Deletion of ENTPD3 does not impair nucleotide hydrolysis in primary somatosensory neurons or spinal cord

Global deletion of NTPDase3 protects against diet-induced obesity by increasing basal energy metabolism

BACKGROUND

Ecto-nucleoside triphosphate diphosphohydrolase 3 (NTPDase3), also known as CD39L3, is the dominant ectonucleotidase expressed by beta cells in the islet of Langerhans and on nerves. NTPDase3 catalyzes the conversion of extracellular ATP and ADP to AMP and modulates purinergic signaling. Previous studies have shown that NTPDase3 decreases insulin release from beta-cells in vitro. This study aims to determine the impact of NTPDase3 in diet-induced obesity (DIO) and metabolism in vivo.

METHODS

We developed global NTPDase3 deficient (Entpd3^-/-) and islet beta-cell-specific NTPDase-3 deficient mice (Entpd3^{flox/flox,InsCre}) using Ins1-Cre targeted gene editing to compare metabolic phenotypes with wildtype (WT) mice on a high-fat diet (HFD).

RESULTS

Entpd3^-/- mice exhibited similar growth rates compared to WT on chow diet. When fed HFD, Entpd3^-/- mice demonstrated significant resistance to DIO. Entpd3^-/- mice consumed more calories daily and exhibited less fecal calorie loss. Although Entpd3^-/- mice had no increases in locomotor activity, the mice exhibited a significant increase in basal metabolic rate when on the HFD. This beneficial phenotype was associated with improved glucose tolerance, but not higher insulin secretion. In fact, Entpd3^{flox/flox,InsCre} mice demonstrated similar metabolic phenotypes and insulin secretion compared to matched controls, suggesting that the expression of NTPDase3 in beta-cells was not the primary protective factor. Instead, we observed a higher expression of uncoupling protein 1 (UCP-1) in brown adipose tissue and an augmented browning in inguinal white adipose tissue with upregulation of UCP-1 and related genes involved in thermogenesis in Entpd3^-/- mice.

CONCLUSIONS

Global NTPDase3 deletion in mice is associated with resistance to DIO and obesity-associated glucose intolerance. This outcome is not driven by the expression of NTPDase3 in pancreatic beta-cells, but rather likely mediated through metabolic changes in adipocytes.

Ectonucleoside triphosphate diphosphohydrolases and ecto-5'-nucleotidase in purinergic signaling: how the field developed and where we are now

Geoffrey Burnstock will be remembered as the scientist who set up an entirely new field of intercellular communication, signaling via nucleotides. The signaling cascades involved in purinergic signaling include intracellular storage of nucleotides, nucleotide release, extracellular hydrolysis, and the effect of the released compounds or their hydrolysis products on target tissues via specific receptor systems. In this context ectonucleotidases play several roles. They inactivate released and physiologically active nucleotides, produce physiologically active hydrolysis products, and facilitate nucleoside recycling. This review briefly highlights the development of our knowledge of two types of enzymes involved in extracellular nucleotide hydrolysis and thus purinergic signaling, the ectonucleoside triphosphate diphosphohydrolases, and ecto-5'-nucleotidase.

History of ectonucleotidases and their role in purinergic signaling

Ectonucleotidases are key for purinergic signaling. They control the duration of activity of purinergic receptor agonists. At the same time, they produce hydrolysis products as additional ligands of purinergic receptors. Due to the considerable diversity of enzymes, purinergic receptor ligands and purinergic receptors, deciphering the impact of extracellular purinergic receptor control has become a challenge. The first group of enzymes described were the alkaline phosphatases - at the time not as nucleotide-metabolizing but as nonspecific phosphatases. Enzymes now referred to as nucleoside triphosphate diphosphohydrolases and ecto-5'-nucleotidase were the first and only nucleotide-specific ectonucleotidases identified. And they were the first group of enzymes related to purinergic signaling. Additional research brought to light a surprising number of ectoenzymes with broad substrate specificity, which can also hydrolyze nucleotides. This short overview traces the development of the field and briefly highlights important results and benefits for therapies of human diseases achieved within nearly a century of investigations.

ATP metabolizing enzymes ENPP1, 2 and 3 are localized in sensory neurons of rat dorsal root ganglion

In dorsal root ganglion (DRG) neurons, ATP is an important neurotransmitter in nociceptive signaling through P2 receptors (P2Rs) such as P2X2/3R, and adenosine is also involved in anti-nociceptive signaling through adenosine A1R. Thus, the clearance system for adenine nucleotide/nucleoside plays a critical role in regulation of nociceptive signaling, but there is little information on it, especially ectoenzyme expression profiles in DRG. In this study, we examined expression and localization of ecto-nucleotide pyrophosphatase/phosphodiesterases (ENPPs), by which ATP is metabolized to AMP, in rat DRG. The mRNA expression levels of ENPP2 were greater than those of ENPP1 and ENPP3 in rat DRGs. On immunohistochemical analysis, ENPP1, 2 and 3 were found in soma of DRG neurons. Immunopositive rate of ENPP3 was greater than that of ENPP1 and ENPP2 in all DRG neurons. ENPP3, as compared with ENPP1 and ENPP2, was expressed mainly by isolectin B4-positive cells, and slightly by neurofilament 200-positive ones. In this way, the expression profile of ENPP1, 2 and 3 was different in DRGs, and they were mainly expressed in small/medium-sized DRG neurons. Moreover, ENPP1-, 2- and 3-immunoreactivities were colocalized with P2X2R, P2X3R and prostatic acid phosphatase (PAP), as an ectoenzyme for metabolism from AMP to adenosine. Additionally, PAP-immunoreactivity was colocalized with equilibrative nucleoside transporter (ENT) 1, as an adenosine uptake system. These results suggest that the clearance system consisted of ENPPs, PAP and ENT1 plays an important role in regulation of nociceptive signaling in sensory neurons.

Neuronal NTPDase3 Mediates Extracellular ATP Degradation in Trigeminal Nociceptive Pathway

ATP induces pain via activation of purinergic receptors in nociceptive sensory nerves. ATP signaling is terminated by ATP hydrolysis mediated by cell surface-localized ecto-nucleotidases. Using enzymatic histochemical staining, we show that ecto-ATPase activity is present in mouse trigeminal nerves. Using immunofluorescence staining, we found that ecto-NTPDase3 is expressed in trigeminal nociceptive neurons and their projections to the brainstem. In addition, ecto-ATPase activity and ecto-NTPDase3 are also detected in the nociceptive outermost layer of the trigeminal subnucleus caudalis. Furthermore, we demonstrate that incubation with anti-NTPDase3 serum reduces extracellular ATP degradation in the nociceptive lamina of both the trigeminal subnucleus caudalis and the spinal cord dorsal horn. These results are consistent with neuronal NTPDase3 activity modulating pain signal transduction and transmission by affecting extracellular ATP hydrolysis within the trigeminal nociceptive pathway. Thus, disruption of trigeminal neuronal NTPDase3 expression and localization to presynaptic terminals during chronic inflammation, local constriction and injury may contribute to the pathogenesis of orofacial neuropathic pain.