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NASA NTRS2002

The retinoic acid signaling pathway regulates anterior/posterior patterning in the nerve cord and pharynx of amphioxus, a chordate lacking neural crest

Amphioxus, the closest living invertebrate relative of the vertebrates, has a notochord, segmental axial musculature, pharyngeal gill slits and dorsal hollow nerve cord, but lacks neural crest. In amphioxus, as in vertebrates, exogenous retinoic acid (RA) posteriorizes the embryo. The mouth and gill slits never form, AmphiPax1, which is normally downregulated where gill slits form, remains upregulated and AmphiHox1 expression shifts anteriorly in the nerve cord. To dissect the role of RA signaling in patterning chordate embryos, we have cloned the single retinoic acid receptor (AmphiRAR), retinoid X receptor (AmphiRXR) and an orphan receptor (AmphiTR2/4) from amphioxus. AmphiTR2/4 inhibits AmphiRAR-AmphiRXR-mediated transactivation in the presence of RA by competing for DR5 or IR7 retinoic acid response elements (RAREs). The 5' untranslated region of AmphiTR2/4 contains an IR7 element, suggesting possible auto- and RA-regulation. The patterns of AmphiTR2/4 and AmphiRAR expression during embryogenesis are largely complementary: AmphiTR2/4 is strongly expressed in the cerebral vesicle (homologous to the diencephalon plus anterior midbrain), while AmphiRAR expression is high in the equivalent of the hindbrain and spinal cord. Similarly, while AmphiTR2/4 is expressed most strongly in the anterior and posterior thirds of the endoderm, the highest AmphiRAR expression is in the middle third. Expression of AmphiRAR is upregulated by exogenous RA and completely downregulated by the RA antagonist BMS009. Moreover, BMS009 expands the pharynx posteriorly; the first three gill slit primordia are elongated and shifted posteriorly, but do not penetrate, and additional, non-penetrating gill slit primordia are induced. Thus, in an organism without neural crest, initiation and penetration of gill slits appear to be separate events mediated by distinct levels of RA signaling in the pharyngeal endoderm. Although these compounds have little effect on levels of AmphiTR2/4 expression, RA shifts pharyngeal expression of AmphiTR2/4 anteriorly, while BMS009 extends it posteriorly. Collectively, our results suggest a model for anteroposterior patterning of the amphioxus nerve cord and pharynx, which is probably applicable to vertebrates as well, in which a low anterior level of AmphiRAR (caused, at least in part, by competitive inhibition by AmphiTR2/4) is necessary for patterning the forebrain and formation of gill slits, the posterior extent of both being set by a sharp increase in the level of AmphiRAR. Supplemental data available on-line.

NASA Discipline Evolutionary Biology↗

NASA NTRS2002

Testing putative hemichordate homologues of the chordate dorsal nervous system and endostyle: expression of NK2.1 (TTF-1) in the acorn worm Ptychodera flava (Hemichordata, Ptychoderidae)

Recent phylogenetic investigations have confirmed that hemichordates and echinoderms are sister taxa. However, hemichordates share several cardinal characterstics with chordates and are thus an important taxon for testing hypotheses of homology between key chordate characters and their putative hemichordate antecedents. The chordate dorsal nervous system (DNS) and endostyle are intriguing characters because both hemichordate larval and adult structures have been hypothesized as homologues. This study attempts to test these purported homologies through examination of the expression pattem of a Ptychodera flava NK2 gene, PfNK2.1, because this gene is expressed both in the DNS and endostyle/thyroid in a wide range of chordate taxa. We found that PfNK2.1 is expressed in both neuronal and pharyngeal structures, but its expression pattem is broken up into distinct embryonic and juvenile phases. During embryogenesis, PfNK2.1 is expressed in the apical ectoderm, with transcripts later detected in presumable neuronal structures, including the apical organ and ciliated feeding band. In the developing juvenile we detected PfNK2.1 signal throughout the pharynx, including the stomochord, and later in the hindgut. We conclude that the similar utilization of NK2.1 in apical organ development and chordate DNS is probably due to a more general role for NK2.1 in neurogenesis and that hemichordates do not possess a homologue of the chordate DNS. In addition, we conclude that P. flava most likely does not possess a true endostyle; rather during the evolution of the endostyle NK2.1 was recruited from its more general role in pharynx development.

NASA Discipline Evolutionary Biology↗

NASA NTRS1989

Anatomical study of the final common pathway for vocalization in the cat

Results are presented of an anatomical study of the neuronal pathways in the cat, via which the periaqueductal gray (PAG) produces excitation of motoneurons involved in vocalization. It is shown that a specific cell group in the lateral part of the caudal PAG and in the tegmentum just lateral to it projects bilaterally to the nucleus retroambiguus (NRA) in the caudal medulla oblongata. Neurons in the NRA in turn project, via a contralateral pathway through the ventral funiculus of the spinal cord, to the motoneuronal cell groups innervating intercostal and abdominal muscles. In the brainstem, the NRA neurons project to the motoneuronal cell groups innervating mouth-opening and perioral muscles as well as to motoneurons innervating the pharynx, soft palate, and tongue. These results indicate that the projections from PAG via NRA to vocalization motoneurons form the final common pathway in vocalization.

Holstege, Gert↗

NASA NTRS1994

Microbiological analysis of debris from Space Transportation System (STS)-55 Spacelab D-2

Filter debris from the Spacelab module D-2 of STS-55 was analyzed for microbial contamination. Debris from cabin and avionics filters was collected by Kennedy Space Center personnel on May 8, 1993, 2 days postflight. Debris weights were similar to those of previous Spacelab missions. Approximately 5.1E+5 colony forming units per gram of debris were enumerated from the cabin and avionics filter debris, respectively. these numbers were similar in previous missions for which the entire contents were analyzed without sorting of the material. Bacterial diversity was small compared to previous missions, with no gram negative bacteria isolated. Only one bacterial species, Corynebacterium pseudodiphtheriticum, was not isolated previously by the laboratory from Spacelab debris. This organism is a normal inhabitant of the pharynx. A table listing all species of bacteria isolated by the laboratory from previous Spacelab air filters debris collection is provided.

Huff, T. L.↗

NASA NTRS2002

AmphiFoxE4, an amphioxus winged helix/forkhead gene encoding a protein closely related to vertebrate thyroid transcription factor-2: expression during pharyngeal development

The full-length sequence and developmental expression of amphioxus AmphiFoxE4 are described. Transcripts of the gene are first detected in the pharyngeal endoderm, where the club-shaped gland is forming and subsequently in the definitive gland itself. AmphiFoxE4 is closely related to vertebrate genes encoding the thyroid-specific transcription factor-2 (TTF2), which plays an early developmental role in the morphogenesis of the thyroid gland and a later role in hormone-mediated control of thyroid function. In amphioxus, AmphiFoxE4 expression is not thyroid specific because the club-shaped gland, the only structure expressing the gene, is not homologous to the vertebrate thyroid; instead, the thyroid homologue of amphioxus is a specialized region of the pharyngeal endoderm called the endostyle. We propose that (a) the pharynx of an amphioxus-like ancestor of the vertebrates included a club-shaped gland that expressed FoxE4 as well as an endostyle that did not, and (b) the club-shaped gland soon disappeared in the vertebrate line of descent but (c) not before there was a homeogenetic transfer of FoxE4 expression from the club-shaped gland to the nearby endostyle. Such a transfer could have provided part of the genetic program enabling the endostyle to separate from the pharyngeal endoderm and migrate away as the rudiment of the thyroid gland.

Non-NASA Center↗

NASA NTRS1998

Role of heat shock protein Hsp25 in the response of the orofacial nuclei motor system to physiological stress

Although expression of the small heat shock protein family member Hsp25 has been previously observed in the central nervous system (CNS), both constitutively and upon induction, its function in the CNS remains far from clear. In the present study we have characterized the spatial pattern of expression of Hsp25 in the normal adult mouse brain as well as the changes in expression patterns induced by subjecting mice to experimental hyperthermia or hypoxia. Immunohistochemical analysis revealed a surprisingly restricted pattern of constitutive expression of Hsp25 in the brain, limited to the facial, trigeminal, ambiguus, hypoglossal and vagal motor nuclei of the brainstem. After hyperthermia or hypoxia treatment, significant increases in the levels of Hsp25 were observed in these same areas and also in fibers of the facial and trigeminal nerve tracts. Immunoblot analysis of protein lysates from brainstem also showed the same pattern of induction of Hsp25. Surprisingly, no other area in the brain showed expression of Hsp25, in either control or stressed animals. The highly restricted expression of Hsp25 implies that this protein may have a specific physiological role in the orofacial motor nuclei, which govern precise coordination between muscles of mastication and the pharynx, larynx, and face. Its rapid induction after stress further suggests that Hsp25 may serve as a specific molecular chaperone in the lower cholinergic motor neurons and along their fibers under conditions of stress or injury. Copyright 1998 Elsevier Science B.V.

Non-NASA Center↗