Extracellular matrix proteins increase the expression of pro-TRH and pro-protein convertase PC1 in fetal hypothalamic neurons in vitro

T3 differentially regulates TRH expression in developing hypothalamic neurons in vitro

Triiodothyronine (T3) plays an important role during development of the central nervous system. T3 effects on gene expression are determined in part by the type of thyroid hormone receptors (TRs) expressed in a given cell type. Previous studies have demonstrated that thyrotropin releasing hormone (TRH) transcription in the adult hypothalamus is subjected to negative regulation by thyroid hormones. However, the role of T3 on the development of TRH expression is unknown. In this study we used primary cultures derived from 17-day-old fetal rat hypothalamus to analyze the effects of T3 on TRH gene expression during development. T3 increased TRH mRNA expression in immature cultures, but decreased it in mature cultures. In addition, T3 up-regulated TRalpha1 and TRbeta2 mRNA expression. TRalpha1 expression coincided chronologically with that of TRH in the rat hypothalamus in vivo. Maturation of TRH expression in the hypothalamus may involve T3 acting through TRalpha1.

Hypoxic neuronal necrosis: protein synthesis-independent activation of a cell death program

Hypoxic necrosis of dentate gyrus neurons in primary culture required the activation of an orderly cell death program independent of protein synthesis. Early mitochondrial swelling and loss of the mitochondrial membrane potential were accompanied by release of cytochrome c and followed by caspase-9-dependent activation of caspase-3. Caspase-3 and -9 inhibitors reduced neuronal necrosis. Calcium directly induced cytochrome c release from isolated mitochondria. Hypoxic neuronal necrosis may be an active process in which the direct effect of hypoxia on mitochondria may lead to the final common pathway of caspase-3-mediated neuronal death.

Secretory proprotein convertases PACE4 and PC6A are heparin-binding proteins which are localized in the extracellular matrix. Potential role of PACE4 in the activation of proproteins in the extracellular matrix

PACE4, PC6 and furin are potent subtilisin-like proprotein convertases (SPCs) which are responsible for the activation of transforming growth factor-beta (TGFbeta)-related factors such as bone morphogenetic proteins. Heparan sulfate proteoglycan within the extracellular matrix (ECM) is known to regulate the biological activity of various differentiation factors including TGFbeta-related molecules. PACE4 binds tightly to heparin and its heparin-binding region was found to be a cationic stretch of amino acids between residues 743 and 760. Furthermore, PACE4 was detected in the extracellular material fraction of the HEK293 cells, defined as the material remaining on the culture plate following the removal of the cells from the plate. PACE4 bound to the extracellular fraction was selectively dislodged by heparin into the culture medium. Heparin has no inhibitory activity against PACE4. Similarly, PC6A is also able to bind to heparin, whereas soluble furin does not. In human placenta, PACE4 is mainly present in syncytiotrophoblasts and can be released by heparin. These results suggest that PACE4 and PC6 are unique SPC family proteases that anchor heparan sulfate proteoglycans at the ECM. The interaction between PACE4 and heparan sulfate proteoglycans might play an important role in the delicate spatiotemporal regulation of TGFbeta-related factors' biological activity.

Development of pro-TRH gene expression in primary cultures of fetal hypothalamic cells

Little is known about the temporal relationship and the sequential steps for peptide biosynthesis during the terminal differentiation of the peptide phenotype in central nervous system. Analysis of the TRH phenotype in primary cultures of rat fetal day 17 hypothalamic cells has shown that TRH levels start increasing only after a week in culture, in contrast with in vivo data showing a steady increase during late fetal life. The purpose of this study was to compare the developmental patterns of TRH and pro-TRH mRNA levels in vitro to determine whether the initial low and steady levels of TRH are due to deficient transcription. Pro-TRH mRNA levels were detected by semi-quantitative RT-PCR through the development of primary cultures of serum-supplemented hypothalamic fetal cells from 17 day old embryos. Pro-TRH mRNA levels per dish increased steadily since the beginning of the culture. In contrast, TRH levels per dish were low and stable during the first week increasing afterwards, but remaining low compared to equivalent in vivo values. Pro-TRH mRNA levels per hypothalamus increased between fetal day 17 and postnatal 14, suggesting that the in vitro pattern of pro-TRH mRNA development mimics that occurring in vivo. These data show that pro-TRH gene expression does not limit TRH accumulation in vitro suggesting that the transcriptional and post-transcriptional programs leading to peptide accumulation are established independently.

BDNF increases the early expression of TRH mRNA in fetal TrkB+ hypothalamic neurons in primary culture

Known effects of neurotrophins in the developing central nervous system include induction or regulation of peptide expression. Hypothalamic postmitotic thyrotropin-releasing hormone (TRH)-producing neurons may require neurotrophins for survival and/or differentiation. This issue was investigated using primary cell cultures derived from 17-day-old fetal rat hypothalamus seeded in serum-free medium and analysed up to 4 days in vitro culture. Neurotrophin receptor (TrkB and TrkC) mRNA expression was detected by RT-PCR in fetal hypothalamus and throughout the culture period. Western blots confirmed the expression of the full-length proteins in vitro. Semi-quantitative RT-PCR showed that the addition of brain-derived neurotrophic factor (BDNF) increases TRH mRNA levels while the addition of neurotrophin-3 does not. TRH cell content was not modified. Studies on the effect of cell density or homologous conditioned medium demonstrated that endogenous factors probably contribute to determine TRH mRNA levels. One of these factors was BDNF because basal TRH mRNA levels were reduced by the addition of a Trk inhibitor or anti-BDNF. TrkB mRNA was expressed in 27% of cells and TRH mRNA in 2% of cells. The number of TRH+ cells was not affected by BDNF treatment. Forty-eight per cent of TRH neurons contained TrkB mRNA; these neurons had higher amounts of TRH mRNA than TrkB- neurons. Only TrkB+ cells responded to BDNF by increasing their TRH mRNA levels suggesting that BDNF may directly affect TRH biosynthesis. In conclusion, fetal hypothalamic TRH neurons are probably heterogeneous in regard to the neurotrophic factors enhancing peptide and mRNA levels. BDNF enhances TRH mRNA levels in a population of TrkB+ fetal hypothalamic TRHergic neurons in primary culture. However, additional influences may be necessary for the establishment of peptide phenotype in the TrkB+ neurons.