A case of leaky brain barrier: how missing a piece of chromosome 22 can lead to schizophrenia

Alexis Crockett was the lead author on this study. She is interested in understanding how the rest of the body affects the brain to change behavior. One way the body signals to the brain and changes its function is through activation of the immune system. Her research focuses on how the immune system can become activated, and tries to understand how this inflammation is able to bypass all the barriers that are supposed to protect the brain from this inflammation. She is currently continuing this line of study in her postdoctoral fellowship at the Cleveland Clinic in the laboratory of Dr. Dimitrios Davalos.

or technically,

Disruption of the blood-brain barrier in 22q11.2 deletion syndrome

[See Original Abstract on Pubmed]

Authors of the study: Alexis M Crockett, Sean K Ryan, Adriana Hernandez Vásquez, Caroline Canning, Nickole Kanyuch, Hania Kebir, Guadalupe Ceja, James Gesualdi, Elaine Zackai, Donna McDonald-McGinn, Angela Viaene, Richa Kapoor, Naïl Benallegue, Raquel Gur, Stewart A Anderson, Jorge I Alvarez

Our brains are like car radios -- they tune into different stations for various thoughts and experiences. However, sometimes the station might change without a person touching the radio knob, leading them to hear sounds or voices that are not real in a way that they can't control. Imagine you are on a road trip with your friends, listening to a carefully curated Taylor Swift soundtrack, when all of the sudden, you only hear Kanye West rapping -- while your friends insist that Kanye hasn’t been playing at all! The idea of hearing something that no one else does is super confusing and frightening, especially because sometimes these stations that only you are tuned into could be ominous -- rather than Kanye rapping, you might hear someone that sounds like a scary character from a horror movie. Alternatively, what if you suddenly have zero interest in listening to Taylor Swift despite being known as her biggest fan for years? Such sudden disconnect-from-reality circumstances and/or the lack of interest and emotions are experienced by people with schizophrenia, a chronic mental illness that can seriously interfere with daily life functions. Medicine and therapy can help to manage symptoms of schizophrenia, but there is currently no cure. One reason for the lack of a cure is that we have yet to fully pinpoint the causes of this disorder, making it difficult to inform therapeutic strategies directly targeting those causes.

Scientists have identified many different genetic mutations that are linked to schizophrenia diagnoses. However, these mutations are not found in all individuals with schizophrenia. In addition, people with these mutations do not necessarily develop schizophrenia. A complex combination of genetic, environmental and lifestyle factors contributes to the development of this disorder. Generally, diseases with strong genetic drivers often have more well-defined biological mechanisms, which makes them easier to study. One of the strongest genetic risk factors in schizophrenia is the deletion of a segment of chromosome 22, herein referred to as 22q11.2 deletion, which results in the loss of 40-50 genes. Strikingly, approximately 25% of people bearing 22q11.2 deletion are diagnosed with schizophrenia, putting these people at much higher risk than the general population. Hence, deciphering the commonality among individuals with 22q11.2 deletion might help us better understand the disease mechanism(s). Dr. Alexis Crockett, a former Neuroscience Graduate Group student in the Alvarez lab at University of Pennsylvania, set out to explore how 22q11.2 deletion alters the brain in the way(s) that might cause schizophrenia.

Unlike most organs in the body, the brain is extremely delicate, with limited ability to regenerate if it is damaged. Therefore, to protect the brain, access of substances in the bloodstream to the brain is tightly controlled by a special filter, referred to as the blood-brain barrier. This structure forms a barrier that is critical for keeping various harmful particles such as bacteria, viruses, and environmental toxins from the brain. This brain barrier is made possible by densely packed endothelial cells, which are specialized cells that make up the blood vessels, and the many proteins between them like bricks and mortar, respectively. Therefore, only select substances are allowed to pass through the tiny pores of this barrier, if they are small enough or being transported by specific proteins from the blood-facing side of the cell to the brain-facing side of the same cell. This tight barrier is further reinforced by astrocytes which are a type of brain cell. Given that many of the deleted genes in the 22q11.2 region are proteins that make up this brain barrier, Dr. Crockett and colleagues hypothesized that the brain barrier is leaky in patients with 22q11.2 deletion.

To explore this hypothesis, they employed a mouse model with a similar 22q11.2 deletion as found in humans. Two proteins in the bloodstream, which are known to normally be kept out of the brain, were instead found in the brain tissue of these mice. Furthermore, they observed a marked increase in the amount of ICAM-1, a protein that aids immune cells in sticking to and migrating across the endothelial cell layer. An intact brain barrier normally restricts entry of the immune cells into the brain to avoid uncontrollable inflammation. However, in the brains of mice with 22q11.2 deletion, there was an increased level of inflammatory proteins in astrocytes of the brain. These evidence indicated a breach of brain barrier along with brain inflammation in the mouse model of 22q11.2 deletion.

Although mice are a valuable animal model for biomedical research, there are important differences between mice and humans. For instance, laboratory mice are quite genetically similar to each other, which fails to reflect the genetic complexity of schizophrenic patients. In order to study 22q11.2 deletion in human cells, Dr. Crockett and colleagues obtained cells from patients with this deletion. They then used established methods to change these cells to resemble the endothelial cells that make up the brain’s barrier, allowing them to examine the integrity of the human brain barrier in the dish. Compared to endothelial-like cells derived from healthy individuals, endothelial-like cells derived from patients with 22q11.2 deletion showed an increase in leakiness. Similar to their findings in mice, there was also a higher level of the adhesion protein ICAM-1 in the human endothelial-like cells with 22q11.2 deletion. Indeed, human immune cells readily crossed endothelial-like cell layer, consistent with known effect of high ICAM-1 level on immune cell migration.

Together, the work led by Dr. Crockett demonstrated that in the context of 22q11.2 deletion, the brain barrier is dysfunctional, permitting the entry of prohibited particles, and subsequently triggering inflammation in the brain. Interestingly, impaired function of the brain barrier has been reported in other cases of schizophrenia without clear genetic mutations, suggesting that a leaky brain barrier might be one of the underlying mechanisms contributing to the development of schizophrenia. Dr. Crockett's findings not only help us further understand the complex origins of this devastating disease, but also may lead to better treatment strategies for schizophrenia by targeting the brain’s barrier.

About the brief writer: Phuong Nguyen

Phuong is a PhD Candidate in Dr. Katy Wellen’s lab at Penn. Her research journey started in her undergraduate study at Drexel University when she performed a drug screening on a fruit fly model of Alzheimer’s disease. She then decided to pursue her PhD training in Neuroscience at Penn. She set out to characterize the brain function of a novel mouse model lacking Acly, an important enzyme for lipid synthesis and various metabolic processes. Interestingly, the brain demonstrated a remarkable resilience to the loss of this enzyme, while the skin of those mice was severely damaged that was associated with fat loss and premature death. Her research work revealed a crosstalk among the skin, the fat tissue, and the dietary lipids. She hopes to continue her research in understanding the complex metabolic crosstalk between organs, especially focusing on the brain, and how nutrition impacts those crosstalks.

Curious to learn more about what Dr. Crockett and colleagues discovered? Check out the details of this work here.

How is the brain fighting HIV?

or technically,

Neuroinflammation associates with antioxidant heme oxygenase-1 response throughout the brain in persons living with HIV

[See Original Abstract on Pubmed]

Authors of the study: Analise L. Gruenewald, Yoelvis Garcia-Mesa, Alexander J Gill, Rolando Garza, Benjamin B. Gelman, Dennis L. Kolson

You might be familiar with Human Immunodeficiency Virus (HIV), but did you know that approximately half of those living with HIV also experience impairments in cognition?¹ HIV is a virus capable of invading our cells, specifically those that play vital roles in our immune system. The immune system is the body’s security force, fighting off invaders and preventing you from getting sick. If HIV is not treated, it will progress into Acquired Immunodeficiency Syndrome (AIDS), in which the immune system is severely weakened and patients become highly vulnerable to complications from other illnesses. While untreated HIV/AIDS is fatal, there have been amazing scientific developments in antiretroviral therapies, which substantially prevent the progression of HIV into AIDS. This therapy prevents the virus from making lots of copies of itself inside cells so that the immune system does not get overwhelmed and can still do its job.² Despite these advancements, there is still much more progress that needs to be made to improve HIV treatment. At the end of 2019, there were roughly 38 million people suffering from HIV/AIDS around the world, and over half a million people died from illnesses related to AIDS. While therapies such as antiretroviral therapy can be successful, a third of people living with HIV still do not have access to these life-saving treatments.³

But how does HIV impact your brainThe brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals.? Besides weakening the immune system, HIV can also impact cognition (the way our brainThe brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. thinks and functions). About half of people with HIV also suffer from HIV-associated neurocognitive disorder (HAND). HAND can impair memory, attention, and decision making. How much a person’s cognition is impaired can vary widely, with mild effects in some cases but detrimental effects to a person’s life in others, such as being unable to learn new things or remember important dates and appointments. Why some with HIV develop HAND, and others do not, is still not well understood, but discoveries made by scientists are starting to shed some light on this question.

Dr. Analise Gruenewald is a recent graduate of the Neuroscience Graduate Group at the University of Pennsylvania and member of the Kolson Lab who is intent on building science’s understanding of why HAND develops in only a subset of people living with HIV. Previous work in the Kolson Lab has suggested that a proteinAn essential molecule found in all cells. DNA contains the recipes the cell uses to make proteins. Examples of proteins include receptors, enzymes, and antibodies. called heme-oxygenase 1 (HO-1) might be key. HO-1 helps the body handle both oxidative stressdamage done to the body due to a build-up of unstable oxygen-containing molecules and inflammationthe state of the body when it is fighting an infection, characterized by the release of specific chemicals and the activation of the immune system, which are both responses of the body to HIV that are thought to contribute to some of its harmful effects. Those with HAND have been shown to have a lower amount of HO-1 proteinAn essential molecule found in all cells. DNA contains the recipes the cell uses to make proteins. Examples of proteins include receptors, enzymes, and antibodies. in a specific brainThe brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. region called the prefrontal cortex.⁴ The prefrontal cortex is a part of the brainThe brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. that is important for decision making, which is disrupted in HAND. Analise built upon these findings to understand how HO-1 might be important in the development of HAND in HIV patients. Specifically, Analise sought to determine how much HO-1 proteinAn essential molecule found in all cells. DNA contains the recipes the cell uses to make proteins. Examples of proteins include receptors, enzymes, and antibodies. is in the brainThe brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. tissue of patients and how this relates to signs of inflammationthe state of the body when it is fighting an infection, characterized by the release of specific chemicals and the activation of the immune system.

Analise examined the brainThe brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. tissue of those who had HIV (but specifically did not have HAND) compared to those who did not have HIV. By examining the brainsThe brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. of those who had HIV but not HAND, she aimed to identify a particular factor that may function to prevent or protect against it. She first observed that the amount of HO-1 proteinAn essential molecule found in all cells. DNA contains the recipes the cell uses to make proteins. Examples of proteins include receptors, enzymes, and antibodies. in the brainsThe brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. of those with HIV but not HAND was higher than the amount in those without HIV. This exciting finding supports the hypothesis that HO-1 is an important factor in how our brainsThe brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. defend against HIV, suggesting high levels of HO-1 proteinAn essential molecule found in all cells. DNA contains the recipes the cell uses to make proteins. Examples of proteins include receptors, enzymes, and antibodies. provide protection against the development of HIV-related HAND. Further understanding of HO-1’s role in the brainThe brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals.’s defense against HAND could enable the development of preventative treatments. Encouraged, Analise decided to explore the connection between HO-1 and HIV-related brainThe brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. inflammationthe state of the body when it is fighting an infection, characterized by the release of specific chemicals and the activation of the immune system.

Figure 1: Potential mechanism by which HO-1 prevents development of HAND. 

HIV leads to inflammation (which involves immune cell activation), and whether or not the individual then develops HAND might depend on the level of HO-1 protein they have. (A) If an individual has a high level of HO-1 protein this might enable them to maintain normal cognition. (B) An individual with a low level of HO-1 protein might not be able to properly respond to the inflammation, and will develop HAND. Image created with BioRender.com

To explore this idea further, Analise investigated how the amount of HO-1 proteinAn essential molecule found in all cells. DNA contains the recipes the cell uses to make proteins. Examples of proteins include receptors, enzymes, and antibodies. in the brainThe brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. correlated with the presence of inflammatory signals. If HO-1 prevents HAND by mediating the body’s response to infection, one might expect these two to be connected. She chose to measure the levels of various proteinsAn essential molecule found in all cells. DNA contains the recipes the cell uses to make proteins. Examples of proteins include receptors, enzymes, and antibodies. involved in inflammationthe state of the body when it is fighting an infection, characterized by the release of specific chemicals and the activation of the immune system – so if the levels of these proteinsAn essential molecule found in all cells. DNA contains the recipes the cell uses to make proteins. Examples of proteins include receptors, enzymes, and antibodies. are higher, that would indicate that there is more inflammationthe state of the body when it is fighting an infection, characterized by the release of specific chemicals and the activation of the immune system present. She found that if the levels of these inflammationthe state of the body when it is fighting an infection, characterized by the release of specific chemicals and the activation of the immune system-related proteinsAn essential molecule found in all cells. DNA contains the recipes the cell uses to make proteins. Examples of proteins include receptors, enzymes, and antibodies. were higher in a given individual, the amount of HO-1 also tended to be higher. These findings provided even stronger support for Analise’s hypothesis that HO-1 might be important in preventing HAND via compensating for the body’s inflammationthe state of the body when it is fighting an infection, characterized by the release of specific chemicals and the activation of the immune system.

To recap how this might work: HIV causes an inflammatory response in infected patients. HO-1 is a proteinAn essential molecule found in all cells. DNA contains the recipes the cell uses to make proteins. Examples of proteins include receptors, enzymes, and antibodies. that is important for ensuring the body can be healthy even during this inflammationthe state of the body when it is fighting an infection, characterized by the release of specific chemicals and the activation of the immune system. Higher HO-1 levels should be protective and allow a person to successfully handle high levels of inflammationthe state of the body when it is fighting an infection, characterized by the release of specific chemicals and the activation of the immune system without a lot of serious negative effects (such as on cognition). In those without HAND, this appears to hold true: if the patient has high inflammationthe state of the body when it is fighting an infection, characterized by the release of specific chemicals and the activation of the immune system, they also have high levels of HO-1.

Analise’s work provides a fascinating insight into a poorly understood yet critical topic: why some people with HIV develop HAND, but others don’t. There is still more work to be done to fully confirm her initial findings; however, her work presents a strong foundation for future research. Her findings indicate that HO-1 proteinAn essential molecule found in all cells. DNA contains the recipes the cell uses to make proteins. Examples of proteins include receptors, enzymes, and antibodies. might be critical in preventing the development of HAND. Not only is this work thrilling in terms of simply furthering scientific understanding of HAND, but it also has promising therapeutic potential. If HO-1 is important for preventing HAND, this opens up the exciting possibility that a treatment that increases the amount of HO-1 proteinAn essential molecule found in all cells. DNA contains the recipes the cell uses to make proteins. Examples of proteins include receptors, enzymes, and antibodies. in those with HIV could prevent HAND. Analise’s work is a wonderful step toward significantly improving the lives of millions of people affected by HIV around the world.

Citations:

1. Saylor, D., Dickens, A. M., Sacktor, N., Haughey, N., Slusher, B., Pletnikov, M., Mankowski, J. L., Brown, A., Volsky, D. J., & McArthur, J. C. (2016). HIV-associated neurocognitive disorder--pathogenesis and prospects for treatment. Nature reviews. Neurology, 12(4), 234–248. https://doi.org/10.1038/nrneurol.2016.27

2. Deeks, S., Overbaugh, J., Phillips, A. & Buchbinder, S. (2015). HIV infection. Nat Rev Dis Primers, 1, 15035. https://doi.org/10.1038/nrdp.2015.35

3. The Global HIV/AIDS Epidemic. (2020, November 25). HIV.gov. Retrieved January 2, 2021, from https://www.hiv.gov/hiv-basics/overview/data-and-trends/global-statistics

   Gill, A. J., Kovacsics, C. E., Cross, S. A., Vance, P. J., Kolson, L. L., Jordan-Sciutto, K. L., Gelman, B. B., & Kolson, D. L. (2014). Heme oxygenase-1 deficiency accompanies neuropathogenesis of HIV-associated neurocognitive disorders. The Journal of clinical investigation, 124(10), 4459–4472. https://doi.org/10.1172/JCI72279

About the brief writer: Katie Copley

Katie is a PhD student in Dr. Jim Shorter’s lab.

Do you want to learn more of the details of Analise’s work? You can find her full paper here.