Pain is an important physiological function, whose primary role is to preserve an organism’s integrity. Disruption of the nociception transduction chain results in the inability to perceive pain. Among these “painlessness” pathologies, Hereditary Sensory and Autonomic Neuropathy type V (HSAN V) is caused by the 661C>T transition in the ngf gene, resulting in the R100W missense mutation in mature Nerve Growth Factor (NGF), in keeping with the key role of this neurotrophin in the development of nociceptors and in their function in the adult . Homozygous HSAN V patients display indifference to noxious stimuli but, no cognitive deficits. In contrast, heterozygous carriers do not show an overt clinical phenotype and have been identified only through pedigree and genetic screening. Considering the particular features of HSAN V patients, I hypothesized that the R100W mutation might cause a dissociation between the actions of NGF on the central and peripheral nervous systems. To test this hypothesis and understand the mechanisms underlying the HSAN V phenotype, I generated a transgenic mouse line harboring the human 661C>T mutation in the human ngf gene. Homozygous NGFR100W/R100W mice were born normal, but failed to reach the first month of age. This early lethality could be due to reduced NGF bioavailability and, indeed, was rescued by continuous treatment, during development and the early postnatal life, with wild type NGF. In contrast, heterozygous NGFR100W/m mice grew normally but displayed impaired nociception, despite Dorsal Root Ganglia (DRGs) neurons being morphologically normal. On the other hand, skin innervation was reduced. The NGFR100W protein showed reduced capability to activate pain-specific signalling, paralleling its reduced ability to induce mechanical allodynia. Surprisingly, NGFR100W/m mice, unlike heterozygous mNGF+/- mice, showed no learning nor memory deficits, despite a reduction in secretion and brain levels of NGF. These results prove the hypothesized dissociation between the peripheral and central actions of NGF, prompting me to investigate if the R100W mutation might affect brain elaboration of pain. To address this issue, I used the fear conditioning test and found that NGFR100W/m mice, despite normal nociceptive responses to a painful conditioning stimulus, showed a deficit in learned fear. Strikingly, their innate fear responses were normal. This was accompanied by a reduced activation of brain regions involved in pain processing and in the generation of task-related motor responses. I also found a decreased density of CGRP-positive fibers in the amygdala, which can provide a mechanistic explanation of the reduced fear response. On the other hand, the expression of endogenous analgesic peptides, namely β-endorphin and oxytocin, was decreased in NGFR100W/m mice, suggesting a different set point of the homeostatic pain/analgesia system, as a consequence of a prolonged reduction of afferent pain signals. Consistent with these findings in mice, data collected in humans showed that heterozygous R100W carriers, despite having a normal pain threshold, had a decreased urgency to react to a painful stimulus, along with impaired ability to integrate sensory information with behavioral task requirements. Functional magnetic resonance imaging (fMRI) revealed, in accordance with mouse data, an altered processing of painful stimuli in brain areas involved in pain processing. These findings demonstrate an uncoupling of nociceptive signals from their central elaboration, leading to altered interpretation and meaning attributed to painful stimuli in human HSAN V carriers and heterozygous NGFR100W/m mice. In addition to clarify the role of NGF in transduction of nociceptive inputs, these data also demonstrate that NGF is at the center of a regulation system linking peripheral nociception to the brain processes responsible for constructing painful perceptions and pain-related memories. Moreover, the peculiar effects of NGFR100W could be exploited to open new avenues for treating conditions of chronic pain.
Understanding Pain Construction from Nociception through a Novel Mutation in Nerve Growth Factor / Testa, Giovanna. - (2019 Jul 25).
|Titolo:||Understanding Pain Construction from Nociception through a Novel Mutation in Nerve Growth Factor|
|Data di pubblicazione:||2019-07-25|
|Corso PhD:||Scienze biologiche|
|Settore Scientifico Disciplinare:||BIO/12 BIOCHIMICA CLINICA E BIOLOGIA MOLECOLARE CLINICA|
|Parole chiave (inglese):||Hereditary Sensory and Autonomic Neuropathy type V (HSAN V)|
Nerve Growth Factor (NGF)
nociception transduction chain
|Editore:||Scuola Normale Superiore|
|Appare nelle tipologie:||9.1 Tesi di Dottorato|