Schistosoma mansoni is a well-adapted blood-dwelling parasitic helminth, persisting for decades in its human host despite being continually exposed to potential immune attack. Here, we describe in detail micro-exon genes (MEG) in S. mansoni, some present in multiple copies, which represent a novel molecular system for creating protein variation through the alternate splicing of short (#36 bp) symmetric exons organized in tandem. Analysis of three closely related copies of one MEG family allowed us to trace several evolutionary events and propose a mechanism for micro-exon generation and diversification. Microarray experiments show that the majority of MEGs are up-regulated in life cycle stages associated with establishment in the mammalian host after skin penetration. Sequencing of RT-PCR products allowed the description of several alternate splice forms of micro-exon genes, highlighting the potential use of these transcripts to generate a complex pool of protein variants. We obtained direct evidence for the existence of such pools by proteomic analysis of secretions from migrating schistosomula and mature eggs. Whole-mount in situ hybridization and immunolocalization showed that MEG transcripts and proteins were restricted to glands or epithelia exposed to the external environment. The ability of schistosomes to produce a complex pool of variant proteins aligns them with the other major groups of blood parasites, but using a completely different mechanism. We believe that our data open a new chapter in the study of immune evasion by schistosomes, and their ability to generate variant proteins could represent a significant obstacle to vaccine development.

The parasitic helminth Schistosoma mansoni is a major public health concern in many developing countries. Glycoconjugates, and in particular the carbohydrate component of these products, represent the main immunogenic challenge to the host and could therefore represent one of the crucial determinants for successful parasite establishment. Here we report a comparative glycomics analysis of the N-and O-glycans derived from glycoproteins present in S. mansoni egg (egg-secreted protein) and cercarial (0 -3-h released protein) secretions by a combination of mass spectrometric techniques. Our results show that S. mansoni secrete glycoproteins with glycosylation patterns that are complex and stage-specific. Cercarial stage secretions were dominated by N-glycans that were core-xylosylated, whereas N-glycans from egg secretions were predominantly core-difucosylated. O-Glycan core structures from cercarial secretions primarily consisted of the core sequence Gal␤133(Gal␤136)GalNAc, whereas egg-secreted O-glycans carried the mucin-type core 1 (Gal␤133GalNAc) and 2 (Gal␤133(GlcNAc␤136)GalNAc) structures. Additionally we identified a novel O-glycan core in both secretions in which a Gal residue is linked to the protein. Terminal structures of N-and O-glycans contained high levels of fucose and include stage-specific structures. These glycan structures identified in S. mansoni secretions are potentially antigenic motifs and ligands for carbohydrate-binding pro-

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An in vitro brain slice preparation of the electrosensory lateral line lobe (ELL) of weakly electric fish was developed. The morphology of this slice was studied and revealed that most ELL neurons and synapses retained their normal appearance for at least 10 h in vitro. The electrophysiological characteristics of the main ELL output neurons, the pyramidal cells, were measured. Extracellular electrode recordings demonstrated that pyramidal cells are capable of spontaneous, rhythmic spike activity. Intracellular recordings showed that intrinsic oscillations in membrane potential underlie the bursting behavior. The majority of pyramidal cells respond to depolarizing current pulses with an initial lag in spike firing followed by a non-accommodating, higher frequency spike train. Time and voltage-dependent properties of pyramidal cell responsiveness, as well as the effects of pharmacological blocking agents indicated that rhythmic activity and repetitive firing are dominated by a persistent, subthreshold sodium conductance (gNa) which activates at depolarizing levels and is the driving force behind the membrane potential oscillations and the sustained (non-accommodating) spike firing. In addition, a transient, outward potassium conductance (gA) is responsible for the lag in spike firing by acting as a 'brake' during the initial 50-200 ms of a depolarizing stimulus. Calcium currents and calcium-dependent potassium conductance add to the interval between spontaneous bursts but appear insufficient for spike frequency accommodation. The in vitro behaviour of pyramidal cells differs substantially from the behaviour of the same cell type in vivo. These observations raise possibilities that intrinsic membrane properties together with local synaptic interactions may regulate pyramidal cell responsiveness.

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