Bidirectional flow of action potentials in axons drives activity dynamics in neuronal cultures

Objective. Recent technological advances are revealing the complex physiology of the axon and challenging long-standing assumptions. Namely, while most action potential (AP) initiation occurs at the axon initial segment in central nervous system neurons, initiation in distal parts of the axon has been reported to occur in both physiological and pathological conditions. The functional role of these ectopic APs, if exists, is still not clear, nor its impact on network activity dynamics.Approach. Using an electrophysiology platform specifically designed for assessing axonal conduction we show here for the first time regular and effective bidirectional axonal conduction in hippocampal and dorsal root ganglia cultures. We investigate and characterize this bidirectional propagation both in physiological conditions and after distal axotomy.Main results.A significant fraction of APs are not coming from the canonical synapse-dendrite-soma signal flow, but instead from signals originating at the distal axon. Importantly, antidromic APs may carry information and can have a functional impact on the neuron, as they consistently depolarize the soma. Thus, plasticity or gene transduction mechanisms triggered by soma depolarization can also be affected by these antidromic APs. Conduction velocity is asymmetrical, with antidromic conduction being slower than orthodromic.Significance.Altogether these findings have important implications for the study of neuronal functionin vitro, reshaping our understanding on how information flows in neuronal cultures.

Response to criticisms of the US FDA parametric approach for withdrawal time estimation: rebuttal and comparison to the nonparametric method proposed by Concordet and Toutain

The benefits and drawbacks of using nonparametric methods for estimating product withdrawal times have been debated for many years. This issue was recently revived by Concordet & Toutain (1997a, b) when they described a nonparametric method for withdrawal time estimation. The authors urged the international adoption of this approach, basing their recommendation on three fundamental concerns: (1) the lack of a consistent official procedure for determining a withdrawal time within the European Union (EU); (2) the need to identify a statistical method for improving the international harmonization of withdrawal times for new chemical entities; and (3) a lack of confidence in the robustness of the US Food and Drug Administration/Center for Veterinary Medicine (US FDA) procedure, particularly with respect to minor violations in the underlying parametric assumptions. Due to the critical nature of these issues, the US FDA considers it vital to respond to these concerns. This paper provides a description of the US FDA parametric procedure. We also examine the statistical concerns expressed by Concordet and Toutain, identifying the reasons for our confidence in the US FDA parametric approach. Finally, using their Monte Carlo simulation models, we generate additional datasets to explore the behaviour of their nonparametric procedure and evaluate its ability to support US FDA regulatory activities.

An olfactory sensory map develops in the absence of normal projection neurons or GABAergic interneurons

Olfactory sensory neurons expressing a given odorant receptor project to two topographically fixed glomeruli in the olfactory bulb. We have examined the contribution of different cell types in the olfactory bulb to the establishment of this topographic map. Mice with a homozygous deficiency in Tbr-1 lack most projection neurons, whereas mice with a homozygous deficiency in Dlx-1 and Dlx-2 lack most GABAergic interneurons. Mice bearing a P2-IRES-tau-lacZ allele and deficient in either Tbr-1 or Dlx-1/Dlx-2 reveal the convergence of axons to one medial and one lateral site at positions analogous to those observed in wild-type mice. These observations suggest that the establishment of a topographic map is not dependent upon cues provided by, or synapse formation with, the major neuronal cell types in the olfactory bulb.

A murine AP-endonuclease gene-targeted deficiency with post-implantation embryonic progression and ionizing radiation sensitivity

Apurinic/apyrimidinic endonuclease (here designated APE/REF) carries out repair incision at abasic or single-strand break damages in mammals. This multifunctional protein also has putative role(s) as a cysteine 'reducing factor' (REF) in cell-stress transcriptional responses. To assess the significance of APE/REF for embryonic teratogenesis we constructed a more precisely targeted Ape/Ref-deficient genotype in mice. Ape/Ref gene replacement in ES cells eliminated the potential of APE/REF protein synthesis while retaining the Ape/Ref bi-directional promoter that avoided potential inactivation of an upstream gene. Chimeric animals crossed into Tac:N:NIHS-BC produced germline transmission. Homozygous null Ape/Ref-embryos exhibited successful implantation and nearly normal developmental progression until embryonic day 7.5 followed by morphogenetic failure and adsorption of embryos by day 9.5. We characterized the cellular events proceeding to embryonic lethality and examined ionizing radiation sensitivity of pre-implantation Ape/Ref-null embryos. After intermating of heterozygotes, Mendelian numbers of putative Ape/Ref-null progeny embryos at day 6.5 displayed a several-fold elevation of pycnotic, fragmenting cell nuclei within the embryo proper-the epiblast. Increased cell-nucleus degeneration occurred within epiblast cells while mitosis continued and before obvious morphogenetic disruption. Mitogenic response to epiblast cell death, if any, was ineffective for replacement of lost cells. Extra-embryonic yolk sac, a trophectoderm derived lineage retained normal appearance to day 9. Explanted homozygous Ape/Ref-null blastocysts displayed increased sensitivity to gamma-irradiation, most likely a manifestation of APE/REF incision defect. Our study establishes that this new Ape/Ref deficiency genotype is definitely capable of post-implantation developmental progression to the onset of gastrulation. Function(s) of APE/REF in base damage incision and also conceivably in mitogenic responses towards epiblast cell death are critical for transit through the gastrulation stage of embryonic growth and development.

Mutations of the homeobox genes Dlx-1 and Dlx-2 disrupt the striatal subventricular zone and differentiation of late born striatal neurons

The striatum has a central role in many neurobiological processes, yet little is known about the molecular control of its development. Inroads to this subject have been made, due to the discovery of transcription factors, such as the Dlx genes, whose expression patterns suggest that they have a role in striatal development. We report that mice lacking both Dlx-1 and Dlx-2 have a time-dependent block in striatal differentiation. In these mutants, early born neurons migrate into a striatum-like region, which is enriched for markers of the striosome (patch) compartment. However, later born neurons accumulate within the proliferative zone. Several lines of evidence suggest that mutations in Dlx-1 and Dlx-2 produce abnormalities in the development of the striatal subventricular zone and in the differentiation of striatal matrix neurons.

Inactivation of mouse alpha-globin gene by homologous recombination: mouse model of hemoglobin H disease

We have disrupted the 5' locus of the duplicated adult alpha-globin genes by gene targeting in the mouse embryonic stem cells and created mice with alpha-thalassemia syndromes. The heterozygous knockout mice (.alpha/alpha alpha) are asymptomatic like the silent carriers in humans whereas the homozygous knockout mice (.alpha/.alpha) show hemolytic anemia. Mice with three dysfunctional alpha-globin genes generated by breeding the 5' alpha-globin knockouts (.alpha/alpha alpha) and the deletion type alpha-thalassemia mice (../alpha alpha) produce severe hemoglobin H disease and they die in utero. These results indicate that the 5' alpha-globin gene is the predominant locus in mice, and suggest that it is even more dominant than its human homologue.

Deletion of beta 1 integrins in mice results in inner cell mass failure and peri-implantation lethality

Integrin receptors for extracellular matrix receptors are important effectors of cell adhesion, differentiation, and migration in cultured cells and are believed to be critical effectors of these processes during development. To determine when beta 1 integrins become critical during embryonic development, we generated mutant mice with a targeted disruption of the beta 1 integrin subunit gene. Heterozygous mutant mice were normal. Homozygous loss of beta 1 integrin expression was lethal during early postimplantation development. Homozygous embryos lacking beta 1 integrins formed normal-looking blastocysts and initiated implantation at E4.5. However, the E4.5 beta 1-null embryos in situ had collapsed blastocoeles, and whereas the trophoblast penetrated the uterine epithelium, extensive invasion of the decidua was not observed. Laminin-positive endoderm cells were detected in the inner cell mass area, but endoderm morphogenesis and migration were defective. By E5.5 beta 1-null embryos had degenerated extensively. In vitro analysis showed that trophoblast function in beta 1-null peri-implantation embryos was largely normal, including expression of tissue-specific markers, and outgrowth on fibronectin- and vitronectin-coated, although not on laminin-coated substrates. In contrast, the inner cell mass region of beta 1-null blastocyst outgrowths, and inner cell masses isolated from beta 1-null blastocysts, showed highly retarded growth and defective extraembryonic endoderm morphogenesis and migration. These data suggest that beta 1 integrins are required for normal morphogenesis of the inner cell mass and are essential mediators of growth and survival of cells of the inner cell mass. Failure of continued trophoblast development in beta 1-null embryos after inner cell mass failure could be attributable to either an intrinsic requirement for beta 1 integrins for later stages of trophoblast development, or to the lack of trophic signals from the beta 1-null inner cell mass.

Epithelial immaturity and multiorgan failure in mice lacking epidermal growth factor receptor

Since the discovery that epidermal growth factor (EGF) can accelerate opening of the eyelids, the EGF receptor (EGF-R) has been extensively studied and is now considered to be a prototype tyrosine kinase receptor. Binding of EGF or of transforming growth factor-alpha (TGF-alpha) or other related factors activates the receptor and induces cell proliferation and differentiation. Although it is not found on haematopoietic cells, the EGF-R is widely expressed in mammals and has been implicated in various stages of embryonic development. Here we investigate the developmental and physiological roles of this receptor and its ligands by inactivating the gene encoding EGF-R. We find that EGF-R-/- mice survive for up to 8 days after birth and suffer from impaired epithelial development in several organs, including skin, lung and gastrointestinal tract.

Percoll purification of chromaffin granules inhibits their ability to take up and maintain calcium

Secretory granules of the adrenal medulla have recently been shown to be able to sequester and release Ca2+, in addition to their previously established role as carriers of secretory products. In order to study the ability of these or any other secretory granules to participate in intracellular calcium homeostasis, it is imperative that they should be free of other contaminating Ca2+ sequestering organelles, and that the Ca2+ uptake and release mechanisms of those granules should remain intact throughout any chosen purification procedure. We report here that chromaffin granules which were purified by the isopycnic gradient medium Percoll, or even incubated with it, showed an attenuated ability to sequester Ca2+.

Cytochrome P-450 metabolic activity in embryonic and extraembryonic tissue lineages of mouse embryos

Mouse morulae, blastocysts, and embryonic and extraembryonic tissue layers were examined for benzo[a]-pyrene metabolism by cytochrome P-450, using the sister chromatid exchange assay. Benzo[a]pyrene exposure in vitro increased sister chromatid exchanges in blastocysts of all genetically responsive mice examined [BALB/cDub, C3H/AnfCum, and outbred Dub:(ICR) strains] but not blastocysts of the nonresponsive AKR/J strain. Benzo[a]pyrene treatment of responsive 7 1/2- and 8 1/2-day (postimplantation-stage) embryos, either intact or as separate tissue layers, increased sister chromatid exchanges in tissues of both embryonic and extraembryonic lineages--i.e., in the embryo proper, in isolated embryonic ectoderm, and in yolk sac, chorion, extraembryonic ectoderm, and extraembryonic endoderm layers. These results indicate that cytochrome P-450 is active in most or all tissues of the early mammalian embryo. It could metabolize xenobiotic molecules reaching the conceptus near the onset of morphogenesis and organogenesis, or it could have another as yet undefined role in normal development.

Cultured mouse embryos metabolize benzo[a]pyrene during early gestation: genetic differences detectable by sister chromatid exchange

Mouse embryos explanted at 7 1/2 or 8 1/2 days of gestation were cultured in medium containing benzo[a]pyrene and supplemented with 5-bromodeoxyuridine to allow detection of sister chromatid exchanges. The murine Ah locus regulates the inducible metabolism of polycyclic hydrocarbons such as benzo[a]pyrene. A high frequency of sister chromatid exchange was induced by benzo[a]pyrene in embryos from three Ah-"responsive" inbred strains (BALB/cDub, C3H/AnfCum, and C57BL/6N); there was little or no increase in two Ah-"nonresponsive" inbred strains (AKR/J and DBA/2J). Benzo[a]pyrene also induced sister chromatid exchanges in the Ah-responsive recombinant inbred line B6NXAKN-12 but not in the Ah-nonresponsive recombinant inbred line B6NXAKN-3. Sister chromatid exchange in cultured Ah-responsive mouse embryos was thus shown to be a sensitive assay. These data provide direct evidence that genetically responsive mouse embryos (early postimplantation stage) possess the subcellular processes necessary for induction of enzymes that metabolize benzo[a]pyrene to its chemically active forms(s). Both the Ah regulatory gene product (a cytoslic receptor) and the structural gene product (inducible cytochrome P1-450) therefore appear to be functional at an early embryonic age. Furthermore, this metabolic capacity may play an important role in the damage to embryonic cells by polycyclic hydracarbons.

Dose-response relationship for X-ray induced dominant lethal mutations detected in mouse embryos in vitro

X-Irradiated male mice sired offspring that expressed dominant lethal mutations during development in vitro. These mutations were expressed as arrest of the embryo either before or after blastocyst formation and were induced in germ cells irradiated as spermatids. When the dose-response data for the induced dominant lethal mutation rate were fit to a linear-quadratic model for dose-response curves, they were found to be compatible with mutation induction by both one-track and two-track processes, and the calculated interaction distance agreed well with those values found for radiation-induced translocations in Vicia faba and mouse spermatogonia. This correlation suggests that dominant lethal mutations in embryos and translocations in germ cells both result from similar radiation-induced chromosome damage.