that mild cognitive impairment in early AD (Alzheimer's disease) may be due to synaptic dysfunction caused by the accumulation of non-fibrillar, oligomeric Abeta (amyloid beta-peptide), long before widespread synaptic loss and neurodegeneration occurs. Soluble Abeta oligomers can rapidly disrupt synaptic memory mechanisms at extremely low concentrations via stress-activated kinases and oxidative/nitrosative stress mediators. Here, we summarize experiments that investigated whether certain putative receptors for Abeta, the alphav integrin extracellular cell matrix-binding protein and the cytokine TNFalpha (tumour necrosis factor alpha) type-1 death receptor mediate Abeta oligomer-induced inhibition of LTP (long-term potentiation). Ligands that neutralize TNFalpha or genetic knockout of TNF-R1s (type-1 TNFalpha receptors) prevented Abeta-triggered inhibition of LTP in hippocampal slices. Similarly, antibodies to alphav-containing integrins abrogated LTP block by Abeta. Protection against the synaptic plasticity-disruptive effects of soluble Abeta was also achieved using systemically administered small molecules targeting these mechanisms in vivo. Taken together, this research lends support to therapeutic trials of drugs antagonizing synaptic plasticity-disrupting actions of Abeta oligomers in preclinical AD.

Evidence of Abeta- and transgene-dependent defects in ERK-CREB signaling in Alzheimer's models

Extracellular-signal regulated kinase (ERK) signaling is critical for memory and tightly regulated by acute environmental stimuli. In Alzheimer disease transgenic models, active ERK is shown to first be increased, then later reduced, but whether these baseline changes reflect disruptions in ERK signaling is less clear. We investigated the influence of the familial Alzheimer's disease transgene APPsw and beta-amyloid peptide (Abeta) immunoneutralization on cannulation injury-associated (i.c.v. infusion) ERK activation. At both 12 and 22 months of age, the trauma-associated activation of ERK observed in Tg(-) mice was dramatically attenuated in Tg(+). In cortices of 22-month-old non-infused mice, a reduction in ERK activation was observed in Tg(+), relative to Tg(-) mice. Intracerebroventricular (i.c.v.) anti-Abeta infusion significantly increased phosphorylated ERK, its substrate cAMP-response element-binding protein (CREB) and a downstream target, the NMDA receptor subunit. We also demonstrated that Abeta oligomer decreased active ERK and subsequently active CREB in human neuroblastoma cells, which could be prevented by oligomer immunoneutralization. Abeta oligomers also inhibited active ERK and CREB in primary neurons, in addition to reducing the downstream post-synaptic protein NMDA receptor subunit. These effects were reversed by anti-oligomer. Our data strongly support the existence of an APPsw transgene-dependent and Abeta oligomer-mediated defect in regulation of ERK activation.

Expression patterns of semaphorin7A and plexinC1 during rat neural development suggest roles in axon guidance and neuronal migration

Background

Although originally identified as embryonic axon guidance cues, semaphorins are now known to regulate multiple, distinct, processes crucial for neuronal network formation including axon growth and branching, dendritic morphology, and neuronal migration. Semaphorin7A (Sema7A), the only glycosylphosphatidylinositol-anchored semaphorin, promotes axon growth in vitro and is required for the proper growth of the mouse lateral olfactory tract in vivo. Sema7A has been postulated to signal through two unrelated receptors, an RGD-dependent α1β1-integrin and a member of the plexin family, plexinC1. β1-integrins underlie Sema7A-mediated axon growth and Sema7A function in the immune system. Sema7A-plexinC1 interactions have also been implicated in immune system function, but the neuronal role of this ligand-receptor pair remains to be explored. To gain further insight into the function(s) of Sema7A and plexinC1 during neural development, we present here a detailed analysis of Sema7A and plexinC1 expression in the developing rat nervous system.

Results

In situ hybridization revealed select expression of Sema7A and plexinC1 in multiple neuronal systems including: the olfactory system, the hypothalamo-hypophysial system, the hippocampus, the meso-diencephalic dopamine system, and the spinal cord. Within these systems, Sema7A and plexinC1 are often expressed in specific neuronal subsets. In general, Sema7A transcript levels increase significantly towards adulthood, whereas plexinC1 expression decreases as development proceeds.

PlexinC1, but not Sema7A, is strongly expressed by distinct populations of migrating neurons. In addition to neuronal expression, Sema7A and plexinC1 transcripts were detected in oligodendrocytes and ependymal cells, respectively.

Conclusion

Sema7A and plexinC1 expression patterns are consistent with these proteins serving both cooperative and separate functions during neural development. The prominent expression of plexinC1 in several distinct populations of migrating neurons suggests a novel role for this plexin family member in neuronal migration.

Alpha v integrins mediate beta-amyloid induced inhibition of long-term potentiation

Beta-amyloid (Abeta) is the principal component of the extracellular plaques present in patients with Alzheimer's disease. Several studies have recently shown that acutely applied Abeta inhibits the induction of LTP in the hippocampus. In the present studies, we have investigated the role of integrins in such Abeta-mediated block of LTP in the dentate gyrus in vitro and in the CA1 in vivo. Selective antibodies to the alpha v integrin subunit were found to prevent the Abeta inhibition of LTP, both in the dentate gyrus in vitro and in the CA1 in vivo. In contrast, two control antibodies did not prevent such action of Abeta. In addition, a small molecule nonpeptide antagonist of alpha v-containing integrins and two other antagonistic ligands of integrins, superfibronectin and the disintegrin echistatin, also prevented the Abeta inhibition of LTP. These studies indicate that alpha v integrins may be important mediators of synaptic dysfunction prior to neurodegeneration in Alzheimer's disease.

Collagen-dependent neurite outgrowth and response to dynamic deformation in three-dimensional neuronal cultures

In vitro models of brain injury that use thick 3-D cultures and control extracellular matrix constituents allow evaluation of cell-matrix interactions in a more physiologically relevant configuration than traditional 2-D cultures. We have developed a 3-D cell culture system consisting of primary rat cortical neurons distributed throughout thick (>500 microm) gels consisting of type IV collagen (Col) conjugated to agarose. Neuronal viability and neurite outgrowth were examined for a range of agarose (AG) percentages (1.0-3.0%) and initial collagen concentrations ([Col](i); 0-600 microg/mL). In unmodified AG, 1.5% gels supported viable cultures with significant neurite outgrowth, which was not found at lower (< or =1.0%) concentrations. Varying [Col](i )in 1.25% AG revealed the formation of dense, 3-D neurite networks at [Col](i) of 300 microg/mL, while neurons in unmodified AG and at higher [Col](i) (600 microg/mL) exhibited significantly less neurite outgrowth; although, neuronal survival did not vary with [Col](i). The effect of [Col](i) on acute neuronal response following high magnitude, high rate shear deformation (0.50 strain, 30 s(-1) strain rate) was evaluated in 1.5% AG for [Col](i) of 30, 150, and 300 microg/mL, which supported cultures with similar baseline viability and neurite outgrowth. Conjugation of Col to AG also increased the complex modulus of the hydrogel. Following high rate deformation, neuronal viability significantly decreased with increasing [Col](i), implicating cell-matrix adhesions in acute mechanotransduction events associated with traumatic loading. These results suggest interrelated roles for matrix mechanical properties and receptor-mediated cell-matrix interactions in neuronal viability, neurite outgrowth, and transduction of high rate deformation. This model system may be further exploited for the elucidation of mechanotransduction mechanisms and cellular pathology following mechanical insult.

Integrin-mediated dendrite branch maintenance requires Abelson (Abl) family kinases

Dendrite arbor structure is a critical determinant of nervous system function that must be actively maintained throughout life, but the signaling pathways that regulate dendrite maintenance are essentially unknown. We report that the Abelson (Abl) and Abl-related gene (Arg) nonreceptor tyrosine kinases are required for maintenance of cortical dendrites in the mouse brain. arg-/- cortical dendrites initially develop normally and are indistinguishable from wild-type dendrites at postnatal day 21. Dendrite branches are not efficiently maintained in arg-/- neurons, leading to a reduction in dendrite arbor size by early adulthood. More severe dendrite loss is observed in abl-/-arg-/- neurons. Elevation of Arg kinase activity in primary cortical neurons promotes axon and dendrite branching. Activation of integrin receptors by adhesion to laminin-1 or Semaphorin 7A also promotes neurite branching in cortical neurons, but this response is absent in arg-/- neurons because of the reduced dynamic behavior of mutant neurite branches. These data suggest that integrin signaling through Abl and Arg support cortical dendrite branch maintenance by promoting dendrite branch dynamics in response to adhesive cues.

Alpha2beta1 and alphaVbeta1 integrin signaling pathways mediate amyloid-beta-induced neurotoxicity

Pathological hallmarks of Alzheimer's disease are the presence of extracellular amyloid plaques, intracellular neurofibrillary tangles, and neurodegeneration. The principal component of amyloid plaques is the amyloid-beta peptide (Abeta). Accumulating evidence indicates that Abeta may play a causal role in Alzheimer's disease. In this report, we demonstrate that Abeta deposition and neurotoxicity in human cortical primary neurons are mediated through alpha2beta1 and alphaVbeta1 integrins using specific integrin-blocking antibodies. An aberrant integrin signaling pathway causing the neurotoxicity is mediated through Pyk2. The role of alpha2beta1 and alphaVbeta1 integrins can be extended to another amyloidosis using an amylin in vitro neurotoxicity model. These results indicate that the alpha2beta1 and alphaVbeta1 integrin signaling pathway may be critical components of neurodegeneration in Alzheimer's disease and that integrins may recognize and be activated by a shared structural motif of polymerizing amyloidogenic proteins.

Gene expression profile of amyloid beta protein-injected mouse model for Alzheimer disease

AIM

To investigate the gene expression profile changes in the cerebral cortex of mice injected icv with amyloid beta-protein (Abeta) fragment 25-35 using cDNA microarray.

METHODS

Balb/c mice were randomly divided into a control group and Abeta-treated group. The Morris water maze test was performed to detect the effect of Abeta-injection on the learning and memory of mice. Atlas Mouse 1.2 Expression Arrays containing 1176 genes were used to investigate the gene expression pattern of each group.

RESULTS

The gene expression profiles showed that 19 genes including TBX1, NF-kB, AP-1/c-Jun, cadherin, integrin, erb-B2, and FGFR1 were up-regulated after 2 weeks of icv administration of Abeta; while 12 genes were down-regulated, including NGF, glucose phosphate isomerase 1, AT motif binding factor 1, Na+/K+-ATPase, and Akt.

CONCLUSIONS

The results provide important leads for pursuing a more complete understanding of the molecular events of Abeta-injection into mice with Alzheimer disease.

Differential regulation of Abeta42-induced neuronal C1q synthesis and microglial activation

Expression of C1q, an early component of the classical complement pathway, has been shown to be induced in neurons in hippocampal slices, following accumulation of exogenous Aβ42. Microglial activation was also detected by surface marker expression and cytokine production. To determine whether C1q induction was correlated with intraneuronal Aβ and/or microglial activation, D-(-)-2-amino-5-phosphonovaleric acid (APV, an NMDA receptor antagonist) and glycine-arginine-glycine-aspartic acid-serine-proline peptide (RGD, an integrin receptor antagonist), which blocks and enhances Aβ42 uptake, respectively, were assessed for their effect on neuronal C1q synthesis and microglial activation. APV inhibited, and RGD enhanced, microglial activation and neuronal C1q expression. However, addition of Aβ10–20 to slice cultures significantly reduced Aβ42 uptake and microglial activation, but did not alter the Aβ42-induced neuronal C1q expression. Furthermore, Aβ10–20 alone triggered C1q production in neurons, demonstrating that neither neuronal Aβ42 accumulation, nor microglial activation is required for neuronal C1q upregulation. These data are compatible with the hypothesis that multiple receptors are involved in Aβ injury and signaling in neurons. Some lead to neuronal C1q induction, whereas other(s) lead to intraneuronal accumulation of Aβ and/or stimulation of microglia.

Endogenous β-amyloid peptide synthesis modulates cAMP response element-regulated gene expression in PC12 cells

Extracellular-regulated kinases play a fundamental role in several neuroplasticity processes. In order to test whether endogenous beta-amyloid peptides play a role in the activation of extracellular-regulated kinase, we investigated the Rap1-extracellular-regulated kinase pathway in PC12 cells expressing human beta-amyloid precursor protein containing familial Alzheimer's disease mutations. In PC12 cells transfected with mutant human beta-amyloid precursor proteins that lead to higher levels of endogenous beta-amyloid, we observed an up-regulation of phospho-extracellular-regulated kinase and higher levels of activity-induced cAMP response element-directed gene expression. These results suggest that moderate levels of endogenous beta-amyloid peptides stimulate cAMP response element-directed gene expression. This stimulation was via a Rap1/MEK/extracellular-regulated kinase signaling pathway, as it was blocked by inhibition of Rap1 and MEK activities, and it requires beta-amyloid precursor protein cleavage at the gamma-site as it was abolished by a gamma-secretase inhibitor. Interestingly, in agreement with the previous observations, micromolar levels of extracellular fibrillar beta-amyloid blocked the cAMP response element-regulated gene expression stimulated by potassium and forskolin. This indicates that beta-amyloid can provoke different responses on cAMP response element-directed gene expression, such that low beta-amyloid levels may play a physiological role favoring synaptic plasticity under normal conditions while it would inhibit this mechanism under pathological conditions.

The role of very late antigen-1 in immune-mediated inflammation

The alpha1beta1 integrin, also known as "very late antigen" (VLA)-1, is normally expressed on mesenchymal cells, some epithelial cells, activated T cells, and macrophages, and interacts, via the I-domain of the extracellular domain of the alpha1 subunit, with collagen molecules in the extracellular matrix (ECM). By "outside-in" transmembranal signaling to the interior of the cell, it mediates adhesion, migration, proliferation, remodeling of the ECM, and cytokine secretion by endothelial cells, mesangial cells, fibroblasts, and immunocytes. Importantly, its expressions and functions are enhanced by inflammatory cytokines including interferon (IFN)gamma and tumor necrosis factor (TNF)alpha, thus augmenting angiogenesis and fibrosis linked, in particular, to inflammation. Moreover, within the immune system, VLA-1 marks effector memory CD4+ and CD8+ T cells that are retained in extralymphatic tissues by interactions of the integrin with collagen and produce high levels of IFNgamma. Thus, immune-mediated inflammation in vivo is inhibited by blockade of the VLA-1-collagen interaction in experimental animal models of arthritis, colitis, nephritis, and graft versus host disease (GVHD), suggesting that inhibiting the interaction of the alpha1 I-domain with its ligands or modulating "outside-in" signaling by VLA-1 would be a useful approach in the human diseases simulated by these experimental models.