SUMMARY Intense noise exposure causes hearing loss by inducing degeneration of spiral ganglia neurites that innervate cochlear hair cells. Nicotinamide adenine dinucleotide (NAD+) exhibits axon-protective effects in cultured neurons, however, its ability to block degeneration in vivo has been difficult to establish due to its poor cell permeability and serum instability. Here, we describe a strategy to increase cochlear NAD+ levels in mice by administering nicotinamide riboside (NR), a recently described NAD+ precursor. We find that administration of NR, even after noise exposure, prevents noise-induced hearing loss (NIHL) and spiral ganglia neurite degeneration. These effects are mediated by the NAD+-dependent mitochondrial sirtuin, SIRT3, since SIRT3-overexpressing mice are resistant to NIHL and SIRT3 deletion abrogates the protective effects of NR and expression of NAD+ biosynthetic enzymes. These findings reveal that administration of NR activates a NAD+-SIRT3 pathway that reduces neurite degeneration caused by noise exposure.

Nicotinamide adenine dinucleotide (NAD + ) is an endogenous enzyme cofactor and cosubstrate that has effects on diverse cellular and physiologic processes, including reactive oxygen species generation, mitochondrial function, apoptosis, and axonal degeneration. A major goal is to identify the NAD + -regulated cellular pathways that may mediate these effects. Here we show that the dynamic assembly and disassembly of microtubules is markedly altered by NAD + . Furthermore, we show that the disassembly of microtubule polymers elicited by microtubule depolymerizing agents is blocked by increasing intracellular NAD + levels. We find that these effects of NAD + are mediated by the activation of the mitochondrial sirtuin sirtuin-3 (SIRT3). Overexpression of SIRT3 prevents microtubule disassembly and apoptosis elicited by antimicrotubule agents and knockdown of SIRT3 prevents the protective effects of NAD + on microtubule polymers. Taken together, these data demonstrate that NAD + and SIRT3 regulate microtubule polymerization and the efficacy of antimicrotubule agents.N icotinamide adenine dinucleotide (NAD + ) is an endogenous dinucleotide that is present in the cytosol, nucleus, and mitochondria. Athough it serves an important role as a redox cofactor in metabolism, NAD + is also a substrate for several families of enzymes, including the poly(ADP ribose) polymerases and the sirtuin deacetylase enzymes (reviewed in refs. 1 and 2). The level of intracellular NAD + is regulated by many factors, including diet and energy status (3), axonal injury (4), DNA damage (5), and certain disease states (6), suggesting that NAD + -dependent signaling is dynamically modulated in diverse contexts.NAD + -dependent signaling can be induced by treatment of cells with exogenous NAD + , which increases intracellular NAD + levels and results in diverse effects in cells and animals. These effects include enhanced oxygen consumption and ATP production (7), as well as protection from genotoxic stress and apoptosis (3). Mice treated with nicotinamide riboside, a NAD + precursor that is metabolized into NAD + , have enhanced oxidative metabolism, increased insulin sensitivity, and protection from high-fat diet-induced obesity (8). These results demonstrate that NAD + -dependent pathways can enhance metabolic function and improve a variety of disease phenotypes.An NAD + -regulated pathway also inhibits axonal degeneration elicited by axonal transection (4). Treatment of axons with 5-20 mM NAD + markedly delays the axon degenerative process (9). Additionally, animals that express the Wallerian degeneration slow (Wld S ) protein, a fusion of the NAD + biosynthetic enzyme Nicotinamide mononucleotide adenylyl transferase 1 and Ube4a, exhibit markedly delayed degeneration of the distal axonal fragment after axonal transection (10), and expression of Wld S mitigates disease phenotypes in several neurodegenerative disease models (11)(12)(13)(14). Thus, understanding the intracellular pathways regulated by NAD + may be important for understanding the pa...

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Continuous, rhythmic beating of the heart requires exquisite control of expression, localization and function of cardiac ion channels -the foundations of the cardiac myocyte action potential. Disruption of any of these processes can alter the shape of the action potential, predisposing to cardiac arrhythmias. These arrhythmias can manifest in a variety of ways depending on both the channels involved and the type of disruption (i.e., gain or loss of function). As much as 1% of the population of developed countries is affected by cardiac arrhythmia each year, and a detailed understanding of the mechanism of each arrhythmia is crucial to developing and prescribing the proper therapies. Many of the antiarrhythmic drugs currently on the market were developed before the underlying cause of the arrhythmia was known, and as a result lack specificity, causing side effects. The majority of the available drugs target the conductance of cardiac ion channels, either by blocking or enhancing current through the channel. In recent years, however, it has become apparent that specific targeting of ion channel conductance may not be the most effective means for treatment. Here we review increasing evidence that suggests defects in ion channel trafficking play an important role in the etiology of arrhythmias, and small molecule approaches to correct trafficking defects will likely play an important role in the future of arrhythmia treatment. Keywordsarrhythmia; atrial fibrillation; hERG; KCNQ1; Kv1.5; LQTS; surface expression; trafficking Execution of an action potential requires the coordinated activity of numerous cardiac ion channels ( Figure 1A) [1]. The initial, depolarizing upstroke of an action potential is generated by the inward movement of positively charged sodium ions through voltage-gated sodium (Nav) channels, primarily Nav1.5 (SCNSA; Phase 0). These channels rapidly inactivate, and the cell depolarization triggers the opening of voltage-gated calcium (Cav) channels (Phase 1). Calcium influx, in addition to calcium release from intracellular stores, facilitates myocardial contraction. Cellular depolarization also triggers voltage-gated potassium (Kv) channels to open, allowing an efflux of potassium down its electrochemical gradient, beginning myocyte repolarization. The initial transient outward current (I to ) is generated by Kv channels such as Kv4.3 (KCND3; Phase 1). Repolarization continues with the delayed rectifier currents (I Kr and I Ks ), generated by the human ether-a-go-go-related © 2010 Expert Reviews Ltd † Author for correspondence: Tel: +1 212 746 6275, Fax: +1 212 746 7984, gwa2001@med.cornell.edu. Financial & competing interests disclosureThe authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed. No writing assistance was utilized in the production of this manuscript. NIH Public Access Author ManuscriptExp...

Transmembrane channel‐like protein isoform 1 (TMC1) is essential for the generation of mechano‐electrical transducer currents in hair cells of the inner ear. TMC1 disruption causes hair cell degeneration and deafness in mice and humans. Although thought to be expressed at the cell surface in vivo, TMC1 remains in the endoplasmic reticulum when heterologously expressed in standard cell lines, precluding determination of its roles in mechanosensing and pore formation. Here, we report that the KCNQ1 Kv channel forms complexes with TMC1 and rescues its surface expression when coexpressed in Chinese Hamster Ovary cells. TMC1 rescue is specific for KCNQ1 within the KCNQ family, is prevented by a KCNQ1 trafficking‐deficient mutation, and is influenced by KCNE β subunits and inhibition of KCNQ1 endocytosis. TMC1 lowers KCNQ1 and KCNQ1‐KCNE1 K+ currents, and despite the surface expression, it does not detectably respond to mechanical stimulation or high salt. We conclude that TMC1 is not intrinsically mechano‐ or osmosensitive but has the capacity for cell surface expression, and requires partner protein(s) for surface expression and mechanosensitivity. We suggest that KCNQ1, expression of which is not thought to overlap with TMC1 in hair cells, is a proxy partner bearing structural elements or a sequence motif reminiscent of a true in vivo TMC1 hair cell partner. Discovery of the first reported strategy to rescue TMC1 surface expression should aid future studies of the TMC1 function and native partners.