Composition and proteolytic processing of corneal deposits associated with mutations in the TGFBI gene

Abstract

Different types of granular corneal dystrophy (GCD)1 and lattice corneal dystrophy (LCD) are associated with mutations in the transforming growth factor beta induced gene (TGFBI). These dystrophies are characterized by the formation of non-amyloid granular deposits (GCDs) and amyloid (LCD type 1 and its variants) in the cornea. Typical corneal non-amyloid deposits from GCD type 2 (R124H), amyloid from a variant of LCD type 1 (V624M) and disease-free tissue controls were procured by laser capture microdissection and analyzed by tandem mass spectrometry. Label-free quantitative comparisons of deposits and controls suggested that the non-amyloid sample (R124H) specifically accumulated transforming growth factor beta induced protein (TGFBIp/keratoepithelin/βig-h3), serum amyloid P-component, clusterin, type III collagen, keratin 3, and histone H3-like protein. The amyloid (V624M) similarly accumulated serum amyloid P-component and clusterin but also a C-terminal fragment of TGFBIp containing residues Y571-R588 derived from the fourth fasciclin-1 domain (FAS1-4), apolipoprotein E and apolipoprotein A-IV. Significantly, analyses of the amyloid sample also revealed the presence of the serine protease Htr (High-temperature requirement) A1 and a number of proteolytic cleavage sites in the FAS1-4 domain of TGFBIp. These cleavage sites were consistent with the ligand binding and proteolytic activity of HtrA1 suggesting that it plays a role in the proteolytic processing of the amyloidogenic FAS1-4 domain. Taken together, the data suggest that the amyloidogenic-prone region of the fourth FAS1 domain of TGFBIp encompasses the Y571-R588 peptide and that HtrA1 is involved in the proteolytic processing of TGFBIp-derived amyloid in vivo.

Human High Temperature Requirement Serine Protease A1 (HTRA1) Degrades Tau Protein Aggregates

“…In line with this hypothesis, accumulation of A␤ was observed in astrocytoma cell culture supernatants following chemical inhibition of HTRA1 and by co-localization of HTRA1 with ␤-amyloid deposits in human brain samples (24). The recently reported association of HTRA1 with corneal amyloid deposits of TGFBI (transforming growth factor ␤-induced gene), however, raises both the possibility of the generation of amyloidogenic fragments and the clearance of amyloid aggregates through proteolysis by HTRA1 (54). Moreover, recent evidence from studying adult macular degeneration suggests that it is the increased activity of HTRA1, resulting from its overexpression that causes disease symptoms (12,13).…”

“…A recent study showed the association of HTRA1 with amyloid deposits in the human cornea (54). Accordingly, the observations that HTRA1 degrades aggregated tau as well as A␤ pep- tides (24), its up-regulation on the transcriptional level and the activation of its proteolytic activity in response to the presence of elevated tau concentrations, respectively, suggest that the protein quality control function of HtrA proteases is conserved.…”

Polymorphic Fibrillation of the Destabilized Fourth Fasciclin-1 Domain Mutant A546T of the Transforming Growth Factor-β-induced Protein (TGFBIp) Occurs through Multiple Pathways with Different Oligomeric Intermediates

“…This has been suggested to be linked to the slow turnover of proteins in the cornea due to a lack of blood vessels (22). Furthermore, a C-terminal fragment of TGFBIp derived from FAS1-4 has been found to accumulate in amyloids of the mutant V624M, forming lattice-type deposits in the cornea (23). The molecular events leading to protein aggregation and accumulation involved in corneal dystrophy are still unclear.…”

Clusterin in the eye: An old dog with new tricks at the ocular surface

“…The deposits are localized to various layers of the cornea depending on the gene involved and its specific mutation, and they affect corneal transparency and visual acuity. CLU has been found colocalized in deposits of two types of superficial and stromal corneal dystrophies: the TGFBIlinked corneal dystrophies [105,106] and the lattice type I corneal dystrophy linked to mutations in the gene for TACSTD2 (Tumor-Associated Calcium Signal Transducer 2) [107]. In addition, CLU is markedly elevated in Fuchs' Endothelial Corneal Dystrophy (FECD), the most common cause of corneal endothelial dysfunction [108][109][110].…”