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Kclo4 Inhibits Thyroidal Activity in the Larval Lamprey Endostyle in Vitro

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Kclo4 Inhibits Thyroidal Activity in the Larval Lamprey Endostyle in Vitro GENERAL AND COMPARATIVE ENDOCRINOLOGY General and Comparative Endocrinology 128 (2002) 214–223 www.academicpress.com KClO4 inhibits thyroidal activity in the larval lamprey endostyle in vitro Richard G. Manzon*,1 and John H. Youson Department of Zoology and Division of Life Sciences, University of Toronto at Scarborough, 1265 Military Trail, Toronto, Ont., Canada MIC 1A4 Accepted 5 July 2002 Abstract An in vitro experimental system was devised to assess the direct effects of the goitrogen, potassium perchlorate (KClO4), on ra- dioiodide uptake and organification by the larval lamprey endostyle. Organification refers to the incorporation of iodide into lamprey thyroglobulin (Tg). Histological and biochemical evidence indicated that endostyles were viable at the termination of a 4 h in vitro incubation. A single iodoprotein, designated as lamprey Tg, was identified in the endostylar homogenates by polyacrylamide gel electrophoresis and Western blotting. Lamprey Tg was immunoreactive with rabbit anti-human Tg serum and had an electrophoretic mobility similar to that of reduced porcine Tg. When KClO4 was added to the incubation medium, both iodide uptake and orga- nification by the endostyle were significantly reduced relative to controls as determined by gamma counting, and gel-autoradiography and densitometry, respectively. Western blotting showed that KClO4 significantly lowered the total amount of lamprey Tg in the endostyle. Based on the results of this in vitro investigation, we conclude that KClO4 acts directly on the larval lamprey endostyle to inhibit thyroidal activity. These data support a previous supposition from in vivo experimentation that KClO4 acts directly on the endostyle to suppress the synthesis of thyroxine and triiodothyronine, resulting in a decrease in the serum levels of these two hormones. Ó 2002 Elsevier Science (USA). All rights reserved. 1. Introduction peroxidase activity (Tsuneki et al., 1983). Biochemical studies with radioiodide indicate that iodide taken up by The larval lamprey endostyle, which gives rise to the endostyle is incorporated into iodoproteins of dif- follicular thyroid tissue during metamorphosis, is the ferent sizes and sedimentation coefficients. Included site of thyroid hormone (TH) synthesis (Hardisty and among these iodoproteins are 19S and 12S proteins that Baker, 1982). Experimental, histological, and biochem- correspond to the homodimer and monomer, respec- ical evidence support the idea that the mechanism of TH tively, of mammalian Tg (Monaco et al., 1978; Suzuki synthesis in the lamprey endostyle is similar to that and Kondo, 1973). Furthermore, hydrolysis of these observed in follicular thyroid tissue. Histological auto- iodoproteins has confirmed that they contain iodothy- radiography at the light and electron microscope levels ronines, TH, and the TH precursors monoiodotyrosine has shown that radioiodide ð125IÀÞ is concentrated and (MIT) and diiodotyrosine (DIT) (Salvatore, 1969; Su- bound in several endostyle cell types, with cell types 2c zuki and Kondo, 1973). These data indicate that the and 3 being the primary iodide binding cells (Fig. 1; for necessary components for TH synthesis, namely, an io- review see Barrington and Sage, 1972; Wright and dide-concentrating mechanism, a Tg-like molecule, and Youson, 1976). These type 2c and type 3 cells are im- peroxidase activity, can be localized to particular cell munoreactive with an antibody against human thyro- types in the larval lamprey endostyle. globulin (Tg) (Wright et al., 1978) and they have The processes involved in the regulation of TH syn- thesis by the larval lamprey endostyle are less clear. Studies conducted to date have not confirmed whether * Corresponding author. Fax: +306-337-2410. TH synthesis and secretion by the larval lamprey en- E-mail address: [email protected] (R.G. Manzon). 1 dostyle are regulated by the hypothalamic–pituitary axis Present address: Department of Biology, University of Regina, 3737 Wascana Parkway, Regina, Sask., Canada S4S OA2. Fax: +306- (Barrington and Sage, 1963a,b, 1966; Clements-Merlini, 337-2410. 1962a; Knowles, 1941; Pickering, 1972). On the other 0016-6480/02/$ - see front matter Ó 2002 Elsevier Science (USA). All rights reserved. PII: S0016-6480(02)00505-1 R.G. Manzon, J.H. Youson / General and Comparative Endocrinology 128 (2002) 214–223 215 Fig. 1. Routine light microscopy (A and C) and autoradiography (B and D) of transverse sections through the anterior portion of the larval lamprey (Lampetra appendix) endostyle following a 4 h in vitro incubation with 3 lCi Na125I. Endostyle cell types 1, 2a, 2b, 2c, 3, and 5 are indicated. (A) The anterior endostyle consists of two straight, epithelium-lined chambers; the most prominent features are the four glandular tracts (Gt), which consist exclusively of type 1 cells. Flanking the openings of the glandular tracts to the lumen of the endostyle are the type 2 cells. Continuous with the type 2c cells are the type 3 endostylar cells. (C) A high magnification of a glandular tract and the surrounding epithelia in the anterior portion of the larval lamprey endostyle. Note that the cells of the epithelia are intact with well-defined cytoplasm and nuclei, confirming that the 4 h incubation with Na125I did not have any visible effects on cellular morphology. (B and D) Light microscopic autoradiography of sections similar to A and C, re- spectively. The type 2c and 3 cells are the primary 125IÀ incorporating cell types. In all four micrographs, the endostyles are oriented with the ventral side on the left. Magnification is Â165 (A and B) and Â330 (C and D). Blood vessels, Bv; Gills, Gi; Pigment, Pi. hand, anti-thyroid agents (goitrogens), known to inhibit Merlini (1962b) found that the goitrogens, thiourea and either iodide uptake or organification in follicular thy- thiocyanate, inhibited radioiodide accumulation and roid tissue, alter both functional and morphological that thiourea inhibited the formation of iodinated ty- aspects of the larval endostyle. Overall, however, the rosines. Subsequent reports from several studies indi- data from these goitrogen studies are not consistent cated that thiourea and thiouracil alter the functional (Salvatore, 1969) and are difficult to interpret. Clements- morphology of all endostyle cell types including those 216 R.G. Manzon, J.H. Youson / General and Comparative Endocrinology 128 (2002) 214–223 not involved in thyroidal activity (Barrington and Sage, vitro experiments, except those conducted for light mi- 1963a,b, 1966). croscopy. Experiments conducted for light microscopy More recent studies have focused on the ability of were performed on larval American brook lampreys, goitrogens to lower serum TH concentrations and in- Lampetra appendix, because sufficient numbers of sea duce precocious metamorphosis in lampreys (Hoheisel lampreys were not available. American brook lampreys and Sterba, 1963; Holmes and Youson, 1993; Holmes greater than 130 mm in length were collected in the fall et al., 1999; Leatherland et al., 1990; Manzon et al., from Duffins Creek in Ajax, Ontario, and housed as 2001; Suzuki, 1986, 1987; Youson et al., 1995). These described above. It has been shown that KClO, also studies support the view that spontaneous metamor- depresses serum TH concentrations and induces meta- phosis is linked to a decline in serum TH concentrations morphosis in L. appendix (Holmes et al., 1999). (Lintlop and Youson, 1983; Wright and Youson, 1977). Furthermore, Manzon et al. (1998) have conclusively 2.2. In vitro experimental protocol shown that elevating serum TH concentrations with exogenous TH prevents potassium perchlorate (KClO4)- Five larval lampreys were anesthetized in 0.025% induced metamorphosis. This result correlates well with tricaine methanesulfonate (MS-222, Syndel Laborato- the observation that exogenous TH can retard sponta- ries, Vancouver, British Columbia, Canada). The en- neous metamorphosis (Youson et al., 1997). However, dostyle (subpharyngeal gland) from each larva was whether goitrogens, such as KClO4, bring about a de- removed with minimal amounts of surrounding con- crease in serum TH concentrations and metamorphosis nective tissue. To simplify tissue processing, endostyles by either: (i) affecting some process underlying both TH used for light microscopy were removed with the ventral titers and metamorphosis; (ii) a direct effect on the en- body wall attached. Each endostyle was rinsed in ice- dostyle; or (iii) causing a generalized chronic debilita- cold incubation buffer (IB) prepared as in Ito et al. tion, has yet to be determined. A partial answer to these (1988): 110 mM NaCl, 1.9 mM KCl, 5 mM NaHCO3, questions could come from a study that determines 10 mM Hepes, 5 mM glucose, 1.1 mM CaCl2, 0.6 mM whether goitrogens can act directly on the endostyle and MgCl2, 0.2% bovine serum albumin, 0.006% penicillin reduce its capacity to synthesize TH. G, and 0.01% streptomycin sulfate, pH 7.4. After rins- Numerous studies have investigated the effects of ing, endostyles were drained on a Kimwipe, weighed, goitrogens on endostyle morphology, serum TH con- and placed in a 25 ml Erlenmeyer flask containing 3 ml centrations, and metamorphosis in larval lampreys. ice-cold IB. However, little is known about the direct effects that The in vitro experimental protocol used in this study goitrogens have on TH synthesis by the larval endostyle. was adapted from the protocol used by Ito et al. (1988) One aim of this study was to develop an in vitro ex- for the incubation of lamprey liver slices. In
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