Identification of a mouse brain beta-spectrin cDNA and distribution of its mRNA in adult tissues

The Spectrinome: The Interactome of a Scaffold Protein Creating Nuclear and Cytoplasmic Connectivity and Function

We provide a review of Spectrin isoform function in the cytoplasm, the nucleus, the cell surface, and in intracellular signaling. We then discuss the importance of Spectrin’s E2/E3 chimeric ubiquitin conjugating and ligating activity in maintaining cellular homeostasis. Finally we present spectrin isoform subunit specific human diseases. We have created the Spectrinome, from the Human Proteome, Human Reactome and Human Atlas data and demonstrated how it can be a useful tool in visualizing and understanding spectrins myriad of cellular functions.

Impact statement

Spectrin was for the first 12 years after its discovery thought to be found only in erythrocytes. In 1981, Goodman and colleagues1found that spectrin-like molecules were ubiquitously found in non-erythroid cells leading to a great multitude of publications over the next thirty eight years. The discovery of multiple spectrin isoforms found associated with every cellular compartment, and representing 2-3% of cellular protein, has brought us to today’s understanding that spectrin is a scaffolding protein, with its own E2/E3 chimeric ubiquitin conjugating ligating activity that is involved in virtually every cellular function. We cover the history, localized functions of spectrin isoforms, human diseases caused by mutations, and provide the spectrinome: a useful tool for understanding the myriad of functions for one of the most important proteins in all eukaryotic cells.

Spectrin's chimeric E2/E3 enzymatic activity

In this minireview, we cover the discovery of the human erythrocyte α spectrin E2/E3 ubiquitin conjugating/ligating enzymatic activity and the specific cysteines involved. We then discuss the consequences when this activity is partially inhibited in sickle cell disease and the possibility that the same attenuation is occurring in multiple organ dysfunction syndrome. We finish by discussing the reasons for believing that nonerythroid α spectrin isoforms (I and II) also have this activity and the importance of testing this hypothesis. If correct, this would suggest that the nonerythroid spectrin isoforms play a major role in protein ubiquitination in all cell types. This would open new fields in experimental biology focused on uncovering the impact that this enzymatic activity has upon protein-protein interactions, protein turnover, cellular signaling, and many other functions impacted by spectrin, including DNA repair.

ELF a beta-spectrin is a neuronal precursor cell marker in developing mammalian brain; structure and organization of the elf/beta-G spectrin gene

Spectrins play a pivotal role in axonal transport, neurite extension, the organization of synaptic vesicles, as well as for protein sorting in the Golgi apparatus and cell membrane. Among spectrins there is great variability in sequence composition, tissue distribution, and function, with two known genes encoding the alpha-chain, and at least five encoding the beta-chain. It remains unclear as to whether novel beta-spectrins such as elf1-4 are distinct genes or beta-G-spectrin isoforms. The role for ELF in the developing nervous system has not been identified to date. In this study we demonstrate the genomic structure of elf-3, as well as the expression of ELF in the developing mouse brain using a peptide specific antibody against its distinctive amino-terminal end. Full genomic structural analyses reveal that elf-3 is composed of 31 exons spanning approximately 67 kb, and confirm that elf and mouse brain beta-G-spectrin share multiple exons, with a complex form of exon/intron usage. In embryonic stages, E9-12, anti-ELF localized to the primary brain vesicular cells that also labeled strongly with anti-nestin but not anti-vimentin. At E12-14, anti-ELF localized to axonal sprouts in the developing neuroblasts of cortex and purkinje cell layer of the cerebellum, as well as in cell bodies in the diencephalon and metencephalon. Double labeling identified significant co-localization of anti-ELF, nestin and dystrophin in sub ventricular zone cells and in stellate-like cells of the developing forebrain. These studies define clearly the expression of ELF, a new isoform of beta-G-spectrin in the developing brain. Based on its expression pattern, ELF may have a role in neural stem cell development and is a marker of axonal sprouting in mid stages of embryonic development.

Mutation of a highly conserved residue of betaI spectrin associated with fatal and near-fatal neonatal hemolytic anemia

We studied an infant with severe nonimmune hemolytic anemia and hydrops fetalis at birth. His neonatal course was marked by ongoing hemolysis of undetermined etiology requiring repeated erythrocyte transfusions. He has remained transfusion-dependent for more than 2 yr. A previous sibling born with hemolytic anemia and hydrops fetalis died on the second day of life. Peripheral blood smears from the parents revealed rare elliptocytes. Examination of their erythrocyte membranes revealed abnormal mechanical stability as well as structural and functional abnormalities in spectrin. Genetic studies revealed that the proband and his deceased sister were homozygous for a mutation of betaIsigma1 spectrin, L2025R, in a region of spectrin that is critical for normal function. The importance of leucine in this position of the proposed triple helical model of spectrin repeats is highlighted by its evolutionary conservation in all beta spectrins from Drosophila to humans. Molecular modeling demonstrated the disruption of hydrophobic interactions in the interior of the triple helix critical for spectrin function caused by the replacement of the hydrophobic, uncharged leucine by a hydrophilic, positively charged arginine. This mutation must also be expressed in the betaIsigma2 spectrin found in muscle, yet pathologic and immunohistochemical examination of skeletal muscle from the deceased sibling was unremarkable.

Meis1, a PBX1-related homeobox gene involved in myeloid leukemia in BXH-2 mice

Leukemia results from the accumulation of multiple genetic alterations that disrupt the control mechanisms of normal growth and differentiation. The use of inbred mouse strains that develop leukemia has greatly facilitated the identification of genes that contribute to the neoplastic transformation of hematopoietic cells. BXH-2 mice develop myeloid leukemia as a result of the expression of an ecotropic murine leukemia virus that acts as an insertional mutagen to alter the expression of cellular proto-oncogenes. We report the isolation of a new locus, Meis1, that serves as a site of viral integration in 15% of the tumors arising in BXH-2 mice. Meis1 was mapped to a distinct location on proximal mouse chromosome 11, suggesting that it represents a novel locus. Analysis of somatic cell hybrids segregating human chromosomes allowed localization of MEIS1 to human chromosome 2p23-p12, in a region known to contain translocations found in human leukemias. Northern (RNA) blot analysis demonstrated that a Meis1 probe detected a 3.8-kb mRNA present in all BXH-2 tumors, whereas tumors containing integrations at the Meis1 locus expressed an additional truncated transcript. A Meis1 cDNA clone that encoded a novel member of the homeobox gene family was identified. The homeodomain of Meis1 is most closely related to those of the PBX/exd family of homeobox protein-encoding genes, suggesting that Meis1 functions in a similar fashion by cooperative binding to a distinct subset of HOX proteins. Collectively, these results indicate that altered expression of the homeobox gene Meis1 may be one of the events that lead to tumor formation in BXH-2 mice.

Brain spectrin: of mice and men

This article reviews our current knowledge of the structure of alpha spectrins and beta spectrins in the brain, as well as their location and expression within neural tissue. We discuss the known protein interactions of brain spectrin isoforms, and then describe results that suggest an important role for spectrin (alpha SpII sigma 1/beta SpII sigma 1) in the Ca(2+)-regulated release of neurotransmitters. Evidence that supports a role for spectrin in the docking of synaptic vesicles to the presynaptic plasma membrane and as a Ca2+ sensor protein that unclamps the fusion machinery is described, along with the Casting the Line model, which summarizes the information. We finish with a discussion of the value of spectrin and ankyrin-deficient mouse models in deciphering spectrin function in neural tissue.

Brain alpha erythroid spectrin: identification, compartmentalization, and beta spectrin associations

Using isoform and subunit specific antibodies we have determined the presence, localization, and beta spectrin associations of alpha erythroid spectrin, alpha SpI sigma*, as well as alpha non-erythroid spectrin, alpha SpII sigma 1, in mouse brain. Peptide specific antibodies against unique sequences within the beta SpII sigma 1, non-erythroid beta spectrin isoform, and within beta SpI sigma 1, erythrocyte beta spectrin isoform were used to compare the immunolocalization of beta spectrin subunit isoforms with that of alpha spectrin subunit isoforms and to immunoprecipitate spectrin tetramers in order to identify the subunit components by immunoblot analysis. The specificity and sensitivity of antibodies for isoform specific alpha and beta subunits was determined by immunodot and immunoblot methods. Immunohistochemical analyses indicated that beta SpI sigma 2 is located in neuronal somata and dendrites in mouse cerebellum. beta SpII sigma 1 is located in the medullary layer, chiefly composed of axonal tracts. Parallel immunohistochemical analysis with antibodies for the alpha and beta spectrin isoforms revealed that antibodies specific for the alpha subunit of erythrocyte spectrin (alpha SpI sigma 1) localized antigen to the somata and dendrites of cerebellar granule cell neurons, a pattern similar to that for the localization of the erythroid beta subunit (beta SpI sigma 2). In contrast antibodies specific for the non-erythroid alpha subunit (alpha SpII sigma 1) localized antigen to axons in the cerebellum corresponding to the pattern for the non-erythroid beta subunit (beta SpII sigma 1). The distinct localization of antigens by antisera which recognize either the alpha subunit of red blood cell spectrin or the alpha subunit of non-erythroid brain spectrin, together with the correspondence of their localization with appropriate beta subunits, clearly indicate that brain contains at least two species of spectrin each with distinct alpha and beta subunits. Immunoprecipitation experiments of cerebellar extracts using beta spectrin peptide specific antibodies followed by immunoblotting analysis confirmed the association of an erythroid alpha subunit isoform with a beta erythroid subunit isoform, as well as the association of non-erythroid alpha and beta subunits. In addition the immunoblot analysis of the immunoprecipitated material suggested there are minor populations of various hybrid tetramers in brain consisting of mixed erythroid and non-erythroid subunits. In summary these data collectively demonstrate that in mouse brain there are at least two alpha spectrin subunits, one erythroid alpha SpI sigma* and one non-erythroid alpha SpII sigma 1; these associate with an erythroid beta SpI sigma 1, and a non-erythroid beta SpII sigma 1 in the cerebellum of mouse.(ABSTRACT TRUNCATED AT 400 WORDS)

The murine mutation jaundiced is caused by replacement of an arginine with a stop codon in the mRNA encoding the ninth repeat of beta-spectrin

The jaundiced, ja/ja, mouse mutant has a severe hemolytic anemia associated with a deficiency of beta-spectrin in erythrocyte ghosts. Genes for the disease phenotype and beta-spectrin colocalize on Chromosome 12. beta-Spectrin mRNA is not detected in reticulocytes or in brain from newborn mutant mice. To locate the nucleotide sequence alteration, the erythroid beta-spectrin transcript from mutant spleen was amplified by reverse transcription PCR and sequenced. A C-to-T alteration is present in the mutant transcript and produces a premature stop codon from an arginine codon in mRNA encoding repeat 9 of beta-spectrin at amino acid position 1160. The point mutation introduces a Dde I site that is present in PCR-amplified DNA of ja/ja and ja/+ mice but not of +/+ control mice from the strain of origin, 129/Sv, or from the two strains, WB/Re and C57BL/6J, in which the mutation has been fixed by over 53 generations of backcrossing. The genetic data confirm that the point mutation is responsible for the severe reductions in beta-spectrin mRNA of jaundiced mice.

Developmental change of alpha-spectrin mRNA in the rat brain

Spectrin is a cytoskeletal protein considered to be a major component of intracellular cohesion. Using an in situ hybridization approach, we have investigated the developmental expression of the mRNA encoding the alpha-subunit of rat brain spectrins, from birth to adulthood. alpha-Subunit mRNA is detectable at birth, in brain areas with perinatal neurogenesis, such as the cerebral cortex, hippocampus, thalamus, and olfactory bulb. alpha-Brain-spectrin mRNA increases gradually during the first postnatal days to reach a plateau between the second and the third week of life. In the young adult brain, the level of alpha-brain spectrin mRNA decreased globally. This spacio-temporal distribution argues for the involvement of the mRNA in the synthesis of both the erythroid and non-erythroid brain spectrin isoforms. We have focused our attention on the hippocampal formation and the cerebellum. In both regions, in situ hybridization signal variations are superimposable with neuronal maturation gradients. This pattern of variation, coupled with the known interaction of brain spectrins with other cytoskeletal proteins, agrees with the notion that brain spectrins may be involved in neuronal differentiation by way of the cytoskeletal lattice organization.

Localization of spectrin isoforms in the adult mouse heart

The distribution of two isoforms of spectrin in the adult mouse heart was investigated by Western blotting and immunocytochemistry by use of monospecific antibodies to erythrocyte spectrin and nonerythroid brain spectrin (240/235). Western blotting revealed proteins analogous to both isoforms of alpha-spectrin in adult heart. Light-microscopic immunocytochemistry indicated that erythroid spectrin was distributed throughout the myocardium, with immunofluorescence localized to plasma membranes, Z-lines, and intercalated discs. Antibodies to brain spectrin (240/235) exhibited staining throughout the heart, with a generally diffuse distribution except for the prominent immunoreactivity associated with the intercalated discs. Nonerythroid spectrin immunofluorescence was detected in the endothelial cells of the endocardium and the mesothelial cell lining of the epicardium. Erythrocyte spectrin was not detected in the endocardium or the epicardium. The identification and localization of spectrin isoforms in the mammalian heart suggest the importance of spectrin proteins in the structural integrity and proper function of cardiac cells and tissues. This is the first demonstration of two different alpha-spectrin subunits in the mammalian heart.

The complete amino acid sequence for brain beta spectrin (beta fodrin): relationship to globin sequences

The amino acid sequence of mouse brain beta spectrin (beta fodrin), deduced from the nucleotide sequence of complementary DNA clones, reveals that this non-erythroid beta spectrin comprises 2363 residues, with a molecular weight of 274,449 Da. Brain beta spectrin contains three structural domains and we suggest the position of several functional domains including f-actin, synapsin I, ankyrin and spectrin self association sites. Analysis of deduced amino acid sequences indicated striking homology and similar structural characteristics of brain beta spectrin repeats beta 11 and beta 12 to globins. In vitro analysis has demonstrated that heme is capable of specific attachment to brain spectrin, suggesting possible new functions in electron transfer, oxygen binding, nitric oxide binding or heme scavenging.

Developmental expression of brain beta-spectrin isoform messenger RNAs

We have investigated the expression of brain beta SpIIa and beta SpIb (previously referred to as the beta-subunits of brain spectrin (240/235) and brain spectrin (240/235E), respectively) during mouse brain development. The 9 kb transcript which encodes beta SpIIa is present in fetal mouse brain tissue and increases to a maximal level in a 30-day-old mouse. There is a coordinate accumulation of the 7.8 kb alpha SpIIa mRNA (with beta SpIIa) during mouse brain development. The coordinate expression of alpha SpIIa and beta SpIIa at the mRNA and protein level allows formation of (alpha SpIIa/beta SpIIa)2 tetramers (brain spectrin(240/235)) early in premitotic neuronal development; and avoids turnover of unassembled alpha and beta-subunits. An 11 kb transcript which encodes beta SpIb is not produced in embryonic tissue, and is first seen in a 6-day-old mouse. The protein translation products beta SpIIa and beta SpIb have previously been demonstrated by our laboratory to first appear in fetal mouse brain tissue and at postnatal day 6-8, respectively [J. Neurosci., 7 (1987) 864-874]. The expression of beta SpIb mRNA on postnatal day 6-8, and the appearance of brain spectrin(240/235E) in postmitotic and postmigratory neurons of the cerebellum at this same time; suggests that brain spectrin(240/235E) is involved in differentiated functions of the neuron (formation of cell-cell contacts, formation of dendritic processes and postsynaptic contacts). Thus, the data from the present study demonstrates that the expression of these two neuronal beta-spectrin isoforms is regulated at the level of mRNA expression.