A cell-specific transgenic approach in Xenopus reveals the importance of a functional p24 system for a secretory cell

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.

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.

Angiogenesis in the intermediate lobe of the pituitary gland alters its structure and function

The pars distalis (PD) and the pars intermedia (PI) have the same embryonic origin, but their morphological and functional characteristics diverge during development. The PD is highly vascularized, whereas the highly innervated PI is essentially non-vascularized. Based on our previous finding that vascular endothelial growth factor-A (VEGF-A) is involved in vascularization of the rat PD, attempt was made to generate transgenic Xenopus expressing VEGF-A specifically in the melanotrope cells of the PI as a model system for studying the significance of vascularization or avascularization for the functional differentiation of the pituitary. The PI of the transgenic frogs, examined after metamorphosis, were distinctly vascularized but poorly innervated. The experimentally induced vascularization in the PI resulted in a marked increase in tissue volume and a decrease in the expression of both alpha-melanophore-stimulating hormone (α-MSH) and prohormone convertase 2, a cleavage enzyme essential for generating α-MSH. The transgenic animals had low plasma α-MSH concentrations and displayed incomplete adaptation to a black background. To our knowledge, this is the first report indicating that experimentally induced angiogenesis in the PI may bring about functional as well as structural alterations in this tissue.

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.

The trafficking protein Tmed2/p24beta(1) is required for morphogenesis of the mouse embryo and placenta

During vesicular transport between the endoplasmic reticulum and the Golgi, members of the TMED/p24 protein family form hetero-oligomeric complexes that facilitate protein-cargo recognition as well as vesicle budding. In addition, they regulate each other's level of expression. Despite analyses of TMED/p24 protein distribution in mammalian cells, yeast, and C. elegans, little is known about the role of this family in vertebrate embryogenesis. We report the presence of a single point mutation in Tmed2/p24beta(1) in a mutant mouse line, 99J, identified in an ENU mutagenesis screen for recessive developmental abnormalities. This mutation does not affect Tmed2/p24beta(1) mRNA levels but results in loss of TMED2/p24beta(1) protein. Prior to death at mid-gestation, 99J homozygous mutant embryos exhibit developmental delay, abnormal rostral-caudal elongation, randomized heart looping, and absence of the labyrinth layer of the placenta. We find that Tmed2/p24beta(1) is normally expressed in tissues showing morphological defects in 99J mutant embryos and that these affected tissues lack the TMED2/p24beta(1) oligomerization partners, TMED7/p24gamma(3) and TMED10/p24delta(1). Our data reveal a requirement for TMED2/p24beta(1) protein in the morphogenesis of the mouse embryo and placenta.

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.

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

Background

The p24 family is thought to be somehow involved in endoplasmic reticulum (ER)-to-Golgi protein transport. A subset of the p24 proteins (p24α₃, -β₁, -γ₃ and -δ₂) is upregulated when Xenopus laevis intermediate pituitary melanotrope cells are physiologically activated to produce vast amounts of their major secretory cargo, the prohormone proopiomelanocortin (POMC).

Methodology/Principal Findings

Here we find that transgene expression of p24α₃ or p24δ₂ specifically in the Xenopus melanotrope cells in both cases causes an effective displacement of the endogenous p24 proteins, resulting in severely distorted p24 systems and disparate melanotrope cell phenotypes. Transgene expression of p24α₃ greatly reduces POMC transport and leads to accumulation of the prohormone in large, ER-localized electron-dense structures, whereas p24δ₂-transgenesis does not influence the overall ultrastructure of the cells nor POMC transport and cleavage, but affects the Golgi-based processes of POMC glycomaturation and sulfation.

Conclusions/Significance

Transgenic expression of two distinct p24 family members has disparate effects on secretory pathway functioning, illustrating the specificity and non-redundancy of our transgenic approach. We conclude that members of the p24 family furnish subcompartments of the secretory pathway with specific sets of machinery cargo to provide the proper microenvironments for efficient and correct secretory protein transport and processing.

The coding sequence of amyloid-beta precursor protein APP contains a neural-specific promoter element

The amyloid-beta precursor protein APP is generally accepted to be involved in the pathology of Alzheimer's disease. Since its physiological role is still unclear, we decided to study the function of APP via stable transgenesis in the amphibian Xenopus laevis. However, the application of constructs encoding (mutant) APP fused to the C-terminus of the green fluorescent protein GFP (GFP-APP), and harboring a tissue-specific or an inducible gene promoter did not result in transgene expression of APP in neuronal and neuroendocrine cells. Surprisingly, a construct encoding either Xenopus or human APP fused to the N-terminus of GFP (APP-GFP) gave fluorescence throughout the whole brain of the tadpole, despite the fact that a proopiomelanocortin gene promoter was used to target transgene expression specifically to the intermediate pituitary cells. Detailed analysis with deletion mutants revealed the presence of a neural-specific, transcriptionally active DNA element within the 3'-end of the APP-coding sequence that gave rise to an aberrant transcript and protein in the APP-GFP transgenic animals. The DNA element appears to prevent proper APP transgene expression in Xenopus neuronal and neuroendocrine cells. Thus, the coding sequences of Xenopus and human APP contain a neural-specific promoter element, the physiological significance of which is at present unclear.

Cell type-specific transgene expression of the prion protein in Xenopus intermediate pituitary cells

The cellular form of prion protein (PrPC) is anchored to the plasma membrane of the cell and expressed in most tissues, but predominantly in the brain, including in the pituitary gland. Thus far, the biosynthesis of PrPC has been studied only in cultured (transfected) tumour cell lines and not in primary cells. Here, we investigated the intracellular fate of PrPCin vivo by using the neuroendocrine intermediate pituitary melanotrope cells of the South-African claw-toed frog Xenopus laevis as a model system. These cells are involved in background adaptation of the animal and produce high levels of its major secretory cargo proopiomelanocortin (POMC) when the animal is black-adapted. The technique of stable Xenopus transgenesis in combination with the POMC gene promoter was used as a tool to express Xenopus PrPC amino-terminally tagged with the green fluorescent protein (GFP-PrPC) specifically in the melanotrope cells. The GFP-PrPC fusion protein was expressed from stage-25 tadpoles onwards to juvenile frogs, the expression was induced on a black background and the fusion protein was subcellularly located mainly in the Golgi apparatus and at the plasma membrane. Pulse-chase metabolic cell labelling studies revealed that GFP-PrPC was initially synthesized as a 45-kDa protein that was subsequently stepwise glycosylated to 48-, 51-, and eventually 55-kDa forms. Furthermore, we revealed that the mature 55-kDa GFP-PrPC protein was sulfated, anchored to the plasma membrane and cleaved to a 33-kDa product. Despite the high levels of transgene expression, the subcellular structures as well as POMC synthesis and processing, and the secretion of POMC-derived products remained unaffected in the transgenic melanotrope cells. Hence, we studied PrPC in a neuroendocrine cell and in a well-defined physiological context.

Transgene expression of prion protein induces crinophagy in intermediate pituitary cells

The cellular form of the prion protein (PrP(C)) is a plasma membrane-anchored glycoprotein whose physiological function is poorly understood. Here we report the effect of transgene expression of Xenopus PrP(C) fused to the C-terminus of the green fluorescent protein (GFP-PrP(C)) specifically in the neuroendocrine intermediate pituitary melanotrope cells of Xenopus laevis. In the transgenic melanotrope cells, the level of the prohormone proopiomelanocortin (POMC) in the secretory pathway was reduced when the cells were (i) exposed for a relatively long time to the transgene product (by physiologically inducing transgene expression), (ii) metabolically stressed, or (iii) forced to produce unfolded POMC. Intriguingly, although the overall ultrastructure was normal, electron microscopy revealed the induction of lysosomes taking up POMC secretory granules (crinophagy) in the transgenic melanotrope cells, likely causing the reduced POMC levels. Together, our results indicate that in neuroendocrine cells transgene expression of PrP(C) affects the functioning of the secretory pathway and induces crinophagy.

Biosynthesis and differential processing of two pools of amyloid-β precursor protein in a physiologically inducible neuroendocrine cell

The amyloid-beta precursor protein (APP) is linked to Alzheimer's disease through its pathological proteolytic processing in the secretory pathway. Nevertheless, surprisingly little is known about the biosynthesis of endogenous APP. We therefore decided to investigate the intracellular fate of newly synthesized APP in a physiologically inducible neuroendocrine cell, the Xenopus intermediate pituitary melanotrope cell. We found that the level of both APP mRNA and protein was about threefold induced in the activated cells of black-adapted animals. Intriguingly, two pools of APP were found, only one of which was up-regulated. This induced pool became readily N- and subsequently O-glycosylated and was eventually proteolytically processed by an alpha-secretase-like cleavage event resulting in a secreted N-terminal and a cell-associated C-terminal APP fragment. Conversely, only the other (non-induced, non-glycosylated and uncleaved) pool became phosphorylated. Thus, we report on the biosynthesis of APP in a physiological context and illuminate the occurrence of two pools of APP, one of which is linked to neuroendocrine cell activation.

LMAN1 and MCFD2 form a cargo receptor complex and interact with coagulation factor VIII in the early secretory pathway

Mutations in LMAN1 (ERGIC-53) and MCFD2 are the causes of a human genetic disorder, combined deficiency of coagulation factor V and factor VIII. LMAN1 is a type 1 transmembrane protein with homology to mannose-binding lectins. MCFD2 is a soluble EF-hand-containing protein that is retained in the endoplasmic reticulum through its interaction with LMAN1. We showed that endogenous LMAN1 and MCFD2 are present primarily in complex with each other with a 1:1 stoichiometry, although MCFD2 is not required for oligomerization of LMAN1. Using a cross-linking-immunoprecipitation assay, we detected a specific interaction of both LMAN1 and MCFD2 with factor VIII, with the B domain as the most likely site of interaction. We also present evidence that this interaction is independent of the glycosylation state of factor VIII but requires native calcium concentration in the endoplasmic reticulum. The interaction of MCFD2 with factor VIII appeared to be independent of LMAN1-MCFD2 complex formation. These results suggest that LMAN1 and MCFD2 form a cargo receptor complex and that the primary sorting signals residing in the B domain direct the binding of factor VIII to LMAN1-MCFD2 through calcium-dependent protein-protein interactions. MCFD2 may function to specifically recruit factor V and factor VIII to sites of transport vesicle budding within the endoplasmic reticulum lumen.