Molecular Simulation Research On Melatonin Disrupting The Stability Of Tau R3-R4 Oligomers

Background:Alzheimer’s disease(AD)is the most common dementia,and plaques formed by extracellular β-amyloid(Aβ)and neurofibrillary tangles(NFTs)formed by intracellular tau protein are the dominant pathological hallmarks of AD.With the continued failure of clinical trials of anti-AD drugs targeting Aβ,tau pathology is also found to occur and develop independently of Aβ and is closely related to the severity of dementia.Therefore,increasing interest has been directed to the important role of tau in anti-AD,and tau is being studied as another potential drug target for the treatment of AD.tau exists in neurons and is most abundant in the central nervous system.Native tau is generally considered a highly soluble and unfolded microtubule-associated protein,and is important in maintaining neuronal structure,neuronal transport and microtubule stability.tau involves an N-terminal region,a proline-rich domain,a microtubule-binding domain,and a C-terminal region.The microtubule-binding domain contains four repeat domains and is the main position where tau binds to microtubules.Abnormally,tau would be released from the surface of microtubules and aggregates into fibrils.The fibrillization of tau undergoes misfolding,nucleation,and fibril elongation stages,and the soluble intermediate oligomers are the major neurotoxic agents.Pathological tau would spread to different brain areas and induce templated misfolding and aggregation of normal tau.tau fibrils show structural polymorphism in different diseases,and paired helical filaments(PHFs)are the dominant pathological tau filaments in AD.The third and fourth repeat regions(R3-R4)are highly amyloidogenic in different tau filaments and are considered the core sequence.To inhibit tau fibrillization is a very promising strategy of AD treatment,and especially small molecules exhibit great potential in preventing tau misfolding,impeding the aggregation and dissolving mature fibrils.Exercise can prevent the occurrence of AD,delay the progress of AD,effectively improve AD pathology,improve memory and cognitive ability.The type,intensity and duration of exercise can affect the level of melatonin(Mel)in the body.As an endogenous small natural molecule,melatonin has been shown to prevent dementia behavior,improve cognitive ability,and alleviate the burden of Aβ and tau.An experiment demonstrated that melatonin can inhibit tau aggregation and disassemble tau fibrils,but the effect of melatonin on tau oligomers and the molecular mechanism of the interaction is still unclear.To clarify the microcosmic mechanism of exercise affecting tau fibrillization and preventing or alleviating AD process,molecular dynamics simulation was used to explore the molecular mechanism of melatonin binding and destabilizing R3-R4 oligomers in this study.This provides a theoretical basis for exercise intervention as a drug alternative therapy or adjuvant therapy for AD.Methods:All-atom molecular dynamics simulation was performed to reveal the mechanisms of melatonin binding and destabilizing protofibrillar tetramer and fibrillar octamer of tau R3-R4.The topological structure of melatonin in this study was obtained by Glyco Bio Chem PRODRG2 server.All simulations used AMBER99SB-ILDN force field and runs were performed by GROMACS software.The analysis was used GROMACS’s programs and self-written programs or scripts.Results:This research explores the molecular mechanism of natural small molecules inhibiting tau aggregation,which is difficult to characterize in experiments,and clarifies the binding behavior and inhibition mechanism of melatonin to tau oligomers.This research will help to explain at the atomic level why exercise improves AD symptoms and delays AD progress.The major results are as follows:Melatonin is able to prevent the formation of the D348-R379 salt bridge and leads to a more extended structure of tau oligomers.Melatonin also destabilizes the association between N-and C-termini by disturbing the ordered arrangement of the hydrophobic V306-I308-L376-F378 cluster.The predominant binding sites are β1 andβ6-β8 regions,and the elongation surface.The tau-melatonin interaction is mainly driven by H-bonding and facilitated by π-π stacking and cation-π interactions.In addition,melatonin is able to occupy the protofilament interface of tetramer through several binding modes,and impedes the H-bonding formation between the residues K331 and Q336 of the opposite protofilaments to destabilize octamer.Conclusions:Melatonin weakens the structural stability of tetramer and octamer of tau R3-R4,which is not conducive to the formation of PHFs.Melatonin is mainly in contact withβ1,β6-β8,and the elongation surface of tau.H-bond plays a leading role in the interaction of melatonin and tau,and π-π stacking and cation-π interaction play a supporting role.Melatonin prevents the association of protofilaments by occupying the interface of tetramer.Melatonin breaks the H-bonds between the residues of the paired protofilaments,thereby reducing the stability of octamer.