Tetrabenazine Facilitates Exocytosis by Enhancing Calcium-Induced Calcium Release through Ryanodine Receptors

Abstract

Vesicular monoamine transporter-2 is expressed in the presynaptic secretory vesicles membrane in the brain. Its blockade by tetrabenazine (TBZ) causes depletion of dopamine at striatal basal ganglia; this is the mechanism underlying its long-standing use in the treatment of Huntington's disease. In the frame of a project aimed at investigating the kinetics of exocytosis from vesicles with partial emptying of their neurotransmitter, we unexpectedly found that TBZ facilitates exocytosis; thus, we decided to characterize such effect. We used bovine chromaffin cells (BCCs) challenged with repeated pulses of high K 1. Upon repeated K 1 pulsing, the exocytotic catecholamine release responses were gradually decaying. However, when cells were exposed to TBZ, responses were mildly augmented and decay rate delayed. Facilitation of exocytosis was not due to Ca 21 entry blockade through voltage-activated calcium channels (VACCs) because, in fact, TBZ mildly blocked the whole-cell Ca 21 current. However, TBZ mimicked the facilitatory effects of exocytosis elicited by BayK8644 (L-subtype VACC agonist), an effect blocked by nifedipine (VACC antagonist). On the basis that TBZ augmented the secretory responses to caffeine (but not those of histamine), we monitored its effects on cytosolic Ca 21 elevations ([Ca 21 ] c) triggered by caffeine or histamine. While the responses to caffeine were augmented twice by TBZ, those of histamine were unaffected; the same happened in rat cortical neurons. Hence, we hypothesize that TBZ facilitates exocytosis by increasing Ca 21 release through the endoplasmic reticulum ryanodine receptor channel (RyR). Confirming this hypothesis are docking results, showing an interaction of TBZ with RyRs. This is consonant with the existence of a healthy Ca 21-induced-Ca 21-release mechanism in BCCs. SIGNIFICANCE STATEMENT A novel mechanism of action for tetrabenazine (TBZ), a drug used in the therapy of Huntington's disease (HD), is described here. Such mechanism consists of facilitation by combining TBZ with the ryanodine receptor of the endoplasmic reticulum, thereby increasing Ca 21-induced Ca 21 release. This novel mechanism should be taken into account when considering the efficacy and/or safety of TBZ in the treatment of chorea associated with HD and other disorders. Additionally, it could be of interest in the development of novel medicines to treat these pathological conditions.

Novel Purine Derivative ITH15004 Facilitates Exocytosis through a Mitochondrial Calcium-Mediated Mechanism

“…We have previously used a mixture of 10 mM caffeine, 1 µM ryanodine and 1 µM thapsigargin (the so-called CRT mixture) to deplete quickly, drastically, and irreversibly the ER Ca 2+ store of CCs [ 14 ]. As ER Ca 2+ mobilisation is involved in the potentiation by ouabain of secretion triggered by K + in BCCs [ 6 ] and tetrabenazine [ 7 ], a possibility exists that ITH15004 was acting in a similar way. Thus, we tested the effects of the compound on secretion in BCCs with their ER Ca 2+ store depleted.…”

“…After recording for a minute to estimate basal [Ca 2+ ] c fluorescence, cells were challenged with 35K + pulses (one pulse per well in 96-well plates). Changes of [Ca 2+ ] c were expressed as arbitrary fluorescence units (AFU) [ 7 ]. Figure 7 A displays a family of [Ca 2+ ] c transient curves in the absence (control) and presence of increasing ITH15004 concentrations.…”

“…Those Ca 2+ microdomains (CMDs) are originated and tightly regulated by endogenous messengers including Ca 2+ itself and various pharmacological ligands acting on Ca 2+ entry through voltage-activated calcium channels (VACCs), their redistribution into the endoplasmic reticulum (ER) Ca 2+ store and mitochondria, and its subsequent release into the cytosol from those CC organelles. For instance, by stimulating the release of ER Ca 2+ in bovine CCs (BCCs), we have previously shown that the Na + ,K + -ATPase pump blocker ouabain [ 6 ] and, more recently, the vesicle dopamine transporter blocker tetrabenazine (TBZ) [ 7 ] facilitated the depolarizing-evoked catecholamine release through the mobilization of ER Ca 2+ . These findings could have interest in the context of safety issues with the handling of these drugs in clinical sets (i.e., ouabain and other digitalis drugs have been, and are being, used in cardiac failure and atrial fibrillation, while TBZ is currently used in the treatment of dystonic movements in Huntington’s disease (HD)).…”

Role of defective calcium regulation in cardiorespiratory dysfunction in Huntington’s disease

“…Neurodegeneration in HD may be associated with impaired synaptic transmission (29,30), reduced brain-derived neurotrophic factor (31,32), mitochondrial dysfunction (33,34), and altered calcium (Ca 2+ ) regulation (35)(36)(37). Increased intracellular Ca 2+ concentration due to NMDA receptor activity or other Ca 2+ sources including the inositol 1,4,5-trisphosphate receptor type 1 (IP3R1) and ryanodine receptor (RyR) may play a role in striatal neurodegeneration in HD (38)(39)(40)(41)(42).…”