The Schlemm’s canal is a VEGF-C/VEGFR-3–responsive lymphatic-like vessel
Aleksanteri Aspelund, Tuomas Tammela, Salli Antila, Harri Nurmi, Veli-Matti Leppänen, Georgia Zarkada, Lukas Stanczuk, Mathias Francois, Taija Mäkinen, Pipsa Saharinen, Ilkka Immonen, Kari Alitalo
Aleksanteri Aspelund, Tuomas Tammela, Salli Antila, Harri Nurmi, Veli-Matti Leppänen, Georgia Zarkada, Lukas Stanczuk, Mathias Francois, Taija Mäkinen, Pipsa Saharinen, Ilkka Immonen, Kari Alitalo
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Research Article

The Schlemm’s canal is a VEGF-C/VEGFR-3–responsive lymphatic-like vessel

Abstract

In glaucoma, aqueous outflow into the Schlemm’s canal (SC) is obstructed. Despite striking structural and functional similarities with the lymphatic vascular system, it is unknown whether the SC is a blood or lymphatic vessel. Here, we demonstrated the expression of lymphatic endothelial cell markers by the SC in murine and zebrafish models as well as in human eye tissue. The initial stages of SC development involved induction of the transcription factor PROX1 and the lymphangiogenic receptor tyrosine kinase VEGFR-3 in venous endothelial cells in postnatal mice. Using gene deletion and function-blocking antibodies in mice, we determined that the lymphangiogenic growth factor VEGF-C and its receptor, VEGFR-3, are essential for SC development. Delivery of VEGF-C into the adult eye resulted in sprouting, proliferation, and growth of SC endothelial cells, whereas VEGF-A obliterated the aqueous outflow system. Furthermore, a single injection of recombinant VEGF-C induced SC growth and was associated with trend toward a sustained decrease in intraocular pressure in adult mice. These results reveal the evolutionary conservation of the lymphatic-like phenotype of the SC, implicate VEGF-C and VEGFR-3 as critical regulators of SC lymphangiogenesis, and provide a basis for further studies on therapeutic manipulation of the SC with VEGF-C in glaucoma treatment.

Authors

Aleksanteri Aspelund, Tuomas Tammela, Salli Antila, Harri Nurmi, Veli-Matti Leppänen, Georgia Zarkada, Lukas Stanczuk, Mathias Francois, Taija Mäkinen, Pipsa Saharinen, Ilkka Immonen, Kari Alitalo

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Figure 2

The SC develops postnatally from transscleral veins.

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The SC develops postnatally from transscleral veins. (A–O) Immunofluores...
(A–O) Immunofluorescence staining with antibodies against PECAM-1 (green), PROX1 (red), and VEGFR-3 (blue) were used to visualize SC development. (P–T) SC developmental stages. Dashed lines denote the subset of confocal z stacks selected for SC visualization. Blue cells, PROX1– BECs; green cells, PROX1+ LEC-like cells. EV, ES vein. For 3D volume renderings of entire confocal z stacks with CCs and ES veins, see Supplemental Videos 1–5. (A–C and P) At P0, lateral sprouting (asterisks and inset in C) of transcleral veins toward adjacent transcleral veins was observed. (D–F and Q) At P1, connection of adjacent transcleral veins by strings of future SC ECs was apparent. (G–I and R) At P2, maturation and induction of PROX1 expression (arrowheads) was observed. (J–L and S) P4 revealed lumenization and expression of PROX1, induction of VEGFR-3 (arrowheads), regression of connections to CCs (hashtags), and lateral sprouting (asterisks). (M–O and Y) Mature SC at P7. Note that the AVs (arrow) did not regress. Scale bars: 50 μm (A–O), 12.5 μm (C, inset).