Dae-Young Park¹, Junyeop Lee¹, Intae Park¹, Dongwon Choi², Sunju Lee², Sukhyun Song¹, Yoonha Hwang³, Ki Yong Hong¹, Yoshikazu Nakaoka⁴, Taija Makinen⁵, Pilhan Kim³, Kari Alitalo⁶, Young-Kwon Hong^2,* and Gou Young Koh^1,*
 
1National Research Laboratory of Vascular Biology and Stem Cells, Graduate School of Medical Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon, Republic of Korea.
²Department of Surgery, Department of Biochemistry and Molecular Biology, Norris Comprehensive Cancer Center, Keck School of Medicine, University of Southern California, Los Angeles, California, USA.
³Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology, Daejeon, Republic of Korea.
⁴Department of Cardiovascular Medicine, Osaka University Graduate School of Medicine, Suita, Osaka, Japan.
⁵Department of Immunology, Genetics and Pathology, Uppsala University, Uppsala, Sweden.
⁶Molecular/Cancer Biology Laboratory, Biomedicum Helsinki, Department of Pathology, Haartman Institute and Helsinki University Central Hospital, University of Helsinki, Helsinki, Finland.
 
^*Address correspondence to: Gou Young Koh, Graduate School of Medical Science and Engineering, KAIST, 373-1, Guseong-dong, Daejeon, 305-701, Korea. Or to: Young Kwon Hong, Departments of Surgery and of Biochemistry & Molecular Biology, Keck School of Medicine, University of Southern California, 1450 Biggy St., Los Angeles, California 90033, USA.

Schlemm canal (SC) is a specialized vascular structure in the eye that functions to drain aqueous humor from the intraocular chamber into systemic circulation. Dysfunction of SC has been proposed to underlie increased aqueous humor outflow (AHO) resistance, which leads to elevated ocular pressure, a factor for glaucoma development in humans. Here, using lymphatic and blood vasculature reporter mice, we determined that SC, which originates from blood vessels during the postnatal period, acquires lymphatic identity through upregulation of prospero homeobox protein 1 (PROX1), the master regulator of lymphatic development. SC expressed lymphatic valve markers FOXC2 and integrin α9 and exhibited continuous vascular endothelial?cadherin (VE-cadherin) junctions and basement membrane, similar to collecting lymphatics. SC notably lacked luminal valves and expression of the lymphatic endothelial cell markers podoplanin and lymphatic vessel endothelial hyaluronan receptor 1 (LYVE-1). Using an ocular puncture model, we determined that reduced AHO altered the fate of SC both during development and under pathologic conditions; however, alteration of VEGF-C/VEGFR3 signaling did not modulate SC integrity and identity. Intriguingly, PROX1 expression levels linearly correlated with SC functionality. For example, PROX1 expression was reduced or undetectable under pathogenic conditions and in deteriorated SCs. Collectively, our data indicate that PROX1 is an accurate and reliable biosensor of SC integrity and identity.