LAUSR

Abstract Thyrotropin-releasing hormone (TRH) is expressed in the brain, and a few peripheral tissues. Apart from controlling TSH secretion from the pituitary, it has an anorexic effect. TRH is a short-lived intercellular signaling molecule, hydrolyzed by the TRH-degrading ectoenzyme (TRH-DE), a narrow specificity peptidase which only known biological substrate is TRH. TRH-DE activity is mainly detected in brain, with an uneven distribution among regions; at the base of the third ventricle, Trhde is expressed by β2 tanycytes whose end-feet form synaptoid contacts with TRH terminals in the external layer of the median eminence (ME). Tanycyte TRH-DE activity is sensitive to thyroid hormone feedback and energy balance clues and may contribute to adjust the thyroid axis to changing energy needs. To test whether this enzyme is relevant for energy balance, we used mice generated in the B6/129S background in which Trhde exon 2 was deleted and backcrossed to C57BL/6NJ background for 11 generations. TRH-DE activity was reduced in heterozygote (HT) and eliminated in homozygote (KO) compared to wild type (WT) animals. Male mice (70-75 days old) were switched from a standard diet to a high fat high fructose diet (HFFD) for 9 weeks. On HHFD, KO animals ingested less kcal and gained less body weight (BW) than WT animals. Bio-impedance data indicated a lower fat mass in KO mice, compared to WT or HT mice. Glucose tolerance was higher in KO mice than in WT or HT mice. To clarify the specific relevance of tanycyte TRH-DE, we used adeno-associated virus (AAV) vectors in male rats to either knock down (with TRH-DE*, a dominant negative isoform) or overexpress TRH-DE in ME tanycytes. Serum TSH concentration increased when TRH-DE activity decreased, and the converse occurred when TRH-DE activity increased. Thus, TRH-DE activity from β2-tanycytes controls the concentration of TSH in the circulation, probably because it regulates the turnover of TRH before entry into the hypothalamus-pituitary portal vessels. However, 2- or 3-weeks treatment with AAV-TRH-DE or AAV-TRH-DE* did not affect BW. To further test the role of peripheral TRH-DE activity, a phosphinic analogue of TRH (P-TRH) was administered to adult male HFFD mice during 28 days through osmotic pumps connected to an intraperitoneal catheter. P-TRH treatment reduced TRH-DE activity in serum, but not inside the blood brain barrier, compared with mice treated with vehicle; however, treatment with P-TRH did not change food consumption and BW. In conclusion, ablation of TRH-DE impedes diet-induced obesity in male mice, possibly through enhanced thyroid axis activity and amplification of the anorectic effect of TRH. Supported in part by grants from DGAPA-UNAM (PAPIIT IN206712, IN206416, IN212719), and CONACYT (CB154931, CB254960 and PN562).