A role for intermediate filaments in the establishment of the primitive epithelia during mammalian embryogenesis

Crystal structure of human peptidylarginine deiminase type VI (PAD6) provides insights into its inactivity

Human peptidylarginine deiminase isoform VI (PAD6), which is predominantly limited to cytoplasmic lattices in the mammalian oocytes in ovarian tissue, is essential for female fertility. It belongs to the peptidylarginine deiminase (PAD) enzyme family that catalyzes the conversion of arginine residues to citrulline in proteins. In contrast to other members of the family, recombinant PAD6 was previously found to be catalytically inactive. We sought to provide structural insight into the human homologue to shed light on this observation. We report here the first crystal structure of PAD6, determined at 1.7 Å resolution. PAD6 follows the same domain organization as other structurally known PAD isoenzymes. Further structural analysis and size-exclusion chromatography show that PAD6 behaves as a homodimer similar to PAD4. Differential scanning fluorimetry suggests that PAD6 does not coordinate Ca2+ which agrees with acidic residues found to coordinate Ca2+ in other PAD homologs not being conserved in PAD6. The crystal structure of PAD6 shows similarities with the inactive state of apo PAD2, in which the active site conformation is unsuitable for catalytic citrullination. The putative active site of PAD6 adopts a non-productive conformation that would not allow protein-substrate binding due to steric hindrance with rigid secondary structure elements. This observation is further supported by the lack of activity on the histone H3 and cytokeratin 5 substrates. These findings suggest a different mechanism for enzymatic activation compared with other PADs; alternatively, PAD6 may exert a non-enzymatic function in the cytoplasmic lattice of oocytes and early embryos.

Mammalian oocytes store proteins for the early embryo on cytoplasmic lattices

Mammalian oocytes are filled with poorly understood structures called cytoplasmic lattices. First discovered in the 1960s and speculated to correspond to mammalian yolk, ribosomal arrays, or intermediate filaments, their function has remained enigmatic to date. Here, we show that cytoplasmic lattices are sites where oocytes store essential proteins for early embryonic development. Using super-resolution light microscopy and cryoelectron tomography, we show that cytoplasmic lattices are composed of filaments with a high surface area, which contain PADI6 and subcortical maternal complex proteins. The lattices associate with many proteins critical for embryonic development, including proteins that control epigenetic reprogramming of the preimplantation embryo. Loss of cytoplasmic lattices by knocking out PADI6 or the subcortical maternal complex prevents the accumulation of these proteins and results in early embryonic arrest. Our work suggests that cytoplasmic lattices enrich maternally provided proteins to prevent their premature degradation and cellular activity, thereby enabling early mammalian development.

PADI6: What we know about the elusive fifth member of the peptidyl arginine deiminase family

Peptidyl arginine deiminase 6 (PADI6) is a maternal factor that is vital for early embryonic development. Deletion and mutations of its encoding gene in female mice or women lead to early embryonic developmental arrest, female infertility, maternal imprinting defects and hyperproliferation of the trophoblast. PADI6 is the fifth and least well-characterized member of the peptidyl arginine deiminases (PADIs), which catalyse the post-translational conversion of arginine to citrulline. It is less conserved than the other PADIs, and currently has no reported catalytic activity. While there are many suggested functions of PADI6 in the early mouse embryo, including in embryonic genome activation, cytoplasmic lattice formation, maternal mRNA and ribosome regulation, and organelle distribution, the molecular mechanisms of its function remain unknown. In this review, we discuss what is known about the function of PADI6 and highlight key outstanding questions that must be answered if we are to understand the crucial role it plays in early embryo development and female fertility. This article is part of the Theo Murphy meeting issue 'The virtues and vices of protein citrullination'.

Human intestinal epithelial cells exhibit a cellular response indicating a potential toxicity upon exposure to hematite nanoparticles

This study examined the effects of different-sized nanoparticles on potential cytotoxicity in intestinal epithelia. Three sizes of hematite nanoparticles were used for the study at a 10 ppm concentration: 17, 53, and, 100 nm. Results indicate that, of the hematite nanoparticles tested, 17 nm was more toxic to the epithelial integrity than 53 or 100 nm. In addition, the epithelial integrity was affected by disruption of epithelial structures such as apical microvilli, and by disruption of the cell-cell junctions leading to reduction in transepithelial electrical resistance measurements (TEER). The drop in TEER was caused by disruption of the adhering junctions not by cell death, as determined by immunocytochemistry, and by using a cell viability assay. Epithelial integrity was also affected at the molecular level as shown by differential expression of genes related to cell junction maintenance, which was assessed by microarray analysis. In conclusion, the 17- and 100-nm hematite nanoparticles caused significant structural changes in the epithelium but not the 53 nm nanoparticles. Also, different-sized hematite nanoparticles each had different effects both at the cellular level and genetic level.

Proteomic characterization of histotroph during the preimplantation phase of the estrous cycle in cattle

Uterine secretions, or histotroph, are a critical component for early embryo survival, functioning as the sole supply of vitamins, minerals, enzymes, and other myriad of nutrients required by the developing conceptus before implantation. Histotroph is therefore a promising source for biomarkers of uterine function and for enhancing our understanding of the environment supporting early embryo development and survival. Utilizing label-free liquid chromatography-tandem mass spectrometry (LC-MS/MS) shotgun proteomics, we characterized the uterine proteome at two key preimplantation stages of the estrous cycle in high fertility cattle. We identified 300 proteins on Day 7 and 510 proteins on Day 13 including 281 proteins shared between days. Five proteins were more abundant (P < 0.05) on Day 7 compared with Day 13 and included novel histotroph proteins cytokeratin 10 and stathmin. Twenty-nine proteins were more abundant (P < 0.05) including 13 unique on Day 13 compared with Day 7 and included previously identified legumain, metalloprotease inhibitor-2, and novel histotroph proteins chromogranin A and pyridoxal kinase. Functional analysis of the 34 differentially expressed proteins (including 14 novel to histotroph) revealed distinct biological roles putatively involved in early pregnancy, including remodelling of the uterine environment in preparation for implantation; nutrient metabolism; embryo growth, development and protection; maintenance of uterine health; and maternal immune modulation. This study is the first reported LC-MS/MS based global proteomic characterization of the uterine environment in any domesticated species before implantation and provides novel information on the temporal alterations in histotroph composition during critical stages for early embryo development and uterine function during the early establishment of pregnancy.

Peptidylarginine deiminase (PAD) 6 is essential for oocyte cytoskeletal sheet formation and female fertility

Peptidylarginine deiminase 6 (PAD6) is an enzyme that is uniquely expressed in male and female germ cells. To study the function of this enzyme in vivo we generated mice deficient for PAD6. Here we show that inactivation of the PAD6 gene in mice leads to female infertility whereas male fertility is not affected. The absence of the PAD6 protein and consequently absence of citrullination activity in oocytes results in dispersal of the cytoskeletal sheets in oocytes, indicating an essential role of these germ cell-specific structures in zygote/embryo development. PAD6 deficient mice do not show any other overt phenotype. Thus, we identify citrullination as a new regulator of fertility.

Type II keratins precede type I keratins during early embryonic development

We and others have recently demonstrated that the keratin (K) gene family in mammals is even more complex than previously thought [Eur. J. Cell Biol. 83, 19-26]. To address the function of keratins during early development, precise information on their spatio-temporal expression is required. Here, we examined the expression of selected mouse keratins from pre-implantation to mid-gestational embryonic stages using RT-PCR and immunofluorescence. At E0.5, transcripts encoding K5, K6, K7, K8, K14, K15, K18, and K19 are apparently absent. We report on a post-transcriptional regulation of type I keratins, preventing filament formation in 8- to 16-cell stage embryos. In these embryos, mRNAs coding for K7, K8, K18, and K19 are present, but only K7 and K8 are translated into protein which is deposited in aggregates. Following the accumulation of K18 protein at E3.5, keratin filaments are formed. Delayed onset of type I keratin protein expression was additionally observed in later embryonic stages for K5 and K14. K5 protein expression starts in the forelimb surface ectoderm as early as E9.25, while the expression of its partner, K14, begins at E9.75. From E9.25 to E9.75, K5 forms atypical filaments with K18. Remarkably, in embryonic K5-/- mice, K14 formed normal filaments until E12.5 despite the absence of its partner K5, due to the presence of K8. Following periderm formation, K14-containing filaments disappeared and K14 became localized in aggregates in basal keratinocytes. Despite the absence of a keratin cytoskeleton, there was no cytolysis. We suggest that the formation of the first embryonic cytoskeleton from soluble keratins is regulated by unknown mechanisms. Whether the premature expression of type II keratins relates to their proposed role in TNF- and Fas-mediated signalling is presently unknown.

Keratin 14 Cre transgenic mice authenticate keratin 14 as an oocyte-expressed protein

Three mouse lines expressing Cre recombinase under the control of the human K14 promoter induced specific deletion of loxP flanked target sequences in the epidermis, in tongue, and thymic epithelium of the offspring where the Cre allele was inherited from the father. Where the mother carried the Cre allele, loxP flanked sequences were completely deleted in all tissues of the offspring, even in littermates that did not inherit the Cre allele. This maternally inherited phenotype indicates that the human K14 promoter is transcriptionally active in murine oocytes and that the enzyme remains active until after fertilization, even when the Cre allele becomes transmitted to the polar bodies during meiosis. Detection of K14 mRNA by RT-PCR in murine ovaries and immunohistochemical identification of the K14 protein in oocytes demonstrates that the human K14 promoter behaves like its murine homolog, thus identifying K14 as an authentic oocytic protein.

Ultrastructure of the embryonic stem cells of the 8-day pig blastocyst before and after in vitro manipulation: development of junctional apparatus and the lethal effects of PBS mediated cell-cell dissociation

Ultrastructural examination of 8-day hatched pig blastocysts (large and small), their cultured inner cell mass (ICM), and cultured epiblast tissue (embryonic stem cells) was undertaken to assess the development of epiblast cell junctions and cytoskeletal elements. In small blastocysts, epiblast cells had no desmosomes or tight junction (TJ) connections and few organized microfilament bundles, whereas in large blastocysts the epiblast cells were connected by TJ and desmosomes with associated microfilaments. ICM isolation by immunodissection damaged the endoderm cells beneath the trophectoderm cells but did not appear to damage the epiblast cells or their associated endoderm cells. Epiblast cells in cultured ICMs were similar in character to those in the intact large blastocyst except that perinuclear microfilaments were observed. Isolated pig epiblasts, cultured for approximately 36 hr on STO feeder layers, formed a monolayer whose cells were connected by TJ, adherens junctions and desmosomes with prominent microfilament bundles running parallel to the apical cytoplasmic membranes. Perinuclear microfilaments were a consistent feature in the approximately 36 hr cultured epiblast cells. A feature characteristic of differentiation into notochordal cells, i.e., a solitary cilium, was also observed in the cultured epiblast. Exposure of the cultured epiblast cells to Ca(++)-Mg(++)-free phosphate buffered saline (PBS) for 5-10 min resulted in extensive cell blebbing and lysis. The results may indicate that pig epiblast cells could be more easily dissociated from early blastocysts ( approximately 400 microm in diameter) if immunodissection damage to the ICM can be avoided. It may be difficult, however, to establish them as embryonic stem cell lines because the cultured pig epiblast cells were easily lysed by standard cell-cell dissociation methods.

Rescuing desmoplakin function in extra-embryonic ectoderm reveals the importance of this protein in embryonic heart, neuroepithelium, skin and vasculature

Desmosomes mediate intercellular adhesion through desmosomal cadherins, which interface with plakoglobin (PG) and desmoplakin (DP) to associate with the intermediate filament (IF) cytoskeleton. Desmosomes first assemble in the E3.5 mouse trophectoderm, concomitant with establishment of epithelial polarity and appearance of a blastocoel cavity. Increasing in size and number, desmosomes continue their prominence in extra-embryonic tissues, but as development proceeds, they also become abundant in a number of embryonic tissues, including heart muscle, epidermis and neuroepithelium. Previously, we explored the functional importance of desmosomes by ablating the Dsp gene. Homozygous Dsp mutant embryos progressed through implantation, but did not survive beyond E6.5, owing to a loss or instability of desmosomes and tissue integrity. We have now rescued the extra-embryonic tissues by aggregation of tetraploid (wild-type) and diploid (Dsp mutant) morulae. These animals survive several days longer, but die shortly after gastrulation, with major defects in the heart muscle, neuroepithelium and skin epithelium, all of which possess desmosomes, as well as the microvasculature, which does not. Interestingly, although wild-type endothelial cells of capillaries do not form desmosomes, they possess unusual intercellular junctions composed of DP, PG and VE-cadherin. The severity in phenotype and the breadth of defects in the Dsp mutant embryo is greater than PG mutant embryos, substantiating redundancy between PG and other armadillo proteins (e.g. beta-catenin). The timing of lethality is similar to that of the VE-cadherin null embryo, suggesting that a participating cause of death may be a defect in vasculature, not reported for PG null embryos.

The initial phase of embryonic patterning in mammals

Although specification of the antero-posterior axis is a critical intial step in development of the fetus, it is not known either how, or at what stage in development, this process begins. Such information is vital for understanding not only normal development in mammals but also monozygotic twinning, which, at least in man, is associated with a significantly increased incidence of birth defects. According to recent studies in the mouse, specification of the fetal anteroposterior axis begins well before gastrulation, and probably even before the conceptus implants. Moreover, evidence is accruing that the origin of relevant asymmetries depends on information that is already present in the zygote before it embarks on cleavage. Hence, early development in mammals does not differ as markedly from that in other animals as has generally been assumed. Consequently, at present, the possibility of adverse effects of techniques used to assist human reproduction cannot be disregarded.

Molecular and biochemical regulation of early mammalian development

Fertilization initiates a rapid series of changes that restructures the egg into the zygote and initiates the program of early development. These changes in the cell occur while the genetic complement of the egg and sperm are in a highly condensed state and unable to participate in transcription. The egg cytoplasm, formed by the maternal genome, contains the necessary components that mediate the early restructuring of egg into zygote. These changes are mediated by a series of cytoplasmic signal transduction events initiated by the rise in [Ca2+]i caused when the sperm penetrates the egg. The structural changes that the egg undergoes are rapid and result in the extensive remodeling of this specialized cell. Protein kinase C (PKC) and calcium/calmodulin-dependent protein kinase II (CaM KII) are two pivotal signaling agents that mediate several of these rapid modifications in cell structure. Studies indicate the meiotic spindle serves as an architectural element in the egg that acts to colocalize elements from several of the key signaling pathways and may provide a means for these pathways to interact. In mammals, transcription begins earlier than in zygotes from other classes of organisms, starting several hours after fertilization in the male and female pronuclei and continuing in the embryonic nuclei. Studies indicate that nuclei undergo an initial state that is permissive for transcription, and then in Gap 2 of the two-cell embryo, enter a transcriptionally repressive state. These changes have been linked to the times during the cell cycle when the DNA is replicated, and also have been proposed as a requirement for proper initiation of the program of early development.

PKC--a pivotal regulator of early development

Oocytes, eggs and blastomeres of the embryo are special cells that undergo rapid changes in structure and function at developmental transitions. These changes are frequently regulated by cytoplasmic signaling events, particularly at the developmental transition of fertilization, because the genome is largely inactivated at this time. Protein kinase C (PKC) is a signaling agent that acts after the sperm-induced rise in calcium and has a central role in the remodeling of the structure of the egg into the zygote in many species. PKC also acts during other developmental transitions. This kinase serves as a chronometer, which can choreograph the cell's remodeling events in both space and time. Several technical advancements discussed in this review have permitted a better understanding of the actions of PKC.

HTLV-I Tax and Cytokeratin: Tax-Expressing Cells Show Morphological Changes in Keratin-Containing Cytoskeletal Networks

Human T cell leukemia virus type I (HTLV-I) has been linked to the development of an aggressive lymphoproliferative disorder (adult T cell leukemia), a chronic neurodegenerative presentation (HTLV-I-associated myelopathy/tropical spastic paraparesis) and numerous less well-defined inflammatory conditions. The viral regulatory protein Tax has been implicated in cellular transformation events leading to the onset of adult T cell leukemia. Details on the stepwise processes through which Tax induces morphological changes in cells are poorly understood. We show here that Tax can bind to a class of intermediate filaments, the cytokeratins (Ker). Tax interacts with the 1B helical coil of keratin 8, a domain critical for higher-order intermediate filament matrix formation. Expression of Tax in epithelial cells visibly altered the structural pattern of the Ker network. In a T lymphocyte cell line, induction of Tax expression resulted in increased cellular adherence/invasion of Matrigel filters. We propose that one aspect of Tax function is the induction of morphological changes in cellular cytoskeletal structures. This finding for Tax-expressing cells might be one factor contributing directly to the pathogenesis of HTLV-I disease(s). Copyright 1997 S. Karger AG, Basel

Oncogenic regulation and function of keratins 8 and 18

Keratin 8 (K8) and keratin 18 (K18) are the most common and characteristic members of the large intermediate filament gene family expressed in 'simple' or single layer epithelial tissues of the body. Their persistent expression in tumor cells derived from these epithelia has led to the wide spread use of keratin monoclonal antibodies as aids in the detection and identification of carcinomas. Oncogenes which activate ras signal transduction pathways stimulate expression of the K18 gene through transcription factors including members of the AP-1 (jun and fos) and ETS families. The persistent expression of K8 and K18 may reflect the integrated transcriptional activation of such transcription factors and, in the cases of ectopic expression, an escape from the suppressive epigenetic mechanisms of DNA methylation and chromatin condensation. Comparison of the mechanisms of transcriptional control of K18 expression with expression patterns documented in both normal and pathological conditions leads to the proposal that persistent K8 and K18 expression is a reflection of the action of multiple different oncogenes converging on the nucleus through a limited number of transcription factors to then influence the expression of a large number of genes including these keratins. Furthermore, correlation of various tumor cell characteristics including invasive behavior and drug sensitivity with K8 and K18 expression has stimulated consideration of the possible functions of these proteins in both normal development and in tumorigenesis. Recent developments in the analysis of the functions of these intermediate filament proteins provide new insights into diverse functions influenced by K8 and K18.