Francesca Di Leva

The plasma membrane Ca2+ ATPase of animal cells : structure, function and regulation

Most important processes in cell life are regulated by calcium (Ca2+). A number of mechanisms have thus been developed to maintain the concentration of free Ca2+ inside cells at the level (100-200nM) necessary for the optimal operation of the targets of its regulatory function. The systems that move Ca2+ back and forth across membranes are important actors in its control. The plasma membrane calcium ATPase (PMCA pump) which ejects Ca2+ from all eukaryotic cell types will be the topic of this contribution. The pump uses a molecule of ATP to transport one molecule of Ca2+ from the cytosol to the external environment. It is a P-type ATPase encoded by four genes (ATP2B1-4), the transcripts of which undergo different types of alternative splicing. Many pump variants thus exist. Their multiplicity is best explained by the specific Ca2+ demands in different cell types. In keeping with these demands, the isoforms are differently expressed in tissues and cell types and have differential Ca2+ extruding properties. At very low Ca2+ concentrations the PMCAs are nearly inactive. They must be activated by calmodulin, by acid phospholipids, by protein kinases, and by other means, e.g., a dimerization process. Other proteins interact with the PMCAs (i.e., MAGUK and NHERF at the PDZ domain and calcineurin A in the main intracellular domain) to sort them to specific regions of the cell membrane or to regulate their function. In some cases the interaction is isoform, or even splice variant specific. PMCAs knock out (KO) mice have been generated and have contributed information on the importance of PMCAs to cells and organisms. So far, only one human genetic disease, hearing loss, has been traced back to a PMCA defect.

The novel mouse mutation Oblivion inactivates the PMCA2 pump and causes progressive hearing loss.

Progressive hearing loss is common in the human population, but we have few clues to the molecular basis. Mouse mutants with progressive hearing loss offer valuable insights, and ENU (N-ethyl-N-nitrosourea) mutagenesis is a useful way of generating models. We have characterised a new ENU-induced mouse mutant, Oblivion (allele symbol Obl), showing semi-dominant inheritance of hearing impairment. Obl/+ mutants showed increasing hearing impairment from post-natal day (P)20 to P90, and loss of auditory function was followed by a corresponding base to apex progression of hair cell degeneration. Obl/Obl mutants were small, showed severe vestibular dysfunction by 2 weeks of age, and were completely deaf from birth; sensory hair cells were completely degenerate in the basal turn of the cochlea, although hair cells appeared normal in the apex. We mapped the mutation to Chromosome 6. Mutation analysis of Atp2b2 showed a missense mutation (2630C→T) in exon 15, causing a serine to phenylalanine substitution (S877F) in transmembrane domain 6 of the PMCA2 pump, the resident Ca2+ pump of hair cell stereocilia. Transmembrane domain mutations in these pumps generally are believed to be incompatible with normal targeting of the protein to the plasma membrane. However, analyses of hair cells in cultured utricular maculae of Obl/Obl mice and of the mutant Obl pump in model cells showed that the protein was correctly targeted to the plasma membrane. Biochemical and biophysical characterisation showed that the pump had lost a significant portion of its non-stimulated Ca2+ exporting ability. These findings can explain the progressive loss of auditory function, and indicate the limits in our ability to predict mechanism from sequence alone.

The novel PMCA2 pump mutation Tommy impairs cytosolic calcium clearance in hair cells and links to deafness in mice.

The mechanotransduction process in hair cells in the inner ear is associated with the influx of calcium from the endolymph. Calcium is exported back to the endolymph via the splice variant w/a of the PMCA2 of the stereocilia membrane. To further investigate the role of the pump, we have identified and characterized a novel ENU-induced mouse mutation, Tommy, in the PMCA2 gene. The mutation causes a non-conservative E629K change in the second intracellular loop of the pump that harbors the active site. Tommy mice show profound hearing impairment from P18, with significant differences in hearing thresholds between wild type and heterozygotes. Expression of mutant PMCA2 in CHO cells shows calcium extrusion impairment; specifically, the long term, non-stimulated calcium extrusion activity of the pump is inhibited. Calcium extrusion was investigated directly in neonatal organotypic cultures of the utricle sensory epithelium in Tommy mice. Confocal imaging combined with flash photolysis of caged calcium showed impairment of calcium export in both Tommy heterozygotes and homozygotes. Immunofluorescence studies of the organ of Corti in homozygous Tommy mice showed a progressive base to apex degeneration of hair cells after P40. Our results on the Tommy mutation along with previously observed interactions between cadherin-23 and PMCA2 mutations in mouse and humans underline the importance of maintaining the appropriate calcium concentrations in the endolymph to control the rigidity of cadherin and ensure the function of interstereocilia links, including tip links, of the stereocilia bundle.

Functional specificity of PMCA isoforms

Abstract:  In mammals, four different genes encode four PMCA isoforms. PMCA1 and PMCA4 are expressed ubiquitously. PMCA2 and PMCA3 are expressed prevalently in the central nervous systems. More than 30 variants are generated by mechanisms of alternative splicing. The physiological meaning of the existence of such elevated number of isoforms is not clear, but it would be plausible to relate it to the cell‐specific demands of Ca2+ homeostasis. To characterize functional specificity of PMCA variants we have investigated two aspects: the effects of the overexpression of the different PMCA variants on cellular Ca2+ handling and the existence of possible isoform‐specific interactions with partner proteins using a yeast two‐hybrid technique. The four basic PMCA isoforms were coexpressed in CHO cells together with the Ca2+‐sensitive recombinant photoprotein aequorin. The effects of their overexpression on Ca2+ homeostasis were monitored in the living cells. They had revealed that the ubiquitous isoforms 1 and 4 are less effective in reducing the Ca2+ peaks generated by cell stimulation as compared to the neuron‐specific isoforms 2 and 3. To establish whether these differences were related to different and new physiological regulators of the pump, the 90 N‐terminal residues of PMCA2 and PMCA4 have been used as baits for the search of molecular partners. Screening of a human brain cDNA library with the PMCA4 bait specified the ɛ‐isoform of protein 14‐3‐3, whereas no 14‐3‐3 ɛ clone was obtained with the PMCA2 bait. Overexpression of PMCA4/14‐3‐3 ɛ (but not of PMCA2/14‐3‐3 ɛ) in HeLa cells together with targeted aequorins showed that the ability of the cells to export Ca2+ was impaired. Thus, the interaction with 14‐3‐3 ɛ inhibited PMCA4 but not PMCA2. The role of PMCA2 has been further characterized by Ca2+ measurements in cells overexpressing different splicing variants. The results indicated that the combination of alternative splicing at two different sites in the pump structure was responsible for different functional characteristics of the pumps.

Physical and Functional Interaction of HIV-1 Tat with E2F-4, a Transcriptional Regulator of Mammalian Cell Cycle

Tat protein of the human immunodeficiency virus type-1 (HIV-1) plays a critical role in the regulation of viral transcription and replication. In addition, Tat regulates the expression of a variety of cellular genes and could account for AIDS-associated diseases including Kaposis Sarcoma and non-Hodgkins lymphoma by interfering with cellular processes such as proliferation, differentiation, and apoptosis. The molecular mechanisms underlying the pleiotropic activities of Tat may include the generation of functional heterodimers of Tat with cellular proteins. By screening a human B-lymphoblastoid cDNA library in the yeast two-hybrid system, we identified E2F-4, a member of E2F family of transcription factors, as a Tat-binding protein. The interaction between Tat and E2F-4 was confirmed by GST pull-down experiments performed with cellular extracts as well as with in vitro translated E2F-4. The physical association of Tat and E2F-4 was confirmed by in vivobinding experiments where Tat·E2F-4 heterodimers were recovered from Jurkat cells by immunoprecipitation and immunoblotting. By using plasmids expressing mutant forms of Tat and E2F-4, the domains involved in Tat·E2F-4 interaction were identified as the regions encompassing amino acids 1–49 of Tat and amino acids 1–184 of E2F-4. Tat·E2F-4 complexes were shown to bind to E2F cis-regions with increased efficiency compared with E2F-4 alone and to mediate the activity of E2F-dependent promoters including HIV-1 long terminal repeat and cyclin A. The data point to Tat as an adaptor protein that recruits cellular factors such as E2F-4 to exert its multiple biological activities.

Identification of a Novel Mutation in the Myosin VIIA Motor Domain in a Family with Autosomal Dominant Hearing Loss (DFNA11)

We ascertained a large Italian family with an autosomal dominant form of non-syndromic sensorineural hearing loss with vestibular involvement. A genome-wide scan found linkage to locus DFNA11. Sequencing of the MYO7A gene in the linked region identified a new missense mutation resulting in an Ala230Val change in the motor domain of the myosin VIIA. Myosin VIIA has already been implicated in several forms of deafness, but this is the third mutation causing a dominant form of deafness, located in the myosin VIIA motor domain in a region never involved in hearing loss until now. A modelled protein structure of myosin VII motor domain provides evidence for a significant functional effect of this missense mutation.

Deletions and mutations in the acidic lipid-binding region of the plasma membrane Ca2+ pump: a study on different splicing variants of isoform 2.

Acidic phospholipids increase the affinity of the plasma membrane Ca2+-ATPase pump for Ca2+. They interact with the C-terminal region of the pump and with a domain in the loop connecting transmembrane domains 2 and 3 (AL region) next to site A of alternative splicing. The contribution of the two phospholipid-binding sites and the possible interference of splicing inserts at site A with the regulation of the ATPase activity of isoform 2 of the pump by phospholipids have been analyzed. The activity of the full-length z/b variant (no insert at site A), the w/b (with insert at site A), and the w/a variant, containing both the 45-amino acid A-site insert and a C-site insert that truncates the pump in the calmodulin binding domain, has been analyzed in microsomal membranes of overexpressing CHO cells. The A-site insertion did not modify the phospholipid sensitivity of the pump, but the doubly inserted w/a variant became insensitive to acidic phospholipids, even if containing the intact AL phospholipid binding domain. Pump mutants in which 12 amino acids had been deleted, or single lysine mutations introduced, in the AL region were studied by monitoring agonist-induced Ca2+ transients in overexpressing CHO cells. The 12-residue deletion completely abolished the ATPase activity of the w/a variant but only reduced that of the z/b variant, which was also affected by the single lysine substitutions in the same domain. A structural interpretation of the interplay of the pump with phospholipids, and of the mechanism of their activation, is proposed on the basis of molecular modeling studies.

Screening for GJB2 and GJB6 gene mutations in patients from Campania region with sensorineural hearing loss

Abstract The aim of this study was to screen 349 patients affected by sensorineural hearing loss (SNHL), mostly from the Campania region (southern Italy), for GJB2 gene mutations and for two deletions of the GJB6 gene (del GJB6 -D13S1830 and del GJB6 -D13S1854). We identified pathogenetic GJB2 mutations in 51 cases (15% of patients). No GJB6 mutation was found. We also examined the audiologic features of the patients for whom we had an etiologic diagnosis, in order to identify correlations between the severity of hearing loss and the type of mutation. Sumario El objetivo de este estudio fue evaluar 349 pacientes afecta-dos de una hipoacusia sensorineural (SNHL), sobre todo de la región de Campania (Italia del sur), buscando muta-ciones en el gen GJB2 y dos deleciones en el gen GJB6 (del GJB6-D13S1830 y del GJB6-13S1854). Identifica-mos mutaciones patogénicas en 51 pacientes (15% de los pacientes). No se encontraron mutaciones del gen GJB6. También examinamos los rasgos audiológicos de aquellos pacientes de quienes teníamos diagnóstico etiológico, para identificar correlaciones entre la severidad de la pérdida audi-tiva y el tipo de mutación.

Human Synaptobrevin-like 1 Gene Basal Transcription Is Regulated through the Interaction of Selenocysteine tRNA Gene Transcription Activating Factor-Zinc Finger 143 Factors with Evolutionary Conserved Cis-elements

The synaptobrevin-like 1 (SYBL1) gene is ubiquitously expressed and codes for an unusual member of the v-SNAREs molecules implicated in cellular exocytosis. This X-linked gene has the peculiarity of also being present on the Y chromosome in a transcriptional inactive status. Moreover, although ubiquitous, the function of SYBL1 is prominent in specific tissues, such as brain. As a first insight into the molecular mechanisms controlling SYBL1 expression, in this report we describe the extent and role of SYBL1 upstream regions and characterize the binding of trans-acting factors. In vivo foot-printing experiments identify three protected regions. Band shift and transient reporter gene assays indicate a strong role of two of these evolutionary conserved regions in regulating SYBL1 transcription. Because one site is the classical CAAT box, we characterized the binding to the other site of the mammalian homologues of the selenocysteine tRNA gene transcription activating factor (Staf) family, zinc-finger transcription factors, and their role in regulating SYBL1 expression. The results reported here clarify that a Staf-zinc finger family factor, together with the CAAT factor, is the major nuclear protein bound to the SYBL1 promoter region and is responsible for its regulation in HeLa cells, thus identifying the basic control of SYBL1 transcription. In vivo binding of Staf proteins to the SYBL1 promoter is confirmed by chromatin immunoprecipitation assays. Our results identify a fourth mRNA promoter stimulated by a member of the Staf-zinc finger family, the function of which on mRNA polymerase II promoters is still very poorly understood.

Otosclerosis: exclusion of linkage to the OTSC1 and OTSC2 loci in four Italian families.

Otosclerosis is the single most common cause of hearing impairment among adult Caucasians. Little is known about its aetiology and its molecular aspects. Until now, genetic linkage in Otosclerosis has been demonstrated in an Indian family and a Belgian family, showing the presence of two Otosclerosis loci, OTSC1 and OTSC2, respectively. Linkage analysis has never been applied to Italian otosclerotic families. We have collected four multigenerational Italian otosclerotic families that show dominant transmission for the pathology. Here, we report a detailed audiological analysis of these families and a genetic linkage study on the OTSC1 and OTSC2 loci. Statistical analysis revealed the absence of linkage between the disease in our families and the OTSC1 and OTSC2 loci. These data strongly suggest the presence of one or more additional loci for Otosclerosis, which still need to be defined. Sumario La otocsclerosis es la causa mas comun de problemas auditivos entre los adullos caucásicos. Poco se conoce acerca de su etiología y de sus aspectos moleculares. Hasta ahora, los enlaces genéticos en la Otoesclerosis han sido demostrados en familias de la India y Bélgica mostrando la prescncia de dos loci de Otoesclerosis, OTSC1 y ORSC2, respectivamente. Nunca se ha realizado el análisis de enlaces en familias con otoeselerosis en Italia. Nosotros coleclamos los datos multigeneracionales de cuatro familias italianas con Otoesclerosis, que mostraron transmisión dominante de esa patologia. En este trabajo reportamos el análisis audiológico detallado de estas familias y el estudio de enlaces genéticos con los loci OTSC1 y ORSC2. El análisis estadistico reveló la auscncia de relación entre la enfermedad en las cuatro familias y los loci OTSC1 y OTSC2. Estos datos sugieren fuertemente la presencia de uno o más loci adicionales para la Otoesclerosis, que aún es necesario definir.