Role of a pro-sequence in the secretory pathway of prothyrotropin-releasing hormone

The α-helical structure of prodomains promotes translocation of intrinsically disordered neuropeptide hormones into the endoplasmic reticulum

Different neuropeptide hormones, which are either too small to adopt a stable conformation or are predicted to be intrinsically disordered, are synthesized as larger precursors containing a prodomain in addition to an N-terminal signal peptide. We analyzed the biogenesis of three unstructured neuropeptide hormones and observed that translocation of these precursors into the lumen of the endoplasmic reticulum (ER) is critically dependent on the presence of the prodomain. The hormone domains could be deleted from the precursors without interfering with ER import and secretion, whereas constructs lacking the prodomain remained in the cytosol. Domain-swapping experiments revealed that the activity of the prodomains to promote productive ER import resides in their ability to adopt an α-helical structure. Removal of the prodomain from the precursor did not interfere with co-translational targeting of the nascent chain to the Sec61 translocon but with its subsequent productive translocation into the ER lumen. Our study reveals a novel function of prodomains to enable import of small or intrinsically disordered secretory proteins into the ER based on their ability to adopt an α-helical conformation.

Role of lysyl oxidase propeptide in secretion and enzyme activity

Lysyl oxidase (LOX) is secreted as a proenzyme (proLOX) that is proteolytically processed in the extracellular milieu to release the propeptide and mature, active LOX. LOX oxidizes lysyl residues of a number of protein substrates in the extracellular matrix and on the cell surface, which impacts several physiological and disease states. Although the LOX propeptide (LOX-PP) is glycosylated, little is known about the role of this modification in LOX secretion and activity. To gain insight into this issue, cells were transfected with native, full-length LOX cDNA (pre-pro-LOX), the N-glycosylation null pre-[N/Q]pro-LOX cDNA and the deletion mutant pre-LOX cDNA, referred to as secretory LOX, in which mature LOX is targeted to the secretory pathway without its N-terminal propeptide sequence. The results show that glycosylation of the LOX-PP is not required for secretion and extracellular processing of pro-LOX but it is required for optimal enzyme activity of the resulting mature LOX. Complete deletion of the propeptide sequence prevents mature LOX from exiting the endoplasmic reticulum (ER). Taken together, our study points out the requirement of the LOX-PP for pro-LOX exit from the ER and is the first to highlight the influence of LOX-PP glycosylation on LOX enzyme activity.

Molecular evolution of the thyrotrophin-releasing hormone precursor in vertebrates: insights from comparative genomics

Human preprothyrotrophin-releasing hormone (ppTRH) includes six copies of the TRH sequence, the rat and mouse precursors have five, and those of non-mammalian vertebrates have up to eight. In the present study, the evolutionary basis of this variation was investigated using ppTRH gene sequences extracted from available vertebrate genomic databases. A structure based on eight TRH repeats appears to be the norm for non-mammalian vertebrates, but in all mammals except monotremes this number is reduced to a maximum of six. In some species, one (or more) of the TRH repeats has been mutated, probably rendering it functionless and, in a few species, one or two copies of the TRH sequence have been deleted completely. Sequences of regions between the TRH sequences are poorly conserved, despite reports that several active peptides are produced from these regions. The 5' untranslated region of ppTRH is also very variable but, in eutherians, the promoter region immediately upstream of the gene is quite strongly conserved. In particular, those sequences identified as being involved in transcriptional regulation are well conserved in most eutherians, although they are largely absent from other vertebrates. In most species, gene order around the ppTRH locus is conserved, although exceptions include man and chimpanzee, as well as rat and mouse. The comparative genomics approach thus provides a wider view than previously available of the range of ppTRH genes in vertebrates, and of the species specificity displayed by this molecule.

Regulation of the hypothalamic thyrotropin releasing hormone (TRH) neuron by neuronal and peripheral inputs

The hypothalamic-pituitary-thyroid (HPT) axis plays a critical role in mediating changes in metabolism and thermogenesis. Thus, the central regulation of the thyroid axis by Thyrotropin Releasing Hormone (TRH) neurons in the paraventricular nucleus of the hypothalamus (PVN) is of key importance for the normal function of the axis under different physiological conditions including cold stress and changes in nutritional status. Before the TRH peptide becomes biologically active, a series of tightly regulated processes occur including the proper folding of the prohormone for targeting to the secretory pathway, its post-translational processing, and targeting of the processed peptides to the secretory granules near the plasma membrane of the cell ready for secretion. Multiple inputs coming from the periphery or from neurons present in different areas of the brain including the hypothalamus are responsible for the activation or inhibition of the TRH neuron and in turn affect the output of TRH and the set point of the axis.