Cloning and expression of a cDNA encoding a brain-specific Na(+)-dependent inorganic phosphate cotransporter

Vesicular glutamate transporter DNPI/VGLUT2 is expressed by both C1 adrenergic and nonaminergic presympathetic vasomotor neurons of the rat medulla

The main source of excitatory drive to the sympathetic preganglionic neurons that control blood pressure is from neurons located in the rostral ventrolateral medulla (RVLM). This monosynaptic input includes adrenergic (C1), peptidergic, and noncatecholaminergic neurons. Some of the cells in this pathway are suspected to be glutamatergic, but conclusive evidence is lacking. In the present study we sought to determine whether these presympathetic neurons express the vesicular glutamate transporter BNPI/VGLUT1 or the closely related gene DNPI, the rat homolog of the mouse vesicular glutamate transporter VGLUT2. Both BNPI/VGLUT1 and DNPI/VGLUT2 mRNAs were detected in the medulla oblongata by in situ hybridization, but only DNPI/VGLUT2 mRNA was present in the RVLM. Moreover, BNPI immunoreactivity was absent from the thoracic spinal cord lateral horn. DNPI/VGLUT2 mRNA was present in many medullary cells retrogradely labeled with Fluoro-Gold from the spinal cord (T2; four rats). Within the RVLM, 79% of the bulbospinal C1 cells contained DNPI/VGLUT2 mRNA. Bulbospinal noradrenergic A5 neurons did not contain DNPI/VGLUT2 mRNA. The RVLM of six unanesthetized rats subjected to 2 hours of hydralazine-induced hypotension contained tenfold more c-Fos-ir DNPI/VGLUT2 neurons than that of six saline-treated controls. c-Fos-ir DNPI/VGLUT2 neurons included C1 and non-C1 neurons (3:2 ratio). In seven barbiturate-anesthetized rats, 16 vasomotor presympathetic neurons were filled with biotinamide and analyzed for the presence of tyrosine hydroxylase immunoreactivity and/or DNPI/VGLUT2 mRNA. Biotinamide-labeled neurons included C1 and non-C1 cells. Most non-C1 (9/10) and C1 presympathetic cells (5/6) contained DNPI/VGLUT2 mRNA. In conclusion, DNPI/VGLUT2 is expressed by most blood pressure-regulating presympathetic cells of the RVLM. The data suggest that these neurons may be glutamatergic and that the C1 adrenergic phenotype is one of several secondary phenotypes that are differentially expressed by subgroups of these cells.

Vesicular glutamate transporter DNPI/VGLUT2 mRNA is present in C1 and several other groups of brainstem catecholaminergic neurons

The mouse glutamate vesicular transporter VGLUT2 has recently been characterized. The rat homolog of VGLUT2, differentiation-associated Na(+)/P(i) cotransporter (DNPI), was examined using a digoxigenin-labeled DNPI/VGLUT2 cRNA probe in the present study to determine which, if any, of the various groups of pontine or medullary monoaminergic neurons express DNPI/VGLUT2 mRNA and, thus, are potentially glutamatergic. DNPI/VGLUT2 mRNA was widely distributed within the brainstem and seemed exclusively neuronal. By using a double in situ hybridization method, the presence of the mRNA for DNPI/VGLUT2 and glutamic acid decarboxylase (GAD)-67 was mutually exclusive. By combining DNPI/VGLUT2 mRNA detection and conventional immunohistochemistry, DNPI/VGLUT2 mRNA was undetectable in lower brainstem cholinergic and serotonergic cells, but it was present in several tyrosine hydroxylase-immunoreactive (TH-ir) cell groups. DNPI/VGLUT2 mRNA was detected in most of the adrenergic neurons of the C1, C2, and C3 groups (75-80% of TH-ir neurons), in the A2 noradrenergic group (80%), and in vast numbers of area postrema cells. Within the A1 region, many fewer TH-ir cells contained DNPI/VGLUT2 (16%). Finally, DNPI/VGLUT2 mRNA was undetectable in the pontine noradrenergic cell groups (A5 and A6/locus coeruleus). In conclusion, the general pattern of DNPI/VGLUT2 expression and its exclusion from GABAergic, cholinergic, and serotonergic neurons supports the notion that DNPI/VGLUT2 mRNA identifies a subset of glutamatergic neurons in the lower brainstem. Within this region several catecholaminergic cell groups appear to be glutamatergic, including but not limited to the adrenergic cell groups C1-C3. Based on the present evidence, the noradrenergic cell groups of the pons (A5 and A6) do not contain either known vesicular glutamate transporter and are most likely not glutamatergic.

Immunohistochemical localization of candidates for vesicular glutamate transporters in the rat brain

Vesicular glutamate transporter 1 (VGluT1) is one of the best markers for glutamatergic neurons, because it accumulates transmitter glutamate into synaptic vesicles. Differentiation-associated Na(+)-dependent inorganic phosphate cotransporter (DNPI) shows 82% amino acid identity to VGluT1, and is another candidate for vesicular glutamate transporters. Here, we report the immunocytochemical localization of DNPI and compare it with that of VGluT1 in the adult rat brain. Both DNPI and VGluT1 immunoreactivities were found mostly in neuropil, presumably in axon terminals, throughout the brain. In the telencephalic regions, intense DNPI immunoreactivity was observed in the glomeruli of the olfactory bulb, layer IV of the neocortex, granular layer of the dentate gyrus, presubiculum, and postsubiculum. In contrast, VGluT1 immunoreactivity was intense in the olfactory tubercle, layers I-III of the neocortex, piriform cortex, entorhinal cortex, hippocampus, dentate gyrus, and subiculum. In the thalamic nuclei, DNPI-immunoreactive terminal-like profiles were much larger than VGluT1-immunoreactive ones, suggesting that DNPI immunoreactivity was subcortical in origin. DNPI immunoreactivity was much more intense than VGluT1 immunoreactivity in many brainstem and spinal cord regions, except the pontine nuclei, interpeduncular nucleus, cochlear nuclei, and external cuneate nucleus. In the molecular layer of the cerebellar cortex, climbing-like fibers showed intense DNPI immunoreactivity, whereas neuropil contained dense VGluT1-immnoreactive deposits. Both DNPI and VGluT1 immunoreactivities were observed as mossy fiber terminal-like profiles in the cerebellar granular layer. DNPI and VGluT1 immunoreactivities appeared associated with synaptic vesicles in the axon terminals forming asymmetric synapses in several regions examined electron microscopically. The present results indicate that DNPI and VGluT1 are used by different neural components in most, if not all, brain regions, suggesting the complementary functions of DNPI and VGluT1.

Identification of the differentiation-associated Na+/PI transporter as a novel vesicular glutamate transporter expressed in a distinct set of glutamatergic synapses

Glutamate transport into synaptic vesicles is a prerequisite for its regulated neurosecretion. Here we functionally identify a second isoform of the vesicular glutamate transporter (VGLUT2) that was previously identified as a plasma membrane Na+-dependent inorganic phosphate transporter (differentiation-associated Na+/P(I) transporter). Studies using intracellular vesicles from transiently transfected PC12 cells indicate that uptake by VGLUT2 is highly selective for glutamate, is H+ dependent, and requires Cl- ion. Both the vesicular membrane potential (Deltapsi) and the proton gradient (DeltapH) are important driving forces for vesicular glutamate accumulation under physiological Cl- concentrations. Using an antibody specific for VGLUT2, we also find that this protein is enriched on synaptic vesicles and selective for a distinct class of glutamatergic nerve terminals. The pathway-specific, complementary expression of two different vesicular glutamate transporters suggests functional diversity in the regulation of vesicular release at excitatory synapses. Together, the two isoforms may account for the uptake of glutamate by synaptic vesicles from all central glutamatergic neurons.

Transporters in the neurohypophysial neuroendocrine system, with special reference to vesicular glutamate transporters (BNPI and DNPI): a review

Recent advances in gene technology have helped to identify novel proteins and allowed study of their distribution and functions in the mammalian brain. One class of these proteins is that of transporters, which exist in plasma and organellar membranes of neurons and other cells to move substances selectively across membranes. These transporters can be categorized further into subclasses by their structural property, substrate selectivity, and site of action. Some of them have been identified in the hypothalamus, which is the only brain site where a neural signal is converted to a humoral one, namely, a hormone for a target organ. This unique neural mechanism has long attracted attention as the neuroendocrine system, part of which has been extensively studied as the hypothalamic-neurohypophysial system involved in secretion of vasopressin and oxytocin. However, transporters in this system have been less well studied. A morphological examination of novel transporters would give us cues to a better understanding of the neuronal organization and function of the system. In this review, we first summarize recent findings on expression of transporter gene and immunoreactivity in the hypothalamus. In the second part, we explain our observations on two vesicular glutamate (inorganic phosphate) transporters (BNPI and DNPI) in the supraoptic and paraventricular nuclei and neurohypophysis. Further study of these and other transporters will provide a basis on which to reevaluate the organization and function of the hypothalamic-neurohypophysial system.

Identification of the differentiation-associated Na+/PI transporter as a novel vesicular glutamate transporter expressed in a distinct set of glutamatergic synapses

Glutamate transport into synaptic vesicles is a prerequisite for its regulated neurosecretion. Here we functionally identify a second isoform of the vesicular glutamate transporter (VGLUT2) that was previously identified as a plasma membrane Na+-dependent inorganic phosphate transporter (differentiation-associated Na+/P(I) transporter). Studies using intracellular vesicles from transiently transfected PC12 cells indicate that uptake by VGLUT2 is highly selective for glutamate, is H+ dependent, and requires Cl- ion. Both the vesicular membrane potential (Deltapsi) and the proton gradient (DeltapH) are important driving forces for vesicular glutamate accumulation under physiological Cl- concentrations. Using an antibody specific for VGLUT2, we also find that this protein is enriched on synaptic vesicles and selective for a distinct class of glutamatergic nerve terminals. The pathway-specific, complementary expression of two different vesicular glutamate transporters suggests functional diversity in the regulation of vesicular release at excitatory synapses. Together, the two isoforms may account for the uptake of glutamate by synaptic vesicles from all central glutamatergic neurons.

Differentiation-associated Na+-dependent inorganic phosphate cotransporter (DNPI) is a vesicular glutamate transporter in endocrine glutamatergic systems

Vesicular glutamate transporter is present in neuronal synaptic vesicles and endocrine synaptic-like microvesicles and is responsible for vesicular storage of L-glutamate. A brain-specific Na(+)-dependent inorganic phosphate transporter (BNPI) functions as a vesicular glutamate transporter in synaptic vesicles, and the expression of this BNPI defines the glutamatergic phenotype in the central nervous system (Bellocchio, E. E., Reimer, R. J., Fremeau, R. T., Jr., and Edwards, R. H. (2000) Science 289, 957-960; Takamori, S., Rhee, J. S., Rosenmund, C., and Jahn, R. (2000) Nature 407, 189-194). However, since not all glutamatergic neurons contain BNPI, an additional transporter(s) responsible for vesicular glutamate uptake has been postulated. Here we report that differentiation-associated Na(+)-dependent inorganic phosphate cotransporter (DNPI), an isoform of BNPI (Aihara, Y., Mashima, H., Onda, H., Hisano, S., Kasuya, H., Hori, T., Yamada, S., Tomura, H., Yamada, Y., Inoue, I., Kojima, I., and Takeda, J. (2000) J. Neurochem. 74, 2622-2625), also transports L-glutamate at the expense of an electrochemical gradient of protons established by the vacuolar proton pump when expressed in COS7 cells. Molecular, biological, and immunohistochemical studies have indicated that besides its presence in neuronal cells DNPI is preferentially expressed in mammalian pinealocytes, alphaTC6 cells, clonal pancreatic alpha cells, and alpha cells of Langerhans islets, these cells being proven to secrete L-glutamate through Ca(2+)-dependent regulated exocytosis followed by its vesicular storage. Pancreatic polypeptide-secreting F cells of Langerhans islets also expressed DNPI. These results constitute evidence that DNPI functions as another vesicular transporter in glutamatergic endocrine cells as well as in neurons.

Molecular and functional analysis of a novel neuronal vesicular glutamate transporter

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. Packaging and storage of glutamate into glutamatergic neuronal vesicles requires ATP-dependent vesicular glutamate uptake systems, which utilize the electrochemical proton gradient as a driving force. VGLUT1, the first identified vesicular glutamate transporter, is only expressed in a subset of glutamatergic neurons. We report here the molecular cloning and functional characterization of a novel glutamate transporter, VGLUT2, from mouse brain. VGLUT2 has all major functional characteristics of a synaptic vesicle glutamate transporter, including ATP dependence, chloride stimulation, substrate specificity, and substrate affinity. It has 75 and 79% amino acid identity with human and rat VGLUT1, respectively. However, expression patterns of VGLUT2 in brain are different from that of VGLUT1. In addition, VGLUT2 activity is dependent on both membrane potential and pH gradient of the electrochemical proton gradient, whereas VGLUT1 is primarily dependent on only membrane potential. The presence of VGLUT2 in brain regions lacking VGLUT1 suggests that the two isoforms together play an important role in vesicular glutamate transport in glutamatergic neurons.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

The expression of vesicular glutamate transporters defines two classes of excitatory synapse

The quantal release of glutamate depends on its transport into synaptic vesicles. Recent work has shown that a protein previously implicated in the uptake of inorganic phosphate across the plasma membrane catalyzes glutamate uptake by synaptic vesicles. However, only a subset of glutamate neurons expresses this vesicular glutamate transporter (VGLUT1). We now report that excitatory neurons lacking VGLUT1 express a closely related protein that has also been implicated in phosphate transport. Like VGLUT1, this protein localizes to synaptic vesicles and functions as a vesicular glutamate transporter (VGLUT2). The complementary expression of VGLUT1 and 2 defines two distinct classes of excitatory synapse.

The essence of excitation

The amino acid glutamate is the major excitatory neurotransmitter in a range of organisms from Caenorhabditis elegans to mammals, and it mediates the information processing that underlies essentially all behavior. Recent advances in our understanding of glutamate storage and release now illuminate how this ubiquitous amino acid can function as a signalling molecule.

Cellular resistance to Evans blue toxicity involves an up-regulation of a phosphate transporter implicated in vesicular glutamate storage

It has recently been suggested that the brain-specific Na+-dependent phosphate inorganic co-transporter (BNPI) is able to support glutamate transport and storage in synaptic vesicles. A procedure for measuring the vesicular pool of glutamate is described and was used to select cell lines according to their glutamate storage capacity. Two cell lines were selected: C6BU-1, with a large intracellular glutamate storage capacity, and NG108-15, devoid of it. Their contents in BNPI mRNA were compared by RT-PCR. We found that both cell lines had BNPI, but in addition C6BU-1 alone expresses the other isoform, DNPI. We also carried out a clonal selection of NG108-15 cells in the presence of the dye Evans blue, a competitive inhibitor of vesicular glutamate transport, very toxic for cells in culture. It was assumed that only those that sequester and eliminate the drug by overexpressing a vesicular glutamate transporter would survive. We found that the NG108-15 clones resistant to Evans blue had an increased storage capacity for glutamate. These cells also up-regulated the BNPI isoform of the phosphate transporter as shown by RT-PCR and northern blot.

Immunocytochemical localization of candidates for vesicular glutamate transporters in the rat cerebral cortex

Brain-specific Na(+)-dependent inorganic phosphate cotransporter (BNPI) was recently reported to serve as a vesicular glutamate transporter (VGluT), and was renamed VGluT1 (Bellocchio et al. [ 2000] Science 289:957-960; Takamori et al. [2000] Nature 407:189-194). Ahead of these reports, cDNA encoding another brain-specific inorganic phosphate transporter, which showed 82% amino acid identity to VGluT1, was cloned and designated differentiation-associated Na(+)-dependent inorganic phosphate cotransporter (DNPI; Aihara et al. [2000] J Neurochem 74:2622-2625). In the present study, we produced a specific antibody against a C-terminal portion of DNPI, and studied the immunohistochemical localization of DNPI in the rat cerebral cortex in comparison with that of VGluT1. DNPI immunoreactivity was enriched in neuropil of layers I and IV and to a lesser extent in the upper portion of layer VI of the cerebral neocortex, whereas VGluT1 immunoreactivity was distributed more evenly in neuropil of the neocortex. Electron microscopic observation revealed that both DNPI and VGluT1 immunoreactivities were mainly located on synaptic vesicles in nerve terminals which made asymmetrical contacts in the neocortex. Furthermore, neither DNPI nor VGluT1 immunoreactivity in the neocortex was colocalized with gamma aminobutyric acid (GABA)ergic axon terminal markers, immunoreactivity for glutamic acid decarboxylase or vesicular GABA transporter. Neuronal depletion in the ventrobasal thalamic nuclei produced by the kainic acid injection resulted in a clear reduction of DNPI immunoreactivity in layers I, IV, and VI of the somatosensory cortex. These results indicate that DNPI is located on the membrane of synaptic vesicles in thalamocortical axon terminals, and that it may be a candidate for VGluT of thalamocortical glutamatergic neurons.

Differential localization and colocalization of two neuron-types of sodium-dependent inorganic phosphate cotransporters in rat forebrain

We studied by immunohistochemistry the distribution of differentiation-associated sodium-dependent inorganic phosphate (Pi) cotransporter (DNPI) in the rat forebrain, in comparison with brain-specific cotransporter (BNPI). DNPI-staining was principally seen in axonal synaptic terminals which showed a widespread but discrete pattern of distribution different from that of the BNPI-staining. In the diencephalon, marked DNPI-staining was seen in the dorsal lateral geniculate, medial geniculate, ventral posterolateral, ventral posteromedial, anterior, and reticular thalamic nuclei without the colocalization with BNPI-staining. DNPI-staining showed a strong mosaical pattern and overlapped well the BNPI-staining in the medial habenular nucleus. DNPI-staining was moderate over the hypothalamus and notably localized in neurosecretory terminals containing corticotropin-releasing hormone in the median eminence. In contrast, the BNPI-staining was region-related and strong in the ventromedial and mammillary nuclei. In the telencephalon, laminar DNPI-staining was seen over the neocortex, corresponding to the thalamocortical termination, and also found in the retrosplenial cortex and the striatum, with the highest intensity in the accumbens nucleus shell. The present results suggest that DNPI serves as a dominant Pi transport system in synaptic terminals of diencephalic neurons including thalamocortical and thalamostriatal pathways as well as the hypothalamic neuroendocrine system in the rat forebrain.

Ecdysteroid-inducible genes in the programmed cell death during insect metamorphosis

The anterior silk gland of the silkworm, Bombyx mori, undergoes programmed cell death (PCD) during pupal metamorphosis and PCD is triggered by 20-hydroxyecdysone (20E) in vitro. In order to identify the genes responsible for the PCD, we subtracted cDNAs prepared from the anterior silk glands incubated in the presence or absence of 20E in vitro. After a series of screenings by dot blot hybridization, DNA sequencing and reverse transcription polymerase chain reaction (RT-PCR), we obtained seven novel genes that were activated by 20E in vitro. Nucleotide sequence analysis indicated that two cDNAs (EN78 and EC08) did not have any obvious region to encode proteins, while five genes, designated EC74, EN86, EN03, EN10 and EN16, encoded proteins that are similar to inorganic phosphate cotransporter, TIA-1-like protein, chitinase-related protein, translation-initiation-factor subunit and annexin, respectively. Expression profiles of the genes after 20E stimulation indicated that four genes could be classified as early genes, while two are delayed early genes. The genes identified may provide insight into the PCD induced by a steroid hormone.

IPF, a vesicular uptake inhibitory protein factor, can reduce the Ca(2+)-dependent, evoked release of glutamate, GABA and serotonin

Synaptic vesicles in the nerve terminal play a pivotal role in neurotransmission. Neurotransmitter accumulation into synaptic vesicles is catalyzed by distinct vesicular transporters, harnessing an electrochemical proton gradient generated by V-type proton-pump ATPase. However, little is known about regulation of the transmitter pool size, particularly in regard to amino acid neurotransmitters. We previously provided evidence for the existence of a potent endogenous inhibitory protein factor (IPF), which causes reduction of glutamate and GABA accumulation into isolated, purified synaptic vesicles. In this study we demonstrate that IPF is concentrated most in the synaptosomal cytosol fraction and that, when introduced into the synaptosome, it leads to a decrease in calcium-dependent exocytotic (but not calcium-independent) release of glutamate in a concentration-dependent manner. In contrast, alpha-fodrin (non-erythroid spectrin), which is structurally related to IPF and thought to serve as the precursor for IPF, is devoid of such inhibitory activity. Intrasynaptosomal IPF also caused reduction in exocytotic release of GABA and the monoamine neurotransmitter serotonin. Whether IPF affects vesicular storage of multiple neurotransmitters in vivo would depend upon the localization of IPF. These results raise the possibility that IPF may modulate synaptic transmission by acting as a quantal size regulator of one or more neurotransmitters.

Neurogenetics of vesicular transporters in C. elegans

The nematode Caenorhabditis elegans has a number of advantages for the analysis of synaptic molecules. These include a simple nervous system in which all cells are identified and synaptic connectivity is known and reproducible, a large collection of mutants and powerful methods of genetic analysis, simple methods for the generation and analysis of transgenic animals, and a number of relatively simple quantifiable behaviors. Studies in C. elegans have made major contributions to our understanding of vesicular transmitter transporters. Two of the four classes of vesicular transporters so far identified (VAChT and VGAT) were first described and cloned in C. elegans; in both cases, the genes were first identified and cloned by means of mutations causing a suggestive phenotype (1, 2). The phenotypes of eat-4 mutants and the cell biology of the EAT-4 protein were critical in the identification of this protein as the vesicular glutamate transporter (3, 4). In addition, the unusual gene structure associated with the cholinergic locus was first described in C. elegans (5). The biochemical properties of the nematode transporters are surprisingly similar to their vertebrate counterparts, and they can be assayed under similar conditions using the same types of mammalian cells (6, 7). In addition, mild and severe mutants (including knockouts) are available for each of the four C. elegans vesicular transporters, which has permitted a careful evaluation of the role(s) of vesicular transport in transmitter-specific behaviors. Accordingly, it seems appropriate at this time to present the current status of the field. In this review, we will first discuss the properties of C. elegans vesicular transporters and transporter mutants, and then explore some of the lessons and insights C. elegans research has provided to the field of vesicular transport.

Regional expression of a gene encoding a neuron-specific Na(+)-dependent inorganic phosphate cotransporter (DNPI) in the rat forebrain

We have analyzed expression of a gene encoding a brain-specific Na(+)-dependent inorganic phosphate cotransporter (DNPI), which was recently cloned from human brain, in rat forebrain using in situ hybridization. The expression of DNPI mRNA showed a widespread but highly heterogeneous pattern of distribution in the forebrain, where hybridization signals were observed in neurons but not in any other types of cells. Neurons expressing the mRNA were far more numerous in the diencephalon than in the telencephalon. In the thalamus, a number of neurons with high levels of signals were localized to all nuclei of the dorsal thalamus, habenular nuclei and subthalamic nucleus, but not the reticular nucleus and zona incerta. Moderate signal levels were seen in many neurons throughout the hypothalamus, particularly the ventromedial, paraventricular, supraoptic and arcuate nuclei, lateral hypothalamic area and mammillary complex. In contrast, expression of DNPI mRNA in the telencephalon was generally at a low level and occurred locally in some restricted regions within the neocortex, retrosplenial cortex, piriform cortex, olfactory regions, hippocampal formation and medial amygdaloid nucleus. The present results suggest that DNPI functions in heterogeneous neuron populations as a neuron-specific Na(+)-dependent inorganic phosphate cotransport system predominantly expressed in the diencephalon of the rat.

Proximal tubular phosphate reabsorption: molecular mechanisms

Renal proximal tubular reabsorption of P(i) is a key element in overall P(i) homeostasis, and it involves a secondary active P(i) transport mechanism. Among the molecularly identified sodium-phosphate (Na/P(i)) cotransport systems a brush-border membrane type IIa Na-P(i) cotransporter is the key player in proximal tubular P(i) reabsorption. Physiological and pathophysiological alterations in renal P(i) reabsorption are related to altered brush-border membrane expression/content of the type IIa Na-P(i) cotransporter. Complex membrane retrieval/insertion mechanisms are involved in modulating transporter content in the brush-border membrane. In a tissue culture model (OK cells) expressing intrinsically the type IIa Na-P(i) cotransporter, the cellular cascades involved in "physiological/pathophysiological" control of P(i) reabsorption have been explored. As this cell model offers a "proximal tubular" environment, it is useful for characterization (in heterologous expression studies) of the cellular/molecular requirements for transport regulation. Finally, the oocyte expression system has permitted a thorough characterization of the transport characteristics and of structure/function relationships. Thus the cloning of the type IIa Na-P(i )cotransporter (in 1993) provided the tools to study renal brush-border membrane Na-P(i) cotransport function/regulation at the cellular/molecular level as well as at the organ level and led to an understanding of cellular mechanisms involved in control of proximal tubular P(i) handling and, thus, of overall P(i) homeostasis.

Identification of a vesicular glutamate transporter that defines a glutamatergic phenotype in neurons

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. Synaptic vesicles are loaded with neurotransmitter by means of specific vesicular transporters. Here we show that expression of BNPI, a vesicle-bound transporter associated with sodium-dependent phosphate transport, results in glutamate uptake by intracellular vesicles. Substrate specificity and energy dependence are very similar to glutamate uptake by synaptic vesicles. Stimulation of exocytosis--fusion of the vesicles with the cell membrane and release of their contents--resulted in quantal release of glutamate from BNPI-expressing cells. Furthermore, we expressed BNPI in neurons containing GABA (gamma-aminobutyric acid) and maintained them as cultures of single, isolated neurons that form synapses to themselves. After stimulation of these neurons, a component of the postsynaptic current is mediated by glutamate as it is blocked by a combination of the glutamate receptor antagonists, but is insensitive to a GABA(A) receptor antagonist. We conclude that BNPI functions as vesicular glutamate transporter and that expression of BNPI suffices to define a glutamatergic phenotype in neurons.

The molecular basis for Na-dependent phosphate transport in human erythrocytes and K562 cells

The kinetics of sodium-stimulated phosphate flux and phosphate-stimulated sodium flux in human red cells have been previously described (Shoemaker, D.G., C.A. Bender, and R.B. Gunn. 1988. J. Gen. Physiol. 92:449-474). However, despite the identification of multiple isoforms in three gene families (Timmer, R.T., and R.B. Gunn. 1998. Am. J. Physiol. Cell Physiol. 274:C757-C769), the molecular basis for the sodium-phosphate cotransporter in erythrocytes is unknown. Most cells express multiple isoforms, thus disallowing explication of isoform-specific kinetics and function. We have found that erythrocyte membranes express one dominant isoform, hBNP-1, to which the kinetics can thus be ascribed. In addition, because the erythrocyte Na-PO(4) cotransporter can also mediate Li-PO(4) cotransport, it has been suggested that this transporter functions as the erythrocyte Na-Li exchanger whose activity is systematically altered in patients with bipolar disease and patients with essential hypertension. To determine the molecular basis for the sodium-phosphate cotransporter, we reasoned that if the kinetics of phosphate transport in a nucleated erythroid-like cell paralleled those of the Na-activated pathway in anucleated erythrocytes and yet were distinct from those known for other Na-PO(4) cotransporters, then the expressed genes may be the same in both cell types. In this study, we show that the kinetics of sodium phosphate cotransport were similar in anuclear human erythrocytes and K562 cells, a human erythroleukemic cell line. Although the erythrocyte fluxes were 750-fold smaller, the half-activation concentrations for phosphate and sodium and the relative cation specificities for activation of (32)PO(4) influx were similar. Na-activation curves for both cell types showed cooperativity consistent with the reported stoichiometry of more than one Na cotransported per PO(4). In K562 cells, external lithium activation of phosphate influx was also cooperative. Inhibition by arsenate, K(I) = 2.6-2.7 mM, and relative inhibition by amiloride, amiloride analogs, phosphonoformate, and phloretin were similar. These characteristics were different from those reported for hNaPi-3 and hPiT-1 in other systems. PCR analysis of sodium-phosphate cotransporter isoforms in K562 cells demonstrated the presence of mRNAs for hPiT-1, hPiT-2, and hBNP-1. The mRNAs for hNaPi-10 and hNaPi-3, the other two known isoforms, were absent. Western analysis of erythrocytes and K562 cells with isoform-specific antibodies detected the presence of only hBNP-1, an isoform expressed in brain neurons and glia. The similarities in the kinetics and the expression of only hBNP-1 protein in the two cell types is strong evidence that hBNP-1 is the erythrocyte and K562 cell sodium-phosphate cotransporter.

Uptake of glutamate into synaptic vesicles by an inorganic phosphate transporter

Previous work has identified two families of proteins that transport classical neurotransmitters into synaptic vesicles, but the protein responsible for vesicular transport of the principal excitatory transmitter glutamate has remained unknown. We demonstrate that a protein that is unrelated to any known neurotransmitter transporters and that was previously suggested to mediate the Na(+)-dependent uptake of inorganic phosphate across the plasma membrane transports glutamate into synaptic vesicles. In addition, we show that this vesicular glutamate transporter, VGLUT1, exhibits a conductance for chloride that is blocked by glutamate.

Molecular cloning of a novel brain-type Na(+)-dependent inorganic phosphate cotransporter

We have isolated a human cDNA encoding a protein, designated DNPI, that shows 82% amino acid identity and 92% similarity to the human brain-specific Na(+)-dependent inorganic phosphate (Na(+)/P(i)) cotransporter (BNPI), which is localized exclusively to neuron-rich regions. Expression of DNPI mRNA in Xenopus oocytes resulted in a significant increase in Na(+)-dependent P(i) transport, indicating that DNPI is a novel Na(+)/P(i) cotransporter. Northern blot analysis shows that DNPI mRNA is expressed predominantly in brain, where the highest levels are observed in medulla, substantia nigra, subthalamic nucleus, and thalamus, all of which express BNPI mRNA at low levels. In contrast, DNPI mRNA is expressed at low levels in cerebellum and hippocampus, where BNPI mRNA is expressed at high levels. No hybridizing signal for DNPI mRNA is observed in the glia-rich region of corpus callosum. In other regions examined, both mRNAs are moderately or highly expressed. These results indicate that BNPI and DNPI, which coordinate Na(+)-dependent P(i) transport in the neuron-rich regions of the brain, may form a new class within the Na(+)/P(i) cotransporter family.

Regulation of intestinal Na+-dependent phosphate co-transporters by a low-phosphate diet and 1,25-dihydroxyvitamin D3

In a study of the rat intestinal P(i) transport system, an activator protein for rat Na/P(i) co-transport system (PiUS) was isolated and characterized. We also investigated the effects of restriction of vitamin D and P(i) (two of the most important physiological and pathophysiological regulators of P(i) absorption in the small intestine) on intestinal P(i) transport activity and the expression of Na/P(i) co-transporters that are expressed in rat small intestine. Rat PiUS encodes a 424-residue protein with a calculated molecular mass of 51463 Da. The microinjection of rat PiUS into Xenopus oocytes markedly stimulated Na(+)-dependent P(i) co-transport activity. In rats fed with a low-P(i) diet, Na(+)-dependent P(i) co-transport activity was increased approx. 2-fold compared with that of rats fed a normal P(i) diet. Kinetic studies demonstrated that this increased activity was due to an elevation of V(max) but not K(m). The PiUS mRNA levels showed an approximate doubling in the rats fed with the low-P(i) diet compared with those fed with the normal P(i) diet. In addition, after the administration of 1, 25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] to vitamin D-deficient animals, the P(i) uptake was significantly increased in the Na(+)-dependent component in the brush border membrane vesicle (BBMV) at 24 and 48 h. In addition, we found a further high-affinity Na/P(i) co-transport system in the BBMV isolated from the vitamin D-replete animals. The levels of type III Na/P(i) co-transporter PiT-2 mRNA were increased 24 and 48 h after 1,25-(OH)(2)D(3) administration to vitamin D-deficient animals, whereas PiUS and the type IIb Na/P(i) co-transporter mRNA levels were unchanged. In conclusion, we first cloned a rat activator protein, PiUS, and then studied its role along with that of other type III Na/P(i) co-transporters. PiUS and PiT-2 might be important components in the regulation of the intestinal P(i) transport system by P(i) restriction and 1,25-(OH)(2)D(3).

Identification of a mammalian brain sulfate transporter

Sulfate is an essential anion involved in many biosynthetic and pharmacological reactions. Sulfate is an important constituent of myelin membranes in the brain; however, very little is known as to how sulfate enters brain cells. In this study, our aim was to determine whether the mammalian brain possesses a sulfate transporter. Injection of rat brain poly A(+) RNA into Xenopus oocytes led to an induction of Na(+)-independent sulfate transport, which was inhibited by oxalate, probenecid, phenol red, thiosulfate and DIDS. Hybrid depletion using sat-1 antisense oligodeoxyribonucleotides led to a complete inhibition of brain mRNA-induced sulfate transport in Xenopus oocytes, suggesting the presence of a functional sat-1 transcript in the brain. By RT-PCR, sat-1 mRNA was detected throughout the rat brain and in situ hybridisation showed highest sat-1 expression in the hippocampus and cerebellum. This is the first study to identify and characterise a functional mammalian brain sulfate transporter.

EAT-4, a homolog of a mammalian sodium-dependent inorganic phosphate cotransporter, is necessary for glutamatergic neurotransmission in caenorhabditis elegans

The Caenorhabditis elegans gene eat-4 affects multiple glutamatergic neurotransmission pathways. We find that eat-4 encodes a protein similar in sequence to a mammalian brain-specific sodium-dependent inorganic phosphate cotransporter I (BNPI). Like BNPI in the rat CNS, eat-4 is expressed predominantly in a specific subset of neurons, including several proposed to be glutamatergic. Loss-of-function mutations in eat-4 cause defective glutamatergic chemical transmission but appear to have little effect on other functions of neurons. Our data suggest that phosphate ions imported into glutamatergic neurons through transporters such as EAT-4 and BNPI are required specifically for glutamatergic neurotransmission.

EAT-4, a homolog of a mammalian sodium-dependent inorganic phosphate cotransporter, is necessary for glutamatergic neurotransmission in caenorhabditis elegans

The Caenorhabditis elegans gene eat-4 affects multiple glutamatergic neurotransmission pathways. We find that eat-4 encodes a protein similar in sequence to a mammalian brain-specific sodium-dependent inorganic phosphate cotransporter I (BNPI). Like BNPI in the rat CNS, eat-4 is expressed predominantly in a specific subset of neurons, including several proposed to be glutamatergic. Loss-of-function mutations in eat-4 cause defective glutamatergic chemical transmission but appear to have little effect on other functions of neurons. Our data suggest that phosphate ions imported into glutamatergic neurons through transporters such as EAT-4 and BNPI are required specifically for glutamatergic neurotransmission.

The localization of the brain-specific inorganic phosphate transporter suggests a specific presynaptic role in glutamatergic transmission

Molecular cloning has recently identified a vertebrate brain-specific Na+-dependent inorganic phosphate transporter (BNPI). BNPI has strong sequence similarity to EAT-4, a Caenorhabditis elegans protein implicated in glutamatergic transmission. To characterize the physiological role of BNPI, we have generated an antibody to the protein. Immunocytochemistry of rat brain sections shows a light microscopic pattern that is suggestive of reactivity in nerve terminals. Excitatory projections are labeled prominently, and ultrastructural analysis confirms that BNPI localizes almost exclusively to terminals forming asymmetric excitatory-type synapses. Although BNPI depends on a Na+ gradient and presumably functions at the plasma membrane, both electron microscopy and biochemical fractionation show that BNPI associates preferentially with the membranes of small synaptic vesicles. The results provide anatomic evidence of a specific presynaptic role for BNPI in glutamatergic neurotransmission, consistent with the phenotype of eat-4 mutants. Because an enzyme known as the phosphate-activated glutaminase produces glutamate for release as a neurotransmitter, BNPI may augment excitatory transmission by increasing cytoplasmic phosphate concentrations within the nerve terminal and hence increasing glutamate synthesis. Expression of BNPI on synaptic vesicles suggests a mechanism for neural activity to regulate the function of BNPI.

The localization of the brain-specific inorganic phosphate transporter suggests a specific presynaptic role in glutamatergic transmission

Molecular cloning has recently identified a vertebrate brain-specific Na+-dependent inorganic phosphate transporter (BNPI). BNPI has strong sequence similarity to EAT-4, a Caenorhabditis elegans protein implicated in glutamatergic transmission. To characterize the physiological role of BNPI, we have generated an antibody to the protein. Immunocytochemistry of rat brain sections shows a light microscopic pattern that is suggestive of reactivity in nerve terminals. Excitatory projections are labeled prominently, and ultrastructural analysis confirms that BNPI localizes almost exclusively to terminals forming asymmetric excitatory-type synapses. Although BNPI depends on a Na+ gradient and presumably functions at the plasma membrane, both electron microscopy and biochemical fractionation show that BNPI associates preferentially with the membranes of small synaptic vesicles. The results provide anatomic evidence of a specific presynaptic role for BNPI in glutamatergic neurotransmission, consistent with the phenotype of eat-4 mutants. Because an enzyme known as the phosphate-activated glutaminase produces glutamate for release as a neurotransmitter, BNPI may augment excitatory transmission by increasing cytoplasmic phosphate concentrations within the nerve terminal and hence increasing glutamate synthesis. Expression of BNPI on synaptic vesicles suggests a mechanism for neural activity to regulate the function of BNPI.

Characterization of the 5' flanking region of the human NPT-1 Na+/phosphate cotransporter gene

To elucidate the expression and regulation of the human type I Na+/phosphate transporter gene (NPT-1), the 5' flanking region of the NPT-1 gene was cloned, and its nucleotide sequence and function were determined. A genomic clone that contained approximately 14.0 kb of the 5'-flanking region of the NPT-1 gene was isolated. A single transcription start site was located 104 base pairs (bp) upstream of the 3' end of exon 1. In addition to the sequence of the 5'-flanking region contained a sequence weakly homologous to a TATA box at position -41 to -36 and many transcriptional regulatory elements. Transient expression revealed that a 45-bp region of proximal to exon 1, which contained TATA-like sequence, was sufficient for promoting luciferase expression in OK-cells derived from opossum kidney proximal tubule.

Cloning of a functional vesicular GABA and glycine transporter by screening of genome databases

The unc-47 locus of Caenorhabditis elegans has been suggested to encode a synaptic vesicle GABA transporter. Here we used hydropathy plot analysis to identify a candidate vesicular GABA transporter in genomic sequences derived from a region of the physical map comprising unc-47. A mouse homologue was identified and cloned from EST database information. In situ hybridization in rat brain revealed codistribution with both GABAergic and glycinergic neuronal markers. Moreover, expression in COS-7 and PC12 cells induced an intracellular, glycine-sensitive GABA uptake activity. These observations, consistent with previous data on GABA and glycine uptake by synaptic vesicles, demonstrate that the mouse clone encodes a vesicular inhibitory amino acid transporter.

Immunohistochemical and RT-PCR detection of Na+-dependent inorganic phosphate cotransporter (NaPi-2) in rat brain

Expression of a renal Na+-dependent inorganic phosphate (Pi) cotransporter (NaPi-2) was studied in rat forebrain with reverse transcription and polymerase chain reaction (RT-PCR) and immunohistochemistry. RT-PCR analysis for total RNA from whole brain and sequencing of the PCR products showed expression of NaPi-2 mRNA in the brain. Immunohistochemical analysis revealed NaPi-2 staining in many nonpyramidal neurons of all six layers throughout neocortical areas and in neurons of proisocortical and periallocortical areas. NaPi-2-immunoreactive neurons were also detectable in the piriform cortex, hippocampal formation, caudate-putamen, amygdaloid nuclei and lateral geniculate nucleus. Furthermore, NaPi-2 staining was shown in ependymal cells and microvascular endothelial cells. The present results suggest that NaPi-2 is synthesized within the brain and involved in maintaining Pi homeostasis of certain neurons and/or the entire brain.

Selective loss of neuronal Na+-dependent phosphate cotransporter mRNA in CA1 pyramidal neuron following global ischemia

A recently identified neuronal Na+-dependent phosphate cotransporter (rBNPI) has been shown to import inorganic phosphate (P(i)) required for the production of high-energy phosphates which are vital to neuronal energy metabolism. In the present study, we have examined the expression of rBNPI mRNA in the hippocampus of rats subjected to 30 min of global ischemia by four-vessel occlusion. In situ hybridization reveals that transient forebrain ischemia results in a selective reduction in rBNPI mRNA expression in CA1 pyramidal neurons of the hippocampus. Expression of rBNPI is significantly reduced by 24 h and completely absent at 72 h following global ischemia when CA1 pyramidal neurons begin to show cell damage. By contrast, there is no change in the expression of Nedd2 mRNA, a developmentally regulated cell death gene, in CA1 pyramidal neurons at these same time points. The loss of rBNPI transcripts appears to be selective for CA1 pyramidal neurons since rBNPI mRNA expression is unchanged in neurons of the CA2-CA4 pyramidal cell layers following global ischemia. Our data indicate that an early reduction of rBNPI transcripts may contribute to a reduction in P(i)-dependent energy metabolism or signal transduction which has been reported in CA1 hippocampal neurons following global ischemia.

Inorganic phosphate enhances phosphonucleotide concentrations in cultured fetal rat cortical neurons

Our laboratory has recently characterized saturable Na(+)-dependent P(i) import into cultured fetal rat cortical neurons and shown that a substantial fraction of the P(i) so accumulated is incorporated into ATP. We now report that the ATP, NADPH and intracellular free P(i) ([P(i)]i) concentrations of cultured fetal rat cortical neurons are dependent on the extracellular P(i) concentration ([P(i)]e). [ATP], [NADPH] and [P(i)]i display a hyperbolic dependence upon [P(i)]e, being significantly increased after incubation with [P(i)]e of > or = 10 microM, and maximal at > or = 500 microM. Increases in both [ATP] and [NADPH] are abolished in the absence of glucose. In the absence of extracellular P(i), both [ATP] and [P(i)]i decline over time. Our data suggest that in cultured fetal rat cortical neurons [P(i)]e has a direct effect on glucose utilization, stimulating both ATP and NADPH synthesis via glycolysis and the pentose phosphate pathway.

Sodium-dependent transport of phosphate in neuronal and related cells

Sodium-dependent phosphate entry into neuronal cells was demonstrated in synaptic plasma membrane vesicles and synaptosomes prepared from rat brains, in PC12 cells and in primary culture of pituitary cells. The extent of the sodium-dependent phosphate transport in the synaptic plasma membrane preparation, at [Na]out = 110 mM and [P(i)]out = 0.1 mM, varied between 0.28 to 1.02 nmol phosphate/mg membrane protein/min. In pituitary cells the value was only about 0.05 nmol P(i)/mg protein/min. In PC12 cells the activity increased from 0.0085 to 0.26 nmol P(i)/mg protein/min in the transit from undifferentiated to differentiated cells. The dependence of phosphate on sodium concentrations fits a model in which two sodium ions are required to transfer the phosphate into the cells with a K[Na]0.5 of 43 mM. The K(m) for the phosphate transport in the synaptic plasma membrane preparations was between 0.1 and 0.45 mM. It is concluded that sodium-driven active transport of phosphate is a ubiquitous activity in various types of neuronal cells.

Cloning and expression of a rat brain interleukin-1beta-converting enzyme (ICE)-related protease (IRP) and its possible role in apoptosis of cultured cerebellar granule neurons

Several members of the IL-1beta-converting enzyme (ICE) family of proteases recently have been implicated in the intracellular cascade mediating the apoptotic death of various cell types. It is unclear, however, whether ICE-related proteases are involved in apoptosis of mammalian neurons and, if so, how they are activated. Here we report the cloning of an ICE-related protease (IRP) from rat brain, which displays strong sequence identity to human CPP32. In situ hybridization histochemistry reveals that this IRP mRNA is expressed in neuron-enriched regions of the developing and adult rat brain but is profoundly downregulated in the adult (compared with developing) brain. To investigate whether this IRP is involved in the death of neurons in the developing brain, we studied IRP expression in cultured cerebellar granule neurons. In cultured cerebellar granule neurons, reduction of extracellular K+ reliably induces apoptosis and stimulates overexpression of IRP mRNA. The latter is especially prominent 4 hr after switching from high K+ to low K+ medium. The expression of IRP mRNA was maintained at this level for at least 8 hr and was followed by apoptotic death of these neurons. Induction of IRP mRNA and cell death are blocked completely by adding depolarizing concentrations of K+ </=90 min after switching to low K+ medium (i.e., before the commitment point for apoptosis) and partially blocked by brain-derived neurotrophic factor (BDNF), which also partially rescues granule neurons from low K+-induced apoptosis. In addition, overexpression of IRP cDNA in HeLa cells results in cell death accompanied by strong internucleosomal cleavage of DNA, a typical feature of apoptosis. Finally, we detected cleavage of the putative death substrate poly (ADP-ribose) polymerase (PARP), beginning 8 hr after changing from high K+ to low K+ medium, coinciding with the time course of induced expression of the IRP gene. Our data suggest that transcriptional activation of IRP could be one of the mechanisms involved in the apoptotic death of cerebellar granule neurons.

Cloning and expression of a rat brain interleukin-1beta-converting enzyme (ICE)-related protease (IRP) and its possible role in apoptosis of cultured cerebellar granule neurons

Several members of the IL-1beta-converting enzyme (ICE) family of proteases recently have been implicated in the intracellular cascade mediating the apoptotic death of various cell types. It is unclear, however, whether ICE-related proteases are involved in apoptosis of mammalian neurons and, if so, how they are activated. Here we report the cloning of an ICE-related protease (IRP) from rat brain, which displays strong sequence identity to human CPP32. In situ hybridization histochemistry reveals that this IRP mRNA is expressed in neuron-enriched regions of the developing and adult rat brain but is profoundly downregulated in the adult (compared with developing) brain. To investigate whether this IRP is involved in the death of neurons in the developing brain, we studied IRP expression in cultured cerebellar granule neurons. In cultured cerebellar granule neurons, reduction of extracellular K+ reliably induces apoptosis and stimulates overexpression of IRP mRNA. The latter is especially prominent 4 hr after switching from high K+ to low K+ medium. The expression of IRP mRNA was maintained at this level for at least 8 hr and was followed by apoptotic death of these neurons. Induction of IRP mRNA and cell death are blocked completely by adding depolarizing concentrations of K+ </=90 min after switching to low K+ medium (i.e., before the commitment point for apoptosis) and partially blocked by brain-derived neurotrophic factor (BDNF), which also partially rescues granule neurons from low K+-induced apoptosis. In addition, overexpression of IRP cDNA in HeLa cells results in cell death accompanied by strong internucleosomal cleavage of DNA, a typical feature of apoptosis. Finally, we detected cleavage of the putative death substrate poly (ADP-ribose) polymerase (PARP), beginning 8 hr after changing from high K+ to low K+ medium, coinciding with the time course of induced expression of the IRP gene. Our data suggest that transcriptional activation of IRP could be one of the mechanisms involved in the apoptotic death of cerebellar granule neurons.

Two cDNAs from potato are able to complement a phosphate uptake-deficient yeast mutant: identification of phosphate transporters from higher plants

Acquisition as well as translocation of phosphate are essential processes for plant growth. In many plants, phosphate uptake by roots and distribution within the plant are presumed to occur via a phosphate/proton cotransport mechanism. Here, we describe the isolation of two cDNAs, StPT1 and StPT2, from potato (Solanum tuberosum) that show homology to the phosphate/proton cotransporter PHO84 from the yeast Saccharomyces cerevisiae. The predicted products of both cDNAs share 35% identity with the PHO84 sequence. The deduced structure of the encoded proteins revealed 12 membrane-spanning domains with a central hydrophilic region. The molecular mass was calculated to be 59 kD for the StPT1 protein and 58 kD for the StPT2 protein. When expressed in a PHO84-deficient yeast strain, MB192, both cDNAs complemented the mutant. Uptake of radioactive orthophosphate by the yeast mutant expressing either StPT1 or StPT2 was dependent on pH and reduced in the presence of uncouplers of oxidative phosphorylation, such as 2,4-dinitrophenol or carbonyl cyanide m-chlorophenylhydrazone. The K(m) for Pi uptake of the StPT1 and StPT2 proteins was determined to be 280 and 130 microM, respectively. StPT1 is expressed in roots, tubers, and source leaves as well as in floral organs. Deprivation of nitrogen, phosphorus, potassium, and sulfur changed spatial expression as well as the expression level of StPT1. StPT2 expression was detected mainly in root organs when plants were deprived of Pi and to a lesser extent under sulfur deprivation conditions. No expression was found under optimized nutrition conditions or when other macronutrients were lacking.

Two cDNAs from potato are able to complement a phosphate uptake-deficient yeast mutant: identification of phosphate transporters from higher plants

Acquisition as well as translocation of phosphate are essential processes for plant growth. In many plants, phosphate uptake by roots and distribution within the plant are presumed to occur via a phosphate/proton cotransport mechanism. Here, we describe the isolation of two cDNAs, StPT1 and StPT2, from potato (Solanum tuberosum) that show homology to the phosphate/proton cotransporter PHO84 from the yeast Saccharomyces cerevisiae. The predicted products of both cDNAs share 35% identity with the PHO84 sequence. The deduced structure of the encoded proteins revealed 12 membrane-spanning domains with a central hydrophilic region. The molecular mass was calculated to be 59 kD for the StPT1 protein and 58 kD for the StPT2 protein. When expressed in a PHO84-deficient yeast strain, MB192, both cDNAs complemented the mutant. Uptake of radioactive orthophosphate by the yeast mutant expressing either StPT1 or StPT2 was dependent on pH and reduced in the presence of uncouplers of oxidative phosphorylation, such as 2,4-dinitrophenol or carbonyl cyanide m-chlorophenylhydrazone. The K(m) for Pi uptake of the StPT1 and StPT2 proteins was determined to be 280 and 130 microM, respectively. StPT1 is expressed in roots, tubers, and source leaves as well as in floral organs. Deprivation of nitrogen, phosphorus, potassium, and sulfur changed spatial expression as well as the expression level of StPT1. StPT2 expression was detected mainly in root organs when plants were deprived of Pi and to a lesser extent under sulfur deprivation conditions. No expression was found under optimized nutrition conditions or when other macronutrients were lacking.

Regulation of rat Na+/Pi cotransporter-1 gene expression: the roles of glucose and insulin

Cytosolic inorganic phosphate (P(i)) is important for glucose metabolism. It plays a role in homeostatic regulation of glucose by insulin and glucagon. Recently, we isolated two cDNA clones for rat Na+/P(i) cotransporter-1 (rNaPi-1) and demonstrated that they are expressed primarily in the rat liver and kidney. We now report that the expression of rNaPi-1 in these tissues is regulated by fasting and streptozotocin-induced diabetes. Using rat hepatocytes in primary culture, we also demonstrate that glucose and insulin upregulate rNaPi-1 expression, whereas glucagon and elevated intracellular adenosine 3',5'-cyclic monophosphate levels downregulate its expression. Because 2-deoxyglucose exhibits no effect on rNaPi-1 gene expression, we suggest that some metabolite accumulated during glucose metabolism may be responsible for the effects of glucose and insulin on rNaPi-1 gene expression. Our data also reveal that other known Na+/P(i) cotransporter genes, NaPi-2 and Ram-1 (a receptor for amphotropic murine retrovirus), are not regulated by insulin and glucose. It is therefore proposed that various subtypes of Na+/P(i) cotransporters are differentially regulated and that each subtype may be involved in a specific cellular function, rNaPi-1 may be responsible for Pi uptake by liver and kidney for glucose metabolism, whereas NaPi-2 may play a key role in P(i) reabsorption in the kidney.

Renal brush border membrane Na/Pi-cotransport: molecular aspects in PTH-dependent and dietary regulation

Inorganic phosphate (Pi) is reabsorbed in renal proximal tubules in a sodium (Na)-dependent manner involving brush border Na/Pi-cotransporter(s). Regulation of renal Pi-reabsorption, such as by parathyroid hormone (PTH) and/or by dietary Pi-deprivation, involves alterations in the rate of Na/Pi-cotransport. Two structurally different Na/Pi-cotransporters have been identified: type I-transporter and type II-transporter. The related mRNAs and proteins are located in the proximal tubule and in the brush border membrane. In heterologous expression systems type I and type II Na/Pi-cotransporters mediate Na/Pi-cotransport. Characterization of the transport properties suggested that the type II transporter is "responsible' for brush border membrane Na/Pi-cotransport (as observed in isolated vesicles). Administration of PTH to rats resulted in an inhibition of brush border membrane Na/Pi-cotransport (vesicles) and in a reduced brush border membrane content of the type II transporter. Feeding low Pi-diets resulted in an up-regulation of Na/Pi-cotransport (vesicles) and of type II transporter content; only after a prolonged exposure to low Pi-diets (more than 4 hr) was an increase in specific mRNA content observed. Refeeding high Pi diets had the opposite effects on Na/Pi-cotransport activity and on type II transporter protein. It is currently the task of future experiments to define the specific mechanisms leading to protein-synthesis-independent (PTH, acute Pi-deprivation, Pi-refeeding) and to protein-synthesis-dependent (prolonged Pi-deprivation) regulation of the type II Na/Pi-cotransporter.

Expression of a renal type I sodium/phosphate transporter (NaPi-1) induces a conductance in Xenopus oocytes permeable for organic and inorganic anions

Two distinct molecular types (I and II) of renal proximal tubular brush border Na+/Pi cotransporters have been identified by expression cloning on the basis of their capacity to induce Na+-dependent Pi influx in tracer experiments. Whereas the type II transporters (e.g., NaPi-2 and NaPi-3) resemble well known characteristics of brush border Na+/Pi cotransport, little is known about the properties of the type I transporter (NaPi-1). In contrast to type II, type I transporters produced electrogenic transport only at high extracellular Pi concentrations (> or =3 mM). On the other hand, expression of NaPi-1 induced a Cl- conductance in Xenopus laevis oocytes, which was inhibited by Cl- channel blockers [5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) > niflumic acid >> 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid]. Further, the Cl- conductance was inhibited by the organic anions phenol red, benzylpenicillin (penicillin G), and probenecid. These organic anions induced outwardly directed currents in the absence of Cl-. In tracer studies, we observed uptake of benzylpenicillin with a Km of 0.22 mM; benzylpenicillin uptake was inhibited by NPPB and niflumic acid. These findings suggest that the type I Na+/Pi cotransporter functions also as a novel type of anion channel permeable not only for Cl- but also for organic anions. Such an apical anion channel could serve an important role in the transport of Cl- and the excretion of anionic xenobiotics.

Renal Na/Pi-cotransporters

Two non-homologous proximal tubular apical Na/Pi-cotransport systems (type I and type II) have been identified thus far by expression cloning. Subsequent studies provided evidence that the type II Na/Pi-cotransporter represents a target for the physiological and pathophysiological regulation of proximal reabsorption of phosphate. The exact role of the type I Na/Pi-cotransporter in proximal Pi-reabsorption and eventually also in the renal handling of other substrates, such as organic anions, is currently less clear and needs further investigation. Evidence was obtained that acute changes of brush border membrane Na/Pi-cotransport involves endo- and exocytic movement of type II Na/Pi-cotransporters. In particular, we elucidated if and how phosphorylation reactions are involved and defined the intracellular structures of the endo/exocytic apparatus involved. At the level of the gene it will be necessary to elucidate its organization in order to understand the mechanisms involved in chronic regulations of Na/Pi-cotransport related to the type II Na/Pi-cotransporter. Furthermore, for structural investigations of these integral membrane proteins, they have to be isolated in sufficient quantities. Thus far the type II cotransporter (NaPi-2) has been expressed in Sf9 insect cells [20], which may eventually allow a purification of this protein.