Effect of phosphatidylcholine vesicles on the activity of DNA polymerase-alpha

Multiple Aspects of PIP2 Involvement in C. elegans Gametogenesis

One of the most studied phosphoinositides is phosphatidylinositol 4,5-bisphosphate (PIP2), which localizes to the plasma membrane, nuclear speckles, small foci in the nucleoplasm, and to the nucleolus in mammalian cells. Here, we show that PIP2 also localizes to the nucleus in prophase I, during the gametogenesis of C. elegans hermaphrodite. The depletion of PIP2 by type I PIP kinase (PPK-1) kinase RNA interference results in an altered chromosome structure and leads to various defects during meiotic progression. We observed a decreased brood size and aneuploidy in progeny, defects in synapsis, and crossover formation. The altered chromosome structure is reflected in the increased transcription activity of a tightly regulated process in prophase I. To elucidate the involvement of PIP2 in the processes during the C. elegans development, we identified the PIP2-binding partners, leucine-rich repeat (LRR-1) protein and proteasome subunit beta 4 (PBS-4), pointing to its involvement in the ubiquitin–proteasome pathway.

Nuclear phosphoinositides and their impact on nuclear functions

Polyphosphoinositides (PPIn) are important lipid molecules whose levels are de-regulated in human diseases such as cancer, neurodegenerative disorders and metabolic syndromes. PPIn are synthesized and degraded by an array of kinases, phosphatases and lipases which are localized to various subcellular compartments and are subject to regulation in response to both extra- and intracellular cues. Changes in the activities of enzymes that metabolize PPIn lead to changes in the profiles of PPIn in various subcellular compartments. Understanding how subcellular PPIn are regulated and how they affect downstream signaling is critical to understanding their roles in human diseases. PPIn are present in the nucleus, and their levels are changed in response to various stimuli, suggesting that they may serve to regulate specific nuclear functions. However, the lack of nuclear downstream targets has hindered the definition of which pathways nuclear PPIn affect. Over recent years, targeted and global proteomic studies have identified a plethora of potential PPIn-interacting proteins involved in many aspects of transcription, chromatin remodelling and mRNA maturation, suggesting that PPIn signalling within the nucleus represents a largely unexplored novel layer of complexity in the regulation of nuclear functions.

DNA-lipid interactions in vitro and in vivo

The data on lipid-nucleic interactions and their role in vitro and in vivo are presented. The results of study of DNA-lipid complexes in absence and in presence of divalent metal cations (triple complexes) are discussed. The triple complexes represent the generation of cellular structures such as pore complexes of eucaryotes and "Bayer's junctions" of procaryotes. The participation of triple complexes in the formation of structure of bacterial and eucaryotic nucleoid and nuclear matrix is analysed. A model of formation of triple complexes and cellular structures and their role in DNA-lipid interactions are discussed.

Morphological evidence of function-related localization of phospholipids in the cell nucleus

The evidence accumulated in recent years on the presence of phospholipids inside the interphase nucleus needs a precise localization of the nuclear sites of accumulation, transport and degradation of these molecules. A very useful approach for monitoring the fine localization of nuclear phospholipids is represented by a recently developed technique using gold-conjugated phospholipases. In fact, in addition to the phospholipids organized in bilayers in the membrane, this technique identifies amorphous lipoprotein complexes present in different cell areas as well as in the nucleus. In this way and using sample preparation systems which reduce lipid removal and translocation, such as cryofixation, cryosectioning, embedding in hydrophylic resins and cryofracturing, we have analyzed the subnuclear localization of phospholipids in different experimental conditions. The results indicate that: in interphase the nuclear phospholipids are localized mainly in the interchromatin spaces and in the nucleolar domain; the observed co-localization of phospholipids and ribonucleoproteins suggests that phospholipids are involved in the mechanism of transport and release of the transcripts; the demonstrated release of ribonucleoproteins after phospholipase digestion suggests that phospholipids mediate the binding between ribonucleoproteins and the nuclear matrix; significant changes of the phospholipid localization occur in the different phases of the cell cycle or in the course of induced cell differentiation.

Interaction of DNA polymerases with phospholipids

Effects of various phospholipids on the in vitro reactions of eukaryotic DNA polymerases alpha, beta and gamma were tested systematically. When phospholipids were added directly to the reaction mixture, neither stimulation nor inhibition was produced. However, when phospholipids were preincubated with enzymes in the absence of template-primer, some of them showed strong inhibition. Cardiolipin strongly inhibited the reactions of all three DNA polymerases and also of terminal deoxynucleotidyl transferase. Phosphatidylinositol selectively inhibited the reaction of DNA polymerase gamma. Phosphatidic acid moderately inhibited DNA polymerase alpha and strongly inhibited DNA polymerase gamma. The inhibition of DNA polymerase gamma by cardiolipin was nearly competitive with template-primer. Since the inhibition was reversed by the addition of 0.05% Triton-X 100 during preincubation, the phospholipid might interact with enzyme protein at the hydrophobic region in competition with template-primer. These results suggest a possible involvement of phospholipids in DNA replication in mitochondria and in nucleus through interaction with DNA polymerase.

Role of chromatin phospholipids on template availability and ultrastructure of isolated nuclei

The influence of phospholipid vesicles has been tested on isolated nuclei by evaluating the endogenous DNA-dependent RNA polymerase and the chromatin ultrastructure. Negatively charged phosphatidylserine liposomes have a stimulating effect on RNA synthesis, while the vesicles obtained with the neutral sphingomyelin, phosphatidylethanolamine and phosphatidylcholine, and the acidic phosphatidylinositol, are inhibitory. The enhancement of transcription by phosphatidylserine seems to be due to both elongation and initiation of RNA transcripts and very likely depends on interactions of the lipid with the template rather than with the enzymes. The morphological analysis indicates deep changes of the chromatin organization, mainly concerning the size and distribution of the fibers, without any variation of the nuclear volume. The rearrangement of the chromatin could account for the variations of the template availability for RNA synthesis induced by the vesicles and might be associated with changes of the nuclear matrix.