Synaptotrophic effects of human amyloid beta protein precursors in the cortex of transgenic mice

Prominent neurodegeneration and increased plaque formation in complement-inhibited Alzheimer's mice

Abnormal accumulation of beta-amyloid (Abeta) in Alzheimer's disease (AD) is associated with prominent brain inflammation. Whereas earlier studies concluded that this inflammation is detrimental, more recent animal data suggest that at least some inflammatory processes may be beneficial and promote Abeta clearance. Consistent with these observations, overproduction of transforming growth factor (TGF)-beta1 resulted in a vigorous microglial activation that was accompanied by at least a 50% reduction in Abeta accumulation in human amyloid precursor protein (hAPP) transgenic mice. In a search for inflammatory mediators associated with this reduced pathology, we found that brain levels of C3, the central component of complement and a key inflammatory protein activated in AD, were markedly higher in hAPP/TGF-beta1 mice than in hAPP mice. To assess the importance of complement in the pathogenesis of AD-like disease in mice, we inhibited C3 activation by expressing soluble complement receptor-related protein y (sCrry), a complement inhibitor, in the brains of hAPP mice. Abeta deposition was 2- to 3-fold higher in 1-year-old hAPP/sCrry mice than in age-matched hAPP mice and was accompanied by a prominent accumulation of degenerating neurons. These results indicate that complement activation products can protect against Abeta-induced neurotoxicity and may reduce the accumulation or promote the clearance of amyloid and degenerating neurons. These findings provide evidence for a role of complement and innate immune responses in AD-like disease in mice and support the concept that certain inflammatory defense mechanisms in the brain may be beneficial in neurodegenerative disease.

Does my mouse have Alzheimer's disease?

Small animal models that manifest many of the characteristic neuropathological and behavioral features of Alzheimer's disease (AD) have been developed and have proven of great value for studying the pathogenesis of this disorder at the molecular, cellular and behavioral levels. The great progress made in our understanding of the genetic factors that either cause or contribute to the risk of developing AD has prompted many laboratories to create transgenic (tg) mice that overexpress specific genes which cause familial forms of the disease. Several of these tg mice display neuropathological and behavioral features of AD including amyloid beta-peptide (A beta) and amyloid deposits, neuritic plaques, gliosis, synaptic alterations and signs of neurodegeneration as well as memory impairment. Despite these similarities, important differences in neuropathology and behavior between these tg mouse models and AD have also been observed, and to date no perfect animal model has emerged. Moreover, ascertaining which elements of the neuropathological and behavioral phenotype of these various strains of tg mice are relevant to that observed in AD continues to be a challenge. Here we provide a critical review of the AD-like neuropathology and behavioral phenotypes of several well-known and utilized tg mice that express human APP transgenes.

Overexpression of wild type but not an FAD mutant presenilin-1 promotes neurogenesis in the hippocampus of adult mice

Mutations in the presenilin-1 (PS-1) gene are one cause of familial Alzheimer's disease (FAD). However, the functions of the PS-1 protein as well as how PS-1 mutations cause FAD are incompletely understood. Here we investigated if neuronal overexpression of wild-type or FAD mutant PS-1 in transgenic mice affects neurogenesis in the hippocampus of adult animals. We show that either a wild-type or an FAD mutant PS-1 transgene reduces the number of neural progenitors in the dentate gyrus. However, the wild-type, but not the FAD mutant PS-1 promoted the survival and differentiation of progenitors leading to more immature granule cell neurons being generated in PS-1 wild type expressing animals. These studies suggest that PS-1 plays a role in regulating neurogenesis in adult hippocampus and that FAD mutants may have deleterious properties independent of their effects on amyloid deposition.

Selective nicotinic receptor consequences in APP(SWE) transgenic mice

The nicotinic (nAChRs) and muscarinic (mAChRs) acetylcholine receptors and acetylcholinesterase (AChE) activity were studied in the brains of APP(SWE) transgenic mice (Tg+) and age-matched nontransgenic controls (Tg-) that were between 4 and 19 months of age. A significant increase in the binding of 125I-labeled alpha-bungarotoxin (alpha7 nAChRs) was observed in most brain regions analyzed in 4-month-old Tg+ mice, preceding learning and memory impairments and amyloid-beta (Abeta) pathology. The enhanced alpha7 receptor binding was still detectable at 17-19 months of age. Increase in [3H]cytisine binding (alpha4beta2 nAChRs) was measured at 17-19 months of age in Tg+ mice, at the same age when the animals showed heavy Abeta pathology. No significant changes in [3H]pirenzepine (M1 mAChRs) or [3H]AFDX 384 (M2 mAChRs) binding sites were found at any age studied. The upregulation of the nAChRs probably reflects compensatory mechanisms in response to Abeta burden in the brains of Tg+ mice.

Phosphatidylinositol-3-kinase-Akt kinase and p42/p44 mitogen-activated protein kinases mediate neurotrophic and excitoprotective actions of a secreted form of amyloid precursor protein

The alpha-secretase-derived form of the amyloid precursor protein (sAPPalpha), which is released from neurons in an activity-dependent manner, has been shown to promote long-term survival of hippocampal and cortical neurons in culture and can protect those neurons against excitotoxic and ischemic injury in culture and in vivo. The signal transduction pathway(s) activated by sAPPalpha has not been established. We now report that sAPPalpha activates the phosphatidylinositol-3-kinase (PI(3)K)-Akt kinase signaling pathway in cultured hippocampal neurons. sAPPalpha also stimulates phosphorylation of p42 (ERK1) and p44 (ERK2) mitogen-activated protein (MAP) kinases by a PI(3)K-independent pathway. Treatment of neurons with sAPPalpha protects them against death induced by trophic factor deprivation and exposure to glutamate, and these survival-promoting effects of sAPPalpha are abolished or attenuated when either PI(3)K or p42/p44 MAP kinases are selectively blocked. Exposure of neurons to sAPPalpha resulted in a decrease in the level of IkappaBbeta and an increase in NF-kappaB DNA binding activity, both of which were blocked by wortmannin, suggesting that the transcription factor NF-kappaB may be a downstream target of the PI(3)K-Akt pathway that may play a role in the cell survival-promoting action of sAPPalpha. These findings suggest that the PI(3)K-Akt pathway and p42/p44 MAP kinases mediate responses of neurons to sAPPalpha in physiological and pathological settings, with implications for synaptic plasticity and the pathogenesis of Alzheimer's disease.

Brain-selective overexpression of angiotensin (AT1) receptors causes enhanced cardiovascular sensitivity in transgenic mice

To examine the physiological importance of brain angiotensin II type 1 (AT1) receptors, we developed a novel transgenic mouse model with rat AT1a receptors targeted selectively to neurons of the central nervous system (CNS). A transgene consisting of 2.8 kb of the rat neuron-specific enolase (NSE) 5' flanking region fused to a cDNA encoding the full open-reading frame of the rat AT1a receptor was constructed and transgenic mice (NSE-AT1a) were generated. Two of six transgenic founder lines exhibited brain-selective expression of the transgene at either moderate or high levels. Immunohistochemistry revealed widespread distribution of AT1 receptors in neurons throughout the CNS. This neuron-targeted overexpression of AT1a receptors resulted in enhanced cardiovascular responsiveness to intracerebroventricular (ICV) angiotensin II (Ang II) injection but not to other central pressor agents, demonstrating functional overexpression of the transgene in NSE-AT1a mice. Interestingly, baseline blood pressure (BP) was not elevated in either transgenic line. However, blockade of central AT1 receptors with ICV losartan caused significant falls in basal BP in NSE-AT1a mice but had no effect in nontransgenic controls. These results suggest that whereas there is an enhanced contribution of central AT1 receptors to the maintenance of baseline BP in NSE-AT1a mice, particularly effective baroreflex buffering prevents hypertension in this model. Used both independently, and in conjunction with mice harboring gene-targeted deletions of AT1a receptors, this new model will permit quantitative and relevant investigations of the role of central AT1a receptors in cardiovascular homeostasis in health and disease.

Maintained synaptophysin immunoreactivity in Tg2576 transgenic mice during aging: correlations with cognitive impairment

Regional loss of synapses, particularly within the neocortex and hippocampus, is characteristic of Alzheimer's Disease (AD) and strongly correlated with extent of cognitive impairment. The Tg2576 transgenic mouse model of AD develops Abeta-containing neuritic plaques by 10-16 months of age and shows cognitive impairment in several tasks. In the present study, synaptophysin immunoreactivity (SYN-IR; a marker for synaptic terminals) was evaluated in the neocortex and hippocampus of behaviorally-tested Tg2576 transgenic (Tg+) mice aged 3, 9, 14, and 19 months of age. In control non-transgenic (Tg-) mice, SYN-IR in both neocortex and hippocampus tended to decrease with age, while SYN-IR in Tg+ mice was maintained with age. Thus, 19M Tg+ mice exhibited significantly greater synaptophysin immunostaining compared to 19M Tg- mice in both inner and outer neocortical regions, as well as in the dentate gyrus' outer molecular layer and polymorphic layer. Over all four age groups collectively, outer cortical SYN-IR was also greater in Tg+ compared to Tg- mice. Multiple factors could be responsible for maintained SYN-IR in aged Tg+ mice, including compensatory changes in synaptic morphology and staining of dystrophic neuritics associated with Abeta deposition. For all animals combined (Tg+ and Tg-), as well as for aged 19M animals alone, hippocampal SYN-IR was correlated with impaired acquisition and spatial reference memory in the Morris water maze task, suggestive that elevated hippocampal SYN-IR is a manifestation of pathophysiologic synaptic processing within the hippocampus. Also for 19M animals alone, hippocampal SYN-IR was highly correlated with impaired visible platform recognition, indicative that elevated SYN-IR is linked to visual agnosia. The results of this study are consistent with the premise that maintained SYN-IR in Tg2576 mice during aging is associated with impaired synaptic function, resulting in cognitive deficits.

Amyloid-associated neuron loss and gliogenesis in the neocortex of amyloid precursor protein transgenic mice

APP23 transgenic mice express mutant human amyloid precursor protein and develop amyloid plaques predominantly in neocortex and hippocampus progressively with age, similar to Alzheimer's disease. We have previously reported neuron loss in the hippocampal CA1 region of 14- to 18-month-old APP23 mice. In contrast, no neuron loss was found in neocortex. In the present study we have reinvestigated neocortical neuron numbers in adult and aged APP23 mice. Surprisingly, results revealed that 8-month-old APP23 mice have 13 and 14% more neocortical neurons compared with 8-month-old wild-type and 27-month-old APP23 mice, respectively. In 27-month-old APP23 mice we found an inverse correlation between amyloid load and neuron number. These results suggest that APP23 mice have more neurons until they develop amyloid plaques but then lose neurons in the process of cerebral amyloidogenesis. Supporting this notion, we found more neurons with a necrotic-apoptotic phenotype in the neocortex of 24-month-old APP23 mice compared with age-matched wild-type mice. Stimulated by recent reports that demonstrated neurogenesis after targeted neuron death in the mouse neocortex, we have also examined neurogenesis in APP23 mice. Strikingly, we found a fourfold to sixfold increase in newly produced cells in 24-month-old APP23 mice compared with both age-matched wild-type mice and young APP23 transgenic mice. However, subsequent cellular phenotyping revealed that none of the newly generated cells in neocortex had a neuronal phenotype. The majority were microglial and to a lesser extent astroglial cells. We conclude that cerebral amyloidosis in APP23 mice causes a modest neuron loss in neocortex and induces marked gliogenesis.

Apolipoprotein D mRNA expression is elevated in PDAPP transgenic mice

Apolipoprotein D (apoD) expression is known to be elevated in select regions of rodent and human brain in association with different types of CNS pathology. To investigate a potential role for apoD in the neuropathology of Alzheimer's disease, we have measured apoD mRNA expression in transgenic mice expressing mutated human amyloid precursor protein under control of platelet-derived growth factor promoter (PDAPP mice). In situ hybridization analysis revealed increased apoD mRNA expression in brains of aged (26 months) PDAPP transgenic mice compared to aged littermate controls. These increases were most prominent in the hippocampal fimbria, corpus callosum and other white matter tracts. No substantial increases in expression were observed in white matter regions in young (6 months) PDAPP transgenic mice compared to young controls. Comparison between aged and young control mice revealed increased apoD expression in similar white matter regions of the aged animals. These findings suggest that, although increases in apoD expression are a normal feature of brain aging, super-increases may represent a glial cell compensatory response to beta-amyloid deposition in Alzheimer's disease.

Early formation of mature amyloid-beta protein deposits in a mutant APP transgenic model depends on levels of Abeta(1-42)

The main objective of the present study was to develop an alternative singly-transgenic (tg) hAPP model where amyloid deposition will occur at an earlier age. For this purpose, we generated lines of tg mice expressing hAPP751 cDNA containing the London (V717I) and Swedish (K670M/N671L) mutations under the regulatory control of the murine (m)Thy-1 gene (mThy1-hAPP751). In the brains of the highest (line 41) and intermediate (lines 16 and 11) expressers, high levels of hAPP expression were found in neurons in layers 4-5 of the neocortex, hippocampal CA1 and olfactory bulb. As early as 3-4 months of age, line 41 mice developed mature plaques in the frontal cortex, whereas at 5-7 months plaque formation extended to the hippocampus, thalamus and olfactory region. Ultrastructural and double-immunolabeling analysis confirmed that most plaques were mature and contained dystrophic neurites immunoreactive with antibodies against APP, synaptophysin, neurofilament and tau. In addition, a decrease in the number of synaptophysin-immunoreactive terminals was most prominent in the frontal cortex of mice from line 41. Mice from line 11 developed diffuse amyloid deposits at 11 months of age, whereas mice from line 16 did not show evidence of amyloid deposition. Analysis of Abeta by ELISA showed that levels of Abeta(1-40) were higher in mice that did not show any amyloid deposits (line 16), whereas Abeta(1-42) was the predominant species in tg animals from the lines showing plaque formation (lines 41 and 11). Taken together this study indicates that early onset plaque formation depends on levels of Abeta(1-42).

Constitutive shedding of the amyloid precursor protein ectodomain is up-regulated by tumour necrosis factor-alpha converting enzyme

The amyloid precursor protein (APP) of Alzheimer's disease is a transmembrane protein that is cleaved within its extracellular domain, liberating a soluble N-terminal fragment (sAPP alpha). Putative mediators of this process include three members of the ADAM (a disintegrin and metalloprotease) family, ADAM9, ADAM10 and ADAM17/TACE (tumour necrosis factor-alpha converting enzyme). Tumour necrosis factor-alpha protease inhibitor (TAPI-1), an inhibitor of ADAMs, reduced constitutive and muscarinic receptor-stimulated sAPP alpha release in HEK-293 cells stably expressing M3 muscarinic receptors. However, the former was less sensitive to TAPI-1 (IC(50)=8.09 microM) than the latter (IC(50)=3.61 microM), suggesting that these processes may be mediated by different metalloproteases. Constitutive sAPP alpha release was increased several-fold in cells transiently transfected with TACE, and this increase was proportional to TACE expression. In contrast, muscarinic-receptor-activated sAPP alpha release was not altered in TACE transfectants. TACE-dependent constitutive release of co-transfected APP(695) was inhibited by TAPI-1 with an IC(50) of 0.92 microm, a value significantly lower than the IC(50)s for inhibition of either constitutive or receptor-regulated sAPP alpha shedding mediated by endogenous secretases. The results indicate that TACE is capable of catalysing constitutive alpha-secretory cleavage of APP, but it is likely that additional members of the ADAM family mediate endogenous constitutive and receptor-coupled release of sAPP alpha in HEK-293 cells.

Constitutive shedding of the amyloid precursor protein ectodomain is up-regulated by tumour necrosis factor-alpha converting enzyme

The amyloid precursor protein (APP) of Alzheimer's disease is a transmembrane protein that is cleaved within its extracellular domain, liberating a soluble N-terminal fragment (sAPP alpha). Putative mediators of this process include three members of the ADAM (a disintegrin and metalloprotease) family, ADAM9, ADAM10 and ADAM17/TACE (tumour necrosis factor-alpha converting enzyme). Tumour necrosis factor-alpha protease inhibitor (TAPI-1), an inhibitor of ADAMs, reduced constitutive and muscarinic receptor-stimulated sAPP alpha release in HEK-293 cells stably expressing M3 muscarinic receptors. However, the former was less sensitive to TAPI-1 (IC(50)=8.09 microM) than the latter (IC(50)=3.61 microM), suggesting that these processes may be mediated by different metalloproteases. Constitutive sAPP alpha release was increased several-fold in cells transiently transfected with TACE, and this increase was proportional to TACE expression. In contrast, muscarinic-receptor-activated sAPP alpha release was not altered in TACE transfectants. TACE-dependent constitutive release of co-transfected APP(695) was inhibited by TAPI-1 with an IC(50) of 0.92 microm, a value significantly lower than the IC(50)s for inhibition of either constitutive or receptor-regulated sAPP alpha shedding mediated by endogenous secretases. The results indicate that TACE is capable of catalysing constitutive alpha-secretory cleavage of APP, but it is likely that additional members of the ADAM family mediate endogenous constitutive and receptor-coupled release of sAPP alpha in HEK-293 cells.

Neuropathology of mice carrying mutant APP(swe) and/or PS1(M146L) transgenes: alterations in the p75(NTR) cholinergic basal forebrain septohippocampal pathway

Cholinergic basal forebrain (CBF) projection systems are defective in late Alzheimer's disease (AD). We examined the brains of 12-month-old singly and doubly transgenic mice overexpressing mutant amyloid precursor protein (APP(swe)) and/or presenilin-1 (PS1(M146L)) to investigate the effects of these AD-related genes on plaque and tangle pathology, astrocytic expression, and the CBF projection system. Two types of beta-amyloid (Abeta)-immunoreactive (ir) plaques were observed: type 1 were darkly stained oval and elongated deposits of Abeta, and type 2 were diffuse plaques containing amyloid fibrils. APP(swe) and PS1(M146L) mouse brains contained some type 1 plaques, while the doubly transgenic (APP(swe)/PS1(M146L)) mice displayed a greater abundance of types 1 and 2 plaques. Sections immunostained for the p75 NGF receptor (p75(NTR)) revealed circular patches scattered throughout the cortex and hippocampus of the APP(swe)/PS1(M146L) mice that contained Abeta, were innervated by p75(NTR)-ir neurites, but displayed virtually no immunopositive neurons. Tau pathology was not seen in any transgenic genotype, although a massive glial response occurred in the APP(swe)/PS1(M146L) mice associated with amyloid plaques. Stereology revealed a significant increase in p75(NTR)-ir medial septal neurons in the APP(swe) and PS1(M146L) singly transgenic mice compared to the APP(swe)/PS1(M146L) mice. No differences in size or optical density of p75(NTR)-ir neurons were observed in these three mutants. p75(NTR)-ir fibers in hippocampus and cortex were more pronounced in the APP(swe) and PS1(M146L) mice, while the APP(swe)/PS1(M146L) mice showed the least p75(NTR)-ir fiber staining. These findings suggest a neurotrophic role for mutant APP and PS1 upon cholinergic hippocampal projection neurons at 12 months of age.

Applications of the Morris water maze in the study of learning and memory

The Morris water maze (MWM) was described 20 years ago as a device to investigate spatial learning and memory in laboratory rats. In the meanwhile, it has become one of the most frequently used laboratory tools in behavioral neuroscience. Many methodological variations of the MWM task have been and are being used by research groups in many different applications. However, researchers have become increasingly aware that MWM performance is influenced by factors such as apparatus or training procedure as well as by the characteristics of the experimental animals (sex, species/strain, age, nutritional state, exposure to stress or infection). Lesions in distinct brain regions like hippocampus, striatum, basal forebrain, cerebellum and cerebral cortex were shown to impair MWM performance, but disconnecting rather than destroying brain regions relevant for spatial learning may impair MWM performance as well. Spatial learning in general and MWM performance in particular appear to depend upon the coordinated action of different brain regions and neurotransmitter systems constituting a functionally integrated neural network. Finally, the MWM task has often been used in the validation of rodent models for neurocognitive disorders and the evaluation of possible neurocognitive treatments. Through its many applications, MWM testing gained a position at the very core of contemporary neuroscience research.

Estrogen induces a rapid secretion of amyloid beta precursor protein via the mitogen-activated protein kinase pathway

The female sex hormone estrogen (17beta-estradiol; E2) may function as a neurohormone and has multiple neuromodulatory functions in the brain. Its potent neuroprotective activities can be dependent and independent of estrogen receptors (ERs). In addition, E2 influences the processing of the amyloid beta precursor protein (APP), one central step in the pathogenesis of Alzheimer's disease. Here, we show: (a) that physiological concentrations of E2 very rapidly cause an increased release of secreted nonamyloidogenic APP (sAPPalpha) in mouse hippocampal HT22 and human neuroblastoma SK-N-MC cells; and (b) that this effect is mediated through E2 via the phosphorylation of extracellular-regulated kinase 1 and 2 (ERK1/2), prominent members of the mitogen-activated protein kinase (MAPK) pathway. Furthermore, we show that the activation of MAPK-signaling pathway and the enhancement of the sAPP release is independent of ERs and could be induced by E2 to a similar extent in neuronal cells either lacking or overexpressing a functional ER.

Transcranial magnetic stimulation as a therapeutic tool in psychiatry: what do we know about the neurobiological mechanisms?

Potential therapeutic properties of repetitive transcranial magnetic stimulation (rTMS) have been suggested in several psychiatric disorders such as depression, mania, obsessive-compulsive disorder, posttraumatic stress disorder and schizophrenia. By inducing electric currents in brain tissue via a time-varying strong magnetic field, rTMS has the potential to either directly or trans-synaptically modulate neuronal circuits thought to be dysfunctional in these psychiatric disorders. However, in order to optimize rTMS for therapeutic use, it is necessary to understand the neurobiological mechanisms involved, particularly the nature of the changes induced and the brain regions affected. Compared to the growing number of clinical studies on its putative therapeutic properties, the studies on the basic mechanisms of rTMS are surprisingly scarce. rTMS currently still awaits clinical routine administration although,there is compelling evidence that it causes changes in neuronal circuits as reflected by behavioural changes and decreases in the activity of the hypothalamic-pituitary-adrenocortical system. Both alterations suggest regional changes in neurotransmitter/neuromodulator release, transsynaptic efficiency, signaling pathways and in gene transcription. Together, these changes are, in part, reminiscent of those accompanying antidepressant drugs.

Reduced expression of amyloid precursor protein, presenilin-1 and rab3a in cortical brain regions in Alzheimer's disease

To study the role of amyloid precursor protein (APP) in the pathogenesis of Alzheimer's disease (AD), the level of APP was analysed by quantitative immunoblotting in 6 AD patients and 6 age-matched controls in 9 brain regions. These were associative cortices (orbital frontal cortex, inferior temporal cortex, inferior parietal cortex), primary cortex (occipital cortex), limbic structures (anterior cingulate gyrus, hippocampus), subcortical structures (putamen, thalamus) and cerebellum. To assess a potential relationship between APP and presenilin-1 (PS-1) and/or synaptic proteins, the levels of PS-1 and rab3a, a specific synaptic vesicle protein, were also determined in the same tissue samples. The level of APP was almost the same in the association cortical regions, primary cortex, and limbic structures and in the subcortical structures, while the lowest level was found in the cerebellum. There were more marked differences in the level of PS-1 and rab3a between different brain regions. The highest levels of PS-1 and rab3a were found in the association cortical areas, while intermediate levels were found in primary cortex, limbic structures and subcortical structures. As for APP, the lowest level was found in cerebellum. We found significantly reduced levels of all three proteins in the association cortices and in hippocampus in AD. Our data show that the protein levels are reduced in specific areas, restricted to neuronal populations that are known to degenerate in AD. Due to the similarity of the expression of APP, PS-1 and rab3a, it is tempting to speculate whether there is a functional relationship between these proteins.

The cerebrospinal fluid levels of tau, growth-associated protein-43 and soluble amyloid precursor protein correlate in Alzheimer's disease, reflecting a common pathophysiological process

Cerebrospinal fluid (CSF) levels of tau (total tau), growth-associated protein-43 (GAP-43), soluble amyloid precursor protein (sAPP; i.e. total sAPP), and beta-amyloid(42) (Abeta(42)) were studied in patients with frontotemporal dementia (FTD; n = 14), Alzheimer's disease (AD; n = 47) and vascular dementia (VAD; n = 16), and in age-matched controls (n = 12). CSF-tau was increased in AD compared to controls and FTD (p < 0.001 for both). CSF-GAP-43 was increased in AD compared to controls (p < 0.05), and both CSF-GAP-43 and CSF-sAPP were increased in AD compared to FTD (p < 0.01). Positive and highly significant correlations were found between CSF-tau and CSF-GAP-43 in all groups and between CSF-tau, CSF-GAP-43 and CSF-sAPP in AD. The correlations found may reflect a common pathophysiologic process such as axonal degeneration.

The functions of the amyloid precursor protein gene

The amyloid precursor protein (APP) gene and its protein products have multiple functions in the central nervous system and fulfil criteria as neuractive peptides: presence, release and identity of action. There is increased understanding of the role of secretases (proteases) in the metabolism of APP and the production of its peptide fragments. The APP gene and its products have physiological roles in synaptic action, development of the brain, and in the response to stress and injury. These functions reveal the strategic importance of APP in the workings of the brain and point to its evolutionary significance.

Secretases as therapeutic targets for the treatment of Alzheimer's disease

The extracellular deposition of short amyloid peptides in the brain of patients is thought to be a central event in the pathogenesis of Alzheimer's Disease. The generation of the amyloid peptide occurs via a regulated cascade of cleavage events in its precursor protein, A beta PP. At least three enzymes are responsible for A beta PP proteolysis and have been tentatively named alpha-, beta- and gamma-secretases. The recent identification of several of these secretases is a major leap in the understanding how these secretases regulate amyloid peptide formation. Members of the ADAM family of metalloproteases are involved in the non-amyloidogenic alpha-secretase pathway. The amyloidogenic counterpart pathway is initiated by the recently cloned novel aspartate protease named BACE. The available data are conclusive and crown BACE as the long-sought beta-secretase. This enzyme is a prime candidate drug target for the development of therapy aiming to lower the amyloid burden in the disease. Finally, the gamma-secretases are intimately linked to the function of the presenilins. These multi-transmembrane domain proteins remain intriguing study objects. The hypothesis that the presenilins constitute a complete novel type of protease family, and are cleaving A beta PP within the transmembrane region, remains an issue of debate. Several questions remain unanswered and direct proof that they exert catalytic activity is still lacking. The subcellular localization of presenilins in neurons, their integration in functional multiprotein complexes and the recent identification of additional modulators of gamma-secretase, like nicastrin, indicate already that several players are involved. Nevertheless, the rapidly increasing knowledge in this area is already paving the road towards selective inhibitors of this secretase as well. It is hoped that such drugs, possibly in concert with the experimental vaccination therapies that are currently tested, will lead to a cure of this inexorable disease.

Survival and plasticity of basal forebrain cholinergic systems in mice transgenic for presenilin-1 and amyloid precursor protein mutant genes

The basalo-cortical cholinergic system was characterized in mice expressing mutant human genes for presenilin-1 (PS1), amyloid precursor protein (APP), and combined PS/APP. Dual immunocytochemistry for ChAT and A beta revealed swollen cholinergic processes within cortical plaques in both APP and PS/APP brains by 12 months, suggesting aberrant sprouting or redistribution of cholinergic processes in response to amyloid deposition. At 8 months, cortical and subcortical ChAT activity was normal (PS/APP) or elevated (PS, APP frontal cortex), while cholinergic cell counts (nBM/SI) and receptor binding were unchanged. ChAT mRNA was up-regulated in the nBM/SI of all three transgenic lines at 8 months. The data indicate that the basal forebrain cholinergic system does not degenerate in mice expressing AD-related transgenes, even in mice with extreme amyloid load. The

Alzheimer's disease as a disorder of mechanisms underlying structural brain self-organization

Mental function has as its cerebral basis a specific dynamic structure. In particular, cortical and limbic areas involved in "higher brain functions" such as learning, memory, perception, self-awareness and consciousness continuously need to be self-adjusted even after development is completed. By this lifelong self-optimization process, the cognitive, behavioural and emotional reactivity of an individual is stepwise remodelled to meet the environmental demands. While the presence of rigid synaptic connections ensures the stability of the principal characteristics of function, the variable configuration of the flexible synaptic connections determines the unique, non-repeatable character of an experienced mental act. With the increasing need during evolution to organize brain structures of increasing complexity, this process of selective dynamic stabilization and destabilization of synaptic connections becomes more and more important. These mechanisms of structural stabilization and labilization underlying a lifelong synaptic remodelling according to experience, are accompanied, however, by increasing inherent possibilities of failure and may, thus, not only allow for the evolutionary acquisition of "higher brain function" but at the same time provide the basis for a variety of neuropsychiatric disorders. It is the objective of the present paper to outline the hypothesis that it might be the disturbance of structural brain self-organization which, based on both genetic and epigenetic information, constantly "creates" and "re-creates" the brain throughout life, that is the defect that underlies Alzheimer's disease (AD). This hypothesis is, in particular, based on the following lines of evidence. (1) AD is a synaptic disorder. (2) AD is associated with aberrant sprouting at both the presynaptic (axonal) and postsynaptic (dendritic) site. (3) The spatial and temporal distribution of AD pathology follows the pattern of structural neuroplasticity in adulthood, which is a developmental pattern. (4) AD pathology preferentially involves molecules critical for the regulation of modifications of synaptic connections, i.e. "morphoregulatory" molecules that are developmentally controlled, such as growth-inducing and growth-associated molecules, synaptic molecules, adhesion molecules, molecules involved in membrane turnover, cytoskeletal proteins, etc. (5) Life events that place an additional burden on the plastic capacity of the brain or that require a particularly high plastic capacity of the brain might trigger the onset of the disease or might stimulate a more rapid progression of the disease. In other words, they might increase the risk for AD in the sense that they determine when, not whether, one gets AD. (6) AD is associated with a reactivation of developmental programmes that are incompatible with a differentiated cellular background and, therefore, lead to neuronal death. From this hypothesis, it can be predicted that a therapeutic intervention into these pathogenetic mechanisms is a particular challenge as it potentially interferes with those mechanisms that at the same time provide the basis for "higher brain function".

A gain-of-function mutation in the sodium channel gene Scn2a results in seizures and behavioral abnormalities

The GAL879-881QQQ mutation in the cytoplasmic S4-S5 linker of domain 2 of the rat brain IIA sodium channel (Na(v)1.2) results in slowed inactivation and increased persistent current when expressed in Xenopus oocytes. The neuron-specific enolase promoter was used to direct in vivo expression of the mutated channel in transgenic mice. Three transgenic lines exhibited seizures, and line Q54 was characterized in detail. The seizures in these mice began at two months of age and were accompanied by behavioral arrest and stereotyped repetitive behaviors. Continuous electroencephalogram monitoring detected focal seizure activity in the hippocampus, which in some instances generalized to involve the cortex. Hippocampal CA1 neurons isolated from presymptomatic Q54 mice exhibited increased persistent sodium current which may underlie hyperexcitability in the hippocampus. During the progression of the disorder there was extensive cell loss and gliosis within the hippocampus in areas CA1, CA2, CA3 and the hilus. The lifespan of Q54 mice was shortened and only 25% of the mice survived beyond six months of age. Four independent transgenic lines expressing the wild-type sodium channel were examined and did not exhibit any abnormalities. The transgenic Q54 mice provide a genetic model that will be useful for testing the effect of pharmacological intervention on progression of seizures caused by sodium channel dysfunction. The human ortholog, SCN2A, is a candidate gene for seizure disorders mapped to chromosome 2q22-24.

A dinucleotide deletion in amyloid precursor protein (APP) mRNA associated with sporadic Alzheimer's disease results in efficient secretion of truncated APP isoforms from neuroblastoma cell cultures

Recently, two dinucleotide deletions were detected in the mRNA of the amyloid precursor protein (APP) from cerebral cortex neurons of patients with sporadic Alzheimer's disease (AD) or Down's syndrome. These deletions resulted in truncation of APP, producing an APP isoform with a 38-kDa N-terminus and a novel carboxyl terminus (APP+1). We investigated the subcellular localization and the processing of APP+1 in the neuroblastoma cell line B103. cDNA constructs were generated encoding fusion proteins of APP+1 or full-length APP with the enhanced green fluorescent protein (eGFP). In transient transfection experiments using B103 cells, the APP+1-eGFP fusion protein showed a reticular localization with intense staining in the Golgi complex. Unlike full-length APP fused to eGFP, the APP+1-eGFP fusion protein did not localize to the perinuclear area or to the plasma membrane. Western blot analysis of cell extracts confirmed the translation of the expected fusion proteins. Analysis of the supernatant by western blot indicated that the APP+1-eGFP fusion protein was efficiently secreted from B103 cells, whereas the secreted form of full-length APP fusion protein (APPs) was hardly detectable. Thus, both dinucleotide deletions in the APP mRNA result in truncated APP+1 that is not membrane associated and is readily secreted from neurons.

Fibulin-1 binds the amino-terminal head of beta-amyloid precursor protein and modulates its physiological function

Genetic studies have implicated amyloid precursor protein (APP) in the pathogenesis of Alzheimer's disease. While accumulating lines of evidence indicate that APP has various functions in cells, little is known about the proteins that modulate its biological activity. Toward this end, we employed a two-hybrid system to identify potential interacting factors. We now report that fibulin-1, which contains repetitive Ca(2+)-binding EGF-like elements, binds to APP at its amino-terminal growth factor-like domain, the region that is responsible for its neurotrophic activities. Fibulin-1 expression in the brain is confined to neurons, and is not expressed significantly by astrocytes or microglia. Direct binding of fibulin-1 to the secreted form of APP (sAPP) was demonstrated with a pull-down assay using fragments of both fibulin-1 fused with glutathione-S transferase and sAPP, produced in bacteria and yeast, respectively. The fibulin-1/sAPP heteromer was shown to form in the conditioned medium of transfected COS-7 cells. Furthermore, fibulin-1 blocks sAPP-mediated proliferation of primary cultured rat neural stem cells. These results suggest that fibulin-1 may play a significant role in modulating the neurotrophic activities of APP.

The role of synaptic proteins in Alzheimer's disease

Synaptic damage is an early event common to neurodegenerative disorders such as Alzheimer's disease (AD) and is the best correlate to the cognitive impairment found in these patients. Recent studies have shown that several of the molecules involved in neurodegenerative disorders are in fact synaptic proteins with amyloidogenic potential (SPWAP). Here we propose a unified theory to explain the neurodegenerative process in AD based on the idea that abnormal folding and/or aggregation of these molecules leads to cell death. The most important predictions of this hypothesis are that: (1) there are other yet unknown SPWAP that might be involved in AD, and their identity can be predicted by defining what makes a protein amyloidogenic; (2) there are endogenous anti-amyloidogenic molecules that regulate the aggregation state of SPWAP; and (3) there might be forms of the disease associated with decreased production of endogenous anti-amyloidogenic molecules or with unbalance of pro- versus anti-amyloidogenic factors.

Mice with combined gene knock-outs reveal essential and partially redundant functions of amyloid precursor protein family members

The amyloid precursor protein (APP) involved in Alzheimer's disease is a member of a larger gene family including amyloid precursor-like proteins APLP1 and APLP2. We generated and examined the phenotypes of mice lacking individual or all possible combinations of APP family members to assess potential functional redundancies within the gene family. Mice deficient for the nervous system-specific APLP1 protein showed a postnatal growth deficit as the only obvious abnormality. In contrast to this minor phenotype, APLP2(-/-)/APLP1(-/-) and APLP2(-/-)/APP(-/-) mice proved lethal early postnatally. Surprisingly, APLP1(-/-)/APP(-/-) mice were viable, apparently normal, and showed no compensatory upregulation of APLP2 expression. These data indicate redundancy between APLP2 and both other family members and corroborate a key physiological role for APLP2. This view gains further support by the observation that APLP1(-/-)/APP(-/-)/APLP2(+/-) mice display postnatal lethality. In addition, they provide genetic evidence for at least some distinct physiological roles of APP and APLP2 by demonstrating that combinations of single knock-outs with the APLP1 mutation resulted in double mutants of clearly different phenotypes, being either lethal, or viable. None of the lethal double mutants displayed, however, obvious histopathological abnormalities in the brain or any other organ examined. Moreover, cortical neurons from single or combined mutant mice showed unaltered survival rates under basal culture conditions and unaltered susceptibility to glutamate excitotoxicity in vitro.

Alzheimer's disease: a dysfunction of the amyloid precursor protein(1)

In this review, we argue that at least one insult that causes Alzheimer's disease (AD) is disruption of the normal function of the amyloid precursor protein (APP). Familial Alzheimer's disease (FAD) mutations in APP cause a disease phenotype that is identical (with the exception that they cause an earlier onset of the disease) to that of 'sporadic' AD. This suggests that there are molecular pathways common to FAD and sporadic AD. In addition, all individuals with Down syndrome, who carry an extra copy of chromosome 21 and overexpress APP several-fold in the brain, develop the pathology of AD if they live past the age of 40. These data support the primacy of APP in the disease. Although APP is the source of the beta-amyloid (Abeta) that is deposited in amyloid plaques in AD brain, the primacy of APP in AD may not lie in the production of Abeta from this molecule. We suggest instead that APP normally functions in the brain as a cell surface signaling molecule, and that a disruption of this normal function of APP is at least one cause of the neurodegeneration and consequent dementia in AD. We hypothesize in addition that disruption of the normal signaling function of APP causes cell cycle abnormalities in the neuron, and that these abnormalities constitute one mechanism of neuronal death in AD. Data supporting these hypotheses have come from investigations of the molecular consequences of neuronal expression of FAD mutants of APP or overexpression of wild type APP, as well as from identification of binding proteins for the carboxyl-terminus (C-terminus) of APP.

Astroglial expression of human alpha(1)-antichymotrypsin enhances alzheimer-like pathology in amyloid protein precursor transgenic mice

Proteases and their inhibitors play key roles in physiological and pathological processes. Cerebral amyloid plaques are a pathological hallmark of Alzheimer's disease (AD). They contain amyloid-ss (Ass) peptides in tight association with the serine protease inhibitor alpha(1)-antichymotrypsin.(1,2) However, it is unknown whether the increased expression of alpha(1)-antichymotrypsin found in AD brains counteracts or contributes to the disease. We used regulatory sequences of the glial fibrillary acidic protein gene(3) to express human alpha(1)-antichymotrypsin (hACT) in astrocytes of transgenic mice. These mice were crossed with transgenic mice that produce human amyloid protein precursors (hAPP) and Ass in neurons.(4,5) No amyloid plaques were found in transgenic mice expressing hACT alone, whereas hAPP transgenic mice and hAPP/hACT doubly transgenic mice developed typical AD-like amyloid plaques in the hippocampus and neocortex around 6 to 8 months of age. Co-expression of hAPP and hACT significantly increased the plaque burden at 7 to 8, 14, and 20 months. Both hAPP and hAPP/hACT mice showed significant decreases in synaptophysin-immunoreactive presynaptic terminals in the dentate gyrus, compared with nontransgenic littermates. Our results demonstrate that hACT acts as an amyloidogenic co-factor in vivo and suggest that the role of hACT in AD is pathogenic.

Dominant negative effects of apolipoprotein E4 revealed in transgenic models of neurodegenerative disease

Apolipoprotein E fulfills fundamental functions in lipid transport and neural tissue repair after injury.(6,8) Its three most common isoforms (E2, E3, and E4) are critical determinants of diverse human diseases, including major cardiovascular and neurodegenerative disorders.(8,14) Apolipoprotein E4 is associated with an increased risk for Alzheimer's disease(3,5) and poor clinical outcome after head injury or stroke.(11,16) The precise role of apolipoprotein E4 in these conditions remains unknown. To characterize the effects of human apolipoprotein E isoforms in vivo, we analysed transgenic Apoe knockout mice that express apolipoprotein E3 or E4 or both in the brain. Hemizygous and homozygous apolipoprotein E3 mice were protected against age-related and excitotoxin-induced neurodegeneration, whereas apolipoprotein E4 mice were not. Apolipoprotein E3/E4 bigenic mice were as susceptible to neurodegeneration as apolipoprotein E4 singly-transgenic mice. At eight months of age neurodegeneration was more severe in homozygous than in hemizygous apolipoprotein E4 mice consistent with a dose effect. Thus, apolipoprotein E4 is not only less neuroprotective than apolipoprotein E3 but also acts as a dominant negative factor that interferes with the beneficial function of apolipoprotein E3. The inhibition of this apolipoprotein E4 activity may be critical for the prevention and treatment of neurodegeneration in APOE varepsilon4 carriers.

Age-related amyloid beta deposition in transgenic mice overexpressing both Alzheimer mutant presenilin 1 and amyloid beta precursor protein Swedish mutant is not associated with global neuronal loss

To analyze the relationship between the deposition of amyloid beta peptides (Abeta) and neuronal loss in transgenic models of Alzheimer's disease (AD), we examined the frontal neocortex (Fc) and CA1 portion of hippocampus (CA1) in PSAPP mice doubly expressing AD-associated mutant presenilin 1 (PS1) and Swedish-type mutant beta amyloid precursor protein (APPsw) by morphometry of Abeta burden and neuronal counts. Deposition of Abeta was detected as early as 3 months of age in the Fc and CA1 of PSAPP mice and progressed to cover 28.3% of the superior frontal cortex and 18.4% of CA1 at 12 months: approximately 20- (Fc) and approximately 40- (CA1) fold greater deposition than in APPsw mice. There was no significant difference in neuronal counts in either CA1 or the frontal cortex between nontransgenic (non-tg), PS1 transgenic, APPsw, and PSAPP mice at 3 to 12 months of age. In the PSAPP mice, there was disorganization of the neuronal architecture by compact amyloid plaques, and the average number of neurons was 8 to 10% fewer than the other groups (NS, P > 0.10) in CA1 and 2 to 20% fewer in frontal cortex (NS, P = 0.31). There was no loss of total synaptophysin immunoreactivity in the Fc or dentate gyrus molecular layer of the 12-month-old PSAPP mice. Thus, although co-expression of mutant PS1 with Swedish mutant betaAPP leads to marked cortical and limbic Abeta deposition in an age-dependent manner, it does not result in the dramatic neuronal loss in hippocampus and association cortex characteristic of AD.

High-level neuronal expression of abeta 1-42 in wild-type human amyloid protein precursor transgenic mice: synaptotoxicity without plaque formation

Amyloid plaques are a neuropathological hallmark of Alzheimer's disease (AD), but their relationship to neurodegeneration and dementia remains controversial. In contrast, there is a good correlation in AD between cognitive decline and loss of synaptophysin-immunoreactive (SYN-IR) presynaptic terminals in specific brain regions. We used expression-matched transgenic mouse lines to compare the effects of different human amyloid protein precursors (hAPP) and their products on plaque formation and SYN-IR presynaptic terminals. Four distinct minigenes were generated encoding wild-type hAPP or hAPP carrying mutations that alter the production of amyloidogenic Abeta peptides. The platelet-derived growth factor beta chain promoter was used to express these constructs in neurons. hAPP mutations associated with familial AD (FAD) increased cerebral Abeta(1-42) levels, whereas an experimental mutation of the beta-secretase cleavage site (671(M-->I)) eliminated production of human Abeta. High levels of Abeta(1-42) resulted in age-dependent formation of amyloid plaques in FAD-mutant hAPP mice but not in expression-matched wild-type hAPP mice. Yet, significant decreases in the density of SYN-IR presynaptic terminals were found in both groups of mice. Across mice from different transgenic lines, the density of SYN-IR presynaptic terminals correlated inversely with Abeta levels but not with hAPP levels or plaque load. We conclude that Abeta is synaptotoxic even in the absence of plaques and that high levels of Abeta(1-42) are insufficient to induce plaque formation in mice expressing wild-type hAPP. Our results support the emerging view that plaque-independent Abeta toxicity plays an important role in the development of synaptic deficits in AD and related conditions.

Transgenic mouse models of Alzheimer's disease

Alzheimer's disease (AD) pathology is characterized by A beta peptide-containing plaques, neurofibrillary tangles consisting of hyperphosphorylated tau, extensive neuritic degeneration, and distinct neuron loss. We generated several transgenic mouse lines expressing the human amyloid precursor protein (APP751) containing the AD-linked KM670/671NL double mutation (Swedish mutation) under the control of a neuron-specific Thy-1 promoter fragment. In the best APP-expressing line (APP23), compact A beta deposits can be detected at 6 months of age. These plaques dramatically increase with age, are mostly Congo Red positive, and accumulate typical plaque-associated proteins such as heparansulfate proteoglycan and apolipoprotein E. Activated astrocytes and microglia indicative of inflammatory processes reminiscent of AD accumulate around the deposits. Furthermore, plaques are surrounded by enlarged dystrophic neurites as visualized by neurofilament or Holmes-Luxol staining. Strong staining for acetylcholinesterase activity is found throughout the plaques and is accompanied by local distortion of the cholinergic fiber network. All congophilic plaques contain hyperphosphorylated tau reminiscent of early tau pathology. Modern stereologic methods demonstrate a significant loss of neurons in the hippocampal CA1 region, correlating with an increasing A beta plaque load. Interestingly, APP23 mice develop cerebral amyloid angiopathy in addition to amyloid plaques even though the APP transgene is only expressed in neurons. Crossbreeding of APP23 mice with transgenic mice carrying AD-linked presenilin mutations but not wild-type presenilin resulted in enhanced formation of pathology. In conclusion, our APP transgenic mice present many pathologic features, similar to those observed in AD and therefore offer excellent tools for studying the contribution of A beta to AD pathogenesis.

Abnormal glutamate transport function in mutant amyloid precursor protein transgenic mice

Recent studies have shown that amyloid precursor protein (APP), which plays a central role in Alzheimer's disease (AD), protects against excitotoxic neuronal injuries by regulating the function of the glial glutamate transporters. The mechanisms underlying these effects and their relationship to the neurodegenerative process in AD are under intense scrutiny. In this context, the main objective of the present study was to determine if overexpression of mutant human APP in transgenic mouse brains results in altered functioning of the excitatory amino acid transporters (EAATs). Transgenic mice expressing the 695 amino acid form of the human APP from the Thy-1 promoter showed a significant decrease in B(max) and K(D) for aspartate uptake when compared to nontransgenic controls. This decrease in glutamate transporter activity was associated with decreased protein expression of glial specific glutamate transporters, EAAT1 and 2, but did not affect mRNA levels. These results suggest that expression of mutant forms of APP disturbs astroglial transport of excitatory amino acids at the posttranscriptional level leading, in turn, to increased susceptibility to glutamate toxicity.

The beta-amyloid precursor protein and its derivatives: from biology to learning and memory processes

Intensive investigation towards the understanding of the biology and physiological functions of the beta-amyloid precursor protein (APP) have been supported since it is known that a 39-43 amino acid fragment of APP, called the beta-amyloid protein (Abeta), accumulates in the brain parenchyma to form the typical lesions associated with Alzheimer's disease (AD). It emerges from extensive data that APP and its derivatives show a wide range of contrasting physiological properties and therefore might be involved in distinct physiological functions. Abeta has been shown to disrupt neuronal activity and to demonstrate neurotoxic properties in a wide range of experimental procedures. In contrast, both in vitro and in vivo studies suggest that APP and/or its secreted forms are important factors involved in the viability, growth and morphological and functional plasticity of nerve cells. Furthermore, several recent studies suggest that APP and its derivatives have an important role in learning and memory processes. Memory impairments can be induced in animals by intracerebral treatment with Abeta. Altered expression of the APP gene in aged animals or in genetically-modified animals also leads to memory deficits. By contrast, secreted forms of APP have recently been shown to facilitate learning and memory processes in mice. These interesting findings open novel perspectives to understand the involvement of APP in the development of cognitive deficits associated with AD. In this review, we summarize the current data concerning the biology and the behavioral effects of APP and its derivatives which may be relevant to the roles of these proteins in memory and in AD pathology.

Developmental regulation of amyloid precursor protein at the neuromuscular junction in mouse skeletal muscle

Amyloid precursor protein (APP), associated with Alzheimer's disease plaques, is known to be present in synapses of the brain and in the adult neuromuscular junction (NMJ). In the present study we examined protein and gene expression of APP during the development of mouse skeletal muscle. Using immunocytochemical approaches, we found that APP is first detected in myotube cytoplasm at embryonic day 16 and becomes progressively concentrated at the NMJ beginning at birth until adulthood. The colocalization between APP and acetylcholine receptors at the NMJ is only partial at birth, but becomes complete upon reaching adulthood. We observed that all APP isoforms, including the Kunitz-containing (protease inhibitor or KPI) forms, are up-regulated from birth to postnatal day 5 and then decreased to reach the low levels observed in the adult. This suggests the involvement of APP during the events which lead to a mature mono-innervated synapse. A 92-kDa band, characteristic of a cleaved APP695 isoform and not due to a new muscle-specific alternative spliced form, was observed from postnatal day 15 following completion of polyneuronal synapse elimination. Taken together, these data suggest that skeletal muscle APP may well play a role in the differentiation of skeletal muscle and in the formation and maturation of NMJs.

Neuroanatomical abnormalities in behaviorally characterized APP(V717F) transgenic mice

Histological analyses were performed on the brains of APP(V717F) transgenic (Tg)mice previously studied in a battery of behavioral tests. We describe here the regional and age-dependent deposition of amyloid in both heterozygous and homozygous Tg mice. We also report that Tg mice show significant and age-dependent changes in synaptic density measured by synaptophysin immunoreactivity. Surprisingly, a rather marked hippocampal atrophy is observed as early as 3 months of age in Tg mice (20-40%). Statistical analyses revealed that the deficits in object recognition memory are related to the number of amyloid deposits in specific brain regions, whereas deficits in spatial reference and working memory are related to the changes in synaptic density and hippocampal atrophy. Our study suggests that the behavioral deficits observed in Tg mice are only in part related to amyloid deposition, but are also related to neuroanatomical alterations secondary to overexpression of the APP(V717F) transgene and independent of amyloid deposition.

Expressions of amyloid precursor protein, synaptophysin and presenilin-1 in the different areas of the developing cerebellum of rat

This study reveals the expressions of Alzheimer's disease-related amyloid precursor protein, presenilin-1, and a presynaptic marker protein, synaptophysin, in the archi-, paleo- and neocerebellum during the postnatal development of the rat. The Western blot results demonstrate a gradual increase in the soluble amyloid precursor protein level in the archicerebellum during the first 3 weeks, while in the neo- and paleocerebellum the levels reach a plateau as early as the 1st week. Immunohistochemically, the protein is present in the deep part of the external granule cell layer and the internal granule cell layer in the newborn animal, while in 3-week-old animals the staining appears mainly in the perikarya and dendrites of the Purkinje cells. The level of synaptophysin increases progressively from postnatal day 7 up to 3 weeks in the archi- and paleocerebellum, and up to 6 weeks in the neocerebellum. Immunohistochemically, the amyloid precursor protein staining appears first in the inner part of the molecular layer and in the internal granule cell layer. In a 3-week-old animal, synaptophysin staining is present in all areas of the cerebellar molecular layer and in the internal granule cell layer. The presenilin-1 immunohistochemical reaction appeared equally in the archi-, paleo- and neocerebellum. Much of the staining is present in the glial cells and Purkinje cells. Less immunoreactivity is observed in the Golgi cells and granule cells. It is concluded that the postnatal expressions of soluble and membrane-bound amyloid precursor protein, synaptophysin and presenilin-1 are regulated differently during the ontogenetical development of the archi-, paleo- and neocerebellum of rat. Further, the amyloid precursor protein and presenilin-1 may be present in cells which do not degenerate in Alzheimer's disease.

Hemisynaptic distribution patterns of presenilins and beta-APP isoforms in the rodent cerebellum and hippocampus

Healthy brain neurons co-express Alzheimer's disease (AD) related proteins presenilins (PS) and beta-amyloid precursor protein (beta-APP). Deposition of beta-amyloid and PS in the senile plaques of AD brain and their ability to interact in vitro suggest that AD pathology could arise from a defect in the physiological interactions between beta-APP and PS within and/or between neurons. The present study compares the immunocytochemical distribution of PS (1 and 2) and beta-APP major isoforms (695 and 751/770) in the synapses of the cerebellum and hippocampus of the adult rat and mouse. In the cerebellar cortex of both species, the four molecules are immunodetected in the presynaptic or the postsynaptic compartments of synapses, suggesting that they are involved in interneuronal relationships. In contrast, PS and beta-APP are postsynaptic in almost all the immunoreactive synapses of the hippocampus. The different distribution patterns of these proteins in cerebellar and hippocampal synapses may reflect specific physiological differences, responsible for differential vulnerability of neurons to AD synaptic pathology. Defective interactions between beta-APP and PS at the synapses could impede the synaptic functions of beta-APP, inducing the selective loss of synapses that accounts for cognitive impairment in AD.

Corticotropin-releasing hormone-mediated neuroprotection against oxidative stress is associated with the increased release of non-amyloidogenic amyloid beta precursor protein and with the suppression of nuclear factor-kappaB

The neuropeptide CRH is the central regulator of the hypothalamic-pituitary-adrenal (HPA) stress response system and is implicated in various stress-related conditions. In the neurodegenerative disorder Alzheimer's disease (AD), levels of CRH are decreased. AD pathology is characterized by the deposition of the nonsoluble amyloid beta protein (A beta), oxidative stress, and neuronal cell death. Employing primary neurons and clonal cells, we demonstrate that CRH has a neuroprotective activity in CRH-receptor type 1 (CRH-R1)-expressing neurons against oxidative cell death. The protective effect of CRH was blocked by selective and nonselective CRH-R1 antagonists and by protein kinase A inhibitors. Overexpression of CRH-R1 in clonal hippocampal cells lacking endogenous CRH-receptors established neuroprotection by CRH. The activation of CRH-R1 and neuroprotection are accompanied by an increased release of non-amyloidogenic soluble A beta precursor protein. At the molecular level CRH caused the suppression of the DNA-binding activity and transcriptional activity of the transcription factor NF-kappaB. Suppression of NF-kappaB by overexpression of a super-repressor mutant form of IkappaB-alpha, a specific inhibitor of NF-kappaB, led to protection of the cells against oxidative stress. These data demonstrate a novel cytoprotective effect of CRH that is mediated by CRH-R1 and downstream by suppression of NF-kappaB and indicate CRH as an endogenous protective neuropeptide against oxidative cell death in addition to its function in the HPA-system. Moreover, the protective function of CRH proposes a molecular link between oxidative stress-related degenerative events and the CRH-R1 system.

Differential expression of beta-amyloid precursor and Bcl-2 proto-oncogene proteins in the developing dog retina

Previous studies of rat retinas have not only provided evidence that beta-amyloid precursor (APP) and B-cell lymphoma proto-oncogene (Bcl-2) proteins are colocalized in retinal Müller glial cells, but have also indicated that common mechanisms regulate their expression in these cells (Chen, S.T., Garey, L.J., Jen, L.S., 1994. Bcl-2 proto-oncogene protein immunoreactivity in normally developing and axotomised rat retinas. Neurosci. Lett. 172, 11 14; Chen, S.T., Gentleman, S.M., Garey, L.J., Jen, L.S., 1996. Distribution of beta-amyloid precursor and B-cell lymphoma proto-oncogene proteins in the rat retina after optic nerve transection or vascular lesion. J. Neuropathol. Exp. Neurol. 55, 1073-1082; Chen, S.T., Garey, L.J., Jen, L.S., 1997. Expression of beta-amyloid precursor protein immunoreactivity in the retina of the rat during normal development and after neonatal optic tract lesion. NeuroReport 8, 713-717). This investigation attempts to resolve whether or not the pattern observed in rats also applies to other higher mammalian species by examining the expression of immunoreactivity to APP and Bcl-2 in developing as well as mature dog retinas using immunocytochemical methods. Experimental results indicate that the immunoreactivity of both APP and Bcl-2 is located primarily in the inner retina, particularly in the ganglion cells and their axons in late fetal and neonatal stages. From the second postnatal week (the time of eye opening) onwards, immunoreactivity to APP, but not Bcl-2, is localized primarily in the endfeet and proximal part of the radial process of retinal Müller glial cells. Although the findings show both APP and Bcl-2 are expressed in ganglion cells and their processes suggest that the molecules have a role in the differentiation of neurons in the central nervous tissue, the lack of Bcl-2 in the Müller glial cells in dog retinas further suggests that the two molecules may have different biological roles with respect to glial function.

Expression of beta-amyloid precursor and Bcl-2 proto-oncogene proteins in rat retinas after intravitreal injection of aminoadipic acid

In order to investigate the role of glia in relation to factors that affect the expression of beta-amyloid precursor protein (betaAPP) and B cell lymphoma oncogene protein (Bcl-2) in the central nervous tissue, the patterns of expression of betaAPP and Bcl-2 in developing and mature rat retinas were studied immunocytochemically after intravitreal injection of alpha-aminoadipic acid (alpha-AAA), a glutamate analogue and gliotoxin that is known to cause injury of retinal Müller glial cells. In normal developing retinas, betaAPP and Bcl-2 were expressed primarily but transiently in a small number of neurons in the ganglion cell layer during the first postnatal week. Immunoreactivity of betaAPP and Bcl-2 appeared in the endfeet and proximal part of the radial processes of Müller glial cells from the second postnatal week onwards. In rats that received intravitreal injection of alpha-AAA at birth, there was a loss of immunoreactivity to vimentin, and a delayed expressed on betaAPP or Bcl-2 in Muller glial cells until 3-5 weeks post-injection. Immunoreactive neurons were also observed in the inner retina especially in the ganglion cell layer from 5 to 35 days after injection. A significant reduction in numerical density of cells with large somata in the ganglion cell layer was observed in the neonatally injected retinas at P56, which was accompanied by an increased immunostaining in radial processes of Müller glial cells. In contrast, no detectable changes in the expression of betaAPP and Bcl-2 were observed in retina that received alpha-AAA as adults. These results indicate that the gliotoxin alpha-AAA has long lasting effects on the expression of betaAPP and Bcl-2 in Müller glial cells as well as neurons in the developing but not mature retinas. The loss of vimentin and delayed expression of betaAPP and Bcl-2 in developing Müller glial cells suggests that the metabolic integrity of Müller cells was temporarily compromised, which may have adverse effects on developing neurons that are vulnerable or dependent on trophic support from the Müller glial cells.

Elimination of the class A scavenger receptor does not affect amyloid plaque formation or neurodegeneration in transgenic mice expressing human amyloid protein precursors

The class A scavenger receptor (SR) is expressed on reactive microglia surrounding cerebral amyloid plaques in Alzheimer's disease (AD). Interactions between the SR and amyloid beta peptides (Abeta) in microglial cultures elicit phagocytosis of Abeta aggregates and release of neurotoxins. To assess the role of the SR in amyloid clearance and Abeta-associated neurodegeneration in vivo, we used the platelet-derived growth factor promoter to express human amyloid protein precursors (hAPPs) in neurons of transgenic mice. With increasing age, hAPP mice develop AD-like amyloid plaques. We bred heterozygous hAPP (hAPP(+/-)) mice that were wild type for SR (SR(+/+)) with SR knockout (SR(-/-)) mice. Crosses among the resulting hAPP(+/-)SR(+/-) offspring yielded hAPP(+/-) and hAPP(-/-) littermates that were SR(+/+) or SR(-/-). These second-generation mice were analyzed at 6 and 12 months of age for extent of cerebral amyloid deposition and loss of synaptophysin-immunoreactive presynaptic terminals. hAPP(-/-)SR(-/-) mice showed no lack of SR expression, plaque formation, or synaptic degeneration, indicating that lack of SR expression does not result in significant accumulation of endogenous amyloidogenic or neurotoxic factors. In hAPP(+/-) mice, ablation of SR expression did not alter number, extent, distribution, or age-dependent accumulation of plaques; nor did it affect synaptic degeneration. Our results do not support a critical pathogenic role for microglial SR expression in neurodegenerative alterations associated with cerebral beta amyloidosis.

The Drosophila beta-amyloid precursor protein homolog promotes synapse differentiation at the neuromuscular junction

Although abnormal processing of beta-amyloid precursor protein (APP) has been implicated in the pathogenic cascade leading to Alzheimer's disease, the normal function of this protein is poorly understood. To gain insight into APP function, we used a molecular-genetic approach to manipulate the structure and levels of the Drosophila APP homolog APPL. Wild-type and mutant forms of APPL were expressed in motoneurons to determine the effect of APPL at the neuromuscular junction (NMJ). We show that APPL was transported to motor axons and that its overexpression caused a dramatic increase in synaptic bouton number and changes in synapse structure. In an Appl null mutant, a decrease in the number of boutons was found. Examination of NMJs in larvae overexpressing APPL revealed that the extra boutons had normal synaptic components and thus were likely to form functional synaptic contacts. Deletion analysis demonstrated that APPL sequences responsible for synaptic alteration reside in the cytoplasmic domain, at the internalization sequence GYENPTY and a putative G(o)-protein binding site. To determine the likely mechanisms underlying APPL-dependent synapse formation, hyperexcitable mutants, which also alter synaptic growth at the NMJ, were examined. These mutants with elevated neuronal activity changed the distribution of APPL at synapses and partially suppressed APPL-dependent synapse formation. We propose a model by which APPL, in conjunction with activity-dependent mechanisms, regulates synaptic structure and number.

Nicergoline stimulates protein kinase C mediated alpha-secretase processing of the amyloid precursor protein in cultured human neuroblastoma SH-SY5Y cells

We investigated the ability of the antidementia agents, nicergoline, aniracetam and hydergine to stimulate PKC mediated alpha-secretase amyloid precursor protein (APP) processing in cultured human neuroblastoma SH-SY5Y cells. Western immunoblotting of cell conditioned media using the Mabs 22C11 and 6E10 revealed the presence of 2 bands with molecular mass of 90 and 120 kDa, corresponding to possible alternatively glycosylated forms of secreted APP (APPs). Short-term (30 min and 2 h) treatment of cells with nicergoline gave an increased intensity of both bands, compared to non-treated cells. Maximal nicergoline effects, of the order of 150-200% over basal APPs release, were seen at concentrations between 1 and 10 microM. Under the same condition, 1 microM PdBu, used as a positive control, gave 500-1000% increases of basal APPs release. In contrast, aniracetam and hydergine, did not show any effect on APPs secretion. 2 h treatment with nicergoline had no effect on cellular full-length APP levels, as determined by immunoblotting of cell extracts with 22C11 and CT15 antibodies. Immunoblotting with PKC isoform specific antibodies of soluble and membrane fractions prepared from 2 h treated cells, showed that nicergoline (50 microM) and PdBu (1 microM) both induced translocation of PKC alpha, gamma and epsilon, but not PKC beta. The involvement of PKC in mediating nicergoline stimulated APPs release was also studied using specific inhibitors. 1 microM calphostin C, a broad range PKC inhibitor, significantly reduced both PdBu (1 microM) and nicergoline (10 microM) induced APPs release. In contrast, Go6976 (1 microM), a selective PKC alpha and beta1 inhibitor, as well as the cAMP-dependent protein kinase inhibitor, H89 (1 microM) were without effect. These results indicate that nicergoline can modulate alpha-secretase APP processing by a PKC dependent mechanism that is likely to involve the gamma and epsilon isoforms of this enzyme.