Spectrin assembly in avian erythroid development is determined by competing reactions of subunit homo- and hetero-oligomerization

The sorting of blood group active proteins during enucleation

Enucleation represents the critical stage during red blood cell development when the nucleus is extruded from an orthochromatic erythroblast in order to generate a nascent immature reticulocyte. Extrusion of the nucleus results in loss of a proportion of the erythroblast plasma membrane, which surrounds the nucleus, the bulk of the endoplasmic reticulum and a small region of cytoplasm. For this reason enucleation provides an important point in erythroblast differentiation at which proteins not required for the function of the erythrocyte can be lost, whilst those that are important for the structure-function properties of the mature erythrocyte must be efficiently retained in the reticulocyte plasma membrane. Disturbances in protein distribution during enucleation are envisaged to occur during human diseases such as Hereditary Spherocytosis. This article will discuss the current knowledge of erythroblast enucleation in the context of retention and loss of proteins that display antigenic blood group sites and that exist within multiprotein complexes within the erythrocyte membrane.

MAP1B and clathrin are novel interacting partners of the giant cyto-linker dystonin

Dystonin is a large multidomain cytoskeletal-associated protein that plays an essential role in the nervous system. Loss of dystonin results in neuromuscular dysfunction and early death in a mouse mutant called dystonia musculorum. Conserved among related proteins, the plakin domain is a defining feature of all major dystonin isoforms, yet its interactions have not been explored in detail. The purpose of the present study was to identify novel interacting partners of the plakin domain of the neuronal isoform of dystonin (dystonin-a). Newly identified interacting proteins discovered through a pull-down assay were validated using coimmunoprecipitation, coimmunofluorescence, and proximity ligation assays. Microtubule associated protein 1B (MAP1B), a microtubule stabilizing protein, and clathrin heavy chain, the major component of the clathrin triskelion, were identified as interaction partners for dystonin-a. Increased levels of phosphorylated MAP1B suggest a misregulation of MAP1B and a potentially novel component of the dt pathology. This work will further facilitate our understanding of how cytoskeletal proteins can affect and regulate neurodegenerative disorders.

Intermediate filaments: molecular structure, assembly mechanism, and integration into functionally distinct intracellular Scaffolds

The superfamily of intermediate filament (IF) proteins contains at least 65 distinct proteins in man, which all assemble into approximately 10 nm wide filaments and are principal structural elements both in the nucleus and the cytoplasm with essential scaffolding functions in metazoan cells. At present, we have only circumstantial evidence of how the highly divergent primary sequences of IF proteins lead to the formation of seemingly similar polymers and how this correlates with their function in individual cells and tissues. Point mutations in IF proteins, particularly in lamins, have been demonstrated to lead to severe, inheritable multi-systemic diseases, thus underlining their importance at several functional levels. Recent structural work has now begun to shed some light onto the complex fine tuning of structure and function in these fibrous, coiled coil forming multidomain proteins and their contribution to cellular physiology and gene regulation.

Hereditary spherocytosis—defects in proteins that connect the membrane skeleton to the lipid bilayer

The molecular causes of hereditary spherocytosis (HS) have been unraveled in the past decade. No frequent defect is found, and nearly every family has a unique mutation. In dominant HS, nonsense and frameshift mutations of ankyrin, band 3, and beta-spectrin predominate. Recessive HS is most often due to compound heterozygosity of defects in ankyrin, alpha-spectrin, or protein 4.2. Common combinations include a defect in the promoter or 5'-untranslated region of ankyrin paired with a missense mutation, a low expression allele of alpha-spectrin plus a missense mutation, and various mutations in the gene for protein 4.2. In most patients' red cells, no abnormal protein is present. Only rare missense mutations, like ankyrin Walsrode (V463I) or beta-spectrin Kissimmee (W202R), have given any insight into the functional domains of the respective proteins. Although the eminent role of the spleen in the premature hemolysis of red cells in HS is unquestioned, the molecular events that cause splenic conditioning of spherocytes are unclear. Electron micrographs show that small membrane vesicles are shed during the formation of spherocytes. Animal models give further insight into the pathogenetic consequences of membrane protein defects as well as the causes of the variability of disease severity.

Spectrins in developing rat hippocampal cells

We studied the spectrins in developing hippocampal tissue in vivo and in vitro to learn how they contribute to the organization of synaptic and extrasynaptic regions of the neuronal plasma membrane. beta-Spectrin, but not beta-fodrin or alpha-fodrin, increased substantially during postnatal development in the hippocampus, where it was localized in neurons but not in astrocytes. Immunoprecipitations from neonatal and adult hippocampal extracts suggest that while both beta-spectrin and beta-fodrin form heteromers with alpha-fodrin, oligomers containing all three subunits are also present. At the subcellular level, beta-fodrin and alpha-fodrin were present in the cell bodies, dendrites, and axons of pyramidal-like neurons in culture, as well as in astrocytes. beta-Spectrin, by contrast, was absent from axons but present in cell bodies and dendrites, where it was organized in a loose, membrane-associated meshwork that lacked alpha-fodrin. A similar meshwork was also apparent in pyramidal neurons in vivo. At some dendritic spines, alpha-fodrin was present in the necks but not in the heads, whereas beta-spectrin was present at significant levels in the spine heads. The presence of significant amounts of beta-spectrin without an accompanying alpha-fodrin subunit was confirmed by immunoprecipitations from extracts of adult hippocampus. Our results suggest that the spectrins in hippocampal neurons can assemble to form different membrane-associated structures in distinct membrane domains, including those at synapses.

Developmental expression of spectrins in rat skeletal muscle

Skeletal muscle contains spectrin (or spectrin I) and fodrin (or spectrin II), members of the spectrin supergene family. We used isoform-specific antibodies and cDNA probes to investigate the molecular forms, developmental expression, and subcellular localization of the spectrins in skeletal muscle of the rat. We report that beta-spectrin (betaI) replaces beta-fodrin (betaII) at the sarcolemma as skeletal muscle fibers develop. As a result, adult muscle fibers contain only alpha-fodrin (alphaII) and the muscle isoform of beta-spectrin (betaISigma2). By contrast, other types of cells present in skeletal muscle tissue, including blood vessels and nerves, contain only alpha- and beta-fodrin. During late embryogenesis and early postnatal development, skeletal muscle fibers contain a previously unknown form of spectrin complex, consisting of alpha-fodrin, beta-fodrin, and the muscle isoform of beta-spectrin. These complexes associate with the sarcolemma to form linear membrane skeletal structures that otherwise resemble the structures found in the adult. Our results suggest that the spectrin-based membrane skeleton of muscle fibers can exist in three distinct states during development.

Combination of two mutant alpha spectrin alleles underlies a severe spherocytic hemolytic anemia

We studied a patient with a severe spherocytic hemolytic anemia without family history of spherocytosis. Analysis of patient's erythrocyte membrane proteins revealed spectrin deficiency and a truncated alpha spectrin protein. We determined that the patient is a compound heterozygote with two mutations in alpha spectrin gene. Mutation in the paternal allele, designated alpha spectrin(PRAGUE), is a transition A to G in the penultimate position of intron 36 that leads to skipping of exon 37, frameshift, and production of the truncated alpha spectrin protein. The maternal allele, designated alpha spectrin(LEPRA), contains transition C-->T in position -99 of intron 30. This mutation enhances an alternative acceptor splice site 70 nucleotides upstream from the regular site. The alternative splicing causes a frameshift and premature termination of translation leading to a significant decrease in alpha spectrin production. The alpha(LEPRA) mutation is linked to a spectrin alphaIIa marker that was found to be associated with recessive or nondominant spectrin-deficient hereditary spherocytosis in approximately 50% of studied families. We conclude that the alpha(LEPRA) mutation combined in trans with the alpha(PRAGUE) mutation underlie the severe hemolytic anemia in the proband. We suggest that allele alpha spectrin(LEPRA) may be frequently involved in pathogenesis of recessive or nondominant spectrin-deficient hereditary spherocytosis.

An unusual beta-spectrin associated with clustered acetylcholine receptors

The clustering of acetylcholine receptors (AChR) in the postsynaptic membrane is an early event in the formation of the neuromuscular junction. The mechanism of clustering is still unknown, but is generally believed to be mediated by the postsynaptic cytoskeleton. We have identified an unusual isoform of beta-spectrin which colocalizes with AChR in AChR clusters isolated from rat myotubes in vitro. A related antigen is present postsynaptically at the neuromuscular junction of the rat. Immunoprecipitation, peptide mapping and immunofluorescence show that the beta-spectrin in AChR clusters resembles but is distinct from the beta-spectrin of human erythrocytes. alpha-Spectrin appears to be absent from AChR clusters. Semiquantitative immunofluorescence techniques indicate that there are from two to seven beta-spectrin molecules present for every clustered AChR, the higher values being obtained from rapidly prepared clusters, the lower values from clusters that require several minutes or more for isolation. Upon incubation of isolated AChR clusters for 1 h at room temperature, beta-spectrin is slowly depleted and the AChR redistribute into microaggregates. The beta-spectrin that remains associated with the myotube membrane is concentrated at these microaggregates. beta-Spectrin is quantitatively lost from clusters upon digestion with chymotrypsin, which causes AChR to redistribute in the plane of the membrane. These results suggest that AChR in clusters is closely linked to an unusual isoform of beta-spectrin.

Contributions of the beta-subunit to spectrin structure and function

The three avian spectrins that have been characterized consist of a common alpha-subunit (240 kD) paired with an isoform-specific beta-subunit from either erythrocyte (220 or 230 kD), brain (235 kD), or intestinal brush border (260 kD). Analysis of avian spectrins, with their naturally occurring "subunit replacement" has proved useful in assessing the relative contribution of each subunit to spectrin function. In this study we have completed a survey of avian spectrin binding properties and present morphometric analysis of the relative flexibility and linearity of various avian and human spectrin isoforms. Evidence is presented that, like its mammalian counterpart, avian brain spectrin binds human erythroid ankyrin with low affinity. Cosedimentation analysis demonstrates that 1) avian erythroid protein 4.1 stimulates spectrin-actin binding of both mammalian and avian erythrocyte and brain spectrins, but not the TW 260/240 isoform, 2) calpactin I does not potentiate actin binding of either TW 260/240 or brain spectrin, and 3) erythrocyte adducin does not stimulate the interaction of TW 260/240 with actin. In addition, a morphometric analysis of rotary-shadow images of spectrin isoforms, individual subunits, and reconstituted complexes from isolated subunits was performed. This analysis revealed that the overall flexibility and linearity of a given spectrin heterodimer and tetramer is largely determined by the intrinsic rigidity and linearity of its beta-spectrin subunit. No additional rigidity appears to be imparted by noncovalent associations between the subunits. The scaled flexural rigidity of the most rigid spectrin analyzed (human brain) is similar to that reported for F-actin.

Functional diversity among spectrin isoforms

The purpose of this review on spectrin is to examine the functional properties of this ubiquitous family of membrane skeletal proteins. Major topics include spectrin-membrane linkages, spectrin-filament linkages, the subcellular localization of spectrins in various cell types and a discussion of major functional differences between erythroid and nonerythroid spectrins. This includes a summary of studies from our own laboratories on the functional and structural comparison of avian spectrin isoforms which are comprised of a common alpha subunit and a tissue-specific beta subunit. Consequently, the observed differences among these spectrins can be assigned to differences in the properties of the beta subunits.

Spectrin and related molecules

This review begins with a complete discussion of the erythrocyte spectrin membrane skeleton. Particular attention is given to our current knowledge of the structure of the RBC spectrin molecule, its synthesis, assembly, and turnover, and its interactions with spectrin-binding proteins (ankyrin, protein 4.1, and actin). We then give a historical account of the discovery of nonerythroid spectrin. Since the chicken intestinal form of spectrin (TW260/240) and the brain form of spectrin (fodrin) are the best characterized of the nonerythroid spectrins, we compare these molecules to RBC spectrin. Studies establishing the existence of two brain spectrin isoforms are discussed, including a description of the location of these spectrin isoforms at the light- and electron-microscope level of resolution; a comparison of their structure and interactions with spectrin-binding proteins (ankyrin, actin, synapsin I, amelin, and calmodulin); a description of their expression during brain development; and hypotheses concerning their potential roles in axonal transport and synaptic transmission.

Changes in erythroid membrane proteins during erythropoietin-mediated terminal differentiation

Membrane and membrane skeleton proteins were examined in erythroid progenitor cells during terminal differentiation. The employed model system of erythroid differentiation was that in which proerythroblasts from mice infected with the anemia-inducing strain of Friend virus differentiate in vitro in response to erythropoietin (EP). With this system, developmentally homogeneous populations of cells can be examined morphologically and biochemically as they progress from proerythroblasts through enucleated reticulocytes. alpha and beta spectrins, the major proteins of the erythrocyte membrane skeleton, are synthesized in the erythroblasts both before and after EP exposure. At all times large portions of the newly synthesized spectrins exist in and are turned over in the cytoplasm. The remaining newly synthesized spectrin is found in a cellular fraction containing total membranes. Pulse-chase experiments show that little of the cytoplasmic spectrins become membrane associated, but that the proportion of newly synthesized spectrin which is membrane associated increases as maturation proceeds. A membrane fraction enriched in plasma membranes has significant differences in the stoichiometry of spectrin accumulation as compared to total cellular membranes. Synthesis of band 3 protein, the anion transporter, is induced only after EP addition to the erythroblasts. All of the newly synthesized band 3 is membrane associated. A two-dimensional gel survey was conducted of newly synthesized proteins in the plasma membrane enriched fraction of the erythroblasts as differentiation proceeded. A majority of the newly synthesized proteins remain in the same proportion to each other during maturation; however, a few newly synthesized proteins greatly increase following EP induction while others decrease markedly. Of the radiolabeled proteins observed in two dimensional gels, only the spectrins, band 3 and actin become major proteins of the mature erythrocyte membrane. Examination of total proteins of the plasma membrane enriched fractions of EP-treated erythroblasts using silver staining and 32P autoradiography show that many proteins and phosphoproteins are selectively eliminated from this fraction late in the course of differentiation during the reticulocyte stage. The selective removal of many proteins at the reticulocyte stage of development combined with previous selective synthesis and accumulation of some specific proteins such as alpha and beta spectrin and band 3 in the differentiating erythroblasts lead to the final mammalian erythrocyte membrane structure.

Gelsolin is expressed in early erythroid progenitor cells and negatively regulated during erythropoiesis

We have identified an approximately 85-kD protein in chicken erythrocytes which is immunologically, structurally, and functionally related to the gelsolin found in many muscle and nonmuscle cell types. Cell fractionation reveals a Ca2+-dependent partitioning of gelsolin into the soluble cytoplasm and the membrane-associated cytoskeleton of differentiating or mature erythrocytes. Depending on either the presence of Ca2+ during cell lysis or on the preincubation of the intact cells with the Ca2+-ionophore A23187, up to 40% of the total cellular gelsolin is found associated with the membrane skeleton. Expression of gelsolin shows a strong negative regulation during erythroid differentiation. From quantitations of its steady-state molar ratio to actin, gelsolin is abundant in early progenitor cells as revealed from avian erythroblastosis virus- and S13 virus-transformed cells which are arrested at the colony forming unit erythroid (CFU-e) stage of erythroid development. In these cells, which have a rudimentary and unstable membrane skeleton, gelsolin remains quantitatively cytoplasmic, irrespective of the Ca2+ concentration. During chicken embryo development and maturation, the expression of gelsolin decreases by a factor of approximately 10(3) in erythroid cells. This down regulation is independent from that of actin, which is considerably less, and is observed also when S13-transformed erythroid progenitor cells are induced to differentiate under conditions where the actin content of these cells does not change. In mature erythrocytes of the adult the amount of gelsolin is low, and significantly less than required for potentially capping of all membrane-associated actin filaments. We suggest that the gelsolin in erythroid cells is involved in the assembly of the actin filaments present in the membrane skeleton, and that it may provide for a mechanism, by means of its severing action on actin filaments, to extend the meshwork of the spectrin-actin-based membrane skeleton in erythroid cells during erythropoiesis.

Assignment of mouse beta-spectrin gene to chromosome 12

The structural gene for the beta-subunit of the mouse erythrocyte spectrin, hereinafter designated as Sp-b, was assigned to the mouse chromosome 12. This assignment was made by Southern analysis of genomic DNA from mouse X Chinese hamster hybrid cells using cloned mouse erythrocyte beta-spectrin cDNA as a probe. In the PstI-digested genomic hamster cell DNA a single band of 2.0 kb was detected, whereas PstI-digested mouse DNA gave a band of 4.2 kb, when probed with the mouse erythroid beta-spectrin cDNA clone. This allowed us to analyze a panel of mouse X Chinese hamster somatic cell hybrids to map this gene to chromosome 12. Interestingly, this assignment is different from that observed for the alpha-subunit of spectrin, which has been mapped to chromosome 1 in mouse. These results serve as a basis for further genetic characterization of the mouse hemolytic anemias.