Disparate effects of p24alpha and p24delta on secretory protein transport and processing

Transmembrane emp24 domain proteins in development and disease

Regulated transport through the secretory pathway is essential for embryonic development and homeostasis. Disruptions in this process impact cell fate, differentiation and survival, often resulting in abnormalities in morphogenesis and in disease. Several congenital malformations are caused by mutations in genes coding for proteins that regulate cargo protein transport in the secretory pathway. The severity of mutant phenotypes and the unclear aetiology of transport protein-associated pathologies have motivated research on the regulation and mechanisms through which these proteins contribute to morphogenesis. This review focuses on the role of the p24/transmembrane emp24 domain (TMED) family of cargo receptors in development and disease.

Function of a p24 Heterodimer in Morphogenesis and Protein Transport in Penicillium oxalicum

The lignocellulose degradation capacity of filamentous fungi has been widely studied because of their cellulase hypersecretion. The p24 proteins in eukaryotes serve important functions in this secretory pathway. However, little is known about the functions of the p24 proteins in filamentous fungi. In this study, four p24 proteins were identified in Penicillium oxalicum. Six p24 double-deletion strains were constructed, and further studies were carried out with the ΔerpΔpδ strain. The experimental results suggested that Erp and Pδ form a p24 heterodimer in vivo. This p24 heterodimer participates in important morphogenetic events, including sporulation, hyphal growth, and lateral branching. The results suggested that the p24 heterodimer mediates protein transport, particularly that of cellobiohydrolase. Analysis of the intracellular proteome revealed that the ΔerpΔpδ double mutant is under secretion stress due to attempts to remove proteins that are jammed in the endomembrane system. These results suggest that the p24 heterodimer participates in morphogenesis and protein transport. Compared with P. oxalicum Δerp, a greater number of cellular physiological pathways were impaired in ΔerpΔpδ. This finding may provide new insights into the secretory pathways of filamentous fungi.

Expression of tmp21 in normal adult human tissues

TMP21, known as p23 protein, is one important member of the p24 protein families. The degradation of TMP21 is mediated by the ubiquitin-proteasome pathway, as with the other presenilin-associated γ-secretase complex members. NFAT plays a very important role in regulation of human TMP21 gene expression. Compared with the function of TMP21, the studies about the distribution of this protein in human tissues are limited. We collected 19 normal adult human tissues from a healthy adult man died in a traffic accident and did examination of all the tissues collected for ICH, western blot and RT-PCR. It was shown that the expression of TMP21 is at high levels in heart, liver, lung, kidney and adrenal gland; moderate levels in brain, pancreas, prostate gland, testicle, small intestine, colon, stomach, gall bladder, thyroid gland and trachea; low levels in skeletal muscle, skin and lymphonodus. TMP21 is widely existed in normal adult human tissues. The current study provided for the first time a comprehensive expression of TMP21 in normal adult human tissues. It will benefit on helping in the design and interpretation of future studies focused on expounding the function of TMP21.

Isoform-selective oligomer formation of Saccharomyces cerevisiae p24 family proteins

p24 family proteins are evolutionarily conserved transmembrane proteins involved in the early secretory pathway. Saccharomyces cerevisiae has 8 known p24 proteins that are classified into four subfamilies (p24α, -β, -γ, and -δ). Emp24 and Erv25 are the sole members of p24β and -δ, respectively, and deletion of either destabilizes the remaining p24 proteins, resulting in p24 null phenotype (p24Δ). We studied genetic and physical interactions of p24α (Erp1, -5, and -6) and γ (Erp2, -3, and -4). Deletion of the major p24α (Erp1) partially inhibited p24 activity as reported previously. A second mutation in either Erp5 or Erp6 aggravated the erp1Δ phenotype, and the triple mutation gave a full p24Δ phenotype. Similar genetic interactions were observed among the major p24γ (Erp2) and the other two γ members. All the p24α/γ isoforms interacted with both p24β and -δ. Interaction between p24β and -δ was isoform-selective, and five major α/γ pairs were detected. These results suggest that the yeast p24 proteins form functionally redundant αβγδ complexes. We also identified Rrt6 as a novel p24δ isoform. Rrt6 shows only limited sequence identity (∼15%) to known p24 proteins but was found to have structural properties characteristic of p24. Rrt6 was induced when cells were grown on glycerol and form an additional αβγδ complex with Erp3, Erp5, and Emp24. This complex was mainly localized to the Golgi, whereas the p24 complex containing Erv25, instead of Rrt6 but otherwise with the same isoform composition, was found mostly in the ER.

Drosophila melanogaster p24 trafficking proteins have vital roles in development and reproduction

p24 proteins comprise a family of type-I transmembrane proteins of ~24kD that are present in yeast and plants as well as metazoans ranging from Drosophila to humans. These proteins are most commonly localized to the endoplasmic reticulum (ER)-Golgi interface and are incorporated in anterograde and retrograde transport vesicles. Little is known about how disruption of p24 signaling affects individual tissue function or whole animals. Drosophila melanogaster express nine p24 genes, grouped into four subfamilies. Based upon our mRNA and protein expression data, Drosophila p24 family members are expressed in a variety of tissues. To identify functions for particular Drosophila p24 proteins, we used RNA interference (RNAi) to reduce p24 expression. Ubiquitous reduction of most p24 genes resulted in complete or partial lethality during development. We found that reducing p24 levels in adults caused defects in female fecundity (egg laying) and also reduced male fertility. We attributed reduced female fecundity to decreased neural p24 expression. These results provide the first genetic analysis of all p24 family members in a multicellular animal and indicate vital roles for Drosophila p24s in development and reproduction, implicating neural expression of p24s in the regulation of female behavior.

Subclass-specific localization and trafficking of Arabidopsis p24 proteins in the ER-Golgi interface

We describe a comprehensive analysis of the subcellular localization and in vivo trafficking of Arabidopsis p24 proteins. In Arabidopsis, there are 11 p24 proteins, which fall into only δ and β subfamilies. Interestingly, the δ subfamily of p24 proteins in Arabidopsis is elaborated spectacularly in evolution, which can be grouped into two subclasses: p24δ1 and p24δ2. We found that, although all p24δ proteins possess classic COPII/COPI binding motifs in their cytosolic C-termini, p24δ1 proteins are localized to the endoplasmic reticulum (ER), p24δ2 proteins are localized to both ER and Golgi. Two p24β proteins reside largely in Golgi. Similar to Atp24 (termed p24δ1c in this study), p24δ2d also cycles between the ER and Golgi. Interestingly, coexpression with p24β1 could retain p24δ2d, but not p24δ1d in Golgi. We revealed that the lumenal coiled-coil domain of p24δ2d is required for its steady-state localization in Golgi, probably through its interaction with p24β1. In p24β1, there is no classic COPII or COPI binding motif in its C-terminus. However, the protein also cycles between the ER and Golgi. We found that a conserved RV motif located at the extreme end of the C-terminus of p24β1 plays an important role in its Golgi target.

p24 Proteins from the same subfamily are functionally nonredundant

The p24 proteins function in early secretory pathway transport processes, but their exact role is unclear. In physiologically activated Xenopus melanotrope cells, a representative of each p24 subfamily (p24α(3), -β(1), -γ(3), -δ(2)) is upregulated coordinately with the major melanotrope cargo, proopiomelanocortin (POMC), whereas two other p24s (p24γ(2) and -δ(1)) are also expressed, but not coordinately with POMC. Using melanotrope-specific transgene expression, we here find that the roles of both p24γ(2) and p24δ(1) in the transport, glycosylation, sulphation and cleavage of POMC are different from those of their upregulated subfamily relatives (p24γ(3) and p24δ(2), respectively). Thus, even p24 proteins from the same subfamily have distinct functions in secretory cargo biosynthesis.

Dilysine retrieval signal-containing p24 proteins collaborate in inhibiting γ-cleavage of amyloid precursor protein

γ-Secretase mediates intramembranous γ-cleavage and ε-cleavage of β-amyloid precursor protein (APP) to liberate β-amyloid peptide (Aβ) and APP intracellular domain respectively from the membrane. Although the regulatory mechanism of γ-secretase cleavage remains unresolved, a member of the p24 cargo protein family, named p24δ(1) or TMP21, has been identified as an activity-modulating component. The p24 family proteins are divided into four subfamilies (p24α, β, δ and γ). In contrast to p24δ(1), p24β(1) has reportedly no effect on γ-cleavage. In this study, we determined whether p24α(2), p24γ(3) or p24γ(4) modulates APP processing. Knockdown of cellular p24α(2) induced a significant increase in Aβ generation but not in APP intracellular domain production in cell-based and cell-free assays, whereas p24α(2) over-expression suppressed Aβ secretion. By contrast, Aβ secretion was not altered by p24γ(3) or p24γ(4) knockdown. Endogenous p24α(2) co-immunoprecipitated with core components of the γ-secretase complex, and the anti-p24α(2) immunoprecipitate exhibited γ-secretase activity. Mutational disruption of the conserved dilysine ER-retrieval motifs of p24α(2) and p24δ(1) perturbed inhibition of γ-cleavage. Simultaneous knockdown, or co-over-expression, of these proteins had no additive or synergistic effect on Aβ generation. Our findings suggest that dilysine ER-retrieval signal-containing p24 proteins, p24α(2) and p24δ(1), bind with γ-secretase complexes and collaborate in attenuating γ-cleavage of APP.

V-ATPase-mediated granular acidification is regulated by the V-ATPase accessory subunit Ac45 in POMC-producing cells

The regulation of the V-ATPase, the proton pump mediating intraorganellar acidification, is still elusive. We find that excess of the neuroendocrine V-ATPase accessory subunit Ac45 reduces the intragranular pH and consequently disturbs prohormone convertase activation and prohormone processing. Thus, Ac45 represents the first V-ATPase regulator.

The vacuolar (H⁺)-ATPase (V-ATPase) is an important proton pump, and multiple critical cell-biological processes depend on the proton gradient provided by the pump. Yet, the mechanism underlying the control of the V-ATPase is still elusive but has been hypothesized to involve an accessory subunit of the pump. Here we studied as a candidate V-ATPase regulator the neuroendocrine V-ATPase accessory subunit Ac45. We transgenically manipulated the expression levels of the Ac45 protein specifically in Xenopus intermediate pituitary melanotrope cells and analyzed in detail the functioning of the transgenic cells. We found in the transgenic melanotrope cells the following: i) significantly increased granular acidification; ii) reduced sensitivity for a V-ATPase-specific inhibitor; iii) enhanced early processing of proopiomelanocortin (POMC) by prohormone convertase PC1; iv) reduced, neutral pH–dependent cleavage of the PC2 chaperone 7B2; v) reduced 7B2-proPC2 dissociation and consequently reduced proPC2 maturation; vi) decreased levels of mature PC2 and consequently reduced late POMC processing. Together, our results show that the V-ATPase accessory subunit Ac45 represents the first regulator of the proton pump and controls V-ATPase-mediated granular acidification that is necessary for efficient prohormone processing.

Using transgenic animal models in neuroendocrine research: lessons from Xenopus laevis

Transgenic animals are commonly employed to explore the function of individual proteins. Transgenic animal models include the mouse, the zebrafish, and the South African clawed toad Xenopus laevis. In contrast to mice and zebrafish, with Xenopus transgenesis DNA integration is mostly achieved in the one-cell stage. Moreover, Xenopus (as well as zebrafish) eggs are relatively large, the embryos are transparent, a large offspring is generated, and maintenance of the offspring is easy. In our transgenic studies in Xenopus, we focus on the well-characterized neuroendocrine melanotrope cells of the pituitary pars intermedia that are regulated during the process of adaptation of Xenopus to a changing environment. When the animal is placed on a black background, the melanotrope cells produce and process large amounts of the prohormone proopiomelanocortin (POMC). We apply stable melanotrope-specific transgenesis that is achieved by mixing a Xenopus POMC-promoter/transgene construct with sperm nuclei and injecting this mixture into unfertilized eggs. Since in the melanotrope cells the POMC promoter is much more active in black-adapted animals, the level of transgene expression can be manipulated by placing the animal on either a black or a white background. In this paper we review the possibilities of the Xenopus melanotrope-specific transgenic approach. Following a brief overview of the functioning of Xenopus melanotrope cells, stable melanotrope-specific transgenesis is discussed and our transgenic studies on brain-derived neurotrophic factor and secretory pathway components are described as examples of the transgenic approach in a physiological context and close to the in vivo situation.

Incomplete posttranslational prohormone modifications in hyperactive neuroendocrine cells

BACKGROUND

In black-background-adapted Xenopus laevis, the intermediate pituitary melanotrope cells are hyperactive, producing large amounts of their major secretory cargo proopiomelanocortin (POMC, representing ~80% of all newly synthesised proteins), whereas in white-adapted frogs these cells are only basally active. Here we explored in the hyperactive and basally active melanotrope cells the capacity for posttranslational POMC processing events in the secretory pathway.

RESULTS

We found that the hyperactive cells produced mainly non-complex N-glycosylated POMC, whereas in the basally active cells POMC was mostly complex N-glycosylated. Furthermore, the relative level of POMC sulphation was ~5.5-fold lower in the hyperactive than in the basally active cells. When the cargo load in the secretory pathway of the hyperactive cells was pharmacologically reduced, the relative amount of complex glycosylated POMC markedly increased.

CONCLUSION

Collectively, our data show that the secretory pathway in hyperactive neuroendocrine secretory cells lacks the capacity to fully comply with the high demands for complex glycosylation and sulphation of the overload of secretory cargo. Thus, a hyperactive secretory cell may run short in providing an output of correctly modified biological signals.

Functional diversity among p24 subfamily members

BACKGROUND INFORMATION

The p24 protein family plays an important but unclear role at the ER (endoplasmic reticulum)-Golgi interface. A p24 member from each subfamily (p24alpha(3), beta(1), gamma(3) and delta(2)) is upregulated with the prohormone POMC (pro-opiomelanocortin) when Xenopus laevis intermediate pituitary melanotrope cells are physiologically activated. Here we explored the role of p24 by generating and analysing Xenopus with melanotrope cell-specific transgene expression of p24beta(1) or p24gamma(3), two of the p24 proteins coexpressed with POMC, and compared the results with those previously reported for the two other coexpressed p24s (p24alpha(3) and p24delta(2)).

RESULTS

The transgene expression of p24beta(1) or p24gamma(3) did not affect the endogenous p24 proteins or affected only endogenous p24gamma(3) respectively, whereas in transgenics expressing p24alpha(3) and p24delta(2), the levels of all endogenous p24 proteins were strongly decreased. Nevertheless, as for p24alpha(3) but albeit to a lesser extent, in the p24beta(1)-transgenic melanotrope cells the rate of cargo cleavage was reduced, probably reflecting reduced cargo transport from the ER, and POMC glycosylation and sulfation in the Golgi were not affected. The p24gamma(3)-transgenic cells displayed features of both the p24alpha(3)-transgenics (reduced cargo cleavage, normal POMC sulfation) and the p24delta(2)-transgenics (affected POMC glycosylation).

CONCLUSIONS

Our results show that the four upregulated proteins p24alpha(3), beta(1), gamma(3) and delta(2) have non-redundant roles in the early secretory pathway, and suggest that each p24 subfamily member provides a proper ER/Golgi subcompartmental microenvironment, together allowing correct secretory protein transport and processing.

Accessory subunit Ac45 controls the V-ATPase in the regulated secretory pathway

The vacuolar (H(+))-ATPase (V-ATPase) is crucial for multiple processes within the eukaryotic cell, including membrane transport and neurotransmitter secretion. How the V-ATPase is regulated, e.g. by an accessory subunit, remains elusive. Here we explored the role of the neuroendocrine V-ATPase accessory subunit Ac45 via its transgenic expression specifically in the Xenopus intermediate pituitary melanotrope cell model. The Ac45-transgene product did not affect the levels of the prohormone proopiomelanocortin nor of V-ATPase subunits, but rather caused an accumulation of the V-ATPase at the plasma membrane. Furthermore, a higher abundance of secretory granules, protrusions of the plasma membrane and an increased Ca(2+)-dependent secretion efficiency were observed in the Ac45-transgenic cells. We conclude that in neuroendocrine cells Ac45 guides the V-ATPase through the secretory pathway, thereby regulating the V-ATPase-mediated process of Ca(2+)-dependent peptide secretion.

Novel oxidative stress-responsive gene ERS25 functions as a regulator of the heat-shock and cell death response

Members of the yeast p24 family, including Emp24p and Erv25p, exist as heteromeric complexes that have been proposed to cycle between the endoplasmic reticulum (ER) and Golgi compartments. The specific functions and sites of action of p24 proteins are still unknown. Here we identified a human homolog of the yeast p24 family of proteins, named ERS25 (endoplasmic reticulum stress-response protein 25), and investigated its role in stress response. ERS25 is predicted to have an ER localization signal peptide, a GOLD (Golgi dynamics) domain, which is found in several eukaryotic Golgi and lipid-trafficking proteins, a coiled-coil region, and a transmembrane domain. We demonstrate that ERS25 is localized to the ER and is induced by ER-specific stress, heat shock, and oxidative stress. The selective induction of ERS25 by brefeldin A, but not tunicamycin, implicates the involvement of ERS25 in protein trafficking between the ER and the Golgi. Small interfering RNA-mediated inhibition of ERS25 results in a significant decrease in apoptosis as well as a reduction of reactive oxygen species induced by oxidative stress. Moreover, ERS25 depletion results in a significant increase in the levels of the ER chaperone HSP70 in response to heat-shock stress through increased levels of HSF-1. We also found that inhibition of ERS25 induction in response to heat shock enhanced the binding of HSP70 to Apaf-1, which is likely to interfere in stress-mediated apoptosis. Together, the data presented here demonstrate that ERS25 may play a critical role in regulation of heat-shock response and apoptosis.