Decisive role of Reelin signaling during early stages of Alzheimer's disease

REELIN ameliorates Alzheimer's disease, but how?

Alzheimer's disease (AD) is the most prevalent type of dementia; therefore, there is a high demand for therapeutic medication targeting it. In this context, extensive research has been conducted to identify molecular targets for drugs. AD manifests through two primary pathological signs: senile plaques and neurofibrillary tangles, caused by accumulations of amyloid-beta (Aβ) and phosphorylated tau, respectively. Thus, studies concerning the molecular mechanisms underlying AD etiology have primarily focused on Aβ generation and tau phosphorylation, with the anticipation of uncovering a signaling pathway impacting these molecular processes. Over the past two decades, studies using not only experimental model systems but also examining human brains have accumulated fragmentary evidences suggesting that REELIN signaling pathway is deeply involved in AD. Here, we explore REELIN signaling pathway and its involvement in memory function within the brain and review studies investigating molecular connections between REELIN signaling pathway and AD etiology. This review aims to understand how the manipulation (activation) of this pathway might ameliorate the disease's etiology.

Causal associations and genetic overlap between COVID-19 and intelligence

OBJECTIVE

COVID-19 might cause neuroinflammation in the brain, which could decrease neurocognitive function. We aimed to evaluate the causal associations and genetic overlap between COVID-19 and intelligence.

METHODS

We performed Mendelian randomization (MR) analyses to assess potential associations between three COVID-19 outcomes and intelligence (N = 269 867). The COVID phenotypes included severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection (N = 2 501 486), hospitalized COVID-19 (N = 1 965 329) and critical COVID-19 (N = 743 167). Genome-wide risk genes were compared between the genome-wide association study (GWAS) datasets on hospitalized COVID-19 and intelligence. In addition, functional pathways were constructed to explore molecular connections between COVID-19 and intelligence.

RESULTS

The MR analyses indicated that genetic liabilities to SARS-CoV-2 infection (odds ratio [OR]: 0.965, 95% confidence interval [CI]: 0.939-0.993) and critical COVID-19 (OR: 0.989, 95% CI: 0.979-0.999) confer causal effects on intelligence. There was suggestive evidence supporting the causal effect of hospitalized COVID-19 on intelligence (OR: 0.988, 95% CI: 0.972-1.003). Hospitalized COVID-19 and intelligence share 10 risk genes within 2 genomic loci, including MAPT and WNT3. Enrichment analysis showed that these genes are functionally connected within distinct subnetworks of 30 phenotypes linked to cognitive decline. The functional pathway revealed that COVID-19-driven pathological changes within the brain and multiple peripheral systems may lead to cognitive impairment.

CONCLUSIONS

Our study suggests that COVID-19 may exert a detrimental effect on intelligence. The tau protein and Wnt signaling may mediate the influence of COVID-19 on intelligence.

Morphological and biomolecular targets in retina and vitreous from Reelin-deficient mice (Reeler): Potential implications for age-related macular degeneration in Alzheimer’s dementia

The neurosensory retina is an outgrowth of the Central Nervous System (CNS), and the eye is considered “a window to the brain.” Reelin glycoprotein is directly involved in neurodevelopment, in synaptic plasticity, learning and memory. Consequently, abnormal Reelin signaling has been associated with brain neurodegeneration but its contributing role in ocular degeneration is still poorly explored. To this aim, experimental procedures were assayed on vitreous or retinas obtained from Reeler mice (knockout for Reelin protein) at different postnatal days (p) p14, p21 and p28. At p28, a significant increase in the expression of Amyloid Precursor Protein (APP) and its amyloidogenic peptide (Aβ1-42 along with truncated tau fragment (i.e., NH2htau)- three pathological hallmarks of Alzheimer’s disease (AD)-were found in Reeler mice when compared to their age-matched wild-type controls. Likewise, several inflammatory mediators, such as Interleukins, or crucial biomarkers of oxidative stress were also found to be upregulated in Reeler mice by using different techniques such as ELLA assay, microchip array or real-time PCR. Taken together, these findings suggest that a dysfunctional Reelin signaling enables the expression of key pathological features which are classically associated with AD neurodegenerative processes. Thus, this work suggests that Reeler mouse might be a suitable animal model to study not only the pathophysiology of developmental processes but also several neurodegenerative diseases, such as AD and Age-related Macular Degeneration (AMD), characterized by accumulation of APP and/or Aβ1-42, NH2htau and inflammatory markers.

Alzheimer’s disease: Pathophysiology and dental pulp stem cells therapeutic prospects

Alzheimer’s disease (AD) is a destructive neurodegenerative disease with the progressive dysfunction, structural disorders and decreased numbers of neurons in the brain, which leads to long-term memory impairment and cognitive decline. There is a growing consensus that the development of AD has several molecular mechanisms similar to those of other neurodegenerative diseases, including excessive accumulation of misfolded proteins and neurotoxic substances produced by hyperactivated microglia. Nonetheless, there is currently a lack of effective drug candidates to delay or prevent the progression of the disease. Based on the excellent regenerative and reparative capabilities of stem cells, the application of them to repair or replace injured neurons carries enormous promise. Dental pulp stem cells (DPSCs), originated from ectomesenchyme of the cranial neural crest, hold a remarkable potential for neuronal differentiation, and additionally express a variety of neurotrophic factors that contribute to a protective effect on injured neuronal cells. Notably, DPSCs can also express immunoregulatory factors to control neuroinflammation and potentiate the regeneration and recovery of injured neurons. These extraordinary features along with accessibility make DPSCs an attractive source of postnatal stem cells for the regeneration of neurons or protection of existing neural circuitry in the neurodegenerative diseases. The present reviews the latest research advance in the pathophysiology of AD and elaborate the neurodifferentiation and neuroprotective properties of DPSCs as well as their application prospects in AD.

Non-Canonical Roles of Tau and Their Contribution to Synaptic Dysfunction

Tau plays a central role in a group of neurodegenerative disorders collectively named tauopathies. Despite the wide range of diverse symptoms at the onset and during the progression of the pathology, all tauopathies share two common hallmarks, namely the misfolding and aggregation of Tau protein and progressive synaptic dysfunctions. Tau aggregation correlates with cognitive decline and behavioural impairment. The mechanistic link between Tau misfolding and the synaptic dysfunction is still unknown, but this correlation is well established in the human brain and also in tauopathy mouse models. At the onset of the pathology, Tau undergoes post-translational modifications (PTMs) inducing the detachment from the cytoskeleton and its release in the cytoplasm as a soluble monomer. In this condition, the physiological enrichment in the axon is definitely disrupted, resulting in Tau relocalization in the cell soma and in dendrites. Subsequently, Tau aggregates into toxic oligomers and amyloidogenic forms that disrupt synaptic homeostasis and function, resulting in neuronal degeneration. The involvement of Tau in synaptic transmission alteration in tauopathies has been extensively reviewed. Here, we will focus on non-canonical Tau functions mediating synapse dysfunction.

Synaptic basis of Alzheimer's disease: Focus on synaptic amyloid beta, P-tau and mitochondria

Alzheimer's disease (AD) is a progressive and synaptic failure disease. Despite the many years of research, AD still harbors many secrets. As more of the world's population grows older, researchers are striving to find greater information on disease progression and pathogenesis. Identifying and treating the markers of this disease, or better yet, preventing it all together, are the hopes of those investing in this field of study. Several years of research revealed that synaptic pathology and mitochondrial oxidative damage are early events in disease progression. Loss of synapses and synaptic damage are the best correlates of cognitive deficits found in AD patients. As the disease progresses, there are significant changes at the synapse. These changes can both shed greater light onto the progression of the disease and serve as markers and therapeutic targets. This article addresses the mechanisms of synaptic action, mitochondrial regulation/dysregulation, resulting synaptic changes caused by amyloid beta and phosphorylated tau in AD progression. This article also highlights recent developments of risk factors, genetics and ApoE4 involvement, factors related to synaptic damage and loss, mislocalization of amyloid beta and phosphorylated tau, mitophagy, microglial activation and synapse-based therapies in AD. Furthermore, impairments in LTD and reactivation of microglia are discussed.

Disease-Specific Changes in Reelin Protein and mRNA in Neurodegenerative Diseases

Reelin is an extracellular glycoprotein that modulates neuronal function and synaptic plasticity in the adult brain. Decreased levels of Reelin activity have been postulated as a key factor during neurodegeneration in Alzheimer’s disease (AD) and in aging. Thus, changes in levels of full-length Reelin and Reelin fragments have been revealed in cerebrospinal fluid (CSF) and in post-mortem brains samples of AD patients with respect to non-AD patients. However, conflicting studies have reported decreased or unchanged levels of full-length Reelin in AD patients compared to control (nND) cases in post-mortem brains and CSF samples. In addition, a compelling analysis of Reelin levels in neurodegenerative diseases other than AD is missing. In this study, we analyzed brain levels of RELN mRNA and Reelin protein in post-mortem frontal cortex samples from different sporadic AD stages, Parkinson’s disease with dementia (PDD), and Creutzfeldt-Jakob disease (sCJD), obtained from five different Biobanks. In addition, we measured Reelin protein levels in CSF samples of patients with mild cognitive impairment (MCI), dementia, or sCJD diagnosis and a group of neurologically healthy cases. The results indicate an increase in RELN mRNA in the frontal cortex of advanced stages of AD and in sCJD(I) compared to controls. This was not observed in PDD and early AD stages. However, Reelin protein levels in frontal cortex samples were unchanged between nND and advanced AD stages and PDD. Nevertheless, they decreased in the CSF of patients with dementia in comparison to those not suffering with dementia and patients with MCI. With respect to sCJD, there was a tendency to increase in brain samples in comparison to nND and to decrease in the CSF with respect to nND. In conclusion, Reelin levels in CSF cannot be considered as a diagnostic biomarker for AD or PDD. However, we feel that the CSF Reelin changes observed between MCI, patients with dementia, and sCJD might be helpful in generating a biomarker signature in prodromal studies of unidentified dementia and sCJD.

Reelin reverts biochemical, physiological and cognitive alterations in mouse models of Tauopathy

Reelin is an extracellular protein crucial for adult brain plasticity. Moreover, Reelin is protective against amyloid-β (Aβ) pathology in Alzheimer's Disease (AD), reducing plaque deposition, synaptic loss and cognitive decline. Given that Tau protein plays a key role in AD pathogenesis, and that the Reelin pathway modulates Tau phosphorylation, here we explored the involvement of Reelin in AD-related Tau pathology. We found that Reelin overexpression modulates the levels of Tau phosphorylation in AD-related epitopes in VLW mice expressing human mutant Tau. in vitro, Reelin reduced the Aβ-induced missorting of axonal Tau and neurofilament proteins to dendrites. Reelin also reverted in vivo the toxic somatodendritic localization of phosphorylated Tau. Finally, overexpression of Reelin in VLW mice improved long-term potentiation and long-term memory cognitive performance thus masking the cognitive and physiological deficits in VLW mice. These data suggest that the Reelin pathway, which is also protective against Aβ pathology, modulates fundamental traits of Tau pathology, strengthening the potential of Reelin as a therapeutic target in AD.

Context-sensitive network analysis identifies food metabolites associated with Alzheimer's disease: an exploratory study

BACKGROUND

Diet plays an important role in Alzheimer's disease (AD) initiation, progression and outcomes. Previous studies have shown individual food-derived substances may have neuroprotective or neurotoxic effects. However, few works systematically investigate the role of food and food-derived metabolites on the development and progression of AD.

METHODS

In this study, we systematically investigated 7569 metabolites and identified AD-associated food metabolites using a novel network-based approach. We constructed a context-sensitive network to integrate heterogeneous chemical and genetic data, and to model context-specific inter-relationships among foods, metabolites, human genes and AD.

RESULTS

Our metabolite prioritization algorithm ranked 59 known AD-associated food metabolites within top 4.9%, which is significantly higher than random expectation. Interestingly, a few top-ranked food metabolites were specifically enriched in herbs and spices. Pathway enrichment analysis shows that these top-ranked herb-and-spice metabolites share many common pathways with AD, including the amyloid processing pathway, which is considered as a hallmark in AD-affected brains and has pathological roles in AD development.

CONCLUSIONS

Our study represents the first unbiased systems approach to characterizing the effects of food and food-derived metabolites in AD pathogenesis. Our ranking approach prioritizes the known AD-associated food metabolites, and identifies interesting relationships between AD and the food group "herbs and spices". Overall, our study provides intriguing evidence for the role of diet, as an important environmental factor, in AD etiology.

Therapeutic correction of ApoER2 splicing in Alzheimer's disease mice using antisense oligonucleotides

Apolipoprotein E receptor 2 (ApoER2) is an apolipoprotein E receptor involved in long‐term potentiation, learning, and memory. Given its role in cognition and its association with the Alzheimer's disease (AD) risk gene, apoE, ApoER2 has been proposed to be involved in AD, though a role for the receptor in the disease is not clear. ApoER2 signaling requires amino acids encoded by alternatively spliced exon 19. Here, we report that the balance of ApoER2 exon 19 splicing is deregulated in postmortem brain tissue from AD patients and in a transgenic mouse model of AD. To test the role of deregulated ApoER2 splicing in AD, we designed an antisense oligonucleotide (ASO) that increases exon 19 splicing. Treatment of AD mice with a single dose of ASO corrected ApoER2 splicing for up to 6 months and improved synaptic function and learning and memory. These results reveal an association between ApoER2 isoform expression and AD, and provide preclinical evidence for the utility of ASOs as a therapeutic approach to mitigate Alzheimer's disease symptoms by improving ApoER2 exon 19 splicing.

Hypervulnerability of the adolescent prefrontal cortex to nutritional stress via reelin deficiency

Overconsumption of high-fat diets (HFDs) can critically affect synaptic and cognitive functions within telencephalic structures such as the medial prefrontal cortex (mPFC). The underlying mechanisms, however, remain largely unknown. Here we show that adolescence is a sensitive period for the emergence of prefrontal cognitive deficits in response to HFD. We establish that the synaptic modulator reelin (RELN) is a critical mediator of this vulnerability because (1) periadolescent HFD (pHFD) selectively downregulates prefrontal RELN⁺ cells and (2) augmenting mPFC RELN levels using transgenesis or prefrontal pharmacology prevents the pHFD-induced prefrontal cognitive deficits. We further identify N-methyl-d-aspartate-dependent long-term depression (NMDA-LTD) at prefrontal excitatory synapses as a synaptic signature of this association because pHFD abolishes NMDA-LTD, a function that is restored by RELN overexpression. We believe this study provides the first mechanistic insight into the vulnerability of the adolescent mPFC towards nutritional stress, such as HFDs. Our findings have primary relevance to obese individuals who are at an increased risk of developing neurological cognitive comorbidities, and may extend to multiple neuropsychiatric and neurological disorders in which RELN deficiency is a common feature.

Secreted Metalloproteinase ADAMTS-3 Inactivates Reelin

The secreted glycoprotein Reelin regulates embryonic brain development and adult brain functions. It has been suggested that reduced Reelin activity contributes to the pathogenesis of several neuropsychiatric and neurodegenerative disorders, such as schizophrenia and Alzheimer's disease; however, noninvasive methods that can upregulate Reelin activity in vivo have yet to be developed. We previously found that the proteolytic cleavage of Reelin within Reelin repeat 3 (N-t site) abolishes Reelin activity in vitro, but it remains controversial as to whether this effect occurs in vivo Here we partially purified the enzyme that mediates the N-t cleavage of Reelin from the culture supernatant of cerebral cortical neurons. This enzyme was identified as a disintegrin and metalloproteinase with thrombospondin motifs-3 (ADAMTS-3). Recombinant ADAMTS-3 cleaved Reelin at the N-t site. ADAMTS-3 was expressed in excitatory neurons in the cerebral cortex and hippocampus. N-t cleavage of Reelin was markedly decreased in the embryonic cerebral cortex of ADAMTS-3 knock-out (KO) mice. Importantly, the amount of Dab1 and the phosphorylation level of Tau, which inversely correlate with Reelin activity, were significantly decreased in the cerebral cortex of ADAMTS-3 KO mice. Conditional KO mice, in which ADAMTS-3 was deficient only in the excitatory neurons of the forebrain, showed increased dendritic branching and elongation in the postnatal cerebral cortex. Our study shows that ADAMTS-3 is the major enzyme that cleaves and inactivates Reelin in the cerebral cortex and hippocampus. Therefore, inhibition of ADAMTS-3 may be an effective treatment for neuropsychiatric and neurodegenerative disorders.SIGNIFICANCE STATEMENT ADAMTS-3 was identified as the protease that cleaves and inactivates Reelin in the cerebral cortex and hippocampus. ADAMTS-3 was expressed in the excitatory neurons of the embryonic and postnatal cerebral cortex and hippocampus. Cleavage by ADAMTS-3 is the major contributor of Reelin inactivation in vivo Tau phosphorylation was decreased and dendritic branching and elongation was increased in ADAMTS-3-deficient mice. Therefore, inhibition of ADAMTS-3 upregulates Reelin activity and may be a potential therapeutic strategy for the prevention or treatment of neuropsychiatric and neurodegenerative disorders, such as schizophrenia and Alzheimer's disease.

Reelin Expression in Creutzfeldt-Jakob Disease and Experimental Models of Transmissible Spongiform Encephalopathies

Reelin is an extracellular glycoprotein involved in key cellular processes in developing and adult nervous system, including regulation of neuronal migration, synapse formation, and plasticity. Most of these roles are mediated by the intracellular phosphorylation of disabled-1 (Dab1), an intracellular adaptor molecule, in turn mediated by binding Reelin to its receptors. Altered expression and glycosylation patterns of Reelin in cerebrospinal and cortical extracts have been reported in Alzheimer's disease. However, putative changes in Reelin are not described in natural prionopathies or experimental models of prion infection or toxicity. With this is mind, in the present study, we determined that Reelin protein and mRNA levels increased in CJD human samples and in mouse models of human prion disease in contrast to murine models of prion infection. However, changes in Reelin expression appeared only at late terminal stages of the disease, which prevent their use as an efficient diagnostic biomarker. In addition, increased Reelin in CJD and in in vitro models does not correlate with Dab1 phosphorylation, indicating failure in its intracellular signaling. Overall, these findings widen our understanding of the putative changes of Reelin in neurodegeneration.

Canonical and Non-canonical Reelin Signaling

Reelin is a large secreted glycoprotein that is essential for correct neuronal positioning during neurodevelopment and is important for synaptic plasticity in the mature brain. Moreover, Reelin is expressed in many extraneuronal tissues; yet the roles of peripheral Reelin are largely unknown. In the brain, many of Reelin's functions are mediated by a molecular signaling cascade that involves two lipoprotein receptors, apolipoprotein E receptor-2 (Apoer2) and very low density-lipoprotein receptor (Vldlr), the neuronal phosphoprotein Disabled-1 (Dab1), and members of the Src family of protein tyrosine kinases as crucial elements. This core signaling pathway in turn modulates the activity of adaptor proteins and downstream protein kinase cascades, many of which target the neuronal cytoskeleton. However, additional Reelin-binding receptors have been postulated or described, either as coreceptors that are essential for the activation of the "canonical" Reelin signaling cascade involving Apoer2/Vldlr and Dab1, or as receptors that activate alternative or additional signaling pathways. Here we will give an overview of canonical and alternative Reelin signaling pathways, molecular mechanisms involved, and their potential physiological roles in the context of different biological settings.

Loss of Reelin protects against atherosclerosis by reducing leukocyte-endothelial cell adhesion and lesion macrophage accumulation

The multimodular glycoprotein Reelin controls neuronal migration and synaptic transmission by binding to apolipoprotein E receptor 2 (Apoer2) and very low density lipoprotein receptor (Vldlr) on neurons. In the periphery, Reelin is produced by the liver, circulates in blood, and promotes thrombosis and hemostasis. To investigate if Reelin influences atherogenesis, we studied atherosclerosis-prone low-density lipoprotein receptor-deficient (Ldlr(-/-)) mice in which we inducibly deleted Reelin either ubiquitously or only in the liver, thus preventing the production of circulating Reelin. In both types of Reelin-deficient mice, atherosclerosis progression was markedly attenuated, and macrophage content and endothelial cell staining for vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) were reduced at the sites of atherosclerotic lesions. Intravital microscopy revealed decreased leukocyte-endothelial adhesion in the Reelin-deficient mice. In cultured human endothelial cells, Reelin enhanced monocyte adhesion and increased ICAM1, VCAM1, and E-selectin expression by suppressing endothelial nitric oxide synthase (eNOS) activity and increasing nuclear factor κB (NF-κB) activity in an Apoer2-dependent manner. These findings suggest that circulating Reelin promotes atherosclerosis by increasing vascular inflammation, and that reducing or inhibiting circulating Reelin may present a novel approach for the prevention of cardiovascular disease.

Docosahexaenoic Acid and Cognition throughout the Lifespan

Docosahexaenoic acid (DHA) is the predominant omega-3 (n-3) polyunsaturated fatty acid (PUFA) found in the brain and can affect neurological function by modulating signal transduction pathways, neurotransmission, neurogenesis, myelination, membrane receptor function, synaptic plasticity, neuroinflammation, membrane integrity and membrane organization. DHA is rapidly accumulated in the brain during gestation and early infancy, and the availability of DHA via transfer from maternal stores impacts the degree of DHA incorporation into neural tissues. The consumption of DHA leads to many positive physiological and behavioral effects, including those on cognition. Advanced cognitive function is uniquely human, and the optimal development and aging of cognitive abilities has profound impacts on quality of life, productivity, and advancement of society in general. However, the modern diet typically lacks appreciable amounts of DHA. Therefore, in modern populations, maintaining optimal levels of DHA in the brain throughout the lifespan likely requires obtaining preformed DHA via dietary or supplemental sources. In this review, we examine the role of DHA in optimal cognition during development, adulthood, and aging with a focus on human evidence and putative mechanisms of action.

Docosahexaenoic Acid and Cognition throughout the Lifespan

Docosahexaenoic acid (DHA) is the predominant omega-3 (n-3) polyunsaturated fatty acid (PUFA) found in the brain and can affect neurological function by modulating signal transduction pathways, neurotransmission, neurogenesis, myelination, membrane receptor function, synaptic plasticity, neuroinflammation, membrane integrity and membrane organization. DHA is rapidly accumulated in the brain during gestation and early infancy, and the availability of DHA via transfer from maternal stores impacts the degree of DHA incorporation into neural tissues. The consumption of DHA leads to many positive physiological and behavioral effects, including those on cognition. Advanced cognitive function is uniquely human, and the optimal development and aging of cognitive abilities has profound impacts on quality of life, productivity, and advancement of society in general. However, the modern diet typically lacks appreciable amounts of DHA. Therefore, in modern populations, maintaining optimal levels of DHA in the brain throughout the lifespan likely requires obtaining preformed DHA via dietary or supplemental sources. In this review, we examine the role of DHA in optimal cognition during development, adulthood, and aging with a focus on human evidence and putative mechanisms of action.

Determination of cleavage site of Reelin between its sixth and seventh repeat and contribution of meprin metalloproteases to the cleavage

Reelin is a secreted glycoprotein whose function is regulated by proteolysis. One of the specific cleavage sites of Reelin, called C-t, is located approximately between the sixth and seventh Reelin repeat but its exact site was unknown. We here show that a metalloprotease present in the culture supernatant of cerebellar granular neurons (CGN) cleaves Reelin between Ala2688 and Asp2689. A Reelin mutant in which Asp2689 is replaced by Lys (Reelin-DK) is resistant to C-t cleavage by culture supernatant of CGN. From biochemical characteristics and the cleavage site preference, meprin α and meprin β were suggested candidate proteases and both were confirmed to cleave Reelin at the C-t site. Meprin α cleaved Reelin-DK but meprin β did not. Actinonin, a meprin α and meprin β inhibitor, did not inhibit the Reelin-cleaving activity of CGN and the amount of Reelin fragments in brains of meprin β knock-out mice was not significantly different from that of the wild-type, indicating that meprin β does not play a major role in Reelin cleavage under basal conditions. We propose that meprin α and meprin β join the modulators of Reelin signalling as they cleave Reelin at a specific site and are upregulated under specific pathological conditions.

Learning from the past and looking to the future: Emerging perspectives for improving the treatment of psychiatric disorders

Modern neuropsychopharmacology commenced in the 1950s with the serendipitous discovery of first-generation antipsychotics and antidepressants which were therapeutically effective yet had marked adverse effects. Today, a broader palette of safer and better-tolerated agents is available for helping people that suffer from schizophrenia, depression and other psychiatric disorders, while complementary approaches like psychotherapy also have important roles to play in their treatment, both alone and in association with medication. Nonetheless, despite considerable efforts, current management is still only partially effective, and highly-prevalent psychiatric disorders of the brain continue to represent a huge personal and socio-economic burden. The lack of success in discovering more effective pharmacotherapy has contributed, together with many other factors, to a relative disengagement by pharmaceutical firms from neuropsychiatry. Nonetheless, interest remains high, and partnerships are proliferating with academic centres which are increasingly integrating drug discovery and translational research into their traditional activities. This is, then, a time of transition and an opportune moment to thoroughly survey the field. Accordingly, the present paper, first, chronicles the discovery and development of psychotropic agents, focusing in particular on their mechanisms of action and therapeutic utility, and how problems faced were eventually overcome. Second, it discusses the lessons learned from past successes and failures, and how they are being applied to promote future progress. Third, it comprehensively surveys emerging strategies that are (1), improving our understanding of the diagnosis and classification of psychiatric disorders; (2), deepening knowledge of their underlying risk factors and pathophysiological substrates; (3), refining cellular and animal models for discovery and validation of novel therapeutic agents; (4), improving the design and outcome of clinical trials; (5), moving towards reliable biomarkers of patient subpopulations and medication efficacy and (6), promoting collaborative approaches to innovation by uniting key partners from the regulators, industry and academia to patients. Notwithstanding the challenges ahead, the many changes and ideas articulated herein provide new hope and something of a framework for progress towards the improved prevention and relief of psychiatric and other CNS disorders, an urgent mission for our Century.

Kainic Acid-Induced Golgi Complex Fragmentation/Dispersal Shifts the Proteolysis of Reelin in Primary Rat Neuronal Cells: An In Vitro Model of Early Stage Epilepsy

The endoplasmic reticulum-lysosome-Golgi network plays an important role in Reelin glycosylation and its proteolytic processing. Golgi complex fragmentation is associated with the separation of Reelin from this network. Kainic acid (KA) is an excitotoxic agent commonly used to induce epilepsy in rodents. The relationship between KA-induced neuronal damage and Golgi complex fragmentation has not been investigated, leaving a major gap in our understanding of the molecular mechanism underlying the development of pathophysiology in epilepsy. We cultured primary rat cortical neurons eitherin ambient condition (control) or treated with a range of KA doses to reveal whether Golgi complex fragmentation impaired neuronal function. The half-life maximal inhibitory concentration (IC₅₀) value of KA was detected to be approximately 5 μM, whereby at these concentrations, KA impaired neuronal viability, which was closely associated with initial Golgi complex fragmentation and subsequent reduction in both the expression and glycosylation patterns of Reelin. These findings implicate that Golgi complex fragmentation and Reelin dysfunction are key contributors to neuronal cell death in the early stage of epilepsy pathophysiology, thereby representing as novel disease biomarkers, as well as potent therapeutic targets for epilepsy.

More than cholesterol transporters: lipoprotein receptors in CNS function and neurodegeneration

Members of the low-density lipoprotein (LDL) receptor gene family have a diverse set of biological functions that transcend lipid metabolism. Lipoprotein receptors have broad effects in both the developing and adult brain and participate in synapse development, cargo trafficking, and signal transduction. In addition, several family members play key roles in Alzheimer's disease (AD) pathogenesis and neurodegeneration. This Review summarizes our current understanding of the role lipoprotein receptors play in CNS function and AD pathology, with a special emphasis on amyloid-independent roles in endocytosis and synaptic dysfunction.

Characterization of hippocampal Cajal-Retzius cells during development in a mouse model of Alzheimer's disease (Tg2576)

Cajal-Retzius cells are reelin-secreting neurons in the marginal zone of the neocortex and hippocampus. The aim of this study was to investigate Cajal-Retzius cells in Alzheimer's disease pathology. Results revealed that the number of Cajal-Retzius cells markedly reduced with age in both wild type and in mice over-expressing the Swedish double mutant form of amyloid precursor protein 695 (transgenic (Tg) 2576 mice). Numerous reelin-positive neurons were positive for activated caspase 3 in Tg2576 mice, suggesting that Cajal-Retzius neuronal loss occurred via apoptosis in this Alzheimer's disease model. Compared with wild type, the number of Cajal-Retzius cells was significantly lower in Tg2576 mice. Western blot analysis confirmed that reelin levels were markedly lower in Tg2576 mice than in wild-type mice. The decline in Cajal-Retzius cells in Tg2576 mice was found to occur concomitantly with the onset of Alzheimer's disease amyloid pathology and related behavioral deficits. Overall, these data indicated that Cajal-Retzius cell loss occurred with the onset and development of Alzheimer's disease.

Genes associated with the progression of neurofibrillary tangles in Alzheimer's disease

The spreading of neurofibrillary tangles (NFTs), intraneuronal aggregates of highly phosphorylated microtubule-associated protein tau, across the human brain is correlated with the cognitive severity of Alzheimer's disease (AD). To identify genes relevant to NFT expansion defined by the Braak stage, we conducted whole-genome exon array analysis with an exploratory sample set consisting of 213 human post-mortem brain tissue specimens from the entorinal, temporal and frontal cortices of 71 brain-donor subjects: Braak NFT stages 0 (N=13), I-II (N=20), III-IV (N=19) and V-VI (N=19). We identified eight genes, RELN, PTGS2, MYO5C, TRIL, DCHS2, GRB14, NPAS4 and PHYHD1, associated with the Braak stage. The expression levels of three genes, PHYHD1, MYO5C and GRB14, exhibited reproducible association on real-time quantitative PCR analysis. In another sample set, including control subjects (N=30), and in patients with late-onset AD (N=37), dementia with Lewy bodies (N=17) and Parkinson disease (N=36), the expression levels of two genes, PHYHD1 and MYO5C, were obviously associated with late-onset AD. Protein-protein interaction network analysis with a public database revealed that PHYHD1 interacts with MYO5C via POT1, and PHYHD1 directly interacts with amyloid beta-peptide 42. It is thus likely that functional failure of PHYHD1 and MYO5C could lead to AD development.

Dendritic planarity of Purkinje cells is independent of Reelin signaling

The dendritic planarity of Purkinje cells is critical for cerebellar circuit formation. In the absence of Crk and CrkL, the Reelin pathway does not function resulting in partial Purkinje cell migration and defective dendritogenesis. However, the relationships among Purkinje cell migration, dendritic development and Reelin signaling have not been clearly delineated. Here, we use synchrotron X-ray microscopy to obtain 3-D images of Golgi-stained Purkinje cell dendrites. Purkinje cells that failed to migrate completely exhibited conical dendrites with abnormal 3-D arborization and reduced dendritic complexity. Furthermore, their spines were fewer in number with a distorted morphology. In contrast, Purkinje cells that migrated successfully displayed planar dendritic and spine morphologies similar to normal cells, despite reduced dendritic complexity. These results indicate that, during cerebellar formation, Purkinje cells migrate into an environment that supports development of dendritic planarity and spine formation. While Reelin signaling is important for the migration process, it does not make a direct major contribution to dendrite formation.

Reelin delays amyloid-beta fibril formation and rescues cognitive deficits in a model of Alzheimer's disease

Reelin is an extracellular matrix protein that is crucial for neural development and adult brain plasticity. While the Reelin signalling cascade has been reported to be associated with Alzheimer's disease (AD), the role of Reelin in this pathology is not understood. Here we use an in vitro approach to show that Reelin interacts with amyloid-β (Aβ42) soluble species, delays Aβ42 fibril formation and is recruited into amyloid fibrils. Furthermore, Reelin protects against both the neuronal death and dendritic spine loss induced by Aβ42 oligomers. In mice carrying the APP(Swe/Ind) mutation (J20 mice), Reelin overexpression delays amyloid plaque formation and rescues the recognition memory deficits. Our results indicate that by interacting with Aβ42 soluble species, delaying Aβ plaque formation, protecting against neuronal death and dendritic spine loss and preventing AD cognitive deficits, the Reelin pathway deserves consideration as a therapeutic target for the treatment of AD pathogenesis.

The airbag problem-a potential culprit for bench-to-bedside translational efforts: relevance for Alzheimer's disease

For the last 20 years, the "amyloid cascade hypothesis" has dominated research aimed at understanding, preventing, and curing Alzheimer's disease (AD). During that time researchers have acquired an enormous amount of data and have been successful, more than 300 times, in curing the disease in animal model systems by treatments aimed at clearing amyloid deposits. However, to date similar strategies have not been successful in human AD patients. Hence, before rushing into further clinical trials with compounds that aim at lowering amyloid-beta (Aβ) levels in increasingly younger people, it would be of highest priority to re-assess the initial assumption that accumulation of Aβ in the brain is the primary pathological event driving AD. Here we question this assumption by highlighting experimental evidence in support of the alternative hypothesis suggesting that APP and Aβ are part of a neuronal stress/injury system, which is up-regulated to counteract inflammation/oxidative stress-associated neurodegeneration that could be triggered by a brain injury, chronic infections, or a systemic disease. In AD, this protective program may be overridden by genetic and other risk factors, or its maintenance may become dysregulated during aging. Here, we provide a hypothetical example of a hypothesis-driven correlation between car accidents and airbag release in analogy to the evolution of the amyloid focus and as a way to offer a potential explanation for the failure of the AD field to translate the success of amyloid-related therapeutic strategies in experimental models to the clinic.

Reelin immunoreactivity in neuritic varicosities in the human hippocampal formation of non-demented subjects and Alzheimer's disease patients

BACKGROUND

Reelin and its downstream signaling members are important modulators of actin and microtubule cytoskeleton dynamics, a fundamental prerequisite for proper neurodevelopment and adult neuronal functions. Reductions in Reelin levels have been suggested to contribute to Alzheimer's disease (AD) pathophysiology. We have previously reported an age-related reduction in Reelin levels and its accumulation in neuritic varicosities along the olfactory-limbic tracts, which correlated with cognitive impairments in aged mice. Here, we aimed to investigate whether a similar Reelin-associated neuropathology is observed in the aged human hippocampus and whether it correlated with dementia status.

RESULTS

Our immunohistochemical stainings revealed the presence of N- and C-terminus-containing Reelin fragments in corpora amylacea (CAm), aging-associated spherical deposits. The density of these deposits was increased in the molecular layer of the subiculum of AD compared to non-demented individuals. Despite the limitation of a small sample size, our evaluation of several neuronal and glial markers indicates that the presence of Reelin in CAm might be related to aging-associated impairments in neuronal transport leading to accumulation of organelles and protein metabolites in neuritic varicosities, as previously suggested by the findings and discussions in rodents and primates.

CONCLUSIONS

Our results indicate that aging- and disease-associated changes in Reelin levels and proteolytic processing might play a role in the formation of CAm by altering cytoskeletal dynamics. However, its presence may also be an indicator of a degenerative state of neuritic compartments.