Subtractive hybridization reveals tissue-specific expression of ahnak during embryonic development

Intensive morphometric analysis of enormous alterations in skeletal bone system with micro-CT for AHNAK-/- mice

AHNAK has been reported to be involved in actin cytoskeleton rearrangement of some cell types, calcium homeostasis, and activation of T cells. Although the functional role of AHNAK in muscle cells, epidermis, and brain has been determined, its association with apparent clinical impairment has not been found yet. During phenotypic analysis of AHNAK knock out (KO) mice for many years, we observed that AHNAK KO mice showed very slow growth. Snouts of these animals were very short, and their bones were easily broken compared to normal mice. It is known that AHNAK is closely related to calcium. However, intensive morphological studies on phenotypes of bone have yet been reported for AHNAK. Thus, the objective of the present study was to analyze the morphology of skull, mandibular, limbs, and caudal bones of AHNAK KO mice intensively using micro-CT with many factors for various ages of these mice (6 weeks, 18 weeks, and 40 weeks). As a result, it was found that the facial region of AHNAK KO mouse showed a large difference in mandible than skull. Their both femur and tibia were shortened, and bone strength was also significantly decreased compared to normal mice. Particularly, the tail bone of AHNAK KO mice exhibited morphological abnormality by age. Taken together, these results suggest that AHNAK plays an important role in bone shape, development, and metabolism. Although our results demonstrated that AHNAK has a distinct role in bone, further investigations are needed to determine various features of bone metabolism related to AHNAK in the future.

A Systems-level Characterization of the Differentiation of Human Embryonic Stem Cells into Mesenchymal Stem Cells

Mesenchymal stem/stromal cells (MSCs) are self-renewing multipotent cells with regenerative, secretory and immunomodulatory capabilities that are beneficial for the treatment of various diseases. To avoid the issues that come with using tissue-derived MSCs in therapy, MSCs may be generated by the differentiation of human embryonic stems cells (hESCs) in culture. However, the changes that occur during the differentiation process have not been comprehensively characterized. Here, we combined transcriptome, proteome and phosphoproteome profiling to perform an in-depth, multi-omics study of the hESCs-to-MSCs differentiation process. Based on RNA-to-protein correlation, we determined a set of high confidence genes that are important to differentiation. Among the earliest and strongest induced proteins with extensive differential phosphorylation was AHNAK, which we hypothesized to be a defining factor in MSC biology. We observed two distinct expression waves of developmental HOX genes and an AGO2-to-AGO3 switch in gene silencing. Exploring the kinetic of noncoding ORFs during differentiation, we mapped new functions to well annotated long noncoding RNAs (CARMN, MALAT, NEAT1, LINC00152) as well as new candidates which we identified to be important to the differentiation process. Phosphoproteome analysis revealed ESC and MSC-specific phosphorylation motifs with PAK2 and RAF1 as top predicted upstream kinases in MSCs. Our data represent a rich systems-level resource on ESC-to-MSC differentiation that will be useful for the study of stem cell biology.

Distinct mesoderm migration phenotypes in extra-embryonic and embryonic regions of the early mouse embryo

In mouse embryo gastrulation, epiblast cells delaminate at the primitive streak to form mesoderm and definitive endoderm, through an epithelial-mesenchymal transition. Mosaic expression of a membrane reporter in nascent mesoderm enabled recording cell shape and trajectory through live imaging. Upon leaving the streak, cells changed shape and extended protrusions of distinct size and abundance depending on the neighboring germ layer, as well as the region of the embryo. Embryonic trajectories were meandrous but directional, while extra-embryonic mesoderm cells showed little net displacement. Embryonic and extra-embryonic mesoderm transcriptomes highlighted distinct guidance, cytoskeleton, adhesion, and extracellular matrix signatures. Specifically, intermediate filaments were highly expressed in extra-embryonic mesoderm, while live imaging for F-actin showed abundance of actin filaments in embryonic mesoderm only. Accordingly, Rhoa or Rac1 conditional deletion in mesoderm inhibited embryonic, but not extra-embryonic mesoderm migration. Overall, this indicates separate cytoskeleton regulation coordinating the morphology and migration of mesoderm subpopulations.

Mesothelium of the murine allantois exhibits distinct regional properties

The rodent allantois is thought to be unique amongst mammals in not having an endodermal component. Here, we have investigated the mesothelium, or outer surface, of murine umbilical precursor tissue, the allantois (∼7.25-8.5 days postcoitum, dpc) to discover whether it exhibits the properties of an epithelium. A combination of morphology, challenge with biotinylated dextran amines (BDAs), and immunohistochemistry revealed that the mesothelium of the mouse allantois exhibits distinct regional properties. By headfold stages (∼7.75-8.0 dpc), distal mesothelium was generally squamous in shape, and highly permeable to BDA challenge, whereas ventral proximal mesothelium, referred to as "ventral cuboidal mesothelium" (VCM) for the characteristic cuboidal shape of its cells, was relatively impermeable. Although "dorsal cuboidal mesothelium" (DCM) resembled the VCM in cell shape, its permeability to BDA was intermediate between the other two regions. Results of immunostaining for Zonula Occludens-1 (ZO-1) and Epithelial-cadherin (E-cadherin), together with transmission electron microscopy (TEM), suggested that impermeability in the VCM may be due to greater cellular contact area between cells and close packing rather than to maturity of tight junctions, the latter of which, by comparison with the visceral yolk sac, appeared to be rare or absent from the allantoic surface. Both VCM and DCM exhibited an ultrastructure more favorable for protein synthesis than did the distal squamous mesothelium; however, at most stages, VCM exhibited robust afadin (AF-6), whereas the DCM uniquely contained alpha-4-integrin. These observations demonstrate that the allantoic mesothelium is not a conventional epithelium but possesses regional ultrastructural, functional and molecular differences that may play important roles in the correct deployment of the umbilical cord and its associated vascular, hematopoietic, and other cell types.

A scaffold protein, AHNAK1, is required for calcium signaling during T cell activation

Engagement of the T cell antigen receptor (TCR) during antigen presentation initiates a coordinated action of a large number of signaling proteins and ion channels. AHNAK1 is a scaffold protein, highly expressed by CD4+ T cells, and is a critical component for calcium signaling. We showed that AHNAK1-deficient mice were highly susceptible to Leishmania major infection. AHNAK1-deficient CD4+ T cells responded poorly to TCR stimulation in vitro with low proliferation and low Interleukin-2 production. Furthermore, AHNAK1 deficiency resulted in a reduced calcium influx upon TCR crosslinking and subsequent poor activation of the transcription factor NFAT. AHNAK1 was required for plasma membrane expression of L-type calcium channels alpha 1S (Cav1.1), probably through its interaction with the beta regulatory subunit. Thus, AHNAK1 plays an essential role in T cell Ca2+ signaling through Cav1 channels, triggered via TCR activation; therefore, AHNAK1 is a potential target for therapeutic intervention.

The murine allantois: emerging paradigms in development of the mammalian umbilical cord and its relation to the fetus

The fertilized egg of the mammal gives rise to the embryo and its extraembryonic structures, all of which develop in intimate relation with each other. Yet, whilst the past several decades have witnessed a vast number of studies on the embryonic component of the conceptus, study of the extraembryonic tissues and their relation to the fetus have been largely ignored. The allantois, precursor tissue of the mature umbilical cord, is a universal feature of all placental mammals that establishes the vital vascular bridge between the fetus and its mother. The allantois differentiates into the umbilical blood vessels, which become secured onto the chorionic component of the placenta at one end and onto the fetus at the other. In this way, fetal blood is channeled through the umbilical cord for exchange with the mother. Despite the importance of this vascular bridge, little is known about how it is made. The aim of this review is to address current understanding of the biology of the allantois in the mouse and genetic control of its features and functions, and to highlight new paradigms concerning the developmental relationship between the fetus and its umbilical cord.

Spinal cord injury-induced up-regulation of AHNAK, expressed in cells delineating cystic cavities, and associated with neoangiogenesis

To investigate the molecular basis for the poor regenerative capacity of the mammalian central nervous system (CNS) after injury, we searched for genes whose expression was affected by an adult rat spinal cord hemi-section. Differential screening of a rat spinal cord expression library was performed using polyclonal antibodies raised against lesioned spinal cord tissue. A striking overexpression was found for ahnak, encoding a 700-kDa protein, in normal CNS present only in the blood-brain barrier (BBB) forming vascular endothelial cells. Indeed, very early after spinal cord injury (SCI), high levels of membrane-associated AHNAK are observed on non-neuronal cells invading the lesion site. With time, AHNAK distribution spreads rostrally and caudally concomitant with the process of tissue inflammation and axon degeneration, delineating the interior surface of cystic cavities, mainly in front of barrier-forming astrocytes. Strong overexpression is also observed on vascular endothelial cells reacting to BBB breakdown. Based on our detailed analysis of its spatiotemporal and cellular expression, and its previously described function in BBB, we suggest that AHNAK expression is associated with cell types displaying tissue-protective barrier properties. Our study may thus contribute to the elucidation of the precise molecular and cellular events that eventually render traumatic spinal cord tissue non-permissive for regeneration.

Ahnak/Desmoyokin is dispensable for proliferation, differentiation, and maintenance of integrity in mouse epidermis

Desmoyokin was first isolated from bovine muzzle epidermis and thought to be an epidermal desmosome-related protein. We previously demonstrated that the Desmoyokin gene is identical to the Ahnak gene, which is expressed ubiquitously and downregulated in neuroblastomas. It was assumed Ahnak/Desmoyokin was associated with epidermal cell adhesion, tumorigenesis, cell proliferation and differentiation, and embryonic development. To determine the precise biological function of Ahnak/Desmoyokin, we generated a null mutation in ES cells and mice. The resultant Ahnak/Desmoyokin-deficient ES cells normally differentiated into embryoid bodies and neural cells. The mutant mice were viable and fertile and showed no gross developmental defects. Electron microscopic examination of skin sections demonstrated that the ultrastructure of epidermal intercellular junctions, including desmosomes, of the mutant mice was indistinguishable from that of wild-type mice. Two-stage chemical skin carcinogenesis experiments showed no difference in frequency or onset of cutaneous tumor formation between wild-type and mutant mice. Moreover, no tumorigenesis was observed in other tissues and organs of mutant mice up to 2 y of age. These results lead us to conclude that Ahnak/Desmoyokin deficiency has only a minimal effect on epidermal cell adhesion, tumorigenesis, cell proliferation and differentiation, and overall mouse development.

The zebrafish moonshine gene encodes transcriptional intermediary factor 1gamma, an essential regulator of hematopoiesis

Hematopoiesis is precisely orchestrated by lineage-specific DNA-binding proteins that regulate transcription in concert with coactivators and corepressors. Mutations in the zebrafish moonshine (mon) gene specifically disrupt both embryonic and adult hematopoiesis, resulting in severe red blood cell aplasia. We report that mon encodes the zebrafish ortholog of mammalian transcriptional intermediary factor 1gamma (TIF1gamma) (or TRIM33), a member of the TIF1 family of coactivators and corepressors. During development, hematopoietic progenitor cells in mon mutants fail to express normal levels of hematopoietic transcription factors, including gata1, and undergo apoptosis. Three different mon mutant alleles each encode premature stop codons, and enforced expression of wild-type tif1gamma mRNA rescues embryonic hematopoiesis in homozygous mon mutants. Surprisingly, a high level of zygotic tif1gamma mRNA expression delineates ventral mesoderm during hematopoietic stem cell and progenitor formation prior to gata1 expression. Transplantation studies reveal that tif1gamma functions in a cell-autonomous manner during the differentiation of erythroid precursors. Studies in murine erythroid cell lines demonstrate that Tif1gamma protein is localized within novel nuclear foci, and expression decreases during erythroid cell maturation. Our results establish a major role for this transcriptional intermediary factor in the differentiation of hematopoietic cells in vertebrates.

The carboxyl-terminal ahnak domain induces actin bundling and stabilizes muscle contraction

Ahnak, a 700 kDa protein, is expressed in a variety of cells and has been implicated in different cell-type-specific functions. In the human heart, we observed an endogenous carboxyl-terminal 72 kDa ahnak fragment that copurified with myofibrillar proteins. Immunocytochemistry combined with confocal microscopy localized this fragment to the intercalated discs and close to the Z-line of cardiomyocytes. No endogenous myofibrillar ahnak fragment was observed in the skeletal muscle. We elucidated the role of the recombinant carboxyl-terminal ahnak fragment (ahnak-C2) in actin filament organization and in the function of muscle fibers. Addition of ahnak-C2 to actin filaments induced filament bundling into paracrystalline-like structures as revealed by electron microscopy. Incubation of demembranated skeletal muscle fibers with ahnak-C2 attenuated the decline in isometric force development upon repeated contraction-relaxation cycles. Our results suggest that the carboxyl-terminal ahnak domain exerts a stabilizing effect on muscle contractility via its interaction with actin of thin filaments.

Calcium Current in Rat Cardiomyocytes Is Modulated by the Carboxyl-terminal Ahnak Domain

Ahnak, a protein of 5643 amino acids, interacts with the regulatory beta-subunit of cardiac calcium channels and with F-actin. Recently, we defined the binding sites among the protein partners in the carboxyl-terminal domain of ahnak. Here we further narrowed down the beta(2)-interaction sites to the carboxyl-terminal 188 amino acids of ahnak by the recombinant ahnak protein fragments P3 (amino acids 5456-5556) and P4 (amino acids 5556-5643). The effects of these P3 and P4 fragments on the calcium current were investigated under whole-cell patch clamp conditions on rat ventricular cardiomyocytes. P4 but not P3 increased significantly the current amplitude by 22.7 +/- 5% without affecting its voltage dependence. The slow component of calcium current inactivation was slowed down by both P3 and P4, whereas only P3 slowed significantly the fast one. The composite recombinant protein fragment P3-P4 induced similar modifications to the ones induced by each of the ahnak fragments. In the presence of carboxyl-terminal ahnak protein fragments, isoprenaline induced a similar relative increase in current amplitude and shift in current kinetics. The actin-stabilizing agents, phalloidin and jasplakinolide, did not modify the effects of these ahnak protein fragments on calcium current in control conditions nor in the presence of isoprenaline. Hence, our results suggest that the functional effects of P3, P4, and P3-P4 on calcium current are mediated by targeting the ahnak-beta(2)-subunit interaction rather than by targeting the ahnak-F-actin interaction. More specifically they suggest that binding of the beta(2)-subunit to the endogenous subsarcolemmal giant ahnak protein re-primes the alpha(1C)/beta(2)-subunit interaction and that the ahnak-derived proteins relieve the beta(2)-subunit from this inhibition.

CMF608-a novel mechanical strain-induced bone-specific protein expressed in early osteochondroprogenitor cells

Microarray gene expression analysis was utilized to identify genes upregulated in primary rat calvaria cultures in response to mechanical force. One of the identified genes designated CMF608 appeared to be novel. The corresponding full-length cDNA was cloned and characterized in more details. It encodes a putative 2597 amino acid protein containing N-terminal signal peptide, six leucine-rich repeats (LRRs), and 12 immunoglobulin-like repeats, 10 of which are clustered within the C-terminus. Expression of CMF608 is bone-specific and the main type of CMF608-positive cells is mesenchymal osteochondroprogenitors with fibroblast-like morphology. These cells reside in the perichondral fibrous ring of La Croix, periosteum, endosteum of normal bone as well as in the activated periosteum and early fibrous callus generated postfracture. Expression of CMF608 is notably absent from the regions of endochondral ossification. Mature bone cell types do not produce CMF608 with the exception of chondrocytes of the tangential layer of the articular cartilage, which are thought to be under constant mechanical loading. Ectopic expression of CMF608 in HEK293T cells shows that the protein is subjected to post-translational processing and its N-terminal approximately 90 kDa polypeptide can be found in the conditioned medium. Ectopic expression of either the full-length cDNA of CMF608 or of its N-terminal region in CMF608-negative ROS17/2.8 rat osteosarcoma cells results in transfected clones displaying increased proliferation rate and the characteristics of less-differentiated osteoblasts compared to the control cells. Our data indicate that CMF608 is a unique marker of early osteochondroprogenitor cells. We propose that it could be functionally involved in maintenance of the osteochondroprogenitor cells pool and its down-regulation precedes terminal differentiation.

AHNAK interaction with the annexin 2/S100A10 complex regulates cell membrane cytoarchitecture

Remodelling of the plasma membrane cytoarchitecture is crucial for the regulation of epithelial cell adhesion and permeability. In Madin-Darby canine kidney cells, the protein AHNAK relocates from the cytosol to the cytosolic surface of the plasma membrane during the formation of cell–cell contacts and the development of epithelial polarity. This targeting is reversible and regulated by Ca²⁺-dependent cell–cell adhesion. At the plasma membrane, AHNAK associates as a multimeric complex with actin and the annexin 2/S100A10 complex. The S100A10 subunit serves to mediate the interaction between annexin 2 and the COOH-terminal regulatory domain of AHNAK. Down-regulation of both annexin 2 and S100A10 using an annexin 2–specific small interfering RNA inhibits the association of AHNAK with plasma membrane. In Madin-Darby canine kidney cells, down-regulation of AHNAK using AHNAK-specific small interfering RNA prevents cortical actin cytoskeleton reorganization required to support cell height. We propose that the interaction of AHNAK with the annexin 2/S100A10 regulates cortical actin cytoskeleton organization and cell membrane cytoarchitecture.

Investigation into a role for the primitive streak in development of the murine allantois

Despite its importance as the source of one of three major vascular systems in the mammalian conceptus, little is known about the murine allantois, which will become the umbilical cord of the chorio-allantoic placenta. During gastrulation, the allantois grows into the exocoelomic cavity as a mesodermal extension of the posterior primitive streak. On the basis of morphology, gene expression and/or function, three cell types have been identified in the allantois: an outer layer of mesothelial cells, whose distal portion will become transformed into chorio-adhesive cells, and endothelial cells within the core. Formation of endothelium and chorio-adhesive cells begins in the distal region of the allantois, farthest from the streak. Over time, endothelium spreads to the proximal allantoic region, whilst the distal outer layer of presumptive mesothelium gradually acquires vascular cell adhesion molecule (VCAM1) and mediates chorio-allantoic union. Intriguingly, the VCAM1 domain does not extend into the proximal allantoic region. How these three allantoic cell types are established is not known, although contact with the chorion has been discounted. In this study, we have investigated how the allantois differentiates, with the goal of discriminating between extrinsic mechanisms involving the primitive streak and an intrinsic role for the allantois itself. Exploiting previous observations that the streak contributes mesoderm to the allantois throughout the latter's early development, microsurgery was used to remove allantoises at ten developmental stages. Subsequent whole embryo culture of operated conceptuses resulted in the formation of regenerated allantoises at all time points. Aside from being generally shorter than normal, none of the regenerates exhibited abnormal differentiation or inappropriate cell relationships. Rather, all of them resembled intact allantoises by morphological, molecular and functional criteria. Moreover, fate mapping adjacent yolk sac and amniotic mesoderm revealed that these tissues and their associated bone morphogenetic protein 4 (BMP4) did not contribute to restoration of allantoic outgrowth and differentiation during allantoic regeneration. Thus, on the basis of these observations, we conclude that specification of allantoic endothelium, mesothelium and chorio-adhesive cells does not occur by a streak-related mechanism during the time that proximal epiblast travels through it and is transformed into allantoic mesoderm. Rather, all three cell-types are established by mechanisms intrinsic to the allantois, and possibly include roles for cell age and cell position. However, although chorio-adhesive cells were not specified within the streak, we discovered that the streak nonetheless plays a role in establishing VCAM1's expression domain, which typically began and was thereafter maintained at a defined distance from the primitive streak. When allantoises were removed from contact with the streak, normally VCAM1-negative proximal allantoic regions acquired VCAM1. These results suggested that the streak suppresses formation of chorio-adhesive cells in allantoic mesoderm closest to it. Together with previous results, findings presented here suggest a model of differentiation of allantoic mesoderm that invokes intrinsic and extrinsic mechanisms, all of which appear to be activated once the allantoic bud has formed.

cdx4 mutants fail to specify blood progenitors and can be rescued by multiple hox genes

Organogenesis is dependent on the formation of distinct cell types within the embryo. Important to this process are the hox genes, which are believed to confer positional identities to cells along the anteroposterior axis. Here, we have identified the caudal-related gene cdx4 as the locus mutated in kugelig (kgg), a zebrafish mutant with an early defect in haematopoiesis that is associated with abnormal anteroposterior patterning and aberrant hox gene expression. The blood deficiency in kgg embryos can be rescued by overexpressing hoxb7a or hoxa9a but not hoxb8a, indicating that the haematopoietic defect results from perturbations in specific hox genes. Furthermore, the haematopoietic defect in kgg mutants is not rescued by scl overexpression, suggesting that cdx4 and hox genes act to make the posterior mesoderm competent for blood development. Overexpression of cdx4 during zebrafish development or in mouse embryonic stem cells induces blood formation and alters hox gene expression. Taken together, these findings demonstrate that cdx4 regulates hox genes and is necessary for the specification of haematopoietic cell fate during vertebrate embryogenesis.

Expression of the giant protein AHNAK (desmoyokin) in muscle and lining epithelial cells

Here we report a detailed analysis of the expression and localization of the giant protein AHNAK in adult mouse tissues. We show that AHNAK is widely expressed in muscle cells, including cardiomyocytes, smooth muscle cells, skeletal muscle, myoepithelium, and myofibroblasts. AHNAK is also specifically expressed in epithelial cells of most lining epithelium, but is absent in epithelium with more specialized secretory or absorptive functions. In all adult tissues, the main localization of AHNAK is at the plasma membrane. A role for AHNAK in the specific organization and the structural support of the plasma membrane common to muscle and lining epithelium is discussed.

Multiple developmental roles of Ahnak are suggested by localization to sites of placentation and neural plate fusion in the mouse conceptus

Ahnak is a gigantic (700 kD) phosphoprotein with a unique structure whose expression and cellular localization are dynamically regulated during cell cycle progression. Here, we report that Ahnak is localized to sites of major morphogenesis during mouse placentation and neurulation. Ahnak was found in: (i) derivatives of trophectoderm, including chorionic ectoderm prior to and during union with the ectoplacental cone, presumptive syncytiotrophoblast cells in the chorionic labyrinth, and giant cells at the trophoblast-uterine interface; (ii) the allantois prior to, during, and after union with the chorion; and (iii) the tips of the neural plate during formation of the neural tube. On the basis of these observations, we suggest that Ahnak may play heretofore unrecognized roles in tissue union during normal mouse development.

The carboxyl-terminal region of ahnak provides a link between cardiac L-type Ca2+ channels and the actin-based cytoskeleton

Ahnak is a ubiquitously expressed giant protein of 5643 amino acids implicated in cell differentiation and signal transduction. In a recent study, we demonstrated the association of ahnak with the regulatory beta2 subunit of the cardiac L-type Ca2+ channel. Here we identify the most carboxyl-terminal ahnak region (aa 5262-5643) to interact with recombinant beta2a as well as with beta2 and beta1a isoforms of native muscle Ca2+ channels using a panel of GST fusion proteins. Equilibrium sedimentation analysis revealed Kd values of 55 +/- 11 nM and 328 +/- 24 nM for carboxyl-terminal (aa 195-606) and amino-terminal (aa 1-200) truncates of the beta2a subunit, respectively. The same carboxyl-terminal ahnak region (aa 5262-5643) bound to G-actin and cosedimented with F-actin. Confocal microscopy of human left ventricular tissue localized the carboxyl-terminal ahnak portion to the sarcolemma including the T-tubular system and the intercalated disks of cardiomyocytes. These results suggest that ahnak provides a structural basis for the subsarcolemmal cytoarchitecture and confers the regulatory role of the actin-based cytoskeleton to the L-type Ca2+ channel.

Protein kinase B phosphorylates AHNAK and regulates its subcellular localization

AHNAK is a ubiquitously expressed giant phosphoprotein that was initially identified as a gene product subject to transcriptional repression in neuroblastoma. AHNAK is predominantly nuclear in cells of nonepithelial origin, but is cytoplasmic or associated with plasma membrane in epithelial cells. In this study we show that the extranuclear localization of AHNAK in epithelial cells depends on the formation of cell-cell contacts. We show that AHNAK is a phosphorylation substrate of protein kinase B (PKB) in vitro and in vivo. Nuclear exclusion of AHNAK is mediated through a nuclear export signal (NES) in a manner that depends on the phosphorylation of serine 5535 of AHNAK by PKB, a process that also plays a major role in determining extranuclear localization of AHNAK. AHNAK is a new PKB substrate whose function, though unknown, is likely to be regulated by its localization, which is in turn regulated by PKB.