Background

Schizophrenia is a chronic mental disorder that is disabling to those affected by it. Characteristic symptoms of schizophrenia include psychosis, paranoid delusions, and auditory hallucinations (Insel 2010). Although the actual cause of schizophrenia remains unknown, neuroimmune dysregulation and BBB dysfunction has been previously implicated in neuropsychiatric disorders such as schizophrenia (Aleksovska et al., 2014; Cai et al., 2020; Orlovska-Waast et al., 2019; Pollak et al., 2017). Recently, a team of researchers looked toward the blood brain barrier (BBB) to investigate its relationship with schizophrenia. The researchers specifically investigated if the BBB was compromised in an organism with 22q11.2 deletion syndrome (22qDS) (Gur et al., 2017). The researchers hypothesized that the proteins deleted as the result of 22qDS result in a compromised BBB, which ultimately can lead to schizophrenia (Crockett et al., 2021). Ultimately, they did find that there is a strong relationship between the BBB and 22qDS schizophrenia (Crockett et al., 2021). The research implies that schizophrenia is in part the result of neuroinflammation and suggests that neuromodulation can potentially treat a percentage of individuals affected by schizophrenia. 

Crockett et al. chose to investigate the BBB since this barrier is the immunological interface between the brain and the periphery. One of the BBB’s characteristics is immune privilege, which means that the brain is able to tolerate the introduction of foreign antigens without triggering an inflammatory immune response (Carson et al., 2006). Additionally, they chose to investigate the BBB since its phenotype is affected by 22qDS. 22qDS, also known as DiGeorge’s Syndrome, is the result of a 2.5 megabase hemizygous deletion of about 46 protein coding genes on chromosome 22; several of these proteins include junctional and mitochondrial genes that potentially influence the BBB (Gur et al., 2017; McDonald-McGinn et al., 2015; Abbott et al., 2010). Approximately 25% of patients with 22qDS subsequently develop schizophrenia, making it the most common genetic risk factor for schizophrenia (Gur et al., 2017). By looking at how 22qDS and the BBB are related, Crockett et al. was able to form a link between the BBB and schizophrenia. 

Methods

To investigate the relationship between 22qDS and the BBB, Crockett et al. received human induced pluripotent stem cells (HiPSCs) from five 22qDS subjects (22qDS+SZ) and five age- and sex-matched healthy control subjects. The HiPSCs were differentiated into BBB-like endothelium (iBBB), stored, and cultured for later analysis. The tests were performed in a paired format in order to reduce inconsistencies. Additionally, monocytes from eight healthy volunteers’ blood were isolated. Subsequently, the CD14⁺ cells from one healthy volunteer migrated across the 22qDS+SZ and paired healthy control subject iBBB monolayers in parallel to look at immune cell transmigration across iBB monolayers. Four to six-month old male and female mice were also anesthetized and used. The mice were transcardially perfused with saline, and their brains were cryopreserved or kept in RPMI+GlutaMAX. Lastly, Crockett et al. looked at three 22qDS human post-mortem samples. The samples were of individuals aged 1, 2, and 14 months; all the samples were female. 

Results

One of the group’s goals was to show that the BBB had decreased integrity. Measuring the movement of plasma proteins has been proven to be a reasonable method of testing the permeability of the BBB previously (Sun et al., 2021). By looking at the plasma proteins of mouse models, Crockett et al. found that there was a significant leakage of both immunoglobulin G and fibrinogen in the 22qDS mice. This indicates that there is substantial BBB disruption and a decrease in immune privilege in the 22qDS+SZ iBBB. Crockett et al. also found investigated claudin-5, which is the most densely expressed tight junction molecule in the BBB and is coded by CLDN-5—one of the deleted genes on chromosome 22. In vitro, they found that there was no change in claudin-5 mRNA or protein levels when comparing the 22qDS-SZ and the control, but they did find that the claudin-5 expression was highly disorganized in the 22qDS+SZ iBBB cells. In vivo, they found the exact opposite: claudin-5 mRNA and protein levels were significantly reduced in the brains of 22qDS mice. These results are proven to be specific to claudin-5 since the expression of zona occludens-1 (ZO-1), which is another junctional protein not coded within the deletion, remained unchanged. In addition, elevated expression of the proinflammatory intercellular adhesion molecule (ICAM-1) was seen in the 22qDS+SZ iBBB in comparison to the control, which is suggestive immune activation in those with 22qDS. This finding was confirmed in vivo as well as via in situ analysis of 22qDS and wildtype prefrontal cortex. 

The team attempted to showcase the permissiveness of the 22qDS+SZ iBBB in comparison to the control by migrating naïve human monocytes isolated from healthy volunteers across the 22QDS+SZ and paired healthy control subject iBBB monolayers in parallel. They found that following migration, the monocytes significantly decreased thrombomodulin (CD141) expression. CD141 is an anti-inflammatory molecule that functions to inhibit immune cell adherence and extravasastion. Therefore, this decrease is indicative of compromised immune privilege of the 22qDS-SZ iBBB and suggests that this BBB favors cell migration and activation. During their experiments, the group noticed that the 22qDS mice brains showed an upregulation of glial fibrillary acidic protein (GFAP). Additionally, they noticed that interleukin-6, a cytokine repeatedly shown to be elevated in the blood of both idiopathy schizophrenia and 22qDS+SZ patients, was elevated, which indicates that a compromised BBB may drive neuroinflammation and in turn schizophrenia. Crockett et al. also investigated the human post-mortem brain sections and compared the results to their other results. The group found that all of the results were relatively the same except there was no extravascular leakage of serum proteins in either the 22qDS patients or the control patients, which is different from the other results. 

Despite the fact that there was a slight difference between the group’s results with the mice and the human post-mortem samples, Crockett et al. still concluded that their results confirmed their hypothesis that a compromised BBB with decreased immune privilege can increase the risk for neuropsychiatric diseases such as schizophrenia. These results are significant since they can potentially be extended to other neuropsychiatric disorders and the development of treatments in the future. One important consideration is whether or not these results can be extended to all individuals affected by 22qDS-associated schizophrenia or if these findings are exclusive to those developing schizophrenia. It would be beneficial to conduct a study investigating a group of mice with 22qDS and not schizophrenia to see if the BBB dysfunction can lead to schizophrenia or is just the result of the deletion (Gandal et al., 2018). Regardless, this study is one of the first that links a compromised BBB with 22qDS and in turn neuropsychiatric disorders including schizophrenia. Future studies can further investigate and corroborate these findings and start to develop treatments for not only those affected by schizophrenia but also those affected by similar neuropsychiatric disorders. 

[+] References

[+] Other Work By Tori Davis