Sarah E. Street

Peptidergic CGRPα Primary Sensory Neurons Encode Heat and Itch and Tonically Suppress Sensitivity to Cold

Calcitonin gene-related peptide (CGRP) is a classic molecular marker of peptidergic primary somatosensory neurons. Despite years of research, it is unknown whether these neurons are required to sense pain or other sensory stimuli. Here, we found that genetic ablation of CGRPα-expressing sensory neurons reduced sensitivity to noxious heat, capsaicin, and itch (histamine and chloroquine) and impaired thermoregulation but did not impair mechanosensation or β-alanine itch-stimuli associated with nonpeptidergic sensory neurons. Unexpectedly, ablation enhanced behavioral responses to cold stimuli and cold mimetics without altering peripheral nerve responses to cooling. Mechanistically, ablation reduced tonic and evoked activity in postsynaptic spinal neurons associated with TRPV1/heat, while profoundly increasing tonic and evoked activity in spinal neurons associated with TRPM8/cold. Our data reveal that CGRPα sensory neurons encode heat and itch and tonically cross-inhibit cold-responsive spinal neurons. Disruption of this crosstalk unmasks cold hypersensitivity, with mechanistic implications for neuropathic pain and temperature perception.

Prostatic Acid Phosphatase Reduces Thermal Sensitivity and Chronic Pain Sensitization by Depleting Phosphatidylinositol 4,5-Bisphosphate

Prostatic acid phosphatase (PAP) is expressed in nociceptive dorsal root ganglion (DRG) neurons, functions as an ectonucleotidase, and generates adenosine extracellularly. Here, we found that PAP inhibits noxious thermal sensitivity and sensitization that is associated with chronic pain through sustained activation of the adenosine A1 receptor (A1R) and phospholipase C-mediated depletion of phosphatidylinositol 4,5-bisphosphate (PIP2). In mice, intrathecal injection of PAP reduced PIP2 levels in DRGs, inhibited thermosensation through TRPV1, and enduringly reduced thermal hyperalgesia and mechanical allodynia caused by inflammation, nerve injury, and pronociceptive receptor activation. This included inhibitory effects on lysophosphatidic acid, purinergic (ATP), bradykinin, and protease-activated (thrombin) receptors. Conversely, PIP2 levels were significantly elevated in DRGs from Pap−/− mice, and this correlated with enhanced thermal hyperalgesia and mechanical allodynia in Pap−/− mice. To directly test the importance of PIP2 in nociception, we intrathecally injected PIP2 into mice. This transiently (2 h) elevated PIP2 levels in lumbar DRGs and transiently (2 h) enhanced thermosensation. Additionally, thermal hyperalgesia and mechanical allodynia were enduringly enhanced when PIP2 levels were elevated coincident with injury/pronociceptive receptor stimulation. Nociceptive sensitization was not affected if PIP2 levels were elevated in the absence of ongoing pronociceptive receptor stimulation. Together, our data suggest that PIP2 levels in DRGs directly influence thermosensation and the magnitude of nociceptive sensitization. Moreover, our data suggest there is an underlying “phosphoinositide tone” that can be manipulated by an adenosine-generating ectonucleotidase. This tone regulates how effectively acute nociceptive insults promote the transition to chronic pain.

Tissue-Nonspecific Alkaline Phosphatase Acts Redundantly with PAP and NT5E to Generate Adenosine in the Dorsal Spinal Cord

Prostatic acid phosphatase (PAP) and ecto-5′-nucleotidase (NT5E) hydrolyze extracellular AMP to adenosine in dorsal root ganglia (DRG) neurons and in the dorsal spinal cord. Previously, we found that adenosine production was reduced, but not eliminated, in Pap−/−/Nt5e−/− double knock-out (dKO) mice, suggesting that a third AMP ectonucleotidase was present in these tissues. Here, we found that tissue-nonspecific alkaline phosphatase (TNAP, encoded by the Alpl gene) is expressed and functional in DRG neurons and spinal neurons. Using a cell-based assay, we found that TNAP rapidly hydrolyzed extracellular AMP and activated adenosine receptors. This activity was eliminated by MLS-0038949, a selective pharmacological inhibitor of TNAP. In addition, MLS-0038949 eliminated AMP hydrolysis in DRG and spinal lamina II of dKO mice. Using fast-scan-cyclic voltammetry, we found that adenosine was rapidly produced from AMP in spinal cord slices from dKO mice, but virtually no adenosine was produced in spinal cord slices from dKO mice treated with MLS-0038949. Last, we found that AMP inhibited excitatory neurotransmission via adenosine A1 receptor activation in spinal cord slices from wild-type, Pap−/−, Nt5e−/−, and dKO mice, but failed to inhibit neurotransmission in slices from dKO mice treated with MLS-0038949. These data suggest that triple elimination of TNAP, PAP, and NT5E is required to block AMP hydrolysis to adenosine in DRG neurons and dorsal spinal cord. Moreover, our data reveal that TNAP, PAP, and NT5E are the main AMP ectonucleotidases in primary somatosensory neurons and regulate physiology by metabolizing extracellular purine nucleotides.

The lipid kinase PIP5K1C regulates pain signaling and sensitization.

Numerous pain-producing (pronociceptive) receptors signal via phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis. However, it is currently unknown which lipid kinases generate PIP2 in nociceptive dorsal root ganglia (DRG) neurons and if these kinases regulate pronociceptive receptor signaling. Here, we found that phosphatidylinositol 4-phosphate 5 kinase type 1C (PIP5K1C) is expressed at higher levels than any other PIP5K and, based on experiments with Pip5k1c(+/-) mice, generates at least half of all PIP2 in DRG neurons. Additionally, Pip5k1c haploinsufficiency reduces pronociceptive receptor signaling and TRPV1 sensitization in DRG neurons as well as thermal and mechanical hypersensitivity in mouse models of chronic pain. We identified a small molecule inhibitor of PIP5K1C (UNC3230) in a high-throughput screen. UNC3230 lowered PIP2 levels in DRG neurons and attenuated hypersensitivity when administered intrathecally or into the hindpaw. Our studies reveal that PIP5K1C regulates PIP2-dependent nociceptive signaling and suggest that PIP5K1C is a therapeutic target for chronic pain.

Time and Technology Will Tell: The Pathophysiologic Basis of Neurohormonal Modulation in Heart Failure

The central roles of neurohormonal abnormalities in the pathobiology of heart failure have been defined in recent decades. Experiments have revealed both systemic involvement and intricate subcellular regulation by circulating effectors of the sympathetic nervous system, the renin-angiotensin-aldosterone system, and others. Randomized clinical trials substantiated these findings, establishing neurohormonal antagonists as cornerstones of heart failure pharmacotherapy, and occasionally offering further insight on mode of benefit. This review discusses the use of β-blockers, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, and aldosterone receptor antagonists in the treatment of heart failure, with particular attention to the pathophysiologic basis and mechanisms of action.

Emerging Roles for Ectonucleotidases in Pain-Sensing Neurons

Nociceptive neurons located in the dorsal root ganglia detect painful stimuli and can be sensitized following inflammation or nerve injury. Many analgesics have ‘antinociceptive’ effects, which mean these drugs can reduce noxious thermal and mechanical sensitization—two symptoms that are associated with chronic pain. One drug that has been studied for its antinociceptive effects in rodents and humans is adenosine (Sawynok and Liu, 2003). Adenosine exerts its antinociceptive effects by activating the adenosine A1 receptor (A1R). A1R is expressed by nociceptive neurons and many other cells of the body, suggesting localized activation of this receptor in nociceptive neurons might inhibit pain without producing cardiovascular and other effects that are associated with systemic A1R activation. Recently, several new studies found that A1R can be activated locally near nociceptive neurons or their axons by ectonucleotidases—a class of enzymes that hydrolyze extracellular adenine nucleotides to adenosine. Moreover, this localized A1R activation was sufficient to inhibit chronic pain in animal models.

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

Ectonucleotidases are membrane-bound or secreted proteins that hydrolyze extracellular nucleotides. Recently, we identified three ectonucleotidases that hydrolyze extracellular adenosine 5’-monophosphate (AMP) to adenosine in primary somatosensory neurons. Currently, it is unclear which ectonucleotidases hydrolyze ATP and ADP in these neurons. Ectonucleoside triphosphate diphosphohydrolases (ENTPDs) comprise a class of enzymes that dephosphorylate extracellular ATP and ADP. Here, we found that ENTPD3 (also known as NTPDase3 or CD39L3) was located in nociceptive and non-nociceptive neurons of the dorsal root ganglion (DRG), in the dorsal horn of the spinal cord, and in free nerve endings in the skin. To determine if ENTPD3 contributes directly to ATP and ADP hydrolysis in these tissues, we generated and characterized an Entpd3 knockout mouse. This mouse lacks ENTPD3 protein in all tissues examined, including the DRG, spinal cord, skin, and bladder. However, DRG and spinal cord tissues from Entpd3 -/- mice showed no reduction in histochemical staining when ATP, ADP, AMP, or UTP were used as substrates. Additionally, using fast-scan cyclic voltammetry (FSCV), adenosine production was not impaired in the dorsal spinal cord of Entpd3 -/- mice when the substrate ADP was applied. Further, Entpd3 -/- mice did not differ in nociceptive behaviors when compared to wild-type mice, although Entpd3 -/- mice showed a modest reduction in β-alanine-mediated itch. Taken together, our data indicate that deletion of Entpd3 does not impair ATP or ADP hydrolysis in primary somatosensory neurons or in dorsal spinal cord. Moreover, our data suggest there could be multiple ectonucleotidases that act redundantly to hydrolyze nucleotides in these regions of the nervous system.