A novel pathway regulates thyroid hormone availability in rat and human hypothalamic neurosecretory neurons

Abstract

Hypothalamic neurosecretory systems are fundamental regulatory circuits influenced by thyroid hormone. Monocarboxylate-transporter-8 (MCT8)-mediated uptake of thyroid hormone followed by type 3 deiodinase (D3)-catalyzed inactivation represent limiting regulatory factors of neuronal T3 availability. In the present study we addressed the localization and subcellular distribution of D3 and MCT8 in neurosecretory neurons and addressed D3 function in their axons. Intense D3-immunoreactivity was observed in axon varicosities in the external zone of the rat median eminence and the neurohaemal zone of the human infundibulum containing axon terminals of hypophysiotropic parvocellular neurons. Immuno-electronmicroscopy localized D3 to dense-core vesicles in hypophysiotropic axon varicosities. N-STORM-superresolution-microscopy detected the active center containing C-terminus of D3 at the outer surface of these organelles. Double-labeling immunofluorescent confocal microscopy revealed that D3 is present in the majority of GnRH, CRH and GHRH axons but only in a minority of TRH axons, while absent from somatostatin-containing neurons. Bimolecular-Fluorescence-Complementation identified D3 homodimers, a prerequisite for D3 activity, in processes of GT1-7 cells. Furthermore, T3-inducible D3 catalytic activity was detected in the rat median eminence. Triple-labeling immunofluorescence and immuno-electronmicroscopy revealed the presence of MCT8 on the surface of the vast majority of all types of hypophysiotropic terminals. The presence of MCT8 was also demonstrated on the axon terminals in the neurohaemal zone of the human infundibulum. The unexpected role of hypophysiotropic axons in fine-tuned regulation of T3 availability in these cells via MCT8-mediated transport and D3-catalyzed inactivation may represent a novel regulatory core mechanism for metabolism, growth, stress and reproduction in rodents and humans.

Figure 1. D3 immunoreactivity in the rat mediobasal hypothalamus.

(A) Abundant D3 immunoreactive structures are seen in the external zone of the rat median eminence marked by silver grains. The inset on the left demonstrates the complete disappearance of D3- immunoractivity from the MBH when sections were incubated with D3 antisera previously preabsorbed with the corresponding peptide antigen. The boxed region is enlarged in (B). Black arrowheads indicate immunoreactive loci, which frequently were adjacent to blood vessels. Scale bars: 50 µm in A, 20 µm in B.

Figure 2. Ultrastructure of D3 immunoreactive elements in the rat mediobasal hypothalamus.

(A) D3 immunoreactivity, identified by silver grain deposits, appear primarily in axon varicosities containing dense core vesicles of 80–120 nm diameter in the upper external zone of the median eminence, characteristic of axons of parvocellular (PC) neurons. No or a few silver grains could be observed in association with organelles of magnocellular neurons (MC) or tanycytes (Tc), respectively. (B) D3-positive axons exhibiting various degrees of labeling are mixed with non-labeled fibers (asterisks) in the external zone of the median eminence. (C) Although silver grains occasionally appear in association with the plasma membrane (black arrowheads) and with small clear vesicles (white arrowheads) of the axon varicosities, the majority are not labeled (asterisks). (D) In contrast, the dense core vesicles accumulate most the reaction product (black arrowheads), as visible at high power magnification in the vicinity of the capillaries of the external zone of the median eminence. Unlabeled small clear vesicles are indicated with asterisk. Tc, tanycyte; Scale bars: 1 µm in A–B, 250 nm in C, 100 nm in inset on C, 500 nm in D.

Figure 3. Diameter of immunoreactive clusters in the outer zone of the rat hypothalamic median eminence stained for D3, GnRH or Rab3 detected by N-STORM superresolution microscopy.

**P<0.001; *P<0.05 by ANOVA followed by Newman-Keuls post test. Data are shown as Mean ± SEM (N = 500).

Figure 4. D3 expression in processes of mouse immortalized GT 1-7 GnRH neurons in vitro.

(A) GT 1-7 cells endogenously express D3 mRNA. –RT: minus reverse transcriptase control; + and -: positive and negative controls, respectively (B) Schematic illustration of Bimolecular Fluorescence Complementation used to study D3 homodimerization. YFP(N) and YFP(C) stand for YFP(1-158aa) and YFP(159–238aa), respectively. (C) GT 1-7 cells were transfected with a plasmid encoding the fusion protein D3-YFP(full-length). (D) Co-expression of YFP(1-158)-D3 and YFP-(159–238)-D3 fusion proteins results in fluorescence complementation and demonstrates D3 homodimers in the axon-like processes of GT1-7 cells. Arrows indicate D3 along the processes. Scale bars: 10 uM. (E). Axonal D3 activity is increased by T3-treatment in the median eminence of male Wistar rats. Mean±SEM (n = 9) *P<0.05 by t-test.

Figure 5. Dual-immunofluorescence images illustrate the overlapping distribution of fibers immunoreactive for D3 (green fluorochrome) and GnRH (A) or TRH (D) (red color) in the median eminence.

Sites of overlap (yellow color) occur along the axonal pathway in the median eminence (B,E). High power confocal images demonstrate dual-labeled axon varicosities (yellow color in composite images) immunoreactive for D3 and GnRH (C) or D3 and TRH (F). The D3 immunoreactivity appears as yellow patches (arrowheads) within the axon varicosities. Single channels are also shown in C’, C” and F’, F”, respectively. High power dual-immunofluorescent images are also shown for fibers labeled for D3 (green fluorochrome) and CRH (G), GHRH (H) or somatostatin (I) (red color). These D3-immunoreactive sites (arrowheads) correspond to axon varicosities (G, H). Somatostatin (SS)- immunoreactive axon varicosities show virtually no signal for D3. Scale bars: 20 µm in A, 5 µm in B, 5 µm in C, 10 µm in D,E, 5 µm in F–I.

Figure 6. Quantification of D3 distribution in parvocellular axon terminals in the rat median eminence.

(n = 3; *P<0.01 TRH vs. GnRH, GHRH, CRH and somatostatin (SST) by ANOVA followed by Newman-Keuls post-test.)

Figure 7. MCT8 immunoreactivity in the rodent mediobasal hypothalamus.

Low magnification photograph illustrates the presence of MCT8-immunoreactivity associated with tanycytes (A). In the median eminence, strong MCT8-immunoreactivity is observed in tanycyte processes (B, arrows). In addition to occurring in tanycyte processes (arrows), MCT8-immunoreactivity is also observed in small dot like structures reminiscent of axon varicosities (arrow heads) (C). MCT8 immunoreactivity in the mediobasal hypothalamus of wild-type (D) and MCT8-KO mice (E). III, third ventricle; Scale bars: 200 µm in A, 100 µm in B, 20 µm in C, 50 µm in D,E.

Figure 8. Ultrastructure of MCT8 immunoreactive structures in the rat median eminence.

MCT8-immunoreactivity (silver grains) is associated with tanycyte (A) and axon varicosities (B, C) in the external zone of the median eminence. In the axon varicosities, silver grains accumulate in a small region of the varicosity close to the cytoplasmic membrane (B, C). Scale bar: 500 nM.

Figure 9. MCT8 and D3 immunoreactivities in axon varicosities of the rat parvocellular hypophysiotropic neurons.

Confocal images were subjected to deconvolution. Boxed areas are enlarged in B and C, respectively. The immunofluorescent signal for MCT8 (red) is distributed as small dots throughout the external zone of median eminence and appears (arrowheads) on the surface of gonadotropin-releasing hormone (GnRH, C), thyrotropin-releasing hormone (TRH, D), corticotropin-releasing hormone (E), growth hormone-releasing hormone (GHRH, F) and somatostatin (SST, E) immunofluorescent axon varicosities (green). (H,I) Representative images of triple immunofluorescent labelings demonstrate D3 (blue), MCT8 (red) and a hypophyseotroph hormone (green) in the axons of the median eminence. MCT8-immunoreactive puncta (arrowheads) appear on the surface of the following categories of axon varicosities; single-labeled for D3 (Ha), single-labeled for GnRH (Hb), and double labeled for D3 and GnRH (Hc) or CRH (I). Scale bars: A: 10 µm, B: 5 µm, C-I: 2 µm.

Figure 10. D3-immunoreactivity in the infundibular stalk of the human hypothalamus.

(A) D3 immunoreactive fibers (arrowheads) are present in the human infundibular stalk (IS); these fibers are shown in medium (B) and high (inset in A) power micrographs. opt: tractus opticus, VMH: ventromedial hypothalamic nucleus, III: third ventricle Scale bars: 500 µm in A, 50 µm in B, 10 µm in inset.

Figure 11. MCT8-immunoreactivity in the infundibular stalk of the human hypothalamus.

A dense network of MCT8-immunoractive axons is present in the infundibular stalk (A). A high magnification image illustrates MCT8-immunoractive axon varicosities in the proximity of a putative portal blood vessel (BV) in the neurovascular zone of the infundibular stalk (B, arrows). Long, MCT8-immunoractive axons with large varicosities are frequently encountered (C). Scale bar: 100 µm in A, 20 µm in C (also corresponds to B).

Figure 12. Schematic illustration of axonal uptake and regulation of T3 in the mediobasal hypothalamus.

We suggest that T3 generated by D2 is released from tanycyte processes and taken up by MCT8 into axons of hypophysiotropic neurons. T3 concentrations are subjected to local regulation by D3-containing axon varicosities (At1), but absent in D3-negative axons (At2). T3 could be subjected to retrograde transport to reach the soma and nucleus of hypophysiotropic neurons and/or could act locally by affecting mitochondrial function and local thermogenesis. At, axon terminal; Tc, tanycyte; Pc, portal capillary.

Figure 13. D3 detection by Western Blot in the rat and human hypothalamus.

Note that the human D3 protein runs slightly faster compared to rat D3, in agreement with the calculated ∼3 kDa size difference between the two proteins.