Dexamethasone does not interfere with hormone-sensitive PI hydrolysis

Serum, but not epidermal growth factor (EGF), stimulated the release of radiolabeled inositol phosphates from human embryo palate mesenchyme (HEPM) cells prelabeled with [3H]-myoinositol. Pretreatment of cells with 10(-6) M dexamethasone (DEX) for 48 h had no effect on the release of inositol phosphates in response to serum. Furthermore, although treatment of the glucocorticoid-sensitive A/J strain of mouse embryo palate mesenchyme (MEPM) cells with 10(-6) M DEX inhibited their proliferation by 40%, it had no effect on the activity of phospholipase(s) C. However, DEX did enhance the incorporation of [3H]-myoinositol into membrane lipids. We interpret these data to mean that 1) serum factors enhance metabolism of inositol lipids in HEPM cells, 2) DEX does not interfere with the primary events by which agonists utilize metabolism of inositol lipids as a mechanism for transmembrane signaling, and 3) DEX may affect synthesis of phosphoinositides, as reported by Grove et al. (Biochem. Biophys. Res. Commun. 110:200-207, 1983; J. Craniofac. Genet. Dev. Biol. Suppl. 2:285-292, 1986).     

Sites of serotonin uptake in epithelia of the developing mouse palate, oral cavity, and face: possible role in morphogenesis

With the method of whole mouse embryo culture, together with immunocytochemistry with an antiserum to serotonin (5-HT), sites of 5-HT uptake were found to be transiently expressed in the epithelia of the developing palate, tongue, nasal septum, and maxillary and mandibular prominences during the period of active morphogenesis (embryonic days 12-14; or E12-14). These sites had the ability to take up 5-HT when added to the culture medium in the presence of the MAO inhibitor nialamide and an antioxiant, L-cysteine (NC), and could also be seen after exposure of embryos to the 5-HT precursor L-tryptophan (L-TRP) + NC. These sites were also visible after culturing embryos without any additives, which may have been due to the presence of L-TRP in one component of the culture medium (DMEM) or to 5-HT itself, which is present in relatively high amounts in fetal calf serum. At E12-13, the appearance of 5-HT immunoreactivity (IR) at these sites after treatment with 5-HT + NC was blocked by the 5-HT uptake inhibitor fluoxetine, providing further evidence that these are true sites of 5-HT uptake. However, fluoxetine did not completely block the appearance of these sites in E14 embryos after 5-HT + NC or L-TRP + NC although it was effective with NC alone. This finding could mean that at E14 5-HT uptake into these sites occurs by mechanisms not completely blocked by fluoxetine or that there is some limited capacity for 5-HT synthesis. Taken together with results from previous studies where 1) 5-HT has been reported to stimulate palatal shelf reorientation and palatal mesenchyme cell motility in vitro [Wee et al., J Embryol Exp Morphol 53:75-90, 1979; Zimmerman et al., J Craniofac Genet Dev Biol 3:371-385, 1983] and 2) long-term culturing of mouse embryos in the presence of 5-HT or fluoxetine has been shown to cause malformations of the craniofacial region (Lauder, Thomas, and Sadler, in preparation), the results of the present study suggest that 5-HT could act as a developmental signal in the palate, oral cavity, and face during the period of active morphogenesis.     

Epithelial bridging of the primary palate: I. Characterization of sub-cultured epithelial cells

Primary palatogenesis involves an intricate array of events. Cell migration, proliferation, differentiation, programmed death, and fusion occur. Prior to fusion, the morphology of the epithelium undergoes marked changes. Epithelial projections form and extend across the fusion site attaching by filopodia to the opposite prominence. By appearance, the epithelium plays a critical role in facial development. In order to monitor epithelial activities, a study was done to isolate and characterize epithelial cells derived from the primary palate. The primary palate was microdissected from day 13 Sprague-Dawley rat embryos, and the epithelium and mesenchyme were separated by enzymatic digestion with a 3% trypsin-pancreatin solution (3:1). All explants were cultured in Dulbecco's modified Eagle's medium (DMEM) and Ham's F-12 medium (1:1) supplemented with 10% fetal calf serum (FCS), 20 ng/ml epidermal growth factor (EGF), and antibiotics. Explant cells were gathered by trypsin harvesting and sub-cultured. These sub-cultured cells were further characterized. Transmission and scanning electron microscopy showed that the cells retained many morphological features observed in vivo. In passaged cells, type IV collagen, laminin, and cytokeratins were visualized by immunocytochemistry. Gel electrophoresis analysis of the water-insoluble extracts demonstrated major bands of proteins of 50 kD and 44 kD that were synthesized by the epithelial cells but not by the mesenchymal cells. These cytokeratin types are suggestive of a simple undifferentiated embryonic epithelium. The effect of all-trans retinoic acid (RA) on cell number and [3H]-proline incorporation was assessed. At [10(-4)M] and [10(-6)M] retinoic acid resulted in significant inhibition in cell proliferation and amount of proline incorporated, with the greater inhibition occurring in the mesenchymal cells. In the concentrations studied, retinoic acid has an inhibitory effect on the two differently derived cell types. This study established that sub-cultured epithelial cells maintain their phenotype and can be used to study fusion processes. Part 2 will demonstrate how the morphology of the epithelial cells can be modified to produce the changes that are observed during fusion of the primary palate.     

Tubulin folding cofactor D is a microtubule destabilizing protein

A rapid switch between growth and shrinkage at microtubule ends is fundamental for many cellular processes. The main structural components of microtubules, the alphabeta-tubulin heterodimers, are generated through a complex folding process where GTP hydrolysis [Fontalba et al. (1993) J. Cell Sci. 106, 627-632] and a series of molecular chaperones are required [Sternlicht et al. (1993) Proc. Natl. Acad. Sci. USA 90, 9422-9426; Campo et al. (1994) FEBS Lett. 353, 162-166; Lewis et al. (1996) J. Cell Biol. 132, 1-4; Lewis et al. (1997) Trends Cell Biol. 7, 479-484; Tian et al. (1997) J. Cell Biol. 138, 821-823]. Although the participation of the cofactor proteins along the tubulin folding route has been well established in vitro, there is also evidence that these protein cofactors might contribute to diverse microtubule processes in vivo [Schwahn et al. (1998) Nature Genet. 19, 327-332; Hirata et al. (1998) EMBO J. 17, 658-666; Fanarraga et al. (1999) Cell Motil. Cytoskel. 43, 243-254]. Microtubule dynamics, crucial during mitosis, cellular motility and intracellular transport processes, are known to be regulated by at least four known microtubule-destabilizing proteins. OP18/Stathmin and XKCM1 are microtubule catastrophe-inducing factors operating through different mechanisms [Waters and Salmon (1996) Curr. Biol. 6, 361-363; McNally (1999) Curr. Biol. 9, R274-R276]. Here we show that the tubulin folding cofactor D, although it does not co-polymerize with microtubules either in vivo or in vitro, modulates microtubule dynamics by sequestering beta-tubulin from GTP-bound alphabeta-heterodimers.     

Epithelial-mesenchymal transformation is the mechanism for fusion of the craniofacial primordia involved in morphogenesis of the chicken lip

We have previously demonstrated that epithelial-mesenchymal transformation (EMT) brings about TGF beta 3-induced confluence of craniofacial primordia that derive from the maxillary processes and give rise to the avian palate. The upper lip of the chick embryo forms by confluence of primordia also derived from the maxillary processes, but in this case, they fuse with the intermaxillary segment of the nasofrontal process. Here, we ask whether the bilateral epithelial seams formed when these primordia contact each other in vivo are removed by apoptosis (as formerly was believed to occur in developing palate) or by EMT. We found that, as is the case in the palate, the periderm of the two-layered embryonic epithelium begins to slough shortly before these primordia fuse, bringing the basal epithelial cells into close contact. We show by TUNEL staining and confirm by TEM that apoptosis occurs only in periderm. TEM reveals that basal epithelial cells contacting each other to form the midline seam produce numerous desmosomes with each other. Then, basement membrane begins to disappear, numerous filopodia extend from the basal surfaces of epithelial cells, the space between them enlarges, and the seam breaks apart, leaving mesenchymal cells in its wake. Transformation of the carboxyfluorescein (CCFSE)-labeled epithelial seam is demonstrated in vivo by detection of CCFSE bodies in mesenchymal cells that replace it. This demonstration of EMT in avian lip development lays important groundwork for understanding the causes of human cleft lip and analyzing the mechanism of action of growth factors, such as SHH and BMPs, that have been shown (J. A. Helms et al., 1997, Dev. Biol. 187, 25-35) to be involved in avian lip confluence.     

Dll3 pudgy mutation differentially disrupts dynamic expression of somite genes

Mutations in the notch ligand delta-like 3 have been identified in both the pudgy mouse (Dll3(pu); Kusumi et al.: Nat Genet 19:274-278, 1998) and the human disorder spondylocostal dysostosis (SCD; Bulman et al.: Nat Genet 24:438-441, 2000), and a targeted mutation has been generated (Dll3(neo); Dunwoodie et al.: Development 129:1795-1806, 2002). Vertebral and rib malformations deriving from defects in somitic patterning are key features of these disorders. In the mouse, notch pathway genes such as Lfng, Hes1, Hes7, and Hey2 display dynamic patterns of expression in paraxial mesoderm, cycling in synchrony with somite formation (Aulehla and Johnson: Dev Biol 207:49-61, 1999; Forsberg et al.: Curr Biol 8:1027-1030, 1998; Jouve et al.: Development 127:1421-1429, 2000; McGrew et al.: Curr Biol 8:979-982, 1998; Nakagawa et al.: Dev Biol 216:72-84, 1999). We report here that the Dll3(pu) mutation has different effects on the expression of cycling (Lfng and Hes7) and stage-specific genes (Hey3 and Mesp2). This suggests a more complex situation than a single oscillatory mechanism in somitogenesis and provides an explanation for the unique radiological features of the human DLL3-type of SCD.     

Influence of chromatographic fractions of extracts derived from bovine neural retina on newt (Notophthalmus viridescens) lens regeneration in vitro

Removal of the ocular lens in adult newts (Notophthalmus viridescens) is followed by a series of cellular events leading to regeneration of a new lens by cell type conversion of pigmented iris epithelial cells at the dorsal pupillary margin (Yamada, Curr. Top. Dev. Biol. 2:247-283, 1967). Following depigmentation and five to seven cell divisions, iris epithelial cells redifferentiate into lens fiber cells and synthesize crystallin proteins (Yamada, Curr. Top. Dev. Biol. 2:247-283, 1967). This process is dependent upon neural retina in vivo (Stone, Anat. Rec. 131:151-172, 1958; Reyer, Dev. Biol. 14:214-225, 1966) and in vitro (Yamada et al., Differentiation 1:65-82, 1973). Acting on the hypothesis that the role of the neural retina is to promote passage of iris epithelial cells through the requisite number of cell cycles which will then allow them to redifferentiate as lens fiber cells (Yamada, in: Cell Biology of the Eye. Academic Press, New York, 1982), we undertook testing of the effects of eye-derived mitogenic substances, as well as other mitogens, on regeneration of lens from iris in organ culture. We have previously defined a critical period for the retinal influence in vivo and in vitro, and have shown that crude extracts of retina can enhance regeneration of lenses in culture (Connelly et al., J. Exp. Zool., 240:343-351, 1986). In this paper, we report on the lens regeneration enhancing activity (LRA) of more highly purified fractions of the retinal extracts. Heparin-sepharose chromatography of the crude retinal extract yields three fractions (Courty et al., Biochemie 67:265-269, 1985) called EDGF I, II, and III. EDGF I and II have affinity for heparin, while EDGF III does not. In our bioassay, LRA appears only in the EDGF III fraction. Dialysis of EDGF III against 0.1 N acetic acid yields a fraction which has affinity for cibacron blue sepharose (eluting at 2.15 M salt) and also has significant LRA. Because insulin at high doses has a marginal effect on lens regeneration in culture (Williams and McGlinn, Am. Zool. 19:923, 1979; Connelly, Differentiation 16:85-91, 1980), we tested IGF-I. Because of the putative neurotrophic effects of transferrin (Tf) (Mescher and Munaim, J. Exp. Zool., 230:485-490, 1986), we tested Tf for its ability to enhance regeneration of the lens in culture. IGF-I seems to have an enhancing effect on lens regeneration; Tf does not.     

Polyploid tissues in the nematode Caenorhabditis elegans

During larval development, the number of somatic nuclei in C. elegans hermaphrodites increases from 558 to 959 (J. E. Sulston and H. R. Horvitz, Dev. Biol. 56, 110-156, 1977; J. E. Sulston et al., Dev. Biol. 100, 64-119, 1983). At the same time, the animals increase about 60-fold in volume. We have measured the DNA contents of several classes of nuclei by quantitating the fluorescence of Hoescht 33258 stained DNA (D. G. Albertson et al., Dev. Biol. 63, 165-178, 1978). Probably all embryonic nuclei, including those of neurons, muscles, hypodermis, and intestine, are diploid at hatching. Neurons, muscles, and nondividing hypodermal nuclei remain diploid throughout larval development. The DNA content of the intestinal nuclei doubles at the end of each larval stage, reaching 32C by the adult stage. New hypodermal cells, generated by division of seam cells in the larval stages, undergo an additional round of DNA replication before fusing with the major syncytium (hyp7, Sulston et al., 1983). Thus the larval hyp7 syncytium comprises a fixed number of diploid embryonic nuclei plus an increasing number of tetraploid postembryonic nuclei. Some of the endoreduplications that occur in the intestinal and hypodermal lineages of C. elegans may correspond to nuclear or cellular divisions in another nematode Panagrellus redivivus (P. W. Sternberg and H. R. Horvitz, Dev. Biol. 93, 181-205, 1982).     

Structural and biochemical studies on four sex-linked chorion mutants of Drosophila melanogaster

Four female-sterile mutants, fs(1)K451, fs(1)K1214, fs(1)K575TS, and fs(1)384, were studied in terms of chorion structure and chorion protein composition. The first three of these mutants cause morphological defects, ie, substantial underproduction and disruption of the endochorion, correlated with underproduction of the six major chorion proteins, s15-s38; the phenotypes are consistent with the observation that these mutants interfere with amplification of the major chorion genes that encode the s15-s38 proteins [Orr et al.: Proc Natl Acad Sci USA 81:3773-3777, 1984; Komitopoulou et al.: Dev Genet 7:75-80, 1986]. The fourth mutant, fs(1)384, and its alleles do not interfere with production of the major chorion proteins and the morphologically detectable bulk of the endochorion but lead to failure of endochorionic organization. Apparently this complementation group is responsible for a minor chorion product, which is evidently important morphogenetically and which is processed posttranslationally in a complex manner [Bauer and Waring: Dev Biol 121:349-358, 1987].     

Medial edge epithelium transforms to mesenchyme after embryonic palatal shelves fuse

The disappearance of palatal medial edge epithelium (MEE) after fusion of secondary palatal shelves is often cited as a classical example of embryonic remodeling by programmed cell death. We reinvestigated this phenomenon in 16-day rat embryos, using light and electron microscopy. We confirm reports that the periderm of the two-layered MEE begins to slough after shelves assume horizontal positions. In vitro, peridermal cells are not able to slough and are trapped during the adhesion process. In vivo, however, surface cells shed before the shelves in the anterior palate adhere, allowing junctions to form between opposing basal epithelial cells. Midline seams so formed consist of two layers of basal cells, all of which appear healthy. Even though its cells are dividing, growth of the seam fails to keep pace with palatal growth and it thins to one layer of cells, and then breaks up into small islands. The basal lamina disappears and elongating MEE cells extend filopodia into adjacent connective tissue. Electron micrographs reveal transitional steps in loss of epithelial characteristics and gain of fibroblast-like features by transforming MEE cells. One such feature, observed with the aid of immunofluorescence, is the turn of the mesenchymal cytoskeletal protein, vimentin. No cell death or macrophages are observed after adhesion and thinning over most of the palate. These data indicate that MEE is an ectoderm that retains the ability to transform into mesenchymal cells. Epithelial-mesenchymal transformation may be expressed in other embryonic remodelings (R.L. Trelstad, A. Hayashi, K. Hayashi, and P.K. Donahue, 1982, Dev. Biol. 92, 27), resulting in heretofore unsuspected conservation of embryonic cell populations.     

Heterogeneous nuclear ribonucleoprotein-H plays a suppressive role in visceral myogenesis

Mouse embryonic mesenchymal cells undergo spontaneous smooth muscle (SM) differentiation upon spreading/elongation in culture (Relan et al., J. Cell Biol. 147 (1999) 1341; Yang et al., Development 125 (1998) 2621; Yang et al., Development 126 (1999) 3027). Using these cells we generated a subtracted cDNA library to identify potential suppressors of SM myogenesis. One of the differentially expressed genes was heterogeneous nuclear ribonucleoprotein-H (hnRNP-H), which is involved in pre-mRNA alternative splicing. hnRNP-H was highly expressed in mesenchymal cells prior to the onset of SM differentiation, but its expression rapidly decreased in mesenchymal cells undergoing SM myogenesis. In vivo, the drop in hnRNP-H expression was restricted to visceral SM cells. Antisense oligodeoxynucleotide and antisense RNA were used to inhibit hnRNP-H synthesis in SM-differentiating mesenchymal cells and in embryonic lung explants. A decrease in hnRNP-H levels resulted in upregulation of SM-specific gene expression and increased bronchial SM development in lung explants. hnRNP-H overexpression in cell cultures had the opposite effect. These studies, therefore, indicate a novel role for hnRNP-H in the control of visceral myogenesis.     

Stochastic models of telomere shortening

Shortening of chromosome ends, known as telomeres, is one of the supposed mechanisms of cellular aging and death. We provide a probabilistic analysis of the process of loss of telomere ends. The first work concerned with that issue is the paper by Levy et al. [J. Molec. Biol. 225 (1992) 951-960]. Their deterministic model reproduced the observed frequencies of viable cells in the in vitro experiments. Arino et al. [J. Theor. Biol. 177 (1995) 45-57] reformulated the model of Levy et al. (1992) in the terms of branching processes with denumerable type space. In the present paper, the mathematical results of Arino et al. (1995) are extended to the case in which cell death is present, in cells with telomeres above and below the critical threshold of length, generally with differing probabilities. Both exact and asymptotic results are provided, as well as a discussion of biological relevance of the results.     

Cloning and regulation of Schizosaccharomyces pombe thi2, a gene involved in thiamine biosynthesis.

Biosyntheses of the pyrimidine and thiazole moieties of the thiamine molecule occur by separate pathways. In Schizosaccharomyces pombe, a gene, thi2, is responsible for thiazole synthesis [Schweingruber et al., Curr. Genet. 19 (1991) 249-254]. We have cloned a 3.1-kb genomic S. pombe fragment which can functionally complement a thi2 mutant. The fragment maps genetically at the thi2 site, indicating that it carries thi2. As shown by Northern hybridization analysis, the appearance of thi2 mRNA levels is repressed when cells are grown in the presence of thiamine and 5-(2-hydroxyethyl)-4-methylthiazole. The thi3 gene involved in the biosynthesis of the pyrimidine moiety, is also regulated by thiamine [Maundrell, J. Biol. Chem. 265 (1990) 10857-10864; Schweingruber et al., Curr. Genet. 19 (1991) 249-254]. We previously identified and analyzed four regulatory genes (tnr1, tnr2, tnr3, and thi1) that are responsible for the regulation of thi3 [Schweingruber et al., Genetics (1992) in press]. Mutants defective in these regulatory genes affect expression of thi2 in a similar way to thi3. This indicates that biosynthesis of the pyrimidine and thiazole moieties are under common genetic control in S. pombe.     

Transduction of the chemotactic signal to the actin cytoskeleton of Dictyostelium discoideum

Dictyostelium discoideum amebae chemotax toward folate during vegetative growth and toward extracellular cAMP during the aggregation phase that follows starvation. Stimulation of starving amebae with extracellular cAMP leads to both actin polymerization and pseudopod extension (Hall et al., 1988, J. Cell. Biochem. 37, 285-299). We have identified an actin nucleation activity (NA) from starving amebae that is regulated by cAMP receptors and controls actin polymerization (Hall et al., 1989, J. Cell Biol., in press). We show here that NA from vegetative cells is also regulated by chemotactic receptors for folate. Our studies indicate that NA is an essential effector in control of the actin cytoskeleton by chemotactic receptors. Guided by a recently proposed model for signal transduction from the cAMP receptor (Snaar-Jagalska et al., 1988, Dev. Genet. 9, 215-225), we investigated which of three signaling pathways activates the NA effector. Treatment of whole cells with a commercial pertussis toxin preparation (PT) inhibited cAMP-stimulated NA. However, endotoxin contamination of the PT appears to account for this effect. The synag7 mutation and caffeine treatment do not inhibit activation of NA by cAMP. Thus, neither activation of adenylate cyclase nor a G protein sensitive to PT treatment of whole cells is necessary for the NA response. Actin nucleation activity stimulated with folate is normal in vegetative fgdA cells. However, cAMP suppresses rather than activates NA in starving fgdA cells. This indicates that the components of the actin nucleation effector are present and that a pathway regulating the inhibitor(s) of nucleation remains functional in starving fgdA cells. The locus of the fgdA defect, a G protein implicated in phospholipase C activation, is directly or indirectly responsible for transduction of the stimulatory chemotactic signal from cAMP receptors to the nucleation effector in Dictyostelium.     

High levels of GM1-ganglioside beta-galactosidase in the salivary glands and GM1-like-ganglioside storage in parotids of deficient mice.

We have previously demonstrated high levels of GM1-ganglioside beta-galactosidase (beta-gal) in the salivary glands of Swiss-Webster mice (Nowroozi et al., J Craniofac Genet Dev Biol 18:51, 1998), and suggested that this activity reflects an important role for the lysosome in catabolism of salivary glycoconjugates. Here, we characterized and compared activities of lysosomal glycosidases among the salivary glands, spleen, and muscle of C57BL/6 mice, beta-gal hexosaminidase, and beta-glucuronidase activities are high in all three glands relative to muscle. Enzyme activities in the sublingual gland were substantially higher than in the submandibular and parotid glands. Spleen displays levels of activity that are comparable or higher (for beta-glucuronidase) than those in the salivary glands, whereas muscle displays substantially lower levels of these lysosomal glycosidases. In order to investigate the role of beta-gal in the salivary glands, we further characterized the salivary phenotype of knock-out mice deficient in this enzyme, mimicking human GM1-gangliosidosis. In contrast with the relative levels of beta-gal specific-activity among the salivary glands, only the parotid developed severe, generalized, degenerative histopathological changes in beta-gal-deficient knock-out mice. GM1-like-ganglioside, typically found at high levels only in the nerve tissue, where its exact function is still not clear, was demonstrated in storage vacuoles of the parotid glands of the deficient mice by binding of cholera toxin subunit B. Thus, beta-gal activity observed in the parotid gland most likely reflects its role in GM1-ganglioside catabolism, and this ganglioside, never previously reported in the salivary glands, may have a role in parotid exocrine secretory functions. beta-gal may also serve in secretory glycoprotein catabolism in other salivary glands, but this function may be non-essential for these glands.     

Quantification of random mutations in the mitochondrial genome

Mitochondrial DNA (mtDNA) mutations contribute to the pathology of a number of age-related disorders, including Parkinson disease [A. Bender et al., Nat. Genet. 38 (2006) 515,Y. Kraytsberg et al., Nat. Genet. 38 (2006) 518], muscle-wasting [J. Wanagat, Z. Cao, P. Pathare, J.M. Aiken, FASEB J. 15 (2001) 322], and the metastatic potential of cancers [K. Ishikawa et al., Science 320 (2008) 661]. The impact of mitochondrial DNA mutations on a wide variety of human diseases has made it increasingly important to understand the mechanisms that drive mitochondrial mutagenesis. In order to provide new insight into the etiology and natural history of mtDNA mutations, we have developed an assay that can detect mitochondrial mutations in a variety of tissues and experimental settings [M. Vermulst et al., Nat. Genet. 40 (2008) 4, M. Vermulst et al., Nat. Genet. 39 (2007) 540]. This methodology, termed the Random Mutation Capture assay, relies on single-molecule amplification to detect rare mutations among millions of wild-type bases [J.H. Bielas, L.A. Loeb, Nat. Methods 2 (2005) 285], and can be used to analyze mitochondrial mutagenesis to a single base pair level in mammals.     

Expression of human salivary protein genes

Human proline-rich proteins (PRPs) are polymorphic, homologous in sequence, and linked in a cluster called the human salivary protein complex (SPC). Recently this complex was localized to human chromosome band 12p13.2 (Mamulaet al., Cytogenet. Cell Genet. 39:279, 1985). We have isolated a PRP cDNA, EO27, from a human parotid gland library, identified it by DNA sequencing, and used it to study the molecular and cellular biology of PRP production. Cell-free translation and mRNA characterization with EO27 indicate that the numerous PRPs seen in saliva are produced from relatively few, large precursors, probably by posttranslational cleavage. This supports an hypothesis originally proposed by Friedman and Karn in 1977 (Am. J. Hum. Genet. 29:44A;Biochem. Genet. 15:549) and later supported by biochemical studies (Karnet al., Biochem Genet. 17:1061, 1979) and molecular studies (Mamulaet al., Fed. Proc. 43:1522, 1984; Maedaet al., J. Biol. Chem. 260:1123, 1985). EO27 was also used in this study to localize PRP mRNA production to the acinar cells of the parotid gland byin situ hybridization.     

Connexons and cell adhesion: a romantic phase

Recent evidence indicates, that gap junction forming proteins do not only contribute to intercellular communication (Kanno and Saffitz in Cardiovasc Pathol 10:169-177, 2001; Saez et al. in Physiol Rev 83:1359-1400, 2003), ion homeostasis and volume control (Goldberg et al. in J Biol Chem 277:36725-36730, 2002; Saez et al. in Physiol Rev 83:1359-1400, 2003). They also serve biological functions in a mechanical sense, supporting adherent connections between neighbouring cells of epithelial and non-epithelial tissues (Clair et al. in Exp Cell Res 314:1250-1265, 2008; Shaw et al. in Cell 128:547-560, 2007), where they stabilize migratory pathways in the developing central nervous system (Elias et al. in Nature 448:901-907, 2007; Malatesta et al. in Development 127:5253-5263, 2000; Noctor et al. in Nature 409:714-720, 2001; Rakic in Brain Res 33:471-476, 1971; J Comp Neurol 145:61-83 1972; Science 241:170-176, 1988), or mediate polarized movements and directionality of neural crest cells during organogenesis (Kirby and Waldo in Circ Res 77:211-215, 1995; Xu et al. in Development 133:3629-3639, 2006). Since, most data describing adhesive properties of gap junctions delt with connexin 43 (Cx43) (Beardslee et al. in Circ Res 83:629-635, 1998), we will focus our brief review on this isoform.