Proliferator-Activated Receptor-Gamma Coactivator-1α Haploinsufficiency Promotes Pain Chronification After Burn Injury

Aging-associated decrease of PGC-1α promotes pain chronification

Aging is generally associated with declining somatosensory function, which seems at odds with the high prevalence of chronic pain in older people. This discrepancy is partly related to the high prevalence of degenerative diseases such as osteoarthritis in older people. However, whether aging alters pain processing in the primary somatosensory cortex (S1), and if so, whether it promotes pain chronification is largely unknown. Herein, we report that older mice displayed prolonged nociceptive behavior following nerve injury when compared with mature adult mice. The expression of peroxisome proliferator-activated receptor-gamma coactivator-1α (PGC-1α) in S1 was decreased in older mice, whereas PGC-1α haploinsufficiency promoted prolonged nociceptive behavior after nerve injury. Both aging and PGC-1α haploinsufficiency led to abnormal S1 neural dynamics, revealed by intravital two-photon calcium imaging. Manipulating S1 neural dynamics affected nociceptive behavior after nerve injury: chemogenetic inhibition of S1 interneurons aggravated nociceptive behavior in naive mice; chemogenetic activation of S1 interneurons alleviated nociceptive behavior in older mice. More interestingly, adeno-associated virus-mediated expression of PGC-1α in S1 interneurons ameliorated aging-associated chronification of nociceptive behavior as well as aging-related S1 neural dynamic changes. Taken together, our results showed that aging-associated decrease of PGC-1α promotes pain chronification, which might be harnessed to alleviate the burden of chronic pain in older individuals.

The impact of burn injury on the central nervous system

Burn injuries can be devastating, with life-long impacts including an increased risk of hospitalization for a wide range of secondary morbidities. One area that remains not fully understood is the impact of burn trauma on the central nervous system (CNS). This review will outline the current findings on the physiological impact that burns have on the CNS and how this may contribute to the development of neural comorbidities including mental health conditions. This review highlights the damaging effects caused by burn injuries on the CNS, characterized by changes to metabolism, molecular damage to cells and their organelles, and disturbance to sensory, motor and cognitive functions in the CNS. This damage is likely initiated by the inflammatory response that accompanies burn injury, and it is often long-lasting. Treatments used to relieve the symptoms of damage to the CNS due to burn injury often target inflammatory pathways. However, there are non-invasive treatments for burn patients that target the functional and cognitive damage caused by the burn, including transcranial magnetic stimulation and virtual reality. Future research should focus on understanding the mechanisms that underpin the impact of a burn injury on the CNS, burn severity thresholds required to inflict damage to the CNS, and acute and long-term therapies to ameliorate deleterious CNS changes after a burn.

Sp1 Inhibits PGC-1α via HDAC2-Catalyzed Histone Deacetylation in Chronic Constriction Injury-Induced Neuropathic Pain

BACKGROUND

Our previous study has illuminated that PGC-1α downregulation promoted chronification of pain after burn injury. RNA-seq analysis predicted association between Sp1 and chronic constriction injury (CCI)-provoked neuropathic pain. Further ChIP-Atlas data investigation suggested the binding to Sp1 to PGC-1α. Thereby, we performed this study to illustrate the functional relevance of the Sp1/PGC-1α axis in neuropathic pain.

METHODS

Neuropathic pain was induced by CCI in vivo in rats, followed by assessment of neuropathic pain-like behaviors. The expression of Sp1 and correlated genes was determined in CCI rat spinal cord tissues. Furthermore, microglia were exposed to lipopolysaccharide (LPS) to mimic inflammation and then cocultured with neurons. Knockdown and ectopic expression methods were used in vivo and in vitro to define the role the Sp1/HDAC2/PGC-1α axis.

RESULTS

Sp1 expression was upregulated in spinal cord tissues of CCI rats. Silencing Sp1 ameliorated CCI-induced neuropathic pain, as reflected by elevated paw withdrawal threshold and paw withdrawal latency, as well as alleviated microglia activation, neuronal dysfunction, inflammatory responses, mitochondrial dysfunction, and oxidative stress in spinal cord tissues. Sp1 knockdown also reversed LPS-induced microglial inflammation and neuronal dysfunction. Sp1 promoted histone deacetylation in the PGC-1α promoter and inhibited PGC-1α expression via recruiting HDAC2. PGC-1α overexpression diminished CCI-induced neuropathic pain and LPS-induced inflammation and mitochondrial dysfunction, based on which Sp1 aggravated microglial inflammation and neuronal dysfunction in neuropathic pain.

CONCLUSION

This study elucidated the promoting effects of Sp1 on CCI-induced neuropathic pain via the HDAC2/PGC-1α axis.

Oral Cancer Cells Release Vesicles that Cause Pain

Oral cancer pain is attributed to the release from cancers of mediators that sensitize and activate sensory neurons. Intraplantar injection of conditioned media (CM) from human tongue cancer cell line HSC-3 or OSC-20 evokes nociceptive behavior. By contrast, CM from noncancer cell lines, DOK, and HaCaT are non-nociceptive. Pain mediators are carried by extracellular vesicles (EVs) released from cancer cells. Depletion of EVs from cancer cell line CM reverses mechanical allodynia and thermal hyperalgesia. CM from non-nociceptive cell lines become nociceptive when reconstituted with HSC-3 EVs. Two miRNAs (hsa-miR-21-5p and hsa-miR-221-3p) are identified that are present in increased abundance in EVs from HSC-3 and OSC-20 CM compared to HaCaT CM. The miRNA target genes suggest potential involvement in oral cancer pain of the toll like receptor 7 (TLR7) and 8 (TLR8) pathways, as well as signaling through interleukin 6 cytokine family signal transducer receptor (gp130, encoded by IL6ST) and colony stimulating factor receptor (G-CSFR, encoded by CSF3R), Janus kinase and signal transducer and activator of transcription 3 (JAK/STAT3). These studies confirm the recent discovery of the role of cancer EVs in pain and add to the repertoire of algesic and analgesic cancer pain mediators and pathways that contribute to oral cancer pain.

Sestrin2 overexpression attenuates osteoarthritis pain via induction of AMPK/PGC-1α-mediated mitochondrial biogenesis and suppression of neuroinflammation

BACKGROUND

Our previous study indicated that reactive oxygen species (ROS) are critically involved in chronic pain. Sestrin2 (Sesn2), a novel stress-inducible protein, is evidenced to reduce the generation of ROS. The study examined the role of Sesn2 in osteoarthritis (OA) pain and delineated the underlying molecular mechanisms.

METHODS

In the present study, we investigated the impact of Sesn2 on mitochondrial biogenesis in a rat model of OA pain. After adeno-associated viral (AAV)-Sesn2EGFP was injected for 14 days, OA was induced by intra-articular injection of monosodium iodoacetate (MIA). We assessed pain behaviors (weight-bearing asymmetry and paw withdrawal threshold) and explored possible mechanisms in the L4-6 spinal cord.

RESULTS

Our results showed that overexpression of Sesn2 in the spinal cord alleviated pain behaviors in OA rats. Moreover, overexpression of Sesn2 increased the activity of AMP-activated protein kinase (AMPK) signaling and significantly restored mitochondrial biogenesis. Besides, Sesn2 overexpression inhibited the activation of astrocytes and microglia, and decreased the production of interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) in the spinal cord of the OA pain rats. These effects were significantly reversed by an AMPK inhibitor.

CONCLUSIONS

Collectively, these results suggest that Sesn2 overexpression ameliorates mechanical allodynia and weight-bearing asymmetry in OA rats via activation of AMPK/PGC-1α-mediated mitochondrial biogenesis in the spinal cord. Moreover, Sesn2 overexpression attenuates OA-induced neuroinflammation at least partly by activating AMPK signaling. Sesn2 may become an encouraging therapeutic strategy for OA pain relief and other disorders.

5-HT1F Receptor Agonist Ameliorates Mechanical Allodynia in Neuropathic Pain via Induction of Mitochondrial Biogenesis and Suppression of Neuroinflammation

Neuropathic pain is a devastating disease that affects millions of people worldwide. Serotonin (5-hydroxytryptamine, 5-HT) is involved in pain modulation. Several lines of evidence have indicated that 5-HT1F receptor agonists are potent inducers of mitochondrial biogenesis. In this study, we tested the hypothesis that 5-HT1F receptor agonists ameliorate mechanical allodynia in neuropathic pain via the induction of mitochondrial biogenesis and suppression of neuroinflammation. Male Sprague–Dawley rats were used to establish a neuropathic pain model via spared nerve injury (SNI). The paw withdrawal threshold (PWT) was used to evaluate mechanical allodynia. Real-time polymerase chain reaction was used to examine the mitochondrial DNA (mtDNA) copy number. Western blotting and immunofluorescence were used to examine the expression of target proteins. Our results showed that mitochondrial biogenesis was impaired in the spinal cord of rats with SNI. Moreover, activation of PGC-1α, the master regulator of mitochondrial biogenesis, attenuates established mechanical allodynia in rats with neuropathic pain. In addition, the neuronal 5-HT1F receptor is significantly downregulated in the spinal cord of rats with neuropathic pain. Furthermore, the selective 5-HT1F receptor agonist lasmiditan attenuated established mechanical allodynia in rats with neuropathic pain. Finally, lasmiditan (Las) treatment restored mitochondrial biogenesis and suppressed neuroinflammation in the spinal cord of rats with SNI. These results provide the first evidence that lasmiditan ameliorates mechanical allodynia in neuropathic pain by inducing mitochondrial biogenesis and suppressing neuroinflammation in the spinal cord. Inducers of mitochondrial biogenesis may be an encouraging therapeutic option for the management of neuropathic pain.

The therapeutic potential of Nrf2 inducers in chronic pain: Evidence from preclinical studies

Chronic pain remains an enormous health problem affecting approximatively 30% of the world's population. Opioids as the first line analgesics often leads to undesirable side effects when used long term. Therefore, novel therapeutic targets are urgently needed to the development of more efficacious analgesics. Substantial evidence indicates that excessive reactive oxygen species (ROS) are extremely important to the development of chronic pain. Nuclear factor erythroid 2-related factor 2 (Nrf2) is a master transcription factor regulating endogenous antioxidant defense. Emerging evidence suggests that Nrf2 and its downstream effectors are implicated in chronic inflammatory and neuropathic pain. Notably, controversial results have been reported regarding the expression of Nrf2 and its downstream targets in peripheral and central regions involved in pain transmission. However, our recent studies and results from other laboratories demonstrate that Nrf2 inducers exert potent analgesic effects in various murine models of chronic pain. In this review, we summarized and discussed the preclinical evidence demonstrating the therapeutic potential of Nrf2 inducers in chronic pain. These evidence indicates that Nrf2 activation are beneficial in chronic pain mostly by alleviating ROS-associated pathological processes. Overall, Nrf2-based therapy for chronic pain is an area with great promise, but more research regarding its detailed mechanisms is warranted.