CAP and Metabolic Diseases: A Mini Review on Preclinical Mechanisms and Clinical Efficacy

Capsaicin (CAP) is the chief active ingredient of natural chili peppers. It has culinary and medicinal benefits. CAP activates its receptor, transient receptor potential vanilloid subfamily 1 (TRPV1), which is expressed in the sensory and motor neurons, adipocytes, liver, vascular smooth muscle cells, neuromuscular junction, skeletal muscle, heart and brain. The specificity of CAP to activate TRPV1 is the fundamental mechanism for its medicinal benefits to treat pain, obesity, hypertension, and other diseases. Preclinical data from rodent model of high fat diet-induced obesity collectively suggest that CAP exerts its effects by activating TRPV1 signaling pathway, which stimulates thermogenic mechanisms in the white and brown adipose tissues to induce browning of white adipose tissues and brown adipose tissue thermogenesis. This leads to enhancement of metabolic activity and thermogenesis to counter obesity. Although CAP and its pungent and non-pungent analogs are used in human clinical studies, their effects on satiety and energy expenditure have been the highlights of such studies. The precise mechanism of action of CAP has not been evaluated in humans. This article summarizes these data and suggests that long-term safety and tolerance studies are important for advancing CAP to treat human obesity

Capsaicin protects neuromuscular junctions from the inhibitory effects of botulinum neurotoxin A

Within 24 hrs after injecting botulinum neurotoxin A (BoNT/A) into the hindlimb, mice lost the toe spread reflex and developed progressive muscle weakness. At the same time, the compound muscle action potential amplitude decreased. Injection of capsaicin before BoNT/A significantly reduced these affects and protected the muscle twitch tension of the Extensor digitorum longus (EDL) nerve muscle preparation. Acute in vitro exposure of isolated nerve muscle preparations, as well as Neuro 2a cells, to capsaicin prevented uptake of Alexa 647 BoNT/A. Motor nerve endings as well as Neuro 2a cells express the capsaicin receptor, a transient receptor potential channel of the vanilloid family (TRPV1). Capsaicin as well as disruption of clathrin coated pits (CCPs) reduced Neuro 2a cell uptake of BoNT/A. FM1-43 uptake indicated that exocytosis persists for BoNT/A treated Neuro 2a cells pretreated with capsaicin. Pre-injection of wortmannin (WMN), a PI3Kinase inhibitor, also protected mice from the paralytic effects of BoNT/A. When applied alone, either WMN or capsaicin selectively reduced stimulus-evoked transmitter release from motor nerve endings. We hypothesize that TRPV1 activation reduces PI(4,5)P2 level within the membrane. This prevents CCP formation and uptake of BoNT/A

Rapidly inducible changes in phosphatidylinositol 4,5-bisphosphate levels influence multiple regulatory functions of the lipid in intact living cells

Rapamycin (rapa)-induced heterodimerization of the FRB domain of the mammalian target of rapa and FKBP12 was used to translocate a phosphoinositide 5-phosphatase (5-ptase) enzyme to the plasma membrane (PM) to evoke rapid changes in phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) levels. Rapa-induced PM recruitment of a truncated type IV 5-ptase containing only the 5-ptase domain fused to FKBP12 rapidly decreased PM PtdIns(4,5)P2 as monitored by the PLCδ1PH-GFP fusion construct. This decrease was paralleled by rapid termination of the ATP-induced Ca2+ signal and the prompt inactivation of menthol-activated transient receptor potential melastatin 8 (TRPM8) channels. Depletion of PM PtdIns(4,5)P2 was associated with a complete blockade of transferrin uptake and inhibition of epidermal growth factor internalization. None of these changes were observed upon rapa-induced translocation of an mRFP-FKBP12 fusion protein that was used as a control. These data demonstrate that rapid inducible depletion of PM PtdIns(4,5)P2 is a powerful tool to study the multiple regulatory roles of this phospholipid and to study differential sensitivities of various processes to PtdIns(4,5)P2 depletion

The inositol 1,4,5-trisphosphate receptor in C. elegans

The soil nematode Caenorhabditis elegans is a genetic model organism whose cellular physiology is closely related to that of mammals, with many signaling cascades and second messengers mirroring those found in higher organisms. Due to the genetic, anatomical, and behavioral simplicity of worms, integrative physiological techniques are relatively straightforward and represent a powerful approach to understand the molecular mechanisms underlying more complex system functions. Studies of the nematode inositol 1,4,5-trisphosphate receptor (InsP 3 R) have led to advances in our understanding of its role in development and behavior

Inorganic Polyphosphate Modulates TRPM8 Channels

Polyphosphate (polyP) is an inorganic polymer built of tens to hundreds of phosphates, linked by high-energy phosphoanhydride bonds. PolyP forms complexes and modulates activities of many proteins including ion channels. Here we investigated the role of polyP in the function of the transient receptor potential melastatin 8 (TRPM8) channel. Using whole-cell patch-clamp and fluorescent calcium measurements we demonstrate that enzymatic breakdown of polyP by exopolyphosphatase (scPPX1) inhibits channel activity in human embryonic kidney and F-11 neuronal cells expressing TRPM8. We demonstrate that the TRPM8 channel protein is associated with polyP. Furthermore, addition of scPPX1 altered the voltage-dependence and blocked the activity of the purified TRPM8 channels reconstituted into planar lipid bilayers, where the activity of the channel was initiated by cold and menthol in the presence of phosphatidylinositol 4,5-biphosphate (PtdIns(4,5)P2). The biochemical analysis of the TRPM8 protein also uncovered the presence of poly-(R)-3-hydroxybutyrate (PHB), which is frequently associated with polyP. We conclude that the TRPM8 protein forms a stable complex with polyP and its presence is essential for normal channel activity