Artwork from the Print edition by Allanah Booth.
This article was originally published in the Oxford Scientist’s Print edition, Barriers, in Michaelmas Term 2022, under the title ‘The Blood Brain Barrier—a barrier to medical progress?‘. You can read more of our Print articles here.
The brain is a complex multitude of tens of billions of neurons. There are new connections and changes in organisation taking place constantly as we develop and learn. We are only at the surface of understanding this incredible structure, the epicentre of our consciousness, and sense of self. Understandably, evolution had developed many layers of protection for this three-pound organ; the most obvious is our 7mm thick skull. Below this, there is a protective membrane (the meninges) and a colourless liquid known as cerebrospinal fluid (CSF). Derived from blood plasma, CSF is critical for protection against sudden impact, as well as the removal of waste products. This fluid also provides buoyancy, allowing brain density to be maintained, while reducing the effective weight of the brain from 1,500g to 50 grams—this reduction in weight lessens the force applied to vessels upon injury. CSF has a significantly different composition from blood, and it is important that the two fluids are kept entirely separate from one another. Consequences of the two fluids mixing include an increase in pressure on the brain, a lack of regulation of nutrients reaching the brain, and impaired waste product removal.
The blood-brain barrier (BBB) is a highly specialised and impeccably designed structure suited for the CSF-blood segregation. It protects the brain from any dangerous substances present in our blood while simultaneously allowing it access to vital nutrients. The structure poses a challenge for the treatment of brain conditions, such as multiple sclerosis (MS) and cancer, because medications cannot cross the barrier. The BBB is so tight that 98% of all developed small molecules and most macromolecular drugs (a new class of drugs with macromolecules, such as proteins, chosen as the drug target) cannot get access to the brain efficiently.
Structure and function of the blood-brain barrier
The blood-brain barrier is composed of adjoining endothelial cells allowing the formation of selectively permeable gaps, known as tight junctions, between them. These junctions have highly specific transport proteins embedded within them, as well as enzymes to alter substances during their journey. At isolated sites in the basement membrane, specialised cells known as pericytes are found along the inside of blood vessel walls to coordinate maintenance of the BBB. These cells regulate blood flow to the capillary and optimise its function. Recent research suggests that they inhibit the production of chemicals that increase vascular permeability (the ability to maintain selective exchange between vessels, tissues and organs). In addition, cells known as astrocytes wrap around the capillary using cellular projections known as astrocytic feet, which act to support the capillary wall.
The BBB’s primary function is filtration. Close connection of these vascular cells and neurons—the neurovascular unit—allows dynamic interactions between the complex neuronal network and the bloodstream. It crucially maintains the correct balance of charged molecules in the brain, prevents access to substances that block essential brain functions, such as toxins and disease-causing microorganisms passing from the bloodstream to the CSF, and regulates substances needed in brain metabolism. Vital molecules are permitted entry via two main methods: receptor-mediated transport and passive diffusion. The former uses specific protein receptors to allow ions, neurotransmitters, glucose and amino acids to cross over the barrier. These substances have a high affinity for specific carriers embedded within the BBB as they are present at low concentrations. In current research, these are prime targets for strategies of drug delivery into the brain. Passive diffusion is limited to water, gases such as oxygen, and lipid-soluble substances. For passive diffusion, no protein-binding mechanisms are required, and rate of entry is entirely dependent on blood flow.
How can the BBB become compromised?
Despite its complexity, the BBB can be compromised in a variety of ways. The barrier can be overcome by the environment and genetics through different ingenious methods.
Meningococcal bacteria adopt paracellular penetration; a mechanism by which they attach to the basement membrane and disrupt the tight junctions, resulting in increased permeability. This allows the pathogen, as well as other toxins, to pass through the gap and infect brain tissue, resulting in inflammation and sometimes death. In addition, evidence suggests that MS also begins with the gradual breakdown of the BBB. MS is an autoimmune disease, meaning the body’s immune system attacks itself. In the case of MS, myelin, the protective coating surrounding nerve cells, is destroyed, which causes communication problems between the brain and rest of the body. Upon BBB breakdown, immune cells can more easily cross the barrier and further attack myelin. Factors including stress, inflammation, and chemical processes such as drugs, air pollution and smoking are thought to damage the BBB . Nevertheless, it is important to bear in mind that there is no single proven cause of MS; it is thought to be triggered by a combination of environmental and genetic factors—recent research has also implicated the pathogen Epstein-Barr virus in MS progression.
Mutations associated with genetic variation are vital for evolution, but can also have devastating implications on our health. A curious disease related intimately with the BBB is Phenylketonuria (PKU). Phenylalanine and tyrosine are two of our twenty amino acids: molecules vital in metabolism that make up all the proteins in our body. Tyrosine is a precursor for neurotransmitters such as epinephrine and dopamine, and can be made from phenylalanine using the enzyme phenylalanine hydroxylase (PAH). If the diet is low in tyrosine, the only method of neurotransmitter production is using PAH. PKU is an autosomal recessive disease meaning mutations in both PAH alleles are required to cause symptoms. In this case, the gene responsible for PAH production is affected. A lack of PAH results in a build-up of phenylalanine, which therefore is able to cross the BBB leading to severe changes in the production of neurotransmitters. Symptoms that may be present in individuals diagnosed with PKU include seizures, brain damage and behavioural difficulties.
The blood brain barrier and the drug delivery problem
In the last few decades, knowledge about the brain and its associated structures has increased exponentially. Along with contributing new knowledge, these discoveries have enabled the production of therapeutics for the treatment of disease. The protective nature of the BBB makes it an obstacle to be overcome by the newly developed technologies in relation to novel drugs. All large molecule and most small molecule technologies cannot cross the BBB. Despite the number of potential applications, there are shockingly few pharmaceutical companies which have built a dedicated BBB drug targeting programme.
Drugs must have a small molecular mass (<500 Da) and high lipid solubility to enter the brain through transmembrane diffusion or transporter proteins. Transmembrane diffusion involves the drug melding with the cell membrane—to be taken up by the membrane, the substance must be lipid soluble, but not so soluble that its blood concentration becomes too low. In contrast, transporter proteins have an uptake rate of around 10 times that of transmembrane diffusion. These are protein receptors present in the membrane that transport a drug beyond the BBB. Diffusion by transporter proteins requires drugs that mimic structures of endogenous substrates. For example, a drug used in the treatment of Parkinson’s Disease is a dopamine precursor. Dopamine is a neurotransmitter which plays a role in the feeling of ‘reward’ and influences our levels of motivation. It is found entirely in the brain and the hope is that by mimicking dopamine, and indeed other molecules found in the brain, we can produce drugs with the ability to cross the BBB.
New approaches to drug treatment take advantage of endogenous regulated entry mechanisms into the brain. In the past 5 years, more than 30 BBB “shuttle” proteins have been identified, and their efficiency as well as versatility is shown to be greater than originally thought. Ernest Giralt, an organic chemist based in Barcelona, has created a non-toxic protein from bee venom. Some animal toxins are able to penetrate the BBB, and by removing specific chemical groups, Giralt and his team were able to develop a drug to penetrate the BBB.
Another therapeutic application is the use of nanometrics. Developed by Ijeoma Uchegbu and based at University College London, nanometrics use nanoparticles engineered from biocompatible polymers. Through self-assembly, they form a protective envelope around drugs in a technique known as Molecular Envelope Technology. The nanoparticles stick to capillaries in the brain, which gives the drug longer to diffuse across the layer of epithelial cells that makes up the BBB. So far, this has shown potential for treatment of brain tumours and may be used to deliver chemotherapy drugs into the brain.
The blood-brain barrier is a chief example of the wonders of evolution. It is extraordinary that natural selection can produce such highly specialised structures, which modern medicine struggles to overcome. Developments are being made in the treatment of brain diseases, in the meantime, we can continue learning from the science right in front of us, and indeed within us.