- Suppression of transcytosis regulates zebrafish blood-brain barrier function
- Holes in the leaky migraine blood–brain barrier hypothesis?
- The link between aura and headache
- Is drug translocation to brain tissue target sites important to abort acute attacks?
- Cerebral blood flow alterations and migraine with aura
- Stem cell-derived blood-brain barrier gives more complete picture of Huntington’s disease
- Can Genetics Impact a “Leaky” Blood-Brain Barrier?
- Research Limitations
- “Leaky Brain” Genetics – Is there a Link?
- Alpha 2-Macroglobulin (A2M)
- Learn More in the “Leaky Brain” Four-Part Series
Suppression of transcytosis regulates zebrafish blood-brain barrier function
As an optically transparent model organism with an endothelial blood-brain barrier (BBB), zebrafish offer a powerful tool to study the vertebrate BBB. However, the precise developmental profile of functional zebrafish BBB acquisition and the subcellular and molecular mechanisms governing the zebrafish BBB remain poorly characterized.
Here, we capture the dynamics of developmental BBB leakage using live imaging, revealing a combination of steady accumulation in the parenchyma and sporadic bursts of tracer leakage. Electron microscopy studies further reveal high levels of transcytosis in brain endothelium early in development that are suppressed later.
The timing of this suppression of transcytosis coincides with the establishment of BBB function. Finally, we demonstrate a key mammalian BBB regulator Mfsd2a, which inhibits transcytosis, plays a conserved role in zebrafish, as mfsd2aa mutants display increased BBB permeability due to increased transcytosis.
Our findings indicate a conserved developmental program of barrier acquisition between zebrafish and mice.
Blood vessels in the vertebrate brain are composed of a single layer of endothelial cells that possess distinct functional properties that allow the passage of necessary nutrients yet prevent unwanted entry of specific toxins and pathogens into the brain.
This specialized endothelial layer forms the blood-brain barrier (BBB) and restricts the passage of substances between the blood and the brain parenchyma via two primary mechanisms: 1) specialized tight junction complexes between apposed endothelial cells to prevent intercellular transit (Reese and Karnovsky, 1967; Brightman and Reese, 1969) and 2) suppressing vesicular trafficking or transcytosis to prevent transcellular transit (Ben-Zvi et al., 2014; Andreone et al., 2015; Chow and Gu, 2017). BBB selectivity is further refined with the expression of substrate-specific transporters that dynamically regulate the influx of necessary nutrients and efflux of metabolic waste products (Sanchez-Covarrubias et al., 2014; Umans et al., 2017). While the BBB is comprised of endothelial cells, the surrounding perivascular cells including pericytes and astroglial cells, play a critical role in forming and maintaining barrier properties (Janzer and Raff, 1987; Armulik et al., 2010; Bell et al., 2010; Daneman et al., 2010; Wang et al., 2014). Collectively, endothelial cells and the surrounding perivascular cells form the neurovascular unit.
As the simplest genetic model organism with an endothelial BBB (Jeong et al., 2008), zebrafish offer a powerful tool to study the cellular and molecular properties of the vertebrate BBB (Xie et al., 2010; Vanhollebeke et al., 2015; Umans et al., 2017; O'Brown et al., 2018; Quiñonez-Silvero et al., 2019).
Zebrafish have served as a great model system to study vascular biology due to their large clutch size, rapid and external development, and transparency for in vivo whole organism live-imaging (Lawson and Weinstein, 2002; Jin et al., 2005; Santoro et al., 2007; Armer et al., 2009; Herbert et al., 2009; Phng et al., 2009; Geudens et al., 2010; Herbert et al.
, 2012; Wilkinson and van Eeden, 2014; Franco et al., 2015; Vanhollebeke et al., 2015; Matsuoka et al., 2016; Ulrich et al., 2016; Venero Galanternik et al., 2017; Stratman et al., 2017; Geudens et al., 2018). Additionally, with the advent of CRISPR-Cas9 technology, zebrafish provide an efficient genetic toolkit for targeted mutagenesis (Hwang et al., 2013; Gagnon et al.
, 2014; Ablain et al., 2015; Varshney et al., 2015; Albadri et al., 2017; Hogan and Schulte-Merker, 2017). However, the subcellular and molecular mechanisms governing the formation and maintenance of the zebrafish BBB are only beginning to be characterized.
Expanding our understanding of the zebrafish BBB can thus reveal the mechanistic similarities between the zebrafish and mammalian BBB to further evaluate the position of zebrafish as a model organism for studying the BBB.
Barrier properties of brain endothelial cells are induced by extrinsic signals from other cells in the surrounding microenvironment during development (Stewart and Wiley, 1981). In rodents, the BBB becomes functionally sealed in a spatiotemporal gradient, with the hindbrain and midbrain barriers becoming functional before the cortical barrier (Daneman et al.
, 2010; Ben-Zvi et al., 2014; Sohet et al., 2015). Within the cortex, barrier function is acquired along a ventral-lateral to dorsal-medial gradient (Ben-Zvi et al., 2014). In the zebrafish, existing studies have disagreed over the timing of zebrafish barrier formation, with some suggesting that BBB maturation occurs concurrently with angiogenesis (Jeong et al., 2008; Xie et al.
, 2010; Umans et al., 2017) and others providing a wide range beginning at 3 dpf and extending to 10 dpf (Fleming et al., 2013). These conflicting reports may be due to regional developmental gradients of barrier acquisition, differences in definitions of BBB development, which range from functional tracer restriction (Jeong et al., 2008; Fleming et al.
, 2013) to gene expression of BBB-specific selective transporters, such as glut1b (Umans et al., 2017), or variations in the experimental approaches used to assess BBB permeability such as the molecular weight and properties of tracers and circulation time.
Therefore, we aim to resolve some of these discrepancies through detailed regional and subcellular characterizations of functional barrier acquisition in zebrafish.
Recent work in the mammalian blood-retinal barrier has indicated that the suppression of transcytosis governs functional barrier development (Chow and Gu, 2017).
Interestingly, endothelial cells at the leaky neonatal angiogenic front possess functional tight junction complexes halting the intercellular passage of the tracer protein Horseradish Peroxidase (HRP) at the so-called ‘kissing points’. In contrast, these endothelial cells exhibit high levels of HRP-filled vesicles compared to functionally sealed proximal vessels.
Moreover, these areas of elevated vesicular trafficking continue to correspond with barrier permeability at the angiogenic front until the barrier seals (Chow and Gu, 2017). Work in the mouse BBB has also demonstrated the importance of suppressing transcytosis in determining barrier permeability.
Mice lacking the major facilitator super family domain containing 2a (Mfsd2a) lipid transporter exhibit increased levels of caveolae vesicles in CNS endothelial cells, resulting in increased barrier permeability (Ben-Zvi et al., 2014; Andreone et al., 2017). Whether this suppression of transcytosis also determines BBB function in zebrafish remains unknown.
Here in zebrafish, we find a spatiotemporal gradient of barrier acquisition, and capture the dynamics of BBB leakage as it matures during development using time lapse live imaging. We further find a conserved role for transcytosis suppression in determining barrier function, both during normal development and in mfsd2aa mutants.
To determine when and how the zebrafish BBB becomes functional in different brain regions, we performed intracardiac injections of fluorescently conjugated tracers (1 kDa NHS and 10 kDa Dextran) simultaneously at different developmental stages and imaged live fish after 1 hr of tracer circulation (Figure 1A and B).
NHS is widely used to assess mouse BBB permeability (Sohet et al., 2015; Chow and Gu, 2017; O'Brown et al., 2018). Additionally, NHS has been used successfully to assess junctional defects in occludin- and claudin-deficient animals (Chen et al., 1997; Furuse et al., 2002; Nitta et al.
, 2003), and was previously shown to be restricted within the adult zebrafish cerebral vasculature (Jeong et al., 2008).
We used a combination of different molecular weight tracers to tease apart potential avenues of leakage, as tight junctional defects result specifically in the leakage of low-molecular-weight tracers 1 kDa and below into the brain parenchyma (Nitta et al., 2003; Campbell et al., 2008; Sohet et al., 2015; Yanagida et al., 2017).
To assess BBB permeability, we measured tracer fluorescence intensity in the brain parenchyma and normalized to circulating levels of each tracer, that is to the fluorescence intensity of tracer within brain blood vessels, to account for between fish variation in tracer injections or circulation (details in the Materials and methods section).
At 3 dpf, we observed the presence of both NHS and Dextran throughout the brain parenchyma (average of 8.5 ± 0.3 Tracer Intensity in the midbrain and average of 7.4 ± 0.4 NHS and 5.5 ± 0.6 Dextran Intensity in the hindbrain; Figure 1; Figure 1—figure supplement 1).
These leakage assays revealed that the injected tracers were able to permeate into the brain parenchyma, suggesting that the BBB was not functionally sealed.
In addition to the use of exogenous injected fluorescent tracers, we also assayed BBB permeability with an endogenous transgenic serum DBP-EGFP fusion protein (Tg(l-fabp:DBP-EGFP)) to account for injection artifacts (Xie et al., 2010). At 3 dpf, we observed similar leakage patterns with the transgenic serum protein as we did with the injected tracers (average of 8.9 ± 0.8 DBP-EGFP Intensity in the midbrain and average of 6.1 ± 0.4 DBP-EGFP Intensity in the hindbrain; Figure 1C and D; Figure 1—figure supplement 1). At 4 dpf, we observed a significant decrease in tracer intensity in the hindbrain (average of 4.1 ± 0.2 NHS Intensity, 3.9 ± 0.4 Dextran Intensity and 4.0 ± 0.4 DBP-EGFP Intensity; p
Holes in the leaky migraine blood–brain barrier hypothesis?
This scientific commentary refers to ‘Increased brainstem perfusion, but no blood-brain barrier disruption, during attacks of migraine with aura’, by Hougaard et al.. (doi:10.1093/brain/awx089).
Disruption of the blood–brain barrier (BBB) and inflammation are important contributors to the pathogenesis of neurological disorders. Although inflammation has been implicated in migraine pathogenesis, it is not known whether barrier integrity is compromised during attacks.
It has been posited by Harper and colleagues (1976) that a leaky barrier may allow noxious chemicals within the blood to trigger pain, and explain the pattern of changes in cerebral blood flow during attacks. BBB penetration may also provide a route by which abortive drugs used for acute treatment and normally excluded from the brain (e.g.
triptans) access binding sites in the CNS to achieve their therapeutic effects. Furthermore, leakage of plasma protein and upregulation of matrix metalloproteinases has been observed within rat cortex for 24 h after cortical spreading depression (CSD), the biological substrate for migraine aura (Gursoy-Ozdemir et al., 2004).
In this issue of Brain, Hougaard and colleagues use high resolution imaging techniques in human patients to study the integrity of the BBB within specific brain regions during attacks of migraine with aura (Hougaard et al., 2017).
Their bottom line finding is that the BBB is not compromised during headache in migraine attacks. To reach this conclusion, Hougaard et al.
used a sensitive and validated technique, dynamic contrast-enhanced high-field MRI coupled with gadolinium tracer, to measure BBB function during the headache of migraine aura.
They assessed multiple regions including visual cortex and cortical territories within the anterior, middle and posterior cerebral arteries and brainstem. They found no correlation between permeability change and time of symptom onset or intensity of pain.
The data appeared consistent across 19 subjects within the time window identified on the basis of CSD in an animal model. However, an important next step would be to test the possibility that barrier disruption may occur transiently and early during an attack.
The link between aura and headache
So, if BBB disruption is not a contributing mechanism, then what may link aura and headache? Preclinical experimental studies support the importance of CSD.
CSD is well established as a propagating depolarization of neurons and glia that spreads slowly (3–5 mm/min) and contiguously within grey matter structures followed by many minutes of depression of electrical activity (Pietrobon and Moskowitz, 2014). CSD has been documented in damaged human brain.
The evidence implicating it in migraine aura is strong (Hadjikhani et al., 2001) but there is no evidence of accompanying tissue injury. CSD imposes severe tissue stress and extends and expands tissue damage but does not appear to initiate it, except within already compromised tissue.
It imposes a severe metabolic demand for ATP, which is required to restore ionic gradients in the presence of constrained blood flow (and hence delivery of oxygen and glucose), so called flow/metabolism mismatch (Dreier and Reiffurth, 2015).
Although there is no evidence for brain damage in otherwise normal humans or experimental models after a single spontaneous or evoked attack (and there need not be), it turns out that CSD is noxious and activates/discharges trigeminal axons (Pietrobon and Moskowitz, 2013). Trigeminal axons innervate primarily the ipsilateral meninges (Mayberg et al.
, 1981) as well as midline structures bilaterally, and contain neuromediators such as calcitonin gene-related peptide (Uddman et al., 1986). Activation of the trigeminovascular (TV) system following CSD has been reported by four different laboratories using six different experimental paradigms in two species (Pietrobon and Moskowitz, 2013).
Hence, it may not be coincidental that 16 of 18 patients studied by Hougaard et al. experienced headache on the same side of the head (and 13 of 18 strictly on the same side) as the CSD generating the aura.
Although auras are understudied, there have been no other candidate events proposed or identified ipsilateral to the affected cortex that trigger trigeminal afferents and generate headache unilaterally, the headache after stroke for example. Hence, the anatomy helps to predict the sidedness of the headache.
Contralateral headache would be an expectation if thalamus and cortex on the same side as the CSD were processing transmitted pain signals. CSDs that are less intense or that are limited in spatial distribution (Zandt et al.
, 2015), and/or reflect more efficient tissue clearance of nociceptive molecules may explain why some people with migraine do not experience headache after aura. Consistent with the above formulation, trigeminal axons and cell bodies are known to express multiple 5-HT1 receptor subtypes plus CGRP and its cognate receptor, which figure prominently as therapeutic targets (see below).
Is drug translocation to brain tissue target sites important to abort acute attacks?
Sumatriptan, a 5-HT1 receptor agonist and the first triptan abortive agent, reportedly does not cross the BBB but does diminish headache significantly when given within the average MRI scanning period (especially the earlier time point reported in the Hougaard et al. study).
Furthermore, recent studies have concluded that occupancy of brain CGRP receptors is not essential for relief of acute migraine headache. Telcagepant, a small molecule CGRP receptor antagonist, did not penetrate the brain further (i.e. displace more radiolabelled ligand from its central binding sites) when given at clinically effective doses (Hostetler et al., 2013).
Similar findings were reported for dihydroergotamine during a drug-induced attack.
Questions need to be asked about the therapeutic importance of CNS targets, especially the activity of drugs that do not penetrate the brain (triptans and ergot alkaloids).
Furthermore, the same might be said for high molecular weight antibodies directed against neutralizing CGRP or blocking its receptor that diminish attack frequency.
What all this means is that expression of a receptor or drug target within brain (even within primary pain processing networks) does not by itself ensure its relevance to a therapeutic effect, especially if the drug does not reach target sites at sufficient concentrations to modulate target cell signalling.
In other words, the evidence to date suggests that drug penetration into CNS is not facilitated by BBB disruption during a migraine aura attack. There may be exceptions.
A disrupted barrier and brain oedema were reported in a migraine genetic variant with aura (Dreier, 2016) that may suggest a counterpart to matrix metalloproteinase upregulation and barrier disruption following multiple CSDs in an animal model (Gursoy-Ozdemir et al., 2004).
The greater degree of tissue stress and inflammatory signalling in a multiple CSD animal model may explain why humans with typical migraine aura do not also show BBB disruption.
These exceptions notwithstanding, the findings by Hougaard and colleagues suggest that candidate tissues and cells outside the BBB merit investigation to help identify therapeutically relevant target sites.
Cerebral blood flow alterations and migraine with aura
Harper took their leaky brain formulation one step further by suggesting that noxious circulating molecules normally excluded from brain and cerebrovascular smooth muscle contribute to brain perfusion changes observed in people with migraine during and after attacks (Harper et al., 1976). The literature over the past two decades contradicts this notion.
From animal and a few human studies, it seems clear that deviations in cerebral blood flow during migraine aura are caused by CSD, at least early on. Blood oxygen level-dependent signal changes in functional MRI during a visual aura reveal slowly propagating hypoperfusion, characteristic of the blood flow changes observed in rats during CSD (Hadjikhani et al., 2001).
Low blood flow usually persists for many minutes or longer, and as first described in humans, is sometimes followed by hyperperfusion (Hougaard et al., 2017).
However, decreases in blood flow during aura are usually modest (10–25%) and above ischaemic thresholds, so that if hyperperfusion develops afterwards, it may reflect an integral of prolonged and persistent plus mild blood flow reductions rather than ischaemia-reperfusion, per se.
Hougaard et al. also reported increases in pontine blood flow bilaterally, a finding noted previously.
Pontine activation may reflect the processing of pain and its autonomic accompaniments associated with nausea, vomiting, diaphoresis, changes in heart rate and blood pressure, bladder/bowel disturbance, anxiety, changes in wakefulness, among other characteristics of an altered physiological state; some of these were experienced by patients studied by Hougaard et al. (Table 1). Sorting this out will be difficult without the development of new techniques with higher spatial resolution for imaging brainstem anatomy and improvements in deep phenotyping of migraine patients.
In summary, brain imaging is a powerful tool to interrogate the underlying mechanisms of migraine headache and inform us about what may or may not be important. Although still in its formative stages, MRI in patients has informed us about the role of CSD and helped us characterize attendant blood flow changes that occur during headache.
It has also informed us that brainstem activation is a consistent, albeit poorly understood feature of attacks, whereas BBB disruption does not appear to characterize the sustained headache.
Calcitonin gene related peptide (CGRP): An effective therapeutic target in migraine, CGRP is a neuromediator within the trigeminovascular system as well as within rostrally projecting central pain pathways.
Matrix metalloproteinases: A family of at least nine extracellular matrix-degrading enzymes that impact tissue function after cleaving matrix proteins to reshape the extracellular space.
, , , , , et al. Mechanisms of migraine aura revealed by functional MRI in human visual cortex. ; : –., , , , , .
Increased brainstem perfusion, but no blood-brain barrier disruption, during attacks of migraine with aura. , ., , , .
Perivascular meningeal projections from cat trigeminal ganglia- possible pathway for vascular headaches in man. ; : –., . Pathophysiology of migraine. Annu Rev Physiol ; : –. Scientific Commentaries
Stem cell-derived blood-brain barrier gives more complete picture of Huntington’s disease
a sophisticated security fence, our bodies have evolved a barrier that protects the brain from potentially harmful substances in the blood but still allows the entry of essential molecules blood sugar and oxygen.
Just in other parts of the body, the blood vessels and capillaries in the brain are lined with endothelial cells.
But in the brain, these cells form extremely tight connections with each other making it nearly impossible for most things to passively squeeze through the blood vessel wall and into the brain fluid.
Compared to blood vessels in other parts of the body, brain blood vessels form a much tighter seal to protect the brain.
Image source: Dana and Chris Reeve Foundation
Recent studies have shown defects in the brain-blood barrier are associated with neurodegenerative disorders Huntington’s disease and as a result becomes leakier.
Although the debilitating symptoms of Huntington’s disease – which include involuntary movements, severe mood swings and difficulty swallowing – are primarily due to the gradual death of specific nerve cells, this breakdown in the blood-brain barrier most ly contributes to the deterioration of the Huntington’s brain.
What hasn’t been clear is if mutations in Huntingtin, the gene that is linked to Huntington’s disease, directly impact the specialized endothelial cells within the blood-brain barrier or if these specialized cells are just innocent bystanders of the destruction that occurs as Huntington’s progresses. It’s an important question to answer. If the mutations in Huntingtin directly affect the blood-brain barrier then it could provide a bigger picture of how this incurable, fatal disease works. More importantly, it may provide new avenues for therapy development.
A UC Irvine research team got to the bottom of this question with the help of induced pluripotent stem cells (iPSCs) derived from the skin cells of individuals with Huntington’s disease. Their CIRM-funded study was published this week in Cell Reports.
In a first for a neurodegenerative disease, the researchers coaxed the Huntington’s disease iPSCs in a lab dish to become brain microvascular endothelial cells (BMECs), the specialized cells responsible for forming the blood-brain barrier.
The researchers found that the Huntington’s BMECs themselves were indeed dysfunctional. Compared to BMECs derived from unaffected individuals, the Huntington’s BMECs weren’t as good at making new blood vessels, and the vessels they did make were leakier.
So the Huntingtin mutation in these BMECs appears to be directly responsible for the faulty blood-brain barrier.
The team dug deeper into this new insight by looking for possible differences in gene activity between the healthy and Huntington’s BMECs.
They found that the Wnt group of genes, which plays an important role in the development of the blood-brain barrier, are over active in the Huntington’s BMECs. This altered Wnt activity can explain the leaky defects.
In fact, the use of a drug inhibitor of Wnt fixed the defects. Dr. Leslie Thompson, the team lead, described the significance of this finding in a press release:
“Now we know there are internal problems with blood vessels in the brain. This discovery can be used for possible future treatments to seal the leaky blood vessels themselves and to evaluate drug delivery to patients with HD.”
Study leader, Leslie Thompson. Steve Zylius / UCI
A companion Cell Stem Cell report, also published this week, used the same iPSC-derived blood-brain barrier system.
In that study, researchers at Cedars-Sinai pinpointed BMEC defects as the underlying cause of Allan-Herndon-Dudley syndrome, another neurologic condition that causes mental deficits and movement problems.
Together these results really drive home the importance of studying the blood-brain barrier function in neurodegenerative disease.
Dr. Ryan Lim, the first author on the UC Irvine study, also points to a larger perspective on the implications of this work:
“These studies together demonstrate the incredible power of iPSCs to help us more fully understand human disease and identify the underlying causes of cellular processes that are altered.”
Can Genetics Impact a “Leaky” Blood-Brain Barrier?
In this post, we go into the genetics research behind a highly controversial concept: “leaky brain.” Read on to find out if genetics can play a role in blood-brain barrier integrity.
the gut barrier, the blood-brain barrier is lined with one layer of cells that separate the blood from the brain. It only allows a few substances oxygen, hormones, and certain cytokines in, while blocking out others .
When this protective layer is compromised, the brain is thought to be vulnerable to damage from chemicals, inflammatory cytokines, and immune cells [2, 1].
“Leaky brain” was coined as a non-medical term for increased blood-brain barrier permeability .
This term is still relatively new and research is sparse. The whole theory is theoretical and it has yet to be verified in proper human studies. Findings are still mostly limited to cell culture or brain tissue studies [2, 1].
That said, limited research has linked a compromised blood-brain barrier with “brain fog” or cognitive dysfunction, chronic fatigue, anxiety/depression, neurodegenerative diseases, and other neurological conditions. There’s not enough evidence to support any of these associations [2, 1].
Remember, the SNPs we go over below have only been found to be associated with increased BBB permeability or “leaky brain.” That does not mean having them will necessarily make you more ly to have BBB permeability problems. More work is needed before we determine whether and how much they may raise a person’s risk of increased BBB permeability.
“Leaky Brain” Genetics – Is there a Link?
There are many potential causes of blood barrier issues. These usually involve a complex interaction between genetics and the environment.
Scientists think that the following genes may have some influence on the blood-brain barrier:
- Matrix metalloproteinases (MMPs)
- Tissue Inhibitors of Metalloproteinases (TIMPs)
- Tight junction proteins
- Genes that control oxidative stress
- Genes that control inflammation and susceptibility to autoimmunity
However, the impact of these genes on the BBB in humans is poorly understood.
There’s no direct study that tested the effects of each gene, SNP, or mutation on the strength of the blood-brain barrier. Thus, scientists can only hypothesize the significance of these SNPs from the gene functions and their association with brain disease/injury outcomes in humans.
For example, limited research suggests that MMP2 gene variants (SNPs) may predict to some extent the possible damage done to the brain by ischemic stroke. This hasn’t been proven in large-enough studies [3, 4].
MMP9 SNPs have been associated with obesity [5, 6]:
A SNP of another matrix metalloproteinase, MMP13, was associated with an extremely rare condition that increased the risk of blood-brain barrier disruption and stroke – leukoaraiosis :
TIMP2 – People with a variant of MMP inhibitor TIMP2, -261G/A, appeared to be at an increased risk of bleeding in the brain data from one study. This finding hasn’t been replicated .
Claudin-5 is a tight junction structural protein. It is important for BBB strength. Variants in this gene have been linked to various diseases.
For instance, it was hypothesized that variants of CLDN5 inherited together with the HLA-DQB1 gene (the celiac disease gene) lead to schizophrenia. This theory remains unverified .
Another study carried out among the Chinese population had a similar hypothesis. They suggested that a SNP of CLDN5, rs10314, may be associated with an increased risk of schizophrenia. Large-scale human data are lacking, though .
Another blood-brain barrier protein, P-glycoprotein, transports necessary chemicals through the BBB and protects it from unwanted molecules because it is selective .
ABCB1 is a gene responsible for P-glycoprotein production. The T3435 allele of this gene was associated with an increased risk of major depressive disorder in Japanese people .
In newborns, NOS3 in the cells that line the blood vessel in the brain can mitigate nerve damage due to a lack of oxygen during birth (hypoxic-ischemic encephalopathy). Some experts consider it a disease of a disrupted blood-brain barrier .
rs2070744 appears to significantly affect APGAR scores (a measure of infant wellness), with the C allele being associated with worse outcomes .
rs1800779 and rs1799983 are not associated with the difference in hypoxic-ischemic encephalopathy.
However, rs1800779 was associated with leukoaraiosis after ischemic stroke .
More research is needed.
Mutations in SOD1 are linked to familial ALS .
In addition, mice that have defective SOD1 function have the same leaky blood-brain barrier, blood-spinal cord barriers, and neurovascular units as ALS patients .
The A allele of rs662 has higher enzymatic activity than the G allele, which lowers the risk of heart disease .
This variant is associated with leukoaraiosis (white matter lesions) after stroke, which may be linked to the worsened leaky brain after stroke .
Some research suggests that ApoE4 promotes blood-brain barrier disruption, whereas ApoE2 and ApoE3 seem to protect the blood-brain barrier [16, 17].
ApoE4 activates an inflammatory pathway in pericytes that disrupt the tight junctions, while ApoE2 and ApoE3 suppress this pathway .
ApoE4 is also famously linked to Alzheimer’s disease .
Alpha 2-Macroglobulin (A2M)
The C (minor) allele of A2M rs669 SNP is linked with an increased risk of Parkinson’s disease and Alzheimer’s disease [19, 20].
In addition, it is also linked to an increased risk of complications following tissue plasminogen activator treatment .
This has been suggested to predict the outcome of post-ischemic stroke, but more data are needed .
rs2290608 GG is linked to leukoaraiosis post-ischemic stroke .
Learn More in the “Leaky Brain” Four-Part Series
You can read about the introduction to the blood-brain barrier and possible causes of “leaky brain” in Parts 1 and 2.