The interphotoreceptor matrix (IPM) is a specialized extracellular mesh of molecules surrounding the inner and outer segments of photoreceptor neurons. Interphotoreceptor matrix proteoglycan 1 and 2 (IMPG1 and IMPG2) are major components of the IPM. Both proteoglycans possess SEA (sperm protein, enterokinase and agrin) domains, which may support proteolysis. Interestingly, mutations in the SEA domains of IMPG1 and IMPG2 are associated with vision disease in humans. However, if SEA domains in IMPG molecules undergo proteolysis, and how this contributes to vision pathology is unknown. Therefore, we investigated SEA-mediated proteolysis of IMPG1 and IMPG2 and its significance to IPM physiology. Immunoblot analysis confirmed proteolysis of IMPG1 and IMPG2 in the retinas of wildtype mice. Point mutations mimicking human mutations in the SEA domain of IMPG1 that are associated with vision disease inhibited proteolysis. These findings demonstrate that proteolysis is part of the maturation of IMPG1 and IMPG2, in which deficits are associated with vision diseases. Further, immunohistochemical assays showed that proteolysis of IMPG2 generated two subunits, a membrane-attached peptide and an extracellular peptide. Notably, the extracellular portion of IMPG2 trafficked from the IPM around the inner segment toward the outer segment IPM by an IMPG1-dependent mechanism. This result provides the first evidence of a trafficking system that shuttles IMPG1 and IMPG2 from the inner to outer IPM in a co-dependent manner. In addition, these results suggest an interaction between IMPG1–IMPG2 and propose that mutations affecting one IMPG could affect the localization of the normal IMPG partner, contributing to the disease mechanism of vision diseases associated with defective IMPG molecules.

The interphotoreceptor matrix (IPM) is a specialized extracellular mesh of molecules surrounding the inner and outer segments of the photoreceptor neurons. The interphotoreceptor matrix proteoglycan 1 and 2 (IMPG1 and IMPG2) are major components of the IPM. Both proteoglycans possess SEA (Sperm protein, Enterokinase and Agrin) domains, which are described to support protein proteolysis. Interestingly, humans with mutations in the SEA domains of IMPG1 and IMPG2 are associated with retinitis pigmentosa (RP) vision disease. This work investigates the SEA-mediated proteolysis of IMPG1 and IMPG2 and its significance to IPM physiology. Western blot relative molecular mobility analysis of IMPG1 from retina confirmed protein proteolysis. Point mutations in the SEA domain of IMPG1 inhibit protein proteolysis, as shown in cell culture assays. Likewise, IMPG2 also proteolyzes at the SEA domain generating two subunits: a membrane-attached and an extracellular peptide, as seen by western blot and immunohistochemical assays. Importantly, the extracellular portion of IMPG2 traffics toward the outer segment IPM by a mechanism dependent on IMPG1. These results demonstrate that proteolysis is part of the maturation of IMPG1 and IMPG2, which is important for protein localization and linked to vision diseases. Moreover, the results indicate interdependency between IMPG1 and IMPG2, which helps understand the molecular mechanisms involved in the development of vision diseases in patients with defective IMPG molecules.