Protein secretion: Puzzling receptors

Huttner and colleagues [1xProtein secretion: Puzzling receptors. Thiele, C, Gerdes, H-H, and Huttner, WB. Curr Biol. 1997; 7: R496–R500Abstract | Full Text | Full Text PDF | PubMedSee all References[1] have challenged the views of myself and colleagues on how prohormones are sorted to the granules of the regulated secretory pathway (RSP), at the trans-Golgi network. By studying the prohormone pro-opiomelanocortin (POMC), we obtained evidence for a sorting signal/receptor-mediated mechanism that differed from the accepted passive aggregation hypothesis for protein sorting to the RSP. Our in vitro binding studies identified membrane carboxy-peptidase E (CPE) as the sorting receptor [2xCarboxy-peptidase E is a regulated secretory pathway sorting receptor: genetic obliteration leads to endocrine disorders in Cpefat mice. Cool, DR, Normant, E, Shen, F-S, Chen, H-C, Pannell, L, Zhang, Y, and Loh, YP. Cell. 1997; 88: 73–83Abstract | Full Text | Full Text PDF | PubMedSee all References[2] to which the POMC sorting signal [4xReceptor-mediated targeting of hormones to secretory granules. Role of carboxypeptidase E. Loh, YP, Snell, CR, and Cool, DR. Trends Endocrinol Metab. 1997; 8: 130–137Abstract | Full Text | Full Text PDF | PubMed | Scopus (21)See all References[4] specifically binds. Misrouting of POMC to the constitutive pathway in cells depleted of CPE and in a mutant mouse deficient in CPE [[2]xCarboxy-peptidase E is a regulated secretory pathway sorting receptor: genetic obliteration leads to endocrine disorders in Cpefat mice. Cool, DR, Normant, E, Shen, F-S, Chen, H-C, Pannell, L, Zhang, Y, and Loh, YP. Cell. 1997; 88: 73–83Abstract | Full Text | Full Text PDF | PubMedSee all References, [3]xIntracellular misrouting and abnormal secretion of adreno-corticotropin and growth hormone in Cpefat mice associated with a carboxypeptidase E mutation. Shen, F-S and Loh, YP. Proc Natl Acad Sci USA. 1997; 94: 5314–5319Crossref | PubMed | Scopus (61)See all References] led us to propose that this binding is essential for sorting to the RSP.Huttner and colleagues [1xProtein secretion: Puzzling receptors. Thiele, C, Gerdes, H-H, and Huttner, WB. Curr Biol. 1997; 7: R496–R500Abstract | Full Text | Full Text PDF | PubMedSee all References[1] criticized our work [[2]xCarboxy-peptidase E is a regulated secretory pathway sorting receptor: genetic obliteration leads to endocrine disorders in Cpefat mice. Cool, DR, Normant, E, Shen, F-S, Chen, H-C, Pannell, L, Zhang, Y, and Loh, YP. Cell. 1997; 88: 73–83Abstract | Full Text | Full Text PDF | PubMedSee all References, [3]xIntracellular misrouting and abnormal secretion of adreno-corticotropin and growth hormone in Cpefat mice associated with a carboxypeptidase E mutation. Shen, F-S and Loh, YP. Proc Natl Acad Sci USA. 1997; 94: 5314–5319Crossref | PubMed | Scopus (61)See all References] using primarily four arguments. In the limited space provided, we rebut these points as follows. First, they criticized our use of CPE antisense RNA to produce CPE-depleted cells [2xCarboxy-peptidase E is a regulated secretory pathway sorting receptor: genetic obliteration leads to endocrine disorders in Cpefat mice. Cool, DR, Normant, E, Shen, F-S, Chen, H-C, Pannell, L, Zhang, Y, and Loh, YP. Cell. 1997; 88: 73–83Abstract | Full Text | Full Text PDF | PubMedSee all References[2], suggesting that these cells may have lost their phenotype. However, we have recently verified that these cells have a normal neuroendocrine phenotype [5xDepletion of carboxypeptidase E, a regulated secretory pathway receptor, causes misrouting and constitutive secretion of proinsulin and proenkephalin, but not chromogranin A. Normant, E and Loh, YP. Endocrinology. 1998; : in pressSee all References[5]. Second, they suggested alternative interpretations of our secretion data and argued that secretion of POMC peptides from CPE-depleted cells without stimulation may not be constitutive [2xCarboxy-peptidase E is a regulated secretory pathway sorting receptor: genetic obliteration leads to endocrine disorders in Cpefat mice. Cool, DR, Normant, E, Shen, F-S, Chen, H-C, Pannell, L, Zhang, Y, and Loh, YP. Cell. 1997; 88: 73–83Abstract | Full Text | Full Text PDF | PubMedSee all References[2], but enhanced basal release through the RSP. If this was correct, one would have expected to see some increase in secretion over basal levels with stimulation, but we did not see any [2xCarboxy-peptidase E is a regulated secretory pathway sorting receptor: genetic obliteration leads to endocrine disorders in Cpefat mice. Cool, DR, Normant, E, Shen, F-S, Chen, H-C, Pannell, L, Zhang, Y, and Loh, YP. Cell. 1997; 88: 73–83Abstract | Full Text | Full Text PDF | PubMedSee all References[2]. Third, they also suggested that our stimulated secretion from wild-type cells evoked by elevated potassium [2xCarboxy-peptidase E is a regulated secretory pathway sorting receptor: genetic obliteration leads to endocrine disorders in Cpefat mice. Cool, DR, Normant, E, Shen, F-S, Chen, H-C, Pannell, L, Zhang, Y, and Loh, YP. Cell. 1997; 88: 73–83Abstract | Full Text | Full Text PDF | PubMedSee all References[2] reflected increased basal release, due to increased POMC synthesis caused by the 3 h potassium treatment. However, we found no significant difference in POMC/adrenocorticotropic hormone (ACTH) content, which reflects synthesis, in cells after 3 h potassium treatment [2xCarboxy-peptidase E is a regulated secretory pathway sorting receptor: genetic obliteration leads to endocrine disorders in Cpefat mice. Cool, DR, Normant, E, Shen, F-S, Chen, H-C, Pannell, L, Zhang, Y, and Loh, YP. Cell. 1997; 88: 73–83Abstract | Full Text | Full Text PDF | PubMedSee all References[2], and a 1 h treatment gave the same stimulation (Y.P.L., unpublished data), arguing against their interpretation. Fourth, reinterpreting our secretion kinetics, they argued that since POMC/POMC peptide secretion from neurointermediate lobe cells of normal and CPE-deficient mutant mice were both constitutive, CPE cannot be involved in sorting. However, if they had also considered our data showing that dopamine regulated POMC secretion in normal, but not mutant, mice [2xCarboxy-peptidase E is a regulated secretory pathway sorting receptor: genetic obliteration leads to endocrine disorders in Cpefat mice. Cool, DR, Normant, E, Shen, F-S, Chen, H-C, Pannell, L, Zhang, Y, and Loh, YP. Cell. 1997; 88: 73–83Abstract | Full Text | Full Text PDF | PubMedSee all References[2], they would not have come to these conclusions.We believe that the arguments of Huttner and colleagues are widely off the mark and we encourage interested readers to make up their own minds by reading all of our papers directly.