Homeobox (Hox) genes encode a family of transcription factors that regulate embryonic patterning and organogenesis. In embryos, alterations of the normal pattern of Hox gene expression result in homeotic transformations and malformations. Disruption of the Hoxa1 gene, the most 3 member of the Hoxa cluster and a retinoic acid (RA) direct target gene, results in abnormal ossification of the skull, hindbrain, and inner ear deficiencies, and neonatal death. We have generated Hoxa1 genes positioned at the 3Ј-end are expressed earlier and more anterior, whereas 5Ј-end genes are expressed at later times and more posterior (reviewed in Refs. 1 and 5). Colinear gene expression from these Hox gene clusters may involve an initial change in chromatin structure (histone modification and chromatin decondensation) of the entire loci, which would then facilitate the programmed expression of genes along the clusters by a progressive 3Ј to 5Ј change in higher order chromatin structure (6). Treatment of teratocarcinoma cells or embryonic stem cells with retinoic acid (RA), which acts via the retinoic acid receptors (RAR␣, -␤, and -␥ and their isoforms (reviewed in Refs. 7 and 8), results in the sequential activation of several Hox genes in a manner that resembles their positions in the clusters, e.g. 3Ј genes are activated before 5Ј genes (7-9). Moreover, our laboratory discovered the presence of a retinoic acid response element (RARE) in the 3Ј enhancer of the Hoxa1 gene (10 -13). This RARE is also functional in transgenic mice (14 -16). We also demonstrated that unlike F9 Wt cells, F9 RAR␥ Ϫ/Ϫ cells fail to express the Hoxa1 gene in response to RA (17). Alterations in the normal pattern of Hox gene expression in embryos result in homeotic transformations and malformations, and frequently, in perinatal lethality (3,18). For instance, the targeted inactivation in mice of both alleles of the most 3Ј member of the Hoxa cluster, the Hoxa1 gene, leads to numerous developmental defects, including hindbrain deficiencies and abnormal skull ossification, and ultimately, to neonatal death (19 -22). Numerous studies have demonstrated that the hindbrain defects in Hoxa1 Ϫ/Ϫ mice result in part from the failure of Hoxb1 to reach its anterior limit of expression at the presumptive rhombomere 3/rhombomere 4 (r3/4) boundary, which initiates a cascade of gene misexpression that results in the misspecification of the hindbrain compartments from r2 to r5 (23-26). Furthermore, the ectopic expression of Hoxa1 in transgenic mice leads to the ectopic activation of Hoxb1 in r2, produces anterior abnormalities, including the reorganization of the developing hindbrain, and ultimately results in embryonic death (27). These observations highlight the importance of the Hoxa1 gene and suggest that the regulatory effects of retinoids on cell growth and on embryonic patterning may be * This work was supported by National Institutes of Health Grants R01CA43796 (to L. J. G.) and UR 2U19HDO35466. The costs of publication of this article were defrayed in part by th...

The ability of embryonic stem (ES) cells to differentiate into different cell fates has been extensively evaluated, and several protocols exist for the generation of various types of cells from mouse and human ES cells. We used a differentiation protocol that involves embryoid body formation and all-trans-retinoic acid (RA, 5 microM) treatment (EB/5 microM RA) to test the ability of Hoxa1 null ES cells to adopt a neuronal fate. Hoxa1(-/-) ES cells, when treated in this EB/5 microM RA protocol, failed to differentiate along a neural lineage; Hoxa1(-/-) ES cells express severalfold lower levels of many neuronal differentiation markers, including nestin, beta-tubulin III, and MAP2, and conversely, higher levels of endodermal differentiation markers (i.e., Sox17, Col4a1) than wild type (Wt) cells. Reintroduction of exogenous Hoxa1, under the control of the metallothionein I promoter, into Hoxa1(-/-) ES cells restored their capacity to generate neurons. Moreover, overexpression of Sox17, a gene that regulates endodermal differentiation, in Wt ES cells resulted in endodermal differentiation and in a complete abolition of beta-tubulin III expression. Thus, Hoxa1 activity is essential for the neuronal differentiation of ES cells in the presence of all-trans-RA, and Hoxa1 may promote neural differentiation by inhibiting Sox17 expression. Pharmacological manipulation of Hoxa1 levels may provide a method for promoting neuronal differentiation for therapeutic uses. Furthermore, because mutations in the Hoxa1 gene can cause autism spectrum disorder in humans, these data also provide important mechanistic insights into the early developmental processes that may result in this disorder.

Diepoxybutane (DEB) is the most potent metabolite of the environmental chemical 1, 3-butadiene (BD), which is prevalent in petrochemical industrial areas. BD is a known mutagen and human carcinogen, and possesses multi-systems organ toxicity. We recently reported that DEB-induced cell death in TK6 lymphoblasts was due to the occurrence of apoptosis, and not necrosis. In this study, we investigated the molecular mechanisms responsible for DEB-induced apoptosis in these cells. Bax and Bak were found to be over-expressed and activated, and the mitochondrial trans-membrane potential was attenuated in cells undergoing DEB-induced apoptosis. Cytochrome c was depleted from the mitochondria of TK6 cells undergoing apoptosis, and was released into the cytosol in Jurkat-T lymphoblasts exposed to the same concentrations of DEB. Executioner caspase 3 was deduced to be activated by initiator caspase 9. DEB induced reactive oxygen species (ROS) formation, and the ROS scavenger N-acetyl-L-cysteine effectively blocked DEB-induced apoptosis in TK6 cells.Collectively, these results demonstrate that the mitochondrial apoptotic pathway is activated to mediate DEB-induced apoptosis in human TK6 lymphoblasts. These results further demonstrate that DEB-induced apoptosis is also mediated by the DEB-induced generation of ROS. This is the first report to examine the mechanism of DEB-induced apoptosis in human lymphoblasts.

Embryonic stem (ES) cells are pluripotent cells that can differentiate into all three main germ layers: endoderm, mesoderm, and ectoderm. Although a number of methods have been developed to differentiate ES cells into neuronal phenotypes such as sensory and motor neurons, the efficient generation of GABAergic interneurons from ES cells still presents an ongoing challenge. Because the main output of inhibitory GABAergic interneurons is the gamma-aminobutyric-acid (GABA), a neurotransmitter whose controlled homeostasis is required for normal brain function, the efficient generation in culture of functional interneurons may have future implications on the treatment of neurological disorders such as epilepsy, autism, and schizophrenia. The goal of this work was to examine the generation of GABAergic neurons from mouse ES cells by comparing an embryoid body-based methodology versus a hydrogel-based encapsulation protocol that involves the use of all-trans-retinoid acid (RA). We observed that 1) there was a 2-fold increase in neuronal differentiation in encapsulated versus non-encapsulated cells and 2) there was an increase in the specificity for interneuronal differentiation in encapsulated cells, as assessed by mRNA expression and electrophysiology approaches. Furthermore, our results indicate that most of the neurons obtained from encapsulated mouse ES cells are GABA-positive (~87%). Thus, these results suggest that combining encapsulation of ES cells and RA treatment provide a more efficient and scalable differentiation strategy for the generation in culture of functional GABAergic interneurons. This technology may have implications for future cell replacement therapies and the treatment of CNS disorders.

Exposure to persistent environmental pollutants may constitute an important factor on the onset of a number of neurological disorders such as autism, Parkinson’s disease, and Attention Deficit Disorder (ADD), which have also been linked to reduced GABAergic neuronal function. GABAergic neurons produce γ-aminobutyric acid (GABA), which is the main inhibitory neurotransmitter in the brain. However, the lack of appropriate models has hindered the study of suspected environmental pollutants on GABAergic function. In this work, we have examined the effect of hexachlorobenzene (HCB), a persistent and bioaccumulative environmental pollutant, on the function and morphology of GABAergic neurons generated in vitro from mouse embryonic stem (ES) cells. We observed that: (1) treatment with 0.5 nM HCB did not affect cell viability, but affected the neuronal differentiation of ES cells; (2) HCB induced the production of reactive oxygen species (ROS); and (3) HCB repressed neurite outgrowth in GABAergic neurons, but this effect was reversed by the ROS scavenger N-acetylcysteine (NAC). Our study also revealed that HCB did not significantly interfere with the function of K+ ion channels in the neuronal soma, which indicates that this pollutant does not affect the maturation of the GABAergic neuronal soma. Our results suggest a mechanism by which environmental pollutants interfere with normal GABAergic neuronal function and may promote the onset of a number of neurological disorders such as autism and ADD.