The secreted Wnt antagonist Dickkopf-1 is required for amyloid β-mediated synaptic loss

Neuronal guidance signaling in neurodegenerative diseases: Key regulators that function at neuron-glia and neuroimmune interfaces

The nervous system processes a vast amount of information, performing computations that underlie perception, cognition, and behavior. During development, neuronal guidance genes, which encode extracellular cues, their receptors, and downstream signal transducers, organize neural wiring to generate the complex architecture of the nervous system. It is now evident that many of these neuroguidance cues and their receptors are active during development and are also expressed in the adult nervous system. This suggests that neuronal guidance pathways are critical not only for neural wiring but also for ongoing function and maintenance of the mature nervous system. Supporting this view, these pathways continue to regulate synaptic connectivity, plasticity, and remodeling, and overall brain homeostasis throughout adulthood. Genetic and transcriptomic analyses have further revealed many neuronal guidance genes to be associated with a wide range of neurodegenerative and neuropsychiatric disorders. Although the precise mechanisms by which aberrant neuronal guidance signaling drives the pathogenesis of these diseases remain to be clarified, emerging evidence points to several common themes, including dysfunction in neurons, microglia, astrocytes, and endothelial cells, along with dysregulation of neuron-microglia-astrocyte, neuroimmune, and neurovascular interactions. In this review, we explore recent advances in understanding the molecular and cellular mechanisms by which aberrant neuronal guidance signaling contributes to disease pathogenesis through altered cell-cell interactions. For instance, recent studies have unveiled two distinct semaphorin-plexin signaling pathways that affect microglial activation and neuroinflammation. We discuss the challenges ahead, along with the therapeutic potentials of targeting neuronal guidance pathways for treating neurodegenerative diseases. Particular focus is placed on how neuronal guidance mechanisms control neuron-glia and neuroimmune interactions and modulate microglial function under physiological and pathological conditions. Specifically, we examine the crosstalk between neuronal guidance signaling and TREM2, a master regulator of microglial function, in the context of pathogenic protein aggregates. It is well-established that age is a major risk factor for neurodegeneration. Future research should address how aging and neuronal guidance signaling interact to influence an individual's susceptibility to various late-onset neurological diseases and how the progression of these diseases could be therapeutically blocked by targeting neuronal guidance pathways.

Blocking the Dkk1-LRP6 interaction prevents acute amyloid-β-driven cognitive impairment

Synapse loss driven by amyloid-β (Aβ) is an early event in Alzheimer's disease (AD). Although the mechanism by which Aβ drives synapse loss remain poorly understood data indicate that a disruption of Wnt signalling plays an important part. We have shown that Aβ exerts its effects on synapses through Dickkopf-1 (Dkk1), a secreted protein that acts upon Wnt signalling via a direct interaction with the canonical Wnt pathway co-receptor proteins, LRP5 and LRP6, preventing their interaction with the receptor Frizzled. This antagonises canonical, Wnt/β-catenin, signalling and allows concomitant activation of non-canonical signalling pathways. We contend that it is the switch from canonical to non-canonical Wnt signalling activity that drives synapse loss and subsequent cognitive impairment in AD, driven by Aβ and mediated by Dkk1. Preventing the Dkk1-LRP5/6 interaction could protect synapses and cognition against Aβ by maintaining canonical Wnt signalling. To test this, we mapped the Dkk1-LRP6 interaction by peptide array and identified a small peptide able to disrupt the Dkk1-LRP6 interaction. This Dkk1-LRP6 'disruptor' peptide dose dependently restores canonical Wnt signalling in the presence of Dkk1; blocks Dkk1-driven dendritic spine loss in primary rat cortical cultures and the accompanying increase in endogenous Aβ production; and when administered intracerebroventricularly to a rat acute Aβ model, blocks Aβ-driven cognitive impairment. These data support our contention that the ability of Aβ to induce Dkk1 and the effects of Dkk1 on LRP6 are an important element in AD aetiopathology and establish Dkk1 as a therapeutic target for protecting synapse and cognition in AD.

Amyloid-β can activate JNK signalling via WNT5A-ROR2 to reduce synapse formation in Alzheimer's disease

Wnt signalling is an essential signalling system in neurogenesis, with a crucial role in synaptic plasticity and neuronal survival, processes that are disrupted in Alzheimer's disease (AD). Within this network, the Wnt/β-catenin pathway has been studied for its neuroprotective role, and this is suppressed in AD. However, the involvement of the non-canonical Wnt-planar cell polarity (Wnt/PCP) pathway in AD remains to be determined. This study investigates the role of ROR2, a Wnt/PCP co-receptor, in synaptogenesis. We demonstrate that WNT5A-ROR2 signalling activates the JNK pathway, leading to synapse loss in mature neurons. This effect mirrors the synaptotoxic actions of Aβ1-42 and DKK1, which are elevated in AD. Notably, blocking ROR2 and JNK mitigates Aβ1-42 and DKK1-induced synapse loss, suggesting their dependence on ROR2. In induced pluripotent stem cell (iPSC)-derived cortical neurons carrying a PSEN1 mutation, known to increase the Aβ42/40 ratio, we observed increased WNT5A-ROR2 clustering and reduced numbers of synapses. Inhibiting ROR2 or JNK partially rescued synaptogenesis in these neurons. These findings suggest that, unlike the Wnt/β-catenin pathway, the Wnt/PCP-ROR2 signalling pathway can operate in a feedback loop with Aβ1-42 to enhance JNK signalling and contribute to synapse loss in AD.

Polyphenols as Wnt/β-catenin pathway modulators: A promising strategy in clinical neurodegeneration

Polyphenols, a diverse group of naturally occurring compounds found in plants, have garnered significant attention for their potential therapeutic properties in treating neurodegenerative diseases (NDs). The Wnt/β-catenin (WβC) signaling pathway, a crucial player in neurogenesis, neuronal survival, and synaptic plasticity, is involved in several cellular mechanisms related to NDs. Dysregulation of this pathway is a hallmark in the development of various NDs. This study explores multiple polyphenolic compounds, such as flavonoids, stilbenes, lignans, and phenolic acids, and their potential to protect the nervous system. It provides a comprehensive analysis of their effects on the WβC pathway, elucidating their modes of action. The study highlights the dual function of polyphenols in regulating and protecting the nervous system, providing reassurance about the research benefits. This review provides a comprehensive analysis of the results obtained from both in vitro studies and in vivo research, shedding light on how these substances influence the various components of the pathway. The focus is mainly on the molecular mechanisms that allow polyphenols to reduce oxidative stress, inflammation, and apoptotic processes, ultimately improving the function and survival of neurons. This study aims to offer a thorough understanding of the potential of polyphenols in targeting the WβC signaling pathway, which could lead to the development of innovative therapeutic options for NDs.

Inflammatory signaling pathways in Alzheimer's disease: Mechanistic insights and possible therapeutic interventions

The complex pathophysiology of Alzheimer's disease (AD) poses challenges for the development of therapies. Recently, neuroinflammation has been identified as a key pathogenic mechanism underlying AD, while inflammation has emerged as a possible target for the management and prevention of AD. Several prior studies have demonstrated that medications modulating neuroinflammation might lessen AD symptoms, mostly by controlling neuroinflammatory signaling pathways such as the NF-κB, MAPK, NLRP3, etc, and their respective signaling cascade. Moreover, targeting these inflammatory modalities with inhibitors, natural products, and metabolites has been the subject of intensive research because of their anti-inflammatory characteristics, with many studies demonstrating noteworthy pharmacological capabilities and potential clinical applications. Therefore, targeting inflammation is considered a promising strategy for treating AD. This review comprehensively elucidates the neuroinflammatory mechanisms underlying AD progression and the beneficial effects of inhibitors, natural products, and metabolites in AD treatment.

Wnt/β-catenin Signaling in Central Nervous System Regeneration

The Wnt/β-catenin signaling pathway plays a pivotal role in the development, maintenance, and repair of the central nervous system (CNS). This chapter explores the diverse functions of Wnt/β-catenin signaling, from its critical involvement in embryonic CNS development to its reparative and plasticity-inducing roles in response to CNS injury. We discuss how Wnt/β-catenin signaling influences various CNS cell types-astrocytes, microglia, neurons, and oligodendrocytes-each contributing to repair and plasticity after injury. The chapter also addresses the pathway's involvement in CNS disorders such as Alzheimer's and Parkinson's diseases, psychiatric disorders, and traumatic brain injury (TBI), highlighting potential Wnt-based therapeutic approaches. Lastly, zebrafish are presented as a promising model organism for studying CNS regeneration and neurodegenerative diseases, offering insights into future research and therapeutic development.

Harmony of Wnt pathway in Alzheimer's: Navigating the multidimensional progression from preclinical to clinical stages

The Wnt pathway stands out as a pivotal signal transduction pathway, operating through two distinct modes of signaling: the canonical/β-catenin pathway and the non-canonical pathway. Among these, the canonical pathway assumes a paramount role in various physiological and pathological processes within the human body. Particularly in the brain, Wnt exhibits involvement in fundamental physiological events including neuronal differentiation/survival, axonogenesis, neural stem cell regulation, synaptic plasticity, and cell cycle modulation. Notably, scientific evidence underscores the critical role of the Wnt pathway in the pathogenesis of Alzheimer's disease (AD), correlating with its involvement in key pathological features such as tau tangles, Amyloid-β plaques, synaptic dysfunction, oxidative stress, mitochondrial dysfunction, cognitive impairments, and disruption of the blood-brain barrier integrity. This review aims to comprehensively explore the involvement and significance of Wnt signaling in Alzheimer's. Furthermore, it delves into recent advancements in research on Wnt signaling, spanning from preclinical investigations to clinical trials.

Unlocking hope: GSK-3 inhibitors and Wnt pathway activation in Alzheimer's therapy

Alzheimer's disease (AD) is a complex neurodegenerative disorder characterised by progressive cognitive decline and the accumulation of amyloid-β plaques and tau tangles. The Wnt signalling pathway known for its crucial role in neurodevelopment and adult neurogenesis has emerged as a potential target for therapeutic intervention in AD. Glycogen synthase kinase-3 beta (GSK-3β), a key regulator of the Wnt pathway, plays a pivotal role in AD pathogenesis by promoting tau hyperphosphorylation and neuroinflammation. Several preclinical studies have demonstrated that inhibiting GSK-3β leads to the activation of Wnt pathway thereby promoting neuroprotective effects, and mitigating cognitive deficits in AD animal models. The modulation of Wnt signalling appears to have multifaceted benefits including the reduction of amyloid-β production, tau hyperphosphorylation, enhancement of synaptic plasticity, and inhibition of neuroinflammation. These findings suggest that targeting GSK-3β to activate Wnt pathway may represent a novel approach for slowing or halting the progression of AD. This hypothesis reviews the current state of research exploring the activation of Wnt pathway through the inhibition of GSK-3β as a promising therapeutic strategy in AD.

Osteoporosis and Alzheimer´s disease (or Alzheimer´s disease and Osteoporosis)

Alzheimer's disease (AD) and osteoporosis are two diseases that mainly affect elderly people, with increases in the occurrence of cases due to a longer life expectancy. Several epidemiological studies have shown a reciprocal association between both diseases, finding an increase in incidence of osteoporosis in patients with AD, and a higher burden of AD in osteoporotic patients. This epidemiological relationship has motivated the search for molecules, genes, signaling pathways and mechanisms that are related to both pathologies. The mechanisms found in these studies can serve to improve treatments and establish better patient care protocols.

Enhancing motor functional recovery in spinal cord injury through pharmacological inhibition of Dickkopf-1 with BHQ880 antibody

BACKGROUND

Mounting experimental evidence has underscored the remarkable role played by the Wnt family of proteins in the spinal cord functioning and therapeutic potential in spinal cord injury (SCI). We aim to provide a therapeutic prospect associated with the modulation of canonical Wnt signaling, examining the spatio-temporal expression pattern of Dickkopf-1 (Dkk1) and its neutralization after SCI. We employ an intraparenchymal injection of the clinically validated Dkk1-blocking antibody, BHQ880, to elucidate its effects in SCI.

METHODS

A rat model of contusion SCI was used. Histological analyses were performed, wherein Dkk1 protein was sought, and ELISA analyses were employed for Dkk1 detection in cerebrospinal fluid and serum. To ascertain the BHQ880 therapeutic effect, rats were subjected to SCI and then injected with the antibody in the lesion epicenter 24 hours post-injury (hpi). Subsequent evaluation of motor functional recovery extended up to 56 days post-injury (dpi). qRT-PCR and histological analyses were conducted.

RESULTS

We demonstrate the presence of Dkk1 in the healthy rat spinal cord, with pronounced alterations observed following injury, primarily concentrated in the epicenter regions. Notably, a significative upregulation of Dkk1 was detected at 24 hpi, peaking at 3 dpi and remaining elevated until 42 dpi. Moreover, we revealed that early administration of BHQ880 considerably improved motor functional recovery, promoted preservation of myelinated tissue, and reduced astroglial and microglia/macrophage reactivity. Furthermore, there was a decrease in the acute expression of different inflammatory genes.

CONCLUSIONS

Collectively, our findings highlight the therapeutic potential of BHQ880 treatment in the context of SCI.

Clusterin is a Potential Therapeutic Target in Alzheimer's Disease

In recent years, Clusterin, a glycosylated protein with multiple biological functions, has attracted extensive research attention. It is closely associated with the physiological and pathological states within the organism. Particularly in Alzheimer's disease (AD) research, Clusterin plays a significant role in the disease's occurrence and progression. Numerous studies have demonstrated a close association between Clusterin and AD. Firstly, the expression level of Clusterin in the brain tissue of AD patients is closely related to pathological progression. Secondly, Clusterin is involved in the deposition and formation of β-amyloid, which is a crucial process in AD development. Furthermore, Clusterin may affect the pathogenesis of AD through mechanisms such as regulating inflammation, controlling cell apoptosis, and clearing pathological proteins. Therefore, further research on the relationship between Clusterin and AD will contribute to a deeper understanding of the etiology of this neurodegenerative disease and provide a theoretical basis for developing early diagnostic and therapeutic strategies for AD. This also makes Clusterin one of the research focuses as a potential biomarker for AD diagnosis and treatment monitoring.

Bone-organ axes: bidirectional crosstalk

In addition to its recognized role in providing structural support, bone plays a crucial role in maintaining the functionality and balance of various organs by secreting specific cytokines (also known as osteokines). This reciprocal influence extends to these organs modulating bone homeostasis and development, although this aspect has yet to be systematically reviewed. This review aims to elucidate this bidirectional crosstalk, with a particular focus on the role of osteokines. Additionally, it presents a unique compilation of evidence highlighting the critical function of extracellular vesicles (EVs) within bone-organ axes for the first time. Moreover, it explores the implications of this crosstalk for designing and implementing bone-on-chips and assembloids, underscoring the importance of comprehending these interactions for advancing physiologically relevant in vitro models. Consequently, this review establishes a robust theoretical foundation for preventing, diagnosing, and treating diseases related to the bone-organ axis from the perspective of cytokines, EVs, hormones, and metabolites.

Targeting of wnt signalling pathway by small bioactive molecules for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is the most occurring neurodegenerative disorder that destroys learning, memory, and thinking skills. Although the pathophysiology of the disease is least understood, the post-mortem brain of AD patients as well as animal models revealed the part of down regulated Wnt signalling in progression of the disease. The deficit in the Wnt signalling leads to the accumulation of amyloid beta peptides, phosphorylation of tau proteins, and synaptic dysfunctions, which are regarded as the major pathological features of AD. As the available drugs for AD are only able to mitigate the symptoms and are also associated with several side effects, the therapeutic potential of the bioactive compounds is being explored for their efficacies in managing the major pathologies. Consequently, a few bioactive compounds fundamentally isolated from Garcinia species are established as promising neuroprotective agents in AD, however; their potential to regulate the Wnt signalling pathway is yet to be discovered. Considering the neuroprotective properties, in the present study efficiency of six small bioactive compounds viz., amentoflavone, isovitexin, orientin, apigenin, kaempferol, and garcinol have been investigated in modulating the receptor proteins (LRP6, DKK1, WIF1 and GSK3β) of the Wnt signalling pathway by molecular docking technique. While all the bioactive compounds could efficiently interact with the target proteins, amentoflavone, orientin, and isovitexin interact with all the target proteins viz., LRP6, DKK1, WIF1, and GSK3β with higher free energy of binding, more number of interactions, and similar mode of binding in comparison to their known or reported modulators. Thus, the present study set forth the investigated small bioactive molecules as potential drug candidates in AD therapeutics.

The evolving roles of Wnt signaling in stem cell proliferation and differentiation, the development of human diseases, and therapeutic opportunities

The evolutionarily conserved Wnt signaling pathway plays a central role in development and adult tissue homeostasis across species. Wnt proteins are secreted, lipid-modified signaling molecules that activate the canonical (β-catenin dependent) and non-canonical (β-catenin independent) Wnt signaling pathways. Cellular behaviors such as proliferation, differentiation, maturation, and proper body-axis specification are carried out by the canonical pathway, which is the best characterized of the known Wnt signaling paths. Wnt signaling has emerged as an important factor in stem cell biology and is known to affect the self-renewal of stem cells in various tissues. This includes but is not limited to embryonic, hematopoietic, mesenchymal, gut, neural, and epidermal stem cells. Wnt signaling has also been implicated in tumor cells that exhibit stem cell-like properties. Wnt signaling is crucial for bone formation and presents a potential target for the development of therapeutics for bone disorders. Not surprisingly, aberrant Wnt signaling is also associated with a wide variety of diseases, including cancer. Mutations of Wnt pathway members in cancer can lead to unchecked cell proliferation, epithelial-mesenchymal transition, and metastasis. Altogether, advances in the understanding of dysregulated Wnt signaling in disease have paved the way for the development of novel therapeutics that target components of the Wnt pathway. Beginning with a brief overview of the mechanisms of canonical and non-canonical Wnt, this review aims to summarize the current knowledge of Wnt signaling in stem cells, aberrations to the Wnt pathway associated with diseases, and novel therapeutics targeting the Wnt pathway in preclinical and clinical studies.

Downregulation of Dickkopf-3, a Wnt antagonist elevated in Alzheimer's disease, restores synapse integrity and memory in a disease mouse model

Increasing evidence supports a role for deficient Wnt signaling in Alzheimer's disease (AD). Studies reveal that the secreted Wnt antagonist Dickkopf-3 (DKK3) colocalizes to amyloid plaques in AD patients. Here, we investigate the contribution of DKK3 to synapse integrity in healthy and AD brains. Our findings show that DKK3 expression is upregulated in the brains of AD subjects and that DKK3 protein levels increase at early stages in the disease. In hAPP-J20 and hAPPNL-G-F/NL-G-F mouse AD models, extracellular DKK3 levels are increased and DKK3 accumulates at dystrophic neuronal processes around plaques. Functionally, DKK3 triggers the loss of excitatory synapses through blockade of the Wnt/GSK3β signaling with a concomitant increase in inhibitory synapses via activation of the Wnt/JNK pathway. In contrast, DKK3 knockdown restores synapse number and memory in hAPP-J20 mice. Collectively, our findings identify DKK3 as a novel driver of synaptic defects and memory impairment in AD.

Harnessing the Stem Cell Niche in Regenerative Medicine: Innovative Avenue to Combat Neurodegenerative Diseases

Regenerative medicine harnesses the body's innate capacity for self-repair to restore malfunctioning tissues and organs. Stem cell therapies represent a key regenerative strategy, but to effectively harness their potential necessitates a nuanced understanding of the stem cell niche. This specialized microenvironment regulates critical stem cell behaviors including quiescence, activation, differentiation, and homing. Emerging research reveals that dysfunction within endogenous neural stem cell niches contributes to neurodegenerative pathologies and impedes regeneration. Strategies such as modifying signaling pathways, or epigenetic interventions to restore niche homeostasis and signaling, hold promise for revitalizing neurogenesis and neural repair in diseases like Alzheimer's and Parkinson's. Comparative studies of highly regenerative species provide evolutionary clues into niche-mediated renewal mechanisms. Leveraging endogenous bioelectric cues and crosstalk between gut, brain, and vascular niches further illuminates promising therapeutic opportunities. Emerging techniques like single-cell transcriptomics, organoids, microfluidics, artificial intelligence, in silico modeling, and transdifferentiation will continue to unravel niche complexity. By providing a comprehensive synthesis integrating diverse views on niche components, developmental transitions, and dynamics, this review unveils new layers of complexity integral to niche behavior and function, which unveil novel prospects to modulate niche function and provide revolutionary treatments for neurodegenerative diseases.

Exercise Intervention for Alzheimer's Disease: Unraveling Neurobiological Mechanisms and Assessing Effects

Alzheimer's disease (AD) is a progressive neurodegenerative disease and a major cause of age-related dementia, characterized by cognitive dysfunction and memory impairment. The underlying causes include the accumulation of beta-amyloid protein (Aβ) in the brain, abnormal phosphorylation, and aggregation of tau protein within nerve cells, as well as neuronal damage and death. Currently, there is no cure for AD with drug therapy. Non-pharmacological interventions such as exercise have been widely used to treat AD, but the specific molecular and biological mechanisms are not well understood. In this narrative review, we integrate the biology of AD and summarize the knowledge of the molecular, neural, and physiological mechanisms underlying exercise-induced improvements in AD progression. We discuss various exercise interventions used in AD and show that exercise directly or indirectly affects the brain by regulating crosstalk mechanisms between peripheral organs and the brain, including "bone-brain crosstalk", "muscle-brain crosstalk", and "gut-brain crosstalk". We also summarize the potential role of artificial intelligence and neuroimaging technologies in exercise interventions for AD. We emphasize that moderate-intensity, regular, long-term exercise may improve the progression of Alzheimer's disease through various molecular and biological pathways, with multimodal exercise providing greater benefits. Through in-depth exploration of the molecular and biological mechanisms and effects of exercise interventions in improving AD progression, this review aims to contribute to the existing knowledge base and provide insights into new therapeutic strategies for managing AD.

Synthetic and Natural Bioactive Molecules in Balancing the Crosstalk among Common Signaling Pathways in Alzheimer's Disease: Understanding the Neurotoxic Mechanisms for Therapeutic Intervention

The structure and function of the brain greatly rely on different signaling pathways. The wide variety of biological processes, including neurogenesis, axonal remodeling, the development and maintenance of pre- and postsynaptic terminals, and excitatory synaptic transmission, depends on combined actions of these molecular pathways. From that point of view, it is important to investigate signaling pathways and their crosstalk in order to better understand the formation of toxic proteins during neurodegeneration. With recent discoveries, it is established that the modulation of several pathological events in Alzheimer's disease (AD) due to the mammalian target of rapamycin (mTOR), Wnt signaling, 5'-adenosine monophosphate activated protein kinase (AMPK), peroxisome proliferator-activated receptor gamma coactivator 1α (PGC-1α), and sirtuin 1 (Sirt1, silent mating-type information regulator 2 homologue 1) are central to the key findings. These include decreased amyloid formation and inflammation, mitochondrial dynamics control, and enhanced neural stability. This review intends to emphasize the importance of these signaling pathways, which collectively determine the fate of neurons in AD in several ways. This review will also focus on the role of novel synthetic and natural bioactive molecules in balancing the intricate crosstalk among different pathways in order to prolong the longevity of AD patients.

The Molecular Basis of Wnt/β-Catenin Signaling Pathways in Neurodegenerative Diseases

Defective Wnt signaling is found to be associated with various neurodegenerative diseases. In the canonical pathway, the Frizzled receptor (Fzd) and the lipoprotein receptor-related proteins 5/6 (LRP5/LRP6) create a seven-pass transmembrane receptor complex to which the Wnt ligands bind. This interaction causes the tumor suppressor adenomatous polyposis coli gene product (APC), casein kinase 1 (CK1), and GSK-3β (glycogen synthase kinase-3 beta) to be recruited by the scaffold protein Dishevelled (Dvl), which in turn deactivates the β-catenin destruction complex. This inactivation stops the destruction complex from phosphorylating β-catenin. As a result, β-catenin first builds up in the cytoplasm and then migrates into the nucleus, where it binds to the Lef/Tcf transcription factor to activate the transcription of more than 50 Wnt target genes, including those involved in cell growth, survival, differentiation, neurogenesis, and inflammation. The treatments that are currently available for neurodegenerative illnesses are most commonly not curative in nature but are only symptomatic. According to all available research, restoring Wnt/β-catenin signaling in the brains of patients with neurodegenerative disorders, particularly Alzheimer's and Parkinson's disease, would improve the condition of several patients with neurological disorders. The importance of Wnt activators and modulators in patients with such illnesses is to mainly restore rather than overstimulate the Wnt/β-catenin signaling, thereby reestablishing the equilibrium between Wnt-OFF and Wnt-ON states. In this review, we have tried to summarize the significance of the Wnt canonical pathway in the pathophysiology of certain neurodegenerative diseases, such as Alzheimer's disease, cerebral ischemia, Parkinson's disease, Huntington's disease, multiple sclerosis, and other similar diseases, and as to how can it be restored in these patients.

WNT-β Catenin Signaling as a Potential Therapeutic Target for Neurodegenerative Diseases: Current Status and Future Perspective

Wnt/β-catenin (WβC) signaling pathway is an important signaling pathway for the maintenance of cellular homeostasis from the embryonic developmental stages to adulthood. The canonical pathway of WβC signaling is essential for neurogenesis, cell proliferation, and neurogenesis, whereas the noncanonical pathway (WNT/Ca²⁺ and WNT/PCP) is responsible for cell polarity, calcium maintenance, and cell migration. Abnormal regulation of WβC signaling is involved in the pathogenesis of several neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS), and spinal muscular atrophy (SMA). Hence, the alteration of WβC signaling is considered a potential therapeutic target for the treatment of neurodegenerative disease. In the present review, we have used the bibliographical information from PubMed, Google Scholar, and Scopus to address the current prospects of WβC signaling role in the abovementioned neurodegenerative diseases.

The Multifaceted Role of WNT Signaling in Alzheimer's Disease Onset and Age-Related Progression

The evolutionary conserved WNT signaling pathway orchestrates numerous complex biological processes during development and is critical to the maintenance of tissue integrity and homeostasis in the adult. As it relates to the central nervous system, WNT signaling plays several roles as it relates to neurogenesis, synaptic formation, memory, and learning. Thus, dysfunction of this pathway is associated with multiple diseases and disorders, including several neurodegenerative disorders. Alzheimer's disease (AD) is characterized by several pathologies, synaptic dysfunction, and cognitive decline. In this review, we will discuss the various epidemiological, clinical, and animal studies that demonstrate a precise link between aberrant WNT signaling and AD-associated pathologies. In turn, we will discuss the manner in which WNT signaling influences multiple molecular, biochemical, and cellular pathways upstream of these end-point pathologies. Finally, we will discuss how merging tools and technologies can be used to generate next generation cellular models to dissect the relationship between WNT signaling and AD.

Identification of a Novel Wnt Antagonist Based Therapeutic and Diagnostic Target for Alzheimer's Disease Using a Stem Cell-Derived Model

Currently, all the existing treatments for Alzheimer's disease (AD) fail to stall progression due to longer duration of time between onset of the symptoms and diagnosis of the disease, raising the necessity of effective diagnostics and novel treatment. Specific molecular regulation of the onset and progression of disease is not yet elucidated. This warranted investigation of the role of Wnt signaling regulators which are thought to be involved in neurogenesis. The AD model was established using amyloid beta (Aβ) in human mesenchymal stem cells derived from amniotic membranes which were differentiated into neuronal cell types. In vivo studies were carried out with Aβ or a Wnt antagonist, AD201, belonging to the sFRP family. We further created an AD201-knockdown in vitro model to determine the role of Wnt antagonism. BACE1 upregulation, ChAT and α7nAChR downregulation with synapse and functionality loss with increases in ROS confirmed the neurodegeneration. Reduced β-catenin and increased AD201 expression indicated Wnt/canonical pathway inhibition. Similar results were exhibited in the in vivo study along with AD-associated behavioural and molecular changes. AD201-knockdown rescued neurons from Aβ-induced toxicity. We demonstrated for the first time a role of AD201 in Alzheimer's disease manifestation, which indicates a promising disease target and biomarker.

A genetic variant of the Wnt receptor LRP6 accelerates synapse degeneration during aging and in Alzheimer's disease

Synapse loss strongly correlates with cognitive decline in Alzheimer's disease (AD), but the underlying mechanisms are poorly understood. Deficient Wnt signaling contributes to synapse dysfunction and loss in AD. Consistently, a variant of the LRP6 receptor, (LRP6-Val), with reduced Wnt signaling, is linked to late-onset AD. However, the impact of LRP6-Val on the healthy and AD brain has not been examined. Knock-in mice, generated by gene editing, carrying this Lrp6 variant develop normally. However, neurons from Lrp6-val mice do not respond to Wnt7a, a ligand that promotes synaptic assembly through the Frizzled-5 receptor. Wnt7a stimulates the formation of the low-density lipoprotein receptor-related protein 6 (LRP6)-Frizzled-5 complex but not if LRP6-Val is present. Lrp6-val mice exhibit structural and functional synaptic defects that become pronounced with age. Lrp6-val mice present exacerbated synapse loss around plaques when crossed to the NL-G-F AD model. Our findings uncover a previously unidentified role for Lrp6-val in synapse vulnerability during aging and AD.

Microglial Expression of the Wnt Signaling Modulator DKK2 Differs between Human Alzheimer's Disease Brains and Mouse Neurodegeneration Models

Wnt signaling is crucial for synapse and cognitive function. Indeed, deficient Wnt signaling is causally related to increased expression of DKK1, an endogenous negative Wnt regulator, and synapse loss, both of which likely contribute to cognitive decline in Alzheimer's disease (AD). Increasingly, AD research efforts have probed the neuroinflammatory role of microglia, the resident immune cells of the CNS, which have furthermore been shown to be modulated by Wnt signaling. The DKK1 homolog DKK2 has been previously identified as an activated response and/or disease-associated microglia (DAM/ARM) gene in a mouse model of AD. Here, we performed a detailed analysis of DKK2 in mouse models of neurodegeneration, and in human AD brain. In APP/PS1 and APP^NL-G-F AD mouse model brains as well as in SOD1^G93A ALS mouse model spinal cords, but not in control littermates, we demonstrated significant microgliosis and microglial Dkk2 mRNA upregulation in a disease-stage-dependent manner. In the AD models, these DAM/ARM Dkk2⁺ microglia preferentially accumulated close to βAmyloid plaques. Furthermore, recombinant DKK2 treatment of rat hippocampal primary neurons blocked WNT7a-induced dendritic spine and synapse formation, indicative of an anti-synaptic effect similar to that of DKK1. In stark contrast, no such microglial DKK2 upregulation was detected in the postmortem human frontal cortex from individuals diagnosed with AD or pathologic aging. In summary, the difference in microglial expression of the DAM/ARM gene DKK2 between mouse models and human AD brain highlights the increasingly recognized limitations of using mouse models to recapitulate facets of human neurodegenerative disease.

Crosstalk between the Hippo Pathway and the Wnt Pathway in Huntington's Disease and Other Neurodegenerative Disorders

The Hippo pathway consists of a cascade of kinases that controls the phosphorylation of the co-activators YAP/TAZ. When unphosphorylated, YAP and TAZ translocate into the nucleus, where they mainly bind to the TEAD transcription factor family and activate genes related to cell proliferation and survival. In this way, the inhibition of the Hippo pathway promotes cell survival, proliferation, and stemness fate. Another pathway can modulate these processes, namely the Wnt/β-catenin pathway that is indeed involved in cellular functions such as proliferation and cell survival, as well as apoptosis, growth, and cell renewal. Wnt signaling can act in a canonical or noncanonical way, depending on whether β-catenin is involved in the process. In this review, we will focus only on the canonical Wnt pathway. It has emerged that YAP/TAZ are components of the β-catenin destruction complex and that there is a close relationship between the Hippo pathway and the canonical Wnt pathway. Furthermore, recent data have shown that both of these pathways may play a role in neurodegenerative diseases, such as Huntington's disease, Alzheimer's disease, or Amyotrophic Lateral Sclerosis. Thus, this review analyzes the Hippo pathway and the Wnt pathway, their crosstalk, and their involvement in Huntington's disease, as well as in other neurodegenerative disorders. Altogether, these data suggest possible therapeutic approaches targeting key players of these pathways.

The Role of Clusterin Transporter in the Pathogenesis of Alzheimer’s Disease at the Blood–Brain Barrier Interface: A Systematic Review

Alzheimer’s disease (AD) is considered a chronic and debilitating neurological illness that is increasingly impacting older-age populations. Some proteins, including clusterin (CLU or apolipoprotein J) transporter, can be linked to AD, causing oxidative stress. Therefore, its activity can affect various functions involving complement system inactivation, lipid transport, chaperone activity, neuronal transmission, and cellular survival pathways. This transporter is known to bind to the amyloid beta (Aβ) peptide, which is the major pathogenic factor of AD. On the other hand, this transporter is also active at the blood–brain barrier (BBB), a barrier that prevents harmful substances from entering and exiting the brain. Therefore, in this review, we discuss and emphasize the role of the CLU transporter and CLU-linked molecular mechanisms at the BBB interface in the pathogenesis of AD.

Genetic analyses identify pleiotropy and causality for blood proteins and highlight Wnt/β-catenin signalling in migraine

Migraine is a common complex disorder with a significant polygenic SNP heritability (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${h}_{{SNP}}^{2}$$\end{document}hSNP2). Here we utilise genome-wide association study (GWAS) summary statistics to study pleiotropy between blood proteins and migraine under the polygenic model. We estimate \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${h}_{{SNP}}^{2}$$\end{document}hSNP2 for 4625 blood protein GWASs and identify 325 unique proteins with a significant \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${h}_{{SNP}}^{2}$$\end{document}hSNP2 for use in subsequent genetic analyses. Pleiotropy analyses link 58 blood proteins to migraine risk at genome-wide, gene and/or single-nucleotide polymorphism levels—suggesting shared genetic influences or causal relationships. Notably, the identified proteins are largely distinct from migraine GWAS loci. We show that higher levels of DKK1 and PDGFB, and lower levels of FARS2, GSTA4 and CHIC2 proteins have a significant causal effect on migraine. The risk-increasing effect of DKK1 is particularly interesting—indicating a role for downregulation of β-catenin-dependent Wnt signalling in migraine risk, suggesting Wnt activators that restore Wnt/β-catenin signalling in brain could represent therapeutic tools against migraine.

Understanding of the causes and treatment of migraine is incomplete. Here, the authors detect pleiotropic genetic effects and causal relationships between migraine and 58 proteins that are largely distinct from migraine-associated loci identified by GWAS.

Molecular mechanisms of developmental pathways in neurological disorders: a pharmacological and therapeutic review

Developmental signalling pathways such as Wnt/β-catenin, Notch and Sonic hedgehog play a central role in nearly all the stages of neuronal development. The term 'embryonic' might appear to be a misnomer to several people because these pathways are functional during the early stages of embryonic development and adulthood, albeit to a certain degree. Therefore, any aberration in these pathways or their associated components may contribute towards a detrimental outcome in the form of neurological disorders such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and stroke. In the last decade, researchers have extensively studied these pathways to decipher disease-related interactions, which can be used as therapeutic targets to improve outcomes in patients with neurological abnormalities. However, a lot remains to be understood in this domain. Nevertheless, there is strong evidence supporting the fact that embryonic signalling is indeed a crucial mechanism as is manifested by its role in driving memory loss, motor impairments and many other processes after brain trauma. In this review, we explore the key roles of three embryonic pathways in modulating a range of homeostatic processes such as maintaining blood-brain barrier integrity, mitochondrial dynamics and neuroinflammation. In addition, we extensively investigated the effect of these pathways in driving the pathophysiology of a range of disorders such as Alzheimer's, Parkinson's and diabetic neuropathy. The concluding section of the review is dedicated to neurotherapeutics, wherein we identify and list a range of biological molecules and compounds that have shown enormous potential in improving prognosis in patients with these disorders.

Canonical Wnt Signaling in the Pathology of Iron Overload-Induced Oxidative Stress and Age-Related Diseases

Iron accumulates in the vital organs with aging. This is associated with oxidative stress, inflammation, and mitochondrial dysfunction leading to age-related disorders. Abnormal iron levels are linked to neurodegenerative diseases, liver injury, cancer, and ocular diseases. Canonical Wnt signaling is an evolutionarily conserved signaling pathway that regulates many cellular functions including cell proliferation, apoptosis, cell migration, and stem cell renewal. Recent evidences indicate that iron regulates Wnt signaling, and iron chelators like deferoxamine and deferasirox can inhibit Wnt signaling and cell growth. Canonical Wnt signaling is implicated in the pathogenesis of many diseases, and there are significant efforts ongoing to develop innovative therapies targeting the aberrant Wnt signaling. This review examines how intracellular iron accumulation regulates Wnt signaling in various tissues and their potential contribution in the progression of age-related diseases.

Dendrobium nobile Lindl. Alkaloids Ameliorate Aβ25-35-Induced Synaptic Deficits by Targeting Wnt/β-Catenin Pathway in Alzheimer’s Disease Models

Background: Dendrobium nobile Lindl. alkaloids (DNLA) are effective in ameliorating cognitive deficit in SAMP8, AβPP/PS1, and LPS-induced AD animal models, and prevented Aβ-induced synaptic degeneration in cultured hippocampal neurons. However, the underlying mechanisms remain unexplored. Objective: This study investigated the protective effects of DNLA on synaptic damage in an Aβ 25-35-induced rat AD model, in primary cortical neuron cultures, and in PC12 cells transfected with human AβPP695, focusing on the Wnt/β-catenin pathway. Methods: Sprague-Dawley rats received a single Aβ 25-35 injection (10μg) into the bilateral hippocampi. DNLA (40 and 80 mg/kg/d) was intragastrically administrated 7d prior to Aβ injection and continued for 28 days. The spatial learning and memory, synaptic morphology, synapse-related proteins, and Wnt signaling components GSK3β and β-catenin phosphorylation were evaluated. Rat primary cortical neuron cultures and AβPP695-PC12 cells were used to evaluate axonal mitochondria distribution, reactive oxygen species production, amyloidogenesis, and Wnt pathway in the protection. Results: DNLA ameliorated Aβ-induced cognitive impairment, increased the number of synapses, elevated the postsynaptic density thickness and expression of synapsin and PSD95 in the hippocampus, and suppressed Aβ-mediated GSK3β activity and the β-catenin phosphorylation. In primary neurons and AβPP695-PC12 cells, DNLA restored Aβ 25-35 induced mitochondrial dysfunction and inhibited reactive oxygen species production and amyloidogenesis. Furthermore, the Wnt/β-catenin pathway inhibitor Dkk-1 blocked the effect of DNLA on the expression of Aβ 1-42 and PSD95. Conclusion: DNLA rescued Aβ-mediated synaptic and mitochondrial injury and inhibited amyloidogenesis in vivo and in vitro, probably through the activation of Wnt/β-catenin signaling pathway to protect synaptic integrity.

Epigenetic repression of Wnt receptors in AD: a role for Sirtuin2-induced H4K16ac deacetylation of Frizzled1 and Frizzled7 promoters

Growing evidence supports a role for deficient Wnt signalling in Alzheimer's disease (AD). First, the Wnt antagonist DKK1 is elevated in AD brains and is required for amyloid-β-induced synapse loss. Second, LRP6 Wnt co-receptor is required for synapse integrity and three variants of this receptor are linked to late-onset AD. However, the expression/role of other Wnt signalling components remain poorly explored in AD. Wnt receptors Frizzled1 (Fzd1), Fzd5, Fzd7 and Fzd9 are of interest due to their role in synapse formation/plasticity. Our analyses showed reduced FZD1 and FZD7 mRNA levels in the hippocampus of human early AD stages and in the hAPP^NLGF/NLGF mouse model. This transcriptional downregulation was accompanied by reduced levels of the pro-transcriptional histone mark H4K16ac and a concomitant increase of its deacetylase Sirt2 at Fzd1 and Fzd7 promoters in AD. In vitro and in vivo inhibition of Sirt2 rescued Fzd1 and Fzd7 mRNA expression and H4K16ac levels at their promoters. In addition, we showed that Sirt2 recruitment to Fzd1 and Fzd7 promoters is dependent on FoxO1 activity in AD, thus acting as a co-repressor. Finally, we found reduced levels of SIRT2 inhibitory phosphorylation in nuclear samples from human early AD stages with a concomitant increase in the SIRT2 phosphatase PP2C. This results in hyperactive nuclear Sirt2 and favours Fzd1 and Fzd7 repression in AD. Collectively, our findings define a novel role for nuclear hyperactivated SIRT2 in repressing Fzd1 and Fzd7 expression via H4K16ac deacetylation in AD. We propose SIRT2 as an attractive target to ameliorate AD pathology.

Menopause and development of Alzheimer's disease: Roles of neural glucose metabolism and Wnt signaling

Late onset Alzheimer´s disease (AD) is a neurodegenerative disease with gender differences in its onset and progression, being the prevalence predominant in women and at an earlier age than in men. The pathophysiology of the menopausal condition has been associated to this dementia, playing major roles regarding both endocrine and glucose metabolism changes, amongst other mechanisms. In the current review we address the role of estrogen deficiency in the processes involved in the development of AD, including amyloid precursor protein (APP) processing to form senile plaques, Tau phosphorylation forming neurofibrillary tangles, Wnt signaling and AD neuropathology, the role of glucose brain metabolism, Wnt signaling and glucose transport in the brain, and our research contribution to these topics.

Exploring the Role of CLU in the Pathogenesis of Alzheimer's Disease

Alzheimer's disease (AD) is a chronic and devastating neurodegenerative disorder that is affecting elderly people at an increasing rate. Clusterin (CLU), an extracellular chaperone, is an ubiquitously expressed protein that can be identified in various body fluids and tissues. Expression of CLU can lead to various processes including suppression of complement system, lipid transport, chaperone function, and also controlling neuronal cell death and cell survival mechanisms. Studies have confirmed that the level of CLU expression is increased in AD. Furthermore, CLU also decreased the toxicity and aggregation of amyloid beta (Aβ). However when the Aβ level was far greater than CLU, then the amyloid generation was increased. CLU was also found to incorporate in the amyloid aggregates, which were more harmful as compared with the Aβ42 aggregates alone. Growing evidence indicates that CLU plays roles in AD pathogenesis via various processes, including aggregation and clearance of Aβ, neuroinflammation, lipid metabolism, Wnt signaling, copper homeostasis, and regulation of neuronal cell cycle and apoptosis. In this article, we represent the critical interaction of CLU and AD based on recent advances. Furthermore, we have also focused on the Aβ-dependent and Aβ-independent mechanisms by which CLU plays a role in AD pathogenesis.

Misregulation of Wnt Signaling Pathways at the Plasma Membrane in Brain and Metabolic Diseases

Wnt signaling pathways constitute a group of signal transduction pathways that direct many physiological processes, such as development, growth, and differentiation. Dysregulation of these pathways is thus associated with many pathological processes, including neurodegenerative diseases, metabolic disorders, and cancer. At the same time, alterations are observed in plasma membrane compositions, lipid organizations, and ordered membrane domains in brain and metabolic diseases that are associated with Wnt signaling pathway activation. Here, we discuss the relationships between plasma membrane components-specifically ligands, (co) receptors, and extracellular or membrane-associated modulators-to activate Wnt pathways in several brain and metabolic diseases. Thus, the Wnt-receptor complex can be targeted based on the composition and organization of the plasma membrane, in order to develop effective targeted therapy drugs.

On the Common Journey of Neural Cells through Ischemic Brain Injury and Alzheimer's Disease

Ischemic brain injury and Alzheimer's disease (AD) both lead to cell death in the central nervous system (CNS) and thus negatively affect particularly the elderly population. Due to the lack of a definitive cure for brain ischemia and AD, it is advisable to carefully study, compare, and contrast the mechanisms that trigger, and are involved in, both neuropathologies. A deeper understanding of these mechanisms may help ameliorate, or even prevent, the destructive effects of neurodegenerative disorders. In this review, we deal with ischemic damage and AD, with the main emphasis on the common properties of these CNS disorders. Importantly, we discuss the Wnt signaling pathway as a significant factor in the cell fate determination and cell survival in the diseased adult CNS. Finally, we summarize the interesting findings that may improve or complement the current sparse and insufficient treatments for brain ischemia and AD, and we delineate prospective directions in regenerative medicine.

A Role for Frizzled and Their Post-Translational Modifications in the Mammalian Central Nervous System

The Wnt pathway is a key signalling cascade that regulates the formation and function of neuronal circuits. The main receptors for Wnts are Frizzled (Fzd) that mediate diverse functions such as neurogenesis, axon guidance, dendritogenesis, synapse formation, and synaptic plasticity. These processes are crucial for the assembly of functional neuronal circuits required for diverse functions ranging from sensory and motor tasks to cognitive performance. Indeed, aberrant Wnt-Fzd signalling has been associated with synaptic defects during development and in neurodegenerative conditions such as Alzheimer's disease. New studies suggest that the localisation and stability of Fzd receptors play a crucial role in determining Wnt function. Post-translational modifications (PTMs) of Fzd are emerging as an important mechanism that regulates these Wnt receptors. However, only phosphorylation and glycosylation have been described to modulate Fzd function in the central nervous system (CNS). In this review, we discuss the function of Fzd in neuronal circuit connectivity and how PTMs contribute to their function. We also discuss other PTMs, not yet described in the CNS, and how they might modulate the function of Fzd in neuronal connectivity. PTMs could modulate Fzd function by affecting Fzd localisation and stability at the plasma membrane resulting in local effects of Wnt signalling, a feature particularly important in polarised cells such as neurons. Our review highlights the importance of further studies into the role of PTMs on Fzd receptors in the context of neuronal connectivity.

The transcriptional landscape of Alzheimer's disease and its association with Wnt signaling pathway

Alzheimer's disease (AD) is a neurological disorder primarily affecting the elderly. The disease manifests as progressive deterioration in cognitive functions, leading to a loss of autonomy. The identification of transcriptional changes in susceptible signaling pathways has provided clues to the origin and progression of AD and has pinpointed synapse loss as the prominent event in early stages of the disease. Synapse failure represents a key pathological correlate of cognitive decline in patients. Genetics and transcriptomics studies have also identified novel genes, processes, and pathways associated with AD. This evidence suggests that a deficiency in Wnt signaling pathway contributes to AD pathogenesis by inducing synaptic dysfunction and neuronal degeneration. In the adult nervous system, Wnt signaling plays a crucial role in synaptic physiology, modulating the synaptic vesicle cycle, trafficking neurotransmitter receptors, and modulating the expression of different genes associated with these processes. In this review, we describe the general transcriptional landscape associated with AD, specifically transcriptional changes associated with the Wnt signaling pathway and their effects in the context of disease.

Alzheimer’s Disease, Neural Plasticity, and Functional Recovery

Alzheimer’s disease (AD) is the most common and devastating neurodegenerative condition worldwide, characterized by the aggregation of amyloid-β and phosphorylated tau protein, and is accompanied by a progressive loss of learning and memory. A healthy nervous system is endowed with synaptic plasticity, among others neural plasticity mechanisms, allowing structural and physiological adaptations to changes in the environment. This neural plasticity modification sustains learning and memory, and behavioral changes and is severely affected by pathological and aging conditions, leading to cognitive deterioration. This article reviews critical aspects of AD neurodegeneration as well as therapeutic approaches that restore neural plasticity to provide functional recoveries, including environmental enrichment, physical exercise, transcranial stimulation, neurotrophin involvement, and direct electrical stimulation of the amygdala. In addition, we report recent behavioral results in Octodon degus, a promising natural model for the study of AD that naturally reproduces the neuropathological alterations observed in AD patients during normal aging, including neuronal toxicity, deterioration of neural plasticity, and the decline of learning and memory.

The neuroprotective effects of alpha-lipoic acid on an experimental model of Alzheimer's disease in PC12 cells

With the growth of the aging population, the prevalence of Alzheimer's disease (AD) has increased and influenced the work and daily life of AD patients, imposing a heavy burden on society and the patients' families. AD is a progressive disease with a long duration, and the pathogenesis is very complicated. Here, we found that alpha-lipoic acid (LA), an endogenous, naturally synthesized compound, could attenuate amyloid beta fragment (Aβ_25-35 )-induced PC12 cell toxicity. Aβ_25-35 treatment largely decreased the viability of PC12 cells, increased reactive oxygen species (ROS) levels, and increased the percentage of apoptotic cells, which were accompanied by changes in the expression of the apoptosis-related genes. Further, the Wnt pathway was inactivated, and the expression of Wnt pathway-related proteins such as Frizzled2, GSK3β, and phosphorylated GSK3β were dysregulated after Aβ_25-35 treatment. LA efficiently attenuated Aβ_25-35 -induced PC12 cell apoptosis and downregulated the phosphorylation-mediated degradation of β-catenin as well as GSK3β. Our results demonstrate that LA rescues Aβ_25-35 -induced neurocytotoxicity through the Wnt-β-catenin pathway.

Striatal Synapse Degeneration and Dysfunction Are Reversed by Reactivation of Wnt Signaling

Synapse degeneration in the striatum has been associated with the early stages of Parkinson’s and Huntington’s diseases (PD and HD). However, the molecular mechanisms that trigger synaptic dysfunction and loss are not fully understood. Increasing evidence suggests that deficiency in Wnt signaling triggers synapse degeneration in the adult brain and that this pathway is affected in neurodegenerative diseases. Here, we demonstrate that endogenous Wnt signaling is essential for the integrity of a subset of inhibitory synapses on striatal medium spiny neurons (MSNs). We found that inducible expression of the specific Wnt antagonist Dickkopf-1 (Dkk1) in the adult striatum leads to the loss of inhibitory synapses on MSNs and affects the synaptic transmission of D2-MSNs. We also discovered that re-activation of the Wnt pathway by turning off Dkk1 expression after substantial loss of synapses resulted in the complete recovery of GABAergic and dopamine synapse number. Our results also show that re-activation of the Wnt pathway leads to a recovery of amphetamine response and motor function. Our studies identify the Wnt signaling pathway as a potential therapeutic target for restoring neuronal circuits after synapse degeneration.

Bone-to-Brain: A Round Trip in the Adaptation to Mechanical Stimuli

Besides the classical ones (support/protection, hematopoiesis, storage for calcium, and phosphate) multiple roles emerged for bone tissue, definitively making it an organ. Particularly, the endocrine function, and in more general terms, the capability to sense and integrate different stimuli and to send signals to other tissues, has highlighted the importance of bone in homeostasis. Bone is highly innervated and hosts all nervous system branches; bone cells are sensitive to most of neurotransmitters, neuropeptides, and neurohormones that directly affect their metabolic activity and sensitivity to mechanical stimuli. Indeed, bone is the principal mechanosensitive organ. Thanks to the mechanosensing resident cells, and particularly osteocytes, mechanical stimulation induces metabolic responses in bone forming (osteoblasts) and bone resorbing (osteoclasts) cells that allow the adaptation of the affected bony segment to the changing environment. Once stimulated, bone cells express and secrete, or liberate from the entrapping matrix, several mediators (osteokines) that induce responses on distant targets. Brain is a target of some of these mediator [e.g., osteocalcin, lipocalin2, sclerostin, Dickkopf-related protein 1 (Dkk1), and fibroblast growth factor 23], as most of them can cross the blood-brain barrier. For others, a role in brain has been hypothesized, but not yet demonstrated. As exercise effectively modifies the release and the circulating levels of these osteokines, it has been hypothesized that some of the beneficial effects of exercise on brain functions may be associated to such a bone-to-brain communication. This hypothesis hides an interesting clinical clue: may well-addressed physical activities support the treatment of neurodegenerative diseases, such as Alzheimer’s and Parkinson’s diseases?

Wnt5a promotes hippocampal postsynaptic development and GluN2B-induced expression via the eIF2α HRI kinase

Wnt signaling plays a key role in neurodevelopment and neuronal maturation. Specifically, Wnt5a stimulates postsynaptic assemblies, increases glutamatergic neurotransmission and, through calcium signaling, generates nitric oxide (NO). Trying to unveil the molecular pathway triggering these postsynaptic effects, we found that Wnt5a treatment induces a time-dependent increases in the length of the postsynaptic density (PSD), elicits novel synaptic contacts and facilitates F-actin flow both in in vitro and ex vivo models. These effects were partially abolished by the inhibition of the Heme-regulated eukaryotic initiation factor 2α (HRI) kinase, a kinase which phosphorylates the initiation translational factor eIF2α. When phosphorylated, eIF2α normally avoids the translation of proteins not needed during stress conditions, in order to avoid unnecessary energetic expenses. However, phosphorylated eIF2α promotes the translation of some proteins with more than one open reading frame in its 5′ untranslated region. One of these proteins targeted by Wnt-HRI-eIF2α mediated translation is the GluN2B subunit of the NMDA receptor. The identified increase in GluN2B expression correlated with increased NMDA receptor function. Considering that NMDA receptors are crucial for excitatory synaptic transmission, the molecular pathway described here contributes to the understanding of the fast and plastic translational mechanisms activated during learning and memory processes.

Therapies for Alzheimer's disease: a metabolic perspective

Alzheimer's disease (AD) is one of the most common forms of dementia in the elderly. Currently, there are over 50 million cases of dementia worldwide and it is expected that it will reach 136 million by 2050. AD is described as a neurodegenerative disease that gradually compromises memory and learning capacity. Patients often exhibit brain glucose hypometabolism and are more susceptible to develop type 2 diabetes or insulin resistance in comparison with age-matched controls. This suggests that there is a link between both pathologies. Glucose metabolism and the tricarboxylic acid cycle are tightly related to mitochondrial performance and energy production. Impairment of both these pathways can evoke oxidative damage on mitochondria and key proteins linked to several hallmarks of AD. Glycation is also another type of post-translational modification often reported in AD, which might impair the function of proteins that participate in metabolic pathways thought to be involved in this illness. Despite needing further research, therapies based on insulin treatment, usage of anti-diabetes drugs or some form of dietary intervention, have shown to be promising therapeutic approaches for AD in its early stages of progression and will be unveiled in this paper.

Interaction of NF-κB and Wnt/β-catenin Signaling Pathways in Alzheimer's Disease and Potential Active Drug Treatments

The most important neuropathological features of Alzheimer's disease (AD) are extracellular amyloid-β protein (Aβ) deposition, tau protein hyperphosphorylation and activation of neurometabolic reaction in the brain accompanied by neuronal and synaptic damage, and impaired learning and memory function. According to the amyloid cascade hypothesis, increased Aβ deposits in the brain to form the core of the senile plaques that initiate cascade reactions, affecting the synapses and stimulating activation of microglia, resulting in neuroinflammation. A growing number of studies has shown that NF-κB and Wnt/β-catenin pathways play important roles in neurodegenerative diseases, especially AD. In this review, we briefly introduce the connection between neuroinflammation-mediated synaptic dysfunction in AD and elaborated on the mechanism of these two signaling pathways in AD-related pathological changes, as well as their interaction. Based on our interest in natural compounds, we also briefly introduce and conduct preliminary screening of potential therapeutics for AD.

WNT Signaling Is a Key Player in Alzheimer's Disease

The cellular processes regulated by WNT signaling have been mainly studied during embryonic development and cancer. In the last two decades, the role of WNT in the adult central nervous system has been the focus of interest in our laboratory. In this chapter, we will be summarized β-catenin-dependent and -independent WNT pathways, then we will be revised WNT signaling function at the pre- and post-synaptic level. Concerning Alzheimer's disease (AD) initially, we found that WNT/β-catenin signaling activation exerts a neuroprotective mechanism against the amyloid β (Αβ) peptide toxicity. Later, we found that WNT/β-catenin participates in Tau phosphorylation and in learning and memory. In the last years, we demonstrated that WNT/β-catenin signaling is instrumental in the amyloid precursor protein (APP) processing and that WNT/β-catenin dysfunction results in Aβ production and aggregation. We highlight the importance of WNT/β-catenin signaling dysfunction in the onset of AD and propose that the loss of WNT/β-catenin signaling is a triggering factor of AD. The WNT pathway is therefore positioned as a therapeutic target for AD and could be a valid concept for improving AD therapy. We think that metabolism and inflammation will be relevant when defining future research in the context of WNT signaling and the neurodegeneration associated with AD.

Regulatory Roles of Bone in Neurodegenerative Diseases

Osteoporosis and neurodegenerative diseases are two kinds of common disorders of the elderly, which often co-occur. Previous studies have shown the skeletal and central nervous systems are closely related to pathophysiology. As the main structural scaffold of the body, the bone is also a reservoir for stem cells, a primary lymphoid organ, and an important endocrine organ. It can interact with the brain through various bone-derived cells, mostly the mesenchymal and hematopoietic stem cells (HSCs). The bone marrow is also a place for generating immune cells, which could greatly influence brain functions. Finally, the proteins secreted by bones (osteokines) also play important roles in the growth and function of the brain. This article reviews the latest research studying the impact of bone-derived cells, bone-controlled immune system, and bone-secreted proteins on the brain, and evaluates how these factors are implicated in the progress of neurodegenerative diseases and their potential use in the diagnosis and treatment of these diseases.

Dysregulated Wnt Signalling in the Alzheimer's Brain

The Wnt signalling system is essential for both the developing and adult central nervous system. It regulates numerous cellular functions ranging from neurogenesis to blood brain barrier biology. Dysregulated Wnt signalling can thus have significant consequences for normal brain function, which is becoming increasingly clear in Alzheimer's disease (AD), an age-related neurodegenerative disorder that is the most prevalent form of dementia. AD exhibits a range of pathophysiological manifestations including aberrant amyloid precursor protein processing, tau pathology, synapse loss, neuroinflammation and blood brain barrier breakdown, which have been associated to a greater or lesser degree with abnormal Wnt signalling. Here we provide a comprehensive overview of the role of Wnt signalling in the CNS, and the research that implicates dysregulated Wnt signalling in the ageing brain and in AD pathogenesis. We also discuss the opportunities for therapeutic intervention in AD via modulation of the Wnt signalling pathway, and highlight some of the challenges and the gaps in our current understanding that need to be met to enable that goal.

Dickkopf-related protein-1 inhibition attenuates amyloid-beta pathology associated to Alzheimer's disease

Alzheimer's disease (AD) constitutes the leading cause of dementia worldwide. It is associated to amyloid-β (Aβ) aggregation and tau hyper-phosphorylation, accompanied by a progressive cognitive decline. Evidence suggests that the canonical Wnt pathway is deregulated in AD. Pathway activity is mediated by β-catenin stabilization in the cytosol, and subsequent translocation to the nucleus to regulate the expression of several genes implicated in brain homeostasis and functioning. It was recently proposed that Dickkopf-related protein-1 (DKK1), an endogenous antagonist of the pathway, might be implicated in AD pathogenesis. Here, we hypothesized that canonical Wnt pathway deactivation associated to DKK1 induction contributes to late-onset AD pathogenesis, and thus DKK1 neutralization could attenuate AD pathology. For this purpose, human post-mortem AD brain samples were used to assess pathway activity, and aged APPswe/PS1 mice were used to investigate DKK1 in late-onset AD-like pathology and therapy. Our findings indicate that β-catenin levels progressively decrease in the brain of AD patients, correlating with the duration of symptoms. Next, we found that Aβ pathology in APPswe/PS1 mediates DKK1 induction in the brain. Pharmacological neutralization of DKK1's biological activity in APPswe/PS1 mice restores pathway activity by stabilizing β-catenin, attenuates Aβ pathology, and ameliorates the memory of mice. Attenuation of AD-like pathology upon DKK1 inhibition is accompanied by a reduced protein expression of beta-site amyloid precursor protein (APP) cleaving enzyme-1 (BACE1). Moreover, DKK1 inhibition enhances vascular density, promotes blood-brain barrier (BBB) integrity by increasing claudin 5, glucose transporter-1 (GLUT1), and ATP-binding cassette sub-family B member-1 (ABCB1) protein expression, as well as ameliorates synaptic plasticity by increasing brain-derived neurotrophic factor (BDNF), and postsynaptic density protein-95 (PSD-95) protein expression. DKK1 conditional induction reduces claudin 5, abcb1, and psd-95 mRNA expression, validating its inhibition effects. Our results indicate that neutralization of DKK1's biological activity attenuates AD-like pathology by restoring canonical Wnt pathway activity.

Unique signatures of stress-induced senescent human astrocytes

Senescence was recently linked to neurodegeneration and astrocytes are one of the major cell types to turn senescent under neurodegenerative conditions. Senescent astrocytes were detected in Parkinson's disease (PD) patients' brains besides reactive astrocytes, yet the difference between senescent and reactive astrocytes is unclear. We aimed to characterize senescent astrocytes in comparison to reactive astrocytes and investigate differences and similarities. In a cell culture model of human fetal astrocytes, we determined a unique senescent transcriptome distinct from reactive astrocytes, which comprises dysregulated pathways. Both, senescent and reactive human astrocytes activated a proinflammatory pattern. Astrocyte senescence was at least partially depending on active mechanistic-target-of-rapamycin (mTOR) and DNA-damage response signaling, both drivers of senescence. To further investigate how PD and senescence connect to each other, we asked if a PD-linked environmental factor induces senescence and if senescence impairs midbrain neurons. We could show that the PD-linked pesticide rotenone causes astrocyte senescence. We further delineate, that the senescent secretome exaggerates rotenone-induced neurodegeneration in midbrain neurons differentiated from human induced pluripotent stem cells (hiPSC) of PD patients with alpha-synuclein gene (SNCA) locus duplication.