Plasma-membrane calcium pumps and hereditary deafness

De novo sequencing and initial annotation of the Mongolian gerbil (Meriones unguiculatus) genome

The Mongolian gerbil (Meriones unguiculatus) is a member of the rodent family that displays several features not found in mice or rats, including sensory specializations and social patterns more similar to those in humans. These features have made gerbils a valuable animal for research studies of auditory and visual processing, brain development, learning and memory, and neurological disorders. Here, we report the whole gerbil annotated genome sequence, and identify important similarities and differences to the human and mouse genomes. We further analyze the chromosomal structure of eight genes with high relevance for controlling neural signaling and demonstrate a high degree of homology between these genes in mouse and gerbil. This homology increases the likelihood that individual genes can be rapidly identified in gerbil and used for genetic manipulations. The availability of the gerbil genome provides a foundation for advancing our knowledge towards understanding evolution, behavior and neural function in mammals. ACCESSION NUMBER: The Whole Genome Shotgun sequence data from this project has been deposited at DDBJ/ENA/GenBank under the accession NHTI00000000. The version described in this paper is version NHTI01000000. The fragment reads, and mate pair reads have been deposited in the Sequence Read Archive under BioSample accession SAMN06897401.

Maintenance of neuronal size gradient in MNTB requires sound-evoked activity

Neurons of the medial nucleus of the trapezoid body (MNTB) act as fast-spiking inhibitory interneurons within the auditory brain stem. The MNTB is topographically organized, with low sound frequencies encoded laterally and high frequencies medially. We discovered a cell size gradient along this axis: lateral neurons are larger than medial neurons. The absence of this gradient in deaf mice lacking plasma membrane calcium ATPase 2 suggests an activity-dependent, calcium-mediated mechanism that controls neuronal soma size.

The medial nucleus of the trapezoid body (MNTB) is an important source of inhibition during the computation of sound location. It transmits fast and precisely timed action potentials at high frequencies; this requires an efficient calcium clearance mechanism, in which plasma membrane calcium ATPase 2 (PMCA2) is a key component. Deafwaddler (dfw^2J) mutant mice have a null mutation in PMCA2 causing deafness in homozygotes (dfw^2J/dfw^2J) and high-frequency hearing loss in heterozygotes (+/dfw^2J). Despite the deafness phenotype, no significant differences in MNTB volume or cell number were observed in dfw^2J homozygous mutants, suggesting that PMCA2 is not required for MNTB neuron survival. The MNTB tonotopic axis encodes high to low sound frequencies across the medial to lateral dimension. We discovered a cell size gradient along this axis: lateral neuronal somata are significantly larger than medially located somata. This size gradient is decreased in +/dfw^2J and absent in dfw^2J/dfw^2J. The lack of acoustically driven input suggests that sound-evoked activity is required for maintenance of the cell size gradient. This hypothesis was corroborated by selective elimination of auditory hair cell activity with either hair cell elimination in Pou4f3 DTR mice or inner ear tetrodotoxin (TTX) treatment. The change in soma size was reversible and recovered within 7 days of TTX treatment, suggesting that regulation of the gradient is dependent on synaptic activity and that these changes are plastic rather than permanent.

NEW & NOTEWORTHY Neurons of the medial nucleus of the trapezoid body (MNTB) act as fast-spiking inhibitory interneurons within the auditory brain stem. The MNTB is topographically organized, with low sound frequencies encoded laterally and high frequencies medially. We discovered a cell size gradient along this axis: lateral neurons are larger than medial neurons. The absence of this gradient in deaf mice lacking plasma membrane calcium ATPase 2 suggests an activity-dependent, calcium-mediated mechanism that controls neuronal soma size.

Evaluation of plasma membrane calcium/calmodulin-dependent ATPase isoform 4 as a potential target for fertility control

The array of contraceptives currently available is clearly inadequate and does not meet consumer demands since it is estimated that up to a quarter of all pregnancies worldwide are unintended. There is, therefore, an overwhelming global need to develop new effective, safe, ideally non-hormonal contraceptives for both male and female use. The contraceptive field, unlike other areas such as cancer, has a dearth of new targets. We have addressed this issue and propose that isoform 4 of the plasma membrane calcium ATPase is a potentially exciting novel target for fertility control. The plasma membrane calcium ATPase is a ubiquitously expressed calcium pump whose primary function in the majority of cells is to extrude calcium to the extracellular milieu. Two isoforms of this gene family, PMCA1 and PMCA4, are expressed in spermatozoa, with PMCA4 being the predominant isoform. Although this gene is ubiquitously expressed, its function is highly tissue-specific. Genetic deletion of PMCA4, in PMCA4 knockout mice, led to 100% infertility specifically in the male mutant mice due to a selective defect in sperm motility. It is important to note that the gene deletion did not affect normal mating characteristics in these mice. This phenotype was mimicked in wild-type sperm treated with the non-specific PMCA inhibitor 5-(and 6-) carboxyeosin diacetate succinimidyl ester; a proof-of-principle that inhibition of PMCA4 has potential importance in the control of fertility. This review outlines the potential for PMCA4 to be a novel target for fertility control by acting to inhibit sperm motility. It will outline the characteristics that make this target drugable and will describe methodologies to identify and validate novel inhibitors of this target.

atp2b1a regulates Ca(2+) export during differentiation and regeneration of mechanosensory hair cells in zebrafish

The molecular mechanisms of development of mechanosensory hair cells have been tackled successfully due to in vivo studies in the zebrafish lateral line. The enhancer trap (ET) transgenic line, SqET4 was instrumental in these studies even despite a lack of a link of its GFP expression pattern to a particular gene(s). We mapped the Tol2 transposon insertion of the SqET4 transgenics onto Chr. 4 next to a gene encoding Atp2b1a (Pmca1) - one of the four PMCAs acting to export Ca(2+) from a cell. atp2b1a expression recapitulates that of GFP during the development of mechanoreceptors of the inner ear and lateral line. atp2b1a expression correlates with the regeneration of these cells. Thus, SqET4 represents the Tg:atp2b1a-GFP line, which links Ca(2+) metabolism and the differentiation of mechanoreceptors. The morpholino-mediated knockdown of atp2b1a blocks Ca(2+) export and affects the division of hair cell progenitors, resulting in their accumulation. Under the control of a master gene of hair cells, Atoh1a, Atp2b1a functions during progenitor cell proliferation and hair cell differentiation. Given the similarity between the phenotypes of atp2b1a morphants and embryos treated with the pan-PMCA inhibitor 5(6)-carboxyeosin, Atp2b1a emerges as member of the Atp2b family responsible for Ca(2+) export during the development of hair cells.

Role of plasma membrane calcium ATPases in calcium clearance from olfactory sensory neurons

Odorants cause Ca(2+) to rise in olfactory sensory neurons (OSNs) first within the ciliary compartment, then in the dendritic knob, and finally in the cell body. Ca(2+) not only excites but also produces negative feedback on the transduction pathway. To relieve this Ca(2+)-dependent adaptation, Ca(2+) must be cleared from the cilia and dendritic knob by mechanisms that are not well understood. This work focuses on the roles of plasma membrane calcium pumps (PMCAs) through the use of inhibitors and mice missing 1 of the 4 PMCA isoforms (PMCA2). We demonstrate a significant contribution of PMCAs in addition to contributions of the Na(+)/Ca(2+) exchanger and endoplasmic reticulum (ER) calcium pump to the rate of calcium clearance after OSN stimulation. PMCAs in neurons can shape the Ca(2+) signal. We discuss the contributions of the specific PMCA isoforms to the shape of the Ca(2+) transient that controls signaling and adaptation in OSNs.

Plasma membrane Ca(2+)-ATPase: from a housekeeping function to a versatile signaling role

Plasma membrane Ca(2+)-ATPases (PMCAs) are high-affinity calcium pumps that contribute to the maintenance of intracellular Ca(2+) homeostasis by exporting Ca(2+) from the cytosol to the extracellular environment. Mammals have four genes encoding the proteins PMCA1 through PMCA4. Each gene transcript is alternatively spliced to generate several variants. Their distribution is tissue- and cell-specific and undergoes regulation during cell development and differentiation. Traditionally, these pumps have been considered to play a housekeeping role in controlling basal Ca(2+) levels, but more recently, it became clear that the presence (and the co-expression) of different isoforms must be related to a more specialized function. Only one of the four genes (encoding PMCA2) has been causally linked to disease in mammals: Several spontaneous mutations are responsible for deafness and ataxia. Other complex human disease phenotype like hearing loss, cardiac function, and infertility are likely to be associated with PMCA function, but no spontaneous mutations in other PMCA genes than PMCA2 are so far identified. The evidence of their involvement in disease phenotypes comes from studies on isoform-specific knockout mice. In this review, I will discuss briefly the general role of PMCA as essential component of Ca(2+) homeostasis machinery and focus on its emerging role as signaling molecule with particular attention on the diseases caused by PMCA dysfunction.