Evolution of the Na-P(i) cotransport systems

Effects of a transient lack of dietary mineral phosphorus on renal gene expression and plasma metabolites in two high-yielding laying hen strains

BACKGROUND

There is an emerging body of evidence that current poultry feed is formulated in excess for phosphorus (P), which results in unnecessarily high P excretions. Sustainable concepts for agricultural P flows should trigger animal-intrinsic mechanisms for efficient P utilization. In the current study, Lohmann Brown (LB) and Lohmann Selected Leghorn (LSL) laying hens were fed either a high P diet (P+) with 1 g/kg mineral P supplement or a low P diet (P-) with 0 g/kg mineral P supplement for a period of 4 weeks prior to sampling. Before and after onset of laying, i.e., at 19 and 24 weeks of life, kidney and plasma samples were collected to investigate the endogenous P utilization in response to restricted dietary P, laying hen strain, and sexual maturation.

RESULTS

Plasma analyses of minerals and metabolites confirmed the response to a low P diet, which was characterized by a significant reduction in plasma P levels at week 19 in both strains. The plasma calcium (Ca) levels were tightly regulated throughout the entire experimental period. Notably, there was a numerical trend of increased plasma calcitriol levels in P- fed birds of both strains compared to the P + group, which might have mediated a substantial role regarding the adaptive responses to low P supply. At week 19, RNA sequencing of kidney identified 1,114 and 556 differentially expressed genes (DEGs) unique to the LB and LSL strains, respectively. The number of DEGs declined with increasing maturity of the hens culminating in 90 and 146 DEGs for LB and LSL strains at week 24. Analyses revealed an enrichment of pathways related to energy metabolism and cell cycle, particularly at week 19 in both strains. The diet-specific expression of target genes involved in P homeostasis highlighted transcripts related to active (SLC34A1, SLC20A2) and passive mineral transport (CLDN14, CLDN16), Ca utilization (STC1, CALB1), and acid-base balance (CA2, SLC4A1).

CONCLUSIONS

Results suggest that both laying hen strains adapted to the lack of mineral P supplements and achieved a physiological Ca: P-ratio in body compartments through endogenous regulation as evidenced via the endocrine profile.

Regulation of eukaryotic transcription initiation in response to cellular stress

Transcription initiation is a vital step in the regulation of eukaryotic gene expression. It can be dysregulated in response to various cellular stressors which is associated with numerous human diseases including cancer. Transcription initiation is facilitated via many gene-specific trans-regulatory elements such as transcription factors, activators, and coactivators through their interactions with transcription pre-initiation complex (PIC). These trans-regulatory elements can uniquely facilitate PIC formation (hence, transcription initiation) in response to cellular nutrient stress. Cellular nutrient stress also regulates the activity of other pathways such as target of rapamycin (TOR) pathway. TOR pathway exhibits distinct regulatory mechanisms of transcriptional activation in response to stress. Like TOR pathway, the cell cycle regulatory pathway is also found to be linked to transcriptional regulation in response to cellular stress. Several transcription factors such as p53, C/EBP Homologous Protein (CHOP), activating transcription factor 6 (ATF6α), E2F, transforming growth factor (TGF)-β, Adenomatous polyposis coli (APC), SMAD, and MYC have been implicated in regulation of transcription of target genes involved in cell cycle progression, apoptosis, and DNA damage repair pathways. Additionally, cellular metabolic and oxidative stressors have been found to regulate the activity of long non-coding RNAs (lncRNA). LncRNA regulates transcription by upregulating or downregulating the transcription regulatory proteins involved in metabolic and cell signaling pathways. Numerous human diseases, triggered by chronic cellular stressors, are associated with abnormal regulation of transcription. Hence, understanding these mechanisms would help unravel the molecular regulatory insights with potential therapeutic interventions. Therefore, here we emphasize the recent advances of regulation of eukaryotic transcription initiation in response to cellular stress.

An intracellular phosphorus-starvation signal activates the PhoB/PhoR two-component system in Salmonella enterica

Bacteria acquire P primarily as inorganic orthophosphate (Pi, PO43-). Once internalized, Pi is rapidly assimilated into biomass during the synthesis of ATP. Because Pi is essential, but excessive ATP is toxic, the acquisition of environmental Pi is tightly regulated. In the bacterium Salmonella enterica (Salmonella), growth in Pi-limiting environments activates the membrane sensor histidine kinase PhoR, leading to the phosphorylation of its cognate transcriptional regulator PhoB and subsequent transcription of genes involved in adaptations to low Pi. Pi limitation promotes PhoR kinase activity by altering the conformation of a membrane signaling complex comprised of PhoR, the multicomponent Pi transporter system PstSACB and the regulatory protein PhoU. However, the identity of the Pi-starvation signal and how it controls PhoR activity remain unknown. Here, we identify conditions where the PhoB and PhoR signal transduction proteins can be maintained in an inactive state when Salmonella is grown in media lacking Pi. Our results demonstrate that PhoB/PhoR is activated by an intracellular P-insufficiency signal.IMPORTANCEIn enteric bacteria, the transcriptional response to phosphorus (P) starvation is controlled by a specialized signal transduction system comprised of a membrane-bound, multicomponent signal sensor, and a cytoplasmic transcriptional factor. Whereas this system has been primarily studied in the context of phosphate (Pi) starvation, it is currently unknown how this stress initiates signal transduction. In the current study, we establish that this signaling system is regulated by a cytoplasmic signal arising from insufficient P. We demonstrate that rather than responding to extracellular conditions, cells couple the activation of their P starvation response to the availability of cytoplasmic P. This regulatory logic may enable cells to prevent toxicity resulting from excessive Pi acquisition and hinder the onset of a P starvation response when their metabolic demands are being met through the consumption of P sources other than Pi.

Interaction of calcium responsive proteins and transcriptional factors with the PHO regulon in yeasts and fungi

Phosphate and calcium ions are nutrients that play key roles in growth, differentiation and the production of bioactive secondary metabolites in filamentous fungi. Phosphate concentration regulates the biosynthesis of hundreds of fungal metabolites. The central mechanisms of phosphate transport and regulation, mediated by the master Pho4 transcriptional factor are known, but many aspects of the control of gene expression need further research. High ATP concentration in the cells leads to inositol pyrophosphate molecules formation, such as IP3 and IP7, that act as phosphorylation status reporters. Calcium ions are intracellular messengers in eukaryotic organisms and calcium homeostasis follows elaborated patterns in response to different nutritional and environmental factors, including cross-talking with phosphate concentrations. A large part of the intracellular calcium is stored in vacuoles and other organelles forming complexes with polyphosphate. The free cytosolic calcium concentration is maintained by transport from the external medium or by release from the store organelles through calcium permeable transient receptor potential (TRP) ion channels. Calcium ions, particularly the free cytosolic calcium levels, control the biosynthesis of fungal metabolites by two mechanisms, 1) direct interaction of calcium-bound calmodulin with antibiotic synthesizing enzymes, and 2) by the calmodulin-calcineurin signaling cascade. Control of very different secondary metabolites, including pathogenicity determinants, are mediated by calcium through the Crz1 factor. Several interactions between calcium homeostasis and phosphate have been demonstrated in the last decade: 1) The inositol pyrophosphate IP3 triggers the release of calcium ions from internal stores into the cytosol, 2) Expression of the high affinity phosphate transporter Pho89, a Na+/phosphate symporter, is controlled by Crz1. Also, mutants defective in the calcium permeable TRPCa7-like of Saccharomyces cerevisiae shown impaired expression of Pho89. This information suggests that CrzA and Pho89 play key roles in the interaction of phosphate and calcium regulatory pathways, 3) Finally, acidocalcisomes organelles have been found in mycorrhiza and in some melanin producing fungi that show similar characteristics as protozoa calcisomes. In these organelles there is a close interaction between orthophosphate, pyrophosphate and polyphosphate and calcium ions that are absorbed in the polyanionic polyphosphate matrix. These advances open new perspectives for the control of fungal metabolism.

Inherited Fanconi syndrome

BACKGROUND

Fanconi-Debré-de Toni syndrome (also known as Fanconi renotubular syndrome, or FRST) profoundly increased the understanding of the functions of the proximal convoluted tubule (PCT) and provided important insights into the pathophysiology of several kidney diseases and drug toxicities.

DATA SOURCES

We searched Pubmed and Scopus databases to find relevant articles about FRST. This review article focuses on the physiology of the PCT, as well as on the physiopathology of FRST in children, its diagnosis, and treatment.

RESULTS

FRST encompasses a wide variety of inherited and acquired PCT alterations that lead to impairment of PCT reabsorption. In children, FRST often presents as a secondary feature of systemic disorders that impair energy supply, such as Lowe's syndrome, Dent's disease, cystinosis, hereditary fructose intolerance, galactosemia, tyrosinemia, Alport syndrome, and Wilson's disease. Although rare, congenital causes of FRST greatly impact the morbidity and mortality of patients and impose diagnostic challenges. Furthermore, its treatment is diverse and considers the ability of the clinician to identify the correct etiology of the disease.

CONCLUSION

The early diagnosis and treatment of pediatric patients with FRST improve the prognosis and the quality of life.

New insights in the genetic variant spectrum of SLC34A2 in pulmonary alveolar microlithiasis; a systematic review

Pulmonary alveolar microlithiasis (PAM) is a rare autosomal recessive lung disease caused by variants in the SLC34A2 gene encoding the sodium-dependent phosphate transport protein 2B, NaPi-2b. PAM is characterized by deposition of calcium phosphate crystals in the alveoli. Onset and clinical course vary considerably; some patients remain asymptomatic while others develop severe respiratory failure with a significant symptom burden and compromised survival. It is likely that PAM is under-reported due to lack of recognition, misdiagnosis, and mild clinical presentation. Most patients are genetically uncharacterized as the diagnostic confirmation of PAM has traditionally not included a genetic analysis. Genetic testing may in the future be the preferred tool for diagnostics instead of invasive methods. This systematic review aims to provide an overview of the growing knowledge of PAM genetics. Rare variants in SLC34A2 are found in almost all genetically tested patients. So far, 34 allelic variants have been identified in at least 68 patients. A majority of these are present in the homozygous state; however, a few are found in the compound heterozygous form. Most of the allelic variants involve only a single nucleotide. Half of the variants are either nonsense or frameshifts, resulting in premature termination of the protein or decay of the mRNA. There is currently no cure for PAM, and the only effective treatment is lung transplantation. Management is mainly symptomatic, but an improved understanding of the underlying pathophysiology will hopefully result in development of targeted treatment options. More standardized data on PAM patients, including a genetic diagnosis covering larger international populations, would support the design and implementation of clinical studies to the benefit of patients. Further genetic characterization and understanding of how the molecular changes influence disease phenotype will hopefully allow earlier diagnosis and treatment of the disease in the future.

An intracellular phosphorus-starvation signal activates the PhoB/PhoR two-component system in Salmonella enterica

Bacteria acquire P primarily as inorganic orthophosphate (Pi, PO43-). Once internalized, Pi is rapidly assimilated into biomass during the synthesis of ATP. Because Pi is essential, but excessive ATP is toxic, the acquisition of environmental Pi is tightly regulated. In the bacterium Salmonella enterica (Salmonella), growth in Pi-limiting environments activates the membrane sensor histidine kinase PhoR, leading to the phosphorylation of its cognate transcriptional regulator PhoB and subsequent transcription of genes involved in adaptations to low Pi. Pi limitation is thought to promote PhoR kinase activity by altering the conformation of a membrane signaling complex comprised by PhoR, the multicomponent Pi transporter system PstSACB and the regulatory protein PhoU. However, the identity of the low Pi signal and how it controls PhoR activity remain unknown. Here we characterize the PhoB-dependent and independent transcriptional changes elicited by Salmonella in response to P starvation, and identify PhoB-independent genes that are required for the utilization of several organic-P sources. We use this knowledge to identify the cellular compartment where the PhoR signaling complex senses the Pi-limiting signal. We demonstrate that the PhoB and PhoR signal transduction proteins can be maintained in an inactive state even when Salmonella is grown in media lacking Pi. Our results establish that PhoR activity is controlled by an intracellular signal resulting from P insufficiency.

The Role of Inorganic Phosphate Transporters in Highly Proliferative Cells: From Protozoan Parasites to Cancer Cells

In addition to their standard inorganic phosphate (Pi) nutritional function, Pi transporters have additional roles in several cells, including Pi sensing (the so-called transceptor) and a crucial role in Pi metabolism, where they control several phenotypes, such as virulence in pathogens and tumour aggressiveness in cancer cells. Thus, intracellular Pi concentration should be tightly regulated by the fine control of intake and storage in organelles. Pi transporters are classified into two groups: the Pi transporter (PiT) family, also known as the Pi:Na⁺ symporter family; and the Pi:H⁺ symporter (PHS) family. Highly proliferative cells, such as protozoan parasites and cancer cells, rely on aerobic glycolysis to support the rapid generation of biomass, which is equated with the well-known Warburg effect in cancer cells. In protozoan parasite cells, Pi transporters are strongly associated with cell proliferation, possibly through their action as intracellular Pi suppliers for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) activity. Similarly, the growth rate hypothesis (GRH) proposes that the high Pi demands of tumours when achieving accelerated proliferation are mainly due to increased allocation to P-rich nucleic acids. The purpose of this review was to highlight recent advances in understanding the role of Pi transporters in unicellular eukaryotes and tumorigenic cells, correlating these roles with metabolism in these cells.

Phosphate balance during dialysis and after kidney transplantation in patients with chronic kidney disease

PURPOSE OF REVIEW

In patients with chronic kidney disease (CKD), hyperphosphatemia is associated with several adverse outcomes, including bone fragility and progression of kidney and cardiovascular disease. However, there is a knowledge gap regarding phosphate balance in CKD. This review explores its current state, depending on the stage of CKD, dialysis modalities, and the influence of kidney transplantation.

RECENT FINDINGS

Adequate phosphate control is one of the goals of treatment for CKD-mineral and bone disorder. However, ongoing studies are challenging the benefits of phosphate-lowering treatment. Nevertheless, the current therapy is based on dietary restriction, phosphate binders, and optimal removal by dialysis. In the face of limited adherence, due to the high pill burden, adjuvant options are under investigation. The recent discovery that intestinal absorption of phosphate is mostly paracellular when the intraluminal concentration is adequate might help explain why phosphate is still well absorbed in CKD, despite the lower levels of calcitriol.

SUMMARY

Future studies could confirm the benefits of phosphate control. Greater understanding of the complex distribution of phosphate among the body compartments will help us define a better therapeutic strategy in patients with CKD.

Newly identified HMO-2011-type phages reveal genomic diversity and biogeographic distributions of this marine viral group

Viruses play critical roles in influencing biogeochemical cycles and adjusting host mortality, population structure, physiology, and evolution in the ocean. Marine viral communities are composed of numerous genetically distinct subfamily/genus-level viral groups. Among currently identified viral groups, the HMO-2011-type group is known to be dominant and broadly distributed. However, only four HMO-2011-type cultivated representatives that infect marine SAR116 and Roseobacter strains have been reported to date, and the genetic diversity, potential hosts, and ecology of this group remain poorly elucidated. Here, we present the genomes of seven HMO-2011-type phages that were isolated using four Roseobacter strains and one SAR11 strain, as well as additional 207 HMO-2011-type metagenomic viral genomes (MVGs) identified from various marine viromes. Phylogenomic and shared-gene analyses revealed that the HMO-2011-type group is a subfamily-level group comprising at least 10 discernible genus-level subgroups. Moreover, >2000 HMO-2011-type DNA polymerase sequences were identified, and the DNA polymerase phylogeny also revealed that the HMO-2011-type group contains diverse subgroups and is globally distributed. Metagenomic read-mapping results further showed that most HMO-2011-type phages are prevalent in global oceans and display distinct geographic distributions, with the distribution of most HMO-2011-type phages being associated with temperature. Lastly, we found that members in subgroup IX, represented by pelagiphage HTVC033P, were among the most abundant HMO-2011-type phages, which implies that SAR11 bacteria are crucial hosts for this viral group. In summary, our findings substantially expand current knowledge regarding the phylogenetic diversity, evolution, and distribution of HMO-2011-type phages, highlighting HMO-2011-type phages as major ecological agents that can infect certain key bacterial groups.

The temporal gene expression profiles of calcium and phosphorus transporters in Hy-Line Brown layers

Calcium and phosphorus homeostasis is crucial for the performance and bone health of laying hens. The calcium and phosphorus transporters play an important role in calcium and phosphorus absorption, reabsorption, and excretion. In the present study, Hy-Line Brown layers were sampled at brooding period (1, 4, 6 wk), growing and developing period (12, 18 wk) and laying period (20, 28, 80 wk) respectively, and the calcium transporters CaBP-D28k and PMCA1b and phosphorus transporters NPt2a and NPt2b were respectively measured in duodenum, jejunum, ileum and kidney. The result showed that serum calcium increased (P < 0.0001) and phosphorus level fluctuated (P = 0.0019), while alkaline phosphatase activity decreased with age (P < 0.0001). The mRNA and protein expressions of CaBP-D28k in small intestine elevated after maturity (P ≤ 0.0001). In contrast, the PMCA1b mRNA showed a trend to increase with age in jejunum (P = 0.0059) and ileum (P = 0.0825) whereas there was a decrease for PMCA1b protein in 12-18 wk (P ≤ 0.0009). The peak of NPt2b mRNA were observed at 28 wk in duodenum (P = 0.0001) and jejunum (P = 0.0622) and 1 wk in ileum (P < 0.0001). The NPt2b protein expression reached the top point at 4 or 6 wk and 20 wk and decreased to the lowest point around 12 wk (P ≤ 0.0850). In kidney, CaBP-D28k mRNA was not influenced by age (P = 0.4999), while PMCA1b highly expressed in 6-12 wk (P = 0.0003). The protein expressions of CaBP-D28k (P = 0.0148) and PMCA1b (P = 0.0003) decreased with age and lowly expressed in 12-18 wk and increased thereafter. In contrast, NPt2a expression increased steadily with age and decreased at 80 wk (P < 0.0001). In conclusion, the expressions of intestinal calcium and phosphorus transporters were changed by age, intestinal CaBP-D28k and renal NPt2a showed a dramatic increase after maturity, which coincide with the increased calcium and phosphorus requirement for egg production.

The Role of Diet in Bone and Mineral Metabolism and Secondary Hyperparathyroidism

Bone disorders are a common complication of chronic kidney disease (CKD), obesity and gut malabsorption. Secondary hyperparathyroidism (SHPT) is defined as an appropriate increase in parathyroid hormone (PTH) secretion, driven by either reduced serum calcium or increased phosphate concentrations, due to an underlying condition. The available evidence on the effects of dietary advice on secondary hyperparathyroidism confirms the benefit of a diet characterized by decreased phosphate intake, avoiding low calcium and vitamin D consumption (recommended intakes 1000-1200 mg/day and 400-800 UI/day, respectively). In addition, low protein intake in CKD patients is associated with a better control of SHPT risk factors, although its strength in avoiding hyperphosphatemia and the resulting outcomes are debated, mostly for dialyzed patients. Ultimately, a consensus on the effect of dietary acid loads in the prevention of SHPT is still lacking. In conclusion, a reasonable approach for reducing the risk for secondary hyperparathyroidism is to individualize dietary manipulation based on existing risk factors and concomitant medical conditions. More studies are needed to evaluate long-term outcomes of a balanced diet on the management and prevention of secondary hyperparathyroidism in at-risk patients at.

Prolonged scotophase within a 24 hour light regime improves eggshell quality by enhancing calcium deposition in laying hens

Calcium (Ca) and phosphorus (P) transporters are responsible for their absorption and transport in small intestine and kidney, contributing to eggshell formation. The light-dark cycle is a primary cue in the reproduction of laying hen. In this study, we investigated the effect of different light-dark programs on eggshell quality and the expression of genes related to Ca and P transportation in laying hens. Seventy-two 56-week-old laying hens were randomly divided into two groups and reared at 16-h light and 8-h dark (control) or 9-h light and 15-h dark regime (long dark phase, LDP). The expressions of calcium transporter calbindin-D28k (CaBP-D28k), plasma membrane Ca ATPase 1b (PMCA1b), and phosphorus transporter NaPi-IIb (NPt2b) and NaPi-IIa (NPt2a) were measured in the small intestine, kidney, and eggshell gland. The results showed that feed intake (P < 0.001) and egg weight (P = 0.05) were decreased by LDP treatment. Compared with control, the eggshell hardness was increased (P = 0.011) by LDP treatment, but the eggshell thickness and the percentage of eggshell were not changed. The Ca and P contents in eggshell were increased by LDP treatment. During the scotophase, LDP-hens showed higher serum Ca (P = 0.0056) and P levels (P = 0.079) but lower alkaline phosphatase (ALP) activity than that of control hens. In the duodenum, the relative higher expression of CaBP-D28k and PMCA1b in scotophase compared to photophase was masked by LDP treatment. The expression of CaBP-D28k and osteopontin (OPN) in the eggshell gland were increased by LDP treatment, compared to control hens. In the jejunum, the protein expression levels of CaBP-D28k and PMCA1b decreased during photophase in LDP-hens. The result indicates that the increased blood Ca and P concentration during scotophase by LPD treatment is beneficial to the deposition of Ca and P in the eggshell. The result offers an alternative strategy for managing laying hens with poor eggshell quality.

Stoichiometric niche, nutrient partitioning and resource allocation in a solitary bee are sex-specific and phosphorous is allocated mainly to the cocoon

Life histories of species may be shaped by nutritional limitations posed on populations. Yet, populations contain individuals that differ according to sex and life stage, each of which having different nutritional demands and experiencing specific limitations. We studied patterns of resource assimilation, allocation and excretion during the growth of the solitary bee Osmia bicornis (two sexes) under natural conditions. Adopting an ecological perspective, we assert that organisms ingest mutable organic molecules that are transformed during physiological processes and that the immutable atoms of the chemical elements composing these molecules may be allocated to specific functions, thereby influencing organismal fitness and life history. Therefore, using the framework of ecological stoichiometry, we investigated the multielemental (C, N, S, P, K, Na, Ca, Mg, Fe, Zn, Mn, Cu) compositions of six components of the bee elemental budget: food (pollen), eggs, pupae, adults, cocoons and excreta. The sexes differed fundamentally in the assimilation and allocation of acquired atoms, elemental phenotypes, and stoichiometric niches for all six components. Phosphorus, which supports larval growth, was allocated mainly (55-75%) to the cocoon after larval development was complete. Additionally, the majority (60-99%) of the Mn, Ca, Mg and Zn acquired during larval development was allocated to the cocoon, probably influencing bee fitness by conferring protection. We conclude that for holometabolous insects, considering only the chemical composition of the adult body within the context of nutritional ecology does not provide a complete picture. Low ratios of C to other nutrients, low N:P and high Na concentrations in excreta and cocoons may be important for local-scale nutrient cycling. Limited access to specific nutritional elements may hinder bee development in a sex-dependent manner, and N and P limitations, commonly considered elsewhere, may not play important roles in O. bicornis. Sexual dimorphism in nutritional limitations due to nutrient scarcity during the larval stage may influence bee population function and should be considered in bee conservation efforts.

Isolation and Characterization of Three Sodium-Phosphate Cotransporter Genes and Their Transcriptional Regulation in the Grass Carp Ctenopharyngodon idella

It is important to explore the regulatory mechanism of phosphorus homeostasis in fish, which help avoid the risk of P toxicity and prevent P pollution in aquatic environment. The present study obtained the full-length cDNA sequences and the promoters of three SLC20 members (slc20a1a, slc20a1b and slc20a2) from grass carp Ctenopharyngodon idella, and explored their responses to inorganic phosphorus (Pi). Grass carp SLC20s proteins possessed conservative domains and amino acid sites relevant with phosphorus transport. The mRNAs of three slc20s appeared in the nine tissues, but their expression levels were tissue-dependent. The binding sites of three transcription factors (SREBP1, NRF2 and VDR) were predicted on the slc20s promoters. The mutation and EMSA analysis indicated that: (1) SREBP1 binding site (-783/-771 bp) negatively but VDR (-260/-253 bp) binding site positively regulated the activities of slc20a1a promoter; (2) SREBP1 (-1187/-1178 bp), NRF2 (-572/-561 bp) and VDR(615/-609 bp) binding sites positively regulated the activities of slc20a1b promoter; (3) SREBP1 (-987/-977 bp), NRF2 (-1469/-1459 bp) and VDR (-1124/-1117 bp) binding sites positively regulated the activities of the slc20a2 promoter. Moreover, Pi incubation significantly reduced the activities of three slc20s promoters, and Pi-induced transcriptional inactivation of slc20s promoters abolished after the mutation of the VDR element but not SREBP1 and NRF2 elements. Pi incubation down-regulated the mRNA levels of three slc20s. For the first time, our study elucidated the transcriptional regulatory mechanisms of SLC20s and their responses to Pi, which offered new insights into the Pi homeostatic regulation and provided the basis for reducing phosphorus discharge into the waters.

Thifluzamide induces the toxic effects on zebrafish (Danio rerio) via inhibition of succinate dehydrogenase (SDH)

Thifluzamide is widely used in treatment of rice diseases and has potential toxicity on aquatic organism. Although previous studies have focused on the toxic effect of thifluzamide in zebrafish, no consistent conclusions have been reached. To help to elucidate the toxic mechanism, qualities of liver and mitochondria were evaluated. The global changes in the transcriptome of zebrafish after exposure to thifluzamide were measured. Based on this, the expression and activities of chitinase and succinate dehydrogenase (SDH) were further assayed. And the targeted site of thifluzamide in zebrafish was confirmed by dock study and co-exposure study. Here we report that developmental inhibition was observed along with presence of liver and mitochondrial damage. The expression of SDHa-d and genes related to mitochondrial DNA (mtDNA) replicate and mitochondrial complexes were significantly altered. And, as the top differentially expressed genes, the expression of chia.1-6 did show apparent changes, but differences of chitinase activity between exposure groups and the controls did not reach significance. In line with that, dock study showed that the binding potentials of thifluzamide toward zebrafish chitinase and SDH exhibited in the following order: SDH> chitinase. And sdhb-sdhc-sdhd (Qp site) showed the highest binding activity toward thifluzamide. The joint exposure (thifluzamide + Q10) significantly improved the survival of zebrafish compared with single thifluzamide exposure. These results indicate that SDH, especially Qp-site, may be the target of thifluzamide in zebrafish and inhibition of SDH activity may be at least in partial responsible for the toxicity of thifluzamide in zebrafish. In addition, the antagonistic effect of Q10 on thifluzamide toxicity in zebrafish suggests that Q10 may be a useful adjunct to detoxification.

Disruption of Phosphate Homeostasis Sensitizes Staphylococcus aureus to Nutritional Immunity

To control infection, mammals actively withhold essential nutrients, including the transition metal manganese, by a process termed nutritional immunity. A critical component of this host response is the manganese-chelating protein calprotectin. While many bacterial mechanisms for overcoming nutritional immunity have been identified, the intersection between metal starvation and other essential inorganic nutrients has not been investigated. Here, we report that overexpression of an operon encoding a highly conserved inorganic phosphate importer, PstSCAB, increases the sensitivity of Staphylococcus aureus to calprotectin-mediated manganese sequestration. Further analysis revealed that overexpression of pstSCAB does not disrupt manganese acquisition or result in overaccumulation of phosphate by S. aureus However, it does reduce the ability of S. aureus to grow in phosphate-replete defined medium. Overexpression of pstSCAB does not aberrantly activate the phosphate-responsive two-component system PhoPR, nor was this two-component system required for sensitivity to manganese starvation. In a mouse model of systemic staphylococcal disease, a pstSCAB-overexpressing strain is significantly attenuated compared to wild-type S. aureus This defect is partially reversed in a calprotectin-deficient mouse, in which manganese is more readily available. Given that expression of pstSCAB is regulated by PhoPR, these findings suggest that overactivation of PhoPR would diminish the ability of S. aureus to resist nutritional immunity and cause infection. As PhoPR is also necessary for bacterial virulence, these findings imply that phosphate homeostasis represents a critical regulatory node whose activity must be precisely controlled in order for S. aureus and other pathogens to cause infection.

The seventh macronutrient: how sodium shortfall ramifies through populations, food webs and ecosystems

Of the 25 elements required to build most organisms, sodium has a unique set of characteristics that ramify through terrestrial ecology. In plants, sodium is found in low concentrations and has little metabolic function; in plant consumers, particularly animals, sodium is essential to running costly Na-K ATPases. Here I synthesise a diverse literature from physiology, agronomy and ecology, towards identifying sodium's place as the '7th macronutrient', one whose shortfall targets two trophic levels - herbivores and detritivores. I propose that sodium also plays a central, though unheralded role in herbivore digestion, via its importance to maintaining microbiomes and denaturing tannins. I highlight how sodium availability is a key determinant of consumer abundance and the geography of herbivory and detritivory. And I propose a re-appraisal of the assumption that, because sodium is metabolically unimportant to most plants, it is of little use. Instead, I suggest that sodium's critical role in limiting herbivore performance makes it a commodity used by plants to manipulate their herbivores and mutualists, and by consumers like bison and elephants to generate grazing lawns: dependable sources of sodium.

Phosphate Transport in Epithelial and Nonepithelial Tissue

Phosphate is an essential nutrient for life and is a critical component of bone formation, a major signaling molecule, and structural component of cell walls. Phosphate is also a component of high-energy compounds (i.e., AMP, ADP, and ATP) and essential for nucleic acid helical structure (i.e., RNA and DNA). Phosphate plays a central role in the process of mineralization, normal serum levels being associated with appropriate bone mineralization, while high and low serum levels are associated with soft tissue calcification. The serum concentration of phosphate and the total body content of phosphate are highly regulated, a process that is accomplished by the coordinated effort of two families of sodium-dependent transporter proteins. The three isoforms of the SLC34 family (SLC34A1-A3) show very restricted tissue expression and regulate intestinal absorption and renal excretion of phosphate. SLC34A2 also regulates the phosphate concentration in multiple lumen fluids including milk, saliva, pancreatic fluid, and surfactant. Both isoforms of the SLC20 family exhibit ubiquitous expression (with some variation as to which one or both are expressed), are regulated by ambient phosphate, and likely serve the phosphate needs of the individual cell. These proteins exhibit similarities to phosphate transporters in nonmammalian organisms. The proteins are nonredundant as mutations in each yield unique clinical presentations. Further research is essential to understand the function, regulation, and coordination of the various phosphate transporters, both the ones described in this review and the phosphate transporters involved in intracellular transport.

Effects of available phosphorus source and concentration on performance and expression of sodium phosphate type IIb cotransporter, vitamin D-1α-hydroxylase, and vitamin D-24-hydroxylase mRNA in broiler chicks

This experiment was conducted to examine the effect of 2 phosphorus (P) sources on broiler performance to day 14. The P bioavailability was estimated using bird performance and tibia ash measurements, whereas P digestibility, intestinal P transporter, kidney vitamin D-1α-hydroxylase, and vitamin D-24-hydroxylase mRNA abundances were also determined. Slope regression analysis was used to determine the bioavailability of dicalcium phosphate (Dical P) and nanocalcium phosphate (Nano P) with dietary available P (AvP) set to 0.20% P (control) using AvP from the major ingredients and Dical P. The experimental treatments were achieved by supplementation with either Dical P or Nano P to generate 0.24, 0.28, 0.32, and 0.36% AvP. A total of 648-day-old unsexed broiler chicks were divided into 72 birds per treatment (8 replicate cages of 9 birds). Slope regression analysis showed positive linear relationships between BW, feed intake (FI), tibia ash weight (TAW), and tibia ash percentage (TAP) with dietary Dical P and Nano P levels. Comparisons between regression slopes for Dical P and Nano P fed birds were not significantly different for BW, feed intake, tibia ash weight, and tibia ash percentage, indicating similar P bioavailability from Dical P and Nano P. There were interactions between P source and AvP for feed efficiency (FE) and apparent ileal P digestibility (AIPD). Dicalcium phosphate had greater FE than Nano P at 0.28% AvP and greater AIPD than Nano P at 0.24% AvP. The addition of AvP from Dical P and Nano P resulted in reduced sodium phosphate cotransporter mRNA abundance in the duodenum in a dose–dependent response. In the kidney, vitamin D-1α-hydroxylase mRNA abundance was greater at 0.36% Nano P compared with control, but there was no difference with Dical P. There was no difference in vitamin D-24-hydroxylase mRNA abundance between control and supplementation with Nano P or Dical P. In conclusion, Nano P and Dical P had the same bioavailability but had different effects on gene expression.

Restriction of dietary non-phytate phosphorus on growth performance and expression of intestinal phosphate cotransporter genes in broilers

Effects of dietary non-phytate phosphorus (nPP) restriction on growth and duodenal type IIb sodium-dependent phosphate cotransporter (NaPi-IIb) genes were observed. A total of 432 one-day old Cobb500 male broiler chickens in 36 cage pens were divided into 6 groups with each group containing 6 pens. Each group was treated with one of the diets containing 0.33, 0.37, 0.41, 0.45, 0.49, and 0.53% of nPP up to 14 D. During 15 to 31 D, birds were treated with one of the diets containing 0.23, 0.27, 0.31, 0.35, 0.39, and 0.43% of nPP. Level of Ca was kept the same across all treatments. Dietary nPP level influenced (P < 0.001) weight gain and feed intake in both growth phases, whereas effect on feed per gain ratio was seen only in the second phase. Toe ash, tibia ash, and tibia breaking strength responded to treatments (P < 0.01) at 14 D. Only tibia ash content was significantly improved (P < 0.001) at 31 D. Growth and bone parameters linearly improved with an increase in dietary nPP content (P < 0.05). Above dietary nPP 0.41% and 0.31% for first phase and second phase, respectively, no significant improvement was seen. Duodenal NaPi-IIb mRNA overexpressed with a decrease in dietary nPP in both phases (P < 0.05). Relative expression of NaPi-IIb in lowest nPP group were 2.2 folds higher in the first phase and 3.6 folds higher in the second phase compared to respective highest nPP groups of each phase. No significant change in NaPi-IIb expression was seen above 0.37% of dietary nPP for 14 D and 0.31% of dietary nPP for 31 D. Dietary requirements of nPP 0.41% for 0 to 14 D and 0.31% for 15 to 31 D were adequate for optimal growth and bone parameters. This study fills the gap in understanding of intestinal NaPi-IIb expression in response to dietary nPP restriction in broilers older than 21 D of age.

Distinct functional properties of two electrogenic isoforms of the SLC34 Na-Pi cotransporter

Inorganic phosphate (Pi ) is crucial for proper cellular function in all organisms. In mammals, type II Na-Pi cotransporters encoded by members of the Slc34 gene family play major roles in the maintenance of Pi homeostasis. However, the molecular mechanisms regulating Na-Pi cotransporter activity within the plasma membrane are largely unknown. In the present study, we used two approaches to examine the effect of changing plasma membrane phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2 ) levels on the activities of two electrogenic Na-Pi cotransporters, NaPi-IIa and NaPi-IIb. To deplete plasma membrane PI(4,5)P2 in Xenopus oocytes, we utilized Ciona intestinalis voltage-sensing phosphatase (Ci-VSP), which dephosphorylates PI(4,5)P2 to phosphatidylinositol 4-phosphate (PI(4)P). Upon activation of Ci-VSP, NaPi-IIb currents were significantly decreased, whereas NaPi-IIa currents were unaffected. We also used the rapamycin-inducible Pseudojanin (PJ) system to deplete both PI(4,5)P2 and PI(4)P from the plasma membrane of cultured Neuro 2a cells. Depletion of PI(4,5)P2 and PI(4)P using PJ significantly reduced NaPi-IIb activity, but NaPi-IIa activity was unaffected, which excluded the possibility that NaPi-IIa is equally sensitive to PI(4,5)P2 and PI(4)P. These results indicate that NaPi-IIb activity is regulated by PI(4,5)P2 , whereas NaPi-IIa is not sensitive to either PI(4,5)P2 or PI(4)P. In addition, patch clamp recording of NaPi-IIa and NaPi-IIb currents in cultured mammalian cells enabled kinetic analysis with higher temporal resolution, revealing their distinct kinetic properties.

Role of phosphate sensing in bone and mineral metabolism

Inorganic phosphate (P_i) is essential for signal transduction and cell metabolism, and is also an essential structural component of the extracellular matrix of the skeleton. P_i is sensed in bacteria and yeast at the plasma membrane, which activates intracellular signal transduction to control the expression of P_i transporters and other genes that control intracellular P_i levels. In multicellular organisms, P_i homeostasis must be maintained in the organism and at the cellular level, requiring an endocrine and metabolic P_i-sensing mechanism, about which little is currently known. This Review will discuss the metabolic effects of P_i, which are mediated by P_i transporters, inositol pyrophosphates and SYG1-Pho81-XPR1 (SPX)-domain proteins to maintain cellular phosphate homeostasis in the musculoskeletal system. In addition, we will discuss how P_i is sensed by the human body to regulate the production of fibroblast growth factor 23 (FGF23), parathyroid hormone and calcitriol to maintain serum levels of P_i in a narrow range. New findings on the crosstalk between iron and P_i homeostasis in the regulation of FGF23 expression will also be outlined. Mutations in components of these metabolic and endocrine phosphate sensors result in genetic disorders of phosphate homeostasis, cardiomyopathy and familial basal ganglial calcifications, highlighting the importance of this newly emerging area of research.

Dietary supplemental vitamin D3enhances phosphorus absorption and utilisation by regulating gene expression of related phosphate transporters in the small intestine of broilers

The present study was carried out to evaluate the effect of dietary supplemental vitamin D3(VD3) on P absorption and utilisation as well as its related mechanisms in the small intestine of broilers. A total of 384 1-d-old Arbor Acres male broilers were assigned randomly into four treatments following a completely randomised design with a 2 (dietary non-phytate P (NPP) contents: 0·43 and 0·22 %)×2 (dietary VD3supplemental levels: 0 and 87·5 μg/kg) factorial arrangement. The experiment lasted for 22 d. The results showed that P contents in serum from the hepatic portal vein and tibia ash of broilers were higher (P<0·05) for 0·43 % NPP than for 0·22 % NPP. The type IIb Na-dependent phosphate cotransporter (NaP-IIb) protein expressions in the duodenum and ileum were higher (P<0·05) also for 0·43 % NPP than 0·22 % NPP. Supplementation of VD3enhanced (P<0·05) tibia P retention rate and type III Na-dependent phosphate cotransporter (PiT)-1 protein expression in the duodenum of all broilers. Moreover, VD3supplementation decreased (P<0·002) mortality and increased (P<0·02) serum P content from the hepatic portal vein after 4 h of feeding, tibia ash content, tibia ash P content and protein expressions of NaP-IIb and PiT-1 in the jejunum of broilers fed diet with 0·22 % NPP. Thus, dietary supplemental VD3promoted intestinal P absorption and bone P utilisation, and this effect might be associated with enhanced PiT-1 levels in the duodenum and PiT-1 and NaP-IIb levels in the jejunum respectively when dietary NPP is limiting.

Phosphate as a Signaling Molecule and Its Sensing Mechanism

In mammals, phosphate balance is maintained by influx and efflux via the intestines, kidneys, bone, and soft tissue, which involves multiple sodium/phosphate (Na+/Pi) cotransporters, as well as regulation by several hormones. Alterations in the levels of extracellular phosphate exert effects on both skeletal and extra-skeletal tissues, and accumulating evidence has suggested that phosphate itself evokes signal transduction to regulate gene expression and cell behavior. Several in vitro studies have demonstrated that an elevation in extracellular Piactivates fibroblast growth factor receptor, Raf/MEK (mitogen-activated protein kinase/ERK kinase)/ERK (extracellular signal-regulated kinase) pathway and Akt pathway, which might involve the type III Na+/Picotransporter PiT-1. Excessive phosphate loading can lead to various harmful effects by accelerating ectopic calcification, enhancing oxidative stress, and dysregulating signal transduction. The responsiveness of mammalian cells to altered extracellular phosphate levels suggests that they may sense and adapt to phosphate availability, although the precise mechanism for phosphate sensing in mammals remains unclear. Unicellular organisms, such as bacteria and yeast, use some types of Pitransporters and other molecules, such as kinases, to sense the environmental Piavailability. Multicellular animals may need to integrate signals from various organs to sense the phosphate levels as a whole organism, similarly to higher plants. Clarification of the phosphate-sensing mechanism in humans may lead to the development of new therapeutic strategies to prevent and treat diseases caused by phosphate imbalance.

Effects of Dietary Phosphorus Level on the Expression of Calcium and Phosphorus Transporters in Laying Hens

The transport of calcium and phosphorus is mainly relied on their corresponding transporters. The aim of this study was to determine the effect of dietary phosphorus level on the expression of the relevant calcium and phosphorus transporters in laying hens, which has a large amount of calcium and phosphorus input from intestine and output from kidney and eggshell gland. Thirty-six 25-week-old Hy-line Brown hens were fed diets with different available phosphorus level (AP, 0.15, 0.41, and 0.82%), respectively. The expression of phosphorus transporters type IIa and type IIb Na/Pi co-transporter (NPt2a, NPt2b), calcium transporter calbindin-D28k (CaBP-D28k), and plasma membrane Ca ATPase 1b (PMCA1b) were measured in small intestine, kidney, and eggshell gland by RT-PCR and western blot. The results showed that serum calcitriol and PTH concentrations were not affected (P > 0.05) by dietary AP levels. Duodenum had the highest mRNA and protein expression level of NPt2b than jejunum and ileum (P < 0.05). The protein expression abundance of CaBP-D28k and PMCA1b were higher in duodenum than that in jejunum and ileum (P < 0.05). 0.15%-AP diet upregulated the ileal mRNA expression level of NPt2b and renal mRNA expression level of NPt2a (P < 0.05), while downregulated the protein abundance of NPt2b and CaBP-D28k mRNA expression in shell gland (P < 0.05). In conclusion, both the Ca and P transporters were highly expressed in duodenum. Low AP diet decreased protein expression abundance of NPt2b in duodenum while upregulated the mRNA expression level of NPt2a in kidney. The result suggests that both the phosphorus absorption in proximal intestine and its reabsorption in kidney are involved in the adaption to low AP diet.

Acquisition of the Phosphate Transporter NptA Enhances Staphylococcus aureus Pathogenesis by Improving Phosphate Uptake in Divergent Environments

During infection, pathogens must obtain all inorganic nutrients, such as phosphate, from the host. Despite the essentiality of phosphate for all forms of life, how Staphylococcus aureus obtains this nutrient during infection is unknown. Differing from Escherichia coli, the paradigm for bacterial phosphate acquisition, which has two inorganic phosphate (P_i) importers, genomic analysis suggested that S. aureus possesses three distinct P_i transporters: PstSCAB, PitA, and NptA. While pitA and nptA are expressed in phosphate-replete media, expression of all three transporters is induced by phosphate limitation. The loss of a single transporter did not affect S. aureus However, disruption of any two systems significantly reduced P_i accumulation and growth in divergent environments. These findings indicate that PstSCAB, PitA, and NptA have overlapping but nonredundant functions, thus expanding the environments in which S. aureus can successfully obtain P_i Consistent with this idea, in a systemic mouse model of disease, loss of any one transporter did not decrease staphylococcal virulence. However, loss of NptA in conjunction with either PstSCAB or PitA significantly reduced the ability of S. aureus to cause infection. These observations suggest that P_i acquisition via NptA is particularly important for the pathogenesis of S. aureus While our analysis suggests that NptA homologs are widely distributed among bacteria, closely related less pathogenic staphylococcal species do not possess this importer. Altogether, these observations indicate that P_i uptake by S. aureus differs from established models and that acquisition of a third transporter enhances the ability of the bacterium to cause infection.

Two Distinct Regulatory Mechanisms of Transcriptional Initiation in Response to Nutrient Signaling

SAGA (Spt-Ada-Gcn5-Acetyltransferase) and TFIID (transcription factor IID) have been previously shown to facilitate the formation of the PIC (pre-initiation complex) at the promoters of two distinct sets of genes. Here, we demonstrate that TFIID and SAGA differentially participate in the stimulation of PIC formation (and hence transcriptional initiation) at the promoter of PHO84, a gene for the high-affinity inorganic phosphate (P_i) transporter for crucial cellular functions, in response to nutrient signaling. We show that transcriptional initiation of PHO84 occurs predominantly in a TFIID-dependent manner in the absence of P_i in the growth medium. Such TFIID dependency is mediated via the NuA4 (nucleosome acetyltransferase of H4) histone acetyltransferase (HAT). Intriguingly, transcriptional initiation of PHO84 also occurs in the presence of P_i in the growth medium, predominantly via the SAGA complex, but independently of NuA4 HAT. Thus, P_i in the growth medium switches transcriptional initiation of PHO84 from NuA4-TFIID to SAGA dependency. Further, we find that both NuA4-TFIID- and SAGA-dependent transcriptional initiations of PHO84 are facilitated by the 19S proteasome subcomplex or regulatory particle (RP) via enhanced recruitment of the coactivators SAGA and NuA4 HAT, which promote TFIID-independent and -dependent PIC formation for transcriptional initiation, respectively. NuA4 HAT does not regulate activator binding to PHO84, but rather facilitates PIC formation for transcriptional initiation in the absence of Pi in the growth medium. On the other hand, SAGA promotes activator recruitment to PHO84 for transcriptional initiation in the growth medium containing Pi. Collectively, our results demonstrate two distinct stimulatory pathways for PIC formation (and hence transcriptional initiation) at PHO84 by TFIID, SAGA, NuA4, and 19S RP in the presence and absence of an essential nutrient, P_i, in the growth media, thus providing new regulatory mechanisms of transcriptional initiation in response to nutrient signaling.

Ectopically expressed Slc34a2a sense-antisense transcripts cause a cerebellar phenotype in zebrafish embryos depending on RNA complementarity and Dicer

Natural antisense transcripts (NATs) are complementary to protein coding genes and potentially regulate their expression. Despite widespread occurrence of NATs in the genomes of higher eukaryotes, their biological role and mechanism of action is poorly understood. Zebrafish embryos offer a unique model system to study sense-antisense transcript interplay at whole organism level. Here, we investigate putative antisense transcript-mediated mechanisms by ectopically co-expressing the complementary transcripts during early zebrafish development. In zebrafish the gene Slc34a2a (Na-phosphate transporter) is bi-directionally transcribed, the NAT predominantly during early development up to 48 hours after fertilization. Declining levels of the NAT, Slc34a2a(as), coincide with an increase of the sense transcript. At that time, sense and antisense transcripts co-localize in the endoderm at near equal amounts. Ectopic expression of the sense transcript during embryogenesis leads to specific failure to develop a cerebellum. The defect is RNA-mediated and dependent on sense-antisense complementarity. Overexpression of a Slc34a2a paralogue (Slc34a2b) or the NAT itself had no phenotypic consequences. Knockdown of Dicer rescued the brain defect suggesting that RNA interference is required to mediate the phenotype. Our results corroborate previous reports of Slc34a2a-related endo-siRNAs in two days old zebrafish embryos and emphasize the importance of coordinated expression of sense-antisense transcripts. Our findings suggest that RNAi is involved in gene regulation by certain natural antisense RNAs.

Segmental diversity of phosphate transport along the intestinal axis in horses

For horses, distinct differences in intestinal phosphate transport have been postulated to account for the unique features of hind gut fermentation compared to other monogastric animals and ruminants. So far published data on mechanisms and underlying transport proteins involved in intestinal phosphate transport in the horse are still missing. Therefore we investigated intestinal phosphate transport in horses at both functional and molecular levels. Segmental diversity of intestinal phosphate transport along the intestinal axis was documented using the Ussing chamber technique. A transcellular phosphate secretion in the jejunum was confirmed. Furthermore, 2 sodium-dependent phosphate cotransporters, NaPiIIb and PiT1, were first detected in the equine intestine at mRNA level with PiT1 being expressed in both the small and large intestine, and NaPiIIb being solely expressed in the large intestine. In the colon, unidirectional net flux rates of phosphate were significantly greater compared to flux rates in other segments ( < 0.005) suggesting the colon as a major site for phosphate absorption in horses. Phosphate transport in the colon was mainly transcellular and mediated by a sodium-gradient as documented by Ussing chamber experiments and uptake of phosphate into colonic brush border membrane vesicles. In summary, the present study demonstrated mechanisms and transporters of intestinal phosphate transport in equine intestinal tissues with distinct differences between intestinal segments providing a new basis for a better understanding of intestinal phosphate transport in horses.

Functional PTB phosphate transporters are present in streptophyte algae and early diverging land plants

Two inorganic phosphate (Pi) uptake mechanisms operate in streptophytes and chlorophytes, the two lineages of green plants. PHOSPHATE TRANSPORTER B (PTB) proteins are hypothesized to be the Na⁺ /Pi symporters catalysing Pi uptake in chlorophytes, whereas PHOSPHATE TRANSPORTER 1 (PHT1) proteins are the H⁺ /Pi symporters that carry out Pi uptake in angiosperms. PHT1 proteins are present in all streptophyte lineages. However, Pi uptake in streptophyte algae and marine angiosperms requires Na⁺ influx, suggesting that Na⁺ /Pi symporters also function in some streptophytes. We tested the hypothesis that Na⁺ /Pi symporters exist in streptophytes. We identified PTB sequences in streptophyte genomes. Core PTB proteins are present at the plasma membrane of the liverwort Marchantia polymorpha. The expression of M. polymorpha core PTB proteins in the Saccharomyces cerevisiae pho2 mutant defective in high-affinity Pi transport rescues growth in low-Pi environments. Moreover, levels of core PTB mRNAs of M. polymorpha and the streptophyte alga Coleochaete nitellarum are higher in low-Pi than in Pi-replete conditions, consistent with a role in Pi uptake from the environment. We conclude that land plants inherited two Pi uptake mechanisms - mediated by the PTB and PHT1 proteins, respectively - from their streptophyte algal ancestor. Both systems operate in parallel in extant early diverging land plants.

Molecular determinants of transport function in zebrafish Slc34a Na-phosphate transporters

The epithelial Na⁺-coupled phosphate cotransporter family Slc34a (NaPi-II) is well conserved in vertebrates and plays an essential role in maintaining whole body levels of inorganic phosphate (P_i). A three-dimensional model of the transport protein has recently been proposed with defined substrate coordination sites. Zebrafish express two NaPi-II isoforms with high sequence identity but a 10-fold different apparent K_m for P_i ([Formula: see text]). We took advantage of the two zebrafish isoforms to investigate the contribution of specific amino acids to P_i coordination and transport. Mutations were introduced to gradually transform the low-affinity isoform into a high-affinity transporter. The constructs were expressed in Xenopus laevis oocytes and functionally characterized. Becaue the cotransport of P_i and Na involves multiple steps that could all influence [Formula: see text], we performed a detailed functional analysis to characterize the impact of the mutations on particular steps of the transport cycle. We used varying concentrations of the substrates P_i and its slightly larger analog, arsenate, as well as the cosubstrate, Na⁺ Moreover, electrogenic kinetics were performed to assess intramolecular movements of the transporter. All of the mutations were found to affect multiple transport steps, which suggested that the altered amino acids induced subtle structural changes rather than coordinating P_i directly. The likely positions of the critical residues were mapped to the model of human Slc34a, and their localization in relation to the proposed substrate binding pockets concurs well with the observed functional data.

Linking Gene Expression in the Intestine to Production of Gametes Through the Phosphate Transporter PITR-1 in Caenorhabditis elegans

Inorganic phosphate is an essential mineral for both prokaryotic and eukaryotic cell metabolism and structure. Its uptake into the cell is mediated by membrane-bound transporters and coupled to Na⁺ transport. Mammalian sodium-dependent Pi cotransporters have been grouped into three families NaPi-I, NaPi-II, and NaPi-III. Despite being discovered more than two decades ago, very little is known about requirements for NaPi-III transporters in vivo, in the context of intact animal models. Here we find that impaired function of the Caenorhabditis elegans NaPi-III transporter, pitr-1, results in decreased brood size and dramatically increased expression of vitellogenin by the worm intestine. Unexpectedly, we found that the effects of pitr-1 mutation on vitellogenin expression in the intestine could only be rescued by expression of pitr-1 in the germline, and not by expression of pitr-1 in the intestine itself. Our results indicate the existence of a signal from the germline that regulates gene expression in the intestine, perhaps linking nutrient export from the intestine to production of gametes by the germline.

Natural antisense transcription from a comparative perspective

Natural antisense transcripts (NATs) can interfere with the expression of complementary sense transcripts with exquisite specificity. We have previously cloned NATs of Slc34a loci (encoding Na-phosphate transporters) from fish and mouse. Here we report the cloning of a human SLC34A1-related NAT that represents an alternatively spliced PFN3 transcript (Profilin3). The transcript is predominantly expressed in testis. Phylogenetic comparison suggests two distinct mechanisms producing Slc34a-related NATs: Alternative splicing of a transcript from a protein coding downstream gene (Pfn3, human/mouse) and transcription from the bi-directional promoter (Rbpja, zebrafish). Expression analysis suggested independent regulation of the complementary Slc34a mRNAs. Analysis of randomly selected bi-directionally transcribed human/mouse loci revealed limited phylogenetic conservation and independent regulation of NATs. They were reduced on X chromosomes and clustered in regions that escape inactivation. Locus structure and expression pattern suggest a NATs-associated regulatory mechanisms in testis unrelated to the physiological role of the sense transcript encoded protein.

Fungal association and utilization of phosphate by plants: success, limitations, and future prospects

Phosphorus (P) is a major macronutrient for plant health and development. The available form of P is generally low in the rhizosphere even in fertile soils. A major proportion of applied phosphate (Pi) fertilizers in the soil become fixed into insoluble, unavailable forms, which restricts crop production throughout the world. Roots possess two distinct modes of P uptake from the soil, direct and indirect uptake. The direct uptake of P is facilitated by the plant's own Pi transporters while indirect uptake occurs via mycorrhizal symbiosis, where the host plant obtains P primarily from the fungal partner, while the fungus benefits from plant-derived reduced carbon. So far, only one Pi transporter has been characterized from the mycorrhizal fungus Glomus versiforme. As arbuscular mycorrhizal fungi cannot be cultured axenically, their Pi transporter network is difficult to exploite for large scale sustainable agriculture. Alternatively, the root-colonizing endophytic fungus Piriformospora indica can grow axenically and provides strong growth-promoting activity during its symbiosis with a broad spectrum of plants. P. indica contains a high affinity Pi transporter (PiPT) involved in improving Pi nutrition levels in the host plant under P limiting conditions. As P. indica can be manipulated genetically, it opens new vistas to be used in P deficient fields.

Hyperphosphatemia Management in Patients with Chronic Kidney Disease

Hyperphosphatemia in chronic kidney disease (CKD) patients is a potentially life altering condition that can lead to cardiovascular calcification, metabolic bone disease (renal osteodystrophy) and the development of secondary hyperparathyroidism (SHPT). It is also associated with increased prevalence of cardiovascular diseases and mortality rates. To effectively manage hyperphosphatemia in CKD patients it is important to not only consider pharmacological and nonpharmacological treatment options but also to understand the underlying physiologic pathways involved in phosphorus homoeostasis. This review will therefore provide both a background into phosphorus homoeostasis and the management of hyperphosphatemia in CKD patients. In addition, it will cover some of the most important reasons for failure to control hyperphosphatemia with emphasis on the effect of the gastric pH on phosphate binders efficiency.

Coregulated expression of the Na+/phosphate Pho89 transporter and Ena1 Na+-ATPase allows their functional coupling under high-pH stress

The yeast Saccharomyces cerevisiae has two main high-affinity inorganic phosphate (Pi) transporters, Pho84 and Pho89, that are functionally relevant at acidic/neutral pH and alkaline pH, respectively. Upon Pi starvation, PHO84 and PHO89 are induced by the activation of the PHO regulon by the binding of the Pho4 transcription factor to specific promoter sequences. We show that PHO89 and PHO84 are induced by alkalinization of the medium with different kinetics and that the network controlling Pho89 expression in response to alkaline pH differs from that of other members of the PHO regulon. In addition to Pho4, the PHO89 promoter is regulated by the transcriptional activator Crz1 through the calcium-activated phosphatase calcineurin, and it is under the control of several repressors (Mig2, Nrg1, and Nrg2) coordinately regulated by the Snf1 protein kinase and the Rim101 transcription factor. This network mimics the one regulating expression of the Na(+)-ATPase gene ENA1, encoding a major determinant for Na(+) detoxification. Our data highlight a scenario in which the activities of Pho89 and Ena1 are functionally coordinated to sustain growth in an alkaline environment.

Structural fold and binding sites of the human Na⁺-phosphate cotransporter NaPi-II

Phosphate plays essential biological roles and its plasma level in humans requires tight control to avoid bone loss (insufficiency) or vascular calcification (excess). Intestinal absorption and renal reabsorption of phosphate are mediated by members of the SLC34 family of sodium-coupled transporters (NaPi-IIa,b,c) whose membrane expression is regulated by various hormones, circulating proteins, and phosphate itself. Consequently, NaPi-II proteins are also potentially important pharmaceutical targets for controlling phosphate levels. Their crucial role in Pi homeostasis is underscored by pathologies resulting from naturally occurring SLC34 mutations and SLC34 knockout animals. SLC34 isoforms have been extensively studied with respect to transport mechanism and structure-function relationships; however, the three-dimensional structure is unknown. All SLC34 transporters share a duplicated motif comprising a glutamine followed by a stretch of threonine or serine residues, suggesting the presence of structural repeats as found in other transporter families. Nevertheless, standard bioinformatic approaches fail to clearly identify a suitable template for molecular modeling. Here, we used hydrophobicity profiles and hidden Markov models to define a structural repeat common to all SLC34 isoforms. Similar approaches identify a relationship with the core regions in a crystal structure of Vibrio cholerae Na(+)-dicarboxylate transporter VcINDY, from which we generated a homology model of human NaPi-IIa. The aforementioned SLC34 motifs in each repeat localize to the center of the model, and were predicted to form Na(+) and Pi coordination sites. Functional relevance of key amino acids was confirmed by biochemical and electrophysiological analysis of expressed, mutated transporters. Moreover, the validity of the predicted architecture is corroborated by extensive published structure-function studies. The model provides key information for elucidating the transport mechanism and predicts candidate substrate binding sites.

Phosphate absorption across multiple epithelia in the Pacific hagfish (Eptatretus stoutii)

Inorganic phosphate (Pi) is an essential nutrient for all organisms, but in seawater, Pi is a limiting nutrient. This study investigated the primary mechanisms of Pi uptake in Pacific hagfish (Eptatretus stoutii) using ex vivo physiological and molecular techniques. Hagfish were observed to have the capacity to absorb Pi from the environment into at least three epithelial surfaces: the intestine, skin, and gill. Pi uptake in all tissues was concentration dependent, and saturable Pi transport was observed in the skin and gill at <2.0 mmol/l Pi. Gill and intestinal Pi uptake was sodium dependent, but Pi uptake into the skin increased under low sodium conditions. Gill Pi transport exhibited an apparent affinity constant ~0.23-0.6 mmol/l Pi. A complete sequence of a type II sodium phosphate cotransporter (Slc34a) was obtained from the hagfish gill. Phylogenetic analysis of the hagfish Slc34a transporter indicates that it is earlier diverging than, and/or ancestral to, the other identified vertebrate Slc34a transporters (Slc34a1, Slc34a2, and Slc34a3). With the use of RT-PCR, the hagfish Slc34a transcript was detected in the intestine, skin, gill, and kidney, suggesting that this may be the transporter involved in Pi uptake into multiple epithelia in the hagfish. This is the first measurement of Pi uptake across the gill or skin of any vertebrate animal and first sodium phosphate cotransporter identified in hagfish.

Treatment of phosphorus balance disorders

Phosphorus (P) homeostasis in ruminants has received increased attention over the past decades. Although environmental concerns associated with excessive P excretion in cattle manure have led to incentives to lower dietary P intake, hypophosphatemia-particularly in the periparturient dairy cow-has been associated with conditions, such as the downer cow syndrome or postparturient hemoglobinuria. The objective of this article is to revisit current understanding of P homeostasis in ruminants, to discuss the pathophysiology and clinical presentation of P balance disorders, and to review different treatment approaches to correct imbalances of the body's P equilibrium.

Inorganic phosphate uptake in unicellular eukaryotes

BACKGROUND

Inorganic phosphate (Pi) is an essential nutrient for all organisms. The route of Pi utilization begins with Pi transport across the plasma membrane.

SCOPE OF REVIEW

Here, we analyzed the gene sequences and compared the biochemical profiles, including kinetic and modulator parameters, of Pi transporters in unicellular eukaryotes. The objective of this review is to evaluate the recent findings regarding Pi uptake mechanisms in microorganisms, such as the fungi Neurospora crassa and Saccharomyces cerevisiae and the parasite protozoans Trypanosoma cruzi, Trypanosoma rangeli, Leishmania infantum and Plasmodium falciparum.

MAJOR CONCLUSION

Pi uptake is the key step of Pi homeostasis and in the subsequent signaling event in eukaryotic microorganisms.

GENERAL SIGNIFICANCE

Biochemical and structural studies are important for clarifying mechanisms of Pi homeostasis, as well as Pi sensor and downstream pathways, and raise possibilities for future studies in this field.

Differential expression of a sodium-phosphate cotransporter among Vibrio vulnificus strains

Vibrio vulnificus is an estuarine bacterium with pathogenic potential. Its three known biotypes differ in host distribution. We have found the nptA gene for a sodium-phosphate cotransporter, which is rare in bacteria, in each biotype. nptA transcript abundance differed significantly among biotypes, leading to the hypothesis that transcript levels differ under environmental conditions associated with estuarine and host environments. nptA transcript abundance was assessed in V. vulnificus biotypes 1 (C and E genotypes), 2 and 3 strains under varied salinity, phosphate concentration, and pH. Differences in transcript abundance separated strains into two groups. Type C and biotype 3 strains formed Group 1, while type E and biotype 2 strains formed Group 2. Group 2 strains had significantly greater nptA RNA transcript abundance than Group 1. Transcript abundance in the two groups also responded differently to pH and salinity, suggesting differential regulation of nptA in response to environmental conditions. Comparison of the deduced amino acid sequences of NptA among strains resulted in strain grouping similar to that based on transcript abundance. Variation in transcript abundance between groups may affect the ability of V. vulnificus strains to colonize hosts and/or to compete as free-living bacteria in various habitats.

The SLC34 family of sodium-dependent phosphate transporters

The SLC34 family of sodium-driven phosphate cotransporters comprises three members: NaPi-IIa (SLC34A1), NaPi-IIb (SLC34A2), and NaPi-IIc (SLC34A3). These transporters mediate the translocation of divalent inorganic phosphate (HPO4 (2-)) together with two (NaPi-IIc) or three sodium ions (NaPi-IIa and NaPi-IIb), respectively. Consequently, phosphate transport by NaPi-IIa and NaPi-IIb is electrogenic. NaPi-IIa and NaPi-IIc are predominantly expressed in the brush border membrane of the proximal tubule, whereas NaPi-IIb is found in many more organs including the small intestine, lung, liver, and testis. The abundance and activity of these transporters are mostly regulated by changes in their expression at the cell surface and are determined by interactions with proteins involved in scaffolding, trafficking, or intracellular signaling. All three transporters are highly regulated by factors including dietary phosphate status, hormones like parathyroid hormone, 1,25-OH2 vitamin D3 or FGF23, electrolyte, and acid-base status. The physiological relevance of the three members of the SLC34 family is underlined by rare Mendelian disorders causing phosphaturia, hypophosphatemia, or ectopic organ calcifications.

Transposon mutagenesis identified chromosomal and plasmid genes essential for adaptation of the marine bacterium Dinoroseobacter shibae to anaerobic conditions

Anaerobic growth and survival are integral parts of the life cycle of many marine bacteria. To identify genes essential for the anoxic life of Dinoroseobacter shibae, a transposon library was screened for strains impaired in anaerobic denitrifying growth. Transposon insertions in 35 chromosomal and 18 plasmid genes were detected. The essential contribution of plasmid genes to anaerobic growth was confirmed with plasmid-cured D. shibae strains. A combined transcriptome and proteome approach identified oxygen tension-regulated genes. Transposon insertion sites of a total of 1,527 mutants without an anaerobic growth phenotype were determined to identify anaerobically induced but not essential genes. A surprisingly small overlap of only three genes (napA, phaA, and the Na(+)/Pi antiporter gene Dshi_0543) between anaerobically essential and induced genes was found. Interestingly, transposon mutations in genes involved in dissimilatory and assimilatory nitrate reduction (napA, nasA) and corresponding cofactor biosynthesis (genomic moaB, moeB, and dsbC and plasmid-carried dsbD and ccmH) were found to cause anaerobic growth defects. In contrast, mutation of anaerobically induced genes encoding proteins required for the later denitrification steps (nirS, nirJ, nosD), dimethyl sulfoxide reduction (dmsA1), and fermentation (pdhB1, arcA, aceE, pta, acs) did not result in decreased anaerobic growth under the conditions tested. Additional essential components (ferredoxin, cccA) of the anaerobic electron transfer chain and central metabolism (pdhB) were identified. Another surprise was the importance of sodium gradient-dependent membrane processes and genomic rearrangements via viruses, transposons, and insertion sequence elements for anaerobic growth. These processes and the observed contributions of cell envelope restructuring (lysM, mipA, fadK), C4-dicarboxylate transport (dctM1, dctM3), and protease functions to anaerobic growth require further investigation to unravel the novel underlying adaptation strategies.

Renal phosphate handling: Physiology

Phosphorus is a common anion. It plays an important role in energy generation. Renal phosphate handling is regulated by three organs parathyroid, kidney and bone through feedback loops. These counter regulatory loops also regulate intestinal absorption and thus maintain serum phosphorus concentration in physiologic range. The parathyroid hormone, vitamin D, Fibrogenic growth factor 23 (FGF23) and klotho coreceptor are the key regulators of phosphorus balance in body.

Conferring electrogenicity to the electroneutral phosphate cotransporter NaPi-IIc (SLC34A3) reveals an internal cation release step

The SLC34 family of Na(+)-dependent inorganic phosphate cotransporters comprises two electrogenic isoforms (NaPi-IIa, NaPi-IIb) and an electroneutral isoform (NaPi-IIc). Both fulfill essential physiological roles in mammalian phosphate homeostasis. By substitution of three conserved amino acids, found in all electrogenic isoforms, at corresponding sites in NaPi-IIc, electrogenicity was re-established and the Na(+)/P i stoichiometry increased from 2:1 to 3:1. However, this engineered electrogenic construct (AAD-IIc) had a reduced apparent P i affinity and different presteady-state kinetics from the wild-type NaPi-IIa/b. We investigated AAD-IIc using electrophysiology and voltage clamp fluorometry to elucidate the compromised behavior. The activation energy for cotransport was threefold higher than for NaPi-IIc and 1.5-fold higher than for NaPi-IIa and the temperature dependence of presteady-state charge displacements suggested that the large activation energy was associated with the empty carrier reorientation. AAD-IIc shows a weak interaction of external Na(+) ions with the electric field, and thus retains the electroneutral cooperative interaction of two Na(+) ions preceding external P i binding of NaPi-IIc. Most of the presteady-state charge movement was accounted for by the empty carrier (in the absence of external P i ), and the cytosolic release of one Na(+) ion (in the presence of P i ). Simulations using a kinetic model recapitulated the presteady-state and steady-state behavior and allowed identification of two critical partial reactions: the final release of Na(+) to the cytosol and external P i binding. Fluorometric recordings from AAD-IIc mutants with Cys substituted at functionally important sites established that AAD-IIc undergoes substrate- and voltage-dependent conformational changes that correlated qualitatively with its presteady-state kinetics.