Abstract
purpose. The objective of this study was to investigate whether leakage of
aquaporin 5 (AQP5) in tear is associated with damage of lacrimal glands
(LGs) in dacryoadenitis models.
methods. Female MRL/lpr (24-week-old), male NOD/Shi Jci (5-, 8-, and
10-week-old), female NFS/s-TX (10-week-old), and lipopolysaccharide
(LPS)-induced dacryoadenitis model mice were used. Tear fluid was
collected by a cotton thread. Tear proteins in the thread were
dissolved in sodium dodecyl sulfate buffer, and AQP5 proteins were
analyzed by the Western blot technique using anti-AQP5 antibody. LGs
were prepared for hematoxylin and eosin staining or immunostaining of
AQP5.
results. In MRL/lpr, NFS/s-TX, 8- and 10-week-old NOD/Shi Jci mice, AQP5 protein
was detected in the tear by Western blot analysis. Inflammatory
lymphocyte infiltrations were observed in LGs of these dacryoadenitis
model mice. In contrast, AQP5 leakage and damage of LG were not
observed in normal mice. In 5-week-old NOD/Shi Jci mice, infiltration
was not seen in LG, and AQP5 leakage was not detected in the tear. In
LPS-induced dacryoadenitis model mice, either tissue destruction with
inflammation in LG or AQP5 leakage in the tear was observed. AQP5 in
the tear and tissue inflammation in LGs was not found in control mice.
These results indicate that AQP5 is leaked in tears when LGs are
damaged by dacryoadenitis.
conclusions. Leakage of AQP5 in the tear was found to be related to LG damage. This
finding suggests that detection of AQP5 in tear is useful for specific
diagnosis of LG disorders with tissue
destruction.
In chronic dacryoadenitis, Sjögren’s syndrome is a
systemic autoimmune disorder characterized by dry eye and dry mouth.
This dryness results from lymphocyte infiltration into lacrimal and
salivary glands.
1 In acute dacryoadenitis, intralobular
edema, lymphocytes, and plasma cell infiltration were observed in the
lacrimal gland.
2 In Sjögren’s syndrome, it is
assumed that infiltrated T cells may recognize unknown self-antigens
and produce inflammatory cytokines and autoantibodies.
1 Histopathologically, lymphocytes penetrate the epithelium of the
salivary gland ducts and cause cytolysis of lacrimal gland
cells.
3 To diagnose these kinds of damage to the lacrimal
gland, an histologic evaluation of biopsy specimens is widely used.
However, this procedure has limitations. The biopsy is not available
for reexamination and causes discomfort to the patients. The evaluation
of reflex tearing is also used for diagnosis of Sicca syndrome. It has
been reported that poor reflex tearing is associated with lymphocyte
infiltrations in the lacrimal gland.
4 Computed tomography
or magnetic resonance imaging of the glands is another procedure used
for the diagnosis of lacrimal tumors.
5 However, these
methods are not always specific or convenient for diagnosis. Hence, a
potent technique that can obviate biopsy, or specific markers to detect
lacrimal gland damage, is needed.
Recently, functional water channels have been identified in mammals,
and this group of proteins has been referred as the
aquaporins.
6 We have shown that aquaporin 5 (AQP5) protein
is specifically localized in the apical membrane of acinar and duct
cells in mouse
7 and human (Tsubota K, Hirai
S, et al., unpublished data, 2000) lacrimal gland. In other
ocular tissues, AQP5 expression was not demonstrated in
conjunctiva.
8 In contrast to AQP5, membrane channel
proteins, such as ion channels or sodium/potassium
adenosinetriphosphatase, have been shown as present not only in the
lacrimal gland
9 10 but also in the corneal
epithelium
11 and the conjunctiva.
12 To the
best of our knowledge, no specifically localized protein in the
lacrimal gland other than AQP5 has been reported. Based on this
evidence, we hypothesize that AQP5 is specifically leaked into tear
fluid when the lacrimal gland is damaged by lymphocyte infiltrations
and inflammation with tissue destruction. In central diabetes
insipidus,
13 impaired water excretion, and
hyponatremia,
14 aquaporin 2 (AQP2), which was excreted
into urine, was found to be related to the pathologic state of the
disease. In this study, we demonstrate that in experimental
dacryoadenitis models, the detection of AQP5 in tear fluid is useful
for specifically diagnosing lacrimal gland damage.