Biomedical subjects

C Chiang

Publications and source records attributed to C Chiang.

At least 19 recordsLinked to original sources

PubMed2001

Manifestation of the limb prepattern: limb development in the absence of sonic hedgehog function.

The secreted protein encoded by the Sonic hedgehog (Shh) gene is localized to the posterior margin of vertebrate limb buds and is thought to be a key signal in establishing anterior-posterior limb polarity. In the Shh(-/-) mutant mouse, the development of many embryonic structures, including the limb, is severely compromised. In this study, we report the analysis of Shh(-/-) mutant limbs in detail. Each mutant embryo has four limbs with recognizable humerus/femur bones that have anterior-posterior polarity. Distal to the elbow/knee joints, skeletal elements representing the zeugopod form but lack identifiable anterior-posterior polarity. Therefore, Shh specifically becomes necessary for normal limb development at or just distal to the stylopod/zeugopod junction (elbow/knee joints) during mouse limb development. The forelimb autopod is represented by a single distal cartilage element, while the hindlimb autopod is invariably composed of a single digit with well-formed interphalangeal joints and a dorsal nail bed at the terminal phalanx. Analysis of GDF5 and Hoxd11-13 expression in the hindlimb autopod suggests that the forming digit has a digit-one identity. This finding is corroborated by the formation of only two phalangeal elements which are unique to digit one on the foot. The apical ectodermal ridge (AER) is induced in the Shh(-/-) mutant buds with relatively normal morphology. We report that the architecture of the Shh(-/-) AER is gradually disrupted over developmental time in parallel with a reduction of Fgf8 expression in the ridge. Concomitantly, abnormal cell death in the Shh(-/-) limb bud occurs in the anterior mesenchyme of both fore- and hindlimb. It is notable that the AER changes and mesodermal cell death occur earlier in the Shh(-/-) forelimb than the hindlimb bud. This provides an explanation for the hindlimb-specific competence to form autopodial structures in the mutant. Finally, unlike the wild-type mouse limb bud, the Shh(-/-) mutant posterior limb bud mesoderm does not cause digit duplications when grafted to the anterior border of chick limb buds, and therefore lacks polarizing activity. We propose that a prepattern exists in the limb field for the three axes of the emerging limb bud as well as specific limb skeletal elements. According to this model, the limb bud signaling centers, including the zone of polarizing activity (ZPA) acting through Shh, are required to elaborate upon the axial information provided by the native limb field prepattern.

Animals↗

PubMed2001

Anorectal malformations caused by defects in sonic hedgehog signaling.

Anorectal malformations are a common clinical problem affecting the development of the distal hindgut in infants. The spectrum of anorectal malformations ranges from the mildly stenotic anus to imperforate anus with a fistula between the urinary and intestinal tracts to the most severe form, persistent cloaca. The etiology, embryology, and pathogenesis of anorectal malformations are poorly understood and controversial. Sonic hedgehog (Shh) is an endoderm-derived signaling molecule that induces mesodermal gene expression in the chick hindgut. However, the role of Shh signaling in mammalian hindgut development is unknown. Here, we show that mutant mice with various defects in the Shh signaling pathway exhibit a spectrum of distal hindgut defects mimicking human anorectal malformations. Shh null-mutant mice display persistent cloaca. Mutant mice lacking Gli2 or Gli3, two zinc finger transcription factors involved in Shh signaling, respectively, exhibit imperforate anus with recto-urethral fistula and anal stenosis. Furthermore, persistent cloaca is also observed in Gli2(-/-);Gli3(+/-), Gli2(+/-);Gli3(-/-), and Gli2(-/-);Gli3(-/-) mice demonstrating a gene dose-dependent effect. Therefore, Shh signaling is essential for normal development of the distal hindgut in mice and mutations affecting Shh signaling produce a spectrum of anorectal malformations that may reveal new insights into their human disease equivalents.

Anal Canal↗

PubMed2001

Cytomegalovirus infection as a cause of pseudomembrane colitis: a report of four cases.

Pseudomembranous colitis is very commonly encountered in patients with acquired immune deficiency syndrome (AIDS), and has been characteristically associated with Clostridium difficile infection. We present four cases of AIDS-related diarrhea and pseudomembrane formation on endoscopy with pathologic features consistent with cytomegalovirus (CMV) colitis. Our findings indicate that CMV colitis should be considered in the differential diagnosis of pseudomembranous colitis in immunocompromised patients.

AIDS-Related Opportunistic Infections↗

PubMed2000

Top-down processing mediated by interareal synchronization.

Perception and cortical responses are not only driven "bottom-up" by the external stimulus but are altered by internal constraints such as expectancy or the current behavioral goal. To investigate neurophysiological mechanisms of such top-down effects, we analyzed the temporal interactions of neurons on different levels of the cortical hierarchy during perception of stimuli with varying behavioral significance. We found that interareal interactions in a middle-frequency range (theta and alpha; 4-12 Hz) strongly depend on the associated behavior, with a phase relationship and a layer specificity indicating a top-down-directed interaction. For novel unexpected stimuli, presumably processed in a feed-forward fashion, no such interactions occurred but high-frequency interactions (gamma; 20-100 Hz) were observed. Thus corticocortical synchronization reflects the internal state of the animal and may mediate top-down processes.

Animals↗

PubMed2000

Involvement of neurogranin in the modulation of calcium/calmodulin-dependent protein kinase II, synaptic plasticity, and spatial learning: a study with knockout mice.

Neurogranin/RC3 is a neural-specific Ca(2+)-sensitive calmodulin (CaM)-binding protein whose CaM-binding affinity is modulated by phosphorylation and oxidation. Here we show that deletion of the Ng gene in mice did not result in obvious developmental or neuroanatomical abnormalities but caused an impairment of spatial learning and changes in hippocampal short- and long-term plasticity (paired-pulse depression, synaptic fatigue, long-term potentiation induction). These deficits were accompanied by a decreased basal level of the activated Ca(2+)/CaM-dependent kinase II (CaMKII) ( approximately 60% of wild type). Furthermore, hippocampal slices of the mutant mice displayed a reduced ability to generate activated CaMKII after stimulation of protein phosphorylation and oxidation by treatments with okadaic acid and sodium nitroprusside, respectively. These results indicate a central role of Ng in the regulation of CaMKII activity with decisive influences on synaptic plasticity and spatial learning.

Animals↗

PubMed2000

Bi-directional interactions between visual areas in the awake behaving cat.

The study of the cooperativity among cortical areas is essential to our understanding of brain functioning. Here we investigated the relative contributions of top-down and bottom-up directed interactions between area 17 and area 7 of the cat visual system. Bipolar local field potentials were recorded while the animals performed a go/no-go task or were in a quiet resting state. The data were analyzed by applying measures of interaction based on the Wiener-Granger causality concept. We found that during the visual task top-down directed interactions were of a similar magnitude as the bottom-up component. Second, interareal couplings tended to increase in conditions requiring a discriminative effort. Third, during behaviors not dominated by visual processing non-directed interactions increased.

Animals↗

PubMed2000

Control of Shh activity and signaling in the neural tube.

The generation of diverse cell types in the neural tube requires inductive signals that are generally derived from tissues adjacent to the neural tube and capable of patterning cell fates at a distance. Shh, a signaling molecule secreted from the notochord and the floor plate, has been shown to induce motor neurons (MN) as well as interneurons, dorsal to the MN, in a concentration-dependent manner. The cellular response to the Shh signal is mediated by receptors, cytoplasmic factors as well as transcription factors, which act both positively and negatively to modulate Shh activity in the patterning of diverse cell types in the ventral neural tube. Additionally, Shh also cooperates with Bmp and Fgf molecules in the control of diverse neuronal cell fates in the brain.

Animals↗

PubMed2000

Specification of ventral neuron types is mediated by an antagonistic interaction between Shh and Gli3.

Specification of distinct neuron types in the ventral spinal cord is thought to be mediated by a graded concentration of Sonic hedgehog (Shh), a secreted signaling protein. Shh is made in the notochord, the most ventral part of the spinal cord, and in mice lacking Shh, ventral cell types are reduced or absent. The response to Shh depends on transcription factors of the Gli family, but the detailed mechanism is not understood. Here we show that Gli3 represses ventral fates in a dose-dependent manner. Whereas Shh -/- mutant mice show reductions in several classes of ventral interneurons and a complete absence of motor neurons, these cell types were rescued in Shh-/-;Gli3 -/- double mutants. This rescue of the Shh null phenotype depended on the level of Gli3 function; a partial rescue was observed in Shh-/-;Gli3 +/- embryos. We propose that Shh is required to antagonize Gli3, which would otherwise repress ventral fates. Differences between rostral and caudal regions suggest that other signaling molecules-in addition to Shh-may be involved in specifying ventral fates, particularly in the caudal region of the spinal cord.

Animals↗

PubMed1999

A sonic hedgehog-independent, retinoid-activated pathway of neurogenesis in the ventral spinal cord.

Sonic hedgehog (Shh) is thought to control the generation of motor neurons and interneurons in the ventral CNS. We show here that a Shh-independent pathway of interneuron generation also operates in the ventral spinal cord. Evidence for this parallel pathway emerged from an analysis of the induction of ventral progenitors that express the Dbx homeodomain proteins and of Evx1/2 (V0) and En1 (V1) neurons. Shh signaling is sufficient to induce Dbx cells and V0 and V1 neurons but is not required for their generation in vitro or in vivo. Retinoids appear to mediate this parallel pathway. These findings reveal an unanticipated Shh-independent signaling pathway that controls progenitor cell identity and interneuron diversity in the ventral spinal cord.

Animals↗

PubMed1999

Essential role for Sonic hedgehog during hair follicle morphogenesis.

The hair follicle is a source of epithelial stem cells and site of origin for several types of skin tumors. Although it is clear that follicles arise by way of a series of inductive tissue interactions, identification of the signaling molecules driving this process remains a major challenge in skin biology. In this study we report an obligatory role for the secreted morphogen Sonic hedgehog (Shh) during hair follicle development. Hair germs comprising epidermal placodes and associated dermal condensates were detected in both control and Shh -/- embryos, but progression through subsequent stages of follicle development was blocked in mutant skin. The expression of Gli1 and Ptc1 was reduced in Shh -/- dermal condensates and they failed to evolve into hair follicle papillae, suggesting that the adjacent mesenchyme is a critical target for placode-derived Shh. Despite the profound inhibition of hair follicle morphogenesis, late-stage follicle differentiation markers were detected in Shh -/- skin grafts, as well as cultured vibrissa explants treated with cyclopamine to block Shh signaling. Our findings reveal an essential role for Shh during hair follicle morphogenesis, where it is required for normal advancement beyond the hair germ stage of development.

Adipose Tissue↗

PubMed1999

Effect of trilinolein on strophanthidin-induced ventricular tachycardia in guinea pigs.

Trilinolein, a triacylglycerol with linoleic acid, was found to inhibit Ca2+ influx in cardiomyocytes. The purpose of this study was to evaluate the effects of trilinolein on ventricular arrhythmia induced by intravenous strophanthidin in guinea pigs anesthetized with intraperitoneal urethane. After strophanthidin- induced ventricular tachycardia, treatment with trilinolein (0.1, 1, 10 and 100 microg/kg) or control (0.04% propylene glycol) did not terminate ventricular tachycardia. However, 1, 10 and 100 microg/kg trilinolein could narrow the width of the QRS complex during ventricular tachycardia. Pretreatment with trilinolein before strophanthidin administration did not prevent the occurrence of ventricular tachycardia; the doses of strophanthidin required to induce arrhythmias (ventricular extrasystole and ventricular tachycardia) were similar in guinea pigs pretreated with trilinolein or control. However, there were fewer ventricular extrasystoles in guinea pigs pretreated with trilinolein than in the control group (ANOVA, p < 0.01). Moreover, the ventricular extrasystoles were fewer in guinea pigs pretreated with higher doses of trilinolein (100 microg/kg, 103 +/- 60; 10 microg/kg, 188 +/- 86) than lower doses of trilinolein (1 microg/kg, 366 +/- 102; 0.1 microg/kg, 436 +/- 145). This study demonstrated that trilinolein was not effective in terminating or preventing strophanthidin-induced ventricular tachycardia. However, trilinolein could improve ventricular depolarization and suppress ventricular extrasystoles.

Analysis of Variance↗

PubMed1999

Sonic hedgehog controls epaxial muscle determination through Myf5 activation.

Sonic hedgehog (Shh), produced by the notochord and floor plate, is proposed to function as an inductive and trophic signal that controls somite and neural tube patterning and differentiation. To investigate Shh functions during somite myogenesis in the mouse embryo, we have analyzed the expression of the myogenic determination genes, Myf5 and MyoD, and other regulatory genes in somites of Shh null embryos and in explants of presomitic mesoderm from wild-type and Myf5 null embryos. Our findings establish that Shh has an essential inductive function in the early activation of the myogenic determination genes, Myf5 and MyoD, in the epaxial somite cells that give rise to the progenitors of the deep back muscles. Shh is not required for the activation of Myf5 and MyoD at any of the other sites of myogenesis in the mouse embryo, including the hypaxial dermomyotomal cells that give rise to the abdominal and body wall muscles, or the myogenic progenitor cells that form the limb and head muscles. Shh also functions in somites to establish and maintain the medio-lateral boundaries of epaxial and hypaxial gene expression. Myf5, and not MyoD, is the target of Shh signaling in the epaxial dermomyotome, as MyoD activation by recombinant Shh protein in presomitic mesoderm explants is defective in Myf5 null embryos. In further support of the inductive function of Shh in epaxial myogenesis, we show that Shh is not essential for the survival or the proliferation of epaxial myogenic progenitors. However, Shh is required specifically for the survival of sclerotomal cells in the ventral somite as well as for the survival of ventral and dorsal neural tube cells. We conclude, therefore, that Shh has multiple functions in the somite, including inductive functions in the activation of Myf5, leading to the determination of epaxial dermomyotomal cells to myogenesis, as well as trophic functions in the maintenance of cell survival in the sclerotome and adjacent neural tube.

Animals↗

PubMed1998

Potent excitatory influence of prefrontal cortex activity on noradrenergic locus coeruleus neurons.

An influence of the prefrontal cortex on noradrenergic locus coeruleus neurons would have profound implications for the function of the locus coeruleus system. Although the medial prefrontal cortex does not substantially innervate the core of the nucleus locus coeruleus, evidence indicates that the medial prefrontal cortex projects to regions containing locus coeruleus dendrites; indirect medial prefrontal cortex-locus coeruleus projections are also possible. Here, we examined influences of prefrontal cortex activity on locus coeruleus firing rates by activating or inactivating the medial prefrontal cortex while recording impulse activity of locus coeruleus neurons extracellularly in anaesthetized rats. Most of our electrical stimulation experiments were conducted in rats which underwent lesions of the ascending dorsal bundle of noradrenergic fibres from the locus coeruleus to eliminate locus coeruleus projections to the prefrontal cortex, because antidromic activation of locus coeruleus from the prefrontal cortex affects even non-driven locus coeruleus neurons through collaterals. Single pulse stimulation (1 mA, 0.3-0.5 ms) of the dorsomedial (frontal region 2) or prelimbic region of the medial prefrontal cortex synaptically activated 13/16 (81%) or 16/56 (29%) locus coeruleus neurons, respectively. Train stimulation (20 Hz for 0.5 s) synaptically activated greater percentages of locus coeruleus cells, 11/12 cells (92%) for the dorsomedial prefrontal cortex, and 41/50 cells (82%) for the prelimbic cortex. No inhibitory responses in the locus coeruleus were obtained with dorsomedial prefrontal stimulation, and weak inhibition was found in 16% of locus coeruleus cells with prelimbic stimulation. Electrical stimulation of more lateral frontal cortex (Fr1 area) had no effects on locus coeruleus activity. Chemical stimulation of the dorsomedial prefrontal cortex with L-glutamate (10 or 100 mM) or D,L-homocysteic acid (10 mM) phasically activated 15/26 (55%) locus coeruleus cells, and 15/68 cells (22%) with prelimbic stimulation; such activation was sometimes followed by long-lasting oscillatory activity. No locus coeruleus cells exhibited purely inhibitory responses with chemical stimulation of any prefrontal cortex site. Inactivation of the dorsomedial or prelimbic region of the prefrontal cortex with lidocaine microinjection (2%, 180 or 300 nl) reduced locus coeruleus firing rates in 6/10 (60%) or 7/19 (37%) locus coeruleus cells, respectively. In no case did lidocaine in any prefrontal cortex site activate a locus coeruleus neuron. These results indicate that the medial prefrontal cortex provides a potent excitatory influence on locus coeruleus neurons. The fact that inactivation of the medial prefrontal cortex suppressed locus coeruleus firing indicates that the medial prefrontal cortex also provides a resting tonic excitatory influence on locus coeruleus activity.

Anesthesia↗

PubMed1998

Mediolateral patterning of somites: multiple axial signals, including Sonic hedgehog, regulate Nkx-3.1 expression.

The axial structures, the notochord and the neural tube, play an essential role in the dorsoventral patterning of somites and in the differentiation of their many cell lineages. Here, we investigated the role of the axial structures in the mediolateral patterning of the somite by using a newly identified murine homeobox gene, Nkx-3.1, as a medial somitic marker in explant in vitro assays. Nkx-3.1 is dynamically expressed during somitogenesis only in the youngest, most newly-formed somites at the caudal end of the embryo. We found that the expression of Nkx-3.1 in pre-somitic tissue explants is induced by the notochord and maintained in newly-differentiated somites by the notochord and both ventral and dorsal parts of the neural tube. We showed that Sonic hedgehog (Shh) is one of the signaling molecules that can reproduce the effect of the axial structures by exposing explants to either COS cells transfected with a Shh expression construct or to recombinant SHH. Shh could induce and maintain Nkx-3.1 expression in pre-somitic mesoderm and young somites but not in more mature, differentiated ones. The effects of Shh on Nkr-3.1 expression were antagonized by a forskolin-induced increase in the activity of cyclic AMP-dependent protein kinase A. Additionally, we confirmed that the expression of the earliest expressed murine myogenic marker, myf 5, is also regulated by the axial structures but that Shh by itself is not capable of inducing or maintaining it. We suggest that the establishment of somitic medial and lateral compartments and the early events in myogenesis are governed by a combination of positive and inhibitory signals derived from the neighboring structures, as has previously been proposed for the dorsoventral patterning of somites.

Animals↗

PubMed1998

Sonic hedgehog is essential to foregut development.

Congenital malformation of the foregut is common in humans, with an estimated incidence of 1 in 3000 live births, although its aetiology remains largely unknown. Mice with a targeted deletion of Sonic hedgehog (Shh) have foregut defects that are apparent as early as embryonic day 9.5, when the tracheal diverticulum begins to outgrow. Homozygous Shh-null mutant mice show oesophageal atresia/stenosis, tracheo-oesophageal fistula and tracheal and lung anomalies, features similar to those observed in humans with foregut defects. The lung mesenchyme shows enhanced cell death, decreased cell proliferation and downregulation of Shh target genes. These results indicate that Shh is required for the growth and differentiation of the oesophagus, trachea and lung, and suggest that mutations in SHH and its signalling components may be involved in foregut defects in humans.

Animals↗

PubMed1997

Expression of murine Lhx5 suggests a role in specifying the forebrain.

A LIM homeobox gene, Lim5, is known to be expressed in the forebrain of Xenopus and zebrafish (Toyama et al. [1995] Dev. Biol. 170:583-593). Results from developmental and comparative studies of its mouse ortholog, Lhx5, indicate that this gene may play important roles in forebrain development. Lhx5 expression is detected in the most anterior portion of the neural tube at the headfold stage, overlapping partially with Otx2 expression domain. After neural tube closure, Lhx5 is expressed as a transverse stripe, covering most of the diencephalic primordium. This expression recedes to restricted areas as Dlx gene expression occurs. By midgestation, both genes, Lhx5 and Dlx5, are expressed in the diencephalon and ventral telencephalon in an alternating complementary pattern. It may be that Dlx inhibits Lhx5, and this may represent a step of early regionalization of the forebrain. Lhx5 is also expressed in midbrain, hindbrain, and spinal cord, overlapping extensively with Lhx1 starting from day E10.5 of gestation. The early, persistent, and dynamic expression of Lhx5 suggests a regulatory function in forebrain formation.

Amino Acid Sequence↗

PubMed1997

Hoxd-12 differentially affects preaxial and postaxial chondrogenic branches in the limb and regulates Sonic hedgehog in a positive feedback loop.

Several 5' members of the Hoxd cluster are expressed in nested posterior-distal domains of the limb bud suggesting a role in regulating anteroposterior pattern of skeletal elements. While loss-of-function mutants have demonstrated a regulatory role for these genes in the developing limb, extensive functional overlaps between various different Hox genes has hampered elucidation of the roles played by individual members. In particular, the function of Hoxd-12 in the limb remains obscure. Using a gain-of-function approach, we find that Hoxd-12 misexpression in transgenic mice produces apparent transformations of anterior digits to posterior morphology and digit duplications, while associated tibial hemimelia and other changes indicate that formation/growth of certain skeletal elements is selectively inhibited. If the digital arch represents an anterior bending of the main limb axis, then the results are all reconcilable with a model in which Hoxd-12 promotes formation of postaxial chondrogenic condensations branching from this main axis (including the anteriormost digit) and selectively antagonizes formation of 'true' preaxial condensations that branch from this main axis (such as the tibia). Hoxd-12 misexpression can also induce ectopic Sonic hedgehog (Shh) expression, resulting in mirror-image polydactyly in the limb. Misexpression of Hoxd-12 in other lateral plate derivatives (sternum, pelvis) likewise phenocopies several luxoid/luxate class mouse mutants that all share ectopic Shh signalling. This suggests that feedback activation of Shh expression may be a major function of Hoxd-12. Hoxd-12 can bind to and transactivate the Shh promoter in vitro. Furthermore, expression of either exogenous Hoxd-11 or Hoxd-12 in cultured limb bud cells, together with FGF, induces expression of the endogenous Shh gene. Together these results suggest that certain 5' Hoxd genes directly amplify the posterior Shh polarizing signal in a reinforcing positive feedback loop during limb bud outgrowth.

Animals↗

PubMed1996

Cyclopia and defective axial patterning in mice lacking Sonic hedgehog gene function.

Targeted gene disruption in the mouse shows that the Sonic hedgehog (Shh) gene plays a critical role in patterning of vertebrate embryonic tissues, including the brain and spinal cord, the axial skeleton and the limbs. Early defects are observed in the establishment or maintenance of midline structures, such as the notochord and the floorplate, and later defects include absence of distal limb structures, cyclopia, absence of ventral cell types within the neural tube, and absence of the spinal column and most of the ribs. Defects in all tissues extend beyond the normal sites of Shh transcription, confirming the proposed role of Shh proteins as an extracellular signal required for the tissue-organizing properties of several vertebrate patterning centres.

Animals↗