Subcellular and Intercellular Traffic of NAD<SUP>+</SUP>, NAD<SUP>+</SUP> Precursors and NAD<SUP>+</SUP>-Derived Signal Metabolites and Second Messengers: Old and New Topological Paradoxes

Abstract

NAD + metabolism and regulation are still incompletely defined in their features. Specifically, several compartmentation problems, both subcellular and intercellular, have emerged in the past years. A topological paradox was identified in the CD38/NAD + /Cyclic ADP-ribose system which represents a means for achieving regulation of intracellular Calcium levels ([Ca 2+ ] i and of Ca 2+ -mediated cell functions. CD38, an ectoenzyme featuring ADP-ribosyl cyclase (ADPRC) activity, is known to catalyze the generation of as many as eight Ca 2+ -active signal metabolites, including cADPR, ADPR, NAADP and three adenine homodinucleotides (Ap2A and two isomers thereof, designated P18 and P24). The first paradox concerns the ectocellular conversion of NAD + to cADPR and the intracellular activity of cADPR as a potent Ca 2+ mobilizer from stores expressing ryanodine receptors/channels on the endoplasmic/sarcoplasmic reticulum. This topological inconsistency was solved with the identification of hexameric Connexin 43 hemichannels (Cx43 HC) as an equilibrative transport system for NAD + , and of redundant equilibrative, and especially concentrative, transport systems for cADPR. Other transporters for NAADP + , P18 and P24 are still unknown. In addition, NAD + itself and some of its derivatives were recognized to be agonists of purinergic receptors, e.g., P2Y 11 and P2X 7 , thereby affecting the [Ca 2+ ] i levels either through protein kinase A-and phospholipase C-mediated pathways or through direct influx of extracellular Ca 2+ , respectively. Finally, recent findings demonstrate that the enzymes involved in NAD + biosynthesis and some of the enzymes involved in the multiple pathways of NAD + utilization localize to distinct subcellular compartments of the same cell and, surprisingly, also to different cell types of the same organism. Therefore, NAD + biosynthesis from several precursors, collectively defined Vitamin B3 forms, is at the same time an organismal and a cellular process, whose individual steps may occur in different cells/tissues/organs. Thus, a paracrine crosstalk is activated via the exchange of intermediate metabolites in biological fluids (e.g., Nicotinamide mononucleotide and Nicotinamide riboside), and the eventual NAD + biosynthesis occurs in selected cells able to utilize it for the display of diverse, fundamental cell functions. These are driven by NAD + -consuming enzymes, as ADPRCs, mono ADP-ribosyltransferases, poly(ADP-ribose) polymerases and different sirtuins (NAD + -dependent protein deacetylases).

Cytosolic interaction of type III human CD38 with CIB1 modulates cellular cyclic ADP-ribose levels

“…A vesicular mechanism has been proposed, positing that CD38 can be endocytosed, together with transport proteins, into endolysosomes. The transporters can then mediate the influx of substrates for CD38 and efflux of cADPR from the endolysosomes to act on the endoplasmic Ca 2+ stores (14,15). Recently, we have shown that CD38, in fact, can exist in two opposite membrane orientations, not only in the expected type II orientation but also in the opposite type III orientation, with the catalytic C-domain facing the cytosol (16).…”

Immunohistochemistry of CD38 in Different Cell Types in the Hypothalamus and Pituitary of Male Mice

“…Thus, an astrocyte-to-neuron calcium signal can be triggered in such cells by the CD38/cADPR system, which, through the activation of (Verderio et al, 2001). Further, co-culture of either cortical or hippocampal astrocytes with neurons results in a significant over-expression of astrocytic CD38, both on the plasma membrane and intracellularly, and revealed a new neuron-to-astrocyte glutamatergic signaling based on the CD38/cADPR system; this affects [Ca 2+ ] i in both cell types, adding further complexity to the bi-directional patterns of communication between astrocytes and neurons (Bruzzone et al, 2012;2004). Not only in cultured astrocytes but also in astrocytes in situ, the expression of CD38 and a second messenger triggering Ca 2+ release from intracellular stores, either cADPR (Banerjee et al, 2008) or NAADP + (Heidemann et al, 2005), has been observed.…”

“…We have shown that secretion of OT is dependent on Ca 2+ amplification through ryanodine receptors sensitive to cyclic ADP-ribose (cADPR) (Bruzzone et al, 2012;Lee, 2012), while that of AVP is not Jin et al, 2007;Liu et al, 2008;Liu et al, 2012). Although the precise mechanism for this discrepancy is not clear, the simplest explanation for this may be the differential expression of CD38 in OT or AVP neurons.…”

“…The paracrine action of cADPR has been proposed through extracellular synthesis by the ectoenzyme activity of CD38 and cellular uptake of cADPR by the nucleotide transporter activity of CD38 (Bruzzone et al, 2012;De Flora et al, 2004), and also by possible other molecules (Rah and Kim, 2012;Walseth et al, 2012). On the other hand, importance of the type III orientation of CD38 (carboxyl-terminal catalytic domain is located inside the cell) compared to type II (catalytic domain is outside of the cell) was reported in its Ca 2+ signaling function by increasing intracellular concentrations of cADPR.…”

Acetylcholine-Induced M/KCNQ Current Inhibition is Regulated by CD38 and A-Kinase Anchoring Protein in Neuroblastoma Cells

“…The key enzyme responsible for synthesis of cADPR is ADP-ribosyl cyclase (Lee et al, 1997, 2012a andHigashida, et al, 1997 and2012a andDe Flora et al, 2004;Podesta et al, 2005;Bruzzone et al, 2012). Intriguingly, this enzyme shows a high degree of identity to the mammalian T cell surface marker CD38 (Takasawa et al, 1993;Malavasi et al, 2008;Lee, 2012a).…”