Novel approaches to target altered pathways in Alzheimer’s disease are based on misfolding protein aggregation, mitochondrial dysfunction, oxidative stress, autophagy impairment, alterations in intracellular Ca2+ homeostasis, neuroinflammation, and neurogenesis impairment, which are involved in neurodegenerative diseases.
The novel acetylcholinesterase (AChE) inhibitors BPT and Kojo tacrine affect autophagy impairment, misfolded proteins, and their aggregates in the intracellular and extracellular brain environment. A03 compound has an antagonist influence on the NMDAR-mediated pathways involved in increasing SIRT1 expression. Other target strategies of autophagy impairment are CDK5 inhibitors and small lanthionine ketamine-ethyl ester (LKE) molecules against increased CRMP2 expression. Neurogenesis impairment is improved by P021, IRL-1620, trehalose, metformin, LKE, amodiaquine, and allopregnanolones (AP) (BR297) treatments. IRL-1620 increases the clearance of Aβ in the bloodstream by influencing the Endothelin B (ETB) receptor. P021 and trehalose have positive influences on misfolded proteins. Diethyl(3,4-dihydroxyphenethylamino) (quinolin-4-yl)methylphosphonate (DDQ) and AP (BR297) increase mitochondrial biogenesis and Aβ clearance. Minocycline increases anti-inflammatory responses and decreases pro-inflammatory responses. Metal ions homeostasis alteration can be rescued by PBT-1, PBT-2, 4-(1-benzylpiperidin-4-yl)thiosemicarbazones (BPT) derivatives, and deferoxamine (DFO).
Antibody-based immunotheraphy against amyloid-beta to trigger its clearance or mitigate its neurotoxicuty has so far been unsuccessful. At present, clinical trials with Monoclonal Antibodies for Alzheimer’s disease are still ongoing: Gantenerumab , Crenezumab, and BAN2401. Earlier immunotherapies caused the failure of immunization of these monoclonal antibodies in AD due to unsuccessful clinical efficacy and major safety problems (e.g., amyloid-related imaging abnormalities) when used at high doses. Other underlining factors are variation in their antibodies epitopes as well as a high variability in the recognition of the structural conformation of Aβ species. Accessibility of N-terminus antibodies immunization of Aβ is more prone to success, compared to hydrophobic C-terminus immunization. Also, N-terminus antibodies have more efficient clearing of the aggregates, since Bapineuzepam, Gantenerumab, and Aducanumab provoke microglial activation and phagocytosis. Phase 3 trials of Aducanumab sought to evaluate the efficacy and saftey of Aducanumab have been discontinued. New trials targeting prodromal and early stage of the disease (e.g., Gantenerumab, Crenezumab, BAN2401) were in the pipeline, since most of previous trails failed. The main reason of this failure is a late intervention in patients when too much Aβ has been accumulated and the Aβ cascade is irreversible. Crenezumab Phase III trials designed to evaluate Crenezumab in early (prodromal to mild) sporadic Alzheimer’s have been stopped. Crenezumab continues to be studied in the Alzheimer’s Prevention Initiative (API) trial investigating the drug in a different population from the CRAD studies, cognitively healthy individuals in Colombia with an autosomal dominant mutation who are at risk of developing familial Alzheimer’s. The Phase II TAURIEL trial is evaluating the anti-tau molecule RG6100, a monoclonal IgG4 antibody that binds to multiple tau species, as well as imaging and fluid-based diagnostic solutions.
Therefore, novel treatments are urgently needed, including both single target and multi target drugs therapies that could act on the molecular pathway links to misfolded proteins (Aβ and tau), synaptic integrity, cognitive impairments, autophagy, and mitochondrial dysfunctions (e.g., oxidative stress, peroxidase induced cytotoxicity), as well as pro- and anti-inflammatory responses related to AD. However, non-invasive administration methods for these drug compounds have been taken into account.
Targeting the Excitotoxicity and Misfolding Protein Aggregations
A few newly developed compounds in preclinical research been proven to have similar properties as acetylcholinesterase inhibitor and NMDAR antagonist/agonist that have established use in the clinic. However, they might have beneficial effects on other dysregulated molecular pathways in AD.
Novel Acetylcholinesterase Inhibitors for Multi-Target Drug Therapy
In AD, a low level of acetylcholine is an important factor in cognitive impairment. Inhibition of acetylcholinesterase (AChE) has been shown to increase cognitive impairment and most 4-(1-benzylpiperidin-4-yl) thiosemicarbazone (BPT) analogues (except 2,3,4-OH-BBPT) have shown potential to act as moderate AChE inhibitors compared to those that are already clinical available AChE inhibitors, like donepezil. Besides this, various 4-(1-benzylpiperidin-4-yl) thiosemicarbazone (BPT) analogues were tested on the other five major hallmarks related to AD: anti-proliferative activity, metal chelation, oxidative stress, dysfunction of autophagy, and protein aggregation. The pyridoxal 4-(1-benzylpiperidin-4-yl)thiosemicarbazone (PBPT) analogue proved to be the best for all of the five major AD hallmarks. A sixth factor was assessed to determine the feasibility of crossing the blood–brain barrier (BBB) by oral administration following the “Lipinski’s Rule of Five”.
The other major factor in AD is the dysregulation of metal ion chelation (copper (Cu2+), zinc (Zn2+), iron (Fe3+)). These ions accumulate within the Aβ plaques, of which Cu2+ and Zn2+ facilitate the self-aggregation of Aβ(1–40) and Aβ(1–42) peptides. Overall, this study demonstrated (in order of decreasing efficacy) that 2,3-OH-BBPT, 8-OH-QBPT, PCBPT, 2,3,4-OH-BBPT, SBPT, QBPT, and PBPT display the ability to inhibit Cu2+-mediated Aβ(1–40) and Aβ(1–42) aggregation. Nevertheless, PBPT analogue and its metal chelators Cu2+ and Fe3+ have demonstrated low anti-proliferative efficacy, which is a desirable characteristic for a long-term AD treatment. Besides this, PBPT showed a greater ability to inhibit 59Fe cellular uptake from 59Fe-transferrin complexes by at least 41% compared to the controls. A look at the effect of iron complexes on ascorbate oxidation demonstrated that Fe3+ complexes of PBPT, NBPT, 8-OH-QBPT, and 2,3-OH-BBPT analogues inhibited ascorbate oxidation more greatly than control samples. These observations suggest that these ligands have the potential to alleviate the Fe-mediated oxidative stress observed in AD. Also, PBPT and SBPT analogues were able to alleviate hydrogen peroxide-mediated cytotoxicity which shows their potential to prevent oxidative stress in AD.
Despite this, the autophagy mechanism, which is dysregulated in AD, plays an important function by eliminating misfolding proteins. However, it was demonstrated that the BPT analogues PBPT, PCBPT, 8-OH-QBPT, and 2,3,4-OH-BBPT increase autophagy flux, while NBPT, 2,3-OH-BBPT, and SBPT inhibit the autophagy degradation pathway. Otherwise, no significant effect of QBPT was observed on the autophagy pathway. This modulation could be a crucial function in increasing the clearance of Aβ aggregated species which may be one of the major problems in AD.
Another acetylcholinesterase inhibitor, Tacrine, a lost multi-target drug entity, was FDA approved. Unfortunately, it was discontinued after other more prone acetylcholinesterase inhibitors were discovered, due to its severe liver toxicity. New insights have shown that Kojo tacrine (KT2D), a tacrine isoform with similar acetylcholinesterase inhibition was developed. This KT2D was synthetized with antioxidant properties and is less hepatotoxic than tacrine, fully completely selective against AChE, and significantly neuroprotective against Aβ. Further properties of various KT2D racemate mixes need to be tested extensively both in vitro and in vivo to determine their clinical translation properties.
Novel NMDA-Receptor Antagonist
The major genetic risk factor in AD is apolipoprotein-E4 (ApoE4), which has a significant potential to reduce Sirtuin 1 (Sirt1) expression. This sirt1 reduction leads to decreases in the FOXO3-mediated oxidative stress response, Coactivator 1-α (PGC1α)-mediated ROS sequestration, and RARβ-mediated ADAM10 expression and increases in P53-mediated apoptosis, NFκB-mediated Aβ toxicity, as well as the acetylation of tau, which leads to microtubule instability and tau pathology. Various Sirt1 enhancers have been identified, such as Alaproclate, Resveratrol, Quercetin, Fisetin, SRT1720, SRT1460, and A03 racemates. A03 was described as a non-competitive NMDAR antagonist. A03 has a similar effect on reducing the excitotoxicity as the FDA approved drug Memantine. Besides this, Memantine does not influence Sirt1 expression. However, A03 has shown beneficial orally pharmacokinetics profiles in the treatment of Sirt1 related pathogeneses in AD. In the amyloidogenic pathway, cleavage fragments of sAPPβ as well as toxic Aβ species have been shown to decrease after A03 treatment in transfected cell cultures either expressing ApoE4 or ApoE3. The decrease of sirt1 expression was greater in ApoE4 compared to ApoE3 transfected cultures and was increased by A03 treatment. No similar effect was seen for sAPPα. Unfortunately, in vivo A03 treatment of E4FAD transgenic animals did not show any significant effect on the cleavage fragments Aβ(1–42), sAPPα, and sAPPβ or the sAPPα/sAPPβ ratio in the hippocampal area, which is crucial area for AD pathology. Besides this, long term oral A03 treatment of AD transgenic animals increased Sirt1 expression in the hippocampus, but not in the frontal cortex, which has been associated with memory improvement. The importance of the above-mentioned compound needs to be further investigated in terms of the relation between sirt1 and sAPPα in dose dependent cases in both preclinical and clinical trials. The Sirt1 function has a major role related to tau pathology, which is an additional risk for progression during AD. The first-in-class ApoE4 targeted therapeutic, A03, which influences Sirt1 levels might be a good candidate for preclinical trails in MCI and AD due to its excellent brain bioavailability and promising efficacy after chronic oral treatment .
Other Target-Receptor Mediated Treatment Strategies
The endothelin B (ETB) receptors is abundant in the CNS and has been shown to play a role in development and neurogenesis. To influence the receptor-mediated function, a highly specific agonist IRL-1620 was used to target the ETB receptor. Endothelin-1 isopeptide (ET-1) plays a central role in the regulation of cardiovascular functions and regional blood flow and may be part of the mechanism by which Aβ interferes with vascular function in AD. It has been shown that Aβ upregulates endothelin converting enzymes 1 and 2 (ECE-1 and ECE-2), which results in increased production and release of ET-1. ECE-1 helps the clearance of Aβ by fragmentation of the peptide. The compound IRL-1620 stimulates the clearance of both ET-1 and Aβ as well as cerebral blood flow which might have a positive influence on the clearing mechanisms of toxic aggregates. Besides this, the compound improves memory deficiency and reduces oxidative stress, which are both caused by Aβ toxicity. IRL-1620 has been demonstrated to increase neural growth factor (NGF) and synapsin I expression, which are both factors involved in neurogenesis and synaptogenesis and which are altered in MCI and AD pathologies.
Targeting Mitochondrial Dysfunction and Related Pathways
The involvement of mitochondrial dysfunction in AD has been investigated by using the pharmacologically developed compound diethyl(3,4-dihydroxyphenethylamino) (quinolin-4-yl)methylphosphonate (DDQ). DDQ has demonstrated positive effects on mRNA and protein levels related to mitochondrial dysfunction and synaptic dysregulation, which are both related to AD. Besides this, DDQ has an effect on the mitochondrial dynamics, related to fission proteins (DRP1 and Lis1), fusion proteins (Mfn1 and 2), and Aβ interactions. DDQ has shown a better docking score than other single existing molecules, like MitoQ, Mdivi1, and SS31. One advantage of DDQ as a novel target is that it binds to the active sites of Aβ and DRP1, inhibiting the Aβ and DRP1 complex formations. The mRNA and protein levels of mitochondrial (PGC1α, Nrf1, Nrf2, TFAM, DRP1, Fis1, Mfn1 and 2) and synaptic activity (Synaptophysin, PSD95, synapsin1 and 2, synaptobrevin1 and 2, synaptopodin, and GAP43) were investigated after Aβ-induced pre-treatment or post-treatment with DDQ. The mRNA and protein levels (PGC1α, Nrf1, Nrf2, and TFAM) were significantly increased after Aβ incubation followed by DDQ treatment. Besides this, reductions of mitochondrial fission proteins (DRP1 and Fis1) and increases of mitochondrial fusion proteins (Mfn1 and 2) were observed after DDQ treatment. Despite this, pre-treatment with DDQ followed by Aβ treatment increased mitochondrial biogenesis mRNA (PGC1α, Nrf1, Nrf2, and TFAM) levels, which suggests that DDQ pre-treatment could serve as prevention agent in AD. DDQ pre-or post-treatment induced Aβ incubation led to a downregulation in mitochondrial fission protein activity (DRP1 and Fis1) and upregulation of fusion activity (Mfn1 and Mfn2). This led to the conclusion that DDQ pre-treatment reduces fission activity (DRP1 and Fis1) and enhances fusion activity (Mfn1 and 2) in the presence of Aβ. However, DDQ treatment-induced Aβ incubation also has a potential enhancing effect on synaptic activity which is downregulated by Aβ pathology. The reduction of DRP1 and Aβ complexes is stronger in DDQ pre-treated compared to post-treated cells. The reduction between Aβ and DRP1 interaction due to DDQ treatment leads to a reduction in mitochondrial fragmentation and maintains the normal count, normal length and normal function of mitochondria, and it might be protective against the Aβ plaque load. Nevertheless, DDQ treatment has been demonstrated to play a neuroprotective role in Aβ toxicity by significantly reducing Aβ(1–42) levels and increasing Aβ(1–40) levels. Despite this, DDQ enhances mitochondrial function and increases cell viability, which leads to an increase in mitochondrial ATP and cytochrome oxidase activity, as well as a reduction in free radicals and oxidative stress, which is dysregulated in AD.
Autophagy dysregulation has been linked indirectly to AD using microtubule-associated protein CRMP2 (collapsin response mediator protein-2) modulation. CRMP2 seems to have same features as tau protein, but, besides this, CRMP2 undergoes profound posttranslational modifications in the brain. CRMP2 is an important adaptor protein that is involved in vesicle trafficking, amyloidogenesis, and autophagy. Tau protein did not show similar involvement in these molecular pathways. Actually, CRMP2 was discovered to be a mediator of neurite retraction and neuron polarization during semaphorin signaling. Besides this, tau has a direct effect and CRPM2 has an indirect effect on the stabilization of actin-based microfilament networks. The difference between CRMP2 and tau is that CRMP2 is involved in endosomal-lysosomal trafficking and autophagy. Although CRMP2 has a highly binding affinity to endocytic adaptor protein (Numb) and MICAL-like protein 1 (MICAL-L1), which are involved in intracellular vesicle movement, as well as the amyloidogenic processing of APP which occurs by endosomal trafficking, it has been speculated that CRMP2 becomes functionally depleted in AD pathology due to both combination of the hyper-phosphorylation of tau; sequestration into nascent neurofibrillary tangles; and oxidative post-translational modification of CRMP2. This includes different multi-facet CRMP2 dependent processes, including amyloidogenic APP trafficking through early and recycled endosomal compartments. Also, it has been demonstrated that the knock-down of CRMP2 expression influences the autophagy flux. Nevertheless, the CRMP2-binding small molecules lanthionine ketamine-ethyl ester (LKE) normalize CRMP2 phosphorylation, reducing the Aβ burden and phosphorylating tau in AD in vitro and in vivo models. Even some derivations of LKE can functionally enhance CRMP2 to promote growth factor-dependent neurite outgrowth as well autophagy-related processes. Despite this, long term LKE treatment increases beclin-1 protein, which is another autophagy-related protein that is downregulated in MCI and AD, which negatively influences the Aβ flux. Besides this, Glia-derived neurotrophic factor (GDNF) can increase CRMP2 expression, which results in microtubule stabilization and enhanced neurite outgrowth.
The activity of mTORC1, the gate-keeper of autophagy, can increase CRMP2 expression which shows that CRMP2 might be a good therapeutic target. Nevertheless, the CRMP2 phosphorylation signaling pathway could be modified by a pharmacological inhibitor of Cdk5, which is under development and which can target the glycogen synthase kinase-3 (GSK3), which is the major compound for regulating the hyper-phosphorylation of either tau or CRMP2. An upstream therapeutic target that might influence CRMP2 expression, as can be seen by axon repulsion factor semaphoring 3A (Sema3A), which binds to the neuropilin-1 receptor (NRP1) and plexin-A co-receptors; its relation in AD needs to be further exploited. Besides that, tau was discovered earlier than CRPM2, and tau is more prone to form non-dissociable high molecular weight complexes than CRMP2. Also, CRMP2 and phosphorylated-CRPM2 (pCRMP2) are not prone to aggregate or form filaments like tau, which later on accumulate to form or associate with NFT-related proteins. Despite this, tau mutations are neuropathic in humans, whereas a CRMP2 mutation has not been discovered yet. Nevertheless, CRMP2 hyper-phosphorylation is a very early event that occurs prior to Aβ accumulation or tau hyper-phosphorylation. This effect of CRMP2 hyper-phosphorylation is a downstream consequence of altered non-amyloidogenic APP processing and perhaps been a more prominent function than tau dysregulation in disorders involving both amyloidopathy and NFT aggregation.
Neuroinflammation is seen as an early event in AD, and drug agents involved in targeting this process could slow down disease progression. Recently, Minocycline, a tetracycline antibiotic, which interferes with the symptoms of neuropsychiatric disorder related to AD. Minocycline has been shown to decrease proinflammatory cytokines, like tumor necrosis factor α (TNF-α) and interleukin 1β (IL-1β), and increase anti-inflammatory cytokines, like interleukin 10 (IL-10), in Aβ(1–42)-treated animals compared to control rats. This neuropsychiatric compound can be considered a new treatment possibility to act against the early effects of AD neuropathology.
Activation of Neurogenesis and Neuronal Survival Pathways
Neurogenesis pathways are downregulated in AD, and various attempts have been made to improve the neurogenic survival pathways in neurodegeneration by interfering with Aβ and tau related mechanisms, boosting dendritogenesis or synaptogenesis and relevant metabolic processes involved in AD pathology. Beneath, some novel compounds that are under preclinical research as possible new treatment strategies are discussed. Nowadays, interest is growing in developing more orally bioavailable compounds as a non-invasive therapeutic tool. These compounds are easily degraded and have the ability to cross the blood brain barrier (BBB).
Orally Bioavailable Compounds
The first orally bioavailable neurotrophic factor is a tetra peptide (P021), which is derived from ciliary neurotrophic factor (CNTF), and which has a suitable biodegradation level and is able to cross the BBB easily. The general functions of P021 are to inhibit leukemia inhibitory factor (LIF) signaling and increase brain-derived neurotrophic factor (BDNF) expression. However, P021 treatment has been able to rescue dendritic and synaptic deficits by boosting neurogenesis, preventing neurodegeneration, Aβ, and tau pathologies, rescuing cognitive impairment in AD transgenic mice, as well as markedly reducing age-related mortality in rat. Besides this, P021 inhibits the GSK3β pathway through the phosphorylation of ser-9 by BDNF. This inhibition has direct effects on Aβ and Tau pathology. Both P021 as well as its parent molecule peptide 6 can increase neurogenesis and synaptic plasticity, which improve cognitive performance in AD transgenic animal models, even in the late stage of the disease progression. Nevertheless, no major side effects have been reported. P021 treatment could serve as an additional therapy strategy to act against AD pathology.
The second orally bioavailable compound which can be classified as a natural disaccharide, also called Trehalose, seems to have a protective role in denaturation and conformational protein changes in neurological disorders. Unfortunately, there is a lot of controversy about trehalose and its derivatives in neurological pathologies. Nevertheless, the major factor in AD is the dysregulation of Aβ protein. It has been reported that Aβ aggregation is inhibited by trehalose and other derivatives. The other major factor involved in AD that was described earlier is metal ion chelation dysregulation. It has been reported that trehalose and its derivatives seem to have a positive influence on Aβ aggregation. Due to this previous relevance, trehalose was studied in the transgenic Tg2576 mouse model for AD. It was shown that intracerebral injection of trehalose improves cognitive impairment in the APP/PSEN-1 model. Unfortunately, this was based on the invasive administration route. However, no statically significant improvement of cognition has been shown by the non-invasive oral administration of trehalose and its derivatives. Also, trehalose treatment did not show any significance in reducing metal ion induced Aβ aggregation, amyloidogenic APP levels, and Aβ fragments, as well as showing no influence on autophagy flux in both the cortex and hippocampus. Nevertheless, trehalose treatment seems to significantly increase the neuromigrating protein doublecortin (DCX), a surrogate indicator for neurogenesis, as well as increasing synaptic activity, especially presynaptic vesicle marker synaptophysin, in the hippocampus and in the cortex. Trehalose treatment seems to increase progranulin expression in both the hippocampus and the cortex. In AD, progranulin, a regulator of neuronal growth and survival, has shown a protective effect against Aβ neurotoxicity.
Other Pro-Neurogenic Compounds Related to Metabolic Disorder
Metabolic disorders like type II diabetes could be genetically linked to AD. In both of these disorders, alterations in neurogenesis are seen. Besides this, the roles of insulin and insulin growth factor, which are important factors in diabetes, have been demonstrated to influence neurogenesis in ex vivo and hippocampal cultured cell lines and might play a crucial role in AD pathology. Compared to an FDA approved drug (Donepezil), Metformin, a type II anti-diabetic drug, has demonstrated both pro-neurogenic potential as well as higher spatial improvement in cognitive impairment in an aluminium chloride (AlCl3)-induced (AlCl3·6H2O) mouse model, a metabolic model for neurodegeneration. Metformin-treated aluminium chloride (AlCl3)·6H2O-induced animals seems to have an irreversible increase in neurogenesis factors (DCX and Neuronal Nuclei (NeuN)), and this can be explained by the effect of metformin on the insulin-mediated Akt signaling pathway compared to donepezil-treated animals. Besides this, metformin has been shown to normalize the aberrant hippocampal proteome signature. Increased levels of the synaptosomal proteins such as calcium/calmodulin-dependent protein kinase type II subunit α (KCC2A), Synapsin-1 (SYN-1), and gamma soluble NSF attachment protein (SNAG) were observed in this metabolically-induced model treated by metformin. All of them play roles in molecular mechanisms related to learning and memory in AD. Moreover, the upregulation of SYN-1 and KCC2A in this metformin-treated metabolically-induced model indicates that metformin facilitates memory formation through synapse plasticity and a possible BDNF-mediated increase of neurogenesis. Other proteins, like ubiquitin-like modifier activating enzyme 1 (UBA1), that are involved in ubiquitin proteasome system (UPS) dysfunction in AD, were shown to increase after metformin treatment. This can be interpreted as a protective response to abnormal or aggregated proteins. Other metabolic proteins, glutathione S transferase 1 (GSTM1) and aconitrate hydratase (ACON) were downregulated in AlCl3·6H2O groups, which is directly linked to an increase in oxidative stress. Following metformin treatment, this downregulation was restored. Alterations in the metabolism-associated proteins in the AlCl3·6H2O-treated group and their restoration by metformin strengthens the idea that AD could be a metabolic disorder. Metabolic drugs could be an alternative therapeutic multi-target strategy to improve the pro-neurogenic effect in AD. Besides this, SYN-1 upregulation-mediated BDNF activation may represent one of the underlying mechanism(s) of metformin-mediated neurogenesis and needs to be further investigated.
Targeting Metal Ions Homeostasis Alteration
Different types of strategies have been developed to recover the metal ions homeostasis alteration. One of these strategies is to decrease the peripheral levels of these metal ions to indirectly decrease their levels in the brain. However, there is not really conclusive evidence of this theory and it could be dangerous to employ this strategy to the entire organism due to the possibility of causing peripheral metal ions deficiency. This is the reason why the drug development against metal ions homeostasis alteration is focused on developing metal-protein-attenuating compounds (MPAC). The MPAC strategy promotes a better distribution of metal ions by the interference of abnormal interaction between some proteins and metal ions in these diseases, allowing for better clearance by the normal endogenous clearance cell processes. Nevertheless, one of the main problems of MPAC development is the difficulty of these drugs to cross the BBB due to their high molecular weight.
One example of MPAC drugs is 5-Chloro-7-iodo-quinolin-8-ol (Clioquinol or PBT-1), a chelator of Cu, Zn, and Fe which is able to cross the BBB. Several studies have been performed analyzing the effect of this drug in AD, including a phase II clinical trial, in which it was confirmed as effective as a disease-modifying therapy as clioquinol. This drug specifically prevents the cognitive decline and decreases the levels of Aβ42 in plasma and amyloid deposition in the brain. Other developing MPAC treatments against AD are PBT-2 (chelator of Cu, Zn, and Fe) or Deferoxamine (DFO), an Fe chelator, which also improves the cognitive decline and reduction of amyloid deposition.
Previous data showed that Aβ and abnormal tau protein alter neurosteroidogenesis in AD models. Natural neurosteroids allopregnanolones (AP) are synthesized following a transformation that involves 5α-reductase activity, which converts progesterone into 5a-dihydroprogesterone (5α-DHP), and also 3α-hydroxysteroid oxidoreductase, which transforms 5α-DHP into AP but also reversibly converts AP back to 5α-DHP. It has been demonstrated to improve steroidogenesis. Despite this, various selected AP derivatives, especially BR297, appear to be the best to mimic the effect of AP on bioenergetics in cellular models. AP and BR297 seem to protect against H2O2-induced cell death by improving mitochondrial bioenergetics by ameliorating cellular ATP production and reducing ROS generation under oxidative stress. Depending on the structural chemical modification of these AP derivatives, they can either increase mitochondrial respiration or induce neurogenic and neuroprotective effects by preventing neuronal cell death without causing the stimulation of cell proliferation. Thus, it is really important to know what the effects of AP derivatives are on the receptor-mediated effects of bioenergetics and cell survival. Here, we focus on the concepts related to AD. The well-known gamma-aminobutyric acid-A receptor (GABA-A) might be modulated through AP derivatives, but the involvement of only GABA-A receptors can be hardly justified. For example, AP promotes the cell proliferation of neural progenitors, induces hippocampal neurogenesis, decreases cerebral Aβ production, and reverses memory deficits in AD triple transgenic mice through acting on the L-type Ca2+ channels and GABA-A receptors .
Future Therapeutic Approaches
In the case of AD, genetic mutations in pathways associated with immune-inflammatory mediated responses, through differential expressions in microglial specific receptors (CD33, TREM2, and CR3) have been demonstrated to be strong risk factors for developing late onset AD (LOAD). Recently, a major microglial transmembrane signaling adaptor polypeptide, also called tyrosine kinase binding protein (TYROBP), which is a direct adaptor for TREM2, CD33, and CR3 activity and seems to be involved in AD, was identified. The function of TYROBP influences TREM2 activity; both factors and their dysregulation may be genetically related risks to AD. However, a deficiency of TYROBP in AD transgenic models was associated with a decrease in microglial activation around the plaques. Also, an alteration in microglial autophagy flux was observed and might be a possible novel therapeutic approach for AD. However, TYROBP deficiency also demonstrated a beneficiary role on other related proteinopathies of AD, like tau, synaptic integrity, and the lysosomal processes.
Besides this, complement factor 3 (C3), which is a ligand for the complement C3 receptor (CR3), has been suggested as a possible factor in AD. C3 deficiency has been demonstrated to increase the Aβ plaque load while it is protective against age- and AD-related synaptic loss and cognitive decline by altering the glial response within the Aβ plaques. This suggests that the increase of fibrillar Aβ plaques is not likely to be the toxic species in AD. This increase of Aβ caused by C3 deficiency also suggests a decrease glial response as well as reduced Aβ phagocytosis. Whether C3 deficiency has a protective role in Aβ toxicity or whether it causes the sequestration/aggregation of Aβ needs to be further investigated.
Both TYROBP deficiency and C3 deficiency need to be further explored to see how they protect or prevent dysregulation of these above-mentioned pathways to select the most promising multi-target drug candidate which might have a beneficiary influence on AD pathology.
Other receptor-mediated therapeutic tools are the toll-like receptor 5 (TLR5) and Nurr1, which both have demonstrated important value in idiopathic related pathways]. A study showed that soluble ectodomains of TLR5, which have a high binding affinity for oligomeric and fibrillary Aβ species, could be a novel immunotherapy against Aβ induced neurotoxicity. Besides this, Nurr1 is highly expressed in the glutamatergic neurons of the hippocampus and is dysregulated in AD and PD. Modulation with the Nurr1 agonist, amodiaquine, led to a reduction in Aβ plaque deposition and enhanced the effect of adult hippocampal neurogenesis, reducing neuronal loss and ameliorating microglia activation.
Both TYROBP deficiency and C3 deficiency, as well as the TLR5 and Nurr1 expression patterns, need to be further explored to see how they protect or prevent dysregulation of these above-mentioned pathways and to select the most promising multi-target drug candidate to encounter AD pathology.
Novel Approaches for the Treatment of Alzheimer’s and Parkinson’s Disease Review, Van Bulck M, et al. Int J Mol Sci 2019