Disruption of the Mouse Blood-Brain Barrier by Small Extracellular Vesicles from Hypoxic Human Placentas

Summary

A protocol is presented to evaluate whether small EVs (sEVs) isolated from placental explants cultured under hypoxic conditions (modeling one aspect of preeclampsia) disrupt the blood-brain barrier in nonpregnant adult female mice.

Abstract

Cerebrovascular complications, including cerebral edema and ischemic and hemorrhagic stroke, constitute the leading cause of maternal mortality associated with preeclampsia. The underlying mechanisms of these cerebrovascular complications remain unclear. However, they are linked to placental dysfunction and blood-brain barrier (BBB) disruption. Nevertheless, the connection between these two distant organs is still being determined. Increasing evidence suggests that the placenta releases signaling molecules, including extracellular vesicles, into maternal circulation. Extracellular vesicles are categorized according to their size, with small extracellular vesicles (sEVs smaller than 200 nm in diameter) considered critical signaling particles in both physiological and pathological conditions. In preeclampsia, there is an increased number of circulating sEVs in maternal circulation, the signaling function of which is not well understood. Placental sEVs released in preeclampsia or from normal pregnancy placentas exposed to hypoxia induce brain endothelial dysfunction and disruption of the BBB. In this protocol, we assess whether sEVs isolated from placental explants cultured under hypoxic conditions (modeling one aspect of preeclampsia) disrupt the BBB in vivo.

Introduction

Approximately 70% of maternal deaths due to preeclampsia, a hypertensive pregnancy syndrome characterized by impaired placentation processes, maternal systemic endothelial dysfunction, and, in severe cases, multi-organ failure^1,2, are associated with acute cerebrovascular complications^3,4. Most maternal deaths occur in low and middle-income countries⁵. However, the underlying mechanisms are still unclear despite the clinical and epidemiological relevance of cerebrovascular complications associated with preeclampsia.

On the other hand, extracellular vesicles (EVs) (diameter ~30-400 nm) are essential mediators of intercellular communication among tissues and organs, including maternal-placental interaction⁶. In addition to proteins and lipids on the external surface, EVs carry cargo within (proteins, RNA, and lipids). EVs can be categorized into (1) exosomes (diameter ~50-150 nm, also named small EVs (sEVs)), (2) medium/large EVs, and (3) apoptotic bodies, which differ by size, biogenesis, content, and potential signaling function. The composition of EVs is determined by the cells from which they originate and the disease type⁷. Syncytiotrophoblast-derived EVs express placental alkaline phosphatase (PLAP)^8,9, which detects placentae-derived circulating small EVs (PDsEVs) in pregnancy. Also, PLAP helps discern changes in the PDsEVs cargo and their effects in preeclampsia versus normotensive pregnancies^{10,11,12,13,14,15}.

The placenta has been recognized as the necessary component in the pathophysiology of preeclampsia¹⁶ or cerebral complications associated with this disease^17,18,19. However, how this distant organ might induce alterations in brain circulation is unknown. Since sEVs play pivotal roles in cell-to-cell communication due to their capacity to transfer bioactive components from donor to recipient cells^6,20,21, a growing number of studies have associated placental sEVs with the generation of maternal endothelial dysfunction^21,22,23,24, including brain endothelial cells^25,26in women with preeclampsia. Thus, the compromise of brain endothelial function may lead to disruption of the blood-brain barrier (BBB), a critical component in cerebrovascular complications associated with preeclampsia^3,27.

Nevertheless, preclinical findings using rat cerebral vessels exposed to serum of women with preeclampsia²⁸ or human brain endothelial cells exposed to plasma of women with preeclampsia²⁹ reported that circulating factor(s) induce disruption of the BBB. Despite several candidates with the potential to harm the BBB present in the maternal circulation during preeclampsia, such as elevated levels of proinflammatory cytokines (i.e., tumor necrosis factor)^18,28 or vascular regulators (i.e., vascular endothelial growth factor (VEGF))^29,30,31, or oxidative molecules such as oxidized-lipoproteins (oxo-LDL)^32,33, among others³⁴, none of them establishes a direct connection between the placenta and the BBB. Recently, sEVs isolated from hypoxic placentas have shown the capacity to disrupt the BBB in nonpregnant female mice²⁵. Since placental sEVs may carry most of the listed circulating factors with the capacity to disrupt the BBB, sEVs are considered suitable candidates to connect the injured placenta, be the carrier of harmful circulating factors, and disrupt the BBB in preeclampsia.

This protocol allows us to investigate whether sEVs isolated from placental explants cultured under hypoxic conditions can disrupt the BBB in nonpregnant female mice as a proxy for understanding the pathophysiology of cerebral complications during preeclampsia.

Protocol

The research was carried out following the principles expressed in the Declaration of Helsinki and under the authorization of the respective Ethical Review Boards. All human participants gave their informed consent before sample collection, as reported previously²⁵. Additionally, the Bioethics and Biosafety Committee of the Bío-Bío University approved this project (Fondecyt grant 1200250). The animal work was conducted in accordance with the cardinal principles of the three R's in the use of animals in experimentation³⁵, and according to the recommendations of the guidelines for the Care and Use of Laboratory Animals published by the US National Institute of Health. Animals were kept in appropriate environments at the Vivarium of the University of Bío-Bío. Fresh placentas (n = 4) were obtained within 1 h after elective cesarean section from mothers (ages 28-31 years old) with normal pregnancies at term (38 to 41 weeks of gestation). Cesarean sections were performed at the Herminda Martin Clinical Hospital, Chillan, Chile, as previously reported²⁵. To apply the in vivo model, 4-6-month-old female nonpregnant mice (strain C57BLACK/6) were used. They were divided into three experimental groups: (1) control (without treatment), (2) treated with sEVs from normoxia (sEVs-Nor), and (3) treated with sEVs from hypoxic cultured placentas (sEVs-Hyp), which were used to evaluate the disruption of the BBB in vivo²⁵. All the injected solutions were sterile. Also, the preparation of sEVs was performed in aseptic conditions and under a class II biosafety hood to avoid contamination.

1. Placental culture explants

1. To extract explants of normal pregnancy placentas, check the method published by Miller et al. 2005³⁶.
2. Carefully remove the clots from the basal plate of the human placenta using sterilized forceps. Extract small explants to obtain a final tissue extraction of 10 g.
3. Ensure that placental explants are removed from the four quadrants of the maternal portion of the placenta. Ensure that devitalized tissue and calcified areas are excluded.
4. To manipulate placental tissue, do so on ice, under sterile conditions, and within a biosafety cabinet.
5. Wash the explants (at least three times) with copious amounts (five volumes of the explants) of cold (4 °C) phosphate buffer solution (PBS 1x, pH 7.4) to remove as much blood as possible. Centrifuge (252 x g for 10 min) at room temperature between washes.
6. Resuspend 10 g of washed explants in 20 mL of culture medium supplemented with 2% fetal bovine serum, 100 IU/mL penicillin, and 100 µg/mL of streptomycin (see Table of Materials).
7. Put the suspension in a 100 mm culture dish. Leave the explants for 2 h in an incubator at 37 °C with 21% oxygen and 5% CO₂.
8. After that, rewash the explants with 1x PBS at 37 °C (three times). Centrifuge (252 x g for 10 min) at room temperature between washes.
9. Further, resuspend the tissue in 20 mL of culture medium previously depleted of nanoparticles.
   NOTE: For nanoparticle depletion, perform ultracentrifugation (120,000 x g for 18 h at room temperature) and microfiltration (0.22 µm filter).
10. Split the resuspended explants into two cultures dishes (100 mm). Each culture dish must include 10 mL of resuspended explants.
11. After that, place one of the dishes containing placental explants in standard culture conditions (37 °C with 8% oxygen and 5% CO₂), while placing the other one in a hypoxic chamber with 1% O₂.
    NOTE: (OPTIONAL) Analysis of tissue viability can be performed using the same tissue extractions and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay (MTT assay) as described elsewhere^37,38. Use total protein concentration measured in the explants homogenates to normalize MTT values.
12. After 18 h in culture, harvest the conditioned media of the explants into a new 15 mL tube.
    ​NOTE: At this step, conditioned media can be frozen (-80 °C) for an extended period (weeks).

2. Placental-derived sEVs isolation

1. Isolate sEVs from the conditioned medium using the differential centrifugation and microfiltration protocol following previously published reports^25,39.
2. Perform sequential centrifugation for the harvested conditioned medium. Sequential steps of centrifugations are (1) 300 x g for 10 min, (2) 2000 x g for 10 min, (3) 10,000 x g for 30 min, and (4) 120,000 x g for 2 h. Perform the centrifugations room temperature.
3. In all these centrifugations, using a 20-100 μL pipette, carefully collect the supernatant and discard the pellet.
4. Once done, pass the last collected supernatant through a 0.22 µm filter (see Table of Materials). Subsequently, perform one additional centrifugation at 120,000 x g for 18 h at room temperature.
5. After that, discard the supernatant while resuspending the pellet (which contains placental sEVs) in 500 µL of PBS (pH 7.4) and again pass through a 0.22 µm filter. Finally, perform one last centrifugation at 120,000 x g for 3 h at room temperature.
6. Next, resuspend the pellet (which contains placental sEVs) in 500 µL of PBS (pH 7.4, previously depleted of sEVs) and pass through a 0.22 µm filter.
7. Label the samples as stock of sEVs-Normoxia (sEVs-Nor) or sEVs-Hypoxia (sEVs-Hyp).
   NOTE: Prepare 50-100 µL aliquots of the isolated sEVs for further experimentation. Small EVs can be stored at -80 °C for an extended period (months).
8. Characterize the placental sEVs by size, count, and sEVs protein marker, as reported previously²⁵. The ideal mean particle diameter of sEVs is 50-150 nm.
   NOTE: sEVs characterization includes positive detection of CD63, Tsg101, Alix, and HSP70 (i.e., to characterize sEVs population enriched in exosomes). Also, use PLAP as a marker of placental origin²⁵.
9. Measure the total amount of protein in the sEVs solution using the BCA protein assay kit following the manufacturer's instructions (see Table of Materials).
   ​NOTE: Figure 1 shows an overview of the placental explant culture and extracellular vesicle isolation.

3. Mice injection

1. Anesthetize the animal in an anesthetic induction chamber with 5% isoflurane distribution (see Table of Materials). Verify the level of anesthesia via a toe pinch.
2. After ~40 s, put the mouse into the anesthesia system using a facemask and maintain an anesthesia level of 2% isoflurane.
3. Next, position the animal on a heating platform (see Table of Materials) at a room temperature of 28 °C. Keep the eyes lubricated with artificial tears.
4. Before injecting, dilute placental sEVs (200 μg of total protein) with phosphate buffer (pH, 7.4) until reaching a final volume of 70 µL.
5. Disinfect the injection area using 70% ethanol and cotton swabs. Ensure to use an insulin syringe with a 30 G needle to inject the sEVs solution.
   NOTE: The syringe can be positioned on the heating platform for 5 min to acquire a physiological temperature.
6. Then, inject the solution into the external jugular vein. This step needs special training.
   NOTE: The procedure must be performed cautiously, slowly retracting the needle and gently aspirating to evidence of blood reflux.
7. After injection, apply light pressure to the injected area for ~15 s with a dry cotton swab. After that, return the animals to their respective cages.
8. Ensure to evaluate consciousness and behavior for 15 min. An indicator of consciousness achievement is the animal recovery of total motor activity.
   ​NOTE: Ensure to keep a warm temperature in animal's cages.

4. Rapid murine coma and behavior scale (RMCBS)

1. Use the RMCBS to evaluate the neurological and well-being parameters of the animals⁴⁰.
   NOTE: RMCBS allows a fast evaluation (~ 3 min) with minimal animal stress due to operator intervention. RMCBS has ten parameters (each score is 0-2).
2. Evaluate mouse well-being with the RMCBS scale at 0 h (before injecting sEVs) and 3 h, 6 h, 12 h, and 24 h after injection.
   ​NOTE: Perform analysis of claudin-5 (at 6 h after sEVs injection) and Evan's blue extravasation (at 24 h after sEVs injection) to evaluate the disruption of the BBB (Figure 2).

5. Analysis of Evan's blue extravasation

1. After placental sEVs injection (24 h), anesthetize the mice as described in step 3.1.
2. Prepare Evan's Blue solution (2%, diluted in phosphate buffer solution, 1x) (see Table of Materials).
3. Using retro-orbital access²⁵, inject Evan's Blue solution at 2 mL/kg.
4. Allow Evan's blue solution to circulate for 20 min under maintenance of anesthesia.
5. Next, perform intracardiac perfusion following previously described protocol⁴¹ with saline solution (~3 mL, 0.9%, w/v) to remove Evan's blue dye from the circulation. This step is mandatory.
   NOTE: For this procedure, increase anesthesia to 5% and verify a deep plane of anesthesia (i.e., drastic reduction in respiratory rate).
6. Expose the heart via medial thoracotomy⁴¹ (Figure 3).
7. After that, fix the whole animal with an intracardial infusion of paraformaldehyde solution (4% in PBS, v/v).
   NOTE: Check the rigidity of the tail for monitoring the appropriate fixation.
8. Further, once fixation is complete, carefully extract the brain, weigh it, and photograph it⁴¹. Take pictures with the whole brain close to a ruler⁴².
9. Next, put the brain in a brain mouse slicer (see Table of Materials).
10. Dissect the brain into nine sections, crane-caudal, including the cerebellum (100 μm each section).
11. After dissection, visualize and capture images of the brain slices in a stereo-zoom microscope (see Table of Materials).
12. Use the captured images to quantify Evan's blue extravasation using ImageJ software (see Table of Materials).
    NOTE: To analyze the presence of Evan's blue dye, set Evan's blue values using a positive control of the brain section (from an animal control animal injected with dye but without intracardial perfusion). Evan's blue values reached from positive control are generally between 75 and 110 in the intensity histogram of the blue channel (Figure 4).
13. Ensure that brain images are analyzed blindly by the respective experimental group to avoid observer bias. Alternatively, quantify Evan's blue extravasation by brain homogenization followed by spectroscopy⁴³.
    NOTE: In separate experiments, using freshly isolated brains from sEVs injected mice (6 h), analyze the protein levels of claudin 5 (CLND-5) (see Table of Materials), a critical tight junction involved in the tightness of the BBB⁴⁴ (Figure 5).

Results

This protocol evaluates the capacity of sEVs derived from placentas cultured in hypoxia to disrupt the BBB in nonpregnant mice. This method allows one to understand better the potential connection between the placenta and the brain in normal and pathological conditions. In particular, this method may constitute a proxy to analyze placental sEVs participation in the onset of cerebral complications in preeclampsia.

Contrary to mice injected with sEVs-Nor, mice injected with sEVs-Hyp show a progr...

Discussion

This study unveils fresh insights into potential harm resulting from sEVs isolated from placental explants cultured in hypoxic conditions on the disruption of the rodent blood-brain barrier. The pathological mechanism involves a reduction in CLND-5 in the posterior brain region²⁵.

Prior investigations have revealed that plasma-sEVs from individuals with preeclampsia induce endothelial dysfunction in various organs using in vitro models⁴⁶...

Materials

Name	Company	Catalog Number	Comments
Adult mice brain slecer matrice 3D printed	Open access file	Adult mice	Adult mice brain slicer. Printed in PLA filament.
Anti β-Actin primary antibody	Sigma-Aldrich	Clon AC-74	Antibody for loading control (Western blot)
Anti-Claudin5 primary antibody	Santa cruz Biotechnology	sc-374221	Primary antibody for tight junction protein CLDN5 of mice BBB (Western blot)
BCA protein kit	Thermo Scientific	23225	Kit for measuring protein concentration
Culture media #200 500 mL	Thermo Fisher Scientific	m200500	Culture media for placental explants
D180 CO2 incubator	RWD Life science	D180	Standard incubator to estabilize explants and culture sEVs-Nor
Evans blue dye  > 75% 10 g	Sigma-Aldrich	E2129.10G	Dye to analize blood brain barrier disruption IN VIVO
Fetal bovine serum 500 mL	Thermo Fisher Scientific	16000044	Additive growth factor for culture media 200
Himac Ultracentrifuge CP100NX	Himac eppendorf group	5720410101	Ultracentrifuge for condicioned media > 1,20,000 x g
ImageJ software	NIH		https://imagej.nih.gov/ij/download.html
Isoflurane x 100 mL	USP Baxter	212-094	Volatile inhalated anaesthesia agent for mice
Kit CellTiter 96 Non-radioactive	Promega	0000105232	In vitro assay for placental explants viability
Mouse IgG Secondary antibody	Thermo Fisher Scientific	MO 63103	Secondary antibody for CLDN5 (western blot)
NanoSight NS300	Malvern Panalytical	90278090	Nanotracking analysis of particles from placental explants condicioned media
Paraformaldehide E 97% solution 500 mL	Thermo Fisher Scientific	A11313.22	Fixative solution for brain tissue slices and intracardial perfusion (once diluted)
PBS 1 X pH 7.4 500 mL	Thermo Fisher Scientific	10010023	Wash solution for placenta explants
Peniciline-streptomicine 100x 20 mL	Thermo Fisher Scientific	10378016	Antiobiotics for placental explants culture media
ProOX C21 Cytocentric O2 and CO2 Subchamber Controller	BioSpherix	SCR_021131	CO2 regulator to induce Hypoxia in sealed chamber for sEVs-Hyp
Sodium Thiopental 1 g	Chemie	7061	humanitarian euthanasia agent
Somnosuite low flow anesthesia system	Kent Scientifics	SS-01	Isoflurane vaporizer for small rodents
Surgical Warming platform	Kent Scientifics	A41166	Warming platform for mainteinance anesthesia in mice
Syringe Filters, Polytetrafluoroethylene (PTFE), Hydrophobic, 0.22 µm, Sterile, 25 mm	Southern labware	10026	Filtration of condicioned media harvested from placental explants
Tabletop High-Speed Micro Centrifuges HITACHI himac CT15E/CT15RE	Hitachi medical systems	6020	Serial centrifugations of condicioned media < 1,20, 000 x g
Trinocular stereomicroscope transmided and reflective light 10x-160x	Center Medical	2597	Stereomicroscope to register brain slices