Molecular Biology

Extracellular Vesicles: How to Shuttle the Metastatic Programme Abstract Purpose of Review This review has the meaning of providing a state-of-the-art in the role of tumour-derived EVs in educating the host microenvironment during the metastatic process. Recent Findings Extracellular vesicles (EVs) now represent another piece in the complex puzzle that is tumorigenesis and metastasis. The indication that EVs are more than just a way for cells to dispose of waste and actually work as active and dynamic structures packaging molecular signals arose in the late '60s, when EV-like structures were involved in the function of coagulation. Since then, a huge amount of information has been collected, and we are now aware that EVs are crucially involved in paracrine and distant cell-cell communication under physiologic and pathologic conditions, such as cancer metastasis to bone. Summary We will focus on the EV-mediated mechanisms regulating bone homeostasis, and we will describe the way these mechanisms are dysregulated by osteotropic cancer cell-derived EVs.

Interplay Between FGF23, Phosphate, and Molecules Involved in Phosphate Sensing Abstract Purpose of Review Despite the important progress made in understanding the regulation of phosphate (Pi) homeostasis over the past 20 years, the mechanisms underlying the very early step leading to the regulating cascade involving multiple hormones (PTH, vitamin D, FGF23) and organs (kidney, intestine, bone, parathyroid glands) are not deciphered. Particularly, knowledge on the Pi-sensing mechanism present within or on the surface of the cell that is able to detect changes in serum or local Pi concentrations and trigger an appropriate FGF23 synthesis/secretion is limited or absent. Recent Findings Several molecular actors have recently been involved as potential key players in Pi sensing and Pi-dependent control of FGF23 secretion. Among them, the PiT1/Slc20a1 and PiT2/Slc20a2 proteins are standing out. Summary We are just beginning to accumulate in vitro and in vivo data that will provide invaluable molecular tools to explore and understand the integrated response of the body to variations of Pi concentration.

Novel and Conventional Preclinical Models to Investigate Bone Metastasis Abstract Purpose of Review The purpose of this review is to emphasize the use of bone metastasis models in preclinical research. As classical models have been thoroughly discussed in recent reviews, we here highlight the most important aspects from these papers with a special focus on novel models developed during the past 5 years. Recent Findings Preclinical mouse models to study bone metastasis can be divided by cancer cell inoculation techniques (spontaneous, systemic, or local) or by immunological background of the mice (immunodeficient, syngeneic, or humanized). Additionally, novel computational, in vitro co-culture, and humanized bone models have been established. Summary Various models can be used to approach distinct research questions. Understanding limitations of the models is essential when planning a study and interpreting the results. Development of novel models will increase understanding of the complex biology and advance the discovery of new therapies targeting bone metastases.

Molecular Control of Growth-Related Sodium-Phosphate Co-transporter (SLC34A3) Abstract The type IIa sodium-dependent inorganic phosphate transporter (NaPi-IIa) has a central regulatory role in inorganic phosphate (Pi) homeostasis. Many studies have reported on the functions and regulatory mechanisms of NaPi-IIa. NaPi-IIc, however, was initially identified as a NaPi transporter required for growth in rodents. The gene encoding NaPi-IIc is causative for hereditary hypophosphatemic rickets with hypercalciuria and considered to be a critical NaPi transporter in the human kidney. However, the physiological roles and regulatory mechanisms of NaPi-IIc are not sufficiently elucidated. Recent studies show that NaPi-IIc is tightly regulated by a variety of agonists and physiological conditions via partially defined molecular mechanisms, including transcriptional and posttranscriptional regulation, protein phosphorylation, trafficking (endocytosis, exocytosis, and recycling), and the association of NaPi-IIc with interacting protein complexes. These data provide further information about understanding of human renal Pi handling. Here, we review recent findings regarding the molecular control of NaPi-IIc transporters.

Animal Models of Phosphorus Homeostasis Abstract Purpose of Review Phosphate homeostasis is a complex process that involves many regulators and multiple organs. In vivo models have been used extensively to study the pathophysiological mechanisms of phosphate disorders. This review focuses on evaluating mouse models generated for the study of disorders of phosphate metabolism. Recent Findings Over the years, several mouse models have been generated by strategies that knockin or knockout one or more genes that encode for the phosphate transporters or other regulatory factors that directly or indirectly influence phosphate homeostasis. These models have shed light on the pathways involved in phosphate metabolism and the mechanisms that lead to phosphate dysregulation in human diseases. Summary Animal models are essential tools to study multisystem disorders that affect multiple organs and cell types. In particular, mouse models generated by a variety of genetic approaches have become the preferred mammalian models to study human diseases. Mouse models of phosphate homeostasis have provided valuable insights and enhanced our understanding of the cross talk between bone, kidney, and intestine and the relationships between the key phosphate regulators FGF23, 1,25(OH)2-vitamin D3, and PTH.

MicroRNAs Are Critical Regulators of Osteoclast Differentiation Abstract Purpose of Review Our goal is to comprehensively review the most recent reports of microRNA (miRNA) regulation of osteoclastogenesis. We highlight validated miRNA-target interactions and their place in the signaling networks controlling osteoclast differentiation and function. Recent Findings Using unbiased approaches to identify miRNAs of interest and reporter-3′UTR assays to validate interactions, recent studies have elucidated the impact of specific miRNA-mRNA interactions during in vitro osteoclastogenesis. There has been a focus on signaling mediators downstream of the RANK and CSF1R signaling, and genes essential for differentiation and function. For example, several miRNAs directly and indirectly target the master osteoclast transcription factor, Nfatc1 (e.g., miR-124 and miR-214) and Rho-GTPases, Cdc42, and Rac1 (e.g., miR-29 family). Summary Validating miRNA expression patterns, targets, and impact in osteoclasts and other skeletal cells is critical for understanding basic bone biology and for fulfilling the therapeutic potential of miRNA-based strategies in the treatment bone diseases.

FGF23 Synthesis and Activity Abstract Purpose of Review The phosphaturic hormone FGF23 is produced primarily in osteoblasts/osteocytes and is known to respond to increases in serum phosphate and 1,25(OH)2 vitamin D (1,25D). Novel regulators of FGF23 were recently identified and may help explain the pathophysiologies of several diseases. This review will focus on recent studies examining the synthesis and actions of FGF23. Recent Findings The synthesis of FGF23 in response to 1,25D is similar to other steroid hormone targets, but the cellular responses to phosphate remain largely unknown. The activity of intracellular processing genes control FGF23 glycosylation and phosphorylation, providing critical functions in determining the serum levels of bioactive FGF23. The actions of FGF23 largely occur through its co-receptor αKlotho (KL) under normal circumstances, but FGF23 has KL-independent activity during situations of high concentrations. Summary Recent work regarding FGF23 synthesis and bioactivity, as well as considerations for diseases of altered phosphate balance, will be reviewed.

MICROmanagement of Runx2 Function in Skeletal Cells Abstract Purpose of Review Precise and temporal expression of Runx2 and its regulatory transcriptional network is a key determinant for the intricate cellular and developmental processes in adult bone tissue formation. This review analyzes how microRNA functions to regulate this network, and how dysregulation results in bone disorders. Recent Findings Similar to other biologic processes, microRNA (miRNA/miR) regulation is undeniably indispensable to bone synthesis and maintenance. There exists a miRNA–RUNX2 network where RUNX2 regulates the transcription of miRs or is post-transcriptionally regulated by a class of miRs, forming a variety of miR-RUNX2 regulatory pathways which regulate osteogenesis. Summary The current review provides insights to understand transcriptional–post-transcriptional regulatory network governed by Runx2 and osteogenic miRs, and is based largely from in vitro and in vivo studies. When taken together, this article discusses a new regulatory layer of bone tissue-specific gene expression by RUNX2 influenced via miRNA.

Regulation of Fibroblast Growth Factor 23 by Iron, EPO, and HIF Abstract Purpose of Review Fibroblast growth factor-23 (FGF23) is the key hormone produced in bone critical for phosphate homeostasis. Elevated serum phosphorus and 1,25-dihydroxyvitamin D stimulates FGF23 production to promote renal phosphate excretion and decrease 1,25-dihydroxyvitamin D synthesis, thus completing the feedback loop and suppressing FGF23. Unexpectedly, studies of common and rare heritable disorders of phosphate handling identified links between iron and FGF23 demonstrating novel regulation outside the phosphate pathway. Recent Findings Iron deficiency combined with an FGF23 cleavage mutation was found to induce the autosomal dominant hypophosphatemic rickets phenotype. Physiological responses to iron deficiency, such as erythropoietin production as well as hypoxia inducible factor activation, have been indicated in regulating FGF23. Additionally, specific iron formulations, used to treat iron deficiency, alter post-translational processing thereby shifting FGF23 protein secretion. Summary Molecular and clinical studies revealed that iron deficiency, through several mechanisms, alters FGF23 at the transcriptional and post-translational level. This review will focus upon the novel discoveries elucidated between iron, its regulators, and their influence on FGF23 bioactivity.

Epigenetics of Multiple Myeloma Bone Disease Abstract Purpose of Review Multiple myeloma bone disease (MMDB) is a devastating clinical manifestation of multiple myeloma associated with excessive bone osteolysis, which results from enhanced osteoclastogenesis and suppression of bone marrow stromal cell (BMSC) differentiation into osteoblasts. Impaired osteogenesis and functional alterations of myeloma-exposed BMSCs (MM-BMSCs) during the course of disease evolution significantly contribute to myeloma growth, metastasis, and chemoresistance. This review highlights new studies demonstrating that epigenetic modalities including chromatin-mediated gene silencing and non-coding RNA contribute to pathogenesis of MM-BMSCs. Recent Findings Inhibitors targeting histone-modifying enzymes EZH2, JMJD3, HDACs, and BET proteins have been successfully used to revert osteogenic suppression of MM-BMSCs. Aberrant expression of non-coding RNA cause functional changes associated with senescence, osteogenic suppression, and tumor-promoting phenotype of MM-BMSCs. Summary Epigenetic events guiding transformations of the surrounding BMSC compartments are ultimately linked to disease onset and progression and open new therapeutic opportunity to target dissemination of MM tumors and reliably repair bone lesions.