Current Pharmaceutical Biotechnology - Volume 10, Issue 3, 2009

Being one of the most abundant cations in nature, calcium plays crucial roles both outside and inside the cells as a signaling ion. Changes in the extracellular free calcium concentration [Ca2+ o] activate the plasmalemmal calcium-sensing receptor (CaSR or CaR), a sevenfold transmembrane spanning protein encoded by genes mapped in human chromosome 3 (and also 19). The CaSR is activated by various ligands, such as divalent and polyvalent cations, aromatic L-amino acids, aminoglycosides, spermine, and amyloid β (Aβ) peptides. A member of family C (or III) of G-protein coupled receptors (GPCRs), acting in the form of a disulfide-linked homodimer, the CaSR signals through both heterotrimeric and monomeric G-proteins, thereby inhibiting adenylate cyclase activity, but increasing the production of diacylglycerol (DAG) and inosiltol-1,4,5-triphosphate (IP3), mobilizing intracellular calcium, activating diverse protein kinases (PKB, PKCs, MAPKs), phospholipases (A2, C, D) and cyclooxygenase-2 (COX-2), and transactivating the EGF receptor (EGFR). CaSR signaling controls mineral ion homeostasis via parathyroid hormone (PTH) and calcitonin (CT) secretion and, in addition, modulates a variety of vital processes including chemotaxis, cell proliferation, differentiation, and malignant transformation, membrane excitability, cell survival, and programmed death (apoptosis). Notably, the expression of CaSR mRNA is widespread, occurring not only in the thyroid and the parathyroid glands (where it was first discovered), but even in kidney, bone, cardiovascular system, liver, gastrointestinal tract, and central nervous system (CNS). This broad expression implies multiple relevant functional roles for the CaSR in the same tissues and organs, the elucidation of which is still ongoing, being somewhat hampered by the facts that (i) not all of the CaSR-expressing cell types (e.g. in the CNS) have hitherto been identified with certainty, and (ii) in each cell-type considered the CaSR-evoked responses to [Ca2+ o] changes exhibit a discrete degree of specificity. It goes without saying that, given this background, in pathologic conditions the CaSR also plays relevant roles, most of which however wait to be clarified. Besides hereditary conditions due to loss-of-function or gain-of-function mutations and polymorphisms of the CaSR, several pathophysiological conditions associated with local and/or general reductions in the [Ca2+ o], oxidative stress and/or extracellular acidification appear to elicit significant changes in CaSR signaling. Therefore, a panoply of agents targeting the CaSR, both calcimimetics (or calcium agonists) and calcilytics (or calcium antagonists), is being developed and tested in specific clinical settings. Accordingly, the CaSR has now fittingly become an exciting topic of the impetuously growing Translational Medicine. This has led to the proposal of collecting in the present issue the reviews from several research groups with the aim of bringing up to date, from different viewpoints, novel facets of the several CaSR roles and the known or possible benefits of the CaSR-targeting drugs concerning the pathophysiology of the the cardiovascular system, bone, kidney, colon, and CNS. The first review of this series, contributed by Drs. Brennan and Conigrave, enlightens the quite intricate CaSR's signaling cascades, which change membrane lipids metabolism, several protein kinases’ activities and their substrates' phosphorylation, and intracellular cAMP, calcium ions and fatty acids levels via G-proteins’ activations. Thus, consistent with its expression site(s) and local signaling pathways, CaSR responses to changes in [Ca2+ o] modulate vital cell activities including, amongst others, peptide hormone secretion, vitamin D metabolism, and ion and water transport. This updated information also underscores the need of further studies to better clarify the intricate yet enticing CaSR signaling mechanisms.

The extracellular calcium-sensing receptor (CaR) is a class III G-protein coupled receptor that coordinates cellular responses to changes in extracellular free Ca2+ or amino acid concentrations as well as ionic strength and pH. It regulates signalling cascades via recruiting and controlling the activities of various heterotrimeric G-proteins, including Gq/11, Gi/0, and G12/13, even Gs in some “unusual” circumstances, thereby inducing changes in the metabolism of membrane lipids, the phosphorylation state of protein kinases and their targets, the activation state of monomeric G-proteins and the levels of intracellular second messengers including cAMP, Ca2+ ions, fatty acids and other small molecules. According to its site(s) of expression and available signalling pathways, the CaR modulates cell proliferation and survival, differentiation, peptide hormone secretion, ion and water transport and various other processes. In this article we consider the complex intracellular mechanisms by which the CaR elicits its cellular functions. We also consider some of the better understood CaR-regulated cell functions and the nature of the signalling mechanisms that support them.

The calcium-sensing receptor (CaSR), which is involved in systemic calcium homeostasis, has also been found to be functionally expressed on cells of the vascular wall. Its activation on perivascular nerves and endothelial cells has been shown to regulate arterial tone, peripheral vascular resistance and possibly local tissue perfusion. The expression of the CaSR on immune cells involved in vascular inflammation, such as macrophages, and its increased expression in inflammation indicates the central role extracellular calcium plays in vascular inflammation and repair. Further detailed analysis will clarify the role the vascular CaSR plays as a therapeutic target for complex disease conditions such as hypertension, tissue hypoperfusion, atherosclerosis and vascular calcification.

Bone cells, particularly osteoblasts and osteoclasts, exhibit functional responses to calcium (Ca2+). The identification of the calcium-sensing receptor (CaR) in parathyroid glands as the master regulator of parathyroid hormone (PTH) secretion proved that cells could specifically respond to changes in divalent cation concentration. Yet, after many years of study, it remains unclear whether this receptor, which has also been identified in bone, has functional import there. Various knockout and transgenic mouse models have been developed, but conclusions about skeletal phenotypes remain elusive. Complex endocrine feedback loops involving calcium, phosphorus, vitamin D, and PTH confound efforts to isolate the effects of a single mineral, hormone, or receptor and most models fail to account for other local factors such as parathyroid hormone related protein (PTHrP). We review the relevant mouse models and discuss the importance of CaR in chondrogenesis and osteogenesis. We present the evidence for a non-redundant role for CaR in skeletal mineralization, including our experience in patients with activating CaR mutations. Additionally, we review emerging research on the importance of the CaR to the regulation of serum calcium homeostasis independent of PTH, the role of the CaR in the hematopoietic stem cell niche with implications for bone marrow transplant, and early evidence that implies a role for the CaR as a factor in skeletal metastasis from breast and prostate cancer. We conclude with a discussion of drugs that target the CaR directly either as agonists (calcimimetics) or antagonists (calcilytics), and the consequences for bone physiology and pathology.

Calcium-sensing receptor (CaSR), a member of family C of the G protein-coupled receptors, is expressed most abundantly in the parathyroid glands and kidney. It plays key role in these two organs because it senses changes in extracellular calcium and regulates PTH secretion and calcium reabsorption to suit the extracellular calcium concentration. In kidney, CaSR is expressed in all nephron segments. It has an inhibitory effect on the reabsorption of calcium, potassium, sodium and water, depending on the particular function of the different tubular tracts. Among its inhibitory effects, CaSR modulates the signaling pathways used by the tubulocytes to activate electrolyte or water reabsorption. The only site where there is no such inhibitory effect is in the proximal tubule, where CaSR enhances phosphate reabsorption to counteract the effect of PTH. CaSR mutations and polymorphisms cause disorders characterized by alterations in renal excretion and serum calcium concentrations. They also can cause sodium and potassium excretion disorders. CaSR also mediates the acute adverse renal effects of hypercalcemia, which include a reduced sodium, potassium and water reabsorption. From a teleological perspective, CaSR seems to protect human tissues against calcium excess in extracellular fluids.

Dietary Ca2+ reduces colon cell proliferation and carcinogenesis, but it becomes ineffective or even tumorpromoting during carcinogenesis. It appears that Ca2+ and the colon cell CaSR together brake the massive cell production in normal colon crypts. The rapid proliferation of the transit-amplifying (TA) progeny of the colon stem cells at the bases of the crypts is driven by the “Wnt” signaling mechanism that stimulates proliferogenic genes and prevents apoptogenesis. It appears that TA cell cycling stops and terminal differentiation starts when the cells reach a higher level in the crypt where there is enough external Ca2+ to stimulate the expression of CaSRs, the signals from which stimulate the expression of E-cadherin. At this point the APC (adenomatous polyposis coli) protein appears and some of it enters the nucleus. There it removes the apoptogenesis shield and stops the β-catenin•Tcf-4 complex from driving further TA cell proliferation by releasing β-catenin from the nucleus, and delivering it to cytoplasmic APC•axin•GSK-3β complexes for ultimate proteasomal destruction. Cytoplasmic β-catenin is prevented from returning to the nucleus by destruction in APC•axin•GSK-3β complexes or locked by the emerging E-cadherin into adherens junctions which link the cell to proliferatively shut-down functioning cells with APC-dependent cytoskeletons moving up and out of the crypt. A common first step in colon carcinogenesis is the loss of functional APC which results in the retention of proliferogenic nuclear β- catenin•Tcf-4. This drives the eventual appearance of mutation accumulating, apoptosis-resistant clones the proliferation of which cannot be inhibited by external Ca2+ because of CaSR-disabling gene mutations.

Although the calcium-sensing receptor (CaSR) is expressed by all types of nerve cells in widespread areas of the human central nervous system (CNS), so far its roles in brain pathophysiology remain largely unknown. Here, we review the available evidence concerning the stages of development of sporadic late-onset Alzheimer's disease (LOAD) and the roles therein played by CaSR signaling. As the brain ages, its ability to dispose of dangerous synapse-targeting soluble amyloid β-(1-42) (sAβ42) oligomers released from normal neuronal activity declines. As their levels slowly rise, these oligomers increasingly target and eliminate synapses and prevent synapse formation, thereby eroding the foundations of memory formation and cognitive functions. In this initial stage, neurons, even though synaptically impaired, remain alive. Concurrently, sAβ42 oligomers by binding to CaSR on human astrocytes induce via mitogen activated protein kinase (MAPK) activity the release of huge amounts nitric oxide (NO), which by itself and after conversion to peroxynitrite (ONOO-) damages neighboring neurons. When the sA&bgr42 oligomers increasingly aggregate into fibrillar plaques, they attract and activate microglial macrophages that, while trying to clear the plaques, produce via A&bgr-activated CaSR signaling several proinflammatory cytokines and reactive oxygen species (ROS). Notably, the microglial cytokines, like sA&bgr42 oligomers, induce human astrocytes to make large amounts of NO and hence ONOO- via CaSR signal-dependent MAPK activity. The microglial cytokines-activated astrocytes might also produce their own sA&bgr42, which would combine with neuron- and microglia-released sAβ42 to increase the fibrillar burden and promote the further production of reactive oxygen species (ROS), NO/ONOO-, and proinflammatory cytokines to efficiently kill both normal and functionally impaired (undead) neurons. But, on a somewhat positive note, we speculate that the astrocytes' CaSR-stimulated MAPK activities might also induce vascular endothelial growth factor (VEGF) expression and production. This might in turn enhance neuronal stem cells neurogenesis at least in the subgranular zone (SGZ) of the hippocampal dentate gyrus.

In recent years, human umbilical cord blood (HUCB) has emerged as an attractive tool for cell-based therapy. Although at present the clinical application of human umbilical cord blood is limited to the fields of hematology and oncology, a rising number of studies show potential for further application in the treatment of non-hematopoietic diseases. Stem cells (SC) from umbilical cord blood (UCB) are now a new reliable alternative to treat different blood diseases, if the samples are frozen at the moment of birth. This procedure is an easy and safe way to preserve genetic materials for future therapeutic uses. It can be used as alternative to bone marrow. Human umbilical cord blood, with its real abundance, simple collection procedure and no serious ethical dilemmas, represents a valuable alternative to the use of other stem cell sources. The aim of this article is to review the literature on human umbilical cord blood (HUCB) and to assess its eventual usability in the treatment of diseases.

“Niche” is defined as a specialized regulatory microenvironment, consisting of components which control the fate specification of stem and progenitor cells, as well as maintaining their development by supplying the requisite factors. Bone marrow (BM) niche has a well-organized architecture and is composed of osteoblasts, osteoclasts, bone marrow endothelial cells, stromal cells, adipocytes and extracellular matrix proteins (ECM). These elements play an essential role in the survival, growth and differentiation of diverse lineages of blood cells, but also provide optimal growth environment for multiple hematological malignancies including multiple myeloma (MM). MM is a neoplastic plasma cell disorder which not only resides in BM but also converts it into specialized neoplastic niche. This niche aids the growth and spreading of tumor cells by a complex interplay of cytokines, chemokines, proteolytic enzymes and adhesion molecules. Moreover, the MM BM microenvironment was shown to confer survival and chemoresistance of MM cells to current therapies. However, our knowledge in this field is still in infancy and many details are unknown. Therefore, there is a strong need to further dissect the MM BM niche and understand the process of how the complex interactions with BM milieu influence MM growth, survival and development of resistance to chemotherapy. A better and more detailed understanding of neoplastic MM niche will provide a guiding model for identifying and validating novel targeted therapies directed against MM. Therefore, in the present review, we have focused principally on the basic features, physical structures, and functions of the BM niche and have highlighted its interaction with MM cells.