Lecture 10: Membrane Function: Bariers and Functional Polarity

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Types of Anatomical Barriers

• Inert

• Physiological

• Specialised

  • All INVOLVED IN TRANSPORT

• Active barriers; selective movement; protective mechanism + immune system response -acts as a barrier to foreign substances/ pathogens

Inert Barrier - Meninges

• Greek for membrane - around the brain

• The dura is the tough outer layer of the meninges

  • Physical barrier – protects the brain

Physiological Barriers

• Skin  

• Intestines, airways, reproductive tract  

• kidneys, liver, exocrine glands,

Specialised Barrier

• Blood-brain,

• blood-CSF,

• blood-retina,

• Blood-testes,

• placenta

Placenta as A Specliased Barrier

• a strong barrier between the mother and baby –

  • tightly regulated to prevent Rhesus disease

Unidirectional Transprot

• Once way transport

  • Uncommon

• E.g.secretion of the salivary gland

Bidirectional Transport

• Two-way transport

  • more common - allows movement in/out cells

2 Stages of Salivary Secretion

• Primary Secretion

• Secondary Modification

Salivary Secretion: Primary Secretion

• Occurs at ACNR cells

• Ions  (Na⁺, Cl^-, HCO₃^-) move into the cell lumen from the blood, and water follows in by osmosis via aquaporins

Salivary Secretion: Secondary Modification

• Reabsorb  Na⁺and Cl^- BUT not  H₂O (no aquaporins)

• Final saliva is hypotonic compared to plasma (↓ NaCl)

• As solution migrates down to form salivate, through the duct cells Na+ and Cl- are quickly reabsorbed

  • No aquaporins present – water trapped in duct cells – gives rise to watery saliva

Role of Nervous System in Secretion

• Increases IP₃ - Increases Ca2+

• Loss of K and Cl - ↓ volume

• HCO^- efflux = ↑ [H⁺]

Role of IP₃ in Secretion:

• PNS acts on  GCPRs and ion channels; ACh binds to receptor to release Ca2+ form IP3 receptor from intracellular store

  • This also results in an efflux of Cl- and K+

Structure of Barriers

• Epithelial cells linked together by junctional complexes

  • Junctional complexes - claudings and occludins - act as gates

Claudins

• Junctioinal complexes that link epithelail cells together to form a barrier

• 23 types

• Determine “tightness” & selectivity of junctions for Paracellular transport

How Do Claudin and Occludins Act As Gates

• Can change their shapes and move cells closer/ further apart to regulate the entry of substances

Paracellular Transport

• Simple diffusion between cells

  • If claudins pull cells further apart – more access

  • If claudin pull cells closer together – less access

Transcellular Transport

• Polarised and asymmetrical transport – substance must pass from outside to inside the cell; inside cell out

• Polarised transport that requires the membrane facing outwards to have different polarity – tightly regulated

Discovery of Selective Blood Brain Barrier

• Identified by Ehrlich in 1855

• IV injections of Evans blue dye in rat could by pass some biological memrbanes

  • All organs, but the brain stained

• Highly selective and impermeable barrier that prevented the access of substances from the blood to the brain

• Goldman injected die into CSF and ventricle and determined a selective barrier between brain and blood BUT free access from the CSF to brain

  • No CSF brarin barrier - permeable

3 Main Barriers of the Brain

• Protect neurons from blood-borne substances (maintaining water homeostasis and an appropriate milieu for neuronal function):

  • The blood brain barrier interface (BBB) - cerebral endothelium

  • The blood-CSF interface (highly selective) - epidymal cells of choroid plexus

  • The CSF-blood interface (highly selective) - avascular arachnoid epithelium

Cerebral Endothelium

• Forms the BBB

  • Largest barrier in the brain

• Endo-‘WITHIN THE BRAIN’

Dura Mater and Pia Mater

• Physical barriers that protects the brain

Cerebrol Spinal Fluid (CSF)

• Produced in the 3rd and 4th lateral ventricle by Chlorid plexus and capillariers

  • faciliated by the high blood flow rate to CP

• Critical for the brain – brings nutrients to the brain and drains out toxins

  • Flow increases in sleep – removes toxins

• same composition as ISF- mix freely across pial surfaces to enter the brain

• Production: ~600ml/day - turno over of fluid multiple times during the day

Formation of Blood-CSF Barrier

• Epithelial cells form around the choroid plexus

  • Formed by tight junctions of ependymal cells of the choroid plex

Avascular Arachnoid Epithelium

• Lies under the dura and completely encases the brain, forming the CSF-blood barrier

  • Highly selective and an active barrier

    • Some CSF is produced here

Formation of Blood Brain Barrier System

• Formed by cerebral endothelial cells that have highly specialized structural and functional properties

  • have apical tight junction complexes that more closely resemble epithelium than endothelium

    • Difference due to specialisation in transport

• Polarised to express specific transporters apically to actively transport nutrients from the blood into the brain

  • Highly restricted, but controllled barrrier to plasma constituents

Transport Across BBB

• Contains transporters that actively transport nutrients into the brain.

• Inactivates and removes toxins from the brain.

• Highly selective and controlled barrie

Structure of BBB: Brain Endothelium

• Characterised as the morphological site of the BBB

  • Reese and Karnovsky

• Polarised and assymetrical: pumps present on either side

Structure of BBB: Tight Junctions and Adheren Junctions

• Connect brain enothelium together

Structure of BBB System: Actin Cytoskeleton

• Forms a continuous membrane that confers high electrical resistance of the BBB (~1500–2000 Ω-cm²) and retention of ions in the vascular lumen

Consequence of Membrane With Interchangable Resistance

• Membrane with interchangeable electrical resistance can slow down the passage of substances by increasing resistance –

  • restricts access – more selective and controlling movement

:) of Cells With Multiple Barrier

• More selective

  • Layer by layer regulate the molecule passing through

BBB System Involvement In Disease States

• Associated with dysregulation of tight junction proteins – caused by changes/ deficiency/ abnormality in transport mechanisms

• Chronic:

  • multiple sclerosis,

  • experimental autoimmune encephalomyelitis,

  • Alzheimer’s disease

• Acute

  • Ischaemic stroke

  • Hypertension

  • Seizure

Seizure

• Disruption of GABAergic transmission –

  • disruption of Cl-/ Cl- RP – increase excitation

Location of The BBB

• Present in cerebral endothelium throughout the brain (including pial arteries and arterioles and veins).

  • BUT absent from the circumventricular organs (CVO).

The circumventricular organs (CVO).

• Highly specialized areas in the brain (area postrema and median eminence, neurohypophysis, pineal gland, sub-fornica organ, and lamina terminalis).

  • Weak BBB – want to detect substances in the blood – less regulated than other areas of the brain

Cerebral Endothelium of CVO

• Fenestrated (leaky) - and do not have BBB properties.

  • Offer less resistance to substances transversing the area

  • Have more access to blood-borne materials – allows the brain to regulate processes and peripheral organs      

• Require significant cross-talk between the brain and peripheral blood, e.g., release and transport of hormones.

CVO Barrier

• Barrier composed of specialised cells that allow substance to diffuse into the the organ but not beyond

  • Tanycytes and ependymal cells

Tanycyctes

• Glial cells that regulate the entry of substances

  • Control and regulate substances that can transverse the parenchyma and rest of the brain

Blood-CSF Barrier

• = Choroiod plexus second most important interface

  • Mainly in the lateral ventricle but also found in the forth cisterna magna ventricle

Capillaries of Choroid Plexus

• Don’t have BBB properties, but are fenstrated and leaky

  • Allows the uptake of substances from the blood – barrier between this and the rest of the brain tissue is highly specialised

Function of The CSF

• Acts as a cushion for the brain and spinal cord and provides important nutrients and signalling conduits (e.g., neurohormones, peptides etc).

  o   Structural support to stop the brain moving around and keep it rigid and resistant to hydraulic pressure

• When the CSF is in the ventricles, it is considered to be outside the cells of the brain and spinal cord, even though it is physically contained within the body.

Choroid Plexus Barrier

• important barrier in the brain.

• Its cells are polarised → have different properties on their apical (facing the ventricle) and basolateral (facing the blood) sides.

• This polarity allows for the selective transport of substances between the blood and the cerebrospinal fluid.

Ependymal Cells of The Choriod Plexus

• Epithelial-like

Apical Surface of Ependymal Cells

• Presence of ion pumps (Na⁺–K⁺ ATPase) produce the chemiosmotic energy for the osmotic gradient for fluid formation by the choroid plexus cells.

  • Chemiosmotic energy allows pumping     

• Water flow follows the osmotic gradient set up by the removal of three Na⁺ ions for 2 K⁺ ions.

  • Changes in osmotic pressure – water enters CSF via osmosis

Hydrocephalus

• Malfunction of pumps on the aplical surface of epidymal cells

• Transport is affected

• CSF accumulates and compresses the brain; decreases cortical hemisphere

CSF and Blood Flow

• Continuous circulation between the brain and the heart

• Blood and CSF are filtered through these barriers the ensure only specialised molecules gain access to the brain

Blood Brain and CSF Barrier System

• Both barriers engage in transport to provide nutrients and remove waste.

  • Signalling conduits (neurohormones, peptides etc)

Role of Astrocytes in BBB:

• Associated cells within the brain parenchyma that have a crucial role in maintaining the blood-brain barrier by:

  • Supporting the structure of endothelial cells

  • Regulating the expression of tight junction proteins

  • Detecting substances in the blood and signaling to endothelial cells

Role of Astrocytes in BBB According to Reese and Karnovsky

• Barrier function is at the level of the cerebral endothelial cells and not astrocytes.

  • Suggest astrocytes act as an additional signal to ensure tight regulation of endothelial function

Astrocyte End Foot

• Wrap around blood vessels and neurons. These end feet help to regulate the blood-brain barrier and modulate neuronal activity

• Regulate neuronal activity by releasing signaling molecules and modulating the extracellular environment. They also play a role in the formation and maintenance of synapses.

AQP-4

• Aqaporin channel highly expressed in the brain on blood vessels

Two-way Interaction of Astrocytes And Cerebral Endothelium

• Endothelium signals astrocytes (e.g.., aquaporin-4 (AQP-4) in astrocytic end-feet surrounding vessels in the brain parenchyma

  • Maintenance of BBB properties and function likely depends on cross-talk between the endothelium and astrocytes.

• In addition, astrocytes have a large number of K⁺ channels (K_ir4.1 and rSloK_Ca) and spatially buffer K⁺ in the perivascular space.

  • Ion channels that activate and inactive atrocytes

Brain Endothelial Cell Barrier

• Cells have adopted an “epithelial” morphology

  • (e.g. “Tight” junctional complexes (claudin 5), polarity & contain manytransporters).

• Asymmetrical transport between blood, CSF, BBB

• Tight assocaiton with microglia

Retinal Blood Blood

• Light moves from inner retinal barrier to outer retinal barrier

  • Barrier of the CNS-  BBB within the retina

Protective Function of CNS Barriers

• Protective against unwanted pathogens and control the immunologic status of the brain.  

• The tight junctions at the BBB do not allow ions to move passively into the brain and thus prevent fluctuations in electrolytes that occur in the blood.

• Prevent proteins (albumin) and circulating blood cells (erythrocytes, leukocytes) from passing into the brain (can damage neuronal tissue and interfere with tightly controlled water homeostasis

Ivermectin

• Drug used to treat parasitic worms in pupies

  • Highly lipid soluble treatment - transverses the BBB

• Actively removed by p-glycoprotein in the brain

Why Can’t Collie Breeds Be Give Ivermectin

• Mutation present that reduces the expresssion of p-glycoprotein

  • Ivermectin accumulates in cells - very neurotoxic

  • not used