PERSPECTIVE NEUROSCIENCE
The Tao of IGF-1: Insulin-Like Growth Factor Receptor Activation Increases Pain by Enhancing T-Type Calcium Channel Activity Patrick L. Stemkowski and Gerald W. Zamponi* T-type calcium channels are important players in the transmission of pain signals in the primary afferent pathway. Indeed, inhibiting or depleting T-type calcium channels in dorsal root ganglion (DRG) neurons mediates analgesia. Conversely, nerve injury or peripheral inflammation have been shown to induce T-type calcium channel activity in DRG neurons, and this in turn has been linked to the development of chronic pain states. The mechanisms that underlie this enhancement of T-type channels remain incompletely understood and may include changes in channel stability in the plasma membrane or alterations in channel function. In this issue of Science Signaling, Zhang and colleagues identify a cell signaling pathway that potently regulates T-type calcium channel activity in afferent neurons and link this process to pain hypersensitivity. Specifically, they show that insulin-like growth factor-1 receptors in DRG neurons mediate a protein kinase C α (PKCα)–dependent enhancement of T-type calcium currents and that interfering with this pathway reduces both mechanical and thermal pain hypersensitivity in rodents. Targeting this process offers a new avenue for developing pain therapeutics.
Pain is an important physiological warning signal that alerts us to injury and infections (1). This process involves the initiation and propagation of action potentials in pain-sensing neurons that innervate tissues, such as the skin or internal organs, and culminates in neurotransmitter release in dorsal horn synapses and activation of second-order neurons that project to the brain, where we perceive pain as an unpleasant sensation (1). In response to nerve injury or inflammation, there is remodeling of ion channels and receptors in the afferent pain pathway (2), which results in its sensitization, thus giving rise to persistent pain that does not fulfill a useful biological purpose and is difficult to treat. This includes T-type calcium channels, which contribute to signaling in afferent neurons in two distinct ways: First, by virtue of their specific biophysical properties, they regulate the electrical excitability of afferent neurons (3). Second, they facilitate the release of neurotransmitters at nerve terminals in the spinal dorsal horn (4). Their enhancement after injury thus facilitates the initiation Department of Physiology and Pharmacology, Hotchkiss Brain Institute, Cumming School of Medicine, University of Calgary, Calgary, Alberta T2N 4N1, Canada. *Corresponding author. E-mail: zamponi@ ucalgary.ca
and transmission of pain-related information, whereas their inhibition produces analgesic effects (1). The dynamic regulation of T-type channel function under normal and pathophysiological conditions is subject to intense investigation and likely involves a combination of different cellular signaling pathways. In their intriguing study, Zhang and colleagues (5) investigated one of these signaling pathways—namely, the coupling of insulin-like growth factor-1 (IGF-1) receptors to T-type calcium channels. This receptor type is expressed in dorsal root ganglion (DRG) neurons, and its activation has been linked to the development of pain hypersensitivity (6), leading the authors to hypothesize that these receptors may perhaps mediate these effects by modulation of T-type channels. The authors first demonstrated that the amplitude of T-type calcium currents expressed endogenously in DRG neurons is enhanced upon acute activation of IGF-1 receptors. Concomitantly, activation of IGF-1 receptors shifts the half-inactivation potential of the channels toward more depolarized voltages. The latter effect is consistent with a direct functional modulation of channel activity, leading to an overall gain of function that is predicted to decrease the firing threshold of DRG neurons (3). Indeed, Zhang and colleagues observed an increase in membrane excitability that is consistent with such a mechanism. The au-
thors then dissected the underlying cell signaling pathway and showed that it involves Gαo signaling, followed by Gβγ-dependent activation of protein kinase C α (PKCα) and its translocation to the plasma membrane. This is particularly interesting because the IGF-1 receptor belongs to the family of tyrosine kinase receptors that normally do not couple directly to heterotrimeric G proteins. This then suggests the possibility that IGF1 receptor activation may perhaps cause the transactivation of a seven-transmembrane Gαo protein–coupled receptor, which in turn could activate PKCα through the Gβγ phospholipase C pathway (Fig. 1). The authors’ observed coimmunoprecipitation between the IGF-1 receptor and Gαo could thus be mediated indirectly through a classical G protein–coupled receptor (GPCR). Importantly, Zhang and colleagues showed that IGF-1 receptor–mediated sensitization to painful stimuli is prevented by blocking Cav3.2 T-type calcium channels. Conversely, inflammatory pain could be alleviated by an IGF-1 receptor antagonist, whose effects were precluded by knockdown of Cav3.2 T-type calcium channels. Altogether, their findings suggest that regulation of T-type calcium channels by the IGF-1 receptor in afferent fibers could be targeted with the development of new analgesics. T-type calcium current amplitude is increased in injured sensory neurons (3) and in DRG neurons of chronic diabetic mice (7), which then contribute to diabetic pain. Furthermore, in experimental models of inflammatory bowel disease, T-type currents are increased in sensory neurons that innervate the colon (8). It has been shown that these changes in T-type channel activity and expression involve multiple molecular mechanisms, including alterations in the glycosylation (9) and the ubiquitination (10) of the channels, both of which alter the trafficking of the channels to and from the plasma membrane. The findings of Zhang and colleagues suggest that T-type channel activity can also be acutely regulated through receptor-mediated changes in the biophysical properties of the channels. Interfering with dysregulation of channel trafficking and altering receptormediated augmentation of channel function both alleviated pain hypersensitivity, thus underscoring the importance of T-type channels in afferent pain signaling. The Research Article by Zhang et al. raises several important questions that are yet to be answered. First, it will be important to determine whether IGF-1 receptor–
www.SCIENCESIGNALING.org
7 October 2014 Vol 7 Issue 346 ps23
1
PERSPECTIVE IGF
Ca2+
IGF-1R
GPCR
?
T-type channel
G𰁠𰁡 G𰁟o
PLC
PKC𰁟
+ Portion of spinal cord cross section
DRG neuron Injury
IGF release
T-type activity
Spinal dorsal horn
Hyperexcitability
Ascending pain pathway
CREDIT: H. MCDONALD/SCIENCE SIGNALING
Fig. 1. Molecular and cellular mechanism that underlies IGF-1–mediated amplification of pain signals. Release of IGF-1 after peripheral injury enhances T-type channel current and DRG neuron excitability in nociceptors through the activation of IGF-1 receptors and the recruitment of a Gβγ-dependent PKCα pathway. This may potentially involve the transactivation of a Gαo-linked GPCR.
mediated regulation of Cav3.2 is important under normal physiological conditions and whether it is aberrantly activated or enhanced in response to nerve injury. Second, it would be interesting to determine whether IGF-1 receptor–mediated modulation can account for long-term enhancement of T-type channel activity in afferent neurons. Third, it remains to be determined whether PKCα directly phosphorylates the T-type channels or acts indirectly through an intermediary signaling molecule. Finally, the noteworthy findings of Zhang and colleagues present a conundrum: On one hand, IGF-1 receptor activation is important for nerve growth and regeneration after injury (11); on the other hand, this receptor contributes to pain hypersensitivity. Hence, if the regulatory mechanisms elucidated by Zhang and colleagues are to be targeted by pain therapeutics, it may be necessary to find means of selectively uncoupling T-type channels from the IGF-1 receptor without interfering with receptor coupling to path-
ways that support nerve regeneration. This could potentially be accomplished by preventing the association of PKCα with the T-type calcium channel or, alternatively, by preventing putative activation of classical GPCR signaling by the IGF-1 receptor. In summary, Zhang and colleagues present an intriguing cell signaling mechanism that couples IGF-1 receptors to T-type calcium channels that could potentially be harnessed for new pain therapeutics. References and Notes 1. E. Bourinet, C. Altier, M. E. Hildebrand, T. Trang, M. W. Salter, G. W. Zamponi, Calcium-permeable ion channels in pain signaling. Physiol. Rev. 94, 81–140 (2014). 2. S. G. Waxman, G. W. Zamponi, Regulating excitability of peripheral afferents: Emerging ion channel targets. Nat. Neurosci. 17, 153–163 (2014). 3. J. Yue, L. Liu, Z. Liu, B. Shu, Y. Zhang, Upregulation of T-type Ca2+ channels in primary sensory neurons in spinal nerve injury. Spine 38, 463–470 (2013). 4. M. O. Jacus, V. N. Uebele, J. J. Renger, S. M. Todorovic, Presynaptic Cav3.2 channels regulate excitatory neurotransmission in nociceptive dorsal horn neurons. J. Neurosci. 32, 9374–9382 (2012).
5. Y. Zhang, W. Qin, Z. Qian, X. Liu, H. Wang, S. Gong, Y.-G. Sun, T. P. Snutch, X. Jiang, J. Tao, Peripheral pain is enhanced by insulin-like growth factor 1 through a G protein–mediated stimulation of T-type calcium channels. Sci. Signal. 7, ra94 (2014). 6. M. Miura, M. Sasaki, K. Mizukoshi, M. Shibasaki, Y. Izumi, G. Shimosato, F. Amaya, Peripheral sensitization caused by insulin-like growth factor 1 contributes to pain hypersensitivity after tissue injury. Pain 152, 888–895 (2011). 7. M. M. Jagodic, S. Pathirathna, M. T. Nelson, S. Mancuso, P. M. Joksovic, E. R. Rosenberg, D. A. Bayliss, V. Jevtovic-Todorovic, S. M. Todorovic, Cell-specific alterations of T-type calcium current in painful diabetic neuropathy enhance excitability of sensory neurons. J. Neurosci. 27, 3305–3316 (2007). 8. F. Marger, A. Gelot, A. Alloui, J. Matricon, J. F. Ferrer, C. Barrère, A. Pizzoccaro, E. Muller, J. Nargeot, T. P. Snutch, A. Eschalier, E. Bourinet, D. Ardid, T-type calcium channels contribute to colonic hypersensitivity in a rat model of irritable bowel syndrome. Proc. Natl. Acad. Sci. U.S.A. 108, 11268–11273 (2011). 9. P. Orestes, H. P. Osuru, W. E. McIntire, M. O. Jacus, R. Salajegheh, M. M. Jagodic, W. Choe, J. Lee, S. S. Lee, K. E. Rose, N. Poiro, M. R. Digruccio, K. Krishnan, D. F. Covey, J. H. Lee, P. Q. Barrett, V. Jevtovic-Todorovic, S. M. Todorovic, Reversal of neuropathic pain in diabetes by targeting glycosylation of CaV3.2 T-type calcium channels. Diabetes 62, 3828–3838 (2013). 10. A. García-Caballero, V. M. Gadotti, P. Stemkowski, N. Weiss, I. A. Souza, V. Hodgkinson, C. Bladen, L. Chen, J. Hamid, A. Pizzoccaro, M. Deage, A. François, E. Bourinet, G. W. Zamponi, The deubiquitinating enzyme USP5 modulates neuropathic and inflammatory pain by enhancing Cav3.2 channel activity. Neuron 83, 1144–1158 (2014). 11. P. J. Apel, J. Ma, M. Callahan, C. N. Northam, T. B. Alton, W. E. Sonntag, Z. Li, Effect of locally delivered IGF-1 on nerve regeneration during aging: An experimental study in rats. Muscle Nerve 41, 335–341 (2010). Funding: Research in the Zamponi laboratory is supported by the Canadian Institutes of Health Research and Alberta Innovates Health Solutions (AI-HS). G.W.Z. holds a Canada Research Chair, and P.C.S. holds an AI-HS Fellowship. Competing Interests: The authors declare that they have no competing interests.
10.1126/scisignal.2005826
Citation: P. L. Stemkowski, G. W. Zamponi, The Tao of IGF-1: Insulin-like growth factor receptor activation increases pain by enhancing T-type calcium channel activity. Sci. Signal. 7, pe23 (2014).
www.SCIENCESIGNALING.org
7 October 2014 Vol 7 Issue 346 ps23
2
The Tao of IGF-1: Insulin-Like Growth Factor Receptor Activation Increases Pain by Enhancing T-Type Calcium Channel Activity Patrick L. Stemkowski and Gerald W. Zamponi* T-type calcium channels are important players in the transmission of pain signals in the primary afferent pathway. Indeed, inhibiting or depleting T-type calcium channels in dorsal root ganglion (DRG) neurons mediates analgesia. Conversely, nerve injury or peripheral inflammation have been shown to induce T-type calcium channel activity in DRG neurons, and this in turn has been linked to the development of chronic pain states. The mechanisms that underlie this enhancement of T-type channels remain incompletely understood and may include changes in channel stability in the plasma membrane or alterations in channel function. In this issue of Science Signaling, Zhang and colleagues identify a cell signaling pathway that potently regulates T-type calcium channel activity in afferent neurons and link this process to pain hypersensitivity. Specifically, they show that insulin-like growth factor-1 receptors in DRG neurons mediate a protein kinase C α (PKCα)–dependent enhancement of T-type calcium currents and that interfering with this pathway reduces both mechanical and thermal pain hypersensitivity in rodents. Targeting this process offers a new avenue for developing pain therapeutics.
Pain is an important physiological warning signal that alerts us to injury and infections (1). This process involves the initiation and propagation of action potentials in pain-sensing neurons that innervate tissues, such as the skin or internal organs, and culminates in neurotransmitter release in dorsal horn synapses and activation of second-order neurons that project to the brain, where we perceive pain as an unpleasant sensation (1). In response to nerve injury or inflammation, there is remodeling of ion channels and receptors in the afferent pain pathway (2), which results in its sensitization, thus giving rise to persistent pain that does not fulfill a useful biological purpose and is difficult to treat. This includes T-type calcium channels, which contribute to signaling in afferent neurons in two distinct ways: First, by virtue of their specific biophysical properties, they regulate the electrical excitability of afferent neurons (3). Second, they facilitate the release of neurotransmitters at nerve terminals in the spinal dorsal horn (4). Their enhancement after injury thus facilitates the initiation Department of Physiology and Pharmacology, Hotchkiss Brain Institute, Cumming School of Medicine, University of Calgary, Calgary, Alberta T2N 4N1, Canada. *Corresponding author. E-mail: zamponi@ ucalgary.ca
and transmission of pain-related information, whereas their inhibition produces analgesic effects (1). The dynamic regulation of T-type channel function under normal and pathophysiological conditions is subject to intense investigation and likely involves a combination of different cellular signaling pathways. In their intriguing study, Zhang and colleagues (5) investigated one of these signaling pathways—namely, the coupling of insulin-like growth factor-1 (IGF-1) receptors to T-type calcium channels. This receptor type is expressed in dorsal root ganglion (DRG) neurons, and its activation has been linked to the development of pain hypersensitivity (6), leading the authors to hypothesize that these receptors may perhaps mediate these effects by modulation of T-type channels. The authors first demonstrated that the amplitude of T-type calcium currents expressed endogenously in DRG neurons is enhanced upon acute activation of IGF-1 receptors. Concomitantly, activation of IGF-1 receptors shifts the half-inactivation potential of the channels toward more depolarized voltages. The latter effect is consistent with a direct functional modulation of channel activity, leading to an overall gain of function that is predicted to decrease the firing threshold of DRG neurons (3). Indeed, Zhang and colleagues observed an increase in membrane excitability that is consistent with such a mechanism. The au-
thors then dissected the underlying cell signaling pathway and showed that it involves Gαo signaling, followed by Gβγ-dependent activation of protein kinase C α (PKCα) and its translocation to the plasma membrane. This is particularly interesting because the IGF-1 receptor belongs to the family of tyrosine kinase receptors that normally do not couple directly to heterotrimeric G proteins. This then suggests the possibility that IGF1 receptor activation may perhaps cause the transactivation of a seven-transmembrane Gαo protein–coupled receptor, which in turn could activate PKCα through the Gβγ phospholipase C pathway (Fig. 1). The authors’ observed coimmunoprecipitation between the IGF-1 receptor and Gαo could thus be mediated indirectly through a classical G protein–coupled receptor (GPCR). Importantly, Zhang and colleagues showed that IGF-1 receptor–mediated sensitization to painful stimuli is prevented by blocking Cav3.2 T-type calcium channels. Conversely, inflammatory pain could be alleviated by an IGF-1 receptor antagonist, whose effects were precluded by knockdown of Cav3.2 T-type calcium channels. Altogether, their findings suggest that regulation of T-type calcium channels by the IGF-1 receptor in afferent fibers could be targeted with the development of new analgesics. T-type calcium current amplitude is increased in injured sensory neurons (3) and in DRG neurons of chronic diabetic mice (7), which then contribute to diabetic pain. Furthermore, in experimental models of inflammatory bowel disease, T-type currents are increased in sensory neurons that innervate the colon (8). It has been shown that these changes in T-type channel activity and expression involve multiple molecular mechanisms, including alterations in the glycosylation (9) and the ubiquitination (10) of the channels, both of which alter the trafficking of the channels to and from the plasma membrane. The findings of Zhang and colleagues suggest that T-type channel activity can also be acutely regulated through receptor-mediated changes in the biophysical properties of the channels. Interfering with dysregulation of channel trafficking and altering receptormediated augmentation of channel function both alleviated pain hypersensitivity, thus underscoring the importance of T-type channels in afferent pain signaling. The Research Article by Zhang et al. raises several important questions that are yet to be answered. First, it will be important to determine whether IGF-1 receptor–
www.SCIENCESIGNALING.org
7 October 2014 Vol 7 Issue 346 ps23
1
PERSPECTIVE IGF
Ca2+
IGF-1R
GPCR
?
T-type channel
G𰁠𰁡 G𰁟o
PLC
PKC𰁟
+ Portion of spinal cord cross section
DRG neuron Injury
IGF release
T-type activity
Spinal dorsal horn
Hyperexcitability
Ascending pain pathway
CREDIT: H. MCDONALD/SCIENCE SIGNALING
Fig. 1. Molecular and cellular mechanism that underlies IGF-1–mediated amplification of pain signals. Release of IGF-1 after peripheral injury enhances T-type channel current and DRG neuron excitability in nociceptors through the activation of IGF-1 receptors and the recruitment of a Gβγ-dependent PKCα pathway. This may potentially involve the transactivation of a Gαo-linked GPCR.
mediated regulation of Cav3.2 is important under normal physiological conditions and whether it is aberrantly activated or enhanced in response to nerve injury. Second, it would be interesting to determine whether IGF-1 receptor–mediated modulation can account for long-term enhancement of T-type channel activity in afferent neurons. Third, it remains to be determined whether PKCα directly phosphorylates the T-type channels or acts indirectly through an intermediary signaling molecule. Finally, the noteworthy findings of Zhang and colleagues present a conundrum: On one hand, IGF-1 receptor activation is important for nerve growth and regeneration after injury (11); on the other hand, this receptor contributes to pain hypersensitivity. Hence, if the regulatory mechanisms elucidated by Zhang and colleagues are to be targeted by pain therapeutics, it may be necessary to find means of selectively uncoupling T-type channels from the IGF-1 receptor without interfering with receptor coupling to path-
ways that support nerve regeneration. This could potentially be accomplished by preventing the association of PKCα with the T-type calcium channel or, alternatively, by preventing putative activation of classical GPCR signaling by the IGF-1 receptor. In summary, Zhang and colleagues present an intriguing cell signaling mechanism that couples IGF-1 receptors to T-type calcium channels that could potentially be harnessed for new pain therapeutics. References and Notes 1. E. Bourinet, C. Altier, M. E. Hildebrand, T. Trang, M. W. Salter, G. W. Zamponi, Calcium-permeable ion channels in pain signaling. Physiol. Rev. 94, 81–140 (2014). 2. S. G. Waxman, G. W. Zamponi, Regulating excitability of peripheral afferents: Emerging ion channel targets. Nat. Neurosci. 17, 153–163 (2014). 3. J. Yue, L. Liu, Z. Liu, B. Shu, Y. Zhang, Upregulation of T-type Ca2+ channels in primary sensory neurons in spinal nerve injury. Spine 38, 463–470 (2013). 4. M. O. Jacus, V. N. Uebele, J. J. Renger, S. M. Todorovic, Presynaptic Cav3.2 channels regulate excitatory neurotransmission in nociceptive dorsal horn neurons. J. Neurosci. 32, 9374–9382 (2012).
5. Y. Zhang, W. Qin, Z. Qian, X. Liu, H. Wang, S. Gong, Y.-G. Sun, T. P. Snutch, X. Jiang, J. Tao, Peripheral pain is enhanced by insulin-like growth factor 1 through a G protein–mediated stimulation of T-type calcium channels. Sci. Signal. 7, ra94 (2014). 6. M. Miura, M. Sasaki, K. Mizukoshi, M. Shibasaki, Y. Izumi, G. Shimosato, F. Amaya, Peripheral sensitization caused by insulin-like growth factor 1 contributes to pain hypersensitivity after tissue injury. Pain 152, 888–895 (2011). 7. M. M. Jagodic, S. Pathirathna, M. T. Nelson, S. Mancuso, P. M. Joksovic, E. R. Rosenberg, D. A. Bayliss, V. Jevtovic-Todorovic, S. M. Todorovic, Cell-specific alterations of T-type calcium current in painful diabetic neuropathy enhance excitability of sensory neurons. J. Neurosci. 27, 3305–3316 (2007). 8. F. Marger, A. Gelot, A. Alloui, J. Matricon, J. F. Ferrer, C. Barrère, A. Pizzoccaro, E. Muller, J. Nargeot, T. P. Snutch, A. Eschalier, E. Bourinet, D. Ardid, T-type calcium channels contribute to colonic hypersensitivity in a rat model of irritable bowel syndrome. Proc. Natl. Acad. Sci. U.S.A. 108, 11268–11273 (2011). 9. P. Orestes, H. P. Osuru, W. E. McIntire, M. O. Jacus, R. Salajegheh, M. M. Jagodic, W. Choe, J. Lee, S. S. Lee, K. E. Rose, N. Poiro, M. R. Digruccio, K. Krishnan, D. F. Covey, J. H. Lee, P. Q. Barrett, V. Jevtovic-Todorovic, S. M. Todorovic, Reversal of neuropathic pain in diabetes by targeting glycosylation of CaV3.2 T-type calcium channels. Diabetes 62, 3828–3838 (2013). 10. A. García-Caballero, V. M. Gadotti, P. Stemkowski, N. Weiss, I. A. Souza, V. Hodgkinson, C. Bladen, L. Chen, J. Hamid, A. Pizzoccaro, M. Deage, A. François, E. Bourinet, G. W. Zamponi, The deubiquitinating enzyme USP5 modulates neuropathic and inflammatory pain by enhancing Cav3.2 channel activity. Neuron 83, 1144–1158 (2014). 11. P. J. Apel, J. Ma, M. Callahan, C. N. Northam, T. B. Alton, W. E. Sonntag, Z. Li, Effect of locally delivered IGF-1 on nerve regeneration during aging: An experimental study in rats. Muscle Nerve 41, 335–341 (2010). Funding: Research in the Zamponi laboratory is supported by the Canadian Institutes of Health Research and Alberta Innovates Health Solutions (AI-HS). G.W.Z. holds a Canada Research Chair, and P.C.S. holds an AI-HS Fellowship. Competing Interests: The authors declare that they have no competing interests.
10.1126/scisignal.2005826
Citation: P. L. Stemkowski, G. W. Zamponi, The Tao of IGF-1: Insulin-like growth factor receptor activation increases pain by enhancing T-type calcium channel activity. Sci. Signal. 7, pe23 (2014).
www.SCIENCESIGNALING.org
7 October 2014 Vol 7 Issue 346 ps23
2
