Comparative expression study of sipa family members during early Xenopus laevis development

Effects of SIPA1L1 on trabecular meshwork extracellular matrix protein accumulation and cellular phagocytosis in POAG

Accumulation of extracellular matrix (ECM) proteins in trabecular meshwork (TM), which leads to increased outflow resistance of aqueous humor and consequently high intraocular pressure, is a major cause of primary open-angle glaucoma (POAG). According to our preliminary research, the RapGAP protein superfamily member, signal-induced proliferation-associated 1-like 1 protein (SIPA1L1), which is involved in tissue fibrosis, may have an impact on POAG by influencing ECM metabolism of TM. This study aims to confirm these findings and identify effects and cellular mechanisms of SIPA1L1 on ECM changes and phagocytosis in human TM (HTM) cells. Our results showed that the expression of SIPA1L1 in HTM cells was significantly increased by TGF-β2 treatment in label-free quantitative proteomics. The aqueous humor and TM cell concentration of SIPA1L1 in POAG patients was higher than that of control. In HTM cells, TGF-β2 increased expression of SIPA1L1 along with accumulation of ECM, RhoA, and p-cofilin 1. The effects of TGF-β2 were reduced by si-SIPA1L1. TGF-β2 decreased HTM cell phagocytosis by polymerizing cytoskeletal actin filaments, while si-SIPA1L1 increased phagocytosis by disassembling actin filaments. Simultaneously, overexpressing SIPA1L1 alone exhibited comparable effects to that of TGF-β2. Our studies demonstrate that SIPA1L1 not only promotes the production of ECM, but also inhibits its removal by reducing phagocytosis. Targeting SIPA1L1 degradation may become a significant therapy for POAG.

An Epha4/Sipa1l3/Wnt pathway regulates eye development and lens maturation

The signal-induced proliferation associated family of proteins comprises four members, SIPA1 and SIPA1L1-1L3. Mutations of the human SIPA1L3 gene result in congenital cataracts. In Xenopus, loss of Sipa1l3 function led to a severe eye phenotype that was distinguished by smaller eyes and lenses including lens fiber cell maturation defects. We found a direct interaction between Sipa1l3 and Epha4, building a functional platform for proper ocular development. Epha4 deficiency phenocopied loss of Sipa1l3 and rescue experiments demonstrated that Epha4 acts up-stream of Sipa1l3 during eye development. Both, Sipa1l3 and Epha4 are required for early eye specification. The ocular phenotype, upon loss of either Epha4 or Sipa1l3, was partially mediated by rax. We demonstrated that canonical Wnt signaling is inhibited downstream of Epha4/Sipa1l3 during normal eye development. Depletion of either Sipa1l3 or Epha4 resulted in an up-regulation of axin2 expression, a direct Wnt/β-catenin target gene. In line with this, Sipa1l3 or Epha4 depletion could be rescued by blocking Wnt/β-catenin or activating non-canonical Wnt signaling. We therefore conclude that this pathomechanism prevents proper eye development and maturation of lens fiber cells resulting in congenital cataracts.