Caterina M. Hernandez

Loss of α7 Nicotinic Receptors Enhances β-Amyloid Oligomer Accumulation, Exacerbating Early-Stage Cognitive Decline and Septohippocampal Pathology in a Mouse Model of Alzheimer's Disease

Early Alzheimers disease (AD) is marked by cholinergic hypofunction, neuronal marker loss, and decreased nicotinic acetylcholine receptor (nAChR) density from the cortex and hippocampus. α7 nAChRs expressed on cholinergic projection neurons and target regions have been implicated in neuroprotection against β-amyloid (Aβ) toxicity and maintenance of the septohippocampal phenotype. We tested the role that α7 nAChRs perform in the etiology of early AD by genetically deleting the α7 nAChR subunit from the Tg2576 mouse model for AD and assessing animals for cognitive function and septohippocampal integrity. Thus, Tg2576 mice transgenic for mutant human amyloid precursor protein (APP) were crossed with α7 nAChR knock-out mice (A7KO) to render an animal with elevated Aβ in the absence of α7 nAChRs (A7KO–APP). We found that learning and memory deficits seen in 5-month-old APP mice are more severe in the A7KO–APP animals. Analyses of animals in early-stage preplaque cognitive decline revealed signs of neurodegeneration in A7KO–APP hippocampus as well as loss of cholinergic functionality in the basal forebrain and hippocampus. These changes occurred concomitant with the appearance of a dodecameric oligomer of Aβ that was absent from all other genotypic groups, generating the hypothesis that increased soluble oligomeric Aβ may underlie additional impairment of A7KO–APP cognitive function. Thus, α7 nAChRs in a mouse model for early-stage AD appear to serve a neuroprotective role through maintenance of the septohippocampal cholinergic phenotype and preservation of hippocampal integrity possibly through influences on Aβ accumulation and oligomerization.

Research update: Alpha7 nicotinic acetylcholine receptor mechanisms in Alzheimer's disease

Aberrant amyloid-β peptide (Aβ) accumulation along with altered expression and function of nicotinic acetylcholine receptors (nAChRs) stand prominently in the etiology of Alzheimers disease (AD). Since the discovery that Aβ is bound to α7 nAChRs under many experimental settings, including post-mortem AD brain, much effort has been expended to understand the implications of this interaction in the disease milieu. This research update will review the current literature on the α7 nAChR-Aβ interaction in vitro and in vivo, the functional consequences of this interaction from sub-cellular to cognitive levels, and discuss the implications these relationships might have for AD therapies.

Prevalence, prevention, and treatment of microalbuminuria and proteinuria in children with sickle cell disease

Microalbuminuria (MA) and proteinuria (P) are believed to be precursors of sickle cell nephropathy. We analyzed our longitudinal data on MA/P in children with sickle cell disease (SS) to define the age of onset, association with age, sex, and hemoglobin, and to explore the safety and efficacy of hydroxyurea and angiotensin converting enzyme inhibitor (ACEI) therapy. Data on 191 patients with SS (ages 3 to 20 y) with a mean follow up of 2.19 years±2.05 were available. Urine MA was measured yearly with follow-up testing if abnormal. Prevalence of MA/P was 19.4%. Increasing age and lower hemoglobin levels were related to MA/P but sex was not. Microalbumin excretion normalized in 44% of patients treated with hydroxyurea and 56% of patients treated with ACEI. Hyperkalemia developed in 4 ACEI patients resulting in discontinuation of treatment in 3 children. In summary, MA/P often develops in childhood and preventive and treatment strategies for sickle cell nephropathy should be a focus of pediatric programs. Our preliminary data suggest that although both hydroxyurea and ACEI therapy may be beneficial for MA/P, hyperkalemia may limit the utility of ACEI.

Cognitive Enhancement with Rosiglitazone Links the Hippocampal PPARγ and ERK MAPK Signaling Pathways

We previously reported that the peroxisome proliferator-activated receptor γ (PPARγ) agonist rosiglitazone (RSG) improved hippocampus-dependent cognition in the Alzheimers disease (AD) mouse model, Tg2576. RSG had no effect on wild-type littermate cognitive performance. Since extracellular signal-regulated protein kinase mitogen-activated protein kinase (ERK MAPK) is required for many forms of learning and memory that are affected in AD, and since both PPARγ and ERK MAPK are key mediators of insulin signaling, the current study tested the hypothesis that RSG-mediated cognitive improvement induces a hippocampal PPARγ pattern of gene and protein expression that converges with the ERK MAPK signaling axis in Tg2576 AD mice. In the hippocampal PPARγ transcriptome, we found significant overlap between peroxisome proliferator response element-containing PPARγ target genes and ERK-regulated, cAMP response element-containing target genes. Within the Tg2576 dentate gyrus proteome, RSG induced proteins with structural, energy, biosynthesis and plasticity functions. Several of these proteins are known to be important for cognitive function and are also regulated by ERK MAPK. In addition, we found the RSG-mediated augmentation of PPARγ and ERK2 activity during Tg2576 cognitive enhancement was reversed when hippocampal PPARγ was pharmacologically antagonized, revealing a coordinate relationship between PPARγ transcriptional competency and phosphorylated ERK that is reciprocally affected in response to chronic activation, compared with acute inhibition, of PPARγ. We conclude that the hippocampal transcriptome and proteome induced by cognitive enhancement with RSG harnesses a dysregulated ERK MAPK signal transduction pathway to overcome AD-like cognitive deficits in Tg2576 mice. Thus, PPARγ represents a signaling system that is not crucial for normal cognition yet can intercede to restore neural networks compromised by AD.

Tau Immunotherapy Modulates Both Pathological Tau and Upstream Amyloid Pathology in an Alzheimer's Disease Mouse Model

In Alzheimers disease (AD), the pathological accumulation of tau appears to be a downstream effect of amyloid β protein (Aβ). However, the relationship between these two proteins and memory loss is unclear. In this study, we evaluated the specific removal of pathological tau oligomers in aged Tg2576 mice by passive immunotherapy using tau oligomer-specific monoclonal antibody. Removal of tau oligomers reversed memory deficits and accelerated plaque deposition in the brain. Surprisingly, Aβ*56 levels decreased, suggesting a link between tau and Aβ oligomers in the promotion of cognitive decline. The results suggest that tau oligomerization is not only a consequence of Aβ pathology but also a critical mediator of the toxic effects observed afterward in AD. Overall, these findings support the potential of tau oligomers as a therapeutic target for AD.

α7 nicotinic acetylcholine receptors in Alzheimer's disease: Neuroprotective, neurotrophic or both?

One of the early signs of Alzheimers disease is the impairment in hippocampus-based episodic memory function, which is improved through the enhancement of cholinergic transmission. Several studies suggest that α7 nicotinic receptor (nAChR) activation represents a useful therapeutic strategy for the cognitive impairments associated with early Alzheimers disease as the α7 subtype of nicotinic acetylcholine receptors are expressed by basal forebrain cholinergic projection neurons as well as by their targets in the hippocampus. The current model for the cholinergic deficit in Alzheimers disease posits that inappropriate accumulation of misfolded oligomeric aggregates of β-amyloid peptide leads to the dysfunction of the signaling mechanisms that support the cholinergic phenotype; this is manifested as an altered function of nicotinic acetylcholine receptors and the nerve-growth factor trophic support system that results in the loss of cholinergic markers and eventually cholinergic neurons from the basal forebrain cholinergic system. A view was confounded by the fact that α7 nAChRs and β-amyloid peptides have been shown to interact in vitro and in vivo, including human post-mortem AD brain. This review will begin with a brief overview of the basal forebrain cholinergic system, followed by a discussion of the current knowledge of the cholinergic deficit in Alzheimers disease, then a summary of the cholinergic phenotype observed in transgenic Alzheimers disease mouse models. We will also present our recent findings that support our hypothesis that the α7 nicotinic acetylcholine receptor performs both the neurotrophic and neuroprotective roles in the maintenance of the cholinergic phenotype and discusses potential mechanisms and implications for Alzheimers disease therapy.

Neutral Sphingomyelinase-2 Deficiency Ameliorates Alzheimer's Disease Pathology and Improves Cognition in the 5XFAD Mouse

Recent evidence implicates exosomes in the aggregation of Aβ and spreading of tau in Alzheimers disease. In neural cells, exosome formation can be blocked by inhibition or silencing of neutral sphingomyelinase-2 (nSMase2). We generated genetically nSMase2-deficient 5XFAD mice (fro;5XFAD) to assess AD-related pathology in a mouse model with consistently reduced ceramide generation. We conducted in vitro assays to assess Aβ42 aggregation and glial clearance with and without exosomes isolated by ultracentrifugation and determined exosome-induced amyloid aggregation by particle counting. We analyzed brain exosome content, amyloid plaque formation, neuronal degeneration, sphingolipid, Aβ42 and phospho-tau levels, and memory-related behaviors in 5XFAD versus fro;5XFAD mice using contextual and cued fear conditioning. Astrocyte-derived exosomes accelerated aggregation of Aβ42 and blocked glial clearance of Aβ42 in vitro. Aβ42 aggregates were colocalized with extracellular ceramide in vitro using a bifunctional ceramide analog preloaded into exosomes and in vivo using anticeramide IgG, implicating ceramide-enriched exosomes in plaque formation. Compared with 5XFAD mice, the fro;5XFAD mice had reduced brain exosomes, ceramide levels, serum anticeramide IgG, glial activation, total Aβ42 and plaque burden, tau phosphorylation, and improved cognition in a fear-conditioned learning task. Ceramide-enriched exosomes appear to exacerbate AD-related brain pathology by promoting the aggregation of Aβ. Reduction of exosome secretion by nSMase2 loss of function improves pathology and cognition in the 5XFAD mouse model. SIGNIFICANCE STATEMENT We present for the first time evidence, using Alzheimers disease (AD) model mice deficient in neural exosome secretion due to lack of neutral sphingomyelinase-2 function, that ceramide-enriched exosomes exacerbate AD-related pathologies and cognitive deficits. Our results provide rationale to pursue a means of inhibiting exosome secretion as a potential therapy for individuals at risk for developing AD.

PPARγ recruitment to active ERK during memory consolidation is required for Alzheimer's disease-related cognitive enhancement.

Cognitive impairment is a quintessential feature of Alzheimers disease (AD) and AD mouse models. The peroxisome proliferator-activated receptor-γ (PPARγ) agonist rosiglitazone improves hippocampus-dependent cognitive deficits in some AD patients and ameliorates deficits in the Tg2576 mouse model for AD amyloidosis. Tg2576 cognitive enhancement occurs through the induction of a gene and protein expression profile reflecting convergence of the PPARγ signaling axis and the extracellular signal-regulated protein kinase (ERK) cascade, a critical mediator of memory consolidation. We therefore tested whether PPARγ and ERK associated in protein complexes that subserve cognitive enhancement through PPARγ agonism. Coimmunoprecipitation of hippocampal extracts revealed that PPARγ and activated, phosphorylated ERK (pERK) associated in Tg2576 in vivo, and that PPARγ agonism facilitated recruitment of PPARγ to pERK during memory consolidation. Furthermore, the amount of PPARγ recruited to pERK correlated with the cognitive reserve in humans with AD and in Tg2576. Our findings implicate a previously unidentified PPARγ–pERK complex that provides a molecular mechanism for the convergence of these pathways during cognitive enhancement, thereby offering new targets for therapeutic development in AD.

Repeated exposure to chlorpyrifos leads to prolonged impairments of axonal transport in the living rodent brain

The toxicity of the class of chemicals known as the organophosphates (OP) is most commonly attributed to the inhibition of the enzyme acetylcholinesterase. However, there is significant evidence that this mechanism may not account for all of the deleterious neurologic and neurobehavioral symptoms of OP exposure, especially those associated with levels that produce no overt signs of acute toxicity. In the study described here we evaluated the effects of the commonly used OP-pesticide, chlorpyrifos (CPF) on axonal transport in the brains of living rats using manganese (Mn(2+))-enhanced magnetic resonance imaging (MEMRI) of the optic nerve (ON) projections from the retina to the superior colliculus (SC). T1-weighted MEMRI scans were evaluated at 6 and 24h after intravitreal injection of Mn(2+). As a positive control for axonal transport deficits, initial studies were conducted with the tropolone alkaloid colchicine administered by intravitreal injection. In subsequent studies both single and repeated exposures to CPF were evaluated for effects on axonal transport using MEMRI. As expected, intravitreal injection of colchicine (2.5μg) produced a robust decrease in transport of Mn(2+) along the optic nerve (ON) and to the superior colliculus (SC) (as indicated by the reduced MEMRI contrast). A single subcutaneous (s.c.) injection of CPF (18.0mg/kg) was not associated with significant alterations in the transport of Mn(2+). Conversely, 14-days of repeated s.c. exposure to CPF (18.0mg/kg/day) was associated with decreased transport of Mn(2+) along the ONs and to the SC, an effect that was also present after a 30-day (CPF-free) washout period. These results indicate that repeated exposures to a commonly used pesticide, CPF can result in persistent alterations in axonal transport in the living mammalian brain. Given the fundamental importance of axonal transport to neuronal function, these observations may (at least in part) explain some of the long term neurological deficits that have been observed in humans who have been repeatedly exposed to doses of OPs not associated with acute toxicity.

Epigenetic changes related to beta-amyloid-implications for Alzheimer's disease

Joy G. Ghosh, Denise Fabian, Ashley Mallat, Steve Ramirez, Mark Burton, Juliet Moncaster, Noel Casey, Anca Mocofanescu, Dean Hartley, Patric Stanton, Lee E. Goldstein, Molecular Aging & Development Laboratory, Boston University School of Medicine, College of Engineering, & Photonics Center, Boston, MA, USA; Rush University Medical Center, Chicago, IL, USA; New York Medical College, New York, NY, USA. Contact e-mail: joyghosh@bu.edu