Phosphoinositide regulation of TRPM channels - TRPM3 joins the club!

Functional Upregulation of TRPM3 Channels Contributes to Acute Pancreatitis-associated Pain and Inflammation

Transient receptor potential melastatin M3 (TRPM3) channels have been recognized as a pain transducer in dorsal root ganglion (DRG) neurons in recent years. TRPM3 activation initiates neurogenic inflammation and is required for the development of inflammatory hyperalgesia. We aimed to evaluate the role of TRPM3 in pancreas sensory afferents in pancreatic nociception, neurogenic inflammation, and acute pancreatitis (AP)-associated pain. AP was induced by intraperitoneal (i.p.) injection of L-arginine in rats. TRPM3 expression in pancreatic DRG neurons, spontaneous or mechanical-stimulation-evoked pain behaviors, and the extent of inflammation were evaluated. We found that TRPM3 channels were expressed on pancreatic primary afferent nerve terminals containing calcitonin gene-related peptide (CGRP). Activation of TRPM3 in the pancreas by injection of its specific agonist CIM0216 (10 μM) induced pain, CGRP and substance P release, and neurogenic inflammation, as evidenced by edema, plasma extravasation, and inflammatory cell accumulation in the pancreas. Increased TRPM3 functional expression was detected in pancreatic DRG neurons from AP rats, and blocking TRPM3 activity with its antagonist (Primidone, 5 mg/kg, i.p.) attenuated AP-associated pain behaviors and pancreatic inflammation. Pre-incubation of pancreatic DRG neurons with nerve growth factor (NGF) enhanced the increase in intracellular Ca2+ induced by the TRPM3 agonist (CIM0216, 1 μM). Our findings indicate that, in addition to TRPV1 and TRPA1 channels, TRPM3 is another pain channel that has a critical role in pancreatic nociception, neurogenic inflammation, and AP-associated pain behaviors. TRPM3 may be a promising pharmaceutical target for AP pain treatment.

TRPM3-mediated dynamic mitochondrial activity in NGF-induced latent sensitization of chronic low back pain

ABSTRACT

The transient receptor potential ion channel TRPM3 is highly prevalent on nociceptive dorsal root ganglion (DRG) neurons, but its functions in neuronal plasticity of chronic pain remain obscure. In an animal model of nonspecific low back pain (LBP), latent spinal sensitization known as nociceptive priming is induced by nerve growth factor (NGF) injection. Here we address the TRPM3-associated molecular basis of NGF-induced latent spinal sensitization at presynaptic level by studying TRPM3-mediated calcium transients in DRG neurons. By investigating TRPM3-expressing HEK cells, we further show the dynamic mitochondrial activity downstream of TRPM3 activation. NGF enhances TRPM3 function, attenuates TRPM3 tachyphylaxis, and slows intracellular calcium clearance; TRPM3 activation triggers more mitochondrial calcium loading than depolarization does, causing a steady-state mitochondrial calcium elevation and a delayed recovery of cytosolic calcium; mitochondrial calcium buffering accounts for approximately 40% of calcium influx subsequent to TRPM3 activation. TRPM3 activation provokes an outbreak of pulsatile superoxide production (mitoflash) that comes in the form of a surge in frequency being tunable. We suggest that mitoflash pulsations downstream of TRPM3 activation might be an early signaling event initiating pain sensitization. Tuning of mitoflash activity would be a novel bottom-up therapeutic strategy for chronic pain conditions such as LBP and beyond.

TRPM3 in Brain (Patho)Physiology

Already for centuries, humankind is driven to understand the physiological and pathological mechanisms that occur in our brains. Today, we know that ion channels play an essential role in the regulation of neural processes and control many functions of the central nervous system. Ion channels present a diverse group of membrane-spanning proteins that allow ions to penetrate the insulating cell membrane upon opening of their channel pores. This regulated ion permeation results in different electrical and chemical signals that are necessary to maintain physiological excitatory and inhibitory processes in the brain. Therefore, it is no surprise that disturbances in the functions of cerebral ion channels can result in a plethora of neurological disorders, which present a tremendous health care burden for our current society. The identification of ion channel-related brain disorders also fuel the research into the roles of ion channel proteins in various brain states. In the last decade, mounting evidence has been collected that indicates a pivotal role for transient receptor potential (TRP) ion channels in the development and various physiological functions of the central nervous system. For instance, TRP channels modulate neurite growth, synaptic plasticity and integration, and are required for neuronal survival. Moreover, TRP channels are involved in numerous neurological disorders. TRPM3 belongs to the melastatin subfamily of TRP channels and represents a non-selective cation channel that can be activated by several different stimuli, including the neurosteroid pregnenolone sulfate, osmotic pressures and heat. The channel is best known as a peripheral nociceptive ion channel that participates in heat sensation. However, recent research identifies TRPM3 as an emerging new player in the brain. In this review, we summarize the available data regarding the roles of TRPM3 in the brain, and correlate these data with the neuropathological processes in which this ion channel may be involved.

Review: Structure and Activation Mechanisms of CRAC Channels

Ca²⁺ release activated Ca²⁺ (CRAC) channels represent a primary pathway for Ca²⁺ to enter non-excitable cells. The two key players in this process are the stromal interaction molecule (STIM), a Ca²⁺ sensor embedded in the membrane of the endoplasmic reticulum, and Orai, a highly Ca²⁺ selective ion channel located in the plasma membrane. Upon depletion of the internal Ca²⁺ stores, STIM is activated, oligomerizes, couples to and activates Orai. This review provides an overview of novel findings about the CRAC channel activation mechanisms, structure and gating. In addition, it highlights, among diverse STIM and Orai mutants, also the disease-related mutants and their implications.

Loss of Transient Receptor Potential Melastatin 3 ion channel function in natural killer cells from Chronic Fatigue Syndrome/Myalgic Encephalomyelitis patients

Background

Chronic Fatigue Syndrome (CFS)/ Myalgic Encephalomyelitis (ME) is a debilitating disorder that is accompanied by reduced cytotoxic activity in natural killer (NK) cells. NK cells are an essential innate immune cell, responsible for recognising and inducing apoptosis of tumour and virus infected cells. Calcium is an essential component in mediating this cellular function. Transient Receptor Potential Melastatin 3 (TRPM3) cation channels have an important regulatory role in mediating calcium influx to help maintain cellular homeostasis. Several single nucleotide polymorphisms have been reported in TRPM3 genes from isolated peripheral blood mononuclear cells, NK and B cells in patients with CFS/ME and have been proposed to correlate with illness presentation. Moreover, a significant reduction in both TRPM3 surface expression and intracellular calcium mobilisation in NK cells has been found in CFS/ME patients compared with healthy controls. Despite the functional importance of TRPM3, little is known about the ion channel function in NK cells and the epiphenomenon of CFS/ME. The objective of the present study was to characterise the TRPM3 ion channel function in NK cells from CFS/ME patients in comparison with healthy controls using whole cell patch-clamp techniques.

Methods

NK cells were isolated from 12 age- and sex-matched healthy controls and CFS patients. Whole cell electrophysiology recording has been used to assess TRPM3 ion channel activity after modulation with pregnenolone sulfate and ononetin.

Results

We report a significant reduction in amplitude of TRPM3 current after pregnenolone sulfate stimulation in isolated NK cells from CFS/ME patients compared with healthy controls. In addition, we found pregnenolone sulfate-evoked ionic currents through TRPM3 channels were significantly modulated by ononetin in isolated NK cells from healthy controls compared with CFS/ME patients.

Conclusions

TRPM3 activity is impaired in CFS/ME patients suggesting changes in intracellular Ca²⁺ concentration, which may impact NK cellular functions. This investigation further helps to understand the intracellular-mediated roles in NK cells and confirm the potential role of TRPM3 ion channels in the aetiology and pathomechanism of CFS/ME.

Impaired calcium mobilization in natural killer cells from chronic fatigue syndrome/myalgic encephalomyelitis patients is associated with transient receptor potential melastatin 3 ion channels

Transient receptor potential melastatin subfamily 3 (TRPM3) ion channels play a role in calcium (Ca²⁺) cell signalling. Reduced TRPM3 protein expression has been identified in chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) patients. However, the significance of TRPM3 and association with intracellular Ca²⁺ mobilization has yet to be determined. Fifteen CFS/ME patients (mean age 48·82 ± 9·83 years) and 25 healthy controls (mean age 39·2 ± 12·12 years) were examined. Isolated natural killer (NK) cells were labelled with fluorescent antibodies to determine TRPM3, CD107a and CD69 receptors on CD56^dimCD16⁺NK cells and CD56^brightCD16^dim/– NK cells. Ca²⁺ flux and NK cytotoxicity activity was measured under various stimulants, including pregnenolone sulphate (PregS), thapsigargin (TG), 2‐aminoethoxydiphenyl borate (2APB) and ionomycin. Unstimulated CD56^brightCD16^dim/– NK cells showed significantly reduced TRPM3 receptors in CFS/ME compared with healthy controls (HC). Ca²⁺ flux showed no significant difference between groups. Moreover, PregS‐stimulated CD56^brightCD16^dim/–NK cells showed a significant increase in Ca²⁺ flux in CFS/ME patients compared with HC. By comparison, unstimulated CD56^dimCD16⁺ NK cells showed no significant difference in both Ca²⁺ flux and TRPM3 expression. PregS‐stimulated CD56^dimCD16⁺ NK cells increased TRPM3 expression significantly in CFS/ME, but this was not associated with a significant increase in Ca²⁺ flux. Furthermore, TG‐stimulated CD56^dimCD16⁺ NK cells increased K562 cell lysis prior to PregS stimulation in CFS/ME patients compared with HC. Differential expression of TRPM3 and Ca²⁺ flux between NK cell subtypes may provide evidence for their role in the pathomechanism involving NK cell cytotoxicity activity in CFS/ME.