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The protocol demonstrates that by performing microtransplantation of synaptic membranes into Xenopus laevis oocytes, it is possible to record consistent and reliable responses of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and γ-aminobutyric acid receptors.
Excitatory and inhibitory ionotropic receptors are the major gates of ion fluxes that determine the activity of synapses during physiological neuronal communication. Therefore, alterations in their abundance, function, and relationships with other synaptic elements have been observed as a major correlate of alterations in brain function and cognitive impairment in neurodegenerative diseases and mental disorders. Understanding how the function of excitatory and inhibitory synaptic receptors is altered by disease is of critical importance for the development of effective therapies. To gain disease-relevant information, it is important to record the electrical activity of neurotransmitter receptors that remain functional in the diseased human brain. So far this is the closest approach to assess pathological alterations in receptors' function. In this work, a methodology is presented to perform microtransplantation of synaptic membranes, which consists of reactivating synaptic membranes from snap frozen human brain tissue containing human receptors, by its injection and posterior fusion into the membrane of Xenopus laevis oocytes. The protocol also provides the methodological strategy to obtain consistent and reliable responses of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and γ-aminobutyric acid (GABA) receptors, as well as novel detailed methods that are used for normalization and rigorous data analysis.
Neurodegenerative disorders affect a large percentage of the population. Although their devastating consequences are well known, the link between the functional alterations of neurotransmitter receptors, which are critical for brain function, and their symptomatology is still poorly understood. Inter-individual variability, chronic nature of the disease, and insidious onset of symptoms are just some of the reasons that have delayed the understanding of the many brain disorders where chemical imbalances are well documented1,2. Animal models have generated invaluable information and expanded our knowledge about the mechanisms underlying physiology and pathophysiology in evolutionary conserved systems; however, several interspecies differences between rodents and humans preclude the direct extrapolation of receptor function from animal models to the human brain3. Thus, initial efforts to study native human receptors were developed by Ricardo Miledi's lab using surgically removed tissue and frozen samples. These initial experiments used whole membranes that include neuronal synaptic and extra synaptic receptors as well as non-neuronal neurotransmitter receptors, and although they provide important information about diseased states, there is a concern that the mix of receptors complicates the interpretation of data4,5,6,7. Importantly, synapses are the major target in many neurodegenerative disorders8,9; therefore, assays to test the functional properties of affected synapses are fundamental to obtain information about disease-relevant changes affecting synaptic communication. Here, a modification of the original method is described: microtransplantation of synaptic membranes (MSM), which focuses on the physiological characterization of enriched synaptic protein preparations and has been successfully applied to study rat and human synaptosomes10,11,12,13,14,15. With this methodology, it is possible to transplant synaptic receptors that were once working in the human brain, embedded in their own native lipids and with their own cohort of associated proteins. Moreover, because MSM data is quantitative, it is possible to use this data to integrate with large proteomic or sequencing datasets10.
It is important to note that many pharmacological and biophysical analyses of synaptic receptors are done on recombinant proteins16,17. While this approach provides better insight into the structure-function relationships of receptors, it cannot provide information about complex multimeric receptor complexes found in neurons and their changes in disease. Therefore, a combination of native and recombinant proteins should provide a more comprehensive analysis of synaptic receptors.
There are many methods to prepare synaptosomes10,11,12,13,14,15 which can be adjusted for the requirements of a lab. The protocol begins with the assumption that synaptosomal enriched preparations were isolated and are ready to be processed for microtransplantation experiments. In the lab, the Syn-Per method is used following the manufacturer instructions. This is done because of high reproducibility in electrophysiological experiments10,11. There is also abundant literature explaining how to isolate Xenopus oocytes18,19, which can also be purchased ready for injection20.
All research is performed in compliance with institutional guidelines and approved by the institutional Animal Care and Use Committee of the University of California Irvine (IACUC-1998-1388) and the University of Texas Medical Branch (IACUC-1803024). Temporal cortex from a non-Alzheimer's disease (AD) brain (female, 74 years old, postmortem interval 2.8 h) and an AD-brain (female, 74 years old, postmortem interval 4.5 h) were provided by the University of California Irvine Alzheimer's disease research center (UCI-ADRC). Informed consent for brain donation was obtained by UCI-ADRC.
NOTE: Unfixed human brain tissue should be treated as a source of blood borne pathogens (BBP). Accordingly, BBP training is needed prior to start experiments. This protocol is performed in a biosafety level 2 (BSL2) laboratory under BSL2 requirements. Guidelines and precautions within the laboratory include: no food or drinks allowed in the laboratory, good laboratory practices must be followed, personal protection equipment (gloves, gown, no open-toed shoes) is required, and the door must be closed at all times.
1. Xenopus oocyte microinjection preparation
2. Loading the sample
3. Injecting the oocytes
NOTE: Standardized rat cortex synaptosomal membranes are also injected in all experiments into a set of oocytes to measure changes in fusion capacity between different batches of oocytes.
4. Recording of ion currents using a two electrode voltage clamp
5. Analyzing TEVC recordings
Within a few hours after injection, the synaptic membranes, carrying their neurotransmitter receptors and ion channels, begin to fuse with the oocyte plasma membrane. Figure 1 shows recordings of AMPA and GABAA receptors microtransplanted into Xenopus oocytes. For most of the analysis, the responses from two or three oocytes per sample were measured, using two or three batches of oocytes from different frogs, for a total of six to nine oocytes per sample. This is done for...
Analysis of native protein complexes from human brains is needed to understand homeostatic and pathological processes in brain disorders and develop therapeutic strategies to prevent or treat diseases. Thus, brain banks containing snap frozen samples are an invaluable source of a large and mostly untapped wealth of physiological information29,30. An initial concern to use postmortem tissue is the clear possibility of mRNA or protein degradation that may confound ...
The authors have no conflicts of interest to disclose.
This work was supported by NIA/NIH grants R01AG070255 and R01AG073133 to AL. We also thank University of California Irvine Alzheimer's disease research center (UCI-ADRC) for providing the human tissue shown in this manuscript. The UCI-ADRC is funded by NIH/NIA grant P30 AG066519.
Name | Company | Catalog Number | Comments |
For Microinjection | |||
3.5" Glass Capillaries | Drummond | 3-000-203-G/X | |
24 well, flat bottom Tissue Culture Plate | Thermofisher | FB012929 | |
Flaming/Brown type micropipette puller | Sutter | P-1000 | |
Injection Dish | Thermofisher | 08-772B | |
Microcentrifuge Tubes | Thermofisher | 02-682-002 | |
Mineral Oil | Thermofisher | O121-1 | |
Nanoject II | Drummond | 3-000-204 | |
Nylon mesh | Industrial Netting | WN0800 | |
Parafilm | Thermofisher | S37440 | |
Stereoscope | Fisher Scientific | 03-000-037 | |
Syringe | Thermofisher | 14-841-31 | |
Ultrasonic cleaning bath | Thermofisher | FS20D | |
Xenopus laevis frogs | Xenopus 1 | 4217 | |
For Two Electrode Voltage clamp | |||
15 cm long fire polished borosilicate glass capillaries | Sutter | B200-116-15 | |
Any PC computer or laptop | |||
Low-pass Bessel Filter | Warner Instruments | LPF-8 | |
Stereoscope | Fisher Scientific | 03-000-037 | |
Two electrode voltage clamp workstation | Warner Instruments | TEV-700 | |
ValveLink 8.2 Perfusion Controller | Automate Scientific | SKU:01-18 | |
WInEDR Free software | University of Strathclyde Glasgow | https://spider.science.strath.ac.uk/sipbs/software_ses.htm | |
X Series Multifunction DAQ | National Instruments | NI USB-6341 | |
Reagents | |||
Calcium dichloride | Thermofisher | C79 | |
Calcium nitrate tetrahydrate | Thermofisher | C109 | |
Collagenase | Sigma-Aldrich | C0130 | |
GABA | Sigma-Aldrich | A2129 | |
HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) | Thermofisher | BP310 | |
Kainic acid | Tocris | 0222 | |
Magnesium sulfate heptahydrate | Thermofisher | M63 | |
Potassium chloride | Thermofisher | P217 | |
Sodium bicarbonate | Thermofisher | S233 | |
Sodium chloride | Thermofisher | S271-1 | |
Ultrafree-0.1 µm MC filter, | Amicon |
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