Blood-Brain Barrier Study Notes

Blood-Brain Barrier Overview

  • Source: Alahmari (2021). Neural Plasticity – Review Article

  • Related Course: MCB5130 – DVM2029 – Block 3

Brain Functions

  • Blood vessels provide essential resources to the brain:

    • Oxygen

    • Nutrients

    • Energy

    • Metabolites

  • Blood vessels also remove metabolic waste products such as CO2.

  • Neurovascular coupling (also known as functional hyperemia):

    • Refers to rapid changes in local blood perfusion that respond to neuronal activity, resulting in increases in:

      • Blood supply

      • Oxygen

  • Introduction of the Blood-Brain Barrier (BBB), also referred to as the hematoencephalic barrier.

Overview of the Blood-Brain Barrier (BBB)

  • Definition: The BBB is a dynamic, semipermeable, and highly selective system existing within the cerebral microvessels of most vertebrates.

Historical Background

  • Paul Ehrlich (Late 19th century): Conducted experiments involving the injection of aniline dyes into the blood of experimental animals, observing that all organs were stained except for the brain, indicating the presence of a barrier.

  • Other historical figures mentioned:

    • Edwin Goldmann

    • Lina Stern

Structural Features of the BBB

  • The BBB is characterized by a selective and semi-permeable membrane separating the blood from the interstitial fluid (extracellular fluid) in the brain.

  • Regulates the movement of molecules and ions between the blood (microvasculature) and the brain.

  • Essential for proper neuronal function.

  • Maintains a stable environment for the brain:

    • Neurons do not regenerate.

    • Environment is evolutionarily conserved across vertebrates.

Mechanism of Action of the BBB

  • Components of the BBB include:

    • Endothelial cells

    • Pericytes

    • Capillary basement membrane (basal lamina)

    • Astrocyte end-feet

    • Microglia

  • These components collectively protect the brain from:

    • Toxins

    • Drugs

    • Microorganisms

  • They also provide nutrients and help maintain a homeostatic internal environment within the central nervous system (CNS).

Neurovascular Unit (NVU)

  • Key components of the NVU:

    • Neurons

    • Pericytes

    • Astrocytes

    • Blood capillaries

    • Tight junctions formed by endothelial cells

    • Basement membrane

Detailed Characteristics of Endothelial Cells

  • Origin: Derived from mesoderm.

  • Unique features related to the BBB include:

    • Absence of fenestrations.

    • Luminal/abluminal polarization.

    • Extensive tight junctions that limit the flux of hydrophilic molecules across the BBB.

    • Sparse pinocytic vesicular transport.

    • Rich mitochondria supporting a unique and active metabolism that changes the cellular environment.

    • Function is regulated by neighboring cells with intercellular communication.

    • Signaling is facilitated through a diverse array of proteins.

Tight Junctions (Zonula Occludens)

  • Endothelial cells in the brain exhibit:

    • A high density of tight junctions, markedly up to 100 times that found in somatic endothelial cells.

    • Highly selective diffusion barrier that prevents blood substances from entering the brain.

    • Blocks the paracellular aqueous diffusion pathways between adjacent endothelial cells.

    • Limits the paracellular flux of hydrophilic molecules and the passive diffusion of proteins and polar solutes to/from the CNS.

  • Composition:

    • Composed of a branching network of sealing strands that are independent from one another.

    • Efficient structure with each strand formed from transmembrane proteins embedded in plasma membranes, where extracellular domains interact directly.

    • Major proteins involved are Claudins and Occludins, which anchor to strands of Actin.

    • Junction adhesion molecules (e.g., JAM 1,3) have potential roles that are yet to be fully elucidated.

Interrelation of Cell Components in the BBB

  • Major components illustrated:

    • Tight junctions are crucial to the functionality of the BBB, providing required structural integrity.

    • The apical plasma membrane supports interaction with the actin cytoskeleton, including proteins such as cingulin, ZO-1, catenins, vinculin, claudin-5, and occludin.

Role of Astrocytes in BBB Regulation

  • Astrocytes are a subtype of glial cells and are the most abundant cells in the CNS.

  • Key functions include:

    • Providing structural support.

    • Metabolic regulation.

    • Signaling, growth, and homeostatic maintenance.

    • Control of blood flow and biochemical maintenance of the BBB.

  • Discusses the foot processes of astrocytes and their impact on the BBB structure and function:

    • A1 astrocyte phenotype: Pro-inflammatory effects (harmful).

    • A2 astrocyte phenotype: Involved in healing (anti-inflammatory).

    • Potential therapeutic implications of astrocyte phenotypic states.

Function and Importance of Pericytes

  • Pericytes are contracting cells located at intervals along capillaries:

    • Important for angiogenesis and maintain structural integrity, differentiation of microvessels, and formation of endothelial tight junctions.

    • Help sustain brain blood flow by relaxing.

    • A role in phagocytosis, but not deeply understood.

    • Linked to microvascular instability when malfunctioning.

Basement Membrane as Part of the BBB

  • Role: Connects cells and intervenes in cellular communication, providing a barrier to chemicals and cells (including inflammatory cells).

  • Offers physical space for various interactions and is composed of extracellular matrix (ECM) proteins.

Extracellular Matrix Proteins

  • Key proteins include:

    • Collagen type IV

    • Laminin

    • Fibronectin

    • Proteoglycans

  • Functions of ECM proteins:

    • Provide structural scaffolding and biochemical cues for cell adhesion, differentiation, and polarization.

    • Facilitate tight junction formation and enhance barrier integrity while reducing permeability.

    • Subject to degradation by metalloproteinases, which are crucial for tissue remodeling, wound healing, and angiogenesis.

Types of Neuroglia

  • In the Central Nervous System (CNS):

    • Ependymal cells

    • Oligodendrocytes

    • Astrocytes

    • Microglia

  • In the Peripheral Nervous System:

    • Satellite cells

    • Schwann cells

Microglia Characteristics

  • Types of microglia mentioned:

    • Microglia (ameboid, active, endocytosis)

    • Enteric glia (with subtypes depending on location)

  • Microglia play a critical role in regulating tight junction expression in the BBB.

Presence of BBB Across Various Structures

  • Not universally present; absent in areas such as:

    • Pineal gland

    • Neurohypophysis (Pituitary)

    • Subcommissural organ (CSF)

    • Subfornical organ (CSF)

    • Organum vasculosum of the lamina terminalis (CSF)

    • Area postrema (vomiting center)

Molecules That Can Cross the BBB

  • Characteristics include:

    • Lipophilic, positively charged molecules with low molecular weight (less than 400 to 600 Da) can cross.

    • Examples of diffusible substances:

      • Water

      • Sodium (Na)

      • Potassium (K)

      • Chloride (Cl)

      • Oxygen (O2)

      • Carbon dioxide (CO2)

      • Anesthetics

      • Barbiturates

      • Ethanol

      • Nicotine

      • Caffeine

  • Role of the BBB is to prevent alterations in concentrations of blood ions, amino acids, peptides, and other elements, including inflammatory cells.

Mechanisms of Transmission Across the BBB

  • Various mechanisms include:

    • Paracellular diffusion

    • Transcellular diffusion

    • Transporter protein-mediated transcytosis

    • Receptor-mediated transcytosis

    • Adsorptive-mediated transcytosis

    • Cell-mediated transcytosis

Efflux Transport at the BBB

  • Substances involved:

    • Oxygen (O2)

    • Carbon dioxide (CO2)

    • Sodium (Na)

    • Potassium (K)

    • Hydroxide ions (OH-)

    • Various peptidases

    • Carriers for drugs

    • Cell surface adsorptive proteins like GLUTS (for glucose)

Detailed Explanation of Paracellular Transport

  • Definition: Passive movement of water, ions, and small molecules between adjacent epithelial cells via tight junctions.

    • The paracellular barrier restricts this movement.

    • Facilitates the transmission of dissolved molecules through an area between two neighboring endothelial cells driven by a negative concentration gradient from the bloodstream to cerebral tissue.

    • Only small water-soluble molecules can cross this area.

Explanation of Transcellular Transport

  • Definition: Movement of solute substances across endothelial cells.

    • Lipid-soluble agents such as oxygen, carbon dioxide, anesthetics, and alcohol can pass through this method.

    • Lipid-soluble substances cross by dissolving in the lipids of endothelial cell plasma membranes.

    • Additional barrier systems exist to safeguard against the harmful lipid-soluble compounds which may permeate directly out of vessel walls, known as efflux pumps.

Key Efflux Transport Proteins at the BBB

  • P-glycoprotein (P-gp/ABCB1): Restricts the uptake of numerous lipid-soluble drugs.

  • Breast Cancer Resistance Protein (BCRP/ABCG2): Exports toxins and drugs along with P-glycoprotein.

  • Multidrug Resistance-associated Proteins (MRPs/ABCCs) and Solute Carrier (SLC) Transporters: Facilitate the efflux of various organic anions.

Molecules Capable of Passing Through the BBB

  • Examples of transport mechanisms:

    • Receptor-mediated transcytosis: Insulin, leptin, IgG, transferrin, TNF-alpha.

    • Adsorptive-mediated transcytosis: Albumin, histones.

    • Transporter-mediated pathway: Glucose, creatine, lactate, pyruvate, and large neutral amino acids.

Drug Delivery to the Brain Challenges

  • Most drugs cannot pass the BBB effectively.

  • Overcoming challenges through invasive techniques:

    • Intra-cerebral-ventricular and intrathecal infusions or injections.

    • Transient disruption of the BBB via:

      • Ultrasound

      • Noxious agents

      • Hyperosmotic solutions

      • Bradykinin and histamine (vasoactive drugs that increase capillary permeability).

Non-Invasive Techniques for Drug Delivery

  • Strategies to enhance drug delivery include:

    • Enhanced transcellular transport.

    • Intranasal delivery utilizing neural pathways connecting the nasal mucosa to the brain.

    • Use of transport/carrier systems.

    • Nanoparticle-based technologies allowing controlled drug release while protecting drugs from metabolism.

    • Transforming water-soluble molecules into lipid-soluble by chemical modifications (e.g. replacing OH- with COO-).

    • Inhibition of efflux transporters that restrict drug delivery.

Examples of Therapeutic Agents Accessible via BBB

  • Types of drugs that can cross include:

    • Anticonvulsants

    • Sedatives

    • Anesthetics

    • Nonsteroidal anti-inflammatory drugs (NSAIDs)

    • Some antibiotics (e.g., chloramphenicol, fluoroquinolones, doxycycline, sulfas/trimethoprim).

    • Influences to consider: molecular size, polarity, lipophilicity, plasma protein binding, and affinity to active transport systems.

Factors Altering the BBB Environment

  • Elements altering this condition include:

    • Astrocytes secreting substances like glutamate, aspartate, taurine, ATP, nitrogen monoxide (NO), TNF-alpha, and IL-beta.

    • Other cells releasing: Bradykinin, histamine, thrombin, and free radicals.

    • Pathological conditions such as inflammation, hypoxia, degeneration, edema, neoplasia, hemorrhage, and necrosis affect the BBB.

    • External factors like microwaves are also noted.

Encephalitis

  • Definition: Inflammation of the brain.

  • Associated Infectious agents:

    • Viruses

    • Bacteria

    • Fungi

    • Parasites

Infectious Encephalitis in Various Species

  • Observations of infectious agents affecting several animal species including horses, dogs, cats, cows, and birds.

Dynamics of Infectious Encephalitis

  • Infections of the brain are rare but can be severe.

  • Reliance on the innate immune system for the brain and CNS immune response as the BBB inhibits antibody and B cell passage.

  • The limited effect of adaptive immune response in CNS follows inflammation increasing BBB permeability.

  • Some bacteria may traverse the barrier utilizing endolysins to disrupt it, such as Treponema spp. and Borrelia spp.

West Nile Virus Impact on the BBB

  • Clinical signs in horses include:

    • Weakness

    • Ataxia

    • Paresis

    • Head tremors

    • Vestibular syndrome

    • Head tilt and pressing

    • Seizures

    • Anorexia, fever

    • Potential abortion

    • Lip droop/paralysis

  • The virus increases BBB permeability and utilizes leukocyte transport for entry.

  • Detection of WNV ds RNA by TLR3 triggers an inflammatory cytokine response (TNF-alpha and IL-6).

Mechanisms for Viral Entry into the CNS

  • Viruses can infect endothelial cells, compromising BBB integrity (West Nile virus is an example).

  • Binding proteins and specific receptors expressed in neurons confer neurotropism.

  • Utilize 'trojan horses' via infected white blood cells, access through cerebral spinal fluid (CSF) via ependymal cells, or axonal transport mechanisms (e.g., rabies and herpes viruses).

Biomarkers of BBB Disruption

  • Definition: A measurable substance in an organism indicating disease, infection, or environmental exposure with potential for diagnosis.

  • Essential features of a biomarker include:

    • High sensitivity and specificity

    • Reliability

    • Quick response

  • Biomarkers include:

    • Occludin (tight junction component)

    • Fibronectin (basal membrane component)

    • Metalloproteinases (extracellular proteases)

    • CSF/Serum albumin ratio

    • Biomarker indicators from brain microvascular endothelial cells.

Summary of Biomarkers of BBB Disruption

  • Reiterated features of molecular biomarkers include their sensitivity, reliability, and diagnostic applicability in the context of BBB integrity assessment.

Questions / Discussion Section

  • A segment reserved for participant queries.