title: Cystatin C uptake in the eye.
abstract: BACKGROUND
  As a secreted protein, cystatin C is assumed to play its role in the extracellular compartment, where it can inhibit virtually all cysteine proteases of families C1 (cathepsin B, L, S) and C13 (mammalian legumain-related proteases). Since many of its potential target enzymes in the eye reside in intracellular compartments, we sought evidence for a cellular uptake of the inhibitor in ocular tissues.
  
  
  METHODS
  Fluorescence-labeled human cystatin C was injected intravitreally into normal rat eyes. Ocular tissues were subsequently examined using ELISA, fluorescence microscopy, and immunohistochemistry. Cystatin C uptake was additionally studied in an in vitro retina model.
  
  
  RESULTS
  Cystatin C administered intravitreally in vivo is taken up into cells of the corneal endothelium and epithelium, the epithelial cells lining the ciliary processes, and into cells in the neuroretina (mostly ganglion cells) and the retinal pigment epithelium. The uptake is demonstrable also in vitro and was, in the neuroretina, found to be a high-affinity system, inhibited by cooling the specimens or by adding the microfilament polymerization inhibitor, cytochalasin D, to the medium.
  
  
  CONCLUSIONS
  There is an active, temperature-dependent uptake system for cystatin C into several cell types in the cornea, ciliary body, and retina. The cell types that take up cystatin C are generally the same that contain endogenous cystatin C, suggesting that much or all cystatin C seen intracellularly in the normal eye may have been taken up from the surrounding extracellular space. The uptake indicates that the inhibitor may exert biological functions in intracellular compartments. It is also possible that this uptake system may regulate the extracellular levels of cystatin C in the eye.
authors.affiliation: Department of Ophthalmology, University Hospital, Lund University, 221 85, Lund, Sweden.
authors.forename: Johan
authors.initials: J
authors.lastname: Wassélius
authors.forename: Kjell
authors.initials: K
authors.lastname: Johansson
authors.forename: Katarina
authors.initials: K
authors.lastname: Håkansson
authors.forename: Magnus
authors.initials: M
authors.lastname: Abrahamson
authors.forename: Berndt
authors.initials: B
authors.lastname: Ehinger
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D002924:Ciliary Body / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D003315:Cornea / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D055316:Cystatin C / Q000008:administration & dosage / Q000493:pharmacokinetics*; D004797:Enzyme-Linked Immunosorbent Assay; D005123:Eye / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D006801:Humans; D007150:Immunohistochemistry; D007267:Injections; D007274:Injections, Intraperitoneal; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D018613:Microscopy, Confocal; D008856:Microscopy, Fluorescence; D051381:Rats; D012160:Retina / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D014822:Vitreous Body
chemical_list: D055316:Cystatin C