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1 From the Institut für Molekularbiologie und Medizinische Chemie; and the 2 Universitätsaugenklinik, OttovonGuerickeUniversität, D-39120 Magdeburg, Germany.
Abstract
PURPOSE. The objective of this study was to determine whether human conjunctival epithelium synthesizes TFF-peptides (formerly P-domain peptides, trefoil factors), a family of mucin-associated secretory peptides of the gastrointestinal tract.
METHODS. Expression of TFF-peptides in human conjunctiva was monitored by reverse transcriptionpolymerase chain reaction analysis. Antisera specific for TFF-peptides were used for immunohistochemistry to determine the presence and distribution of TFF-peptides in human conjunctiva.
RESULTS. mRNA expression of TFF1 and TFF3, but not TFF2, was detected in human conjunctiva. TFF1 and TFF3 but not TFF2 are stored in conjunctival goblet cells only as revealed by immunofluorescence.
CONCLUSIONS. Goblet cells of the human conjunctiva synthesize TFF1 and TFF3. These peptides, together with the secretory ocular mucin MUC5AC, may contribute to the rheological properties of the tear film. They also may influence healing of corneal wounds due to their motogenic properties.
The surface of the eye is overlaid by a complex tear film, which contributes optical clarity, lubrication, and a protective barrier against pathogenic and noxious agents. This film is approximately 35- to 45-µm thick and is composed of three layers: an outer lipid layer, secreted by the meibomian glands; an intermediate aqueous layer, secreted mainly by the lacrimal glands; and an inner mucus layer of approximately 30 µm containing mucins as its major structural component.1 2
Mucins of the inner layer are large glycoconjugates secreted or as transmembrane bound molecules. The secretory mucin MUC5AC is produced by conjunctival goblet cells only.1 3 4 In contrast, the transmembrane mucin MUC4 is synthesized from cells of the stratified conjunctival epithelium, and the transmembrane mucin MUC1 is produced by both corneal and conjunctival epithelia1 4 5 ; both MUC4 and MUC1 transcripts are not detectable in goblet cells. The sialomucin complex (SMC), a transmembrane mucin expressed in the rat, is synthesized particularly abundantly in the cornea and in the conjunctiva but not by conjunctival goblet cells.6 Nonetheless, MUC1 and SMC seem to have different locations within the same population of cells, where SMC has been found more along the apical surfaces of the epithelial cells and MUC1 is predominant in the basal layers.6
Mucins influence the rheological properties of the ocular mucus.1 2 The rheological properties are defined by the tear break-up time, which is changed in various pathologic conditions (e.g. in patients with dry eye symptoms). Alterations of mucin in human conjunctival epithelia of such patients have been reported.7
TFF-peptides8 (formerly P-domain peptides, trefoil factors) like mucins are typical constituents of mucus gels (e.g., from the gastrointestinal9 10 11 12 and the respiratory 13 tracts), and amphibian integumentary mucins contain integral TFF-domains.14 Three TFF-peptides are known to exist in humans: TFF1 (formerly pS2), TFF2 (formerly hSP), and TFF3 (formerly hP1.B/hITF). They show distinct expression patterns; however, they all are typically secreted by mucin-producing cells (e.g., several types of goblet cells). TFF-peptides are thought to modulate the rheological properties of mucus gels by specific interaction with mucins.15 16 Furthermore, all three TFF-peptides are motogens that influence the migration rates of cell lines in wound healing assays in vitro17 18 19 and increase the resistance of animals against gastrointestinal damage in vivo.16 19 20 21 22 23 24 This makes them candidates as factors regulating rapid repair of mucous epithelia by a process called restitution.8
TFF3 has recently been shown to be accumulated in goblet cells of porcine conjunctival epithelia, perhaps serving as a protective agent.25 An analysis of TFF-peptides, expanded to the expression of all three members, in human conjunctival epithelial tissues is presented here.
Materials and Methods
Antisera
The following antisera-monitoring TFF-peptides were used:
Anti-TFF1, a monoclonal mouse antiserum against the 30 C-terminal amino acids of human TFF1, was purchased from Zymed Laboratories (San Francisco, CA), and anti-TFF3, the polyclonal rabbit antiserum (anti-rTFF3-1) against the C terminus of the rat sequence, was described previously.13 26
RNA Extraction and Reverse TranscriptionPolymerase Chain Reaction
Analysis
All investigations followed the tenets of the Declaration of
Helsinki. Pieces of human conjunctivae were excised at the time of
cataract surgery (2 men, 4 women; 65 to 88 years of age) for which
informed consent was obtained from the subjects. All patients were free
of preoperative chronic medication except one patient receiving topical
eye medication for glaucoma. All conjunctiva obtained appeared normal
at the time of preoperative examination. Freshly excised conjunctiva
bulbi tissue was immediately frozen in liquid nitrogen, and RNA was
extracted from the pooled tissue samples using a guanidinium
thiocyanate protocol. RNA purification via CsCl ultracentrifugation and
reverse transcriptionpolymerase chain reaction (RTPCR) analysis
monitoring expression of TFF1, TFF2, and TFF3 were essentially as
described previously,13
with 30 amplification cycles
(Taq DNA polymerase; Boehringer Mannheim gmbH, Mannheim,
Germany). As a control, glyceraldehyde-3-phosphate
dehydrogenase (GAPDH) transcripts were amplified in a parallel reaction
using a specific primer pair.13
General Histology and Immunofluorescence
Human conjunctiva bulbi tissues were obtained in the course of
cataract surgery (3 men, 1 woman; 70 to 83 years of age) or strabismus
surgery (4 females; 6 to 31 years of age). None of the patients were
using extended preoperative eye medication. Tissues were fixed in
HEPES-buffered 4% paraformaldehyde overnight at 4°C, dehydrated in a
series of graded ethanols, and embedded in Technovit 7100 (Heraeus
Kulzer GmbH, Wehrheim, Germany). Two-micrometer-thick
sections were cut on a rotation microtome (Leica RM 2155), put on
"Polysine" microslides (MenzelGläser, Braunschweig,
Germany), and dried for 2 hours.
Mucins were stained using a combination of Alcian blue 8GX at pH 2.5 and the periodic acidSchiff reaction as described previously.27 Nuclei were counterstained with hematoxylin.
For immunofluorescence the fixed sections were treated with 0.1% papain (Merck, Darmstadt, Germany) for 15 minutes at room temperature, blocked with 1% bovine serum albumin for 20 minutes at 37°C and then incubated with the primary antibody (anti-TFF1, 1:5000 dilution, or anti-rTFF3-1, 1:1000 dilution) in 0.5% bovine serum albumin for 12 hours at 37°C. The secondary antibodies, Cy3-labeled sheep anti-mouse IgG (Dianova, Hamburg, Germany) or fluorescein-labeled sheep anti-rabbit IgG (Boehringer Mannheim), were incubated for 1 hour at 37°C. Furthermore, specificity of the staining was checked by competition with the corresponding peptides, that is, 1 ml anti-TFF1 (1:5000) or anti-rTFF3-1 (1:1000 dilution) was preadsorbed with 7 µg recombinant TFF1/dimer or 5 µg synthetic FKPLQEAECTF (representing the C-terminal of human TFF3), respectively, for at least 2 hours at 4°C and then used for immunofluorescence. DNA within the nuclei was stained with 4',6-diamidino-2-phenylindole (DAPI; Sigma, Deisenhofen, Germany), and the sections were covered with fluorescent mounting medium (Dako, gmbH, Hamburg, Germany) after being washed in phosphate-buffered saline and water. For colocalization studies the sections were stained sequentially first with anti-rTFF3-1 (1:1000) and then with anti-TFF1 (1:1000). TFF3 staining was documented before TFF1 staining. Photomicrographs were taken on Kodak Ektachrome EPJ 320T.
Results
RTPCR Analysis
RNA was isolated from pooled conjunctiva tissue of six patients of
which cDNA was amplified by the use of specific primer
pairs,13
testing for TFF1, TFF2, or TFF3 transcripts (Fig. 1
A). TFF1- and TFF3-specific amplification products were clearly visible
after separation on an agarose gel. In contrast, expression of TFF2 was
not detectable in human conjunctiva. As controls, TFF1, TFF2, and TFF3
transcripts were monitored in parallel reactions with cDNA samples from
human stomach and colon, respectively (Fig. 1B)
.
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Figures 1 2 3 clearly demonstrate biosynthesis and storage of the secretory peptides TFF1 and TFF3, but not TFF2, in the human conjunctival epithelium. This result is in agreement with a similar observation concerning TFF3 in porcine material.25 Both TFF1 and TFF3 are localized in the same conjunctival goblet cells, which does not conform to most other mucous epithelia that show a nonoverlapping distribution pattern of TFF-peptides (Table 1) . For example, TFF1 is typically localized in gastric surface cells,27 28 29 whereas TFF3 is found in intestinal and respiratory goblet cells.13 15 30 Thus, the localization of TFF1 in conjunctival goblet cells is remarkable because one would have expected its expression in apical cells of the conjunctival epithelium (together with MUC4) by analogy with its gastric expression.
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It is postulated that TFF-peptides function as "link-peptides" interacting noncovalently with mucins and influencing the rheological properties of mucous gels.15 This hypothesis has been confirmed in preliminary studies with TFF2 and TFF3, both increasing the viscosities of mucin preparations.16 The dimeric structure of TFF131 and TFF332 is ideally suited to form an entangled network33 34 with MUC5AC in the ocular mucus. The precise nature of the interaction between TFF-peptides and mucins is currently not known; generally, proteinprotein and proteinsugar interactions are imaginable. However, the latter has been proposed for TFF2 because of its established three-dimensional structure.35
A second possible function of TFF1 and TFF3 in the eye is that of motogens by analogy to their wound healing properties in the gastrointestinal tract. After a wound in the cornea, migration of surrounding corneal epithelial cells is observed, whereas a total corneal epithelial defect can only be healed by conjunctival epithelial cells covering the denuded cornea.36 37 38 A corneal wound stimulates goblet cell mucus secretion in the same eye by activation of efferent parasympathetic and sympathetic nerves in the rat conjunctiva.39 40 This reflux secretion could also release TFF1 and TFF3.
Mucin-deficiency disorders41 such as ocular cicatricial pemphigoid, Steven-Johnson syndrome, or xerophthalmia could lower levels of TFF-peptides parallel to the decrease in goblet cell densities. In contrast, a higher number of goblet cells42 might enhance TFF-peptide secretion as when contact lenses are worn daily. TFF-peptide secretion might also be influenced by alterations in glycosylation of goblet cell mucins as they occur in patients with dry eye symptoms.7 However, as yet there are no data concerning synthesis and secretion of ocular TFF-peptides in pathologic conditions.
Acknowledgements
The authors thank Ursula Meyer for excellent technical assistance, Felicity E. B. May (Newcastle upon Tyne, United Kingdom) for recombinant TFF1, Antje Wiede for many helpful discussions, and Martin Oertel for comments on the manuscript.
Footnotes
Supported by the "Fonds der Chemischen Industrie" (Grants 0163615 and 500058 to WH).
Submitted for publication February 28, 1999; revised April 19, 1999; accepted April 29, 1999.
Proprietary interest category: N.
Corresponding author: Werner Hoffmann, Institut für Molekularbiologie und Medizinische Chemie, Universitätsklinikum, Leipziger Str. 44, D-39120 Magdeburg, Germany. E-mail: werner.hoffman@medizin.uni-magdeburg.de
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