Building the mammalian cochlea — an overview

The mammalian cochlea is a highly intricate organ responsible for hearing. Numerous specialized cell types residing in the cochlear participate in processing and relaying sound information to the brain. In general, cells in the cochlea are divided into three major types: sensory, neural, and non-sensory. Sensory cells are a group of cells in the organ of Corti consisting of hair cells and supporting cells. Sensory hair cells play a primary role in detecting and processing sound in the form of vibrations. Neural cells are the neurons and glia in the spiral (cochlear) ganglion that relay the processed sound signals in the form of a neurotransmitter to the brain. Other non-sensory cells include all other cell types providing architectural and functional support. Building a functional cochlea requires tightly orchestrated, spatial and temporal regulation of gene expressions. Disruption of the normal gene expression patterns can cause developmental failure of the organ, which can lead to permanent hearing loss. Thus, comprehensive understanding of genes contributing to cochlear development is crucial for elucidating the pathological mechanisms of hearing loss. This article is intended to provide an overview of mammalian cochlear development, focusing on genes involved in its early patterning.     

Leucyl-β-naphthylamide-splitting enzymes in the mammalian endocrine pancreas

The proteolytic capacity of the endocrine pancreas in obese-hyperglycaemic mice was evaluated by using the chromogenic substrate l-leucyl-beta-naphthylamide (LNA). The analytical sensitivity obtained with this substrate in photometric and fluorimetric assays permitted quantitative determinations of C-N-bond-splitting activity in both crude islet homogenates and electrophoretic fractions thereof. The following observations were made: (1) The rate of LNA cleavage was maximal at about pH7 in islets as well as in acinar tissue. An apparent K(m) of 4x10(-5)-6x10(-5)m was calculated for both the endocrine and exocrine pancreas. (2) The level of LNA-splitting enzyme activity was of the same magnitude in the islets as in the liver, and significantly higher in the islets than in the exocrine pancreas. Starving the animals for 7 days did not affect the enzyme activity levels. (3) Two distinct LNA-splitting enzymes could be separated from the pancreatic islets by means of disc electrophoresis, the most rapidly migrating band representing the highest activity. Though similar electrophoresis patterns were obtained with acinar tissue and liver, only one enzyme could be demonstrated in serum. The data suggest that the beta-cells, in addition to being highly specialized for the production of a specific protein, contain a comparatively high capacity for protein catabolism.     

Is the mammalian serine palmitoyltransferase a high-molecular-mass complex?

SPT (serine palmitoyltransferase) catalyses the rate-limiting step for the de novo synthesis of sphingolipids. Mammalian SPT is believed to be a heterodimer composed of two subunits, SPTLC1 and SPTLC2. We reported previously the identification of a new third SPT subunit, SPTLC3. In the present study, we have investigated the structure of the SPT complex in more detail. Pull-down assays with antibodies against SPTLC3 concomitantly co-precipitated SPTLC1 and SPTLC2 in human placenta extracts and SPTLC3 overexpressing human embryonic kidney-293 cells. By size exclusion chromatography, we determined the molecular mass of the functional SPT complex to be approx. 480 kDa. By Blue-native-PAGE experiments we demonstrated that all three SPT subunits (SPTLC1-3) are co-localized within a single SPT complex. On the basis of these results we conclude that the functional SPT is not a dimer, but a higher organized complex, composed of three distinct subunits (SPTLC1, SPTLC2 and SPTLC3) with a molecular mass of 480 kDa. The stoichiometry of SPTLC2 and SPTLC3 in this complex seems not to be fixed and is probably changed dynamically in dependence of the tissue specific SPTLC2 and SPTLC3 expression levels. Based on our own and earlier published data we propose a model of an octameric SPT structure. The observed dynamic composition of the SPT complex could provide a cellular mechanism to adjust SPT activity to tissue specific requirements in sphingolipid synthesis.     

Melatonin in the multi‐oscillatory mammalian circadian world

In mammals, the complex interaction of neural, hormonal, and behavioral outputs from the suprachiasmatic nucleus (SCN) drives circadian expression of events, either directly or through coordination of the timing of peripheral oscillators. Melatonin, one of the endocrine output signals of the clock, provides the organism with circadian information and can be considered as an endogenous synchronizer, able to stabilize and reinforce circadian rhythms and to maintain their mutual phase‐relationship at the different levels of the circadian network. Moreover, exogenous melatonin, through an action on the circadian clock, affects all levels of the circadian network. The molecular mechanisms underlying this chronobiotic effect have also been investigated in rats. REV‐ERB α seems to be the initial molecular target.     

Something in the air? New insights into mammalian pheromones

Olfaction is the dominant sensory modality for most animals and chemosensory communication is particularly well developed in many mammals. Our understanding of this form of communication has grown rapidly over the last ten years since the identification of the first olfactory receptor genes. The subsequent cloning of genes for rodent vomeronasal receptors, which are important in pheromone detection, has revealed an unexpected diversity of around 250 receptors belonging to two structurally different classes. This review will focus on the chemical nature of mammalian pheromones and the complementary roles of the main olfactory system and vomeronasal system in mediating pheromonal responses. Recent studies using genetically modified mice and electrophysiological recordings have highlighted the complexities of chemosensory communication via the vomeronasal system and the role of this system in handling information about sex and genetic identity. Although the vomeronasal organ is often regarded as only a pheromone detector, evidence is emerging that suggests it might respond to a much broader variety of chemosignals.     

Conservation of functional asymmetry in the mammalian MutLα ATPase

The DNA mismatch repair (MMR) protein dimer MutLα is comprised of the MutL homologues MLH1 and PMS2, which each belong to the family of GHL ATPases. These ATPases undergo functionally important conformational changes, including dimerization of the NH₂-termini associated with ATP binding and hydrolysis. Previous studies in yeast and biochemical studies with the mammalian proteins established the importance of the MutLα ATPase for overall MMR function. Additionally, the studies in yeast demonstrated a functional asymmetry between the contributions of the Mlh1 and Pms1 ATPase domains to MMR that was not reflected in the biochemical studies. We investigated the effect of mutating the highly conserved ATP hydrolysis and Mg²⁺ binding residues of MLH1 and PMS2 in mammalian cell lines. Amino acid substitutions in MLH1 intended to impact either ATP binding or hydrolysis disabled MMR, as measured by instability at microsatellite sequences, to an extent similar to MLH1-null mutation. Furthermore, cells expressing these MLH1 mutations exhibited resistance to the MMR-dependent cytotoxic effect of 6-thioguanine (6-TG). In contrast, ATP hydrolysis and binding mutants of PMS2 displayed no measurable increase in microsatellite instability or resistance to 6-TG. Our findings suggest that, in vivo, the integrity of the MLH1 ATPase domain is more critical than the PMS2 ATPase domain for normal MMR functions. These in vivo results are in contrast to results obtained previously in vitro that showed no functional asymmetry within the MutLα ATPase, highlighting the differences between in vivo and in vitro systems.     

Why does the mammalian red blood cell have aquaporins?

Aquaporins are now known to mediate the rapid exchange of water across the plasma membranes of diverse cell types. This exchange has been studied and kinetically characterized in red blood cells (erythrocytes; RBC) from many animal species. In recent years, a favoured method has been one based on NMR spectroscopy. Despite knowledge of their molecular structure the physiological raison d' etre of aquaporins in RBCs is still only speculated upon. Here, we present two hypotheses that account for the fact that the exchange of water is so fast in RBCs. The first is denoted the "oscillating sieve" hypothesis and it posits that known membrane undulations at frequencies up to 30 Hz with displacements up to 0.3 microm are energetically favoured by the high water permeability of the membrane. The second denoted the "water displacement" hypothesis is based on the known rapid exchange across the RBC membrane of ions such as Cl- and HCO3- and solutes such as glucose, all of whose molecular volumes are significantly greater than that of water. The ideas are generalizable to other cell types and organelles.     

Does size matter? Examining the drivers of mammalian vocalizations

Previous studies of the vocalization frequencies of mammals have suggested that it is either body mass or environment that drives these frequencies. Using 193 species across the globe from the terrestrial and aquatic environments and a model selection approach, we identified that the best‐supported model for minimum and maximum frequencies for vocalization included both body mass and environment. The minimum frequencies of vocalizations of species from all environments retained the influence of body mass. For maximum frequency however, aquatic species are released from such a trend with body mass having little constraint on frequencies. Surprisingly, phylogeny did not have a strong impact on the evolution of the maximum frequency of mammal vocalizations, largely due to the pinniped species divergence of frequency from their carnivoran relatives. We demonstrate that the divergence of signal frequencies in mammals has arisen from the need to adapt to their environment.     

Expression of the G-protein α-subunit gustducin in mammalian spermatozoa

Although chemotaxis has been proposed to guide sperm to egg throughout the animal kingdom, sperm attractants released from mammalian eggs have not been identified. Since the G protein subunit α-gustducin is accepted as a marker of chemosensitive cells, attempts were made to explore whether α-gustducin is also expressed in spermatozoa of mammals. Immunohistochemical approaches using an anti-α-gustducin-specific antibody revealed the most intense immunoreactivity in differentiating spermatids. Further evidence for the α-gustducin expression was obtained analyzing testicular and sperm-derived tissue preparations in western blot analyses. To elucidate whether α-gustducin is retained in mature spermatozoa, epididymal mouse and rat sperm were subjected to immunocytochemistry as well as immunogold electron microscopy. A specific staining was obtained within the circumference of the midpiece-localized mitochondria, on the axoneme and the outer dense fibers surrounding the microtubules of this region, whereas no labeling was detectable in the end piece regions. The analysis of ejaculated bovine and human sperm revealed a comparable segmental distribution pattern for α-gustducin. Although a possible function for α-gustducin has yet to be determined, the axonemal-associated localization within the midpiece and principal piece of different mammalian spermatozoa raises the possibility that this G protein α-subunit may process intracellular signals controlling sperm motility. Johanna Fehr and Dorke Meyer contributed equally to this work.     

Is there an acceleration of the CpG transition rate during the mammalian radiation?

MOTIVATION: In this article we build a model of the CpG dinucleotide substitution rate and use it to challenge the claim that, that rate underwent a sudden mammalian-specific increase approximately 90 million years ago. The evidence supporting this hypothesis comes from the application of a model of neutral substitution rates able to account for elevated CpG dinucleotide substitution rates. With the initial goal of improving that model's accuracy, we introduced a modification enabling us to account for boundary effects arising by the truncation of the Markov field, as well as improving the optimization procedure required for estimating the substitution rates. RESULTS: When using this modified method to reproduce the supporting analysis, the evidence of the rate shift vanished. Our analysis suggests that the CpG-specific rate has been constant over the relevant time period and that the asserted acceleration of the CpG rate is likely an artifact of the original model.     

Are Asian monkeys the original mammalian hosts of Trypanosoma conorhini?

It is hypothesized that Asian monkeys were the original hosts of Trypanosoma conorhini because they have been found naturally infected, the vector among rats is a tropicopolitan triatomine bug that belongs to a complex of Asian species, and primates were shown to be more susceptible than rats.     

Genomic rearrangement of the α-globin gene complex during mammalian evolution

In man, the gene for hydroxyacyl glutathione hydrolase (HAGH; glyoxalase II) is closely linked to the α-globin locus (HBα) on Chromosome 16. HAGH polymorphism in the mouse has now enabled the mapping of the murine homologue. Deletion mapping, congenic strain studies, and characterization of 41 recombinant inbred strains establish that the mouseHagh locus lies very close to the α-globin pseudogene (Hba-ps4) in the vicinity of the major histocompatibility locus (H-2) on chromosome 17. Several other loci have been identified previously that are also closely linked to the human α-globin locus but near the α-globin pseudogeneHba-ps4 in the mouse. These linkage relationships suggest that during the evolution of mice a translocation occurred that subdivided the α-globin locus, leaving one inactive α-globin gene still associated with theHagh locus and linked sequences, while moving and inserting the active α-globin locus and all distal sequences into an internal location on another autosome, the predecessor to mouse chromosome 11.     

Evolutionary changes ofα-crystallin and the phylogeny of mammalian orders

Summary The sequences of the A chains of the eye lens protein-crystallin from seventeen mammalian species were compared. They showed a generally slow rate of evolution, but with marked variations in different lineages. Most substitutions have occurred in the C-terminal part of the chain, which probably forms part of the surface of the-crystallin aggregate. The ancestral sequence method of Dayhoff revealed interesting indications about the phylogenetic relationships between the eleven mammalian orders that were represented by the investigated species. Most evident was the divergence of marsupial and placental orders. A notable resemblance between the hyrax and elephant sequences was observed, setting them apart from the ungulates, including whale. Primates, rodents, lagomorphs, insectivores and tupaiids seem to derive from a common stem group. These phylogenetic inferences are discussed in relation to current palaeontological and taxonomical opinions, and compared to evidence from other protein sequence data.     

How does the eye breathe? Evidence for neuroglobin-mediated oxygen supply in the mammalian retina

Visual performance of the vertebrate eye requires large amounts of oxygen, and thus the retina is one of the highest oxygen-consuming tissues of the body. Here we show that neuroglobin, a neuron-specific respiratory protein distantly related to hemoglobin and myoglobin, is present at high amounts in the mouse retina (approximately 100 microm). The estimated concentration of neuroglobin in the retina is thus about 100-fold higher than in the brain and is in the same range as that of myoglobin in the muscle. Neuroglobin is expressed in all neurons of the retina but not in the retinal pigment epithelium. Neuroglobin mRNA was detected in the perikarya of the nuclear and ganglion layers of the neuronal retina, whereas the protein was present mainly in the plexiform layers and in the ellipsoid region of photoreceptor inner segment. The distribution of neuroglobin correlates with the subcellular localization of mitochondria and with the relative oxygen demands, as the plexiform layers and the inner segment consume most of the retinal oxygen. These findings suggest that neuroglobin supplies oxygen to the retina, similar to myoglobin in the myocardium and the skeletal muscle.     

Are more diverse parts of the mammalian skull more?labile?

Morphological variation is unevenly distributed within the mammalian skull; some of its parts have diversified more than others. It is commonly thought that this pattern of variation is mainly the result of the structural organization of the skull, as defined by the pattern and magnitude of trait covariation. Patterns of trait covariation can facilitate morphological diversification if they are aligned in the direction of selection, or these patterns can constrain diversification if oriented in a different direction. Within this theoretical framework, it is thought that more variable parts possess patterns of trait covariation that made them more capable of evolutionary change, that is, are more labile. However, differences in the degree of morphological variation among skull traits could arise despite variation in trait lability if, for example, some traits have evolved at a different rate and/or undergone stabilizing selection. Here, we test these hypotheses in the mammalian skull using 2D geometric morphometrics to quantify skull shape and estimating constraint, rates of evolution, and lability. Contrary to the expectations, more variable parts of the skull across mammalian species are less capable of evolutionary change than are less variable skull parts. Our results suggest that patterns of morphological variation in the skull could result from differences in rate of evolution and stabilizing selection.     

Have gene knockouts caused evolutionary reversals in the mammalian first arch?

Many recent gene knockout experiments cause anatomical changes to the jaw region of mice that several investigators claim are evolutionary reversals. Here we evaluate these mutant phenotypes and the assertions of atavism. We argue that following the knockout of Hoxa-2, Dlx-2, MHox, Otx2, and RAR genes, ectopic cartilages arise as secondary consequences of disruptions in normal processes of cell specification, migration, or differentiation. These disruptions cause an excess of mesenchyme to accumulate in a region through which skeletal progenitor cells usually migrate, and at a site of condensation that is normally present in mammals but that is too small to chondrify. We find little evidence that these genes, when disrupted, cause a reversion to any primitive condition and although changes in their expression may have played a role in the evolution of the mammalian jaw, their function during morphogenesis is not sufficiently understood to confirm such hypotheses. BioEssays 20 :245–255, 1998.© 1998 John Wiley & Sons, Inc.     

R-spondin1—discovery of the long-missing,mammalian female-determining gene?

Until recently, sex determination in mammals has often been described as a male determination process, with male differentiation being the active and dominant pathway, and only in its absence is the passive female pathway followed. This picture has been challenged recently with the discovery that the gene encoding R-spondin1 is mutated in human patients with female-to-male sex reversal. These findings might place R-spondin1 in the exceptional position of being the female-determining gene in mammals. In this review, possible roles of R-spondin1 during sex determination as well as questions arising from this study will be discussed.     

Neurotrophic control of the transmembrane Cl− pump in mammalian muscle fibers

Resting membrane potential of both innervated and denervated rat diaphragm muscle fibers was investigated when the concentration of potential-producing ions was changed in the external fluid and following treatment with furosemide. It was found that equilibrium potential ( ) equalled resting potential level in innervated muscle for Cl^–, but shifts to more positive values compared with resting membrane potential following section of the nerve. Furosemide retards development of post-denervation depolarization of the muscle membrane. It is deduced that trophic influences originating from the motor nerve activates the furosemide-sensitive Cl^– influx system, leading to raised intracellular concentration of Cl^–, a shift in (E_Cl) and depolarization of the muscle membrane.S. V. Kurashov Medical Institute, Minsitry of Health of the RSFSR, Kazan'. Translated from Neirofiziologiya, Vol. 19, No. 6, pp. 766–771, November–December, 1987.