HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Summary Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Santella, L. Search for Related Content PubMed PubMed Citation Articles by Santella, L.

NAADP: A New Second Messenger Comes of Age

Laboratory of Cell Signaling, Stazione Zoologica A. Dohrn, Villa Comunale I-80121, Napoli, Italy

SUMMARY

Nicotinic acid adenine dinucleotide phosphate (NAADP) is one of the most potent stimulators of intracellular Ca²⁺ mobilization in a variety of cell types. Its role in physiological processes is increasingly demonstrated by NAADP increases following cellular stimulation. As a second messenger NAADP shows unique features such as the ability to mobilize Ca²⁺ from stores that are physically distinct from those connected to the Ca²⁺ channels located in the endoplasmic reticulum, namely, the inositol-1,4,5-trisphosphate and the cyclic-ADP-ribose/ ryanodine receptors. Furthermore, the NAADP-induced self-inactivation mechanism is suggestive of an irreversible binding of NAADP to its putative receptor.

One of the hallmarks of signal transduction by Ca²⁺ is complexity, which is mostly a consequence of the multiple, independent Ca²⁺-release mechanisms within cells. Following external stimulation, Ca²⁺ channels in the plasma membrane and on cellular organelles are activated in a specific spatio-temporal pattern to ensure the precise control of local, and eventually global, increases in concentrations of intracellular Ca²⁺ (1). The discovery of 1,4,5-trisphosphate (InsP₃) as an intracellular Ca²⁺ mobilizer has led to a better understanding of the phospholipase C (PLC)–InsP₃-mediated cellular signaling (2). Nicotinic acid adenine dinucleotide phosphate (NAADP) is the newest entry to the list of Ca²⁺-regulating second messengers (3). In NAADP, the nicotinamide ring in the molecule nicotinamide adenine dinucleotide phosphate (NADP) is replaced by nicotinic acid. NAADP mobilizes Ca²⁺ from stores that are now known to be important in a number of physiological processes, the common denominator of which is the interplay of the Ca²⁺ stores operated by the three Ca²⁺-linked second messengers NAADP, cyclic adenosine diphosphate ribose (cADPR), and InsP₃. The stores activated by InsP₃ and by cADPR are most likely the same: the sarco/endoplasmic reticulum. By general consensus, the stores operated by cADPR coincide with that mobilized through the ryanodine receptors (RyRs). Ever since its discovery as a Ca²⁺-mobilizing molecule in sea urchin egg homogenates, NAADP has exhibited a distinct pharmacological profile (4). Depletion of Ca²⁺ stores from endoplasmic reticulum (ER) in sea urchin homogenates or in intact eggs through the use of thapsigargin, a sarcoplasmic/ER calcium ATPase (SERCA) pump inhibitor, does not prevent NAADP-mediated release of Ca²⁺ from these preparations, suggesting that the NAADP-sensitive stores are not located in the ER. These findings are also in line with the distribution profile of NAADP-mediated Ca²⁺ stores, which do not co-migrate with those of the ER in cell fractionation experiments (5).

The most interesting finding in the early studies of NAADP has been its crosstalk with the other two well-characterized messengers of Ca²⁺ flux. Thus, Ca²⁺ release induced by NAADP elicits long-term Ca²⁺ oscillations mediated by the InsP₃ receptor (InsP₃R) and the cADPR/ryanodine receptor, indicating that a triggered Ca²⁺ release (by NAADP) is subsequently amplified by the two other messengers (6–9). Other unique properties of the NAADP-sensitive Ca²⁺ stores include the lack of inhibition by known modulators of InsP₃R or RyR and a self-inactivation mechanism by subthreshold concentrations of NAADP that are, per se, insufficient to release Ca²⁺ (10). This auto-inactivation could reflect the presence of high and low affinity-binding sites in the NAADP receptors whereby a high affinity site mediates channel inactivation, and a low affinity site promotes channel opening. The auto-inactivation mechanism is also relevant to the concept of cross-talk among the three types of Ca²⁺ stores because the desensitization of NAADP-mediated Ca²⁺ release specifically blocks both InsP₃ and cADPR-mediated signaling in T lymphocytes (11) and blocks cholecystokinin-induced Ca²⁺ oscillations in the apical pole of pancreatic acinar cells and the subsequent propagating Ca²⁺ wave (12). Additionally, the prior depletion of ER Ca²⁺ stores with thapsigargin and inhibition of RyRs with ryanodine inhibits the formation of NAADP-induced Ca²⁺ waves, further reinforcing the idea of mutual interaction of the three Ca²⁺ stores (13). But, if the NAADP-sensitive Ca²⁺ stores are not in the ER, where are they? Recent hints have come from copurification of a putative NAADP-sensitive store in a cell lysate fraction enriched with lysosomal markers and from the related observation that NAADP-induced Ca²⁺ release is inhibited by targeting drugs to lysosomes (14). These findings suggest that NAADP releases Ca²⁺ through a mechanism that utilizes a lysosomal-like organelle. However, this suggestion might not apply to all NAADP-sensitive stores—of which there could be more than one—or might not hold true for NAADP-sensitive stores in all cells. For instance, NAADP might directly modulate RyRs (15), possibly through an accessory protein, as shown by the opening of purified RyRs reconstituted in lipid bilayers (16). However, the finding that preparations of isolated pancreatic nuclei devoid of lysosomes or lysosomal-like acidic granules can still release Ca²⁺ into the nucleoplasm when challenged with NAADP is hard to reconcile with the idea that the release of Ca²⁺ by NAADP should originate from a Ca²⁺ store different from that of the ER (17).

The precise spatial distribution of NAADP receptors is essential for the mechanisms that underlie fertilization in starfish oocytes—an ideal system in which to study the details of Ca²⁺ homeostasis. The mechanism of egg activation has been studied intensively for more than a century and Ca²⁺ has been known for quite a while to play a vital role in it (18). When a sperm fertilizes an egg, it triggers a local explosive rise in intracellular Ca²⁺ concentrations that is then translated into a wave that sweeps through the egg. NAADP-activated and InsP₃-activated receptors have recently been explored in detail in the starfish as determinants of the spatiotemporal dynamics of Ca²⁺ signaling during fertilization. In oocytes, the Ca²⁺ wave that follows the interaction with the sperm is preceded by a flash located in the cortical domain of the oocyte (cortical flash) generated during depolarization by the influx of Ca²⁺ through voltage-dependent plasma membrane channels (19). Injection of NAADP into the oocytes also induces a cortical Ca²⁺ flash, which then spreads throughout the cytoplasm as a wave (Figure 1A). A key point is that in starfish oocytes, the cortical flash is dependent on external Ca²⁺ (20, 21). The cortical flash begins with a localized Ca²⁺ increase, originating from a thapsigargin-sensitive NAADP-dependent store, the emptying of which activates an inwardly-directed Ca²⁺ current—movement of extracellular Ca²⁺ into the intracellular space—with properties similar to those of the current triggered by the sperm (Figure 1B) (22). Thus, the roles of NAADP-mediated and InsP₃–mediated receptors in controlling the spatiotemporal dynamics of Ca²⁺ signaling during fertilization, previously assumed to be exclusively under the control of InsP₃R, have now been separated and provide compelling evidence that NAADP triggers the initial circumscribed Ca²⁺ release that initiates the sperm-induced cortical flash, whereas InsP₃Rs are involved in the later propagation of the Ca²⁺ wave in a Ca²⁺-induced Ca²⁺ release (CICR)–type mechanism (21). Supporting this conclusion are the findings that NAADP elicits the fertilization potential in starfish oocytes, and that the desensitization of NAADP Ca²⁺ stores prevents the onset of this fertilization potential (23).

Figure 1. Ca2+ increase and membrane current activated by NAADP uncaging in a starfish oocyte. A. Pseudocolor relative fluorescent images of the Ca2+ increase induced by the global photoactivation of previously injected caged NAADP. NAADP induces a cortical flash that spreads centripetally to the oocyte center, which is dependent on extracellular Ca2+. B. Ca2+-mediated membrane current activated by the light-mediated liberation of NAADP. The current is affected by the preincubation with thapsigargin and injection of BAPTA [1,2-Bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid] (22).

Different results had been instead initially obtained in intact sea urchin eggs, where the Ca²⁺ response to NAADP, it had been claimed, was independent of extracellular Ca²⁺ (7). More recent findings, however, indicate the response involves, as in starfish oocytes, a cortical flash produced by Ca²⁺ entry that is inhibited by the prior injection of NAADP to desensitize the NAADP receptors (24). That NAADP participates in fertilization is also supported by the demonstration that high (micromolar) levels of NAADP are also synthesized in sea urchin spermatozoa that have been activated by seawater (25).

Returning to the role played by NAADP in the fertilization process, it has been suggested that the NAADP-sensitive store activated by the sperm in sea urchin eggs is located in the lysosomes. This suggestion, however, is hard to reconcile with the finding that the NAADP-evoked current in starfish oocytes is not affected by lysosomal inhibitors such as bafilomycin, the vacuolar proton pump inhibitor, and by lycylphenylalanine 2-naphthylamide (GPN), which disrupts lysosomes through osmotic lysis. Bafilomycin, has also been tested in pancreatic acinar cells where it has no effect on the response to NAADP (26). Bafilomycin, however, does eliminate the NAADP-stimulated Ca²⁺ wave in pulmonary artery smooth muscle cells (27). Thus, much remains to be learned on the nature of the Ca²⁺ stores activated by NAADP.

Solving the controversies surrounding the mechanism of action and the spatiotemporal aspects of the function of NAADP is likely to take some time. Apart from the points alluded to above, additional issues need to be addressed. NAADP-induced Ca²⁺ release has now been described in many systems, including humans, but only recently it has been shown that the concentrations of NAADP in cells are controlled by physiological stimuli (8). Increased amounts of NAADP have been reported in pancreatic beta cells stimulated with glucose (28), and micromolar levels of NAADP have been found in activated sea urchin spermatozoa (25), but the signaling pathway leading to NAADP production is still obscure. CD38, a mammalian membrane bound ectoenzyme that catalyzes the synthesis of cADPR, might be involved in the pathway (8). It is noteworthy that CD38 is not exclusively expressed on the cell surface but it is also present in the cytoplasm (29–31) and can be internalised (32). Several tissues, including liver, heart, spleen, lung, and pancreas from CD38-knockout mice are unable to synthesize cADPR and NAADP (33). More information is needed, however, to understand the role played by CD38 and by intracellular soluble ADP-ribosyl cyclases in the regulation of intracellular Ca²⁺ concentrations (via cADPR and NAADP). Adding to this conundrum is the fact that the optimal pH of the reaction is out of the physiological range (34). The pH optimum issue, when coupled to the compartmentalization of nicotinic acid and NADP in an acidic environment such as that of the lysosomes, could strengthen the hypothesis of a lysosomal-like organelle as the NAADP-sensitive Ca²⁺ store (14). Nevertheless, other possible routes for NAADP synthesis in vivo should be explored, such as deamination of NADP or the phosphorylation of NAAD (35).

The molecular identity of the NAADP-sensitive receptors is still obscure, but a radioreceptor assay in sea urchin eggs, together with pharmacological studies, have revealed interesting features in the NAADP binding site. Very importantly, NAADP binding seems to be irreversible, suggesting an explanation for the molecular basis of the inactivation phenomenon (36–38). Whatever their nature, NAADP receptors display peculiar features that make them distinct among effectors of Ca²⁺ stores. Their identification will clearly be an important achievement. Finally, after all that has been discussed here is clarified, one exceptionally important point will have to be addressed: the molecular mechanism by which NAADP cross talks to the receptor molecule(s), wherever they are.

Luigia Santella, PhD, is a senior scientist and group leader at the Cell Signaling Laboratory, Stazione Zoologica "Anton Dohrn", Naples, Italy. Her research focuses on calcium signaling during the meiotic cycle (maturation) and fertilization of starfish oocytes. E-mail santella{at}szn.it; fax +39-081-7641355.

References

1. Carafoli, E., Santella, L., Branca, D., Brini, M. Generation, control, and processing of cellular calcium signals. Crit. Rev. Biochem. Mol. Biol. 36, 107–260 (2001).[CrossRef][Medline]
2. Streb, H., Irvine, R.F., Berridge, M.J, Schulz, I. Release of Ca²⁺ from a nonmitochondrial intracellular store in pancreatic acinar cells by inositol-1,4,5-trisphosphate. Nature 306, 67–69 (1983).[CrossRef][Medline]
3. Lee, H.C. Cyclic ADP-ribose and NAADP. Structures, Metabolism and Functions. Dordrecht, Kluwer Academic Publishers (2002).
4. Clapper, D.L., Walseth, T.F., Dargie, P.J., and Lee, H.C. Pyridine nucleotide metabolites stimulate calcium release from sea urchin egg microsomes desensitized to inositol trisphosphate. J. Biol. Chem. 262, 9561–9568 (1987). The first demonstration that a derivative of the pyridine nucleotide NADP was able to release Ca²⁺ from sea urchin eggs homogenates through a mechanism distinct from that activated by InsP₃. This derivative was later identified as NAADP.[Abstract/Free Full Text]
5. Genazzani, A.A. and Galione, A. Nicotinic acid-adenine dinucleotide phosphate mobilizes Ca²⁺ from a thapsigargin-insensitive pool. Biochem J. 315, 721–725 (1996).
6. Cancela, J.M., Gerasimenko, O.V., Gerasimenko, J.V., Tepikin, A.V., and Petersen, O.H. Two different but converging messenger pathways to intracellular Ca²⁺ release: The roles of nicotinic acid adenine dinucleotide phosphate, cyclic ADP-ribose and inositol trisphosphate. EMBO J. 19, 2549–2557 (2000).[CrossRef][Medline]
7. Churchill, G.C. and Galione, A. NAADP induces Ca²⁺ oscillations via a two-pool mechanism by priming IP₃- and cADPR-sensitive Ca²⁺ stores. EMBO J. 20, 2666–2671 (2001).[CrossRef][Medline]
8. Lee, H.C. Multiplicity of Ca²⁺ messengers and Ca²⁺ stores: A perspective from cyclic ADP-ribose and NAADP. Curr. Mol. Med. 4, 227–37 (2004).[CrossRef][Medline]
9. Schulz, I. and Krause, E. Inositol 1,4,5-trisphosphate and its co-players in the concert of Ca²⁺ signaling–new faces in the line up. Curr. Mol. Med. 4, 313–22 (2004).[CrossRef][Medline]
10. Aarhus, R., Dickey, D.M., Graeff, R.M., Gee, K.R., Walseth, T.F., and Lee, H.C. Activation and inactivation of Ca²⁺ release by NAADP⁺. J Biol Chem. 271, 8513–6 (1996).[Abstract/Free Full Text]
11. Berg, I., Potter, B.V., Mayr, G.W., and Guse, A.H. Nicotinic acid adenine dinucleotide phosphate NAADP+ is an essential regulator of T-lymphocyte Ca²⁺-signaling. J. Cell. Biol. 150, 581–588 (2000).[Abstract/Free Full Text]
12. Cancela, J.M. Specific Ca²⁺ signaling evoked by cholecystokinin and acetylcholine: The roles of NAADP, cADPR, and IP₃. Annu. Rev. Physiol. 63, 99–117 (2001).[CrossRef][Medline]
13. Boittin, F.X., Galione, A., Evans, A.M. Nicotinic acid adenine dinucleotide phosphate mediates Ca²⁺ signals and contraction in arterial smooth muscle via a two-pool mechanism. Circ. Res. 91, 1168–1175 (2002).[Abstract/Free Full Text]
14. Churchill, G.C., Okada, Y., Thomas, J.M., Genazzani, A.A., Patel, S., and Galione, A. NAADP mobilizes Ca²⁺ from reserve granules, lysosome-related organelles, in sea urchin eggs. Cell 111, 703–708 (2002). The NAADP-sensitive store was localized in the reserve granules of sea urchin eggs, which are the functional equivalent of lysosomes. Such a store is filled with Ca²⁺ through an H⁺–Ca²⁺ exchanger driven by the acidic lumen of the granule.[CrossRef][Medline]
15. Langhorst, M.F., Schwarzmann, N., and Guse, A.H. Ca²⁺ release via ryanodine receptors and Ca²⁺ entry: major mechanisms in NAADP-mediated Ca²⁺ signaling in T-lymphocytes. Cell Signal 16, 1283–1289 (2004).[CrossRef][Medline]
16. Hohenegger, M., Suko, J., Gscheidlinger, R., Drobny, H., and Zidar, A. Nicotinic acid-adenine dinucleotide phosphate activates the skeletal muscle ryanodine receptor. Biochem J. 367, 423–431 (2002). This study has employed single-channel recording on lipid bilayers to provide the first evidence that NAADP may activate ryanodine receptors. The question of whether the effect is directly on the receptor or is mediated by an intermediate protein is, however, not answered.[CrossRef][Medline]
17. Gerasimenko, O., and Gerasimenko, J. New aspects of nuclear calcium signaling. J. Cell Sci. 117, 3087–3094 (2004).[Abstract/Free Full Text]
18. Santella, L., Lim, D., and Moccia, F. Calcium and fertilization: the beginning of life. Trends. Biochem. Sci. 29, 400–408 (2000).
19. Lim, D., Lange, K., and Santella, L. Activation of oocytes by latrunculin A. FASEB J. 2002 16, 1050–1056 (2002).
20. Santella, L., Kyozuka, K., Genazzani, A.A., De Riso, L., and Carafoli, E. Nicotinic acid adenine dinucleotide phosphate-induced Ca²⁺ release. Interactions among distinct Ca²⁺ mobilizing mechanisms in starfish oocytes. J. Biol. Chem. 275, 8301–8306 (2000).[Abstract/Free Full Text]
21. Lim, D., Kyozuka, K., Gragnaniello, G., Carafoli, E., and Santella, L. NAADP⁺ initiates the Ca²⁺ response during fertilization of starfish oocytes. FASEB J. 15, 2257–2267 (2001). This paper provides strong evidence that the fertilization-induced Ca²⁺ response in starfish oocytes is initiated by NAADP and propagated by the InsP₃ receptors.[Abstract/Free Full Text]
22. Moccia, F., Lim, D., Nusco, G.A., Ercolano, E., and Santella, L. NAADP activates a Ca²⁺ current that is dependent on F-actin cytoskeleton. FASEB J. 17,1907–1909 (2003). This study is the first demonstration that NAADP activates a Ca²⁺-mediated current that triggers intracellular Ca²⁺ from InsP₃ and ryanodine receptors and shows the modulation of NAADP-dependent channels by actin cytoskeleton.[Abstract/Free Full Text]
23. Moccia, F., Lim, D., Kyozuka, K., and Santella, L. NAADP triggers the fertilization potential in starfish oocytes. Cell Calcium 36, 515–524 (2004).[CrossRef][Medline]
24. Churchill, G.C., O’Neill, J.S., Masgrau, R., Patel, S., Thomas, J.M., Genazzani, A.A., and Galione, A. Sperm deliver a new second messenger: NAADP. Curr. Biol. 13, 125–128 (2003). Conclusive demonstration that NAADP is a genuine second messenger by showing that NAADP levels increase at fertilization of sea urchin eggs.[CrossRef][Medline]
25. Billington, R.A., Ho, A., and Genazzani, A.A. Nicotinic acid adenine dinucleotide phosphate (NAADP) is present at micromolar concentrations in sea urchin spermatozoa. J. Physiol. 544, 107–112 (2002).[Abstract/Free Full Text]
26. Gerasimenko, J.V., Maruyama, Y., Yano, K., Dolman, N.J., Tepikin, A.V., Petersen, O.H., and Gerasimenko, O.V. NAADP mobilizes Ca²⁺ from a thapsigargin-sensitive store in the nuclear envelope by activating ryanodine receptors. J. Cell Biol. 163, 271–282 (2003). This paper shows that NAADP may mobilize Ca²⁺ from a thapsigargin-sensitive pool in the nuclear envelope, including endoplasmic reticulum adherens. The authors further show that the store is quite different from the lysosome and releases Ca²⁺ through ryanodine receptors.[Abstract/Free Full Text]
27. Kinnear, N.P., Boittin, F.X., Thomas, J.M., Galione, A., and Evans, A.M. Lysosome-sarcoplasmic reticulum junctions. A trigger zone for calcium signaling by nicotinic acid adenine dinucleotide phosphate and endothelin- 1. J. Biol. Chem. 279, 54319–54326 (2004).[Abstract/Free Full Text]
28. Masgrau, R., Churchill, G.C., Morgan, A.J., Ashcroft, S.J., and Galione, A. NAADP: A new second messenger for glucose-induced Ca²⁺ responses in clonal pancreatic beta cells. Curr. Biol. 13, 247–251 (2003). Important demonstration that endogenous levels of NAADP in mammalian cells increase upon external stimulation.[CrossRef][Medline]
29. Munshi, C.B., Graeff, R., and Lee, H.C. Evidence for a causal role of CD38 expression in granulocytic differentiation of human HL-60 cells. J. Biol. Chem. 277, 49453–49458 (2002).[Abstract/Free Full Text]
30. Adebanjo, O.A., Anandatheerthavarada, H.K., Koval, A.P. et al. A new function for CD38/ADP-ribosyl cyclase in nuclear Ca²⁺ homeostasis. Nat. Cell. Biol. 1, 409–414 (1999).[CrossRef][Medline]
31. Liang, M., Chini, E.N., Cheng, J., and Dousa, T.P. Synthesis of NAADP and cADPR in mitochondria. Arch. Biochem. Biophys. 371, 317–325 (1999).[CrossRef][Medline]
32. Zocchi, E., Usai, C., Guida, L., Franco, L., Bruzzone, S., Passalacqua, M., and De Flora, A. Ligand-induced internalization of CD38 results in intracellular Ca²⁺ mobilization: role of NAD⁺ transport across cell membranes. FASEB J. 13, 273–283 (1999).[Abstract/Free Full Text]
33. Chini, E.N., Chini, C.C., Kato, I., Takasawa, S., and Okamoto, H. CD38 is the major enzyme responsible for synthesis of nicotinic acid-adenine dinucleotide phosphate in mammalian tissues. Biochem. J. 362, 125–130 (2002).[CrossRef][Medline]
34. Graeff, R.M., Franco, L., De Flora, A, and Lee, H.C. Cyclic GMP-dependent and -independent effects on the synthesis of the calcium messengers cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate. J. Biol. Chem. 273, 118–125 (1998).[Abstract/Free Full Text]
35. Chini, E.N. and De Toledo, F.G. Nicotinic acid adenine dinucleotide phosphate: A new intracellular second messenger? Am. J. Physiol. Cell. Physiol. 282, C1191–C1198 (2002).[Abstract/Free Full Text]
36. Berridge, G., Dickinson, G., Parrington, J., Galione, A, and Patel, S. Solubilization of receptors for the novel Ca²⁺-mobilizing messenger, nicotinic acid adenine dinucleotide phosphate. J. Biol. Chem. 277, 43717–43723 (2002). Solubilization and characterization of the NAADP binding sites (receptors) in sea urchin eggs homogenates. The results show that these receptors consist of proteins smaller than InsP₃ and ryanodine receptors.[Abstract/Free Full Text]
37. Patel, S. NAADP-induced Ca²⁺ release—a new signaling pathway. Biol. Cell 96, 19–28 (2004).[CrossRef][Medline]
38. Billington, R.A., Tron, G.C., Reichenbach, S., Sorba, G., and Genazzani, A.A. Role of the nicotinic acid group in NAADP receptor selectivity. Cell Calcium 37, 81–86 (2005).[CrossRef][Medline]

This article has been cited by other articles:

				F. Malavasi, S. Deaglio, A. Funaro, E. Ferrero, A. L. Horenstein, E. Ortolan, T. Vaisitti, and S. Aydin Evolution and Function of the ADP Ribosyl Cyclase/CD38 Gene Family in Physiology and Pathology Physiol Rev, July 1, 2008; 88(3): 841 - 886. [Abstract] [Full Text] [PDF]

				A. Beck, M. Kolisek, L. A. Bagley, A. Fleig, and R. Penner Nicotinic acid adenine dinucleotide phosphate and cyclic ADP-ribose regulate TRPM2 channels in T lymphocytes FASEB J, May 1, 2006; 20(7): 962 - 964. [Abstract] [Full Text] [PDF]

This Article Summary Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Santella, L. Search for Related Content PubMed PubMed Citation Articles by Santella, L.