Anete Rozkalne

Rapid appearance and local toxicity of amyloid-beta plaques in a mouse model of Alzheimer's disease.

Senile plaques accumulate over the course of decades in the brains of patients with Alzheimer’s disease. A fundamental tenet of the amyloid hypothesis of Alzheimer’s disease is that the deposition of amyloid-β precedes and induces the neuronal abnormalities that underlie dementia. This idea has been challenged, however, by the suggestion that alterations in axonal trafficking and morphological abnormalities precede and lead to senile plaques. The role of microglia in accelerating or retarding these processes has been uncertain. To investigate the temporal relation between plaque formation and the changes in local neuritic architecture, we used longitudinal in vivo multiphoton microscopy to sequentially image young APPswe/PS1d9xYFP (B6C3-YFP) transgenic mice. Here we show that plaques form extraordinarily quickly, over 24 h. Within 1–2 days of a new plaque’s appearance, microglia are activated and recruited to the site. Progressive neuritic changes ensue, leading to increasingly dysmorphic neurites over the next days to weeks. These data establish plaques as a critical mediator of neuritic pathology.

Curcumin labels amyloid pathology in vivo, disrupts existing plaques, and partially restores distorted neurites in an Alzheimer mouse model

Alzheimer’s disease (AD) is characterized by senile plaques and neurodegeneration although the neurotoxic mechanisms have not been completely elucidated. It is clear that both oxidative stress and inflammation play an important role in the illness. The compound curcumin, with a broad spectrum of anti‐oxidant, anti‐inflammatory, and anti‐fibrilogenic activities may represent a promising approach for preventing or treating AD. Curcumin is a small fluorescent compound that binds to amyloid deposits. In the present work we used in vivo multiphoton microscopy (MPM) to demonstrate that curcumin crosses the blood–brain barrier and labels senile plaques and cerebrovascular amyloid angiopathy (CAA) in APPswe/PS1dE9 mice. Moreover, systemic treatment of mice with curcumin for 7 days clears and reduces existing plaques, as monitored with longitudinal imaging, suggesting a potent disaggregation effect. Curcumin also led to a limited, but significant reversal of structural changes in dystrophic dendrites, including abnormal curvature and dystrophy size. Together, these data suggest that curcumin reverses existing amyloid pathology and associated neurotoxicity in a mouse model of AD. This approach could lead to more effective clinical therapies for the prevention of oxidative stress, inflammation and neurotoxicity associated with AD.

Amyloid β Induces the Morphological Neurodegenerative Triad of Spine Loss, Dendritic Simplification, and Neuritic Dystrophies through Calcineurin Activation

Amyloid β (Aβ)-containing plaques are surrounded by dystrophic neurites in the Alzheimers disease (AD) brain, but whether and how plaques induce these neuritic abnormalities remain unknown. We tested the hypothesis that soluble oligomeric assemblies of Aβ, which surround plaques, induce calcium-mediated secondary cascades that lead to dystrophic changes in local neurites. We show that soluble Aβ oligomers lead to activation of the calcium-dependent phosphatase calcineurin (CaN) (PP2B), which in turn activates the transcriptional factor nuclear factor of activated T cells (NFAT). Activation of these signaling pathways, even in the absence of Aβ, is sufficient to produce a virtual phenocopy of Aβ-induced dystrophic neurites, dendritic simplification, and dendritic spine loss in both neurons in culture and in the adult mouse brain. Importantly, the morphological deficits in the vicinity of Aβ deposits in a mouse model of AD are ameliorated by CaN inhibition, supporting the hypothesis that CaN–NFAT are aberrantly activated by Aβ and that CaN–NFAT activation is responsible for disruption of neuronal structure near plaques. In accord with this, we also detect increased levels of an active form of CaN and NFATc4 in the nuclear fraction from the cortex of patients with AD. Thus, Aβ appears to mediate the neurodegeneration of AD, at least in part, by activation of CaN and subsequent NFAT-mediated downstream cascades.

Antagonistic Regulation of Actin Dynamics and Cell Motility by TRPC5 and TRPC6 Channels

Coupling TRPC5 and TRPC6 calcium channels to different Rho GTPases allows calcium to both promote and inhibit cell migration. Signaling Stop and Go Calcium-dependent remodeling of the actin cytoskeleton through members of the Rho family of small guanosine triphosphatases (Rho GTPases) is crucial for cell migration. Tian et al. investigated the upstream regulation of this process in kidney podocytes, a class of cells associated with glomerular capillaries whose contractile function is crucial to maintenance of the kidney filtration barrier. They found that, although angiotensin II elicited calcium influx through both TRPC5 and TRPC6 (transient receptor potential canonical type 5 and 6) channels, TRPC5 signaled through Rac1 to promote cell motility, whereas TRPC6 signaled through RhoA to inhibit it. Mechanistic analyses indicated that differential activation of the two GTPases depended on their location relative to the two channels: TRPC5 was present in a complex with Rac1 and TRPC6 was associated with RhoA, enabling their antagonistic regulation of the cytoskeleton and thereby their opposing effects on cell migration. The Rho family of small guanosine triphosphatases (Rho GTPases: RhoA, Cdc42, and Rac1) regulates many aspects of cell behavior, including actin dynamics and cell migration. The generation of calcium ion (Ca2+) microdomains is critical in promoting cell migration because they control the localized activity of Rho GTPases. We identified receptor-activated TRPC5 and TRPC6 (transient receptor potential canonical type 5 and 6) channels as antagonistic regulators of actin remodeling and cell motility in fibroblasts and kidney podocytes. We show that TRPC5 is in a molecular complex with Rac1, whereas TRPC6 is in a molecular complex with RhoA. TRPC5-mediated Ca2+ influx induces Rac1 activation, thereby promoting cell migration, whereas TRPC6-mediated Ca2+ influx increases RhoA activity, thereby inhibiting cell migration. Our data unveil antagonistic Ca2+ influx pathways as a conserved signaling mechanism for the integrated regulation of cell migration.

Preservation of Neuronal Number Despite Age-Related Cortical Brain Atrophy In Elderly Subjects Without Alzheimer Disease

Cerebral volume loss has long been associated with normal aging, but whether this is due to aging itself or to age-related diseases, including incipient Alzheimer disease, is uncertain. To understand the changes that occur in the aging brain, we examined the cerebral cortex of 27 normal individuals ranging in age from 56 to 103 years. None fulfilled the criteria for the neuropathologic diagnosis of Alzheimer disease or other neurodegenerative disease. Seventeen of the elderly participants had cognitive testing an average of 6.7 months prior to death. We used quantitative approaches to analyze cortical thickness, neuronal number, and density. Frontal and temporal neocortical regions had clear evidence of cortical thinning with age, but total neuronal numbers in frontal and temporal neocortical regions remained relatively constant during a 50-year age range. These data suggest that loss of neuronal and dendritic architecture, rather than loss of neurons, underlies neocortical volume loss with increasing age in the absence of Alzheimer disease.

Passive immunotherapy rapidly increases structural plasticity in a mouse model of Alzheimer disease

Senile plaque-associated changes in neuronal connectivity such as altered neurite trajectory, dystrophic swellings, and synapse and dendritic spine loss are thought to contribute to cognitive dysfunction in Alzheimers disease and mouse models. Immunotherapy to remove amyloid beta is a promising therapy that causes recovery of neurite trajectory and dystrophic neurites over a period of days. The acute effects of immunotherapy on neurite morphology at a time point when soluble amyloid has been cleared but dense plaques are not yet affected are unknown. To examine whether removal of soluble amyloid beta (Abeta) has a therapeutic effect on dendritic spines, we explored spine dynamics within 1 h of applying a neutralizing anti Abeta antibody. This acute treatment caused a small but significant increase in dendritic spine formation in PDAPP brain far from plaques, without affecting spine plasticity near plaques or average dendritic spine density. These data support the hypothesis that removing toxic soluble forms of amyloid-beta rapidly increases structural plasticity possibly allowing functional recovery of neural circuits.

Calcineurin inhibition with FK506 ameliorates dendritic spine density deficits in plaque-bearing Alzheimer model mice

Synapse loss is the strongest correlate of cognitive decline in Alzheimers disease, and synapses are an attractive therapeutic target due to their plastic nature that allows for potential recovery with intervention. We have previously demonstrated in transgenic mice that form senile plaques that dendrites surrounding plaques become dystrophic and lose postsynaptic dendritic spines. Furthermore, we found strong evidence that plaque-associated dendritic changes are mediated by calcineurin, a calcium-dependent phosphatase involved in cell signaling, using in vitro models and genetically encoded inhibitors in mouse models. In this study, we pharmacologically inhibited calcineurin with FK506 treatment to test the hypothesis that calcineurin inhibition will allow recovery of plaque-associated synapse loss. We found that in plaque bearing transgenic mice, short term (1 week) FK506 treatment results in an amelioration of dendritic spine loss. We also observe an effect on spine morphology in wild-type mice with FK506 treatment. These data show that systemic FK506 administration, and hence calcineurin inhibition, may be neuroprotective for amyloid beta induced synaptic alterations.

Apolipoprotein E: isoform specific differences in tertiary structure and interaction with amyloid-β in human Alzheimer brain

We applied a novel application of FLIM-FRET to in situ measurement and quantification of protein interactions to explore isoform specific differences in Aβ-ApoE interaction and ApoE tertiary conformation in senile plaques in human Alzheimer brain. ApoE3 interacts more closely with Aβ than ApoE4, but a greater proportion of Aβ molecules within plaques are decorated with ApoE4 than ApoE3, lending strong support to the hypothesis that isoform specific differences in ApoE are linked with Aβ deposition. We found an increased number of ApoE N-terminal fragments in ApoE4 plaques, consistent with the observation that ApoE4 is more easily cleaved than ApoE3. In addition, we measured a small but significant isoform specific difference in ApoE domain interaction. Based on our in situ data, supported by traditional biochemical data, we propose a pathway by which isoform specific conformational differences increase the level of cleavage at the hinge region of ApoE4, leading to a loss of ApoE function to mediate clearance of Aβ and thereby increase the risk of AD for carriers of the APOEε4 allele.

Calcineurin inhibition with systemic FK506 treatment increases dendritic branching and dendritic spine density in healthy adult mouse brain

Calcineurin has been implicated as part of a critical signaling pathway for learning and memory, and recent data suggest that calcineurin activation mediates some of the neurotoxicity of the Alzheimer related neurotoxin Aβ. Immunosuppression via calcineurin inhibition with the compound FK506 is an important treatment for organ transplant patients. Here we use Golgi impregnation techniques, along with a new survival analysis-based statistical approach for analysis of dendritic complexity, to show that in healthy adult mice one week of treatment with FK506 affects both the branching patterns and dendritic spine density of cortical neurons. These results indicate that calcineurin inhibition leads to readily detectable changes in brain morphology, further implicating calcineurin related pathways in both the function and structure of the adult brain.

Inhibition of Sirtuin 2 with Sulfobenzoic Acid Derivative AK1 is Non-Toxic and Potentially Neuroprotective in a Mouse Model of Frontotemporal Dementia

Tauopathies including tau-associated Frontotemporal dementia (FTD) and Alzheimer’s disease are characterized pathologically by the formation of tau-containing neurofibrillary aggregates and neuronal loss, which contribute to cognitive decline. There are currently no effective treatments to prevent or slow this neural systems failure. The rTg4510 mouse model, which expresses a mutant form of the tau protein associated with FTD with Parkinsonism-17, undergoes dramatic hippocampal and cortical neuronal loss making it an ideal model to study treatments for FTD-related neuronal loss. Sirtuins are a family of proteins involved in cell survival that have the potential to modulate neuronal loss in neurodegenerative disorders. Here we tested the hypothesis that sirtuin 2 (SIRT2) inhibition would be non-toxic and prevent neurodegeneration in rTg4510 brain. In this study we delivered SIRT2 inhibitor AK1 directly to the hippocampus with an osmotic minipump and confirmed that it reached the target region both with histological assessment of delivery of a dye and with a pharmacodynamic marker, ABCA1 transcription, which was upregulated with AK1 treatment. AK1 treatment was found to be safe in wild-type mice and in the rTg4510 mouse model, and further, it provided some neuroprotection in the rTg4510 hippocampal circuitry. This study provides proof-of-concept for therapeutic benefits of SIRT2 inhibitors in both tau-associated FTD and Alzheimer’s disease, and suggests that development of potent, brain permeable SIRT2 inhibitors is warranted.