Inhibition of the calcium-sensing receptor by extracellular phosphate ions and by intracellular phosphorylation

The Highly Selective Discovery of Allosteric Ligands from Herbal Extracts through Their Direct Interactions with the Immobilized Headless CaSR Truncation

Allosteric modulators represent a novel paradigm to therapeutically target G-protein-coupled receptors (GPCRs), but the discovery of novel allosteric ligands from natural products remains challenging. Here, we developed a high-performance affinity chromatography method for screening allosteric ligands toward the human calcium-sensing receptor (CaSR) by immobilizing an extracellular domain-deleted CaSR truncation (ΔCaSR) onto silica gels as solid-phase materials for column packing. The immobilized ΔCaSR column demonstrated the greatest allosteric responsive feature when cinacalcet at 0.50 μM or NPS2143 at 0.25 μM was included in the mobile phase, suggesting that the binding affinity of Ca2+ was increased 8% by cinacalcet and was decreased 77% by NPS2143. The column was applied to screen allosteric ligands from Epimedii Folium, which were identified as epimedin B, epimedin C, and icariin using HPLC-MS. The allosteric binding of the screened compounds was testified through competitive experiments, and their allosteric effects were verified by CaSR downstream signaling events, like the intracellular Ca2+ levels and cAMP production. Our observations indicated that the three compounds exerted an allosteric effect similar to that of cinacalcet and might be potential allosteric ligands. The proposed approach features the immobilization of headless GPCR truncations, in which the transmembrane domain is exposed to interact directly with the ligands, realizing the highly selective discovery of allosteric ligands from complex herbal extracts.

Role of the calcium-sensing receptor in regulating vascular function

Functional expression of the calcium-sensing receptor (CaSR) in calcitropic tissues, for example, parathyroid glands and kidneys, is important for maintaining Ca2+ homeostasis. It is also established that the CaSR is present in tissues previously thought to be noncalcitropic and this review discusses the role of the CaSR in vascular function, focusing mainly on contractility but also outlining its role in cell proliferation and calcification. Stimulation of the CaSR by extracellular Ca2+ concentration ([Ca2+]o) on perivascular sensory nerves and vascular endothelial cells is associated with vasodilatation through the release of vasoactive substances and stimulation of IKCa channels and nitric oxide synthesis, respectively, which mediate endothelium-derived hyperpolarizations and activation of BKCa channels and KATP channels in vascular smooth muscle cells (VSMCs). CaSR-induced vasoconstrictions are mediated by the CaSR expressed in VSMCs, which are coupled to the Gq/11 protein-coupled pathway. In addition, the CaSR expressed on VSMCs also regulates proliferation and calcification. Consequently, the CaSR has been implicated in regulating systemic and pulmonary blood pressure and calcimimetics and calcilytics are potential therapeutic targets for cardiovascular diseases, such as hypertension, pulmonary artery hypertension, and atherosclerosis.

Metformin and vitamin D combination therapy ameliorates type 2 diabetes mellitus-induced renal injury in male Wistar rats

Diabetic kidney disease is a major microvascular diabetes mellitus (DM) complication clinically associated with a gradual renal function decline. Although metformin is a common drug for managing DM, however, monotherapy treatment with any antidiabetic drug will necessitate dosage increment since type 2 DM (T2DM) deteriorates over time due to the increasing pancreatic β-cell dysfunction and will eventually require a combination therapy approach with another antidiabetic medication. Vitamin D is a food supplement that has been proven to have antidiabetic and reno-protective activities. Hence, we explore the combination of vitamin D and metformin on T2DM-induced renal dysfunction. Thirty male Wistar rats were randomized into five (5) groups: control, diabetes untreated, diabetics treated with metformin, vitamin D, and vitamin D + metformin. Vitamin D and metformin significantly reversed DM-induced hyperglycemia, electrolyte imbalance, and dyslipidemia. Also, vitamin D and metformin reversed T2DM-induced increase in serum creatinine and urea and renal lactate, LDH, and oxido-inflammatory response. These observed alterations were accompanied by an increase in proton pump activities and modulation of Nrf2/Nf-κB and XO/UA signaling. This study revealed that vitamin D and/or metformin ameliorated T2DM-induced renal injury.

Fibroblast growth factor 23 - A review with particular reference to the physiology and pathophysiology of phosphate homeostasis in the cat

Fibroblast growth factor-23 (FGF23) is a phosphaturic hormone, discovery of which has transformed our understanding of mineral regulation in healthy mammals, including the cat. It is produced by osteoblasts and osteocytes and its prime role is to regulate phosphate entry into extracellular fluid (from bone and via the gut) and its excretion via the kidney. It interacts with other hormones (calcitriol and parathyroid hormone), inhibiting their activation and secretion respectively and so impacts on calcium as well as phosphate homeostasis. Physiological factors regulating its secretion are not well understood, although phosphate ion sensing is likely to be important. Calcium and magnesium ions are also involved and unravelling the control points and integration of the system regulating bone turnover and mineral balance whilst preventing soft tissue (non-osseous) mineralisation is a future research goal. Calciprotein particle size and number likely play an important role in this system but precisely how remains to be determined. Elevated serum FGF23 is the earliest indicator of mineral bone disorder associated with chronic kidney disease in human patients and in cats, enabling reference-range serum phosphorus to be maintained despite reduction in glomerular filtration rate which limits phosphate excretion. FGF23 also predicts CKD progression and survival in cats. The many factors influencing its secretion at different stages of CKD, including relative iron deficiency, anaemia and chronic systemic inflammation, hypomagnesaemia and α-klotho deficiency are discussed in this review, where the data available in cats with naturally occurring CKD is presented alongside that from rodent models and human CKD patients.

Persistent uncertainties in optimal treatment approaches of secondary hyperparathyroidism and hyperphosphatemia in patients with chronic kidney disease

PURPOSE OF REVIEW

This review is a critical analysis of treatment results obtained in clinical trials conducted in patients with chronic kidney disease (CKD) and secondary hyperparathyroidism (SHPT), hyperphosphatemia, or both.

RECENT FINDINGS

Patients with CKD have a high mortality rate. The disorder of mineral and bone metabolism (CKD-MBD), which is commonly present in these patients, is associated with adverse outcomes, including cardiovascular events and mortality. Clinical trials aimed at improving these outcomes by modifying CKD-MBD associated factors have most often resulted in disappointing results. The complexity of CKD-MBD, where many players are closely interconnected, might explain these negative findings. We first present an historical perspective of current knowledge in the field of CKD-MBD and then examine potential flaws of past and ongoing clinical trials targeting SHPT and hyperphosphatemia respectively in patients with CKD.

Molecular regulation of calcium-sensing receptor (CaSR)-mediated signaling

Calcium-sensing receptor (CaSR), a family C G-protein-coupled receptor, plays a crucial role in regulating calcium homeostasis by sensing small concentration changes of extracellular Ca2+, Mg2+, amino acids (e.g., L-Trp and L-Phe), small peptides, anions (e.g., HCO3 - and PO4 3-), and pH. CaSR-mediated intracellular Ca2+ signaling regulates a diverse set of cellular processes including gene transcription, cell proliferation, differentiation, apoptosis, muscle contraction, and neuronal transmission. Dysfunction of CaSR with mutations results in diseases such as autosomal dominant hypocalcemia, familial hypocalciuric hypercalcemia, and neonatal severe hyperparathyroidism. CaSR also influences calciotropic disorders, such as osteoporosis, and noncalciotropic disorders, such as cancer, Alzheimer's disease, and pulmonary arterial hypertension. This study first reviews recent advances in biochemical and structural determination of the framework of CaSR and its interaction sites with natural ligands, as well as exogenous positive allosteric modulators and negative allosteric modulators. The establishment of the first CaSR protein-protein interactome network revealed 94 novel players involved in protein processing in endoplasmic reticulum, trafficking, cell surface expression, endocytosis, degradation, and signaling pathways. The roles of these proteins in Ca2+-dependent cellular physiological processes and in CaSR-dependent cellular signaling provide new insights into the molecular basis of diseases caused by CaSR mutations and dysregulated CaSR activity caused by its protein interactors and facilitate the design of therapeutic agents that target CaSR and other family C G-protein-coupled receptors.

Inhibition of calcium-sensing receptor by its antagonist promotes gastrointestinal motility in a Parkinson's disease mouse model

BACKGROUND

The Calcium-sensing receptor (CaSR) participates in the regulation of gastrointestinal (GI) motility under normal conditions and might be involved in the regulation of GI dysmotility in patients with Parkinson's disease (PD).

METHODS

CaSR antagonist-NPS-2143 was applied in in vivo and ex vivo experiments to study the effect and underlying mechanisms of CaSR inhibition on GI dysmotility in the MPTP-induced PD mouse model.

FINDINGS

Oral intake of NPS-2143 promoted GI motility in PD mice as shown by the increased gastric emptying rate and shortened whole gut transit time together with improved weight and water content in the feces of PD mice, and the lack of influence on normal mice. Meanwhile, the number of cholinergic neurons, the proportion of serotonergic neurons, as well as the levels of acetylcholine and serotonin increased, but the numbers of nitrergic and tyrosine hydroxylase immunoreactive neurons, and the levels of nitric oxide synthase and dopamine decreased in the myenteric plexus in the gastric antrum and colon of PD mice in response to NPS-2143 treatment. Furthermore, the numbers of c-fos positive neurons in the nucleus tractus solitarius (NTS) and cholinergic neurons in the dorsal motor nucleus of the vagus (DMV) increased in NPS-2143 treated PD mice, suggesting the involvement of both the enteric (ENS) and central (CNS) nervous systems. However, ex vivo results showed that NPS-2143 directly inhibited the contractility of antral and colonic strips in PD mice via a non-ENS mediated mechanism. Further studies revealed that NPS-2143 directly inhibited the voltage gated Ca2+ channels, which might, at least in part, explain its direct inhibitory effects on the GI muscle strips.

INTERPRETATION

CaSR inhibition by its antagonist ameliorated GI dysmotility in PD mice via coordinated neuronal regulation by both ENS and CNS in vivo, although the direct effects of CaSR inhibition on GI muscle strips were suppressive.

Contribution of membrane-associated oscillators to biological timing at different timescales

Environmental rhythms such as the daily light-dark cycle selected for endogenous clocks. These clocks predict regular environmental changes and provide the basis for well-timed adaptive homeostasis in physiology and behavior of organisms. Endogenous clocks are oscillators that are based on positive feedforward and negative feedback loops. They generate stable rhythms even under constant conditions. Since even weak interactions between oscillators allow for autonomous synchronization, coupling/synchronization of oscillators provides the basis of self-organized physiological timing. Amongst the most thoroughly researched clocks are the endogenous circadian clock neurons in mammals and insects. They comprise nuclear clockworks of transcriptional/translational feedback loops (TTFL) that generate ∼24 h rhythms in clock gene expression entrained to the environmental day-night cycle. It is generally assumed that this TTFL clockwork drives all circadian oscillations within and between clock cells, being the basis of any circadian rhythm in physiology and behavior of organisms. Instead of the current gene-based hierarchical clock model we provide here a systems view of timing. We suggest that a coupled system of autonomous TTFL and posttranslational feedback loop (PTFL) oscillators/clocks that run at multiple timescales governs adaptive, dynamic homeostasis of physiology and behavior. We focus on mammalian and insect neurons as endogenous oscillators at multiple timescales. We suggest that neuronal plasma membrane-associated signalosomes constitute specific autonomous PTFL clocks that generate localized but interlinked oscillations of membrane potential and intracellular messengers with specific endogenous frequencies. In each clock neuron multiscale interactions of TTFL and PTFL oscillators/clocks form a temporally structured oscillatory network with a common complex frequency-band comprising superimposed multiscale oscillations. Coupling between oscillator/clock neurons provides the next level of complexity of an oscillatory network. This systemic dynamic network of molecular and cellular oscillators/clocks is suggested to form the basis of any physiological homeostasis that cycles through dynamic homeostatic setpoints with a characteristic frequency-band as hallmark. We propose that mechanisms of homeostatic plasticity maintain the stability of these dynamic setpoints, whereas Hebbian plasticity enables switching between setpoints via coupling factors, like biogenic amines and/or neuropeptides. They reprogram the network to a new common frequency, a new dynamic setpoint. Our novel hypothesis is up for experimental challenge.