Articles written in Journal of Biosciences

• Immunoneutralization of insulin partially modifies dorsoventral patterning in developing frog embryos

Determination of anteroposterior and dorsoventral axes is an important early event in the development of vertebrates involving extensive cellular interactions including inductive events. Recently we showed that insulin plays an essential role in prepancreatic development of the frogMicrohyla ornata. In the present study we have investigated the effects of immunoneutralization of endogenous insulin on the process of pattern formation. Treatment of neurulating embryos with antiserum to insulin caused abnormal pattern formation. The defects included loss of normal architecture of the neural tube, reduction in the size of the neural tube and, most conspicuously, rotation of the dorsoventral axis of the neural tube, notochord and adjoining mesodermal elements. The effects could be alleviated partially by pretreatment of embryos with exogenous insulin. This supports our belief that insulin plays an important role in induction and pattern formation of the amphibian nervous system.

In addition, using 2-deoxy-α-D-glucose, an inhibitor of glucose metabolism, it is shown that the stimulatory effects of exogenous insulin on developing frog embryos are, at least partially, through the glucose metabolism pathway.

• Mesoderm induction in amphibians and chick

Induction is a process in which the developmental pathway of one cell is controlled by signals emitted from another. Mesoderm induction is the first inductive interaction in theXenopus enbryo and probably occurs in all vertebrates. It is a very important event as it is implicated in the regulation of morphogenesis. Nieuwkoop first demonstrated the importance of vegetal endoderm in inducing the mesoderm. Slack and co-workers incorporated the information obtained from experimental embryology in a “three signal” model for mesoderm induction in amphibians (signals arising from ventral vegetal hemisphere, dorsal vegetal hemisphere and the organizer). More recent research has resulted in the detection of mesoderm inducing factors which are members of FGF and TGF--β families. Activin, a member of the TGF-β family, has been shown to induce differential gene expression and cell differentiation in a concentration-dependent manner giving credence to the theory of morphogen gradients. Study of mesoderm induction in the chick embryo is much more difficult due to several reasons. Novel experimental approaches, however, have been used which point to the role of activin and FGF in chick mesoderm induction. The demonstration of mesoderm inducing activity of activin and FGF in other groups of vertebrates, particularly the chick embryo brings out the possibility of a universal mechanism of mesoderm induction being operative in all the vertebrates.

• Hydra constitutively expresses transcripts involved in vertebrate neural differentiation

The diploblastic Hydra is among the most primitive multicellular organisms. Using cross-hybridization withXenopus probes,noggin-like transcripts were detected in the hypostome and basal disc of adult Hydra (Pelmatohydra oligactis), regions with properties similar to that of the amphibian organizer. This points to the possibility of a close molecular similarity between theXenopus and Hydra organizers. The constitutive expression of anoggin-like gene in Hydra may be responsible for its regenerative capacity.

• Ultraviolet irradiation initiates ectopic foot formation in regenerating hydra and promotes budding

We have studied the effects of ultraviolet-C (UVC) and Ultraviolet-B (UVB) on growth and pattern formation inPelmatohydra oligactis. UVC brings about a significant increase in budding in intact hydra while UVB does not exhibit such an effect. Excessive budding could be a response for survival at wavelengths that damage biological tissues. If the head or base piece of a bisected hydra is irradiated and recombined with the unirradiated missing part, regeneration proceeds normally indicating that exposure of a body part with either an intact head or foot to UVC does not influence pattern formation. Most significantly, in the middle piece, but not in the head or the base piece of a trisected hydra, UVC leads to initiation of ectopic feet formation in almost one third of the cases. Thus, UV irradiation interferes with pattern formation in regenerating hydra, possibly by changing positional values, and promotes budding in intact hydra. This is the first report on induction of ectopic feet formation by UV in regenerating hydra and opens up the possibility of using UV irradiation as a tool to understand pattern formation in the enigmatic hydra

• Genome-wide screening reveals the emergence and divergence of RTK homologues in basal Metazoan Hydra magnipapillata

Receptor tyrosine kinases (RTKs) are key components of cell–cell signalling required for growth and development of multicellular organisms. It is therefore likely that the divergence of RTKs and associated components played a significant role in the evolution of multicellular organisms. We have carried out the present study in hydra, a diploblast, to investigate the divergence of RTKs after parazoa and before emergence of triploblast phyla. The domain-based screening using Hidden Markov Models (HMMs) for RTKs in Genomescan predicted gene models of the Hydra magnipapillata genome resulted in identification of 15 RTKs. These RTKs have been classified into eight families based on domain architecture and homology. Only 5 of these RTKs have been previously reported and a few of these have been partially characterized. A phylogeny-based analysis of these predicted RTKs revealed that seven subtype duplications occurred between parazoan–eumetazoan split’ and diploblast–triploblast split’ in animal phyla. These results suggest that most of the RTKs evolved before the radiata–bilateria divergence during animal evolution.

• Evo-devo: Hydra raises its Noggin

Noggin, along with other secreted bone morphogenetic protein (BMP) inhibitors, plays a crucial role in neural induction and neural tube patterning as well as in somitogenesis, cardiac morphogenesis and formation of the skeleton in vertebrates. The BMP signalling pathway is one of the seven fundamental pathways that drive embryonic development and pattern formation in animals. Understanding its evolutionary origin and role in pattern formation is, therefore, important to evolutionary developmental biology (evo-devo).We have studied the evolutionary origin of BMP–Noggin antagonism in hydra, which is a powerful diploblastic model to study evolution of pattern-forming mechanisms because of the unusual cellular dynamics during its pattern formation and its remarkable ability to regenerate. We cloned and characterized the noggin gene from hydra and found it to exhibit considerable similarity with its orthologues at the amino acid level. Microinjection of hydra Noggin mRNA led to duplication of the dorsoventral axis in Xenopus embryos, demonstrating its functional conservation across the taxa. Our data, along with those of others, indicate that the evolutionarily conserved antagonism between BMP and its inhibitors predates bilateral divergence. This article reviews the various roles of Noggin in different organisms and some of our recent work on hydra Noggin in the context of evolution of developmental signalling pathways.

• Xeroderma pigmentosum A homolog from Hydra partially complements DNA repair defect in human XPA-deficient cells

Nucleotide excision repair (NER) pathway is a DNA repair mechanism that rectifies a wide spectrum of DNAlesions. Xeroderma pigmentosum group of proteins (XPA through XPG) orchestrate the NER pathway inhumans. We have earlier studied XPA homolog from Hydra (HyXPA) and found it to be similar to humanXPA. Here, we examined if HyXPA can functionally complement human XPA-deficient cells and reduce theirsensitivity to UV radiation. We found that HyXPAwas able to partially rescue XPA-deficient human cells fromUV by its binding to chromatin of UV-irradiated cells. However, HyXPA failed to bind replication protein A(RPA70), a key interacting partner of human XPA in NER pathway. This could be attributed to changes incertain amino acid residues that have occurred during evolution, leading to prevention of some interactionsbetween Hydra and human proteins.

• # Journal of Biosciences

Volume 46, 2020
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019