Mass Spectrometry Imaging For Metabolites

Metabolites(such as amino acids,nucleotides,fatty acids,and hormones)are essential components in animals;they play an important role in the communication of nerve cells and regulate variously biological processes and behaviors.Abnormal changes in the concentration of metabolites and in metabolic pathways can cause dysfunction of the nervous system and variously central nervous system diseases such as schizophrenia,Alzheimer's disease(AD)and depression.Thus,spatially visualizing the change of metabolite concentrations and the abnormalities of metabolic pathways is critical for understanding the pathogenesis of disease.Mass spectrometry imaging(MSI)is a marker-free imaging technique that allows simultaneous visualization of various metabolites in biological tissues.However,matrix used in the matrix-assisted laser desorption ionization(MALDI)will bring about ion suppression on metabolite signals,and lots of fragments generated by secondary ion mass spectrometry(SIMS)will interfere with the identification of metabolites.Desorption electrospray ionization mass spectrometry(DESI-MS),as an ambient and soft ionization method,does not require any sample pretreatment.So this techniuqe is more suitable for mapping the distribution of metabolites in biological tissue sections.To observe more metabolites in biological tissues and thus to study the change of metabolites concentration and metabolic pathways in brain diseases,the main purpose of this paper is to improve the detection sensitivity of DESI technique and study the change of metabolites concentration and metabolic pathways in AD.Firstly,we developed a data calibration method to solve the problem that the mass spectrometry image was abnormal in morphology compared to optical image.Calibrated with the value of maximum,median and average,the mass spectrometry image was absolutely similar to the ideal image.Additionally,we established a deeper analysis method for the visualization of metabolites subsequently.The first is to explore the correlation of spatial distribution among metabolites from the whole and special brain region in biological tissue.The second is to establish a comprehensive analysis flow to study the abnormality of metabolic pathways in neurological diseases.Secondy,the sensitivity of the DESI-MSI was improved by adjusting the type,ratio and composition of the DESI spray solvent.One was to add ammonium acetate to the spray solvent to increase the sensitivity of the derivatized DESI-MSI.The imaging method with high sensitivity for amino acids is off line derived DESI-MSI,but its sensitivity still needs to be improved.Herein,we added ammonium acetate to the DESI spray solvent based on above method.Taking the derivatives of y-aminobutyric acid,dopamine and glutamate as examples,we explored the optimal concentration of ammonium acetate and the effect of ammonium acetate on the signal intensities of derivatives in different spray solvents.And we explored the efffect of ammonium acetate on the imaging signals of neurotransmitter-derived products on mouse brain slices.The second was to add trifluoroacetic acid to the DESI spray solvent to visualize cholesterol and other metabolites simultaneously.The concentration of cholesterol and other metabolites is abnormally expressed in many brain diseases,so exploring their spatial distribution in biological tissues is helpful for understanding the disease mechanism.We added trifluoroacetic acid to the spray solvent of DESI-MSI,so that the sensitivity of DESI for the detection of cholesterol and metabolites in mouse brain homogenate were improved.Then we explored the effects of trifluoroacetic acid on the signals of metabolites(including cholesterol)and the number of species mapped by DESI-MSI in mouse brain sections.Since more kinds of metabolites were mapped in the brain sections after using trifluoroacetic acid,we can observe the correlationship between the spatial distributions of more metabolites.At the same time,this method was used to explore the concentration changes of cholesterol and other metabolites in AD mice,proving that the method has potential in the fields of biological and medical research.Finally,we explored the aberrant metabolic pathways in AD using above data processing methods and sensitivity-enhanced DESI-MSI.AD is a brain degenerative disease associated with aging and characterized by decreased cognitive function.The etiology is unknown and the pathogenesis is unclear.We combined acid-enhanced DESI-MSI and immunohistochemistry to explore the disorder of metabolic pathways during the development of AD.Multiple metabolic pathways were disordered in the brain of AD model mouse APP23,including neurotransmitters-related pathways(including glutamate-glutamine,glutamate-y-aminobutyric acid,choline-acetylcholine,and choline-phosphocholine),lipid-related pathways(carnitine-acetylcamitine),and purine-related pathways(adenosine-adenine-hypoxanthine).Our study provided the opportunity to map disrupted metabolic pathways in the AD brain,which might simultaneously benefit the identification of early diagnostic markers and therapeutic targets for AD.