Skin-specific expression of ank-3(93), a novel ankyrin-3 splice variant

Exon organization and novel alternative splicing of Ank3 in mouse heart

Ankyrin-G is an adaptor protein that links membrane proteins to the underlying cytoskeletal network. Alternative splicing of the Ank3 gene gives rise to multiple ankyrin-G isoforms in numerous tissues. To date, only one ankyrin-G isoform has been characterized in heart and transcriptional regulation of the Ank3 gene is completely unknown. In this study, we describe the first comprehensive analysis of Ank3 expression in heart. Using a PCR-based screen of cardiac mRNA transcripts, we identify two new exons and 28 alternative splice variants of the Ank3 gene. We measure the relative expression of each splice variant using quantitative real-time PCR and exon-exon boundary spanning primers that specifically amplify individual Ank3 variants. Six variants are rarely expressed (<1%), while the remaining variants display similar expression patterns in three hearts. Of the five first exons in the Ank3 gene, exon 1d is only expressed in heart and skeletal muscle as it was not detected in brain, kidney, cerebellum, and lung. Immunoblot analysis reveals multiple ankyrin-G isoforms in heart, and two ankyrin-G subpopulations are detected in adult cardiomyocytes by immunofluorescence. One population co-localizes with the voltage-gated sodium channel Na_V1.5 at the intercalated disc, while the other population expresses at the Z-line. Two of the rare splice variants excise a portion of the ZU5 motif, which encodes the minimal spectrin-binding domain, and these variants lack β-spectrin binding. Together, these data demonstrate that Ank3 is subject to complex splicing regulation resulting in a diverse population of ankyrin-G isoforms in heart.

Ankyrin protein networks in membrane formation and stabilization

In eukaryotic cells, ankyrins serve as adaptor proteins that link membrane proteins to the underlying cytoskeleton. These adaptor proteins form protein complexes consisting of integral membrane proteins, signalling molecules and cytoskeletal components. With their modular architecture and ability to interact with many proteins, ankyrins organize and stabilize these protein networks, thereby establishing the infrastructure of membrane domains with specialized functions. To this end, ankyrin collaborates with a number of proteins including cytoskeletal proteins, cell adhesion molecules and large structural proteins. This review addresses the targeting and stabilization of protein networks related to ankyrin interactions with the cytoskeletal protein β-spectrin, L1-cell adhesion molecules and the large myofibrillar protein obscurin. The significance of these interactions for differential targeting of cardiac proteins and neuronal membrane formation is also presented. Finally, this review concludes with a discussion about ankyrin dysfunction in human diseases such as haemolytic anaemia, cardiac arrhythmia and neurological disorders.

Ankyrin-linked hereditary spherocytosis in an African-American kindred

Mutations of ankyrin-1 are the most frequent cause of the inherited hemolytic anemia, hereditary spherocytosis (HS), in people of European ancestry. Ankyrin-1, which provides the primary linkage between the erythrocyte membrane skeleton and the plasma membrane, has numerous isoforms generated by alternative splicing, alternate polyadenylation, use of tissue-specific promoters, and alternate NH(2) or COOH-termini. Mutation detection in erythrocyte membrane protein genes, including ankyrin, has been a challenge, primarily due to the large size of these genes, and the apparent frequent occurrence of HS-associated null alleles. Using denaturing high-performance liquid chromatography (DHPLC), we screened the ankyrin gene of the proband of a large, three generation African-American kindred with ankyrin-deficient HS. DHPLC yielded an abnormal chromatogram for exon 1. Examination of the corresponding exon 1 sequence in genomic DNA from the proband revealed heterozygosity for a mutation of the initiator methionine (ATG to ATA Met 1 Ile). Coupled in vitrotranscription/translation studies with rabbit reticulocyte lysates demonstrated that the wild-type ankyrin erythroid cDNA initiates only from the known initiator methionine, indicating that the use of alternate initiator methionine is not a mechanism of isoform diversity in erythroid cells. The mutant ankyrin allele, unlike some initiator methionine mutations that utilize downstream codons for translation initiation, was associated with a null allele. This is the first report describing ankyrin-linked HS in an African-American kindred.

Quantitative immuno-electron microscopic analysis of nuclear respiratory factor 2 alpha and beta subunits: Normal distribution and activity-dependent regulation in mammalian visual cortex

The macaque visual cortex is exquisitely organized into columns, modules, and streams, much of which can be correlated with its metabolic organization revealed by cytochrome oxidase (CO). Plasticity in the adult primate visual system has also been documented by changes in CO activity. Yet, the molecular mechanism of regulating this enzyme remains not well understood. Being one of only four bigenomic enzymes in mammalian cells, the transcriptional regulation of this enzyme necessitates a potential bigenomic coordinator. Nuclear respiratory factor 2 (NRF-2) or GA-binding protein is a transcription factor that may serve such a critical role. The goal of the present study was to determine if the two major subunits of NRF-2, 2alpha and 2beta, had distinct subcellular distribution in neurons of the rat and monkey visual cortex, if major metabolic neuronal types in the macaque exhibited different levels of the two subunits, and if they would respond differently to monocular impulse blockade. Quantitative immuno-electron microscopy was used. In both rats and monkeys, nuclear labeling of alpha and beta subunits was mainly over euchromatin rather than heterochromatin, consistent with their active participation in transcriptional activity. Cytoplasmic labeling was over free ribosomes, the Golgi apparatus, and occasionally the nuclear envelope, signifying sites of synthesis and possible posttranslational modifications. The density of both subunits was much higher in the nucleus than in the cytoplasm for all neurons examined, again indicating that their major sites of cellular action is in the nucleus.