At the interface of sensory and motor dysfunctions and Alzheimer's disease.

Recent evidence indicates that sensory and motor changes may precede the cognitive symptoms of Alzheimer's disease (AD) by several years and may signify increased risk of developing AD. Traditionally, sensory and motor dysfunctions in aging and AD have been studied separately. To ascertain the evidence supporting the relationship between age-related changes in sensory and motor systems and the development of AD and to facilitate communication between several disciplines, the National Institute on Aging held an exploratory workshop titled "Sensory and Motor Dysfunctions in Aging and AD." The scientific sessions of the workshop focused on age-related and neuropathologic changes in the olfactory, visual, auditory, and motor systems, followed by extensive discussion and hypothesis generation related to the possible links among sensory, cognitive, and motor domains in aging and AD. Based on the data presented and discussed at this workshop, it is clear that sensory and motor regions of the central nervous system are affected by AD pathology and that interventions targeting amelioration of sensory-motor deficits in AD may enhance patient function as AD progresses.

γ-secretase binding sites in aged and Alzheimer's disease human cerebrum: the choroid plexus as a putative origin of CSF Aß.

Deposition of ß -amyloid (Aß) peptides, cleavage products of ß-amyloid precursor protein (APP) by ß-secretase-1 (BACE1) and γ-secretase, is a neuropathological hallmark of Alzheimer's disease (AD). γ-Secretase inhibition is a therapeutical anti-Aß approach, although changes in the enzyme's activity in AD brain are unclear. Cerebrospinal fluid (CSF) Aß peptides are thought to derive from brain parenchyma and thus may serve as biomarkers for assessing cerebral amyloidosis and anti-Aß efficacy. The present study compared active γ-secretase binding sites with Aß deposition in aged and AD human cerebrum, and explored the possibility of Aß production and secretion by the choroid plexus (CP). The specific binding density of [(3) H]-L-685,458, a radiolabeled high-affinity γ-secretase inhibitor, in the temporal neocortex and hippocampal formation was similar for AD and control cases with similar ages and post-mortem delays. The CP in post-mortem samples exhibited exceptionally high [(3) H]-L-685,458 binding density, with the estimated maximal binding sites (Bmax) reduced in the AD relative to control groups. Surgically resected human CP exhibited APP, BACE1 and presenilin-1 immunoreactivity, and ß-site APP cleavage enzymatic activity. In primary culture, human CP cells also expressed these amyloidogenic proteins and released Aß40 and Aß42 into the medium. Overall, our results suggest that γ-secretase activity appears unaltered in the cerebrum in AD and is not correlated with regional amyloid plaque pathology. The CP appears to be a previously unrecognised non-neuronal contributor to CSF Aß, probably at reduced levels in AD.

Long-term effects of estradiol replacement in the olfactory system.

Olfactory dysfunction often precedes other clinical symptoms in chronic neurodegenerative diseases like Alzheimer's disease and Parkinson's disease. Estrogen deficiency and apoE genotype are known risk factors in these diseases and these factors also affect olfaction. Therefore we examined the effects of estradiol replacement following ovariectomy on expression of apoE and markers of cell proliferation, neuronal maturation, synaptogenesis and reactive gliosis in the primary olfactory pathway of wild-type (WT) and apoE knockout (KO) mice. Estradiol replacement increased apoE staining in the olfactory nerve and glomerular layers. Estradiol increased astrocyte density and olfactory epithelium (OE) thickness regardless of the genotype. In addition estradiol treatment increased the number of mature neurons in the OE and glomerular synaptophysin in both genotypes, but the magnitude of increase was greater in the WT than in the KO mice. These data suggest that estrogen and apoE act synergistically to minimize the loss of mature sensory neurons and synapses following ovariectomy.

BACE1 elevation is involved in amyloid plaque development in the triple transgenic model of Alzheimer's disease: differential Aß antibody labeling of early-onset axon terminal pathology.

ß-amyloid precursor protein (APP) and presenilins mutations cause early-onset familial Alzheimer's disease (FAD). Some FAD-based mouse models produce amyloid plaques, others do not. ß-Amyloid (Aß) deposition can manifest as compact and diffuse plaques; it is unclear why the same Aß molecules aggregate in different patterns. Is there a basic cellular process governing Aß plaque pathogenesis? We showed in some FAD mouse models that compact plaque formation is associated with a progressive axonal pathology inherent with increased expression of ß-secretase (BACE1), the enzyme initiating the amyloidogenic processing of APP. A monoclonal Aß antibody, 3D6, visualized distinct axon terminal labeling before plaque onset. The present study was set to understand BACE1 and axonal changes relative to diffuse plaque development and to further characterize the novel axonal Aß antibody immunoreactivity (IR), using triple transgenic AD (3xTg-AD) mice as experimental model. Diffuse-like plaques existed in the forebrain in aged transgenics and were regionally associated with increased BACE1 labeled swollen/sprouting axon terminals. Increased BACE1/3D6 IR at axon terminals occurred in young animals before plaque onset. These axonal elements were also co-labeled by other antibodies targeting the N-terminal and mid-region of Aß domain and the C-terminal of APP, but not co-labeled by antibodies against the Aß C-terminal and APP N-terminal. The results suggest that amyloidogenic axonal pathology precedes diffuse plaque formation in the 3xTg-AD mice, and that the early-onset axonal Aß antibody IR in transgenic models of AD might relate to a cross-reactivity of putative APP ß-carboxyl terminal fragments.

Is brain amyloid production a cause or a result of dementia of the Alzheimer's type?

The amyloid cascade hypothesis has guided much of the research into Alzheimer's disease (AD) over the last 25 years. We argue that the hypothesis of amyloid-ß (Aß) as the primary cause of dementia may not be fully correct. Rather, we propose that decline in brain metabolic activity, which is tightly linked to synaptic activity, actually underlies both the cognitive decline in AD and the deposition of Aß. Aß may further exacerbate metabolic decline and result in a downward spiral of cognitive function, leading to dementia. This novel interpretation can tie the disparate risk factors for dementia to a unifying hypothesis and present a roadmap for interventions to decrease the prevalence of dementia in the elderly population.

Reconstitution of the olfactory epithelium following injury in apoE-deficient mice.

ApoE, a protein component of lipoproteins, is extensively expressed in the primary olfactory pathway. Because apoE has been shown to play a vital role in nerve repair and remodeling, we hypothesized that apoE expression will increase in the injured olfactory epithelium (OE), and that apoE deficiency in apoE knockout (KO) mice will lead to delayed/incomplete reconstitution of the OE following injury. To directly test this hypothesis, we compared OE regeneration in wild-type (WT) and KO mice following injury induced by intranasal irrigation of Triton X-100. OE was collected at 0, 3, 7, 21, 42, and 56 days post lesion. The amount and distribution of apoE in the regenerating OE was measured by immunoblotting and immunohistochemistry. Rate of OE reconstitution in WT and KO mice was assessed by using three independent measures: (1) OE thickness was measured in cresyl-violet stained sections, (2) basal cell proliferation was determined by using bromodeoxyuridine (BrdU) staining, and (3) differentiation and maturation of olfactory sensory neurons were measured by immunoblotting and immunohistochemical analysis of growth associated protein (GAP) 43 and olfactory marker protein (OMP). The results revealed that apoE expression in the OE is highly regulated during the entire course of OE reconstitution post injury, and that apoE deficiency in apoE KO mice leads to delayed recovery of mature OMP(+) cells in the reconstituting OE. The data suggest that apoE production increases in the injured OE to facilitate maturation of olfactory sensory neurons.

ß-Secretase-1 elevation in aged monkey and Alzheimer's disease human cerebral cortex occurs around the vasculature in partnership with multisystem axon terminal pathogenesis and ß-amyloid accumulation.

Alzheimer's disease (AD) is the most common dementia-causing disorder in the elderly; it may be related to multiple risk factors, and is characterized pathologically by cerebral hypometabolism, paravascular ß-amyloid peptide (Aß) plaques, neuritic dystrophy, and intra-neuronal aggregation of phosphorylated tau. To explore potential pathogenic links among some of these lesions, we examined ß-secretase-1 (BACE1) alterations relative to Aß deposition, neuritic pathology and vascular organization in aged monkey and AD human cerebral cortex. Western blot analyses detected increased levels of BACE1 protein and ß-site-cleavage amyloid precursor protein C-terminal fragments in plaque-bearing human and monkey cortex relative to controls. In immunohistochemistry, locally elevated BACE1 immunoreactivity (IR) occurred in AD but not in control human cortex, with a trend for increased overall density among cases with greater plaque pathology. In double-labeling preparations, BACE1 IR colocalized with immunolabeling for Aß but not for phosphorylated tau. In perfusion-fixed monkey cortex, locally increased BACE1 IR co-existed with intra-axonal and extracellular Aß IR among virtually all neuritic plaques, ranging from primitive to typical cored forms. This BACE1 labeling localized to swollen/sprouting axon terminals that might co-express one or another neuronal phenotype markers (GABAergic, glutamatergic, cholinergic, or catecholaminergic). Importantly, these BACE1-labeled dystrophic axons resided near to or in direct contact with blood vessels. These findings suggest that plaque formation in AD or normal aged primates relates to a multisystem axonal pathogenesis that occurs in partnership with a potential vascular or metabolic deficit. The data provide a mechanistic explanation for why senile plaques are present preferentially near the cerebral vasculature.

Acute responses to estradiol replacement in the olfactory system of apoE-deficient and wild-type mice.

Epidemiological studies suggest that estrogen therapy protects against clinical expression of chronic neurological diseases. These beneficial effects of estrogen therapy are highly modified by apolipoprotein E (apoE) through an unknown mechanism. We examined the short-term effects of estradiol replacement in ovariectomized mice on apoE expression and markers for cell proliferation, reactive gliosis, neuronal maturation, and synaptogenesis in the primary olfactory pathway of wild-type (WT) and apoE knockout (KO) mice. Three days of estradiol replacement increased apoE expression in the olfactory nerve and in the glomerular layer. Estradiol treatment also increased cell proliferation, total cell numbers, number of mature neurons in the olfactory epithelium, and reactive astrocyte numbers in the olfactory bulb (OB) in both WT and KO mice. We also found that estradiol increased glomerular synaptophysin (Syn), but the magnitude of increase was potentiated by the presence of apoE. These data suggest that apoE may be necessary to elicit the complete effect of estradiol on Syn upregulation.

Functional deprivation promotes amyloid plaque pathogenesis in Tg2576 mouse olfactory bulb and piriform cortex.

Cerebral hypometabolism and amyloid accumulation are principal neuropathological manifestations of Alzheimer's disease (AD). Whether and how brain/neuronal activity might modulate certain pathological processes of AD are interesting topics of recent clinical and basic research in the field, and may be of potential medical relevance in regard to both the disease etiology and intervention. Using the Tg2576 transgenic mouse model of AD, this study characterized a promotive effect of neuronal hypoactivity associated with functional deprivation on amyloid plaque pathogenesis in the olfactory pathway. Unilateral naris-occlusion caused beta-secretase-1 (BACE1) elevation in neuronal terminals in the deprived relative to the non-deprived bulb and piriform cortex in young adult mice. In parallel with the overall age-related plaque development in the forebrain, locally increased BACE1 immunoreactivity co-occurred with amyloid deposition first in the piriform cortex then within the bulb, more prominent on the deprived relative to the non-deprived side. Biochemical analyses confirmed elevated BACE1 protein levels, enzymatic activity and products in the deprived relative to non-deprived bulbs. Plaque-associated BACE1 immunoreactivity in the bulb and piriform cortex was localized preferentially to swollen/sprouting glutamatergic axonal terminals, with Abeta immunoreactivity occurring inside as well as around these terminals. Together, these findings suggest that functional deprivation or neuronal hypoactivity facilitates amyloid plaque formation in the forebrain in a transgenic model of AD, which operates synergistically with age effect. The data also implicate an intrinsic association of amyloid accumulation and plaque formation with progressive axonal pathology.

Doublecortin-expressing cells persist in the associative cerebral cortex and amygdala in aged nonhuman primates.

A novel population of cells that express typical immature neuronal markers including doublecortin (DCX+) has been recently identified throughout the adult cerebral cortex of relatively large mammals (guinea pig, rabbit, cat, monkey and human). These cells are more common in the associative relative to primary cortical areas and appear to develop into interneurons including type II nitrinergic neurons. Here we further describe these cells in the cerebral cortex and amygdala, in comparison with DCX+ cells in the hippocampal dentate gyrus, in three age groups of rhesus monkeys: young adult (12.3 +/- 0.2 years, n = 3), mid-age (21.2 +/- 1.9 years, n = 3) and aged (31.3 +/- 1.8 years, n = 4). DCX+ cells with a heterogeneous morphology persisted in layers II/III primarily over the associative cortex and amygdala in all groups (including in two old animals with cerebral amyloid pathology), showing a parallel decline in cell density with age across regions. In contrast to the cortex and amygdala, DCX+ cells in the subgranular zone diminished in the mid-age and aged groups. DCX+ cortical cells might arrange as long tangential migratory chains in the mid-age and aged animals, with apparently distorted cell clusters seen in the aged group. Cortical DCX+ cells colocalized commonly with polysialylated neural cell adhesion molecule and partially with neuron-specific nuclear protein and gamma-aminobutyric acid, suggesting a potential differentiation of these cells into interneuron phenotype. These data suggest a life-long role for immature interneuron-like cells in the associative cerebral cortex and amygdala in nonhuman primates.

Doublecortin expression in adult cat and primate cerebral cortex relates to immature neurons that develop into GABAergic subgroups.

DCX-immunoreactive (DCX+) cells occur in the piriform cortex in adult mice and rats, but also in the neocortex in adult guinea pigs and rabbits. Here we describe these cells in adult domestic cats and primates. In cats and rhesus monkeys, DCX+ cells existed across the allo- and neocortex, with an overall ventrodorsal high to low gradient at a given frontal plane. Labeled cells formed a cellular band in layers II and upper III, exhibiting dramatic differences in somal size (5-20 microm), shape (unipolar, bipolar, multipolar and irregular), neuritic complexity and labeling intensity. Cell clusters were also seen in this band, and those in the entorhinal cortex extended into deeper layers as chain-like structures. Densitometry revealed a parallel decline of the cells across regions with age in cats. Besides the cellular band, medium-sized cells with weak DCX reactivity resided sparsely in other layers. Throughout the cortex, virtually all DCX+ cells co-expressed polysialylated neural cell adhesion molecule. Medium to large mature-looking DCX+ cells frequently colocalized with neuron-specific nuclear protein and gamma-aminobutyric acid (GABA), and those with a reduced DCX expression also partially co-labeled for glutamic acid decarboxylase, parvalbumin, calbindin, beta-nicotinamide adenine dinucleotide phosphate diaphorase and neuronal nitric oxide synthase. Similar to cats and monkeys, small and larger DCX+ cells were detected in surgically removed human frontal and temporal cortices. These data suggest that immature neurons persist into adulthood in many cortical areas in cats and primates, and that these cells appear to undergo development and differentiation to become functional subgroups of GABAergic interneurons.

Beta-secretase-1 elevation in transgenic mouse models of Alzheimer's disease is associated with synaptic/axonal pathology and amyloidogenesis: implications for neuritic plaque development.

The presence of neuritic plaques is a pathological hallmark of Alzheimer's disease (AD). However, the origin of extracellular beta-amyloid peptide (Abeta) deposits and the process of plaque development remain poorly understood. The present study attempted to explore plaque pathogenesis by localizing beta-secretase-1 (BACE1) elevation relative to Abeta accumulation and synaptic/neuritic alterations in the forebrain, using transgenic mice harboring familial AD (FAD) mutations (5XFAD and 2XFAD) as models. In animals with fully developed plaque pathology, locally elevated BACE1 immunoreactivity (IR) coexisted with compact-like Abeta deposition, with BACE1 IR occurring selectively in dystrophic axons of various neuronal phenotypes or origins (GABAergic, glutamatergic, cholinergic or catecholaminergic). Prior to plaque onset, localized BACE1/Abeta IR occurred at swollen presynaptic terminals and fine axonal processes. These BACE1/Abeta-containing axonal elements appeared to undergo a continuing process of sprouting/swelling and dystrophy, during which extracellular Abeta IR emerged and accumulated in surrounding extracellular space. These data suggest that BACE1 elevation and associated Abeta overproduction inside the sprouting/dystrophic axonal terminals coincide with the onset and accumulation of extracellular amyloid deposition during the development of neuritic plaques in transgenic models of AD. Our findings appear to be in harmony with an early hypothesis that axonal pathogenesis plays a key or leading role in plaque formation.

Impact of apoE deficiency during synaptic remodeling in the mouse olfactory bulb.

In this study we examined the role of apoE on the rate of synaptic recovery in the olfactory bulb (OB) following olfactory epithelium (OE) lesioning in mice. We used both immunoblotting and immunohistochemical techniques to compare the density of OB synaptophysin (Syn, a synaptic marker) in apoE-gene deficient/knockout (KO) mice and wild-type (WT) mice following OE lesion. We found that the whole bulb concentrations of Syn, measured by immunoblotting, declined sharply following injury in both WT and KO mice during the degenerative phase (3-7 days). After this initial decline, the Syn concentration gradually increased to normal levels by 56 days in WT mice. In contrast, Syn concentration in KO mice did not recover by day 56 when Syn density in WT was essentially normal. Glomerular Syn density, measured by immunohistochemistry, found a lower density in KO mice at all time points post-lesion. This lower concentration of whole bulb Syn parallels the slower recovery of glomerular area in KO mice. The data indicate that apoE deficiency in KO mice is associated with a delayed recovery of the glomerular area and a slower recovery in Syn concentration in the OB.

Apolipoprotein E may be a critical factor in hormone therapy neuroprotection.

In this review we examine the evidence for ovarian hormone neuroprotection in chronic neurological diseases, including stroke. We propose that neuroprotection may involve the ability of estrogens to modulate apolipoprotein E (apoE) and its receptor, the low density lipoprotein receptor related protein (LRP). Results from numerous studies have demonstrated that (1) nerve regeneration is severely delayed in apoE-gene knockout (KO) mice as compared to wild-type (WT) littermates; (2) 17beta estradiol replacement in ovariectomized mice resulted in a significant increase in levels of apoE and LRP, in the olfactory bulb (OB) and other brain areas; (3) estradiol treatment increased both apoE and neurite outgrowth in cortical and olfactory neuronal cultures; and (4) estradiol treatment had no effect on neurite outgrowth in cultures deprived of apoE or in the presence of apoE4. In essence these studies suggest that apoE is a critical intermediary for the beneficial effects of 17beta estradiol on nerve repair, which can lead to functional reorganization (plasticity). Future studies of HT should evaluate the effects of apoE genotype and production estradiol on neuroprotection.

Doublecortin-expressing cells are present in layer II across the adult guinea pig cerebral cortex: partial colocalization with mature interneuron markers.

Doublecortin-immunoreactive (DCX+) cells were detected across the allo- and neo-cortical regions in the adult guinea pig cerebrum, localized to layer II specifically at its border with layer I. The density of labeled cells declined with age, whereas no apparent apoptotic activity was detectable over the cortex including layer II. DCX+ cells varied in somal size, labeling intensity, nuclear appearance, and complexity of processes. These cells were often arranged in clusters with cells of similar morphology sometimes packed tightly together. They exhibited complete colocalization with polysialylated neural cell adhesion molecule (PSA-NCAM) and neuron-specific type III beta-tubulin (TuJ1). Medium to large-sized DCX+ cells had well-developed neuritic processes, and expressed neuron-specific nuclear protein (NeuN). Large mature-looking cells with weak DCX reactivity invariably displayed heavy NeuN reactivity, implicating a transitional stage of these labeled cells. These "transitional" cells also consistently exhibited weak reactivity for gamma-aminobutyric acid (GABA), glutamate decarboxylase (GAD67), beta-nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) and neuronal nitric oxide synthase (nNOS), suggestive of them being young GABAergic/nitrinergic interneurons. Our data indicate that DCX+ cells exist widely in the adult guinea pig cerebral cortex, with a predominant localization in upper layer II. The morphological variation and differential expression of neuronal markers in these cells implicate that they might be developing neurons, and that they are probably differentiating into GABAergic interneurons. This population of cells might be involved in interneuron plasticity in the adult mammalian cerebral cortex.

[3H]-L-685,458 as a radiotracer that maps gamma-secretase complex in the rat brain: relevance to Abeta genesis and presence of active presenilin-1 components.

Gamma-secretase is a multimeric enzyme important for normal cell/neuronal proliferation, differentiation and plasticity. Determining in vivo gamma-secretase expression and activity remains a challenge because its subunit proteins can exist in immature and preassembled forms, but may execute cellular roles irrelevant to gamma-site cleavage. In this study, we characterized [3H]-L-685,458 as a radiotracer for the detection of active gamma-secretase in adult rat brain. In vitro autoradiography indicated that [3H]-L-685,458 binding was saturatable, displaceable by peptidomimetic and small molecule gamma-secretase inhibitors, and exhibited rapid association and dissociation kinetics. In cultured hippocampal slices, [3H]-L-685,458 binding density correlated with Abeta reduction following in-dish dosing of this radioligand or a non-radioactive gamma-secretase inhibitor. [3H]-L-685,458 binding sites in the adult brain were differentially distributed across regions and laminas, with heavy binding localized to the olfactory glomeruli, hippocampal CA3 and cerebellar molecular layer, and moderate binding in the cerebral cortex, amygdala and selected subcortical regions. All of these regions showed labeling for presenilin-1 N-terminal fragments (PS1-NTFs). A distinct correlation of dense binding sites with abundant presence of PS1-NTFs was verified in hippocampal mossy fiber terminals and olfactory bulb glomeruli, suggestive of a rich expression of gamma-secretase in the synapses at these locations that are characteristic of dynamic plasticity. Together, [3H]-L-685,458 is an excellent radiotracer for mapping active gamma-secretase complex, and may serve as a useful tool for studying the enzyme in vivo and in vitro.

Mitochondrial respiratory inhibition and oxidative stress elevate beta-secretase (BACE1) proteins and activity in vivo in the rat retina.

Cerebral hypometabolism, oxidative stress and beta-amyloid peptide (Abeta) accumulation are key pathological events in Alzheimer's disease (AD). Beta-secretase (BACE, i.e., BACE1), a prerequisite for Abeta genesis, is elevated in sporadic AD. Recent studies show BACE upregulation in experimental conditions likely associated with energy insufficiency and/or oxidative stress. We investigated the effect of sublethal doses of mitochondrial respiratory inhibitors and potential endogenous oxidative substances on BACE expression in vivo using the retina as a model. Retinas were analyzed biochemically and anatomically 48 h following intraocular applications of mitochondrial complex I, II and IV inhibitors including rotenone, 3-nitropropionic acid and sodium azide, and plaque-containing oxidants including Fe(3+) and Abeta42 fibrils (Abeta42f). All agents caused elevations of BACE proteins and beta-site amyloid precursor protein (APP) cleavage product, beta-CTF, in retinal lysates in a dose-dependant manner. BACE activity and Abeta40 levels were also increased in agent-treated retinas relative to vehicle controls. BACE immunoreactivity in normal adult rat retina was present mostly in the plexiform layers, indicating a localization of the enzyme to synaptic terminals. No apparent change in laminar or cellular distribution of BACE labeling was detected in the experimental retinas. However, signs of neuronal stress including glial activation were observed in agent-treated retinas especially in high dosage groups. Our data suggest that mitochondrial respiratory inhibition and oxidative stress facilitate BACE expression in vivo. In addition, plaque constituents such as Fe(3+) and Abeta42f may participate in a self-enforcing cycle of amyloidogenesis via BACE upregulation.

Estradiol replacement increases the low-density lipoprotein receptor related protein (LRP) in the mouse brain.

Numerous epidemiology studies have shown protective effects of hormone therapy (HT) on chronic neurological diseases. We have proposed that some of the neuroprotective effects of estrogen are mediated by apolipoprotein E (apoE). Polymorphisms of receptors for apoE modify the risk for dementia. To our knowledge, no reports exist showing CNS effects of estrogen replacement on members of the low-density lipoprotein receptor family. The current study focused on the effect of estradiol-17beta (E2) replacement on protein expression of two members of the receptor family, the low-density lipoprotein receptor (LDL-r) and low-density lipoprotein receptor related protein (LRP) in ovariectomized mice. Five days of E2 replacement significantly increased LRP expression in the hippocampus, olfactory bulb and neocortex but not in cerebellum. In contrast, E2 treatment decreased LDL-r protein expression in olfactory bulb. HT modification of both apoE and LRP could have wide-spread effects on cellular function given LRP's manifold signaling functions.

beta-Secretase expression in normal and functionally deprived rat olfactory bulbs: inverse correlation with oxidative metabolic activity.

Cerebral hypometabolism, mitochondrial dysfunction, and beta-amyloid peptide (Abeta) accumulation are well-characterized manifestations of Alzheimer's disease (AD). beta-Secretase (BACE) is a prerequisite for amyloidogenesis, and it is up-regulated in sporadic AD. To explore a potential in vivo mechanism by which Abeta production is modulated by neuronal activity and/or oxidative metabolism, we compared BACE expression with cytochrome c oxidase (CO) or succinic dehydrogenase (SDH) activity in normal and functionally deprived adult rat olfactory bulb. In normal bulb, BACE was expressed predominantly in the glomerular layer, but labeling intensity within individual glomeruli varied substantially. A strong negative correlation existed between BACE labeling intensity and CO or SDH activity among individual glomeruli. Unilateral naris occlusion resulted in elevated glomerular BACE labeling in the deprived bulbs relative to the nondeprived counterparts, which was correlated with decreased CO activity in the same anatomic location. Enhanced BACE labeling was confirmed by measurements of elevated protein levels, enzymatic activity, and beta-site cleavage products of amyloid precursor protein in bulb extracts. Our findings reveal a negative regulation of BACE expression by physiological neuronal activity and an intrinsic inverse correlation between BACE expression and oxidative metabolism at the first synapse on the olfactory pathway. The results point to a biological role of BACE in synapse function and plasticity as well as a potential mechanism whereby reduced neuronal activity or metabolism could lead to amyloid overproduction in synaptic terminals.

The distribution of apolipoprotein E in mouse olfactory epithelium.

Previous studies from our laboratory suggest that apolipoprotein (apoE), a lipid transporting protein, facilitates olfactory nerve regeneration. We have shown that apoE is enriched in the olfactory nerve and around the glomeruli of the olfactory bulb (OB). The studies reported herein were undertaken to identify possible sources of apoE in the olfactory epithelium (OE). Immunoblotting results revealed apoE expression in the OE of wild-type (WT) mice, but not in apoE deficient/knockout (KO) mice. Immunohistochemical studies revealed that the perikarya and processes of sustentacular (Sus) cells expressed apoE-like immunoreactivity. Minimal neuronal apoE immunostaining was seen, although apoE was observed in the interstial spaces between olfactory receptor neurons (ORN). Substantial apoE-like immunoreactivity was localized to the endfeet and terminal process of Sus cells surrounding the basal cells. Double labeling immunocytochemical studies confirmed that the cell bodies and endfeet of Sus cells expressed high levels of apoE. The endothelial cells of blood vessels were intensely stained for apoE in the lamina propria. Cells forming Bowman's gland also immunostained for apoE. The apoE staining in the nerve fascicles was less intense, but was uniformly distributed throughout the core of the nerve bundles. Heavily stained cells, probably ensheathing glia, surrounded the nerve fascicles. These results revealed that apoE is expressed in the adult OE and lamina propria at strategic locations where it could facilitate the differentiation, maturation and axonal growth of the ORN, perhaps by recycling lipids from degenerating ORN for use by growing axons.