Anaerobic pathways in the Porifera: Strombine dehydrogenase, an opine dehydrogenase, from the sponge Suberites domuncula

The study presented here encompasses identification, analysis and characterization of the strombine dehydrogenase (StDH) from the sponge S. domuncula, on the gene and protein level. StDH is an opine dehydrogenase which is involved in opine production pathways found mainly in marine invertebrates. These anaerobic pathways are regarded as analogues to the classical anaerobic glycolytic pathway (lactate production pathway), which is predominant in vertebrates. The StDH was previously annotated as a tauropine dehydrogenase (TaDH) on the basis of its 68% identity with the TaDH protein from Halichondria japonica. Subsequent enzymatic assays showed that S. domuncula opine dehydrogenase is in fact strombine dehydrogenase which possesses specific characteristics not found in other proteins of the same family. It is described here for the first time the StDH gene in Eukaryotes. Two allelic variants have been identified which are present in the different specimens either as a homozygotic or a heterozygotic. Phylogenetic analyses supported with enzymatic assays indicate that S. domuncula StDH is only distantly related to the opine dehydrogenases from marine invertebrates. StDH showed that the protein is highly specific to glycine and inhibited by the substrate pyruvate. Furthermore, S. domunucla StDH has a dimeric structure (~75 kDa) which is not observed in so far described OpDHs that are monomeric proteins. This enzyme showed similarities to the OCD/mu-cristallyin protein family. Results showed that a sponge StDH is unusual enzyme that belongs to the independent enzyme class. In addition, expression studies revealed that the StDH is down-regulated with aeration. Immunohistology analyses showed high expression of the protein in almost all sponge cells. A strong accumulation of the enzyme was seen around the bacteria indicating that under aerobic conditions the bacteria might metabolize strombine (end product of the reaction). In conclusion, the data documented here shed new light on the anaerobic pathways in marine invertebrates. Potential mutual influences between bacteria and sponge are discussed as well. Hopefully, these results could have a small but important contribution to the better understanding of the evolution in the animal kingdom.

The structuring body : a critical study in the description & explanation of perceptual experience

The aims of the dissertation are to find the right description of the structure of perceptual experience and to explore the ways in which the structure of the body might serve to explain it. In the first two parts, I articulate and defend the claim that perceptual experience seems direct and the claim that its objects seem real. I defend these claims as integral parts of a coherent metaphysically neutral conception of perceptual experience. Sense-datum theorists, certain influential perceptual psychologists, and early modern philosophers (most notably Berkeley) all disputed the claim that perceptual experience seems direct. In Part I, I argue that the grounds on which they did so were poor. The aim is then, in Part II, to give a proper appreciation of the distinctive intentionality of perceptual experience whilst remaining metaphysically neutral. I do so by drawing on the early work of Edmund Husserl, providing a characterisation of the perceptual experience of objects as real, qua mind-independent particulars. In Part III, I explore two possible explanations of the structure characterising the intentionality of perceptual experience, both of which accord a distinctive explanatory role to the body. On one account, perceptual experience is structured by an implicit pre-reflective consciousness of oneself as a body engaged in perceptual activity. An alternative account makes no appeal to the metaphysically laden concept of a bodily self. It seeks to explain the structure of perceptual experience by appeal to anticipation of the structural constraints of the body. I develop this alternative by highlighting the conceptual and empirical basis for the idea that a first-order structural affordance relation holds between a bodily agent and certain properties of its body. I then close with a discussion of the shared background assumptions that ought to inform disputes over whether the body itself (in addition to its representation) ought to serve as an explanans in such an account.