Provided here is a protocol for investigating the interactions between native form, prefibrillar, and mature amyloid fibrils of different peptides and proteins with mitochondria isolated from different tissues and various areas of the brain.
A growing body of evidence indicates that membrane permeabilization, including internal membranes such as mitochondria, is a common feature and primary mechanism of amyloid aggregate-induced toxicity in neurodegenerative diseases. However, most reports describing the mechanisms of membrane disruption are based on phospholipid model systems, and studies directly targeting events occurring at the level of biological membranes are rare. Described here is a model for studying the mechanisms of amyloid toxicity at the membrane level. For mitochondrial isolation, density gradient medium is used to obtain preparations with minimal myelin contamination. After mitochondrial membrane integrity confirmation, the interaction of amyloid fibrils arising from α-synuclein, bovine insulin, and hen egg white lysozyme (HEWL) with rat brain mitochondria, as an in vitro biological model, is investigated. The results demonstrate that treatment of brain mitochondria with fibrillar assemblies can cause different degrees of membrane permeabilization and ROS content enhancement. This indicates structure-dependent interactions between amyloid fibrils and mitochondrial membrane. It is suggested that biophysical properties of amyloid fibrils and their specific binding to mitochondrial membranes may provide explanations for some of these observations.
Amyloid-related disorders, known as amyloidoses, constitute a large group of diseases defined by the appearance of insoluble protein deposits in different tissues and organs1,2. Among them, neurodegenerative disorders are the most frequently forms in which protein aggregates appear in the central or peripheral nervous system2. Although a number of mechanisms have been proposed to be involved in the toxicity of amyloid aggregates3, a growing body of evidence points to cell membrane disruption and permeabilization as the primary mechanism of amyloid pathology4,5. In addition to plasma membrane, internal organelles (i.e., mitochondria) may also be affected.
Interestingly, emerging evidence suggests that mitochondrial dysfunction plays a critical role in the pathogenesis of neurodegenerative disorders, including Alzheimer’s and Parkinson’s diseases6,7. In accordance with this issue, numerous reports have indicated binding and accumulation of amyloid β-peptide, α-synuclein, Huntingtin, and ALS-linked mutant SOD1 proteins to mitochondria8,9,10,11. The mechanism of membrane permeabilization by amyloid aggregates is thought to occur either through formation of discrete channels (pores) and/or through a nonspecific detergent-like mechanism5,12,13. It is noteworthy that most of these conclusions have been based on reports involving phospholipid model systems, and studies directly targeting the events occurring in biological membranes are rare. Clearly, these artificial lipid bilayers do not necessarily reflect the intrinsic properties of biological membranes, including those of mitochondria, which are heterogeneous structures and composed of a wide variety of phospholipids and proteins.
In the present study, mitochondria isolated from rat brains are used as an in vitro biological model to examine the destructive effects of amyloid fibrils arising from α-synuclein (as an amyloidogenic protein), bovine insulin (as a model peptide showing significant structural homology with human insulin involved in injection-localized amyloidosis), and hen egg white lysozyme (HEWL; as a common model protein for study of amyloid aggregation). The interactions and possible damage of mitochondrial membranes induced by amyloid fibrils are then investigated by observing the release of mitochondrial malate dehydrogenase (MDH) (located in the mitochondrial matrix) and mitochondria reactive oxygen species (ROS) enhancement.
All animal experiments were performed in accordance with the Institutional Animal Care and Use Committee (IACUC) of Medical Sciences of Tehran University. Maximal efforts were made to minimize suffering and detrimental effects to the rats by sharpening the guillotine blades and applying resolute and swift movements of the blade.
1. Brain homogenization and mitochondrial isolation
NOTE: All reagents for mitochondrial isolation were prepared according to Sims and Anderson14.
2. Protein concentration determination
NOTE: Protein concentration is measured using the method of Lowry et al.16.
3. Mitochondrial membrane integrity determination
NOTE: Mitochondrial membrane integrity is confirmed by measuring malate dehydrogenase (MDH) activity in isolated mitochondria before and after membrane disruption by Triton X-100.
4. MDH activity determination
NOTE: MDH activity was measured spectrophotometrically as described by Sottocasa et al.17.
5. In vitro α-synuclein, bovine insulin, and HEWL fibril formation
6. Treatment of mitochondria with amyloid fibrils, MDH release assay, and ROS measurement
7. Statistical analysis
The protocol describes a model for studying the interactions of amyloid fibril with rat brain mitochondria as an in vitro biological model. For mitochondrial preparation, 15% (v/v) density gradient medium was used to remove myelin as major contamination of brain tissue14. As shown in Figure 1A, centrifugation at 30,700 x g produced two distinct bands of material, myelin (as the major component of band 1) and band 2, which contains enriched mitochondrial fraction.
By modifying the protocol described by Sims and Anderson14, a mitochondrial suspension containing both synaptic and non-synaptic mitochondria was prepared. For preparation of pure non-synaptic mitochondria, Sims and Anderson previously detailed the protocol14. At the final step of centrifugation,10 mg/mL fatty acid-free BSA was added to obtain a firm mitochondrial pellet (Figure 1A). Mitochondrial membrane integrity was assessed by measuring MDH activity in isolated mitochondria before and after membrane disruption and enzyme release by Triton X-100, as described in the protocol.
It was typically found that mitochondrial preparations were about 93% intact (Figure 1B). A ThT fluorescence assay was carried out to monitor the growth of amyloid fibrils. As shown in Figure 2A, the curve for amyloid fibrillation of three proteins showed a typical sigmoidal pattern, which is in line with nucleation-dependent polymerization models of these proteins as reported previously23,24,25. The highest ThT fluorescence emission for α-synuclein, bovine insulin, and HEWL was observed after 96 h, 12 h, and 96 h, respectively, suggesting formation of amyloid fibrils (Figure 2A). This was further confirmed by AFM measurement.
As illustrated in Figure 2B, well-defined mature fibrils with typical amyloid morphology were observed for three proteins incubated under amyloidogenic conditions. Interactions and possible damage and permeabilization of mitochondrial membranes by amyloid fibrils were investigated. This was accomplished by monitoring the release of mitochondrial MDH and mitochondrial ROS measurement upon the addition of α-synuclein, bovine insulin, or HEWL amyloid fibrils to mitochondrial suspensions and following the procedures outlined. The preliminary experiments showed that the presence of ThT (up to 3 µM) had no effect on MDH release and mitochondrial ROS content (data not shown).
As shown in Figure 3, substantial release of MDH was observed upon the addition of α-synuclein amyloid fibrils to mitochondria in a concentration-dependent manner. Although slight release was detected upon the addition of bovine insulin amyloid fibrils, HEWL fibrils were found to be ineffective (Figure 3). While no significant enhancement in mitochondrial ROS content was observed upon the addition of bovine insulin or HEWL amyloid fibrils, treatment with α-synuclein fibrils led to a considerable increase in ROS content of brain mitochondria in a concentration-dependent manner (Figure 4).
Figure 1: Mitochondrial isolation and membrane integrity determination. (A) Left: typical appearance of a centrifuge tube after resuspending the pellet in cold 15% density gradient medium solution following by centrifugation. Myelin is the major component of band 1, which accumulates at the top. Band 2 contains the highly enriched mitochondrial fraction. Right: a firm pellet was produced after the addition of 10 mg/mL fatty-acid-free BSA and centrifugation at 6900 x g. (B) Mitochondrial membrane integrity was determined by measuring MDH activity in isolated mitochondria before and after membrane disruption by Triton X-100. Please click here to view a larger version of this figure.
Figure 2: Amyloid fibrillation of α-synuclein, bovine insulin, and HEWL. (A) Kinetics of amyloid fibril formation indicated by increasing fluorescence intensity of ThT at 485 nm. (B) AFM images of three proteins are shown, which were incubated under amyloidogenic conditions for regular times. Scale bars represent 500 nm. Please click here to view a larger version of this figure.
Figure 3: Mitochondrial MDH release upon interaction with monomer and amyloid fibril of α-synuclein, bovine insulin, and HEWL. Release is expressed as the percentage of the maximum observed upon treatment with 0.5% (v/v) Triton X-100. Each value represents mean ± SD (n = 3; *p < 0.05, **p < 0.01). Please click here to view a larger version of this figure.
Figure 4: Mitochondrial ROS content upon interaction with monomer and amyloid fibril of α-synuclein, bovine insulin, and HEWL. The data are expressed as the percentage of values in control mitochondria. Each value represents the mean ± SD (n = 3; *p < 0.05; **p < 0.01). Please click here to view a larger version of this figure.
Mitochondrial homogenate (1 mg/mL) | α-synuclein monomer (200 µM) | α-synuclein fibril (200 µM) | Isolation buffer | PBS | Triton x-100 20% (v/v) | |
Control | 175 µL | 0 | 0 | 25 µL | 0 | 0 |
PBS | 175 µL | 0 | 0 | 0 | 25 µL | 0 |
Monomer 25 µM | 175 µL | 25 µL | 0 | 0 | 0 | 0 |
Fibril 5 µM | 175 µL | 0 | 5 µL | 20 µL | 0 | 0 |
Fibril 10 µM | 175 µL | 0 | 10 µL | 15 µL | 0 | 0 |
Fibril 20 µM | 175 µL | 0 | 20 µL | 5 µL | 0 | 0 |
Fibril 25 µM | 175 µL | 0 | 25 µL | 0 | 0 | 0 |
Positive control | 175 µL | 0 | 0 | 20 µL | 0 | 5 µL |
Table 1: Treatment of mitochondria with various concentrations of α-synuclein amyloid fibrils.
Mitochondrial homogenate (1 mg/mL) | Bovine insulin monomer (250 µM) | Bovine insulin fibril (250 µM) | Isolation buffer | Glycine buffer | Triton x-100 20% (v/v) | |
Control | 175 µL | 0 | 0 | 25 µL | 0 | 0 |
Glycine buffer | 175 µL | 0 | 0 | 0 | 25 µL | 0 |
Monomer 25 µM | 175 µL | 20 µL | 0 | 5 µL | 0 | 0 |
Fibril 5 µM | 175 µL | 0 | 4 µL | 21 µL | 0 | 0 |
Fibril 10 µM | 175 µL | 0 | 8 µL | 17 µL | 0 | 0 |
Fibril 20 µM | 175 µL | 0 | 16 µL | 9 µL | 0 | 0 |
Fibril 25 µM | 175 µL | 0 | 20 µL | 5 µL | 0 | 0 |
Positive control | 175 µL | 0 | 0 | 20 µL | 0 | 5 µL |
Table 2: Treatment of mitochondria with various concentrations of bovine insulin amyloid fibrils.
Mitochondrial homogenate (1 mg/mL) | HEWL monomer (1 mM) | HEWL fibril (1 mM) | Isolation buffer | Glycine buffer | Triton x-100 20% (v/v) | |
Control | 175 µL | 0 | 0 | 25 µL | 0 | 0 |
Glycine buffer | 175 µL | 0 | 0 | 0 | 25 µL | 0 |
Monomer 25 µM | 175 µL | 5 µL | 0 | 20 µL | 0 | 0 |
Fibril 5 µM | 175 µL | 0 | 1 µL | 24 µL | 0 | 0 |
Fibril 10 µM | 175 µL | 0 | 2 µL | 23 µL | 0 | 0 |
Fibril 20 µM | 175 µL | 0 | 4 µL | 21 µL | 0 | 0 |
Fibril 25 µM | 175 µL | 0 | 5 µL | 20 µL | 0 | 0 |
Positive control | 175 µL | 0 | 0 | 20 µL | 0 | 5 µL |
Table 3: Treatment of mitochondria with various concentrations of HEWL amyloid fibrils.
A wealth of experimental results supports the hypothesis that the cytotoxicity of fibrillar aggregates is significantly associated with their ability to interact with and permeabilize biological membranes4,5. However, most of the data are based on artificial lipid bilayers that do not necessarily reflect the intrinsic properties of biological membranes, which are heterogeneous structures with a wide variety of phospholipids and proteins. Here, using brain mitochondria as an in vitro biological membrane, a model for studying fibrillar aggregates cytotoxicity at the membrane level is described.
During the experiment, it is important to work quickly and keep everything on ice to prepare functionally active mitochondria with intact membranes. Moreover, mitochondrial concentration (based on total protein measurement) is an important factor affecting membrane-amyloid fibril interaction; thus, it must be keep constant throughout the protocol. In the present study, mitochondrial preparations contained both synaptic and non-synaptic mitochondria. For pure non-synaptic mitochondrial preparation, 40%, 23%, and 15% (v/v) density gradient medium should be used, as described by Sims and Anderson14. Because the mitochondria is an organelle with a well-characterized membrane, it may serve as an extremely useful biological model system in molecular studies related to the mechanisms of amyloid cytotoxicity at the membrane level (especially relating to neurodegenerative diseases). This protocol allows investigation of the interactions and extent of mitochondrial membranes permeabilization by looking into the release of various molecules and enzymes located in different compartments (i.e., those embedded in the outer and inner membranes and those located in the intermembrane space or mitochondrial matrix).
After mitochondrial membrane integrity confirmation (Figure 1B) and amyloid fibril formation (Figure 2), the interaction, damage, and permeabilization of mitochondrial membranes upon the addition of α-synuclein, bovine insulin, and HEWL amyloid fibrils was investigated. The results demonstrated clear differences in the extent of membrane permeabilization and mitochondrial ROS enhancement induced by amyloid fibrils (Figure 3 and Figure 4), suggesting variations in the capability of various amyloid fibrils to interact with and damage the mitochondrial membranes.
For membrane permeabilization initiation, amyloid fibrils must reach the mitochondrial surface. It has been shown that association, localization, and insertion of proteins to charged membranes is dependent on nonspecific electrostatic interactions and the hydrophobic nature of proteins26,27,28. In regard to amyloid toxicity, a growing body of evidence strongly implicates a key role of protein aggregate surface hydrophobicity in their cytotoxicity29,30. Although HEWL bears a net positive charge over a broad range of pH levels and has a high affinity for anionic and neutral phospholipids31 (e.g., the mitochondrial membrane23), no mitochondrial membrane permeabilization and ROS enhancement were observed (Figure 3 and Figure 4). An explanation for this observation may be related to a reduction of surface hydrophobicity of HEWL fibrils due to lateral fibril-fibril interactions23,32, leading to loss of the ability to cause membrane damage.
For bovine insulin, despite a significant exposure of hydrophobic regions in the course of fibril formation22, a slight enzyme release was observed upon the addition of 25 µM amyloid fibrils (Figure 3) without significant effects on mitochondrial ROS content (Figure 4). As both brain mitochondria and bovine insulin fibrils bear a negative zeta potential at their surfaces24, it is suggested that repulsive forces between mitochondrial membrane and bovine insulin amyloid fibrils may account for their inability to effectively interact with and damage mitochondrial membranes24. The highest permeabilization and mitochondrial ROS increments were observed upon addition of α-synuclein fibrils (Figure 3 and Figure 4).
This observation may be attributed to the high capacity of α-synuclein for interacting with biological membranes that have negatively charged surfaces33,34, such as mitochondria. To this regard, some studies have shown specific binding and importation of α-synuclein into mitochondria, where they become predominantly associated with the inner membrane35. Interestingly, Ghio et al. reported that binding, mediated by cardiolipin (a phospholipid uniquely enriched in inner mitochondrial membrane), of α-synuclein may be an important mechanism for association and disruption of mitochondrial membranes36.
Interestingly, there is clear evidence for binding and accumulation of various amyloidogenic peptides and proteins including amyloid β-peptide, α-synuclein, Huntingtin, and ALS-linked mutant SOD1 to mitochondria35,37,38,39. Moreover, previous studies have demonstrated that amyloidogenic assemblies of amyloid β (25–35)15 and SOD 140 have the ability to interact with and cause mitochondrial membrane permeabilization. According to the literature, it is suggested that amyloid fibrils are too large to cross the membrane; therefore, average fibril size should not be a factor in the apparent lack of effects induced by bovine insulin and HEWL amyloid fibrils. Although further studies are needed to elucidate the detailed mechanisms involved, it is proposed that the biophysical features of amyloid fibrils (i.e., surface charge, hydrophobicity, and possessing a specific sequence-targeting mitochondrial membrane) may play an important role in their capacity to interact with and damage biological membranes.
The authors have nothing to disclose.
This work was supported by grants from the Research Council of the Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, Iran.
2′,7′-Dichlorodihydrofluorescein diacetate | Sigma | 35845 | |
Ammonium sulfate | Merck | 1012171000 | |
Black 96-well plate | Corning | ||
Black Clear-bottomed 96-well plate | Corning | ||
Bovine insulin | Sigma | I6634 | |
Bovine Serum Albumin (BSA) | Sigma | A2153 | |
BSA essentially fatty acid-free | Sigma | A6003 | |
Centrifuge | Sigma | ||
Crystal clear sealing tape | Corning | ||
CuSO4 | Sigma | 451657 | |
Dialysis bag (cut off 2 KDa) | Sigma | D2272 | |
Dounce homogenizer | Potter Elvehjem | ||
EDTA | Sigma | E9884 | |
Fluorescence plate reader | BioTek | ||
Fluorescence spectrophotometer | Cary Eclipse VARIAN | ||
Folin | Merck | F9252 | |
Glycine | Sigma | G7126 | |
Guillotine | Made in Iran | ||
HCl | Merck | H1758 | |
Hen Egg White Lysozyme (HEWL) | Sigma | L6876 | |
Na2CO3 | Sigma | S7795 | |
NaH2PO4 | Sigma | S7907 | |
NaOH | Merck | S8045 | |
Oxaloacetate | Sigma | O4126 | |
Percoll | GE Healthcare | ||
Phosphate Buffer Saline (PBS) | Sigma | CS0030 | |
PMSF | Sigma | P7626 | |
Potassium sodium tartrate | Sigma | 217255 | |
Quartz cuvette | Sigma | ||
Spectrophotometer | analytik jena | SPEKOL 2000 model | |
Succinate | Sigma | S2378 | |
Sucrose | Merck | 1076871000 | |
Thermomixer | Eppendorph | ||
Thioflavin T | Sigma | T3516 | |
Tris-HCl | Merck | 1082191000 | |
Triton X-100 | Sigma | T9284 | |
Tryptone | QUELAB | ||
Water bath | Memmert | ||
Yeast Extract | QUELAB | ||
β-NADH | Sigma | N8129 |