Micro‐ and nano‐scale ultrastructure of cell walls in Cryogenian microfossils: revealing their biological affinity

Moczyd?owska, M., Schopf, J.W. & Willman, S. 2009: Micro‐ and nano‐scale ultrastructure of cell walls in Cryogenian microfossils: revealing their biological affinity. Lethaia, Vol. 43, pp. 129–136. Recently established protocols and methods in advanced microscopy and spectrometry applied to studies of ancient unicellular organic‐walled microfossils of uncertain biological affinities (acritarchs) provide new evidence of the fine ultrastructure of cell walls and their biochemistry that support the interpretation of some such microfossils as photosynthesizing microalgae. The micro‐scale and nanoscale ultrastructure of the cell walls of late Cryogenian sphaeromorphic acritarchs from the Chichkan Formation (Kazakhstan) revealed by the advanced techniques and studied originally by Kempe et al. (2005) is here further analysed and compared with that of modern microalgal analogues. On the basis of such comparison, we interpret the preserved cell wall ultrastructure to reflect original layering and lamination within sub‐layers of the fossil wall, rather than being a result of taphonomic and diagenetic alteration. The outer thick layer represents the primary wall and the inner layer the secondary wall of the cell, whereas the laminated amorphous sub‐layers, 10–20 nm in thickness and revealed by transmission electron and atomic force microscopy, are recognized as trilaminar sheath structure. Because two‐layered cell walls, trilaminar sheaths and the position of the TLS within the fossil cell wall are characteristic of the mature developmental state in cyst morphogenesis in modern microalgae, we infer that the Chichkan sphaeromorphs are probably resting cells (aplanospores) of chlorophyceaen green microalgae from the order Volvocales. □Biological affinity, cell wall, Cryogenian, microfossils, ultrastructure.     

Alteration of Phosphoinositide Degradation by Cytosolic and Membrane-Bound Phospholipases after Forebrain Ischemia - Reperfussion in Gerbil: Effects of Amyloid Beta Peptide

The reperfusion of previously ischemic brain is associated with exacerbation of cellular injury. Reperfusion occasionally potentates release of intracellular enzymes, influx of Ca²⁺, breakdown of membrane phospholipids, accumulation of amyloid precursor protein or amyloid -(like) proteins, and apolipoprotein E. In this study, the effect of reperfusion injury on the activity of cerebral cortex enzymes acting on phosphatidyl [³H] inositol (PI) and [^l4C-arachidonoyl] PI was investigated. Moreover the effect of amyloid 25–35 on PI degradation by phospholipase(s) of normoxic brain and subjected to ichemia-reperfussion injury was determined. Brain ischemia in gerbils (Meriones unguiculatus) was induced by ligation of both common carotid arteries for 5 min and then brains were perfused for 15 min, 2 h and 7 days. Statistically significant activation of enzyme(s) involved in phosphatidylinositol degradation in gerbils subjected to ischemia-reperfusion injury was observed. Nearly all gerbils showed a higher activity of cytosolic PI phos-pholipase C (PLC) at 15 min after ischemia. Concomitantly, the significant enhancement of the level of DAG and AA radioactivity at this short reperfusion time confirmed the active PI degradation by phospholipase(s) in cerebral cortex and hippocampus. After a prolonged reperfusion time of 7 days after ischemia, both cytosolic and membrane-bound forms of PI-PLC were activated. The question arises if alteration of membranes by the degradation of phospholipids occurring after an ischemic episode potentates the effect of A on membrane-bound enzymes. A neuro-toxic fragment of amyloid, A 25–35, incubated in the presence of endogenous Ca²⁺, increased significantly the PI-PLC activity of normoxic brain. In its non-aggregated form, A 25–35 activates PI-PLC but in the aggregated form the enzymatic activity decreased. Thus, A 25–35 exerts a similar effect on the membrane-bound PI-PLC from normoxic brain or subjected to ischemia reperfussion injury. We conclude that the degradation of phosphatidylinositol by cytosolic phosphoinositide-phospholipase C may contribute to the pathophysiology of delayed neuronal death following cerebral ischemia. Thus, a specific inhibitor of this enzyme(s) may offer therapeutic strategies to protect the brain from damage triggered by ischemia. Ischemia-reperfusion injury had no effect on A-evoked alterations of synaptic plasma membrane-bound PI-PLC.     

Domain mapping of the Rad51 paralog protein complexes

The five human Rad51 paralogs are suggested to play an important role in the maintenance of genome stability through their function in DNA double-strand break repair. These proteins have been found to form two distinct complexes in vivo, Rad51B–Rad51C–Rad51D–Xrcc2 (BCDX2) and Rad51C–Xrcc3 (CX3). Based on the recent Pyrococcus furiosus Rad51 structure, we have used homology modeling to design deletion mutants of the Rad51 paralogs. The models of the human Rad51B, Rad51C, Xrcc3 and murine Rad51D (mRad51D) proteins reveal distinct N-terminal and C-terminal domains connected by a linker region. Using yeast two-hybrid and co-immunoprecipitation techniques, we have demonstrated that a fragment of Rad51B containing amino acid residues 1–75 interacts with the C-terminus and linker of Rad51C, residues 79–376, and this region of Rad51C also interacts with mRad51D and Xrcc3. We have also determined that the N-terminal domain of mRad51D, residues 4–77, binds to Xrcc2 while the C-terminal domain of mRad51D, residues 77–328, binds Rad51C. By this, we have identified the binding domains of the BCDX2 and CX3 complexes to further characterize the interaction of these proteins and propose a scheme for the three-dimensional architecture of the BCDX2 and CX3 paralog complexes.