Transforming growth factor-activated kinase 1 induced in spinal astrocytes contributes to mechanical hypersensitivity after nerve injury

MAPK induces AQP1 expression in astrocytes following injury

Aquaporin-4 (AQP4) is the principle water channel and the primary route for water transport across astrocytic membranes. AQP4 co-localizes with Kir4.1 channels at astrocytic endfeet, and it has been suggested that these channels cooperate in K(+) and water homeostasis. In response to injury, two additional aquaporins, AQP1 and AQP9, can be detected in astrocytes, yet neither is found in cultured astrocytes, and therefore their contribution to astrocyte water uptake and biology is poorly investigated. In this study, we used a cortical stab wound assay to demonstrate an upregulation of AQP1 following injury in reactive glia. We were able to mimic such injury in astrocytic cultures and show that AQP1 expression is induced within 16 h following injury in vitro. This induction could be blocked by inhibition of MEK1/2 using U0126, and suggests that AQP1 is specifically induced in reactive astrocytes via the mitogen-activated protein kinases signaling pathway.

Pain and purinergic signaling

A growing body of evidence indicates that extracellular nucleotides play important roles in the regulation of neuronal and glial functions in the nervous system through P2 purinoceptors. P2 purinoceptors are divided into two families, ionotropic receptors (P2X) and metabotropic receptors (P2Y). P2X receptors (seven types; P2X1-P2X7) contain intrinsic pores that open by binding with ATP, and P2Y receptors (eight types; P2Y1, 2, 4, 6, 11, 12, 13 and 14) are activated by nucleotides and couple to intracellular second-messenger systems through heterotrimeric G-proteins. Nucleotides are released or leaked from non-excitable cells as well as neurons in physiological and pathophysiological conditions. Studies have shown that microglia, a type of glial cells known as resident macrophages in the CNS, express several subtypes of P2X and P2Y receptors, and these receptors play a key role in pain signaling in the spinal cord under pathological conditions such as by peripheral nerve injury (called neuropathic pain). Within the spinal dorsal horn, peripheral nerve injury leads to a progressive series of changes in microglia including morphological hypertrophy of the cell body and proliferation, which are considered indicative of activation. These activated microglia upregulate expression of P2X/Y receptors (e.g., P2X4 and P2Y12). Importantly, pharmacological, molecular and genetic manipulations of the function or expression of these microglial molecules strongly suppress neuropathic pain. We expect that further investigation to determine how ATP signaling via P2X receptors participates in the pathogenesis of chronic pain will lead to a better understanding of the molecular mechanisms of pathological pain and provide clues for the development of new therapeutic drugs.

Interleukin-18-mediated microglia/astrocyte interaction in the spinal cord enhances neuropathic pain processing after nerve injury

Interleukin (IL)-18 is an important regulator of innate and acquired immune responses. Here we show that both the IL-18 and IL-18 receptor (IL-18R), which are induced in spinal dorsal horn, are crucial for tactile allodynia after nerve injury. Nerve injury induced a striking increase in IL-18 and IL-18R expression in the dorsal horn, and IL-18 and IL-18R were upregulated in hyperactive microglia and astrocytes, respectively. The functional inhibition of IL-18 signaling pathways suppressed injury-induced tactile allodynia and decreased the phosphorylation of nuclear factor kappaB in spinal astrocytes and the induction of astroglial markers. Conversely, intrathecal injection of IL-18 induced behavioral, morphological, and biochemical changes similar to those observed after nerve injury. Our results indicate that IL-18-mediated microglia/astrocyte interactions in the spinal cord have a substantial role in the generation of tactile allodynia. Thus, blocking IL-18 signaling in glial cells might provide a fruitful strategy for treating neuropathic pain.

Inverse relation between intensity of GFAP expression in the substantia gelatinosa and degree of chronic mechanical allodynia

Glial cells are known to have a large impact on neuropathic pain conditions. Within the spinal cord, microglia rapidly respond to peripheral nerve injury, resulting in central sensitization and ultimately in the onset of enhanced pain behaviour. Astroglia respond with a short delay and are thought to contribute to the early maintenance of neuropathic pain. Nevertheless, it is unknown whether the roles of these glial cell types can be influenced by the chronicity of the neuropathology. Here, the persistent responses of astroglia and microglia to peripheral nerve injury within central pain networks in the upper dorsal horn laminae were studied. At 12 weeks after complete sciatic nerve injury, upregulation of glial fibrillary acidic protein (GFAP), but not complement receptor-3, could be detected in laminae II and III. Moreover, it was found that neuropathic animals with a higher degree of mechanical allodynia had a lower intensity of GFAP expression in lamina II (substantia gelatinosa). From these data we conclude that the role of astroglial responses in mechanical allodynia after peripheral nerve injury may be less straightforward as previously thought. Although astroglia are known to play a pro-nociceptive role in early neuropathic pain states, this role may shift to anti-nociception in more chronic pain states.

MAP kinase and pain

Mitogen-activated protein kinases (MAPKs) are important for intracellular signal transduction and play critical roles in regulating neural plasticity and inflammatory responses. The MAPK family consists of three major members: extracellular signal-regulated kinases (ERK), p38, and c-Jun N-terminal kinase (JNK), which represent three separate signaling pathways. Accumulating evidence shows that all three MAPK pathways contribute to pain sensitization after tissue and nerve injury via distinct molecular and cellular mechanisms. Activation (phosphorylation) of MAPKs under different persistent pain conditions results in the induction and maintenance of pain hypersensitivity via non-transcriptional and transcriptional regulation. In particular, ERK activation in spinal cord dorsal horn neurons by nociceptive activity, via multiple neurotransmitter receptors, and using different second messenger pathways plays a critical role in central sensitization by regulating the activity of glutamate receptors and potassium channels and inducing gene transcription. ERK activation in amygdala neurons is also required for inflammatory pain sensitization. After nerve injury, ERK, p38, and JNK are differentially activated in spinal glial cells (microglia vs astrocytes), leading to the synthesis of proinflammatory/pronociceptive mediators, thereby enhancing and prolonging pain. Inhibition of all three MAPK pathways has been shown to attenuate inflammatory and neuropathic pain in different animal models. Development of specific inhibitors for MAPK pathways to target neurons and glial cells may lead to new therapies for pain management. Although it is well documented that MAPK pathways can increase pain sensitivity via peripheral mechanisms, this review will focus on central mechanisms of MAPKs, especially ERK.