Vitamin D in Tear Fluid.
OBJECTIVE
To determine the source(s) of vitamin D in tear fluid and examine the expression of the endocytic proteins and putative vitamin D transporters megalin and cubilin in lacrimal and Harderian glands.


METHODS
Wild-type, heterozygous, and vitamin D receptor (VDR) knockout C57BL/6 mice were used, with a subset of knockout mice fed a replenishment diet for some studies. Mouse lacrimal and Harderian glands from each group were used to measure megalin and cubilin by RT-PCR, Western blot, and immunohistochemistry. New Zealand white rabbits were used to collect lacrimal and accessory gland fluid for vitamin D mass spectroscopy measurements.


RESULTS
Ten-week-old knockout mice were significantly (P < 0.05) smaller than wild-type mice. Real-time PCR and Western blot showed decreased expression of megalin and cubilin in select VDR knockout mouse groups. Immunohistochemistry showed apical duct cell megalin staining and weaker megalin staining in VDR knockout mice compared with controls. Vitamin D2 was more prevalent in rabbit lacrimal and accessory gland fluid than vitamin D3, and greater amounts of Vitamin D2 were found in in tear fluid obtained directly from lacrimal and accessory glands as compared with plasma concentrations.


CONCLUSIONS
This is the first study to demonstrate the presence of megalin and cubilin in lacrimal and accessory glands responsible for producing tear fluid. The results strengthen the hypothesis that megalin and cubilin are likely involved in the secretory pathway of vitamin D into tear fluid by the duct cells.
Department of Cell Biology and Anatomy Georgia Regents University, Augusta, Georgia, United States.
Xiaowen
X
Lu
Department of Cell Biology and Anatomy Georgia Regents University, Augusta, Georgia, United States.
Rodolfo A
RA
Elizondo
Department of Biomedicine, Aarhus University, Aarhus, Denmark.
Rikke
R
Nielsen
Department of Biomedicine, Aarhus University, Aarhus, Denmark.
Erik I
EI
Christensen
Department of Entomology and Nematology & UCD Comprehensive Cancer Center, University of California-Davis, Davis, California, United States.
Jun
J
Yang
Department of Entomology and Nematology & UCD Comprehensive Cancer Center, University of California-Davis, Davis, California, United States.
Bruce D
BD
Hammock
Department of Cell Biology and Anatomy Georgia Regents University, Augusta, Georgia, United States.
Mitchell A
MA
Watsky
D000818:Animals; D015153:Blotting, Western; D004195:Disease Models, Animal; D006243:Harderian Gland / Q000378:metabolism; D007765:Lacrimal Apparatus / Q000378:metabolism; D026561:Low Density Lipoprotein Receptor-Related Protein-2 / Q000378:metabolism*; D013058:Mass Spectrometry; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D011817:Rabbits; D018167:Receptors, Calcitriol / Q000172:deficiency; D011956:Receptors, Cell Surface / Q000378:metabolism*; D013666:Tears / Q000378:metabolism*; D014807:Vitamin D / Q000378:metabolism*
D026561:Low Density Lipoprotein Receptor-Related Protein-2; D018167:Receptors, Calcitriol; D011956:Receptors, Cell Surface; C029486:intrinsic factor-cobalamin receptor; D014807:Vitamin D