Super-resolution microscopy: a closer look at synaptic dysfunction in Alzheimer disease

Advancements and Practical Considerations for Biophysical Research: Navigating the Challenges and Future of Super-resolution Microscopy

The introduction of super-resolution microscopy (SRM) has significantly advanced our understanding of cellular and molecular dynamics, offering a detailed view previously beyond our reach. Implementing SRM in biophysical research, however, presents numerous challenges. This review addresses the crucial aspects of utilizing SRM effectively, from selecting appropriate fluorophores and preparing samples to analyzing complex data sets. We explore recent technological advancements and methodological improvements that enhance the capabilities of SRM. Emphasizing the integration of SRM with other analytical methods, we aim to overcome inherent limitations and expand the scope of biological insights achievable. By providing a comprehensive guide for choosing the most suitable SRM methods based on specific research objectives, we aim to empower researchers to explore complex biological processes with enhanced precision and clarity, thereby advancing the frontiers of biophysical research.

The probable role of tissue plasminogen activator/neuroserpin axis in Alzheimer's disease: a new perspective

Alzheimer's disease (AD) is the most common type of dementia associated with amyloid beta (Aβ) deposition. Dysfunction of the neuronal clearance pathway promotes the accumulation of Aβ. The plasminogen-activating system (PAS) is controlled by various enzymes like tissue plasminogen activators (tPA). Neuronal tPA enhances the conversion of plasminogen to plasmin, which cleaves Aβ; this function is controlled by many inhibitors of PAS, including a plasminogen-activating inhibitor (PAI-1) and neuroserpin. Therefore, the objective of the present narrative review was to explore the potential role of tPA/neuroserpin in the pathogenesis of AD. PAI-1 activity is increased in AD, which is involved in accumulating Aβ. Progressive increase of Aβ level during AD neuropathology is correlated with the over-production of PAI-1 with subsequent reduction of plasmin and tPA activities. Reducing plasmin and tPA activities promote Aβ by reducing Aβ clearance. Neuroserpin plays a critical role in the pathogenesis of AD as it regulates the expression and accumulation of Aβ. Higher expression of neuroserpin inhibits the neuroprotective tPA and the generation of plasmin with subsequent reduction in the clearance of Aβ. These observations raise conflicting evidence on whether neuroserpin is neuroprotective or involved in AD progression. Thus, neuroserpin over-expression with subsequent reduction of tPA may propagate AD neuropathology.

An ultra-sensitive platinized nanocavity electrode for analysis of cytosolic catecholamines in one living cell

The catecholamines, mainly dopamine (DA), are present in the cellular cytosol with low abundance, while, play key roles in various neurodegenerative disorders. Here, platinized nanocavity carbon electrodes are employed to analyze cytosolic catecholamines in a single living PC12 cell, which is not easily quantified using the classic electrodes. The confined structure and excellent conductivity in the platinized nanocavity accelerate the electron transfer of the DA, resulting in a low detection limit down to 50 nM. The sensitivity of DA detection is improved to be 10.73 pA mM-1 nm-1 in the response range of 50 nM-100 μM, which guarantees quantitative analysis of cytosolic catecholamines with low abundance. Eventually, the platinized nanocavity electrode is employed to detect cytosolic catecholamines in a single PC12 cell without an obvious interruption of cellular catecholamine level. The cytosolic catecholamines in a single PC12 cell is measured in situ to be 0.1 μM, which is achieved for the first time at the single cell level using the electrochemical method. The results demonstrate that the nanocavity electrode with a high sensitivity could offer a promising means to dynamically track catecholamines in a single cell.

Single-molecule imaging of Tau reveals how phosphorylation affects its movement and confinement in living cells

Tau is a microtubule-associated protein that is regulated by post-translational modifications. The most studied of these modifications is phosphorylation, which affects Tau's aggregation and loss- and gain-of-functions, including the interaction with microtubules, in Alzheimer's disease and primary tauopathies. However, little is known about how Tau's phosphorylation state affects its dynamics and organisation at the single-molecule level. Here, using quantitative single-molecule localisation microscopy, we examined how mimicking or abrogating phosphorylation at 14 disease-associated serine and threonine residues through mutagenesis influences the behaviour of Tau in live Neuro-2a cells. We observed that both pseudohyperphosphorylated Tau (TauE14) and phosphorylation-deficient Tau (TauA14) exhibit a heterogeneous mobility pattern near the plasma membrane. Notably, we found that the mobility of TauE14 molecules was higher than wild-type Tau molecules, while TauA14 molecules displayed lower mobility. Moreover, TauA14 was organised in a filament-like structure resembling cytoskeletal filaments, within which TauA14 exhibited spatial and kinetic heterogeneity. Our study provides a direct visualisation of how the phosphorylation state of Tau affects its spatial and temporal organisation, presumably reflecting the phosphorylation-dependent changes in the interactions between Tau and its partners. We suggest that alterations in Tau dynamics resulting from aberrant changes in phosphorylation could be a critical step in its pathological dysregulation.

miR-429-3p mediates memory decline by targeting MKP-1 to reduce surface GluA1-containing AMPA receptors in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) is a leading cause of dementia in the elderly. Mitogen-activated protein kinase phosphatase 1 (MKP-1) plays a neuroprotective role in AD. However, the molecular mechanisms underlying the effects of MKP-1 on AD have not been extensively studied. MicroRNAs (miRNAs) regulate gene expression at the post-transcriptional level, thereby repressing mRNA translation. Here, we reported that the microRNA-429-3p (miR-429-3p) was significantly increased in the brain of APP23/PS45 AD model mice and N2AAPP AD model cells. We further found that miR-429-3p could downregulate MKP-1 expression by directly binding to its 3'-untranslated region (3' UTR). Inhibition of miR-429-3p by its antagomir (A-miR-429) restored the expression of MKP-1 to a control level and consequently reduced the amyloidogenic processing of APP and Aβ accumulation. More importantly, intranasal administration of A-miR-429 successfully ameliorated the deficits of hippocampal CA1 long-term potentiation and spatial learning and memory in AD model mice by suppressing extracellular signal-regulated kinase (ERK1/2)-mediated GluA1 hyperphosphorylation at Ser831 site, thereby increasing the surface expression of GluA1-containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). Together, these results demonstrate that inhibiting miR-429-3p to upregulate MKP-1 effectively improves cognitive and synaptic functions in AD model mice, suggesting that miR-429/MKP-1 pathway may be a novel therapeutic target for AD treatment.

Neprilysin inhibitors and risk of Alzheimer's disease: A future perspective

Alzheimer's disease (AD) is a heterogeneous neurodegenerative disease with multifaceted neuropathological disorders. AD is characterized by intracellular accumulation of phosphorylated tau proteins and extracellular deposition of amyloid beta (Aβ). Various protease enzymes, including neprilysin (NEP), are concerned with the degradation and clearance of Aβ. Indeed, a defective neuronal clearance pathway due to the dysfunction of degradation enzymes might be a possible mechanism for the accumulation of Aβ and subsequent progression of AD neuropathology. NEP is one of the most imperative metalloproteinase enzymes involved in the clearance of Aβ. This review aimed to highlight the possible role of NEP inhibitors in AD. The combination of sacubitril and valsartan which is called angiotensin receptor blocker and NEP inhibitor (ARNI) may produce beneficial and deleterious effects on AD neuropathology. NEP inhibitors might increase the risk of AD by the inhibition of Aβ clearance, and increase brain bradykinin (BK) and natriuretic peptides (NPs), which augment the pathogenesis of AD. These verdicts come from animal model studies, though they may not be applied to humans. However, clinical studies revealed promising safety findings regarding the use of ARNI. Moreover, NEP inhibition increases various neuroprotective peptides involved in inflammation, glucose homeostasis and nerve conduction. Also, NEP inhibitors may inhibit dipeptidyl peptidase 4 (DPP4) expression, ameliorating insulin and glucagon-like peptide 1 (GLP-1) levels. These findings proposed that NEP inhibitors may have a protective effect against AD development by increasing GLP-1, neuropeptide Y (NPY) and substance P, and deleterious effects by increasing brain BK. Preclinical and clinical studies are recommended in this regard.

STED microscopy reveals dendrite-specificity of spines in turtle cortex

Dendritic spines are key structures for neural communication, learning and memory. Spine size and shape probably reflect synaptic strength and learning. Imaging with superresolution STED microscopy the detailed shape of the majority of the spines of individual neurons in turtle cortex (Trachemys scripta elegans) revealed several distinguishable shape classes. Dendritic spines of a given class were not distributed randomly, but rather decorated significantly more often some dendrites than others. The individuality of dendrites was corroborated by significant inter-dendrite differences in other parameters such as spine density and length. In addition, many spines were branched or possessed spinules. These findings may have implications for the role of individual dendrites in this cortex.

Tau and Amyloid β Protein in Patient-Derived Aqueous Brain Extracts Act Concomitantly to Disrupt Long-Term Potentiation in Vivo

Amyloid β protein (Aβ) and tau, the two main proteins implicated in causing Alzheimer's disease (AD), are posited to trigger synaptic dysfunction long before significant synaptic loss occurs in vulnerable circuits. Whereas soluble Aβ aggregates from AD brain are well recognized potent synaptotoxins, less is known about the synaptotoxicity of soluble tau from AD or other tauopathy patient brains. Minimally manipulated patient-derived aqueous brain extracts contain the more diffusible native forms of these proteins. Here, we explore how intracerebral injection of Aβ and tau present in such aqueous extracts of patient brain contribute to disruption of synaptic plasticity in the CA1 area of the male rat hippocampus. Aqueous extracts of certain AD brains acutely inhibited long-term potentiation (LTP) of synaptic transmission in a manner that required both Aβ and tau. Tau-containing aqueous extracts of a brain from a patient with Pick's disease (PiD) also impaired LTP, and diffusible tau from either AD or PiD brain lowered the threshold for AD brain Aβ to inhibit LTP. Remarkably, the disruption of LTP persisted for at least 2 weeks after a single injection. These findings support a critical role for diffusible tau in causing rapid onset, persistent synaptic plasticity deficits, and promoting Aβ-mediated synaptic dysfunction.SIGNIFICANCE STATEMENT The microtubule-associated protein tau forms relatively insoluble fibrillar deposits in the brains of people with neurodegenerative diseases including Alzheimer's and Pick's diseases. More soluble aggregates of disease-associated tau may diffuse between cells and could cause damage to synapses in vulnerable circuits. We prepared aqueous extracts of diseased cerebral cortex and tested their ability to interfere with synaptic function in the brains of live rats. Tau in these extracts rapidly and persistently disrupted synaptic plasticity and facilitated impairments caused by amyloid β protein, the other major pathologic protein in Alzheimer's disease. These findings show that certain diffusible forms of tau can mediate synaptic dysfunction and may be a target for therapy.

Physiological Roles of β-amyloid in Regulating Synaptic Function: Implications for AD Pathophysiology

The physiological functions of endogenous amyloid-β (Aβ), which plays important role in the pathology of Alzheimer's disease (AD), have not been paid enough attention. Here, we review the multiple physiological effects of Aβ, particularly in regulating synaptic transmission, and the possible mechanisms, in order to decipher the real characters of Aβ under both physiological and pathological conditions. Some worthy studies have shown that the deprivation of endogenous Aβ gives rise to synaptic dysfunction and cognitive deficiency, while the moderate elevation of this peptide enhances long term potentiation and leads to neuronal hyperexcitability. In this review, we provide a new view for understanding the role of Aβ in AD pathophysiology from the perspective of physiological meaning.

Roles of Rac1-Dependent Intrinsic Forgetting in Memory-Related Brain Disorders: Demon or Angel

Animals are required to handle daily massive amounts of information in an ever-changing environment, and the resulting memories and experiences determine their survival and development, which is critical for adaptive evolution. However, intrinsic forgetting, which actively deletes irrelevant information, is equally important for memory acquisition and consolidation. Recently, it has been shown that Rac1 activity plays a key role in intrinsic forgetting, maintaining the balance of the brain's memory management system in a controlled manner. In addition, dysfunctions of Rac1-dependent intrinsic forgetting may contribute to memory deficits in neurological and neurodegenerative diseases. Here, these new findings will provide insights into the neurobiology of memory and forgetting, pathological mechanisms and potential therapies for brain disorders that alter intrinsic forgetting mechanisms.

Clinical relevance of animal models in aging-related dementia research

Alzheimer's disease (AD) and other, less prevalent dementias are complex age-related disorders that exhibit multiple etiologies. Over the past decades, animal models have provided pathomechanistic insight and evaluated countless therapeutics; however, their value is increasingly being questioned due to the long history of drug failures. In this Perspective, we dispute this criticism. First, the utility of the models is limited by their design, as neither the etiology of AD nor whether interventions should occur at a cellular or network level is fully understood. Second, we highlight unmet challenges shared between animals and humans, including impeded drug transport across the blood-brain barrier, limiting effective treatment development. Third, alternative human-derived models also suffer from the limitations mentioned above and can only act as complementary resources. Finally, age being the strongest AD risk factor should be better incorporated into the experimental design, with computational modeling expected to enhance the value of animal models.

Synapse pathology in Alzheimer's disease

Synapse loss and damage are central features of Alzheimer's disease (AD) and contribute to the onset and progression of its behavioural and physiological features. Here we review the literature describing synapse pathology in AD, from what we have learned from microscopy in terms of its impacts on synapse architecture, to the mechanistic role of Aβ, tau and glial cells, mitochondrial dysfunction, and the link with AD risk genes. We consider the emerging view that synapse pathology may operate at a further level, that of synapse diversity, and discuss the prospects for leveraging new synaptome mapping methods to comprehensively understand the molecular properties of vulnerable and resilient synapses. Uncovering AD impacts on brain synapse diversity should inform therapeutic approaches targeted at preserving or replenishing lost and damaged synapses and aid the interpretation of clinical imaging approaches that aim to measure synapse damage.

Bringing synapses into focus: Recent advances in synaptic imaging and mass-spectrometry for studying synaptopathy

Synapses are integral for healthy brain function and are becoming increasingly recognized as key structures in the early stages of brain disease. Understanding the pathological processes driving synaptic dysfunction will unlock new therapeutic opportunities for some of the most devastating diseases of our time. To achieve this we need a solid repertoire of imaging and molecular tools to interrogate synaptic biology at greater resolution. Synapses have historically been examined in small numbers, using highly technical imaging modalities, or in bulk, using crude molecular approaches. However, recent advances in imaging techniques are allowing us to analyze large numbers of synapses, at single-synapse resolution. Furthermore, multiplexing is now achievable with some of these approaches, meaning we can examine multiple proteins at individual synapses in intact tissue. New molecular techniques now allow accurate quantification of proteins from isolated synapses. The development of increasingly sensitive mass-spectrometry equipment means we can now scan the synaptic molecular landscape almost in totality and see how this changes in disease. As we embrace these new technical developments, synapses will be viewed with clearer focus, and the field of synaptopathy will become richer with insightful and high-quality data. Here, we will discuss some of the ways in which synaptic interrogation is being facilitated by methodological advances, focusing on imaging, and mass spectrometry.

Ultrasound as a versatile tool for short- and long-term improvement and monitoring of brain function

Treating the brain with focused ultrasound (FUS) at low intensities elicits diverse responses in neurons, astroglia, and the extracellular matrix. In combination with intravenously injected microbubbles, FUS also opens the blood-brain barrier (BBB) and facilitates focal drug delivery. However, an incompletely understood cellular specificity and a wide parameter space currently limit the optimal application of FUS in preclinical and human studies. In this perspective, we discuss how different FUS modalities can be utilized to achieve short- and long-term improvements, thereby potentially treating brain disorders. We review the ongoing efforts to determine which parameters induce neuronal inhibition versus activation and how mechanoreceptors and signaling cascades are activated to induce long-term changes, including memory improvements. We suggest that optimal FUS treatments may require different FUS modalities and devices, depending on the targeted brain area or local pathology, and will be greatly enhanced by new techniques for monitoring FUS efficacy.

NMDA Receptor GluN2B Subunit Is Involved in Excitotoxicity Mediated by Death-Associated Protein Kinase 1 in Alzheimer's Disease

BACKGROUND

Alzheimer's disease (AD) is the most common form of neurodegenerative dementia among the elderly. Excitotoxicity has been implicated as playing a dominant role in AD, especially related to the hyperactivation of excitatory neurons. Death-associated protein kinase 1 (DAPK1) is a calcium/calmodulin-dependent kinase and involved in the pathogenesis of AD, but the roles and mechanisms of DAPK1 in excitotoxicity in AD are still uncertain.

OBJECTIVE

We mainly explored the underlying mechanisms of DAPK1 involved in the excitotoxicity of AD and its clinical relevance.

METHODS

Differentiated SH-SY5Y human neuroblastoma cells, PS1 V97 L transgenic mice, and human plasma samples were used. Protein expression was assayed by immunoblotting, and intracellular calcium and neuronal damage were analyzed by flow cytometry. Plasma DAPK1 was measured by ELISA.

RESULTS

We found that DAPK1 was activated after amyloid-β oligomers (AβOs) exposure in differentiated SH-SY5Y cells. Besides, we found the phosphorylation of GluN2B subunit at Ser1303 was increased, which contributing to excitotoxicity and Ca2+ overload in SH-SY5Y cells. Inhibiting DAPK1 activity, knockdown of DAPK1 expression, and antagonizing GluN2B subunits could effectively prevent AβOs-induced activation of GluN2B subunit, Ca2+ overload, and neuronal apoptosis. Additionally, we found that DAPK1 was elevated in the brain of AD transgenic mouse and in the plasma of AD patients.

CONCLUSION

Our finding will help to understand the mechanism of DAPK1 in the excitotoxicity in AD and provide a reference for the diagnosis and therapy of AD.

Fyn nanoclustering requires switching to an open conformation and is enhanced by FTLD-Tau biomolecular condensates

Fyn is a Src kinase that controls critical signalling cascades and has been implicated in learning and memory. Postsynaptic enrichment of Fyn underpins synaptotoxicity in dementias such as Alzheimer's disease and frontotemporal lobar degeneration with Tau pathology (FTLD-Tau). The FLTD P301L mutant Tau is associated with a higher propensity to undergo liquid-liquid phase separation (LLPS) and form biomolecular condensates. Expression of P301L mutant Tau promotes aberrant trapping of Fyn in nanoclusters within hippocampal dendrites by an unknown mechanism. Here, we used single-particle tracking photoactivated localisation microscopy to demonstrate that the opening of Fyn into its primed conformation promotes its nanoclustering in dendrites leading to increased Fyn/ERK/S6 downstream signalling. Preventing the auto-inhibitory closed conformation of Fyn through phospho-inhibition or through perturbation of its SH3 domain increased Fyn's nanoscale trapping, whereas inhibition of the catalytic domain had no impact. By combining pharmacological and genetic approaches, we demonstrate that P301L Tau enhanced both Fyn nanoclustering and Fyn/ERK/S6 signalling via its ability to form biomolecular condensates. Together, our findings demonstrate that Fyn alternates between a closed and an open conformation, the latter being enzymatically active and clustered. Furthermore, pathogenic immobilisation of Fyn relies on the ability of P301L Tau to form biomolecular condensates, thus highlighting the critical importance of LLPS in controlling nanoclustering and downstream intracellular signalling events.

Carnosic Acid Attenuates AβOs-Induced Apoptosis and Synaptic Impairment via Regulating NMDAR2B and Its Downstream Cascades in SH-SY5Y Cells

Neuronal death and synaptic loss are principal pathological features of Alzheimer's disease (AD). Amyloid beta oligomers (AβOs) constitute the main neurotoxin underscoring AD pathology. AβOs interact with N-methyl-D-aspartate receptors (NMDARs), resulting in neurotoxic events, including activation of apoptosis and synaptic impairment. Carnosic acid (CA), extracted from Salvia rosmarinus, has been verified its neuroprotective effects in AD. However, the precise mechanisms by which CA induces synaptic protection remain unclear. In this study, we established an in vitro AD model using SH-SY5Y human neuroblastoma cells. We observed that CA improved neuronal survival by suppressing apoptosis. Moreover, CA restored synaptic impairments by increasing expression levels of brain-derived neurotrophic factor (BDNF), postsynaptic density protein-95 (PSD-95), and synaptophysin (Syn). Furthermore, we found these protective effects were dependent on inhibiting the phosphorylation of NMDAR subtype 2B (NMDAR2B), which further suppressed calcium overload and promoted activation of the extracellular signal-regulated kinase (ERK)-cAMP response element-binding protein (CREB) pathway. Administration of N-methyl-D-aspartic acid (NMDA), an agonist of NMDARs, abolished these effects of CA. Our findings demonstrate that CA exerts neuroprotective effects in an in vitro model of AD by regulating NMDAR2B and its downstream cascades, highlighting the therapeutic potential of CA as a NMDARs-targeted candidate in the treatment of AD.

Nanoscale regulation of Ca2+ dependent phase transitions and real-time dynamics of SAP97/hDLG

Synapse associated protein-97/Human Disk Large (SAP97/hDLG) is a conserved, alternatively spliced, modular, scaffolding protein critical in regulating the molecular organization of cell-cell junctions in vertebrates. We confirm that the molecular determinants of first order phase transition of SAP97/hDLG is controlled by morpho-functional changes in its nanoscale organization. Furthermore, the nanoscale molecular signatures of these signalling islands and phase transitions are altered in response to changes in cytosolic Ca²⁺. Additionally, exchange kinetics of alternatively spliced isoforms of the intrinsically disordered region in SAP97/hDLG C-terminus shows differential sensitivities to Ca²⁺ bound Calmodulin, affirming that the molecular signatures of local phase transitions of SAP97/hDLG depends on their nanoscale heterogeneity and compositionality of isoforms.

SAP97/hDLG is a ubiquitous, alternatively spliced, and conserved modular scaffolding protein involved in the organization cell junctions and excitatory synapses. Here, authors confirm that SAP97/hDLG condenses in to nanosized molecular domains in both heterologous cells and hippocampal pyramidal neurons. Authors demonstrate that in vivo and in vitro condensation, molecular signatures of nanoscale condensates and exchange kinetics of SAP97/hDLG is modulated by the local availability of alternatively spliced isoforms. Additionally, SAP97/hDLG isoforms exhibits a differential sensitivity to Ca²⁺ bound Calmodulin, resulting in altered properties of nanocondensates and their real-time regulation

Do tau-synaptic long-term depression interactions in the hippocampus play a pivotal role in the progression of Alzheimer's disease?

Cognitive decline in Alzheimer's disease correlates with the extent of tau pathology, in particular tau hyperphosphorylation that initially appears in the transentorhinal and related regions of the brain including the hippocampus. Recent evidence indicates that tau hyperphosphorylation caused by either amyloid-β or long-term depression, a form of synaptic weakening involved in learning and memory, share similar mechanisms. Studies from our group and others demonstrate that long-term depression-inducing low-frequency stimulation triggers tau phosphorylation at different residues in the hippocampus under different experimental conditions including aging. Conversely, certain forms of long-term depression at hippocampal glutamatergic synapses require endogenous tau, in particular, phosphorylation at residue Ser396. Elucidating the exact mechanisms of interaction between tau and long-term depression may help our understanding of the physiological and pathological functions of tau/tau (hyper)phosphorylation. We first summarize experimental evidence regarding tau-long-term depression interactions, followed by a discussion of possible mechanisms by which this interplay may influence the pathogenesis of Alzheimer's disease. Finally, we conclude with some thoughts and perspectives on future research about these interactions.

Super-resolution imaging and quantitative analysis of microtubule arrays in model neurons show that epothilone D increases the density but decreases the length and straightness of microtubules in axon-like processes

Microtubules are essential for the development of neurons and the regulation of their structural plasticity. Microtubules also provide the structural basis for the long-distance transport of cargo. Various factors influence the organization and dynamics of neuronal microtubules, and disturbance of microtubule regulation is thought to play a central role in neurodegenerative diseases. However, imaging and quantitative assessment of the microtubule organization in the densely packed neuronal processes is challenging. The development of super-resolution techniques combined with the use of nanobodies offers new possibilities to visualize microtubules in neurites in high resolution. In combination with recently developed computational analysis tools, this allows automated quantification of neuronal microtubule organization with high precision. Here we have implemented three-dimensional DNA-PAINT (Point Accumulation in Nanoscale Topography), a single-molecule localization microscopy (SMLM) technique, which allows us to acquire 3D arrays of the microtubule lattice in axons of model neurons (neuronally differentiated PC12 cells) and dendrites of primary neurons. For the quantitative analysis of the microtubule organization, we used the open-source software package SMLM image filament extractor (SIFNE). We found that treatment with nanomolar concentrations of the microtubule-targeting drug epothilone D (EpoD) increased microtubule density in axon-like processes of model neurons and shifted the microtubule length distribution to shorter ones, with a mean microtubule length of 2.39 µm (without EpoD) and 1.98 µm (with EpoD). We also observed a significant decrease in microtubule straightness after EpoD treatment. The changes in microtubule density were consistent with live-cell imaging measurements of ensemble microtubule dynamics using a previously established Fluorescence Decay After Photoactivation (FDAP) assay. For comparison, we determined the organization of the microtubule array in dendrites of primary hippocampal neurons. We observed that dendritic microtubules have a very similar length distribution and straightness compared to microtubules in axon-like processes of a neuronal cell line. Our data show that super-resolution imaging of microtubules followed by algorithm-based image analysis represents a powerful tool to quantitatively assess changes in microtubule organization in neuronal processes, useful to determine the effect of microtubule-modulating conditions. We also provide evidence that the approach is robust and can be applied to neuronal cell lines or primary neurons, both after incorporation of labeled tubulin and by anti-tubulin antibody staining.

Opportunities and challenges in delivering biologics for Alzheimer's disease by low-intensity ultrasound

Low-intensity ultrasound combined with intravenously injected microbubbles (US^+MB) is a novel treatment modality for brain disorders, including Alzheimer's disease (AD), safely and transiently allowing therapeutic agents to overcome the blood-brain barrier (BBB) that constitutes a major barrier for therapeutic agents. Here, we first provide an update on immunotherapies in AD and how US^+MB has been applied to AD mouse models and in clinical trials, considering the ultrasound and microbubble parameter space. In the second half of the review, we compare different in vitro BBB models and discuss strategies for combining US^+MB with BBB modulators (targeting molecules such as claudin-5), and highlight the insight provided by super-resolution microscopy. Finally, we conclude with a short discussion on how in vitro findings can inform the design of animal studies, and how the insight gained may aid treatment optimization in the clinical ultrasound space.

Flying under the radar: CDH2 (N-cadherin), an important hub molecule in neurodevelopmental and neurodegenerative diseases

CDH2 belongs to the classic cadherin family of Ca²⁺-dependent cell adhesion molecules with a meticulously described dual role in cell adhesion and β-catenin signaling. During CNS development, CDH2 is involved in a wide range of processes including maintenance of neuroepithelial integrity, neural tube closure (neurulation), confinement of radial glia progenitor cells (RGPCs) to the ventricular zone and maintaining their proliferation-differentiation balance, postmitotic neural precursor migration, axon guidance, synaptic development and maintenance. In the past few years, direct and indirect evidence linked CDH2 to various neurological diseases, and in this review, we summarize recent developments regarding CDH2 function and its involvement in pathological alterations of the CNS.

The lateral entorhinal cortex is a hub for local and global dysfunction in early Alzheimer's disease states

Functional network activity alterations are one of the earliest hallmarks of Alzheimer's disease (AD), detected prior to amyloidosis and tauopathy. Better understanding the neuronal underpinnings of such network alterations could offer mechanistic insight into AD progression. Here, we examined a mouse model (3xTgAD mice) recapitulating this early AD stage. We found resting functional connectivity loss within ventral networks, including the entorhinal cortex, aligning with the spatial distribution of tauopathy reported in humans. Unexpectedly, in contrast to decreased connectivity at rest, 3xTgAD mice show enhanced fMRI signal within several projection areas following optogenetic activation of the entorhinal cortex. We corroborate this finding by demonstrating neuronal facilitation within ventral networks and synaptic hyperexcitability in projection targets. 3xTgAD mice, thus, reveal a dichotomic hypo-connected:resting versus hyper-responsive:active phenotype. This strong homotopy between the areas affected supports the translatability of this pathophysiological model to tau-related, early-AD deficits in humans.

Single-molecule imaging reveals Tau trapping at nanometer-sized dynamic hot spots near the plasma membrane that persists after microtubule perturbation and cholesterol depletion

Accumulation of aggregates of the microtubule‐binding protein Tau is a pathological hallmark of Alzheimer's disease. While Tau is thought to primarily associate with microtubules, it also interacts with and localizes to the plasma membrane. However, little is known about how Tau behaves and organizes at the plasma membrane of live cells. Using quantitative, single‐molecule imaging, we show that Tau exhibits spatial and kinetic heterogeneity near the plasma membrane of live cells, resulting in the formation of nanometer‐sized hot spots. The hot spots lasted tens of seconds, much longer than the short dwell time (∼ 40 ms) of Tau on microtubules. Pharmacological and biochemical disruption of Tau/microtubule interactions did not prevent hot spot formation, suggesting that these are different from the reported Tau condensation on microtubules. Although cholesterol removal has been shown to reduce Tau pathology, its acute depletion did not affect Tau hot spot dynamics. Our study identifies an intrinsic dynamic property of Tau near the plasma membrane that may facilitate the formation of assembly sites for Tau to assume its physiological and pathological functions.

APOE ε4 in Depression-Associated Memory Impairment—Evidence from Genetic and MicroRNA Analyses

(1) Background: The aim of this study was to replicate a reported interaction between APOE ε4 status and depression on memory function in two independent, nondemented samples from the general population and to examine the potential role of circulating plasma miRNAs. (2) Methods: The impact of the APOE ε4 allele on verbal memory and the interaction with depression is investigated in two large general-population cohorts from the Study of Health in Pomerania (SHIP, total n = 6286). Additionally, biological insights are gained by examining the potential role of circulating plasma miRNAs as potential epigenetic regulators. Analyses are performed using linear regression models adjusted for relevant biological and environmental covariates. (3) Results: Current depression as well as carrying the APOE ε4 allele were associated with impaired memory performance, with increasing effect for subjects with both risk factors. In a subcohort with available miRNA data subjects with current depressive symptoms and carrying APOE e4 revealed reduced levels of hsa-miR-107, a prominent risk marker for early Alzheimer’s Disease. (4) Conclusions: Our results confirm the effect of depressive symptoms and APOE ε4 status on memory performance. Additionally, miRNA analysis identified hsa-miR-107 as a possible biological link between APOE ε4, depressive symptoms, and cognitive impairment.

Emerging insights into synapse dysregulation in Alzheimer's disease

Alzheimer's disease is the leading cause of dementia and a growing worldwide problem, with its incidence expected to increase in the coming years. Since synapse loss is a major pathology and is correlated with symptoms in Alzheimer's disease, synapse dysfunction and loss may underlie pathophysiology. In this context, this review focuses on emerging insights into synaptic changes at the ultrastructural level. The three-dimensional electron microscopy technique unequivocally detects all types of synapses, including multi-synapses, which are indicators of synaptic connectivity between neurons. In recent years it has become feasible to perform sophisticated three-dimensional electron microscopy analyses on post-mortem human Alzheimer's disease brain as tissue preservation and electron microscopy techniques have improved. This ultrastructural analysis found that synapse loss does not always precede neuronal loss, as long believed. For instance, in the transentorhinal cortex and area CA1 of the hippocampus, synapse loss does not precede neuronal loss. However, in the entorhinal cortex, synapse loss precedes neuronal loss. Moreover, the ultrastructural analysis provides details about synapse morphology. For example, changes in excitatory synapses' post-synaptic densities, with fragmented postsynaptic densities increasing at the expense of perforated synapses, are seen in Alzheimer's disease brain. Further, multi-synapses also appear to be altered in Alzheimer's disease by doubling the abundance of multi-innervated spines in the transentorhinal cortex of Alzheimer's disease brain. Collectively, these recent ultrastructural analyses highlight distinct synaptic phenotypes in different Alzheimer's disease brain regions and broaden the understanding of synapse alterations, which may unravel some new therapeutic targets.