Lecture 3

Lecture Plan

  • The Lymphatic System

  • Lymphangiogenesis

  • Growth Factor Signalling and Physiological Drives

  • This lecture will introduce the lymphatic system and what we know about its stimulus to begin growing.

Introduction

  • Lymphatic System: Maintains fluid homeostasis and is essential for various physiological processes.

  • Lymphangiogenesis: Refers to the growth of lymphatic vessels.

  • Lymphatic Vessels Composition: Composed of lymphatic endothelial cells (LECs).

    • Key Point: Lymphatic endothelial cells are specialized endothelial cells.

Generation of Initial Lymphatic Network

  • Transcription Factor Activity: Begins to confer lymphatic identity during the development phase.

  • This lymphatic identity is maintained after development.

Understanding Angiogenesis

  • Angiogenesis involves more than just tip cells and stalk cells.

  • It requires clustering of cell phenotypes around known markers.

  • Reference: Goveia et al, Cell 2020.

Lymphatic Endothelial Cells Characteristics

  • Lymphatic endothelial cells exhibit endothelial cell features but possess unique transcriptomes.

  • Key Markers Identified in Research:

    • Isolectin IB4.

    • Adult rat mesentery markers: FLT4, SEMA3A, LAMP3, CEACAM1, EPB41L3, MMP2, DSP, TIMP3, SRPX2.

    • Identified top 10 genes in vitro through RNASeq analysis.

  • Enrichment noted in both blood vessels and lymphatics.

Cellular Markers and References

  • Key Markers in Adult Mouse Tissues:

    • Ear Tissue: Prox1, CD31, PODOPLANIN.

    • Reference: Prof. T. Makinen, University of Uppsala; Zarah Tabrizi.

    • Trachea: CD31, LYVE1.

The Purpose of the Lymphatic System

  • Capillary Pressure Dynamics:

    • Higher pressure in capillaries proximal to lymphatic capillaries impacts fluid dynamics.

  • Fluid Leakage:

    • Fluid, along with solutes and cells, leaks out due to vascular permeability.

    • Process is highly controlled and influenced by the microenvironment (e.g., fluid pressure, oncotic pressures, colloid pressures, reflection coefficients).

  • Starling Equation:

    • Introduces the mathematical model describing fluid movement across capillaries.

    • Reference: https://en.wikipedia.org/wiki/Starling_equation.

Increased Fluid Filtration Scenario

  • Hypothetical discussion on the effects of increasing fluid filtration in normal capillaries.

Importance of Lymphatic System in Disease

  • The role of the lymphatic system is critical in conditions such as cancer, where lymphatic involvement can facilitate metastasis.

  • Specific markers indicated: AST, CANCER, PECAM.

  • Reference: James et al., 2013.

Functional Mechanism of Lymphatic System

  • System Structure:

    • Composed of blind-ending lymphatic capillaries.

    • Larger, floppy tubes permeate within tissues; fluid is drawn in by negative pressure.

    • Reference: LYVE1 - Benest et al 2008; Zhang et al 2020.

Permeability and Functionality of Lymphatic Endothelium

  • The lymphatic endothelium features are described as highly permeable.

  • Mechanism of Fluid Passage:

    • Presence of ‘zipper’-like punctate junctions yields lower resistance for fluid entry into the lymphatic vessel.

  • Collecting Lymphatics:

    • Contractile in nature yet exhibit very tight junctions to prevent fluid leakage during peristalsis.

    • Valves are present to prevent backflow, ensuring lymph is returned to venous circulation.

    • Reference: Baluk et al, 2007.

Normal Lymphatic Function Response

  • Anchoring Filaments: Attach LECs to the interstitium.

  • Expansion Response Mechanism:

    • If interstitium expands (due to increased fluid leakage), anchoring filaments tighten and consequently open the vessel's interior.

    • The negative relative pressure aids fluid intake.

    • Peristalsis and valves facilitate the removal of fluid into collecting lymphatics.

Responses to Stimulation

  • Reference: Benest et al 2008; Zarah Tabrizi.

VEGFC Signaling Pathway

  • Summary of VEGFC binding with various VEGF receptors:

    • VEGFR2 homodimer and heterodimers (VEGFR1-VEGFR2, VEGFR2-VEGFR3).

    • Interactions leading to complex formation proximal to cell membrane:

      • VEGFR2-NRP1 complex, VEGFR2-integrin complex, and others.

  • Mechanism of Ignition:

    • VEGF binding triggers changes that promote signaling pathways leading to lymphangiogenesis.

    • Reference: Simons et al 2016.

Major Signalling Pathways in Lymphangiogenesis

  • Key pathways and components visualized include:

    • VEGFC, FOXC2, Prox1.

    • Co-receptor nuances, cell-cell and cell-ECM interactions.

    • Proteins secreted by LECs: CCL21, Ccbe1, Ang1, Ang2, Ang 3/4, EphB4, Fibronectin, Integrins.

    • Cytokines and growth factors: TGF-B, COO, VEGF-C, VEGF-D.

    • Lists the different interactions leading to lymphangiogenesis and angiogenesis respectively (e.g., Angiogenesis via VEGF).

Unique Characteristics of Lymphatics

  • The lymphatic system functions as another vascular system, characterized as blind-ended and unidirectional.

  • Notable transcriptomic and structural variations compared to blood vasculature.

  • Critical regulatory mechanism lies in VEGF family signaling.

Conclusion: Molecular Regulation of Lymphangiogenesis

  • Review Paper Citation:

    • Ralf H. Adams and Kari Alitalo describe the role of blood and lymphatic vessels in fluid transport, factor in tissue specialization, and highlight the significant research milestone in understanding lymphangiogenesis.

  • Lymphangiogenesis plays a key role in diverse pathological processes, including tumor metastasis and lymphedema.

    • Emerging insights may lead to novel therapeutic strategies.

    • DOI: 10.1016/j.cell.2010.01.045

Questions Session

  • Any questions?