Sylvia E. Perez

Cholinergic system during the progression of Alzheimer’s disease: therapeutic implications

Alzheimer’s disease (AD) is characterized by a progressive phenotypic downregulation of markers within cholinergic basal forebrain (CBF) neurons, frank CBF cell loss and reduced cortical choline acetyltransferase activity associated with cognitive decline. Delaying CBF neurodegeneration or minimizing its consequences is the mechanism of action for most currently available drug treatments for cognitive dysfunction in AD. Growing evidence suggests that imbalances in the expression of NGF, its precursor proNGF and the high (TrkA) and low (p75NTR) affinity NGF receptors are crucial factors underlying CBF dysfunction in AD. Drugs that maintain a homeostatic balance between TrkA and p75NTR may slow the onset of AD. A NGF gene therapy trial reduced cognitive decline and stimulated cholinergic fiber growth in humans with mild AD. Drugs treating the multiple pathologies and clinical symptoms in AD (e.g., M1 cholinoceptor and/or galaninergic drugs) should be considered for a more comprehensive treatment approach for cholinergic dysfunction.

Distribution of estrogen receptor alpha and beta immunoreactive profiles in the postnatal rat brain

The present study was conducted to identify the localization and possible contribution of the two estrogen receptor (ER) subtypes in the rat brain at postnatal (P) days 3, 7 and 14. Evaluation of the distribution of ERalpha and ERbeta immunoreactive (ir) nuclei did not reveal gender differences at the developmental point times examined. With the exception of the cerebral cortex, the pattern of staining for these ERs was unchanged across the postnatal ages examined. The distribution of ERalpha-ir nuclei was wider than ERbeta-ir during brain development. From P3, ERbeta and ERalpha-ir nuclei were found in different regions of the cerebral cortex, basal forebrain, amygdala, thalamus, hypothalamus, mesencephalon, pons, cerebellum and medulla oblongata. In addition, ERalpha-ir nuclei were exclusively detected in the hippocampal subfields, epithalamus and in several circumventricular organs. ERalpha and ERbeta dual immunofluorescence revealed positive nuclei in the medial part of the bed nucleus of the stria terminalis, periventricular preoptic nucleus and in caudal aspects of the ventrolateral part of the ventromedial hypothalamic nucleus. Although the functional significance of the dual expression of both ERs within the same nuclei remains unknown, it is possible that ERs play different roles in gene regulation within the same cell. The presence of ERs in diverse brain regions through early postnatal periods supports a potential role for estrogens in neural differentiation.

β-Amyloid Deposition and Functional Impairment in the Retina of the APPswe/PS1ΔE9 Transgenic Mouse Model of Alzheimer’s Disease

PURPOSE To determine whether beta-amyloid (Abeta) deposition affects the structure and function of the retina of the APPswe/PS1DeltaE9 transgenic (tg) mouse model of Alzheimers disease. METHODS Retinas from 12- to 19-month old APPswe/PS1DeltaE9 tg and age-matched non-transgenic (ntg) littermates were single or double stained with thioflavine-S and antibodies against Abeta, glial fibrillar acidic protein (GFAP), microglial marker F4/80, choline acetyltransferase (ChAT), and syntaxin 1. Quantification of thioflavine-S positive plaques and retinal layer thickness was analyzed semi-quantitatively, whereas microglial cell size and levels of F4/80 immunoreactivity were evaluated using a densitometry program. Scotopic electroretinogram (ERG) recording was used to investigate retinal physiology in these mice. RESULTS Thioflavine-S positive plaques appeared at 12 months in the retinas of APPswe/PS1DeltaE9 tg mice with the majority of plaques in the outer and inner plexiform layers. Plaques were embedded in the inner plexiform layer strata displaying syntaxin 1 and ChAT. The number and size of the plaques in the retina increased with age. Plaques appeared earlier and in greater numbers in females than in male tg littermate mice. Microglial activity was significantly increased in the retinas of APPswe/PS1DeltaE9 tg mice. Although we did not detect neuronal degeneration in the retina, ERG recordings revealed a significant reduction in the amplitudes of a- and b-waves in aged APPswe/PS1DeltaE9 tg compared to ntg littermates. CONCLUSIONS The present findings suggest that Abeta deposition disrupts retinal structure and may contribute to the visual deficits seen in aged APPswe/PS1DeltaE9 tg mice. Whether Abeta is involved in other forms of age-related retinal dysfunction is unclear.

Mild cognitive impairment: pathology and mechanisms

Mild cognitive impairment (MCI) is rapidly becoming one of the most common clinical manifestations affecting the elderly. The pathologic and molecular substrate of people diagnosed with MCI is not well established. Since MCI is a human specific disorder and neither the clinical nor the neuropathological course appears to follow a direct linear path, it is imperative to characterize neuropathology changes in the brains of people who came to autopsy with a well-characterized clinical diagnosis of MCI. Herein, we discuss findings derived from clinical pathologic studies of autopsy cases who died with a clinical diagnosis of MCI. The heterogeneity of clinical MCI imparts significant challenges to any review of this subject. The pathologic substrate of MCI is equally complex and must take into account not only conventional plaque and tangle pathology but also a wide range of cellular, biochemical and molecular deficits, many of which relate to cognitive decline as well as compensatory responses to the progressive disease process. The multifaceted nature of the neuronal disconnection syndrome associated with MCI suggests that there is no single event which precipitates this prodromal stage of AD. In fact, it can be argued that neuronal degeneration initiated at different levels of the central nervous system drives cognitive decline as a final common pathway at this stage of the dementing disease process.

DHA diet reduces AD pathology in young APPswe/PS1ΔE9 transgenic mice: Possible gender effects

Epidemiological and clinical trial findings suggest that consumption of docosahexaenoic acid (DHA) lowers the risk of Alzhemiers disease (AD). We examined the effects of short‐term (3 months) DHA enriched diet on plaque deposition and synaptic deficts in forebrain of young APPswe/PS1ΔE9 transgenic (tg) and non‐transgenic (ntg) mice. Gas chromatography revealed a significant increase in DHA concomitant with a decrease of arachidonic acid in both brain and liver in mice fed with DHA. Female tg mice consumed relatively more food daily than ntg female mice, independent of diet. Plaque load was significantly reduced in the cortex, ventral hippocampus and striatum of female APPswe/PS1ΔE9 tg mice on DHA diet compared to female tg mice on control diet. Immunoblot quantitation of the APOE receptor, LR11, which is involved in APP trafficking and Aβ production, were unchanged in mice on DHA or control diets. Moreover drebrin levels were significantly increased in the hippocampus of tg mice on the DHA diet. Finally, in vitro DHA treatment prevented amyloid toxicity in cell cultures. Our findings support the concept that increased DHA consumption may play and important role in reducing brain insults in female AD patients.

Nigrostriatal Dysfunction in Familial Alzheimer's Disease-Linked APPswe/PS1ΔE9 Transgenic Mice

Alzheimers disease (AD) is often accompanied by extrapyramidal signs attributed to nigrostriatal dysfunction. The association between amyloid deposition and nigrostriatal degeneration is essentially unknown. We showed previously that the striatum and the substantia nigra of transgenic mice harboring familial AD (FAD)-linked APPswe/PS1ΔE9 mutants exhibit morphological alterations accompanied by amyloid-β (Aβ) deposition (Perez et al., 2004). In the present study, we further investigated the interaction between Aβ deposition and dopaminergic nigrostriatal dysfunction, by correlating morphological and biochemical changes in the nigrostriatal pathway with amyloid deposition pathology in the brains of 3- to 17-month-old APPswe/PS1ΔE9 transgenic mice and age-matched wild-type controls. We show that Aβ deposition is pronounced in the striatum of APPswe/PS1ΔE9 mice at 6 months of age, and the extent of deposition increases in an age-dependent manner. Tyrosine hydroxylase (TH)-positive dystrophic neurites with rosette or grape-like cluster disposition are observed adjacent to Aβ plaques and display multilaminar, multivesicular, and dense-core bodies as well as mitochondria. In addition, an age-dependent increase of TH protein levels are shown in nigral cells in these mutant mice. Using HPLC analysis, we found a reduction in the dopamine metabolite DOPAC in the striatum of these mice. These findings show a close association between amyloid deposition and nigrostriatal pathology and suggest that altered FAD-linked amyloid metabolism impairs, at least in part, the function of dopaminergic neurons.

Hippocampal ProNGF Signaling Pathways and β-Amyloid Levels in Mild Cognitive Impairment and Alzheimer Disease

Abstract Hippocampal precursor of nerve growth factor (proNGF)/NGF signaling occurs in conjunction with &bgr;-amyloid (A&bgr;) accumulations in Alzheimer disease (AD). To assess the involvement of this pathway in AD progression, we quantified these proteins and their downstream pathway activators in postmortem tissues from the brains of subjects with no cognitive impairment (NCI), mild cognitive impairment (MCI), and AD using immunoblotting and ELISA. Hippocampal proNGF was significantly greater in AD cases compared with those in NCI and MCI cases. TrkA was significantly reduced in MCI compared with those in NCI and AD, whereas p75 neurotrophin receptor, sortilin, and neurotrophin receptor homolog 2 remained stable. Akt decreased from NCI to MCI to AD, whereas phospho-Akt and phospho-Akt–to–Akt ratio were elevated in AD compared with those in MCI and NCI. No differences were found in phospho-Erk, Erk, or their ratio across groups. Although c-jun kinase (JNK) remained stable across groups, phospho-JNK and the phospho-JNK–to–JNK ratio increased significantly in AD compared with those in NCI and MCI. Expression levels of A&bgr;1–40, A&bgr;1–42, and A&bgr;40/42 ratio were stable. Statistical analysis revealed a strong positive correlation between proNGF and phospho-JNK, although only proNGF was negatively correlated with cognitive function and only TrkA was negatively associated with pathologic criteria. These findings suggest that alterations in the hippocampal NGF signaling pathway in MCI and AD favor proNGF-mediated proapoptotic pathways, and that this is independent of A&bgr; accumulation during AD progression.

Staging of Alzheimer's pathology in triple transgenic mice: a light and electron microscopic analysis.

The age-related pathological cascade underlying intraneuronal tau formation in 3xTg-AD mice, which harbor the human APPSwe, PS1M126V , and TauP301L gene mutations, remains unclear. At 3 weeks of age, AT180, Alz50, MC1, AT8, and PHF-1 intraneuronal immunoreactivity appeared in the amygdala and hippocampus and at later ages in the cortex of 3xTg-AD mice. AT8 and PHF-1 staining was fixation dependent in young mutant mice. 6E10 staining was seen at all ages. Fluorescent immunomicroscopy revealed CA1 neurons dual stained for 6E10 and Alz50 and single Alz50 immunoreactive neurons in the subiculum at 3 weeks and continuing to 20 months. Although electron microscopy confirmed intraneuronal cytoplasmic Alz50, AT8, and 6E10 reaction product in younger 3xTg-AD mice, straight filaments appeared at 23 months of age in female mice. The present data suggest that other age-related biochemical mechanisms in addition to early intraneuronal accumulation of 6E10 and tau underlie the formation of tau filaments in 3xTg-AD mice.

Cholinotrophic basal forebrain system alterations in 3xTg-AD transgenic mice

The cholinotrophic system, which is dependent upon nerve growth factor and its receptors for survival, is selectively vulnerable in Alzheimers disease (AD). But, virtually nothing is known about how this deficit develops in relation to the hallmark lesions of this disease, amyloid plaques and tau containing neurofibrillary tangles. The vast majority of transgenic models of AD used to evaluate the effect of beta amyloid (Aβ) deposition upon the cholinotrophic system over-express the amyloid precursor protein (APP). However, nothing is known about how this system is affected in triple transgenic (3xTg)-AD mice, an AD animal model displaying Aβ plaque- and tangle-like pathology in the cortex and hippocampus, which receive extensive cholinergic innervation. We performed a detailed morphological and biochemical characterization of the cholinotrophic system in young (2-4 months), middle-aged (13-15 months) and old (18-20 months) 3xTg-AD mice. Cholinergic neuritic swellings increased in number and size with age, and were more conspicuous in the hippocampal-subicular complex in aged female than in 3xTg-AD male mice. Stereological analysis revealed a reduction in choline acetyltransferase (ChAT) positive cells in the medial septum/vertical limb of the diagonal band of Broca in aged 3xTg-AD mice. ChAT enzyme activity levels decreased significantly in the hippocampus of middle-aged 3xTg-AD mice compared to age-matched non-transgenic (or wild type) mice. ProNGF protein levels increased in the cortex of aged 3xTg-AD mice, whereas TrkA protein levels were reduced in a gender-dependent manner in aged mutant mice. In contrast, p75(NTR) protein cortical levels were stable but increased in the hippocampus of aged 3xTg-AD mice. These data demonstrate that cholinotrophic alterations in 3xTg-AD mice are age- and gender-dependent and more pronounced in the hippocampus, a structure more severely affected by Aβ plaque pathology.

Hippocampal plasticity during the progression of Alzheimer's disease.

Neuroplasticity involves molecular and structural changes in central nervous system (CNS) throughout life. The concept of neural organization allows for remodeling as a compensatory mechanism to the early pathobiology of Alzheimers disease (AD) in an attempt to maintain brain function and cognition during the onset of dementia. The hippocampus, a crucial component of the medial temporal lobe memory circuit, is affected early in AD and displays synaptic and intraneuronal molecular remodeling against a pathological background of extracellular amyloid-beta (Aβ) deposition and intracellular neurofibrillary tangle (NFT) formation in the early stages of AD. Here we discuss human clinical pathological findings supporting the concept that the hippocampus is capable of neural plasticity during mild cognitive impairment (MCI), a prodromal stage of AD and early stage AD.