Endothelial Cells Lecture Notes

Lecture Overview

  • Module: PHSI3X10

  • Lecture Number: 15

  • Presenter: A/Prof Anna Waterhouse

  • Focus: Endothelial Cells

  • Context: Chronic Diseases

Last Lecture Summary

  • Discussion on collagen layers in heart valves and their roles:

    • Spongiosa: Contains hyaluronan and proteoglycans

    • Ventricularis: Composed of elastin, fibrillins, and fibulin

    • Fibrosa: Contains collagens I, III, V

    • Basement membrane: Composed of collagen IV, laminin, perlecan, fibronectin

    • Endomysial collagen: Allows for stress-bearing coaptation, compression, and extension

Today's Lecture Objectives

  1. Features and functions of endothelial cells

  2. Role of the endothelium in homeostasis

  3. Mechanical forces on endothelial cells, their sensory pathways for shear and pressure

  4. Causes and consequences of endothelial dysfunction

Characteristics of Endothelium

  • Structure:

    • Inner cellular lining of blood vessels and lymphatic system, 1 cell layer thick

    • Contact with fluids (blood/lymph), the body’s largest organ (>1 kg, 1-2 trillion cells, ~4000-7000 m² surface area)

  • Morphology:

    • 1-2 μm thick, 10-20 μm diameter, 'cobblestone' appearance

    • Glycocalyx: made of proteoglycans with carbohydrate chains

    • Tight junctions: Maintain vessel wall integrity but are disrupted by toxins (e.g., nicotine) leading to disease

Endothelial Cell Functions

  • Contains vesicles for transport (pinocytosis and macropinocytosis)

  • Facilitates bulk exchange of gases, nutrients, etc.

  • Caveolae: Specialized vesicles that mediate endocytosis and transcytosis, containing caveolin

  • Weibel-Palade Bodies: Storage granules for von Willebrand Factor; play role in thrombosis and inflammation

  • Vesicles

Arrangement of Endothelial and Smooth Muscle Cells (SMCs)

  • ECs align with blood flow, responding to biochemical and physical changes

  • SMCs: Organized circumferentially, providing pulsatile force

  • Barrier Function: ECs act as a semipermeable barrier between blood and SMCs

Mechanical Forces on Endothelial Cells

  • Contact-derived stresses and flow-derived stresses:

    • Shear Stress: Range of 0.1-5 Pa

    • Transmural and interstitial pressure: Affects ECs significantly

    • Tensile strain: 5-20%; arterial pressures ~120/80 mmHg

Roles of Endothelium in Cardiovascular Function

  • Vascular tone regulation: Responds to hormones and factors (e.g., NO, Prostacyclin, Thromboxane A2)

    • ECs control vascular tone by responding to various hormones,

      neurotransmitters and vasoactive factors

    • Vasodilatory factors

      • Nitric Oxide (NO)

      • Prostacyclin (PGI2)

      • Endothelium derived hyperpolarizing factor (EDHF)

    • Vasoconstrictive factors

      • Thromboxane A2 (TXA2)

      • Endothelin-1 (ET-1)

      • Catecholamines and others

  • Involved in coagulation processes: produces activators and inhibitors of thrombosis

    • Anticoagulants (inhibit coagulation)

      • Thrombomodulin/Protein C

      • Sequesters ATIII on HS

      • ATPase/ADPase (CD39)

      • TFPI

      • PAI-1

    • Anti-platelets (inhibit platelets)

      • Nitric Oxide (NO)

      • PGI2

    • Factors to break down fibrin

      • tPA activates plasmin

  • Inflammation regulation: Leukocyte adhesion and migration mediated through endothelial interactions

    • Leukocytes can attach an roll on the endothelium, particularly when the endothelium is inflamed/injured.

      • Tethering and rolling = selectins

      • Slow rolling and arrest = integrins

      • Leukocytes transmigrate into the surrounding tissue.

Mechanotransduction in Endothelial Cells

  1. Physical forces create biochemical responses through changes in cell membrane and cytoskeleton

    1. Physical Force

    1. ECM, Glycocalyx

    2. Tight junctions, Ion channels

    2. Receptor/protein conformational change/activation

    3. Protein phosphorylation

    4. Cytoskeletal rearrangement

    5. Activation of signalling molecules

    6. Changes in transcription

  1. Glycocalyx and transmembrane proteins play roles in sensing mechanical stimuli

Barrier Function

Maintains barrier function through specific proteins:

  • Endothelial specific tight junctions:

    • Claudin-5

    • Occludin (linked to cytoskeleton)

    • JAM

    • ZO (not on image)

  • Adherens junctions

    • VE-Cadherin

    • Nectin (evidence only in vitro)

  • Not on image:

    • Connexins (gap junction proteins)

  • Other receptors:

    • Tie-2 and S1P1 indirectly stabilize

Shear Sensing by the Endothelium

  • Mechanosensory complex/Mechanosome:

    • VE-cadherin, VEGFR2 and platelet EC adhesion molecule (PECAM-1 aka CD31)

    • Shear stress causes conformational changes in PECAM-1

    • VE-cadherin acts as a mechanoadaptor to transmit signal to VEGFR2

    • VEGFR2 activates PI3K/Akt

Shear and Pressure Sensing by the Endothelium

  • Piezo1 is also located on endothelial cell membranes

  • Ion channel that mediates Ca influx

  • Multiple roles identified so far:

    • Required for vascular development

    • Causes polarisation of ECs in direction of flow

    • Involved in blood pressure regulation

    • Regulates endothelial barrier function in the lungs…

Endothelial Dysfunction

  • Causes: Oxidative stress, hypertension, inflammation, unhealthy lifestyles

  • Effects: Impaired NO signaling, increased permeability, thrombosis, linked to atherothrombosis

  • Can lead to pathologies affecting multiple organ systems

Conclusion

  • Glycocalyx: Key role in signaling and maintaining barrier function; adaptations can lead to pathology

  • Upcoming topics: Smooth Muscle Cells

  • Ends with an invitation for questions.