Experimental Study Of Charge Effect On The Interaction Between β-amyloid Protein And Phospholipid Membranes

Alzheimer’s disease is a degenerative disease of nervous system.The memory function and cognitive ability of patients will decline rapidly with the development of the disease.Now there is still no perfect pre-diagnosis and treatment method for the disease.There are many hypotheses about the pathogenesis of Alzheimer’s disease,among which the amyloid cascade hypothesis is highly recognized.This hypothesis holds that β-amyloid protein(Aβ)is deposited in the brain,causing neuroinflammation and eventually leading to the death of neurons.Experiments have shown that the existence of cell membrane can not only provide a suitable interface for the aggregation of Aβ in vivo,but also regulate the aggregation behavior of Aβ.Therefore,the interaction between β-amyloid protein and cell membrane and its influencing factors are studied.It is of great significance for drug research and development of new treatments for Alzheimer’s disease and other amyloidosis diseases.Considering that most animal cell membranes are negatively charged and Aβ is charged under physiological p H,the effect of electrical properties on peptide membrane interactions is worth studying.In this paper,the supported lipid bilayers were used as the cell membrane model,and the charge of the supported lipid bilayers were changed by adding different molar ratios of negative phospholipid POPS.The changes of morphology and mechanical properties of the supported lipid bilayers before and after the action of Aβ(25-35),the full length functional segment of Aβ,was characterized by atomic force microscopy.The experimental results showed that under the condition of low peptide-lipid ratio,all the supported lipid bilayers with different negative ratios interacted with Aβ(25-35).There were only a few oligomers on the neutral phospholipid POPE membrane surface after the action of Aβ(25-35),and no obvious holes were observed.Holes appeared on the surface of negatively charged phospholipid membranes,and the average diameter and depth of the holes were the largest when the negative electric ratio was 10%,followed by when the negative electric ratio was 20% and 30%.In this paper,it was considered that the interaction effect between the membrane and Aβ(25-35)was determined by the lateral interaction between membrane chains and the electrostatic force between membranes and peptides.With the increase of the proportion of negative phospholipid,the electrostatic interaction between membrane and Aβ(25-35)increased,and the phospholipid molecules in the membrane were easier to be extracted.However,the increased negative phospholipid also increased the lateral interaction between membrane chains and reduced the fluidity of the membrane,thus protecting the phospholipid molecules from being extracted.As a result of the game of two effects,the proportion of negative phospholipid increased,and the proportion of pores on the membrane surface increased first and then decreased.Under the condition of high peptide-lipid ratio,a large number of Aβ(25-35)aggregates appeared on the surface of membranes with different negative ratios after the action of Aβ(25-35).The average height and the proportion of aggregates accumulated on negatively charged phospholipid membranes were larger than those on POPE membrane.The electrostatic interaction between the negatively charged head group of phospholipid POPS and the positively charged side chains of Aβ(25-35)increased the local peptide concentration and promoted Aβ(25-35)nucleation process,resulting in accelerated aggregation and deposition of peptides on the membrane surface.The deposition of Aβ(25-35)on the membrane surface changed the originally close arrangement of phospholipid molecules in the membrane,making the phospholipid membranes thinner.The accumulated peptide can also decompose the membrane by destroying the curvature of the membrane.When the negative charge ratio was 20% and30%,the phospholipid membranes tent to disintegrate after interacting with Aβ(25-35).In this paper,it was considered that the mechanism of interaction between negatively charged phospholipid membrane and Aβ(25-35)was "Detergent effect" under the condition of low peptide-lipid ratio.The phospholipid molecules were extracted by Aβ(25-35)in a detergent-like way,which formed holes on the membrane surface and destroyed the original structure of membrane.Under the condition of high peptide-lipid ratio,the interaction mechanism was "Carpeting effect".Aβ(25-35)covered the surface of membrane and combined with it.The accumulated peptides reduced the thickness of the membrane and disturbed it.The mechanism was different from that of peptide membrane with low peptide to lipid ratio.This was because peptides preferentially formed aggregates when the concentration of peptide was high.The breakthrough force decreased after the action of Aβ(25-35),and the change of the breakthrough force of the negative phospholipid membrane was greater than that of the neutral phospholipid membrane,which indicated that the surface charge of the membrane was an important factor affecting the interaction between them.