A novel conditional knockout strategy applied to serotonin receptors

Here we demonstrate the feasibility of a doubly regulatable transgenic mouse design that allows for gene manipulation by both Cre-recombinase and the tetracycline inducible system. Using a knock-in strategy to insert both elements of the tetracycline inducible system and a neomycin (neo) cassette flanked by loxP sequences (floxed) into the wild-type locus, we generated mice that express the 5-HT(1B) receptor in a conditional manner. In the presence of a floxed neo-cassette, receptor expression was silenced. Removal of this cassette by Cre-mediated recombination led to 5-HT(1B) receptor expression, which was highly regulatable when doxycycline, a derivative of tetracycline, was administered to the mice. This system allowed for a determination of an in vivo time course of receptor half-life and recovery. Physiological studies also demonstrated that rescued 5-HT(1B) receptors were functional, and that this functionality was reversible upon treatment with doxycycline. Crossing mice where the 5-HT(1B), or the 5-HT(1A), receptors were silenced by the neo-cassette, with mice expressing either Cre-recombinase or the tetracycline transactivator (tTA) under the control of tissue-specific promoters, led to tissue-specific re-expression of these receptors. Our studies thus demonstrate the potential of this strategy for achieving both a classic knockout, as well as subsequent tissue-specific and/or inducible knockouts.

Agrp, a novel gene implicated in the control of feeding

Rapid progress has recently been made to characterise a new pathway involved in the control of feeding behaviour, referred to as the melanocortinergic pathway. Studies of the obese phenotype of the yellow mouse mutant (Ay) suggested the existence of this feeding circuit for some time, but the precise molecular and biochemical regulation was unclear. Through pharmacological and genetic approaches, specific melanocortin receptors expressed within the brain are now known to play a pivotal role. Since receptor agonists decrease food intake, and blocking receptor activity (through antagonists or mutations) increases food intake, this pathway is thought to provide a tonic inhibitory effect on food consumption. A unique feature of this pathway seems to be the regulation of receptor activity by an endogenous protein antagonist, agouti-related protein (AGRP). The molecular characterisation and biochemical activity of this gene suggest that it plays a role in the regulation of feeding behaviour.

Ocular motor signs in an infant with carbohydrate-deficient glycoprotein syndrome type Ia

PURPOSE

To document the evolution of ocular motor abnormalities in an infant with carbohydrate-deficient glycoprotein syndrome.

METHODS

Case report. An infant with carbohydrate-deficient glycoprotein syndrome type 1a underwent magnetic resonance imaging and infrared eye movement recording.

RESULTS

A 10-month-old male with carbohydrate-deficient glycoprotein syndrome type Ia had rapid horizontal oscillations of the eyes when startled or awakened from sleep. Clinical examination confirmed this finding and disclosed congenital ocular motor apraxia with a reduced vestibulo-ocular reflex. Infrared eye movement recording showed ocular flutter and square wave jerks superimposed on a horizontal pendular nystagmus. Magnetic resonance imaging showed diffuse cerebellar hypoplasia.

CONCLUSION

Carbohydrate-deficient glycoprotein syndrome type Ia can be associated with multiple cerebellar eye signs including ocular flutter, square-wave jerks, and congenital ocular motor apraxia.

Dll4, a novel Notch ligand expressed in arterial endothelium

We report the cloning and characterization of a new member of the Delta family of Notch ligands, which we have named Dll4. Like other Delta genes, Dll4 is predicted to encode a membrane-bound ligand, characterized by an extracellular region containing several EGF-like domains and a DSL domain required for receptor binding. In situ analysis reveals a highly selective expression pattern of Dll4 within the vascular endothelium. The activity and expression of Dll4 and the known actions of other members of this family suggest a role for Dll4 in the control of endothelial cell biology.

Notch signaling is essential for vascular morphogenesis in mice

The Notch gene family encodes large transmembrane receptors that are components of an evolutionarily conserved intercellular signaling mechanism. To assess the role of the Notch4 gene, we generated Notch4-deficient mice by gene targeting. Embryos homozygous for this mutation developed normally, and homozygous mutant adults were viable and fertile. However, the Notch4 mutation displayed genetic interactions with a targeted mutation of the related Notch1 gene. Embryos homozygous for mutations of both the Notch4 and Notch1 genes often displayed a more severe phenotype than Notch1 homozygous mutant embryos. Both Notch1 mutant and Notch1/Notch4 double mutant embryos displayed severe defects in angiogenic vascular remodeling. Analysis of the expression patterns of genes encoding ligands for Notch family receptors indicated that only the Dll4 gene is expressed in a pattern consistent with that expected for a gene encoding a ligand for the Notch1 and Notch4 receptors in the early embryonic vasculature. These results reveal an essential role for the Notch signaling pathway in regulating embryonic vascular morphogenesis and remodeling, and indicate that whereas the Notch4 gene is not essential during embryonic development, the Notch4 and Notch1 genes have partially overlapping roles during embryogenesis in mice.

Knockout mice reveal opposite roles for serotonin 1A and 1B receptors in prepulse inhibition

The serotonergic system is involved in the modulation of prepulse inhibition (PPI) and habituation of startle, which are deficient in schizophrenia patients. PPI is the reduction in startle amplitude that occurs when a weak "prepulse" precedes a startling stimulus by 30-500 msec. The roles of 5-HT(1A) and 5-HT(1B) receptors in modulating PPI and habituation were examined using wild-type (WT), 5-HT(1A) knockout (1AKO), and 5-HT(1B) knockout (1BKO) mice. The 5-HT(1A/1B) agonist RU24969 reduced PPI and habituation in WT and 1AKO, but not 1BKO mice, whereas the 5-HT(1A) agonist 8-OH-DPAT increased PPI in WT and 1BKO, but not in 1AKO mice. Similarly, the selective 5-HT(1B) agonist anpirtoline reduced PPI in WT, but not in 1BKO mice. In experiments using intact 129Sv mice, the 5-HT(1A) agonist flesinoxan increased PPI while anpirtoline decreased PPI and habituation. Findings suggest that 5-HT(1B) receptor activation decreases PPI and habituation, and 5-HT(1A) receptor activation increases PPI in mice.

Visual recovery after a year of craniopharyngioma-related amaurosis: report of a nine-year-old child and a review of pathophysiologic mechanisms

BACKGROUND

The probability of visual recovery in tumor-related optic neuropathy usually correlates with the severity and duration of optic pathway compromise. Recovery of visual acuity to normal levels is unexpected after profound loss of vision extending for a period of weeks and months.

METHODS

A 9-year-old girl who had neurosurgical resection of a craniopharyngioma compressing the optic chiasm and optic tract was followed up serially with neuroimaging and clinical examinations over a 6-year period.

RESULTS

Within 3 months of the diagnosis of craniopharyngioma, the girl's vision was reduced to no-light-perception blindness when she viewed with the more involved eye. The blindness correlated with an amaurotic (i.e., >3.6 log unit) relative afferent pupillary defect and an absence of any response when tested with visual field perimetry. After more than a year of total blindness and cessation of all neurosurgical and radiation therapy, visual acuity recovered to a normal level (20/25), the afferent pupillary defect improved, and sensitivity in a portion of the temporal hemivisual field was restored. In the follow-up that has extended for 5 years from the time of recovery, stability of the restored vision has been documented.

CONCLUSION

Children who have tumor-related loss of vision due to damage to the anterior visual pathways may be capable of recovery after intervals of blindness that would be considered irreversible in adults. The mechanism of the recovery in our patient may have been decompression-related restoration of axoplasmic flow, followed by gradual remyelination of visual fibers, which allowed reorganization of connections to the lateral geniculate nucleus to optimize synaptic transmission.

Disulfide assignment of the C-terminal cysteine knot of agouti-related protein (AGRP) by direct sequencing analysis

We have assigned the disulfide structure of Md-65 agouti-related protein (Md65-AGRP) using differential reduction and alkylation followed by direct sequencing analysis. The mature human AGRP is a single polypeptide chain of 112 amino acid residues, consisting of an N-terminal acidic region and a unique C-terminal cysteine-rich domain. The C-terminal domain, a 48 amino acid peptide named Md65-AGRP, was expressed in Escherichia coil cells and refolded under different conditions from the mature recombinant protein. The disulfide bonds in the cystine knot structure of Md65-AGRP were partially reduced using tris(2-carboxyethyl) phosphine (TCEP) under acidic conditions, followed by alkylation with N-ethylmaleimide (NEM). The procedure generated several isoforms with varying degrees of NEM alkylation. The multiple forms of Md65-AGRP generated by partial reduction and NEM modification were then completely reduced and carboxymethylated to identify unreactive disulfide bonds. Differentially labeled Md65-AGRP were directly sequenced and analyzed by MALDI mass spectrometry. The results confirmed that Md65-AGRP contained the same disulfide structure as that of Md5-AGRP reported previously [Bures, E. J., Hui, J. O., Young, Y. et al. (1998) Biochemistry 37, 12172-12177].

Differential addressing of 5-HT1A and 5-HT1B receptors in epithelial cells and neurons

The 5-HT1A and 5-HT1B serotonin receptors are expressed in a variety of neurons in the central nervous system. While the 5-HT1A receptor is found on somas and dendrites, the 5-HT1B receptor has been suggested to be localized predominantly on axon terminals. To study the intracellular addressing of these receptors, we have used in vitro systems including Madin-Darby canine kidney (MDCK II) epithelial cells and primary neuronal cultures. Furthermore, we have extended these studies to examine addressing in vivo in transgenic mice. In epithelial cells, 5-HT1A receptors are found on both apical and basolateral membranes while 5-HT1B receptors are found exclusively in intracellular vesicles. In hippocampal neuronal cultures, 5-HT1A receptors are expressed on somatodendritic membranes but are absent from axons. In contrast, 5-HT1B receptors are found on both dendritic and axonal membranes, including growth cones where they accumulate. Using 5-HT1A and 5-HT1B knockout mice and the binary tTA/tetO system, we generated mice expressing these receptors in striatal neurons. These in vivo experiments demonstrate that, in striatal medium spiny neurons, the 5-HT1A receptor is restricted to the somatodendritic level, while 5-HT1B receptors are shipped exclusively toward axon terminals. Therefore, in all systems we have examined, there is a differential sorting of the 5-HT1A and 5-HT1B receptors. Furthermore, we conclude that our in vivo transgenic system is the only model that reconstitutes proper sorting of these receptors.

Inducible knockout strategies to probe the functions of 5-HT receptors

Gene targeting has proven to be extremely powerful in various fields of biological research. Through this technique, knockout mice lacking a particular gene and thus a particular protein, can be generated. One limitation to this technique is the fact that mice develop without the protein of interest and therefore, developmental compensations may have taken place, contributing to an observed phenotype. Inducible strategies, those which allow the timing of expression of a gene to be regulated, are currently being developed and should prove useful when applied to gene targeting technology. In order to begin to apply such new technologies to the field of gene targeting, we first created and tested several reporter constructions using the tetracycline inducible system. Here we describe the creation of several beta-galactosidase reporter constructions and the results of in vitro testing in Cos-7 cells. We then discuss future knockout strategies based upon our observations.

Biochemical, biophysical, and pharmacological characterization of bacterially expressed human agouti-related protein

The agouti-related protein gene (Agrp) is a novel gene implicated in the control of feeding behavior. The hypothalamic expression of Agrp is regulated by leptin, and overexpression of Agrp in transgenic animals results in obesity and diabetes. By analogy with the known actions of agouti, these data suggest a role for the Agrp gene product in the regulation of melanocortin receptors expressed in the central nervous system. The availability of recombinant, highly purified protein is required to fully address this potential interaction. A nearly full-length form of AGRP (MKd5-AGRP) was expressed in the cytosolic or soluble fraction of Escherichia coli and appeared as large intermolecular disulfide-bonded aggregates. Following oxidation, refolding, and purification, this protein was soluble, and eluted as a single symmetric peak on RP-HPLC. Circular dichroism studies indicated that the purified protein contains primarily random coil and beta-sheet secondary structure. Sedimentation velocity studies at neutral pH demonstrated that MKd5-AGRP is monomeric at low micromolar concentrations. Mobility shifts observed using SDS-PAGE under reducing and nonreducing conditions for bacterially expressed and mammalian expressed AGRP were identical, an indication of a similar disulfide structure. The purification to homogeneity of a second, truncated form of AGRP (Md65-AGRP) which was expressed in the insoluble or inclusion body fraction is also described. Both forms act as competitive antagonists of alpha-melanocyte stimulating hormone (alpha-MSH) at melanocortin-3 (MC-3) and melanocortin-4 receptors (MC-4). The demonstration that AGRP is an endogenous antagonist with respect to these receptors is a unique mechanism within the central nervous system, and has important implications in the control of feeding.

Determination of disulfide structure in agouti-related protein (AGRP) by stepwise reduction and alkylation

The agouti-related protein gene (Agrp) plays an important role in body weight regulation. The mature human protein is a single polypeptide chain of 112 amino acid residues, consisting of an N-terminal acidic region and a unique C-terminal cysteine-rich domain. The disulfide structure of recombinant human AGRP was determined by chemical methods using partial reduction with tris(2-carboxyethyl)phosphine under acidic conditions, followed by direct alkylation with N-ethylmaleimide or fluorescein-5-maleimide. Partial reduction and alkylation provided several forms of AGRP that were modified in a stepwise fashion. The resulting proteins were characterized by peptide mapping, sequence analysis, and mass spectrometry, showing that AGRP contained a highly reducible disulfide bond, C85-C109, followed by less reactive ones, C90-C97, C74-C88, C67-C82, and C81-C99, respectively. The chemically defined disulfide connectivity of the recombinant human AGRP was homologous to that of omega-agatoxin IVB except for an additional disulfide bond, C85-C109.

Novel strategies to probe the functions of serotonin receptors

Gene targeting has proven to be extremely powerful in various fields of biological research. Through this technique, knockout mice lacking a particular gene, and thus a particular protein, can be generated. One limitation to this technique is the fact that mice develop without the protein of interest and therefore, developmental compensations may have taken place, contributing to an observed phenotype. Inducible strategies, those which allow the timing of expression of a gene to be regulated, are currently being developed and should prove useful when applied to gene targeting technology. To begin to apply such new technologies to the field of gene targeting, we first created and tested several reporter constructions using the tetracycline inducible system. Here we describe the creation of several beta-galactosidase reporter constructions and the results of in vitro testing in Cos-7 cells. We then discuss future knockout strategies based upon our observations.

Utilization of microhomologous recombination in yeast to generate targeting constructs for mammalian genes

We have developed a new procedure utilizing microhomologous recombination in yeast to generate targeting constructs for producing targeted mutations in mice. This procedure is rapid and efficient, and should be directly applicable to all mammalian genes. Moreover, only minimal information about the locus being targeted is required. The feasibility of this approach was demonstrated by producing another allele of the mouse Tg737 polycystic kidney gene.

Anemia and perinatal death result from loss of the murine ecotropic retrovirus receptor mCAT-1

The mCAT-1 gene encodes a basic amino acid transporter that also acts as the receptor for murine ecotropic leukemia viruses. Targeted mutagenesis in embryonic stem cells has been used to introduce a germ-line null mutation into this gene. This mutation removes a domain critical for virus binding and inactivates amino acid transport activity. Homozygous mutant pups generated from these cells were approximately 25% smaller than normal littermates, very anemic, and died on the day of birth. Peripheral blood from homozygotes contained 50% fewer red blood cells, reduced hemoglobin levels, and showed a pronounced normoblastosis. Histological analyses of bone marrow, spleen, and liver showed a decrease in both erythroid progenitors and mature red blood cells. Mutant fetal liver cells behaved normally in in vitro hematopoietic colony-forming assays but generated an anemia when transplanted into irradiated C.B.-17 SCID mice. Furthermore, reconstitution of the white cell compartment of SCID mice by mutant fetal liver cells was less complete than that observed with a mixed population of wild-type and heterozygous fetal liver cells. Primary embryo fibroblasts from mutant mice were completely resistant to ecotropic retrovirus infection. Thus, mCAT-1 not only appears to be the sole receptor for a group of murine ecotropic retroviruses associated with hematological disease but also plays a critical role in both hematopoiesis and growth control during mouse development.

Hypothalamic expression of ART, a novel gene related to agouti, is up-regulated in obese and diabetic mutant mice

We have isolated cDNA clones that encode a novel human gene related to agouti. Sequence analysis of this gene, named ART, for agouti-related transcript, predicts a 132-amino-acid protein that is 25% identical to human agouti. The highest degree of identity is within the carboxyl terminus of both proteins. Like agouti, ART contains a putative signal sequence and a cysteine rich carboxyl terminus, but lacks the region of basic residues and polyproline residues found in the middle of the agouti protein. Both agouti and ART contain 11 cysteines, and 9 of these are conserved spatially. ART is expressed primarily in the adrenal gland, subthalamic nucleus, and hypothalamus, with a lower level of expression occurring in testis, lung, and kidney. The murine homolog of ART was also isolated and is predicted to encode a 131-amino-acid protein that shares 81% amino acid identity to humans. The mouse was found to have the same expression pattern as human when assessed by RT-PCR. Examination by in situ hybridization using mouse tissues showed localized expression in the arcuate nucleus of the hypothalamus, the median eminence, and the adrenal medulla. In addition, the hypothalamic expression of ART was elevated approximately 10-fold in ob/ob and db/db mice. ART was mapped to human chromosome 16q22 and to mouse chromosome 8D1-D2. The expression pattern and transcriptional regulation of ART, coupled with the known actions of agouti, suggests a role for ART in the regulation of melanocortin receptors within the hypothalamus and adrenal gland, and implicates this novel gene in the central control of feeding.

Wnt-3a regulates somite and tailbud formation in the mouse embryo

Amphibian studies have implicated Wnt signaling in the regulation of mesoderm formation, although direct evidence is lacking. We have characterized the expression of 12 mammalian Wnt-genes, identifying three that are expressed during gastrulation. Only one of these, Wnt-3a, is expressed extensively in cells fated to give rise to embryonic mesoderm, at egg cylinder stages. A likely null allele of Wnt-3a was generated by gene targeting. All Wnt-3a-/Wnt-3a- embryos lack caudal somites, have a disrupted notochord, and fail to form a tailbud. Thus, Wnt-3a may regulate dorsal (somitic) mesoderm fate and is required, by late primitive steak stages, for generation of all new embryonic mesoderm. Wnt-3a is also expressed in the dorsal CNS. Mutant embryos show CNS dysmorphology and ectopic expression of a dorsal CNS marker. We suggest that dysmorphology is secondary to the mesodermal and axial defects and that dorsal patterning of the CNS may be regulated by inductive signals arising from surface ectoderm.

FGFR-4, a new member of the fibroblast growth factor receptor family, expressed in the definitive endoderm and skeletal muscle lineages of the mouse

We have used the polymerase chain reaction to clone from fetal cerebellar RNA a novel member of the fibroblast growth factor receptor family, FGFR-4. cDNAs encoding a full-length receptor were isolated and RNA expression examined in adult and fetal tissues by RNA blot analysis. Transcripts were detected in adult lung, liver and kidney and in fetal RNAs from 11.5 to 16.5 days post coitum (p.c.). In situ hybridization was performed to examine developmental expression. FGFR-4 RNA was expressed in definitive endoderm of the developing gut and extraembryonic endoderm of the yolk-sac from 8.5 to 14.5 days p.c. At early somite stages, FGFR-4 was also expressed in the myotomal component of the somite, and by 14.5 days p.c. in the myotomally derived skeletal muscle. No expression was seen at any stage in cardiac muscle. Several endodermal derivatives, the liver, lung and pancreas, expressed FGFR-4 at 14.5 days p.c. In addition, FGFR-4 RNA was detected in the adrenal cortex, collecting tubules of the kidney and condensing cartilage at this time. These results suggest that FGFR-4 is likely to have diverse roles in development, which may include regulation of definitive endoderm and skeletal muscle lineages.

Cytochrome P450-dependent bioactivation of prodysmorphogens in cultured conceptuses

These investigations were undertaken to determine the extent to which tissues of cultured rat conceptuses contain cytochrome P450 isoforms in sufficient quantities to significantly influence the capacity of certain chemicals to elicit dysmorphogenic effects in vitro. Investigations with highly sensitive probe substrates/inhibitors and with immunologic methods enabled the detection of at least four separate P450 isoforms in tissues of the visceral yolk sac, ectoplacental cone, and embryo proper. One of the isoforms was identified as P450IA1 and was found to be inducible by polycyclic aromatic hydrocarbons in all three tissues. Other isoforms exhibited properties differing from characterized adult rat hepatic isoforms. Each of the isoforms was detectable in conceptuses on gestational days 10, 11, 12, and 14 and was present in the highest concentrations in the visceral yolk sac. Conceptal P450IA1 catalyzed the conversion of dysmorphogenically inactive 2-acetylaminofluorene to 7-hydroxy-2-acetylaminofluorene, a proximate dysmorphogen. Investigations with microinjections suggested that visceral yolk sac hydroxylation was largely responsible for the bioactivation reaction in vitro. The same isoform exhibited no capacity to influence the dysmorphogenic activity of cyclophosphamide. The results demonstrated that tissues of cultured rat conceptuses may contain P450 isoforms in sufficient amounts to markedly influence the dysmorphogenic activity of substrates of the corresponding isoforms.

Dysmorphogenesis elicited by microinjected acetaminophen analogs and metabolites in rat embryos cultured in vitro

Direct additions of acetaminophen (APAP), 3,5-dimethylacetaminophen, 3-hydroxyacetaminophen or 3-methoxyacetaminophen to the medium of cultured embryos each produced an increased incidence of morphologically similar, abnormally open anterior neuropores. Approximate concentrations required to produce an equal incidence were 0.5 mM, 1.0 mM, 0.1 mM and 0.75 mM, respectively. In contrast, 2.6-dimethylacetaminophen and N-acetyl-p-benzoquinoneimine failed to produce elevated incidences of abnormal neurulation unaccompanied by marked growth retardation. However, with intra-amniotic microinjections, 3-hydroxyacetaminophen and N-acetyl-p-benzoquinoneimine were roughly equipotent for eliciting abnormal neurulation, whereas 3-methoxyacetaminophen required greater than 30-fold higher concentrations. This suggests that N-acetyl-p-benzoquinoneimine does not readily transit the visceral yolk sac and would likely not be a major factor in APAP-elicited neural tube abnormalities unless generated in target tissues. The differential effects produced by two dimethylated (2.6 and 3.5) APAP analogs further suggest that sulfhydryl oxidation is associated more closely than sulfhydryl conjugation with the neurulation defect. Intra-amniotic microinjections of large quantities (3500 ng) of 7-hydroxy-2-acetylaminofluorene (7-OH-AAF) or APAP failed to produce the specific neurulation defect. Microinjections of 7-OH-AAF into the exocoelomic cavity effected the characteristic abnormal neurulation. Conversion by conceptal homogenates of 7-OH-AAF was roughly 7- to 8-fold more rapid than conversion of APAP to respective catechol metabolites, and specific activities in yolk sac tissues were greater than those in the embryo. Rates of conceptal conversion to the quinoneimine were approximately 2- to 3-fold lower than catechol generation.(ABSTRACT TRUNCATED AT 250 WORDS)

Abnormal neurulation induced by 7-hydroxy-2-acetylaminofluorene and acetaminophen: evidence for catechol metabolites as proximate dysmorphogens

Direct additions to culture media of either acetaminophen (APAP) or 7-hydroxy-2-acetylaminofluorene (7-OH-AAF) resulted in abnormal closure of the anterior neuropores of cultured rat embryos in the absence of an exogenous bioactivation system. Concentrations required to produce a 50% incidence of the defect were approximately 500 and 250 microM for APAP and 7-OH-AAF, respectively. Losses of viability were not evident at these concentrations but 7-OH-AAF elicited a somewhat greater effect on growth parameters and generalized embryotoxicity. Transplacental induction with 3-methylcholanthrene (MC) of P450IA1 in subsequently cultured rat embryos did not detectably alter the capacity of APAP or 7-OH-AAF to effect embryotoxicity or neuropore closure. However, additions to the culture medium of exogenous hepatic bioactivating systems (S9) from MC-induced vs phenobarbital (PB)-induced adult rats produced profoundly different effects on neuropore closure. Coincubation with S9 from MC-induced rats reduced the incidence of 7-OH-AAF-elicited abnormal neuropores from 45 to 19%, whereas coincubation with S9 from PB-induced rats increased the incidence to 77%. Coincubation with MC-induced S9 produced no statistically significant effect on APAP-elicited neuropore abnormalities but, with PB-induced S9, resulted in a significant increase from 60 to 86%. Additions of 3-OH-APAP (0.1-0.2 mM) but not N-acetyl-p-benzoquinoneimine (NAPQI, 0.1-0.5 mM) to the culture medium elicited the typical neuropore abnormality. Experiments with APAP and 7-OH-AAF as substrates demonstrated that embryonic enzymes catalyzed their conversion to the corresponding catechols. Considered together, the results provided evidence that embryonic conversion of APAP or 7-OH-AAF to the corresponding catechol metabolites may be instrumental in effecting the abnormal anterior neuropore closure observed after exposure of embryos to the respective parent compounds.

Microinjection of cultured rat embryos: a new technique for studies in chemical dysmorphogenesis

The utility of a new technique for exposure of cultured whole rat embryos to potential dysmorphogens was demonstrated with nitrosofluorene (NF), a cytotoxic and mutagenic metabolite of 2-acetylaminofluorene (AAF). At an initial concentration in the culture medium of 41 microM, NF produced a 100% incidence of defects in axial rotation with no significant effect on prosencephalic development, consistent with previous reports. This route of exposure was also associated with a significant decrease in yolk sac vasculature and protein content. However, when 2 to 20 ng of NF was microinjected directly into the amniotic space, the predominant malformation observed was prosencephalic hypoplasia. Injection of 10 ng NF resulted in approximately equivalent decreases in viability as 41 microM NF dissolved in the culture medium, but produced only a 41% incidence of rotation defects and a 27% incidence of open neural tubes in the rhombencephalic region. The protein content of injected conceptuses was significantly reduced in the embryo, but not in the visceral yolk sac. When 10 ng of NF was injected inside the yolk sac but outside the amnion, the incidence of abnormal rotation was increased to 75%, and the severity of prosencephalic hypoplasia as well as the incidence of neural tube abnormalities was attenuated. The protein content of both the embryo and yolk sac was significantly decreased relative to that of the controls. The data are consistent with the suggestion that NF elicits defects in axial rotation primarily via its effects on the visceral yolk sac and demonstrate the capacity of this technique to provide insights into mechanistic aspects of chemical dysmorphogenesis.

Influence of electrophilic character and glutathione depletion on chemical dysmorphogenesis in cultured rat embryos

To examine the importance of reduced intracellular glutathione (GSH) in the modulation of dysmorphogenesis and to gain insight into the electrophilic character of the embryotoxic intermediates generated in the rat embryo from N-acetoxy-2-acetylaminofluorene (AAAF) and acetaminophen (APAP) in cultured embryos, the effects of GSH depletion on the embryotoxicity, dysmorphogenesis and covalent binding of these agents were examined. Both AAAF (90 microM) and APAP (500 microM) produced concentration-dependent, statistically significant (P less than or equal to 0.05) decreases in embryonic length as well as embryonic and visceral yolk sac protein content when rat embryos were exposed in vitro between days 10 and 11 of gestation. The predominant malformations observed upon exposure to AAAF and APAP were prosencephalic hypoplasia and abnormal neurulation respectively. Exposure of conceptuses to [3H]APAP followed by separation and fractionation of the cellular RNA, DNA and protein via density gradient centrifugation resulted in detectable binding in fractions that contained protein, but not DNA or RNA. This suggested that the rat conceptus is capable of bioactivating APAP to a soft electrophile that selectively arylates protein. In contrast, conceptuses exposed to [3H]AAAF exhibited detectable binding to RNA, DNA and protein, indicative of conversion to both hard and soft electrophiles. Depletion of GSH was accomplished by pretreating conceptuses with 500 microM L-buthionine-S,R-sulfoximine (BSO) from the start of the culture period (day 9.5) until the morning of day 10. When conceptuses were depleted previously of GSH by BSO, exposure to APAP resulted in significant potentiation (relative to APAP alone) of the observed embryotoxicity. These conceptuses displayed further decreases in both embryonic size and protein content of the embryo and yolk sac, as well as increased incidence of abnormally open anterior neuropores and increased binding (3-fold) of [3H]APAP to protein. In contrast, pretreatment with BSO did not potentiate the AAAF-elicited decreases in embryonic size or protein content, nor the severity of prosencephalic hypoplasia, although a slight increase in binding of [3H]AAAF to DNA was observed. Taken together, these data are consistent with the concept that abnormal neurulation elicited by APAP results from the generation of one or more soft electrophilic species, whereas elicitation of prosencephalic hypoplasia by AAAF appears to be a consequence of conversion to a relatively hard electrophile(s).

Biotransformation, estrogenicity, and steroid structure as determinants of dysmorphogenic and generalized embryotoxic effects of steroidal and nonsteroidal estrogens

A series of nine chemicals of varying structure and estrogenicity was investigated for biochemical determinants of their relative capacities to alter normal embryonic growth and developmental patterns during organogenesis in rats. In order to circumvent the potentially confounding influences of maternal factors, the direct effects of steroidal and nonsteroidal estrogens on cultured whole embryos were compared at concentrations producing readily measurable embryotoxicity but low embryolethality (2-20%). Nonsteroidal estrogens included were diethylstilbestrol (DES), hexestrol (HES), E,E-dienestrol (alpha-DIES), and tamoxifen (TAM). Steroidal estrogens were estradiol 17 beta (E2), estrone (E1), and 17 alpha-ethinylestradiol 17 beta (EE). For comparative purposes, the effects of two essentially nonestrogenic phenols, Z,Z-dienestrol (beta-DIES) and phenol, were also studied. TAM, a weak estrogen which also exhibits antiestrogenic properties, was studied for possible interactive effects with potent estrogens. Prosencephalic hypoplasia was the abnormality most consistently observed and was elicited by each of the chemicals investigated. Embryotoxicity was neither attenuated by TAM nor related to estrogenic potency or steroidal structure, but was strongly and unpredictably influenced by biotransformational determinants. Presence of a cytochrome P450-dependent oxidizing system in the culture medium resulted in marked increases in embryotoxicity of E1, E2, and phenol, only minor increases for beta-DIES and alpha-DIES, but in strikingly decreased effects of EE, TAM, and HES. It produced no statistically significant differences in effects of DES. The results obtained were compatible with the concept that effects of these agents on growth and development during the earlier stages of organogenesis are independent of steroid structure or estrogenic activity but strongly dependent upon pathways and rates of biotransformation of some (but not all) of the parent chemicals.

Modulation of the embryotoxicity and cytotoxicity elicited by 7-hydroxy-2-acetylaminofluorene and acetaminophen via deacetylation

Acetaminophen (APAP) and 7-hydroxy-2-acetylaminofluorene (7-OH-AAF) each produced a similar incidence of, as well as a qualitatively similar, abnormal closure of the anterior neuropore at similar concentrations when added to the medium of cultured rat embryos. At concentrations producing a 50-65% incidence of abnormal neurulation, the affected embryos displayed relatively complete embryonic development as assessed from measurements of protein, axial rotation, and embryonic length. The neural tube defect produced by these agents consisted of elevated neural folds remaining separated by approximately 45 degrees as well as the presence of a mitotically active neural epithelium. In contrast, the nonacetylated structures, p-aminophenol (PAP) and 7-hydroxyaminofluorene (7-OH-AF), were embryotoxic at concentrations 10-fold lower than the corresponding acetylated compounds; each produced a greater incidence of abnormal axial rotation and a greater decrease in embryonic protein than APAP or 7-OH-AAF. In addition, the embryos exposed to PAP or 7-OH-AF were morphologically and histologically dissimilar to those exposed to the acetylated compounds. The neural folds of the latter remained elevated and in apposition, but lacked complete fusion of the folds of neural epithelium and were accompanied by marked cytotoxicity. Addition of active deacetylase sources (guinea pig liver microsomes or commercially obtained, purified carboxylic-ester hydrolase) to the culture medium of conceptuses exposed to 7-OH-AAF or APAP resulted in an increased embryotoxicity which was indistinguishable from that produced by the nonacetylated compounds alone. The increases in toxicity were effectively blocked by the addition of paraoxon, indicating that catalysis of the deacetylation of APAP and 7-OH-AAF was the causative factor. No evidence could be found for deacetylation of 7-OH-AAF or APAP mediated by the Day 10 conceptus itself. When examined for cytotoxicity in F9 embryonal carcinoma cells, APAP and 7-OH-AAF each produced observable cell death only if reduced glutathione (GSH) had previously been depleted and if a deacetylase source were present; this cytotoxicity was also blocked by addition of paraoxon. The nonacetylated metabolites were directly cytotoxic, although GSH depletion greatly increased the incidence of cell death. Therefore, deacetylation of APAP and 7-OH-AAF produced an increase in generalized embryotoxicity and cytotoxicity relative to abnormal neurulation, suggesting that APAP and 7-OH-AAF are capable of eliciting neural tube defects via a mechanism(s) that is distinguishable from the generalized embryotoxicity or cytotoxicity produced by their nonacetylated counterparts.

Glutathione status and the incidence of neural tube defects elicited by direct acting teratogens in vitro

Valproic acid (VPA), cytochalasin D (CD) and 7-hydroxy-2-acetylaminofluorene (7-OH-AAF) each caused abnormal closure of the anterior neuropore in rat embryos cultured in vitro in the absence of an exogenous bioactivation system. Morphological comparisons showed that although all three compounds prevented normal neural tube closure, each did so in a distinctive manner. Modulation of GSH in cultured rat conceptuses was evaluated to determine whether common responses occurred relative to the ability of different chemicals to elicit neural tube defects. Malformation incidence in embryos (10-14 somites) varied widely following exposure to CD (44%), 7-OH-AAF (29%) or VPA (17%). The incidence of CD-elicited malformations was increased by 50% following GSH depletion by L-buthionine-S, R-sulfoximine (BSO) and was decreased by nearly 60% when the cysteine pro-drug 2-oxothiazolidine-4-carboxylate (OTC) was added to the culture medium. GSH modulation also exerted significant effects on the incidence of abnormal neurulation caused by VPA or 7-OH-AAF. A relatively low incidence of open neural tubes produced by VPA or 7-OH-AAF alone was shown to be a function of the state of maturation in the embryos. Conceptuses cultured in the presence of VPA or 7-OH-AAF from an earlier gestational age (6-10 somites) showed 2-3 fold increases in the number of embryos with open neural tubes. Differential alterations in protein and DNA content were observed among embryos and yolk sacs after various treatments indicating possible differences in the site of embryotoxicity. These results demonstrate the role of GSH status on the capacity of three chemically diverse compounds to elicit abnormal neurulation in cultured rat embryos and suggest some possible mechanisms by which normal neurulation may be compromised.

Embryotoxicity elicited by inhibition of gamma-glutamyltransferase by Acivicin and transferase antibodies in cultured rat embryos

Acivicin (also known as AT-125) and IgG isolated from goat anti-gamma-glutamyltransferase antiserum were used to inhibit the activity of gamma-glutamyltransferase (GGT, EC 2.3.2.2) in rat conceptuses cultured from Days 10 to 11 of gestation. Inhibition of GGT by either Acivicin or anti-GGT IgG produced embryotoxicity and malformations, although each compound produced a unique spectrum of effects. Acivicin, at an initial concentration in the culture medium of 5 microM, produced a marked decrease in yolk sac vasculature and was associated with embryonic malformations such as neural tube necrosis, microophthalmia, and cephalic edema after 24 hr exposure. These malformations were accompanied by significant decreases in both embryonic and yolk sac protein, yolk sac GGT activity, as well as embryonic glutathione (GSH) levels. In contrast, anti-GGT IgG produced no apparent effects on yolk sac vasculature or protein after exposure of conceptuses to an initial concentration of 50 micrograms IgG/ml culture medium, even though equal inhibition of yolk sac GGT (30%) was achieved by each inhibitor. Exposure to IgG (50 micrograms/ml) for 24 hr was associated with decreased embryonic protein; decreased levels of GSH in the embryo were observed after both 3 and 24 hr. The dichotomy of effects on the yolk sac by the two compounds indicates that Acivicin produced these effects by mechanisms other than by GGT inhibition alone. These results demonstrate that inhibition of GGT in rat embryos undergoing organogenesis can elicit embryotoxic effects and produce alterations in GSH levels. The capacity of the anti-GGT antibody to inhibit the GGT activity in the yolk sac (while having no apparent effect on yolk sac morphology), and yet influence the embryo by decreasing protein and GSH levels, underscores the important role of the yolk sac during the highly sensitive stages of organogenesis.