Aleksander F. Sikorski

Spectrin and calpain: a ‘target’ and a ‘sniper’ in the pathology of neuronal cells

Abstract.It is well documented that activation of calpain, a calcium-sensitive cysteine protease, marks the pathology of naturally and experimentally occuring neurodegenerative conditions. Calpain-mediated proteolysis of major membrane-skeletal protein, αII-spectrin, results in the appearance of two unique and highly stable breakdown products, which is an early event in neural cell pathology. This review focuses on spectrin degradation by calpain within neurons induced by diverse conditions, emphasizing a current picture of multi-pattern neuronal death and a recent success in the development of spectrin-based biomarkers. The issue is presented in the context of the major structural and functional properties of the two proteins.

Human DHHC proteins: a spotlight on the hidden player of palmitoylation.

Palmitoylation is one of the most common posttranslational lipid modifications of proteins and we now know quite a lot about it. However, the state of knowledge about the enzymes that catalyze this process is clearly insufficient. This review is focused on 23 human DHHC genes and their products - protein palmitoyltransferases. Here we describe mainly the structure and function of these proteins, but also, to a lesser degree, what the substrates of the enzymes are and whether they are related to various diseases. The main aim of this review was to catalogue existing information concerning the human DHHC family of genes/proteins, making them and their functions easier to understand.

Spectrin-based skeleton as an actor in cell signaling.

This review focuses on the recent advances in functions of spectrins in non-erythroid cells. We discuss new data concerning the commonly known role of the spectrin-based skeleton in control of membrane organization, stability and shape, and tethering protein mosaics to the cellular motors and to all major filament systems. Particular effort has been undertaken to highlight recent advances linking spectrin to cell signaling phenomena and its participation in signal transduction pathways in many cell types.

Spectrins: A structural platform for stabilization and activation of membrane channels, receptors and transporters☆

This review focuses on structure and functions of spectrin as a major component of the membrane skeleton. Recent advances on spectrin function as an interface for signal transduction mediation and a number of data concerning interaction of spectrin with membrane channels, adhesion molecules, receptors and transporters draw a picture of multifaceted protein. Here, we attempted to show the current depiction of multitask role of spectrin in cell physiology. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Spectrin (βSpIIΣ1) is an essential component of synaptic transmission

The cellular mechanism that underlies the regulated release of synaptic vesicles during neurotransmission is not fully known. Our previous data has shown that brain spectrin (alphaSpIIsigma1/betaSpIIsigma1)2 is localized in axons and nerve terminals and we have shown that the beta subunit (betaSpIIsigma1) contains a synapsin-binding domain capable of interacting with synapsin and small synaptic vesicles in vitro and in vivo. These findings suggested a role for brain beta-spectrin in synaptic neurotransmission. To examine this possibility further, peptide-specific antibodies directed against epitopes within the synapsin-binding domain of brain beta-spectrin, or against flanking regions, were injected into the presynaptic neuron of synaptically paired rat hippocampal neurons in culture. Here, we show that the antibodies directed against the synapsin-binding domain specifically blocked synaptic neurotransmission.

Brain spectrin (fodrin) interacts with phospholipids as revealed by intrinsic fluorescence quenching and monolayer experiments

We demonstrate that phospholipid vesicles affect the intrinsic fluorescence of isolated brain spectrin. In the present studies we tested the effects of vesicles prepared from phosphatidylcholine (PtdCho) alone, in addition to vesicles containing PtdCho mixed with other phospholipids [phosphatidylethanolamine (PtdEtn) and phosphatidylserine] as well as from total lipid mixture extracted from brain membrane. The largest effect was observed with PtdEtn/PtdCho (3:2 molar ratio) vesicles; the effect was markedly smaller when vesicles were prepared from egg yolk PtdCho alone. Brain spectrin injected into a subphase induced a substantial increase in the surface pressure of monolayers prepared from phospholipids. Results obtained with this technique indicated that the largest effect is again observed with monolayers prepared from a PtdEtn/PtdCho mixture. The greatest effect was observed when the monolayer contained 50-60% PtdEtn in a PtdEtn/PtdCho mixture. This interaction occurred at salt and pH optima close to physiological conditions (0.15 M NaCl, pH7.5). Experiments with isolated spectrin subunits indicated that the effect of the beta subunit on the monolayer surface pressure resembled that measured with the whole molecule. Similarly to erythrocyte spectrin-membrane interactions, brain spectrin interactions with PtdEtn/PtdCho monolayer were competitively inhibited by isolated erythrocyte ankyrin. This also suggests that the major phospholipid-binding site is located in the beta subunit and indicates the possible physiological significance of this interaction.

Toward a magic or imaginary bullet? Ligands for drug targeting to cancer cells: principles, hopes, and challenges

There are many problems directly correlated with the systemic administration of drugs and how they reach their target site. Targeting promises to be a hopeful strategy as an improved means of drug delivery, with reduced toxicity and minimal adverse side effects. Targeting exploits the high affinity of cell-surface-targeted ligands, either directly or as carriers for a drug, for specific retention and uptake by the targeted diseased cells. One of the most important parameters which should be taken into consideration in the selection of an appropriate ligand for targeting is the binding affinity (KD). In this review we focus on the importance of binding affinities of monoclonal antibodies, antibody derivatives, peptides, aptamers, DARPins, and small targeting molecules in the process of selection of the most suitable ligand for targeting of nanoparticles. In order to provide a critical comparison between these various options, we have also assessed each technology format across a range of parameters such as molecular size, immunogenicity, costs of production, clinical profiles, and examples of the level of selectivity and toxicity of each. Wherever possible, we have also assessed how incorporating such a targeted approach compares with, or is superior to, original treatments.

Membrane rafts as a novel target in cancer therapy.

Membrane rafts are distinct plasma membrane microdomains that are enriched in sphingolipids and cholesterol. They organize receptors and their downstream molecules and regulate a number of intracellular signaling pathways. This review presents information on the dependence of several growth factor receptor signaling pathways on membrane rafts. It also discusses the involvement of rafts in the regulation of differentiation, apoptosis and cell migration connected with invasiveness and metastasis. Examples of known synthetic and naturally occurring substances that are known to affect lateral membrane organization in tumor cell growth are discussed as potential or actual therapeutics.

microRNAs: fine tuning of erythropoiesis

Cell proliferation and differentiation is a complex process involving many cellular mechanisms. One of the best-studied phenomena in cell differentiation is erythrocyte development during hematopoiesis in vertebrates. In recent years, a new class of small, endogenous, non-coding RNAs called microRNAs (miRNAs) emerged as important regulators of gene expression at the post-transcriptional level. Thousands of miRNAs have been identified in various organisms, including protozoa, fungi, bacteria and viruses, proving that the regulatory miRNA pathway is conserved in evolution. There are many examples of miRNA-mediated regulation of gene expression in the processes of cell proliferation, differentiation and apoptosis, and in cancer genesis. Many of the collected data clearly show the dependence of the proteome of a cell on the qualitative and quantitative composition of endogenous miRNAs. Numerous specific miRNAs are present in the hematopoietic erythroid line. This review attempts to summarize the state of knowledge on the role of miRNAs in the regulation of different stages of erythropoiesis. Original experimental data and results obtained with bioinformatics tools were combined to elucidate the currently known regulatory network of miRNAs that guide the process of differentiation of red blood cells.

Ankyrin inhibits binding of erythrocyte spectrin to phospholipid vesicles.

The studies on binding of erythrocyte spectrin to frozen and thawed phospholipid liposomes and its inhibition by ankyrin were performed. It was found that ankyrin inhibited up to 60% binding of spectrin by phosphatidylethanolamine/phosphatidylcholine vesicles. It was able to dissociate up to 40% of spectrin from this complex. Ankyrin inhibition of binding of phosphatidylserine/phosphatidylcholine vesicles by spectrin, although much lower, was also observed.