• Shweta Saran

      Articles written in Journal of Biosciences

    • The determination of spatial pattern inDictyostelium discoideum

      Vidyanand Nanjundiah Shweta Saran

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      Free-living amoebae of the cellular slime mouldDictyostelium discoideum aggregate when starved and give rise to a long and thin multicellular structure, the slug. The slug resembles a metazoan embryo, and as with other embryos it is possible to specify a fate map. In the case ofDictyostelium discoideum the map is especially simple: cells in the anterior fifth of the slug die and form a stalk while the majority of those in the posterior differentiate into spores. The genesis of this anterior-posterior distinction is the subject of our review. In particular, we ask: what are the relative roles of individual pre-aggregative predispositions and post-aggregative position in determining cell fate? We review the literature on the subject and conclude that both factors are important. Variations in nutritional status, or in cell cycle phase at starvation, can bias the probability that an amoeba differentiates into a stalk cell or a spore. On the other hand, isolates, or slug fragments, consisting of only prestalk cells or only prespore cells can regulate so as to result in a normal range of both cell types. We identify three levels of control, each being responsible for guiding patterning in normal development: (i) ‘coin tossing’, whereby a cell autonomously exhibits a preference for developing along either the stalk or the spore pathway with relative probabilities that can be influenced by the environment; (ii) ‘chemical kinetics’, whereby prestalk and prespore cells originate from undifferentiated amoebae on a probabilistic basis but, having originated, interact (e.g. via positive and negative feedbacks), and the interaction influences the possibility of conversion of one cell type into the other; and (iii) ‘positional information’, in which the spatial distribution of morphogens in the slug influences the pathway of differentiation. In the case of possibilities (i) and (ii), sorting out of like cell types leads to the final spatial pattern. In the case of possibility (iii), the pattern arisesin situ

    • Analysis of Rheb in the cellular slime mold Dictyostelium discoideum: Cellular localization, spatial expression and overexpression

      Pynskhem Bok Swer Pooja Bhadoriya Shweta Saran

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      Dictyostelium discoideum encodes a single Rheb protein showing sequence similarity to human homologues of Rheb. The DdRheb protein shares 52% identity and 100% similarity with the human Rheb1 protein. Fluorescence of Rheb yellow fluorescent protein fusion was detected in the D. discoideum cytoplasm. Reverse transcription-polymerase chain reaction and whole-mount in situ hybridization analyses showed that rheb is expressed at all stages of development and in prestalk cells in the multicellular structures developed. When the expression of rheb as a fusion with lacZ was driven under its own promoter, the 𝛽-galactosidase activity was seen in the prestalk cells. D. discoideum overexpressing Rheb shows an increase in the size of the cell. Treatment of the overexpressing Rheb cells with rapamycin confirms its involvement in the TOR signalling pathway.

    • Classification and expression analyses of homeobox genes from Dictyostelium discoideum

      Himanshu Mishra Shweta Saran

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      Homeobox genes are compared between genomes in an attempt to understand the evolution of animal development. The ability of the protist, Dictyostelium discoideum, to shift between uni- and multicellularity makes this group ideal for studying the genetic changes that may have occurred during this transition. We present here the first genome-wide classification and comparative genomic analysis of the 14 homeobox genes present in D. discoideum. Based on the structural alignment of the homeodomains, they can be broadly divided into TALE and non-TALE classes. When individual homeobox genes were compared with members of known class or family, we could further classify them into 3 groups, namely, TALE, OTHER and NOVEL classes, but no HOX family was found. The 5 members of TALE class could be further divided into PBX, PKNOX, IRX and CUP families; 4 homeobox genes classified as NOVEL did not show any similarity to any known homeobox genes; while the remaining 5 were classified as OTHERS as they did show certain degree of similarity to few known homeobox genes. No unique RNA expression pattern during development of D. discoideum emerged for members of an individual group. Putative promoter analysis revealed binding sites for few homeobox transcription factors among many probable factors.

    • Deletion of Dictyostelium discoideum Sir2A impairs cell proliferation and inhibits autophagy


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      Sirtuins are a family of deacetylases (Class III histone deacetylases) with evolutionarily conserved functions in cellularmetabolism and chromatin regulation. Out of the seven human Sirtuins, the function of Sirt2 is the least understood. Thepurpose of the present study was to investigate the role of Sir2A, a homolog of human Sirt2 in Dictyostelium discoideum(Dd), a lower eukaryote. We created both overexpressing and deletion strains of Ddsir2A to analyse its functions. Weobserved sir2A mRNA expression throughout development and the transcript was present in the prespore/spore region ofmulticellular structures developed. They show a preference towards prestalk/stalk pathway when co-developed with wildtypecells during chimera formation. Deletion strain showed a multi-tipped phenotype, decrease in cell proliferation andinhibition of autophagy. In conclusion, our results show low cAMP levels, reduced cell-adhesion, weak cell migration andimpaired autophagy to be responsible for the phenotype shown by the null cells. This study provides new insights into thefunctions of Ddsir2A.

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