Belluardo N, Mudò G, Caniglia G, Corsaro M, Cheng Q, Frasca F, Belfiore A, Condorelli D F
Institute of Human Physiology, Faculty of Medicine, University of Palermo, Palermo, Italy.
Cell Tissue Res. 1999 Mar;295(3):467-75. doi: 10.1007/s004410051252.
Levels of mRNA for neurotrophins (brain-derived neurotrophic factor, BDNF; neurotrophin 3, NT-3; neurotrophin 4, NT-4) and their receptors (trkA, trkB, trkC) and for glial cell line-derived neurotrophic factor (GDNF) and its receptors (ret, GDNFR-alpha) were measured in rat thyroid tissue by ribonuclease protection assays. In thyroid tissue the NT-3 mRNA level was threefold lower and the NT-4 mRNA level sixfold higher than those detected in adult rat hippocampus, while BDNF mRNA was undetectable. Very low levels of mRNA for truncated trkB and trkC receptors and no catalytic trkA, trkB or trkC were found. In conclusion NT-3 and NT-4, but not the corresponding functional receptors, are expressed in the thyroid tissue. Therefore, it is unlikely that these factors serve a direct local autocrine or paracrine function in thyroid cell types, and a target-derived mode of action on neurons innervating the thyroid tissue is suggested. An opposite result has been found for the neurotrophic factor GDNF: thyroid tissue showed a high level of transcripts for the GDNF receptor subunits (GDNFR-alpha and Ret), while GDNF mRNA was undetectable. The in situ hybridization analysis of GDNFR-alpha and ret mRNA revealed an interesting difference in the cell distribution of these transcripts: ret mRNA is selectively expressed in a subpopulation of cells scattered in the follicular epithelium and in the interfollicular spaces, while GDNFR-alpha expression is more homogeneous and widespread, including the more abundant cell type of the thyroid gland: the follicular cell. Double-labeling in situ hybridization/immunocytochemistry experiments, with a specific marker (calcitonin), showed that parafollicular cells express ret but not GDNFR-alpha. This differential distribution of the GDNF receptor components (GDNFR-alpha and ret) may reflect a peculiar biological role in intercellular communication in the thyroid gland.
通过核糖核酸酶保护试验测定了大鼠甲状腺组织中神经营养因子（脑源性神经营养因子，BDNF；神经营养因子3，NT-3；神经营养因子4，NT-4）及其受体（trkA、trkB、trkC）以及胶质细胞系源性神经营养因子（GDNF）及其受体（ret、GDNFR-α）的mRNA水平。在甲状腺组织中，NT-3 mRNA水平比成年大鼠海马体中检测到的水平低三倍，NT-4 mRNA水平高六倍，而BDNF mRNA未检测到。发现截短的trkB和trkC受体的mRNA水平非常低，且未发现有催化活性的trkA、trkB或trkC。总之，NT-3和NT-4在甲状腺组织中表达，但相应的功能性受体不表达。因此，这些因子不太可能在甲状腺细胞类型中发挥直接的局部自分泌或旁分泌功能，提示其对支配甲状腺组织的神经元具有靶源性作用方式。对于神经营养因子GDNF则发现了相反的结果：甲状腺组织显示GDNF受体亚基（GDNFR-α和Ret）的转录本水平很高，而GDNF mRNA未检测到。GDNFR-α和ret mRNA的原位杂交分析揭示了这些转录本在细胞分布上的有趣差异：ret mRNA选择性地在散在于滤泡上皮和滤泡间空间的细胞亚群中表达，而GDNFR-α的表达更均匀且广泛，包括甲状腺中数量较多的细胞类型：滤泡细胞。用特异性标志物（降钙素）进行的原位杂交/免疫细胞化学双标记实验表明，滤泡旁细胞表达ret但不表达GDNFR-α。GDNF受体成分（GDNFR-α和ret）的这种差异分布可能反映了其在甲状腺细胞间通讯中的特殊生物学作用。
