Rationally Designed Peptoid Inhibitors of Amyloid-ß Oligomerization.

While plaques comprised of fibrillar Aß aggregates are hallmarks of Alzheimer's disease, soluble Aß oligomers present higher neurotoxicity. Thus, one therapeutic approach is to prevent the formation of Aß oligomers and reduce their associated harmful effects. We have proposed a peptoid mimic of the Aß hydrophobic KLVFF core as an ideal candidate aggregation inhibitor due to its ability to evade proteolytic degradation via repositioning of the side chain from the α-carbon to the amide nitrogen. This peptoid, JPT1, utilizes chiral sidechains to achieve a helical structure, while C-terminal addition of two phenylalanine residues places aromatic groups on two sides of the helix with spacing designed to facilitate interaction with amyloid ß-sheet structure. We have previously shown that JPT1 modulates Aß fibril formation. Here, we demonstrate that JPT1 also modulates Aß oligomerization, and we explore the role of the charge on the linker between the KLVFF mimic and the extended aromatic residues. Additionally, we demonstrate that peptoid-induced changes in Aß oligomerization correlate with attenuation of oligomer-induced nuclear factor-κB activation in SH-SY5Y human neuroblastoma cells. These findings support the therapeutic potential of peptoids to target early stages of Aß aggregation and impact the associated Aß-induced cellular response.

Modulating the RAGE-Induced Inflammatory Response: Peptoids as RAGE Antagonists.

While the primary pathology of Alzheimer's disease (AD) is defined by brain deposition of amyloid-ß (Aß) plaques and tau neurofibrillary tangles, chronic inflammation has emerged as an important factor in AD etiology. Upregulated cell surface expression of the receptor for advanced glycation end-products (RAGE), a key receptor of innate immune response, is reported in AD. In parallel, RAGE ligands, including Aß aggregates, HMGB1, and S100B, are elevated in AD brain. Activation of RAGE by these ligands triggers release of inflammatory cytokines and upregulates cell surface RAGE. Despite such observation, there are currently no therapeutics that target RAGE for treatment of AD-associated neuroinflammation. Peptoids, a novel class of potential AD therapeutics, display low toxicity, facile blood-brain barrier permeability, and resistance to proteolytic degradation. In the current study, peptoids were designed to mimic Aß, a ligand that binds the V-domain of RAGE, and curtail RAGE inflammatory activation. We reveal the nanomolar binding capability of peptoids JPT1 and JPT1a to RAGE and demonstrate their ability to attenuate lipopolysaccharide-induced pro-inflammatory cytokine production as well as upregulation of RAGE cell surface expression. These results support RAGE antagonist peptoid-based mimics as a prospective therapeutic strategy to counter neuroinflammation in AD and other neurodegenerative diseases.

Design and Synthesis of Ranitidine Analogs as Multi-Target Directed Ligands for the Treatment of Alzheimer's Disease.

The aggregation of amyloid ß (Aß) peptides and deposition of amyloid plaques are implicated in the pathogenesis of Alzheimer's disease (AD). Therefore, blocking Aß aggregation with small molecules has been proposed as one therapeutic approach for AD. In the present study, a series of ranitidine analogs containing cyclic imide isosteres were synthesized and their inhibitory activities toward Aß aggregation were evaluated using in vitro thioflavin T assays. The structure-activity relationship revealed that the 1,8-naphthalimide moiety provided profound inhibition of Aß aggregation and structural modifications on the other parts of the parent molecule (compound 6) maintained similar efficacy. Some of these ranitidine analogs also possessed potent inhibitory activities of acetylcholinesterase (AChE), which is another therapeutic target in AD. These ranitidine analogs, by addressing both Aß aggregation and AChE, offer insight into the key chemical features of a new type of multi-target directed ligands for the pharmaceutical treatment of AD.

Cysteine-rich granulin-3 rapidly promotes amyloid-ß fibrils in both redox states.

Granulins (GRNs 1-7) are cysteine-rich proteolytic products of progranulin (PGRN) that have recently been implicated in neurodegenerative diseases including frontotemporal dementia (FTD) and Alzheimer's disease (AD). Their precise mechanism in these pathologies remains uncertain, but both inflammatory and lysosomal roles have been observed for GRNs. Among the seven GRNs, GRN-3 is well characterized and is implicated within the context of FTD. However, the relationship between GRN-3 and amyloid-ß (Aß), a protein relevant in AD pathology, has not yet been explored. To gain insight into this mechanism, we investigated the effect of both oxidized and reduced GRN-3 on Aß aggregation and found that both GRN-3 (oxidized) and rGRN-3 (reduced) bind to monomeric and oligomeric Aß42 to promote rapid fibril formation with subtle rate differences. As low molecular weight oligomers of Aß are well-established neurotoxins, rapid promotion of fibrils by GRN-3 mitigates Aß42-induced cellular apoptosis. These data provide valuable insights in understanding GRN-3's ability to modulate Aß-induced toxicity under redox control and presents a new perspective toward AD pathology. These results also prompt further investigation into the role(s) of other GRNs in AD pathogenesis.

Modulating amyloid-ß aggregation: The effects of peptoid side chain placement and chirality.

Alzheimer's disease (AD) is characterized by the buildup of insoluble aggregated amyloid-ß protein (Aß) into plaques that accumulate between the neural cells in the brain. AD is the sixth leading cause of death in the United States and is the only cause of death among the top ten that cannot currently be treated or cured (Alzheimer's Association, 2011; Selkoe, 1996). Researchers have focused on developing small molecules and peptides to prevent Aß aggregation; however, while some compounds appear promising in vitro, the research has not resulted in a viable therapeutic treatment. We previously reported a peptoid-based mimic (JPT1) of the peptide KLVFF (residues 16-20 of Aß) that modulates Aß40 aggregation, specifically reducing the total number of fibrillar, ß-sheet structured aggregates formed. In this study, we investigate two new variants of JPT1 that probe the importance of aromatic side chain placement (JPT1s) and side chain chirality (JPT1a). Both JPT1s and JPT1a modulate Aß40 aggregation by reducing total ß-sheet aggregates. However, JPT1a also has a pronounced effect on the morphology of fibrillar Aß40 aggregates. These results suggest that Aß40 aggregation may follow a different pathway in the presence of peptoids with different secondary structures. A better understanding of the interactions between peptoids and Aß will allow for improved design of AD treatments.

Conformational Dynamics of Specific Aß Oligomers Govern Their Ability To Replicate and Induce Neuronal Apoptosis.

Oligomers of amyloid-ß (Aß) have emerged as the primary toxic agents responsible for early synaptic dysfunction and neuronal death in Alzheimer's disease (AD). Characterization of oligomers is an important step in the progress toward delineating the complex molecular mechanisms involved in AD pathogenesis. In our previous reports, we established that a distinct 12-24mer neurotoxic oligomer of Aß42, called Large Fatty Acid derived Oligomers (LFAOs), exhibits a unique property of replication in which LFAOs directly duplicate to quantitatively larger amounts upon interacting with monomers. This self-propagative process of replication is somewhat reminiscent of prion propagation. In this report, we sought to investigate the concentration-dependent conformational dynamics LFAOs undergo and how such transitions manifest in their ability to replicate and induce neuronal apoptosis. The results indicate that LFAOs undergo a concentration-dependent transition between 12mers and disperse 12-24mers with a dissociation constant (Kd) of 0.1 µM. The two species differ in their respective tertiary/quaternary structures but not their secondary structures. This conformational dynamics of LFAOs correlates with their ability to replicate and to induce apoptosis in SH-SY5Y human neuroblastoma cells, with 12mers being more neurotoxic and prone to replication than 12-24mers. The latter result implicates the replication process dominates at low physiological concentrations. The observations made in this report may have profound significance in deciphering the elusive roles of Aß oligomer phenotypes and in determining their prion-type behavior in AD pathology.

Capillary electrophoresis for the analysis of the effect of sample preparation on early stages of Aß1-40 aggregation.

Aggregation of the amyloid-ß protein (Aß) contributes to the neurodegeneration characteristic of Alzheimer's disease. Of particular importance are the early stages of aggregation, which involve the formation of soluble oligomers and protofibrils. In these studies, we demonstrate the potential for CE with UV detection using a polyethylene oxide separation matrix to identify the evolution of various oligomeric species of Aß1-40 . To demonstrate the efficacy of this technique, UV-CE was utilized to compare two methods commonly used to prepare Aß for aggregation experiments and their effect on the formation of early aggregates. SEC-purified Aß1-40 initially contained more small species, including monomer, than did freshly dissolved Aß1-40 pretreated with hexafluoroisopropanol. Strikingly, the lag time to oligomer formation for SEC-isolated Aß1-40 samples was ∼23 h shorter compared to freshly dissolved Aß1-40 samples. Furthermore, oligomers formed from the aggregation of SEC-purified Aß1-40 persisted within solution for a longer period of time. These results indicate that the initial sample preparation has a drastic influence on the early stages of Aß1-40 aggregation. This is the first report of the use of UV-CE with a separation matrix to study the effect of sample preparation on early aggregation of Aß1-40 . UV-CE was also used in parallel with dot blot analysis and inhibitory compounds to discern structural characteristics of individual oligomer peaks, demonstrating the capacity of UV-CE as a complimentary technique to further understand the aggregation process.

Human in vitro blood barrier models: architectures and applications.

Blood barriers serve as key points of transport for essential molecules as well as lines of defense to protect against toxins. In vitro modeling of these barriers is common practice in the study of their physiology and related diseases. This review describes a common method of using an adaptable, low cost, semipermeable, suspended membrane to experimentally model three blood barriers in the human body: the blood-brain barrier (BBB), the gut-blood barrier (GBB), and the air-blood barrier (ABB). The GBB and ABB both protect from the outside environment, while the BBB protects the central nervous system from potential neurotoxic agents in the blood. These barriers share several commonalities, including the formation of tight junctions, polarized cellular monolayers, and circulatory system contact. Cell architectures used to mimic barrier anatomy as well as applications to study function, dysfunction, and response provide an overview of the versatility enabled by these cultural systems.

Soluble and insoluble protein aggregates, endoplasmic reticulum stress, and vascular dysfunction in Alzheimer's disease and cardiovascular diseases.

Dementia refers to a particular group of symptoms characterized by difficulties with memory, language, problem-solving, and other thinking skills that affect a person's ability to perform everyday activities. Alzheimer's disease (AD) is the most common form of dementia, affecting about 6.2 million Americans aged 65 years and older. Likewise, cardiovascular diseases (CVDs) are a major cause of disability and premature death, impacting 126.9 million adults in the USA, a number that increases with age. Consequently, CVDs and cardiovascular risk factors are associated with an increased risk of AD and cognitive impairment. They share important age-related cardiometabolic and lifestyle risk factors, that make them among the leading causes of death. Additionally, there are several premises and hypotheses about the mechanisms underlying the association between AD and CVD. Although AD and CVD may be considered deleterious to health, the study of their combination constitutes a clinical challenge, and investigations to understand the mechanistic pathways for the cause-effect and/or shared pathology between these two disease constellations remains an active area of research. AD pathology is propagated by the amyloid ß (Aß) peptides. These peptides give rise to small, toxic, and soluble Aß oligomers (SPOs) that are nonfibrillar, and it is their levels that show a robust correlation with the extent of cognitive impairment. This review will elucidate the interplay between the effects of accumulating SPOs in AD and CVDs, the resulting ER stress response, and their role in vascular dysfunction. We will also address the potential underlying mechanisms, including the possibility that SPOs are among the causes of vascular injury in CVD associated with cognitive decline. By revealing common mechanistic underpinnings of AD and CVD, we hope that novel experimental therapeutics can be designed to reduce the burden of these devastating diseases. Graphical abstract Alzheimer's disease (AD) pathology leads to the release of Aß peptides, and their accumulation in the peripheral organs has varying effects on various components of the cardiovascular system including endoplasmic reticulum (ER) stress and vascular damage. Image created with BioRender.com.

Unraveling the early events of amyloid-ß protein (Aß) aggregation: techniques for the determination of Aß aggregate size.

The aggregation of proteins into insoluble amyloid fibrils coincides with the onset of numerous diseases. An array of techniques is available to study the different stages of the amyloid aggregation process. Recently, emphasis has been placed upon the analysis of oligomeric amyloid species, which have been hypothesized to play a key role in disease progression. This paper reviews techniques utilized to study aggregation of the amyloid-ß protein (Aß) associated with Alzheimer's disease. In particular, the review focuses on techniques that provide information about the size or quantity of oligomeric Aß species formed during the early stages of aggregation, including native-PAGE, SDS-PAGE, Western blotting, capillary electrophoresis, mass spectrometry, fluorescence correlation spectroscopy, light scattering, size exclusion chromatography, centrifugation, enzyme-linked immunosorbent assay, and dot blotting.

Inhibition of amyloid-ß aggregation by coumarin analogs can be manipulated by functionalization of the aromatic center.

Aggregation of the amyloid-ß protein (Aß) plays a pathogenic role in the progression of Alzheimer's disease, and small molecules that attenuate Aß aggregation have been identified toward a therapeutic strategy that targets the disease's underlying cause. Compounds containing aromatic structures have been repeatedly reported as effective inhibitors of Aß aggregation, but the functional groups that influence inhibition by these aromatic centers have been less frequently explored. The current study identifies analogs of naturally occurring coumarin as novel inhibitors of Aß aggregation. Derivatization of the coumarin structure is shown to affect inhibitory capabilities and to influence the point at which an inhibitor intervenes within the nucleation dependent Aß aggregation pathway. In particular, functional groups found within amyloid binding dyes, such as benzothiazole and triazole, can improve inhibition efficacy. Furthermore, inhibitor intervention at early or late stages within the amyloid aggregation pathway is shown to correlate with the ability of these functional groups to recognize and bind amyloid species that appear either early or late within the aggregation pathway. These results demonstrate that functionalization of small aromatic molecules with recognition elements can be used in the rational design of Aß aggregation inhibitors to not only enhance inhibition but to also manipulate the inhibition mechanism.

Monitoring insulin aggregation via capillary electrophoresis.

Early stages of insulin aggregation, which involve the transient formation of oligomeric aggregates, are an important aspect in the progression of Type II diabetes and in the quality control of pharmaceutical insulin production. This study is the first to utilize capillary electrophoresis (CE) with ultraviolet (UV) detection to monitor insulin oligomer formation at pH 8.0 and physiological ionic strength. The lag time to formation of the first detected species in the aggregation process was evaluated by UV-CE and thioflavin T (ThT) binding for salt concentrations from 100 mM to 250 mM. UV-CE had a significantly shorter (5-8 h) lag time than ThT binding (15-19 h). In addition, the lag time to detection of the first aggregated species via UV-CE was unaffected by salt concentration, while a trend toward an increased lag time with increased salt concentration was observed with ThT binding. This result indicates that solution ionic strength impacts early stages of aggregation and ß-sheet aggregate formation differently. To observe whether CE may be applied for the analysis of biological samples containing low insulin concentrations, the limit of detection using UV and laser induced fluorescence (LIF) detection modes was determined. The limit of detection using LIF-CE, 48.4 pM, was lower than the physiological insulin concentration, verifying the utility of this technique for monitoring biological samples. LIF-CE was subsequently used to analyze the time course for fluorescein isothiocyanate (FITC)-labeled insulin oligomer formation. This study is the first to report that the FITC label prevented incorporation of insulin into oligomers, cautioning against the use of this fluorescent label as a tag for following early stages of insulin aggregation.

Comparative study of inhibition at multiple stages of amyloid-beta self-assembly provides mechanistic insight.

The "amyloid cascade hypothesis," linking self-assembly of the amyloid-beta protein (Abeta) to the pathogenesis of Alzheimer's disease, has led to the emergence of inhibition of Abeta self-assembly as a prime therapeutic strategy for this currently unpreventable and devastating disease. The complexity of Abeta self-assembly, which involves multiple reaction intermediates related by nonlinear and interconnected nucleation and growth mechanisms, provides multiple points for inhibitor intervention. Although a number of small-molecule inhibitors of Abeta self-assembly have been identified, little insight has been garnered concerning the point at which these inhibitors intervene within the Abeta assembly process. In the current study, a julolidine derivative is identified as an inhibitor of Abeta self-assembly. To gain insight into the mechanistic action of this inhibitor, the inhibition of fibril formation from monomeric protein is assessed quantitatively and compared with the inhibition of two distinct mechanisms of growth for soluble Abeta aggregation intermediates. This compound is observed to significantly inhibit soluble aggregate growth by lateral association while having little effect on soluble aggregate elongation via monomer addition. In addition, inhibition of soluble Abeta aggregate association exhibits an IC(50) with a somewhat lower stoichiometric ratio than the IC(50) determined for inhibition of fibril formation from monomeric Abeta. This quantitative comparison of inhibition within multiple Abeta self-assembly assays suggests that this compound binds the lateral surface of on-pathway intermediates exhibiting a range of sizes to prevent their association with other aggregates, which is required for further assembly into mature fibrils.

Soluble aggregates of the amyloid-beta protein activate endothelial monolayers for adhesion and subsequent transmigration of monocyte cells.

Increasing evidence suggests that the deposition of amyloid plaques, composed primarily of the amyloid-beta protein (Abeta), within the cerebrovasculature is a frequent occurrence in Alzheimer's disease and may play a significant role in disease progression. Accordingly, the pathogenic mechanisms by which Abeta can alter vascular function may have therapeutic implications. Despite observations that Abeta elicits a number of physiological responses in endothelial cells, ranging from alteration of protein expression to cell death, the Abeta species accountable for these responses remains unexplored. In the current study, we show that isolated soluble Abeta aggregation intermediates activate human brain microvascular endothelial cells for both adhesion and subsequent transmigration of monocyte cells in the absence of endothelial cell death and monolayer disruption. In contrast, unaggregated Abeta monomer and mature Abeta fibril fail to induce any change in endothelial adhesion or transmigration. Correlations between average Abeta aggregate size and observed increases in adhesion illustrate that smaller soluble aggregates are more potent activators of endothelium. These results support previous studies demonstrating heightened neuronal activity of soluble Abeta aggregates, including Abeta-derived diffusible ligands, oligomers, and protofibrils, and further show that soluble aggregates also selectively exhibit activity in a vascular cell model.

Soluble aggregates of the amyloid-beta protein selectively stimulate permeability in human brain microvascular endothelial monolayers.

Cerebral amyloid angiopathy associated with Alzheimer's disease is characterized by cerebrovascular deposition of the amyloid-beta protein (Abeta). Abeta elicits a number of morphological and biochemical alterations in the cerebral microvasculature, which culminate in hemorrhagic stroke. Among these changes, compromise of the blood-brain barrier has been described in Alzheimer's disease brain, transgenic animal models of Alzheimer's disease, and cell culture experiments. In the current study, presented data illustrates that isolated soluble Abeta(1-40) aggregates, but not unaggregated monomer or mature fibril, enhance permeability in human brain microvascular endothelial monolayers. Abeta(1-40)-induced changes in permeability are paralleled by both a decrease in transendothelial electrical resistance and a re-localization of the tight junction-associated protein zonula occludin-1 away from cell borders and into the cytoplasm. Small soluble Abeta(1-40) aggregates are confirmed to be the most potent stimulators of endothelial monolayer permeability by establishing an inverse relationship between average aggregate size and stimulated changes in diffusional permeability coefficients. These results support previous findings demonstrating that small soluble Abeta(1-40) aggregates are also primarily responsible for endothelial activation, suggesting that these same species may elicit other changes in the cerebrovasculature associated with cerebral amyloid angiopathy and Alzheimer's disease.

Quartz crystal microbalance analysis of growth kinetics for aggregation intermediates of the amyloid-beta protein.

Evidence linking soluble aggregation intermediates of the amyloid-beta protein (A beta), as well as the ongoing growth of A beta aggregates, to physiological responses characteristic of Alzheimer's disease (AD) indicates that a kinetic description A beta aggregation intermediate growth may be fundamental to understanding disease progression. Although the growth of mature A beta fibrils has been investigated using several experimental platforms, the growth of A beta aggregation intermediates has been less thoroughly explored. In the current study, a quartz crystal microbalance (QCM) was employed to analyze the real-time growth of A beta(1-40) aggregation intermediates selectively immobilized on the crystal surface. Immobilization permitted quantitative evaluation of A beta(1-40) aggregation intermediate growth under controlled solution conditions. Elongation of A beta(1-40) aggregation intermediates via monomer addition proceeded in a nonsaturable and reversible fashion. The rate of elongation was observed to vary linearly with both monomer concentration and immobilized aggregate density, to be elevated by increases in solution ionic strength, and to increase as solution pH became more acidic. Elongation was consistent with a first-order kinetic model for the single growth phase observed. These findings extend previous kinetic studies involving the growth of mature A beta fibrils to describe the growth of A beta(1-40) aggregation intermediates via monomer addition.

Determining the Potential of Mean Force for Amyloid-ß Dimerization: Combining Self-Consistent Field Theory with Molecular Dynamics Simulation.

Amyloid-ß (Aß) protein aggregates through a complex pathway to progress from monomers to soluble oligomers and ultimately insoluble fibrils. Because of the dynamic nature of aggregation, it has proven exceedingly difficult to determine the precise interactions that lead to the formation of transient oligomers. Here, a statistical thermodynamic model has been developed to elucidate these interactions. Aß1-42 was simulated using fully atomistic replica exchange molecular dynamics. We use an ensemble of approximately 5 × 105 configurations taken from simulation as input in a self-consistent field theory that explicitly accounts for the size, shape, and charge distribution of both the amino acids comprising Aß and all molecular species present in solution. The solution of the model equations provides a prediction of the probabilities of the configurations of the Aß dimer and the potential of mean force between two monomers during the dimerization process. This model constitutes a reliable methodology to elucidate the underlying physics of the Aß dimerization process as a function of pH, temperature, and salt concentration. The results obtained with this new model could be valuable in the design of Aß oligomerization inhibitors, a prospective therapeutic for Alzheimer's disease.

Anthoxanthin Polyphenols Attenuate Aß Oligomer-induced Neuronal Responses Associated with Alzheimer's Disease.

AIMS: Epidemiological evidence implicates polyphenols as potential natural therapeutics for Alzheimer's disease (AD). To investigate this prospect, five anthoxanthin polyphenols were characterized for their ability to reduce amyloid-ß (Aß) oligomer-induced neuronal responses by two mechanisms of action, modulation of oligomerization and antioxidant activity, as well as the synergy between these two mechanisms. METHODS: Anthoxanthin oligomerization modulation and antioxidant capabilities were evaluated and correlated with anthoxanthin attenuation of oligomer-induced intracellular reactive oxygen species (ROS) and caspase activation using human neuroblastoma cell treatments designed to isolate these mechanisms of action and to achieve dual-action. RESULTS: While modulation of oligomerization resulted in only minor reductions to neuronal responses, anthoxanthin antioxidant action significantly attenuated oligomer-induced intracellular ROS and caspase activation. Kaempferol uniquely exhibited synergism when the two mechanisms functioned in concert, leading to a pronounced reduction in both ROS and caspase activation. CONCLUSIONS: Together, these findings identify the dominant mechanism by which these anthoxanthins attenuate Aß oligomer-induced neuronal responses, elucidate their prospective synergy, and demonstrate the potential of anthoxanthin polyphenols as natural AD therapeutics.

Influence of gold nanoparticle surface chemistry and diameter upon Alzheimer's disease amyloid-ß protein aggregation.

BACKGROUND: Deposits of aggregated amyloid-ß protein (Aß) are a pathological hallmark of Alzheimer's disease (AD). Thus, one therapeutic strategy is to eliminate these deposits by halting Aß aggregation. While a variety of possible aggregation inhibitors have been explored, only nanoparticles (NPs) exhibit promise at low substoichiometric ratios. With tunable size, shape, and surface properties, NPs present an ideal platform for rationally designed Aß aggregation inhibitors. In this study, we characterized the inhibitory capabilities of gold nanospheres exhibiting different surface coatings and diameters. RESULTS: Both NP diameter and surface chemistry were found to modulate the extent of aggregation, while NP electric charge influenced aggregate morphology. Notably, 8 nm and 18 nm poly(acrylic acid)-coated NPs abrogated Aß aggregation at a substoichiometric ratio of 1:2,000,000. Theoretical calculations suggest that this low stoichiometry could arise from altered solution conditions near the NP surface. Specifically, local solution pH and charge density are congruent with conditions that influence aggregation. CONCLUSIONS: These findings demonstrate the potential of surface-coated gold nanospheres to serve as tunable therapeutic agents for the inhibition of Aß aggregation. Insights gained into the physiochemical properties of effective NP inhibitors will inform future rational design of effective NP-based therapeutics for AD.

Molecular Signaling Mechanisms of Natural and Synthetic Retinoids for Inhibition of Pathogenesis in Alzheimer's Disease.

Retinoids, which are vitamin A derivatives, interact through retinoic acid receptors (RARs) and retinoid X receptors (RXRs) and have profound effects on several physiological and pathological processes in the brain. The presence of retinoic acid signaling is extensively detected in the adult central nervous system, including the amygdala, cortex, hypothalamus, hippocampus, and other brain areas. Retinoids are primarily involved in neural patterning, differentiation, and axon outgrowth. Retinoids also play a key role in the preservation of the differentiated state of adult neurons. Impairment in retinoic acid signaling can result in neurodegeneration and progression of Alzheimer's disease (AD). Recent studies demonstrated severe deficiencies in spatial learning and memory in mice during retinoic acid (vitamin A) deprivation indicating its significance in preserving memory function. Defective cholinergic neurotransmission plays an important role in cognitive deficits in AD. All-trans retinoic acid is known to enhance the expression and activity of choline acetyltransferase in neuronal cell lines. Activation of RAR and RXR is also known to impede the pathogenesis of AD in mice by inhibiting accumulation of amyloids. In addition, retinoids have been shown to inhibit the expression of chemokines and pro-inflammatory cytokines in microglia and astrocytes, which are activated in AD. In this review article, we have described the chemistry and molecular signaling mechanisms of natural and synthetic retinoids and current understandings of their therapeutic potentials in prevention of AD pathology.