Genetic variant in the aquaporin 9 gene is associated with bone mineral density in postmenopausal women

Effects of imatinib and nilotinib on the whole transcriptome of cultured murine osteoblasts

Numerous clinical observations have confirmed that breakpoint cluster region-abelson fusion oncoprotein tyrosine kinase inhibitors used in leukemia treatment alter bone physiology in a complex manner. The aim of the present study was to analyze the whole transcriptome of cultured murine osteoblasts and determine the changes following treatment with imatinib and nilotinib using Sequencing by Oligonucleotide Ligation and Detection next generation RNA sequencing. This study also aimed to identify candidate signaling pathways and network regulators by multivariate Ingenuity Pathway Analysis. Based on the right-tailed Fisher's exact test, significantly altered pathways including upstream regulators were defined for each drug. The correlation between these pathways and bone metabolism was also examined. The preliminary results suggest the two drugs have different mechanisms of action on osteoblasts, and imatinib was shown to have a greater effect on gene expression. Data also indicated the potential role of a number of genes and signaling cascades that may contribute to identifying novel targets for the treatment of metabolic bone diseases.

Bone Marrow Lipid Profiles from Peripheral Skeleton as Potential Biomarkers for Osteoporosis: A 1H-MR Spectroscopy Study

RATIONALE AND OBJECTIVES

To characterize the lipidic profile of bone marrow in the calcaneus and femoral neck of healthy, osteopenic, and osteoporotic women, by using magnetic resonance spectroscopy (MRS) at 3T. The final goal was to identify specific metabolites with the potential ability to discriminate between healthy, osteopenic, and osteoporotic subjects.

MATERIALS AND METHODS

Sixty-two and thirty three postmenopausal women recruited to investigate calcaneus and femoral neck, respectively, underwent a bone mineral density (BMD) measurement to be classified as healthy subjects (n = 22), osteopenic (n = 45), or osteoporotic (n = 28) patients. MRS spectra were used to quantify and compare bone marrow fat resonances between the three BMD groups. Between-group differences were tested using a Welch analysis of variance. Multiple comparisons were made with the Games-Howell correction. Relationships between pairs of variables were assessed with linear correlation analysis. Reproducibility analysis was performed for all the lipid resonances in both sites.

RESULTS

The reproducibility was satisfactory. In femoral neck, methylene (L13), glycerol (L41, L43), and total lipid resonances were significantly lower in healthy as compared to osteoporotic subjects. On the other hand, in calcaneus, L13/glycerol significantly discriminated between osteopenic and osteoporotic subjects whereas L13/(unsaturated lipid) discriminated between healthy and osteopenic group. However, the reproducibility of both unsaturated lipid and glycerol resonances were less optimal.

CONCLUSIONS

MRS of bone marrow lipid profiles from peripheral skeletal sites may be a promising tool for screening of large population to identify individuals with or at risk for developing osteoporosis. Moreover, it provides information about the metabolic changes occurring in bone marrow with the development of osteoporosis, which are skeletal site dependent.

Role of aquaglyceroporins and caveolins in energy and metabolic homeostasis

Aquaglyceroporins and caveolins are submicroscopic integral membrane proteins that are particularly abundant in many mammalian cells. Aquaglyceroporins (AQP3, AQP7, AQP9 and AQP10) encompass a subfamily of aquaporins that allow the movement of water, but also of small solutes, such as glycerol, across cell membranes. Glycerol constitutes an important metabolite as a substrate for de novo synthesis of triacylglycerols and glucose as well as an energy substrate to produce ATP via the mitochondrial oxidative phosphorylation. In this sense, the control of glycerol influx/efflux in metabolic organs by aquaglyceroporins plays a crucial role with the dysregulation of these glycerol channels being associated with metabolic diseases, such as obesity, insulin resistance, non-alcoholic fatty liver disease and cardiac hypertrophy. On the other hand, caveolae have emerged as relevant plasma membrane sensors implicated in a wide range of cellular functions, including endocytosis, apoptosis, cholesterol homeostasis, proliferation and signal transduction. Caveolae-coating proteins, namely caveolins and cavins, can act as scaffolding proteins within caveolae by concentrating signaling molecules involved in free fatty acid and cholesterol uptake, proliferation, insulin signaling or vasorelaxation, among others. The importance of caveolae in whole-body homeostasis is highlighted by the link between homozygous mutations in genes encoding caveolins and cavins with metabolic diseases, such as lipodystrophy, dyslipidemia, muscular dystrophy and insulin resistance in rodents and humans. The present review focuses on the role of aquaglyceroporins and caveolins on lipid and glucose metabolism, insulin secretion and signaling, energy production and cardiovascular homeostasis, outlining their potential relevance in the development and treatment of metabolic diseases.

The evolutionary origin of the vasopressin/V2-type receptor/aquaporin axis and the urine-concentrating mechanism

In this mini-review, current evidence for how the vasopressin/V2-type receptor/aquaporin axis developed co-evolutionary as a crucial part of the urine-concentrating mechanism will be presented. The present-day human kidney, allowing the concentration of urine up to a maximal osmolality around 1200 mosmol kg(-1)-or urine to plasma osmolality ratio around 4-with essentially no sodium secreted is the result of up to 3 billion years evolution. Moving from aquatic to terrestrial habitats required profound changes in kidney morphology, most notable the loops of Henle modifying the kidneys from basically a water excretory system to a water conserving system. Vasopressin-like molecules has during the evolution played a significant role in body fluid homeostasis, more specifically, the osmolality of body liquids by controlling the elimination/reabsorption of fluid trough stimulating V2-type receptors to mobilize aquaporin water channels in the renal collector tubules. Recent evidence supports that all components of the vasopressin/V2-type receptor/aquaporin axis can be traced back to early precursors in evolutionary history. The potential clinical and pharmacological implications of a better phylogenetic understanding of these biological systems so essential for body fluid homeostasis relates to any pathological aspects of the urine-concentrating mechanism, in particular deficiencies of any part of the vasopressin-V2R-AQP2 axis causing central or nephrogenic diabetes insipidus-and for broader patient populations also in preventing and treating disturbances in human circadian regulation of urine volume and osmolality that may lead to enuresis and nocturia.

AQP9: a novel target for bone loss induced by microgravity

The aim of current study was to elucidate whether aquaporin-9 (AQP9) expression was involved in the progression of bone loss induced by microgravity. We used the hind-limb suspension (HLS) mice model to simulate microgravity and induce bone loss. It was found that HLS exposure decreased femur bone mineral density (BMD), and enhanced femur AQP9 mRNA and protein levels. Then, the relationship between AQP9 mRNA expression and BMD was studied and it was showed that femur AQP9 mRNA level was negatively related to femur BMD in mice exposed to HLS. We sought to exam the function of AQP9 in the femur using the AQP9-null mice. It was found that AQP9 knockout attenuated bone loss and inhibited osteoclastogenesis under the condition of HLS exposure, but had no similar effect on bone under normal physiological conditions. In addition, it was found that exposure to simulated hypergravity or exercise training, main countermeasures against microgravity, reduced AQP9 mRNA and protein levels in femur of mice. Moreover, it was found that both aging and estrogen deprivation, another two risk factors of bone loss, had no significant effect on femur AQP9 expression. In conclusion, AQP9 plays an important role in the development of microgravity-induced bone loss, and may be a potential target for the prevention or management of microgravity-induced bone loss.

Aquaglyceroporins serve as metabolic gateways in adiposity and insulin resistance control

Aquaglyceroporins (AQP3, AQP7, AQP9 and AQP10) encompass a subfamily of aquaporins that allow the movement of water and other small solutes, especially glycerol, through cell membranes. Adipose tissue constitutes a major source of glycerol via AQP7. We have recently reported that, in addition to the well-known expression of AQP7 in adipose tissue, AQP3 and AQP9 are also expressed in omental and subcutaneous fat depots. Moreover, insulin and leptin act as regulators of aquaglyceroporins through the PI3K/Akt/mTOR pathway. AQP3 and AQP7 appear to facilitate glycerol efflux from adipose tissue while reducing the glycerol influx into hepatocytes via AQP9 to prevent the excessive lipid accumulation and the subsequent aggravation of hyperglycemia in human obesity. This Extra View focuses on the control of glycerol release by aquaglyceroporins in the adipose tissue and briefly discusses the importance of glycerol as a substrate for hepatic gluconeogenesis, pancreatic insulin secretion and cardiac ATP production.