Increased fatty acid metabolism and decreased glycolysis are hallmarks of metabolic reprogramming within microglia in degenerating white matter during recovery from experimental stroke.

The goal of this study was to evaluate changes in metabolic homeostasis during the first 12 weeks of recovery in a distal middle cerebral artery occlusion mouse model of stroke. To achieve this goal, we compared the brain metabolomes of ipsilateral and contralateral hemispheres from aged male mice up to 12 weeks after stroke to that of age-matched naïve and sham mice. There were 707 biochemicals detected in each sample by liquid chromatography-mass spectroscopy (LC-MS). Mitochondrial fatty acid ß-oxidation, indicated by acyl carnitine levels, was increased in stroked tissue at 1 day and 4 weeks following stroke. Glucose and several glycolytic intermediates were elevated in the ipsilateral hemisphere for 12 weeks compared to the aged naïve controls, but pyruvate was decreased. Additionally, itaconate, a glycolysis inhibitor associated with activation of anti-inflammatory mechanisms in myeloid cells, was higher in the same comparisons. Spatial transcriptomics and RNA in situ hybridization localized these alterations to microglia within the area of axonal degeneration. These results indicate that chronic metabolic differences exist between stroked and control brains, including alterations in fatty acid metabolism and glycolysis within microglia in areas of degenerating white matter for at least 12 weeks after stroke.

Post-Stroke Administration of the p75 Neurotrophin Receptor Modulator, LM11A-31, Attenuates Chronic Changes in Brain Metabolism, Increases Neurotransmitter Levels, and Improves Recovery.

The aim of this study was to test whether poststroke oral administration of a small molecule p75 neurotrophin receptor (p75NTR) modulator (LM11A-31) can augment neuronal survival and improve recovery in a mouse model of stroke. Mice were administered LM11A-31 for up to 12 weeks, beginning 1 week after stroke. Metabolomic analysis revealed that after 2 weeks of daily treatment, mice that received LM11A-31 were distinct from vehicle-treated mice by principal component analysis and had higher levels of serotonin, acetylcholine, and dopamine in their ipsilateral hemisphere. LM11A-31 treatment also improved redox homeostasis by restoring reduced glutathione. It also offset a stroke-induced reduction in glycolysis by increasing acetyl-CoA. There was no effect on cytokine levels in the infarct. At 13 weeks after stroke, adaptive immune cell infiltration in the infarct was unchanged in LM11A-31-treated mice, indicating that LM11A-31 does not alter the chronic inflammatory response to stroke at the site of the infarct. However, LM11A-31-treated mice had less brain atrophy, neurodegeneration, tau pathology, and microglial activation in other regions of the ipsilateral hemisphere. These findings correlated with improved recovery of motor function on a ladder test, improved sensorimotor and cognitive abilities on a nest construction test, and less impulsivity in an open field test. These data support small molecule modulation of the p75NTR for preserving neuronal health and function during stroke recovery. SIGNIFICANCE STATEMENT: The findings from this study introduce the p75 neurotrophin receptor as a novel small molecule target for promotion of stroke recovery. Given that LM11A-31 is in clinical trials as a potential therapy for Alzheimer's disease, it could be considered as a candidate for assessment in stroke or vascular dementia studies.

Repeated Administration of 2-Hydroxypropyl-ß-Cyclodextrin (HPßCD) Attenuates the Chronic Inflammatory Response to Experimental Stroke.

Globally, more than 67 million people are living with the effects of ischemic stroke. Importantly, many stroke survivors develop a chronic inflammatory response that may contribute to cognitive impairment, a common and debilitating sequela of stroke that is insufficiently studied and currently untreatable. 2-Hydroxypropyl-ß-cyclodextrin (HPßCD) is an FDA-approved cyclic oligosaccharide that can solubilize and entrap lipophilic substances. The goal of the present study was to determine whether the repeated administration of HPßCD curtails the chronic inflammatory response to stroke by reducing lipid accumulation within stroke infarcts in a distal middle cerebral artery occlusion mouse model of stroke. To achieve this goal, we subcutaneously injected young adult and aged male mice with vehicle or HPßCD 3 times per week, with treatment beginning 1 week after stroke. We evaluated mice at 7 weeks following stroke using immunostaining, RNA sequencing, lipidomic, and behavioral analyses. Chronic stroke infarct and peri-infarct regions of HPßCD-treated mice were characterized by an upregulation of genes involved in lipid metabolism and a downregulation of genes involved in innate and adaptive immunity, reactive astrogliosis, and chemotaxis. Correspondingly, HPßCD reduced the accumulation of lipid droplets, T lymphocytes, B lymphocytes, and plasma cells in stroke infarcts. Repeated administration of HPßCD also preserved NeuN immunoreactivity in the striatum and thalamus and c-Fos immunoreactivity in hippocampal regions. Additionally, HPßCD improved recovery through the protection of hippocampal-dependent spatial working memory and reduction of impulsivity. These results indicate that systemic HPßCD treatment following stroke attenuates chronic inflammation and secondary neurodegeneration and prevents poststroke cognitive decline.SIGNIFICANCE STATEMENT Dementia is a common and debilitating sequela of stroke. Currently, there are no available treatments for poststroke dementia. Our study shows that lipid metabolism is disrupted in chronic stroke infarcts, which causes an accumulation of uncleared lipid debris and correlates with a chronic inflammatory response. To our knowledge, these substantial changes in lipid homeostasis have not been previously recognized or investigated in the context of ischemic stroke. We also provide a proof of principle that solubilizing and entrapping lipophilic substances using HPßCD could be an effective strategy for treating chronic inflammation after stroke and other CNS injuries. We propose that using HPßCD for the prevention of poststroke dementia could improve recovery and increase long-term quality of life in stroke sufferers.

IgA natural antibodies are produced following T-cell independent B-cell activation following stroke.

Up to 30% of stroke patients experience cognitive decline within one year of their stroke. There are currently no FDA-approved drugs that can prevent post-stroke cognitive decline, in part due to a poor understanding of the mechanisms involved. We have previously demonstrated that a B-lymphocyte response to stroke, marked by IgA + cells, can cause delayed cognitive dysfunction in mice and that a similar adaptive immune response occurs in the brains of some human stroke patients that suffer from vascular dementia. The stimuli which trigger B-lymphocyte activation following stroke, and their target antigens, are still unknown. Therefore, to learn more about the mechanisms by which B-lymphocytes become activated following stroke we first characterized the temporal kinetics of the B-lymphocyte, T-lymphocyte, and plasma cell (PC) response to stroke in the brain by immunohistochemistry (IHC). We discovered that B-lymphocyte, T-lymphocyte, and plasma cell infiltration within the infarct progressively increases between 2 and 7 weeks after stroke. We then compared the B-lymphocyte response to stroke in WT, MHCII-/-, CD4-/-, and MyD88-/- mice to determine if B-lymphocytes mature into IgA + PCs through a T-lymphocyte and MyD88 dependent mechanism. Our data from a combination of IHC and flow cytometry indicate that following stroke, a population of IgA + PCs develops independently of CD4 + helper T-lymphocytes and MyD88 signaling. Subsequent sequencing of immunoglobulin genes of individual IgA + PCs present within the infarct identified a novel population of natural antibodies with few somatic mutations in complementarity-determining regions. These findings indicate that a population of IgA + PCs develops in the infarct following stroke by B-lymphocytes interacting with one or more thymus independent type 2 (TI-2) antigens, and that they produce IgA natural antibodies.

Liquefaction of the Brain following Stroke Shares a Similar Molecular and Morphological Profile with Atherosclerosis and Mediates Secondary Neurodegeneration in an Osteopontin-Dependent Mechanism.

Here we used mouse models of heart and brain ischemia to compare the inflammatory response to ischemia in the heart, a protein rich organ, to the inflammatory response to ischemia in the brain, a lipid rich organ. We report that ischemia-induced inflammation resolves between one and four weeks in the heart compared to between eight and 24 weeks in the brain. Importantly, we discovered that a second burst of inflammation occurs in the brain between four and eight weeks following ischemia, which coincided with the appearance of cholesterol crystals within the infarct. This second wave shares a similar cellular and molecular profile with atherosclerosis and is characterized by high levels of osteopontin (OPN) and matrix metalloproteinases (MMPs). In order to test the role of OPN in areas of liquefactive necrosis, OPN-/- mice were subjected to brain ischemia. We found that at seven weeks following stroke, the expression of pro-inflammatory proteins and MMPs was profoundly reduced in the infarct of the OPN-/- mice, although the number of cholesterol crystals was increased. OPN-/- mice exhibited faster recovery of motor function and a higher number of neuronal nuclei (NeuN) positive cells in the peri-infarct area at seven weeks following stroke. Based on these findings we propose that the brain liquefies after stroke because phagocytic cells in the infarct are unable to efficiently clear cholesterol rich myelin debris, and that this leads to the perpetuation of an OPN-dependent inflammatory response characterized by high levels of degradative enzymes.

Alzheimer's associated amyloid and tau deposition co-localizes with a homeostatic myelin repair pathway in two mouse models of post-stroke mixed dementia.

The goal of this study was to determine the chronic impact of stroke on the manifestation of Alzheimer's disease (AD) related pathology and behavioral impairments in mice. To accomplish this goal, we used two distinct models. First, we experimentally induced ischemic stroke in aged wildtype (wt) C57BL/6 mice to determine if stroke leads to the manifestation of AD-associated pathological ß-amyloid (Aß) and tau in aged versus young adult wt mice. Second, we utilized a transgenic (Tg) mouse model of AD (hAPP-SL) to determine if stroke leads to the worsening of pre-existing AD pathology, as well as the development of pathology in brain regions not typically expressed in AD Tg mice. In the wt mice, there was delayed motor recovery and an accelerated development of cognitive deficits in aged mice compared to young adult mice following stroke. This corresponded with increased brain atrophy, increased cholinergic degeneration, and a focal increase of Aß in areas of axonal degeneration in the ipsilateral hemisphere of the aged animals. By contrast, in the hAPP-SL mice, we found that ischemia induced aggravated behavioral deficits in conjunction with a global increase in Aß, tau, and cholinergic pathology compared to hAPP-SL mice that underwent a sham stroke procedure. With regard to a potential mechanism, in both models, we found that the stroke-induced Aß and tau deposits co-localized with increased levels of ß-secretase 1 (BACE1), along with its substrate, neuregulin 1 (NGR1) type III, both of which are proteins integral for myelin repair. Based on these findings, we propose that the chronic sequelae of stroke may be ratcheting-up a myelin repair pathway, and that the consequent increase in BACE1 could be causing an inadvertent cleavage of its alternative substrate, AßPP, resulting in greater Aß seeding and pathogenesis.

Gut Microbiota Contributes to Resistance Against Pneumococcal Pneumonia in Immunodeficient Rag-/- Mice.

Streptococcus pneumoniae causes infection-related mortality worldwide. Immunocompromised individuals, including young children, the elderly, and those with immunodeficiency, are especially vulnerable, yet little is known regarding S. pneumoniae-related pathogenesis and protection in immunocompromised hosts. Recently, strong interest has emerged in the gut microbiota's impact on lung diseases, or the "gut-lung axis." However, the mechanisms of gut microbiota protection against gut-distal lung diseases like pneumonia remain unclear. We investigated the role of the gut commensal, segmented filamentous bacteria (SFB), against pneumococcal pneumonia in immunocompetent and immunocompromised mouse models. For the latter, we chose the Rag-/- model, with adaptive immune deficiency. Immunocompetent adaptive protection against S. pneumoniae infection is based on antibodies against pneumococcal capsular polysaccharides, prototypical T cell independent-II (TI-II) antigens. Although SFB colonization enhanced TI-II antibodies in C57BL/6 mice, our data suggest that SFB did not further protect these immunocompetent animals. Indeed, basal B cell activity in hosts without SFB is sufficient for essential protection against S. pneumoniae. However, in immunocompromised Rag-/- mice, we demonstrate a gut-lung axis of communication, as SFB influenced lung protection by regulating innate immunity. Neutrophil resolution is crucial to recovery, since an unchecked neutrophil response causes severe tissue damage. We found no early neutrophil recruitment differences between hosts with or without SFB; however, we observed a significant drop in lung neutrophils in the resolution phase of S. pneumoniae infection, which corresponded with lower CD47 expression, a molecule that inhibits phagocytosis of apoptotic cells, in SFB-colonized Rag-/- mice. SFB promoted a shift in lung neutrophil phenotype from inflammatory neutrophils expressing high levels of CD18 and low levels of CD62L, to pro-resolution neutrophils with low CD18 and high CD62L. Blocking CD47 in SFB(-) mice increased pro-resolution neutrophils, suggesting CD47 down-regulation may be one neutrophil-modulating mechanism SFB utilizes. The SFB-induced lung neutrophil phenotype remained similar with heat-inactivated S. pneumoniae treatment, indicating these SFB-induced changes in neutrophil phenotype during the resolution phase are not simply secondary to better bacterial clearance in SFB(+) than SFB(-) mice. Together, these data demonstrate that the gut commensal SFB may provide much-needed protection in immunocompromised hosts in part by promoting neutrophil resolution post lung infection.

Glial scars are permeable to the neurotoxic environment of chronic stroke infarcts.

Following stroke, the damaged tissue undergoes liquefactive necrosis, a stage of infarct resolution that lasts for months although the exact length of time is currently unknown. One method of repair involves reactive astrocytes and microglia forming a glial scar to compartmentalize the area of liquefactive necrosis from the rest of the brain. The formation of the glial scar is a critical component of the healing response to stroke, as well as other central nervous system (CNS) injuries. The goal of this study was to evaluate the toxicity of the extracellular fluid present in areas of liquefactive necrosis and determine how effectively it is segregated from the remainder of the brain. To accomplish this goal, we used a mouse model of stroke in conjunction with an extracellular fluid toxicity assay, fluorescent and electron microscopy, immunostaining, tracer injections into the infarct, and multiplex immunoassays. We confirmed that the extracellular fluid present in areas of liquefactive necrosis following stroke is toxic to primary cortical and hippocampal neurons for at least 7 weeks following stroke, and discovered that although glial scars are robust physical and endocytic barriers, they are nevertheless permeable. We found that molecules present in the area of liquefactive necrosis can leak across the glial scar and are removed by a combination of paravascular clearance and microglial endocytosis in the adjacent tissue. Despite these mechanisms, there is delayed atrophy, cytotoxic edema, and neuron loss in regions adjacent to the infarct for weeks following stroke. These findings suggest that one mechanism of neurodegeneration following stroke is the failure of glial scars to impermeably segregate areas of liquefactive necrosis from surviving brain tissue.

[18F]GE-180 PET Detects Reduced Microglia Activation After LM11A-31 Therapy in a Mouse Model of Alzheimer's Disease.

Microglial activation is a key pathological feature of Alzheimer's disease (AD). PET imaging of translocator protein 18 kDa (TSPO) is a strategy to detect microglial activation in vivo. Here we assessed flutriciclamide ([18F]GE-180), a new second-generation TSPO-PET radiotracer, for its ability to monitor response to LM11A-31, a novel AD therapeutic in clinical trials. AD mice displaying pathology were treated orally with LM11A-31 for 3 months. Subsequent [18F]GE-180-PET imaging revealed significantly lower signal in cortex and hippocampus of LM11A-31-treated AD mice compared to those treated with vehicle, corresponding with decreased levels of TSPO immunostaining and microglial Iba1 immunostaining. In addition to detecting decreased microglial activation following LM11A-31 treatment, [18F]GE-180 identified activated microglia in AD mice with greater sensitivity than another second-generation TSPO radiotracer, [18F]PBR06. Together, these data demonstrate the promise of [18F]GE-180 as a potentially sensitive tool for tracking neuroinflammation in AD mice and for monitoring therapeutic modulation of microglial activation.

Multiplex immunoassay characterization and species comparison of inflammation in acute and non-acute ischemic infarcts in human and mouse brain tissue.

This study provides a parallel characterization of the cytokine and chemokine response to stroke in the human and mouse brain at different stages of infarct resolution. The study goal was to address the hypothesis that chronic inflammation may contribute to stroke-related dementia. We used C57BL/6 and BALB/c mice to control for strain related differences in the mouse immune response. Our data indicate that in both mouse strains, and humans, there is increased granulocyte macrophage colony-stimulating factor (GM-CSF), interleukin-6 (IL-6), interleukin-12 p70 (IL-12p70), interferon gamma-induced protein-10 (IP-10), keratinocyte chemoattractant/interleukin-8 (KC/IL-8), monocyte chemoattractant protein-1 (MCP-1), macrophage inflammatory protein-1α (MIP-1α), macrophage inflammatory protein-1ß (MIP-1ß), regulated on activation, normal T cell expressed and secreted (RANTES), and Tumor necrosis factor-α (TNF-α) in the infarct core during the acute time period. Nevertheless, correlation and two-way ANOVA analyses reveal that despite this substantial overlap between species, there are still significant differences, particularly in the regulation of granulocyte colony-stimulating factor (G-CSF), which is increased in mice but not in humans. In the weeks after stroke, during the stage of liquefactive necrosis, there is significant resolution of the inflammatory response to stroke within the infarct. However, CD68+ macrophages remain present, and levels of IL-6 and MCP-1 remain chronically elevated in infarcts from both mice and humans. Furthermore, there is a chronic T cell response within the infarct in both species. This response is differentially polarized towards a T helper 1 (Th1) response in C57BL/6 mice, and a T helper 2 (Th2) response in BALB/c mice, suggesting that the chronic inflammatory response to stroke may follow a different trajectory in different patients. To control for the fact that the average age of the patients used in this study was 80 years, they were of both sexes, and many had suffered from multiple strokes, we also present findings that reveal how the chronic inflammatory response to stroke is impacted by age, sex, and multiple strokes in mice. Our data indicate that the chronic cytokine and chemokine response to stroke is not substantially altered in 18-month old compared to 3-month old C57BL/6 mice, although T cell infiltration is attenuated. We found a significant correlation in the chronic cytokine response to stroke in males and females. However, the chronic cytokine response to stroke was mildly exacerbated by a recurrent stroke in both C57BL/6 and BALB/c mice.

B-lymphocyte-mediated delayed cognitive impairment following stroke.

Each year, 10 million people worldwide survive the neurologic injury associated with a stroke. Importantly, stroke survivors have more than twice the risk of subsequently developing dementia compared with people who have never had a stroke. The link between stroke and the later development of dementia is not understood. There are reports of oligoclonal bands in the CSF of stroke patients, suggesting that in some people a B-lymphocyte response to stroke may occur in the CNS. Therefore, we tested the hypothesis that a B-lymphocyte response to stroke could contribute to the onset of dementia. We discovered that, in mouse models, activated B-lymphocytes infiltrate infarcted tissue in the weeks after stroke. B-lymphocytes undergo isotype switching, and IgM, IgG, and IgA antibodies are found in the neuropil adjacent to the lesion. Concurrently, mice develop delayed deficits in LTP and cognition. Genetic deficiency, and the pharmacologic ablation of B-lymphocytes using an anti-CD20 antibody, prevents the appearance of delayed cognitive deficits. Furthermore, immunostaining of human postmortem tissue revealed that a B-lymphocyte response to stroke also occurs in the brain of some people with stroke and dementia. These data suggest that some stroke patients may develop a B-lymphocyte response to stroke that contributes to dementia, and is potentially treatable with FDA-approved drugs that target B cells.

PET imaging of translocator protein (18 kDa) in a mouse model of Alzheimer's disease using N-(2,5-dimethoxybenzyl)-2-18F-fluoro-N-(2-phenoxyphenyl)acetamide.

UNLABELLED: Herein we aimed to evaluate the utility of N-(2,5-dimethoxybenzyl)-2-(18)F-fluoro-N-(2-phenoxyphenyl)acetamide ((18)F-PBR06) for detecting alterations in translocator protein (TSPO) (18 kDa), a biomarker of microglial activation, in a mouse model of Alzheimer's disease (AD). METHODS: Wild-type (wt) and AD mice (i.e., APP(L/S)) underwent (18)F-PBR06 PET imaging at predetermined time points between the ages of 5-6 and 15-16 mo. MR images were fused with PET/CT data to quantify (18)F-PBR06 uptake in the hippocampus and cortex. Ex vivo autoradiography and TSPO/CD68 immunostaining were also performed using brain tissue from these mice. RESULTS: PET images showed significantly higher accumulation of (18)F-PBR06 in the cortex and hippocampus of 15- to 16-mo-old APP(L/S) mice than age-matched wts (cortex/muscle: 2.43 ± 0.19 vs. 1.55 ± 0.15, P < 0.005; hippocampus/muscle: 2.41 ± 0.13 vs. 1.55 ± 0.12, P < 0.005). And although no significant difference was found between wt and APP(L/S) mice aged 9-10 mo or less using PET (P = 0.64), we were able to visualize and quantify a significant difference in (18)F-PBR06 uptake in these mice using autoradiography (cortex/striatum: 1.13 ± 0.04 vs. 0.96 ± 0.01, P < 0.05; hippocampus/striatum: 1.266 ± 0.003 vs. 1.096 ± 0.017, P < 0.001). PET results for 15- to 16-mo-old mice correlated well with autoradiography and immunostaining (i.e., increased (18)F-PBR06 uptake in brain regions containing elevated CD68 and TSPO staining in APP(L/S) mice, compared with wts). CONCLUSION: (18)F-PBR06 shows great potential as a tool for visualizing TSPO/microglia in the progression and treatment of AD.

Small molecule p75NTR ligands reduce pathological phosphorylation and misfolding of tau, inflammatory changes, cholinergic degeneration, and cognitive deficits in AßPP(L/S) transgenic mice.

The p75 neurotrophin receptor (p75NTR) is involved in degenerative mechanisms related to Alzheimer's disease (AD). In addition, p75NTR levels are increased in AD and the receptor is expressed by neurons that are particularly vulnerable in the disease. Therefore, modulating p75NTR function may be a significant disease-modifying treatment approach. Prior studies indicated that the non-peptide, small molecule p75NTR ligands LM11A-31, and chemically unrelated LM11A-24, could block amyloid-ß-induced deleterious signaling and neurodegeneration in vitro, and LM11A-31 was found to mitigate neuritic degeneration and behavioral deficits in a mouse model of AD. In this study, we determined whether these in vivo findings represent class effects of p75NTR ligands by examining LM11A-24 effects. In addition, the range of compound effects was further examined by evaluating tau pathology and neuroinflammation. Following oral administration, both ligands reached brain concentrations known to provide neuroprotection in vitro. Compound induction of p75NTR cleavage provided evidence for CNS target engagement. LM11A-31 and LM11A-24 reduced excessive phosphorylation of tau, and LM11A-31 also inhibited its aberrant folding. Both ligands decreased activation of microglia, while LM11A-31 attenuated reactive astrocytes. Along with decreased inflammatory responses, both ligands reduced cholinergic neurite degeneration. In addition to the amelioration of neuropathology in AD model mice, LM11A-31, but not LM11A-24, prevented impairments in water maze performance, while both ligands prevented deficits in fear conditioning. These findings support a role for p75NTR ligands in preventing fundamental tau-related pathologic mechanisms in AD, and further validate the development of these small molecules as a new class of therapeutic compounds.

Small molecule p75NTR ligand prevents cognitive deficits and neurite degeneration in an Alzheimer's mouse model.

The p75 neurotrophin receptor (p75(NTR)) is associated with multiple mechanisms linked to Alzheimer's disease (AD); hence, modulating its function might confer therapeutic effects. In previous in vitro work, we developed small molecule p75(NTR) ligands that inhibited amyloid-ß-induced degenerative signaling and prevented neurite degeneration. In the present study, a prototype p75(NTR) ligand, LM11A-31, was administered orally to the Thy-1 hAPP(Lond/Swe) (APP(L/S)) AD mouse model. LM11A-31 reached brain concentrations known to inhibit degenerative signaling without toxicity or induction of hyperalgesia. It prevented deficits in novel object recognition after 2.5 months and, in a separate cohort, deficits in Y-maze performance after 3 months of treatment. Stereology studies found that the number and size of basal forebrain cholinergic neurons, which are normal in APP(L/S) mice, were unaffected. Neuritic dystrophy, however, was readily apparent in the basal forebrain, hippocampus and cortex, and was significantly reduced by LM11A-31, with no effect on amyloid levels. These studies reveal that p75(NTR) is an important and tractable in vivo drug target for AD, with LM11A-31 representing a novel class of therapeutic candidates.

Thy1-hAPP(Lond/Swe+) mouse model of Alzheimer's disease displays broad behavioral deficits in sensorimotor, cognitive and social function.

Alzheimer's disease (AD), the most common form of dementia, is an age-dependent progressive neurodegenerative disorder. ß-amyloid, a metabolic product of the amyloid precursor protein (APP), plays an important role in the pathogenesis of AD. The Thy1-hAPP(Lond/Swe+) (line 41) transgenic mouse overexpresses human APP751 and contains the London (V717I) and Swedish (K670M/N671L) mutations. Here, we used a battery of behavioral tests to evaluate general activity, cognition, and social behavior in six-month-old male Thy1-hAPP(Lond/Swe+) mice. We found hyperactivity in a novel environment as well as significant deficits in spontaneous alternation behavior. In fear conditioning (FC), Thy1-hAPP(Lond/Swe+) mice did not display deficits in acquisition or in memory retrieval in novel context of tone-cued FC, but they showed significant memory retrieval impairment during contextual testing in an identical environment. Surprisingly, in a standard hidden platform water maze, no significant deficit was detected in mutant mice. However, a delayed-matching-to-place paradigm revealed a significant deficit in Thy1-hAPP(Lond/Swe+) mice. Lastly, in the social novelty session of a three-chamber test, Thy1-hAPP(Lond/Swe+) mice exhibited a significantly decreased interest in a novel versus a familiar stranger compared to control mice. This could possibly be explained by decreased social memory or discrimination and may parallel disturbances in social functioning in human AD patients. In conclusion, the Thy1-hAPP(Lond/Swe+) mouse model of AD displayed a behavioral phenotype that resembles, in part, the cognitive and psychiatric symptoms experienced in AD patients.

Dihydrotestosterone activates CREB signaling in cultured hippocampal neurons.

Although androgens induce numerous actions in brain, relatively little is known about which cell signaling pathways androgens activate in neurons. Recent work in our laboratory showed that the androgens testosterone and dihydrotestosterone (DHT) activate androgen receptor (AR)-dependent mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) signaling. Since the transcription factor cyclic AMP response element binding protein (CREB) is a downstream effector of MAPK/ERK and androgens activate CREB in non-neuronal cells, we investigated whether androgens activate CREB signaling in neurons. First, we observed that DHT rapidly activates CREB in cultured hippocampal neurons, as evidenced by CREB phosphorylation. Further, we observed that DHT-induced CREB phosphorylation is AR-dependent, as it occurs in PC12 cells stably transfected with AR but in neither wild-type nor empty vector-transfected cells. Next, we sought to identify the signal transduction pathways upstream of CREB phosphorylation using pharmacological inhibitors. DHT-induced CREB phosphorylation in neurons was found to be dependent upon protein kinase C (PKC) signaling but independent of MAPK/ERK, phosphatidylinositol 3-kinase, protein kinase A, and Ca(2+)/calmodulin-dependent protein kinase IV. These results demonstrate that DHT induces PKC-dependent CREB signaling, which may contribute to androgen-mediated neural functions.

The p75 neurotrophin receptor promotes amyloid-beta(1-42)-induced neuritic dystrophy in vitro and in vivo.

Oligomeric forms of amyloid-beta (Abeta) are thought to play a causal role in Alzheimer's disease (AD), and the p75 neurotrophin receptor (p75(NTR)) has been implicated in Abeta-induced neurodegeneration. To further define the functions of p75(NTR) in AD, we examined the interaction of oligomeric Abeta(1-42) with p75(NTR), and the effects of that interaction on neurite integrity in neuron cultures and in a chronic AD mouse model. Atomic force microscopy was used to ascertain the aggregated state of Abeta, and fluorescence resonance energy transfer analysis revealed that Abeta oligomers interact with the extracellular domain of p75(NTR). In vitro studies of Abeta-induced death in neuron cultures isolated from wild-type and p75(NTR-/-) mice, in which the p75(NTR) extracellular domain is deleted, showed reduced sensitivity of mutant cells to Abeta-induced cell death. Interestingly, Abeta-induced neuritic dystrophy and activation of c-Jun, a known mediator of Abeta-induced deleterious signaling, were completely prevented in p75(NTR-/-) neuron cultures. Thy1-hAPP(Lond/Swe) x p75(NTR-/-) mice exhibited significantly diminished hippocampal neuritic dystrophy and complete reversal of basal forebrain cholinergic neurite degeneration relative to those expressing wild-type p75(NTR). Abeta levels were not affected, suggesting that removal of p75(NTR) extracellular domain reduced the ability of excess Abeta to promote neuritic degeneration. These findings indicate that although p75(NTR) likely does not mediate all Abeta effects, it does play a significant role in enabling Abeta-induced neurodegeneration in vitro and in vivo, establishing p75(NTR) as an important therapeutic target for AD.

Androgens regulate neprilysin expression: role in reducing beta-amyloid levels.

Age-related testosterone depletion in men is a risk factor for Alzheimer's disease. Prior studies suggest that androgens affect Alzheimer's disease risk by regulating accumulation of beta-amyloid protein (Abeta) by an undefined mechanism. In this study, we investigated the role of the Abeta-catabolizing enzyme neprilysin (NEP) in this process. First, we observed that androgens positively regulate neural expression of NEP in adult male rats. Next, we investigated androgen regulatory effects on both NEP expression and Abeta levels using cultured hippocampal neurons and neuronally differentiated rat pheochromocytoma cell 12 with or without androgen receptor (AR). Dihydrotestosterone (DHT) induced a time-dependent increase in NEP expression. DHT also significantly decreased levels of Abeta in AR-expressing cells transfected with amyloid precursor protein, but did not affect levels of either full-length or non-amyloidogenic, soluble amyloid precursor protein. Importantly, the DHT induced decrease of Abeta was blocked by pharmacological inhibition of NEP. The DHT-mediated increase in NEP expression and decrease in Abeta levels were (i) not observed in rat pheochromocytoma cell 12 lacking AR and (ii) blocked in AR-expressing cells by the antagonists, cyproterone acetate and flutamide. Together, these findings suggest that androgen regulation of Abeta involves an AR-dependent mechanism requiring up-regulation of the Abeta catabolizing enzyme NEP.

Androgen cell signaling pathways involved in neuroprotective actions.

As a normal consequence of aging in men, testosterone levels significantly decline in both serum and brain. Age-related testosterone depletion results in increased risk of dysfunction and disease in androgen-responsive tissues, including brain. Recent evidence indicates that one deleterious effect of age-related testosterone loss in men is increased risk for Alzheimer's disease (AD). We discuss recent findings from our laboratory and others that identify androgen actions implicated in protecting the brain against neurodegenerative diseases and begin to define androgen cell signaling pathways that underlie these protective effects. Specifically, we focus on the roles of androgens as (1) endogenous negative regulators of beta-amyloid accumulation, a key event in AD pathogenesis, and (2) neuroprotective factors that utilize rapid non-genomic signaling to inhibit neuronal apoptosis. Continued elucidation of cell signaling pathways that contribute to protective actions of androgens should facilitate the development of targeted therapeutic strategies to combat AD and other age-related neurodegenerative diseases.