An x-ray diffraction investigation of corneal structure in lumican-deficient mice

The corneas of mice homozygous for a null mutation in lumican, a keratan sulfate-containing proteoglycan, are not as clear as normal. In the present study, mutant corneas were examined by synchrotron x-ray diffraction to see what structural changes might lie behind the loss of transparency.

X-ray diffraction patterns were obtained from the corneas of 6-month-old and 2-month-old lumican-null and wild-type mice. Measured in each cornea were the average collagen fibril diameter, average collagen fibril spacing, and the level of order in the collagen array.

The x-ray reflection arising from regularly packed collagen was well-defined on all x-ray patterns from 6-month-old wild-type corneas. Patterns from 6-month-old lumican-deficient corneas, however, contained interfibrillar reflections that were measurably more diffuse, a fact that points to a widespread alteration in the way the collagen fibrils are configured. The same distinction between mutant and wild-type corneas was also noted at 2-months of age. Average collagen fibril spacing was marginally higher in corneas of 6-month-old lumican-null mice than in corneas of normal animals. Unlike x-ray patterns from wild-type corneas, patterns from lumican-deficient corneas of both ages registered no measurable subsidiary x-ray reflection, evidence of a wider than normal range of fibril diameters.

The spatial arrangement of stromal collagen in the corneas of lumican-deficient mice is in disarray. There is also a considerable variation in the diameter of the hydrated collagen fibrils. These abnormalities, seen at 2 months as well as 6 months of age, probably contribute to the reduced transparency.