1 Congenital Pain Syndromes

From the evolutionary aspect pain is important to protect the human being from noxious stimuli. However, when pain becomes chronic (often caused by genetic diseases), it requires (sometimes lifelong) medical attention. About every fifth person in the world suffers from chronic pain (WHO-Guidelines 2008). To date often no satisfactory medication for chronic pain is available, which underlines the ongoing need for drug development. Recent model systems allow patient-specific genetic disease modeling, which can provide adapted medication and illustrates a way to personalized medicine. Voltage-gated sodium channels (Navs) are in the focus of recent analgesic drug development, as several mutations were identified in patients suffering from inherited pain syndromes. Navs are, among others, located on sensory neurons of the peripheral nervous system. Nociceptors are specialized sensory neurons, which detect potentially harmful (noxious) stimuli from the periphery or internal organs and transduce the information via long neurites to the spinal cord. Several Nav subtypes were identified as potentially disease causing when mutated: Nav1.7, Nav1.8, and Nav1.9, all of which are expressed in nociceptors (Leipold et al. 2013; Lampert et al. 2014).

1.1 Nav1.7 in Human Pain Syndromes

The first mutation to be linked to a congenital pain syndrome was described in the SCN9A gene, which codes for Nav1.7 (Yang et al. 2004). In the years to follow, several distinct inherited pain syndromes were identified to result from mutations in Nav1.7: paroxysmal extreme pain disorder [PEPD (Fertleman et al. 2006)], inherited erythromelalgia (IEM), and small fiber neuropathy [SFN (Faber et al. 2012a)]. All are characterized by gain-of-function mutations of Nav1.7, and patients suffer from intense burning pain. PEPD has a very early onset, sometimes already in utero, and is characterized by physical assaults of severe pain in proximal parts of the body such as the face and the anogenital region. Potential triggers of the attacks are cold, mechanical stress, or intestinal peristalsis, and attacks are often associated with reddening of the skin. IEM may also start during childhood but mostly manifests when patients are young adults. Patients show typically intense, burning pain and erythema in the extremities triggered by exposure to warmth or mild exercise. In contrast, the onset of SFN is usually later in life and correlates with a reduction of small nerve fibers in the skin epidermis (Cazzato and Lauria 2017), which seems to be contradictory to the burning pain sensation reported by the patients. Chronic insensitivity to pain (CIP) can be caused by homozygous functional knockout mutations of Nav1.7, rendering the channel protein nonfunctional. These patients do not feel pain and suffer from self-inflicted injuries. Interestingly, gain-of-function mutations of Nav1.9 can have similar phenotypes, which cannot be explained intuitively.

To date several dozens of pain-linked Nav mutations were described, and numerous reviews have summarized correlations between the clinical manifestations and the biophysical gating effects of the respective mutations (Drenth and Waxman 2007; Lampert et al. 2010, 2014; Dabby 2012; Hoeijmakers et al. 2012; Dib-Hajj et al. 2013; Waxman 2013; Bennett and Woods 2014; Brouwer et al. 2014; Habib et al. 2015; Vetter et al. 2017). It is quite clear that humans carrying nonfunctional Nav1.7 suffer from CIP, stressing the importance of Nav1.7 for human pain perception (Cox et al. 2006). A loss of Nav1.7 may result in the inability to produce action potentials in nociceptors, thereby a lack of signal transduction to the CNS, and thus the inability to sense pain. Still, the exact mechanism how a loss of Nav1.7 leads to insensitivity to pain remains to be determined, especially as reliable knockout mouse models are scarce. Recently, a Nav1.7 CIP patient was able to feel pain for the first time in her life after administration of naloxone, an opioid antagonist (Minett et al. 2015). This raises the question whether the endogenous opioid system may play an important role in Nav1.7 knockdown-mediated human loss of pain.

A characteristic feature of PEPD mutations is a depolarized, and thus impaired, fast channel inactivation, and every mutation tested so far showed enhanced resurgent currents. These are depolarizing sodium currents, which occur during the falling phase of the action potential and may thus support hyperexcitability (Cannon and Bean 2010). Resurgent currents are thought to occur due to an open channel block and can physiologically be found in some small sensory neurons and may be enhanced under pathological conditions (Jarecki et al. 2010; Theile et al. 2011; Eberhardt et al. 2014; Sittl et al. 2012).

IEM can be clearly linked to a facilitated channel activation of Nav1.7. Under physiological conditions, Navs open when the membrane potential reaches a certain voltage threshold, which occurs due to a depolarizing stimulus. Sodium ions enter the cell, and an action potential is initiated. In case of IEM, the activation threshold is shifted to more hyperpolarized potentials by most of the mutations. Thus, a smaller difference in the membrane potential is needed to open the channels for induction of action potentials, and therefore smaller stimuli are sufficient to elicit a nociceptive signal. To date more than 30 mutations in the Nav1.7 channel gene have been characterized, leading to IEM. 27 of them were shown to have a hyperpolarized activation threshold, when investigated by overexpression in cell lines. Unfortunately, up to now this shift was only shown in heterologous expression systems, as human sensory neurons are not easily available.

Electrophysiological analyses of SFN mutations paint a more complex picture. Some of these mutations show an impaired slow inactivation, while it remains unchanged in others. Increased resurgent currents were reported, but also a shift of fast inactivation to more depolarized potentials, which could render the expressing neuron hyperexcitable. When overexpressed in rodent sensory neurons, all SFN-linked Nav1.7 mutations produce an increased firing rate. Thus, a clear link to a biophysical characteristic, as it was proposed for IEM or PEPD, cannot be claimed for SFN. It also remains to be discovered, how a functional hyperexcitability may lead to degeneration of the small nerve fibers in the skin. Some data point toward a dysfunctional calcium homeostasis (Rolyan et al. 2016), but an impairment of skin-nerve signaling could also be involved.

Genetic screens of neuropathic pain patients led to the identification of mutations and genetic variations in other Nav channels, such as the already mentioned Nav1.8 and Nav1.9 subtypes, as well as Nav1.1 and Nav1.6. All of these may also play an important role in peripheral nociception. Nav1.8 and Nav1.9 will be discussed in the following section.

1.2 Nav1.8 and Nav1.9 in Pain (Less) Disorders

Animal and human studies have shown a role for Nav1.8 (encoded by the SCN10A gene) in pain symptoms. Mutations described in this gene are mostly linked to SFN (Faber et al. 2012b), but some are also described for IEM (Kist et al. 2016). Nav1.8 is important for the fast upstroke of the action potential and thus may have an influence on cellular excitability. Additionally, Nav1.8 knockout mice show impairments in the electrocardiogram [ECG (Yang et al. 2012)], suggesting a more general role for Nav1.8 in regulation of physiological functions.

Nav1.9 (encoded by the SCN11A gene), like Nav1.8, is resistant to the pufferfish toxin tetrodotoxin (TTX). Mutations in Nav1.9 were linked to gain of pain as well as to loss of pain. The channel is special as it is slowly gating and produces large persistent currents. Its open probability can be enhanced by G proteins, which are also part of the second messenger pathways of inflammatory processes that occur during tissue damage (Vanoye et al. 2013).

Patients suffering from pain were also reported to carry gain-of-function mutations of Nav1.9 (Zhang et al. 2013; Huang et al. 2014; Leipold et al. 2015; Han et al. 2017). The link between a gain of function and pain is intuitive, as the enhanced persistent current of Nav1.9 may support higher frequency firing. On the other hand, the L811P mutation, a very strong gain-of-function mutation, was described in 2013 in a patient suffering from CIP (Leipold et al. 2013). Nav1.9 channels with this mutation showed a slowed deactivation and strongly hyperpolarized voltage dependence of activation, suggesting that cells expressing this mutated Nav1.9 would be hyperexcitable. This apparent discrepancy between channel gain of function and clinical picture (loss-of-pain) was solved when the authors showed that the strong increase in activity of Nav1.9 led to a depolarized resting membrane potential and thus, after initial strong firing, to a depolarization block (Leipold et al. 2015). A similar mechanism was described for the L1302F mutation, which was identified in a patient with with impaired pain sensation, for which an U-shaped relationship between the resting membrane potential and the neuronal action potential threshold was reported (Phatarakijnirund et al. 2016; Huang et al. 2017). Also here, a strong gain-of-function phenotype determined the clinical symptoms characterized by a loss of pain. Electrophysiological studies showed that this is most likely due to the inactivation of voltage-gated ion channels by depolarization of the resting membrane potential. Patients carrying one of these two mutations suffer not only from CIP but also from hyperflexibility, gastrointestinal symptoms, itch, and bone defects, suggesting a wide range of physiological functions for Nav1.9.

Nav1.9 has been notoriously hard to express in heterologous cell systems. Nav1.8 knockout sensory neurons of mice were used successfully to express the TTX-resistant Nav1.9 for electrophysiological recordings (e.g., Leipold et al. 2013), and only very few groups managed to express this channel heterologously in neuronal cell lines (Leipold et al. 2013; Vanoye et al. 2013). Larger currents can be achieved by using a chimera of Nav1.9 and Nav1.4 (Goral et al. 2015), and only recently Lin et al. managed with major efforts to create a stable Nav1.9 cell line using human embryonic kidney (HEK293) cells, which is also ambenable to drug testing (Lin et al. 2016).

In order to investigate the impact of a Nav mutation, heterologous expression systems are valuable, and in some cases, as for many IEM mutations, 100% of clinical penetrance of the mutations can be observed, clearly showing the link between the mutation and the disease. Other genetic variants give a more complex picture and the identification of pain-associated SNPs stresses the importance of the patient’s specific genetic background when assessing the potential disease causing effects.

The pathological importance of genetic modifications is not easy to interpret, especially when focusing on single genetic variations in isolation, i.e., expressing one mutation, SNP, or genetic variation in a non-neuronal, non-patient-specific cellular background. Patient sensory neurons contain all relevant gene products of the specific person, and the combination of genetic variations may impact cell function and disease.

2 Translation from Dish to Rodent to Human

Over the past five decades, animal models have become an invaluable tool to study cellular, molecular, and mechanistic aspects of somatosensation and pain. However, which aspects of this knowledge can be translated to devise new analgesic strategies has become a major focus of discussion. In particular, the discrepancy between the number of successfully conducted preclinical studies and the number of clinical trials that had to be terminated or did not reveal any measurable improvement for the patients sparked a discussion among scientists and clinicians about the reasons for these failures and the possibilities to develop alternative, new approaches for future studies (Mogil 2009; Burma et al. 2017).

One of the possible reasons why compounds that did well in animal models failed to show beneficial effects in clinical trials may stem from the fact that too little is known about differences of fundamental processes that trigger pain responses and maladaptations in humans compared to lower vertebrates (Dib-Hajj 2014). Besides the psychological component of pain perception, which can be very different in human beings according to social upbringing, personal stress levels, and other factors – and which is currently not possible to model in animals – working with humans or tissues derived from humans has been very difficult in the past. The latter is due to the impeded accessibility of dorsal root ganglia or trigeminal ganglia, the native sensory tissue that houses the cell bodies of nociceptive neurons. Therefore, studying the molecular and functional make up of sensory neurons, the primary transducers of nociceptive and innocuous stimuli that can evoke pain, was hampered by limited access to human tissue – particularly in conditions that would allow functional characterization.

2.1 Heterologous Expression of Human Proteins

As native human sensory tissue is difficult to obtain, an easy way to investigate the function of a human protein is introducing an expression vector harboring the genetic information of the target molecule into a cell line such as HEK 293 cells. In many instances heterologous expression of the protein of interest is unproblematic, and the function of the introduced protein can be analyzed using different techniques, such as the patch-clamp technique. Many of the results described in the first part of this chapter were gathered in this way. In some cases, however, the target protein cannot be adequately and functionally expressed in conventionally used expression systems. Nevertheless, heterologous expression in cellular systems has been widely used in the past to gain information about human proteins and to compare them to their rodent counterparts. In the case of Nav channel physiology, researchers also used rodent dorsal root ganglion neurons as target cells to express and investigate the human channels in a more native environment. Using this method, differences between human and rodent Nav1.8 have been described (Han et al. 2015). The researchers took DRG neurons from a Nav1.8 knockout mouse line and introduced either human or rat Nav1.8. Thereby, they found that human Nav1.8 displays larger persistent, ramp, and window currents than their rat counterparts, which also affected action potential characteristics (Han et al. 2015).

The data clearly demonstrate species-specific differences between rodent and human Nav1.8, which have to be taken into account when inferring therapeutic possibilities based on rodent Nav1.8. Heterologous cell systems are important research tools for identifying a starting point for investigating the function of human channels. Nevertheless, the properties in native human neurons were still shown to be different, which argues for the importance of a natural environment of the channel to reveal its full physiological function. Therefore, new strategies to utilize human sensory neurons and native expression levels of the receptors and ion channels under investigation are of imminent importance.

2.2 Human DRGs as a Model System

Differences as well as similarities between human and rodent sensory neurons have already been established. Some literature is available, showing that marker profiles can be obtained from and electrophysiological recordings can be performed with sensory tissue collected from postmortem patients and were compared to rodent neurons. Those differences include, for example, a nearly complete absence of P2X2 in human nociceptors, while the orthologous purinergic receptor in rodents has been proposed to play a prominent role in models of chronic pain (Serrano et al. 2012).

A comparative study assessing the presence of specific nociceptive markers in tissue slices of mouse and human DRGs using in situ hybridization was recently performed in our laboratory (Rostock et al. 2017). The expression of sensory markers such as TRPV1, RET, Nav1.7, Nav1.8, and Nav1.9 was analyzed in relation to the expression of TRKA, an accepted marker for peptidergic nociceptors: we observed a higher percentage of TRKA positive neurons expressing TRPV1, RET, Nav1.8, and Nav1.9 in human versus mouse DRG sections. No difference in the expression of Nav1.7 was observed. Another striking difference is the almost universal expression of the neurofilament heavy subunit (NF200) in human sensory neurons compared to a rather confined expression of the filament in myelinated medium to large diameter mouse sensory neurons that give rise to Aβ and Aδ fibers (Vega et al. 1994; Rostock et al. 2017). Already from these few studies one can learn that the molecular makeup of human sensory neurons cannot automatically be inferred from expression data that has been obtained from rodent cells. Additionally, further studies need to be performed to assess if these differences do have significant implications on the functional level of the sensory neurons.

Unfortunately, so far functional data from cultured human DRG neurons is scarce. Two recent studies assessed some fundamental electrophysiological properties of cultured small-diameter human nociceptors and found the majority of cells to exhibit a “shoulder” on the falling phase of the action potential that has been proposed to be the result of calcium as well as sodium influx (Davidson et al. 2014; Han et al. 2015). This electrophysiological feature has also been found in small-diameter rodent nociceptors (Ritter and Mendell 1992; Gold et al. 1996; McCarthy and Lawson 1997). The “shoulder” phenomenon in rodents acts to prolong action potential duration (Blair and Bean 2002) and can be correlated with the presence of the isolectin-B4 (IB4), a marker widely used in rodent models to label non-peptidergic, small-diameter nociceptors (Stucky and Lewin 1999). However, Davidson et al. were not able to show binding of IB4 to cultured human DRGs (Davidson et al. 2014), which possibly is another species difference, although other groups did observe IB4 binding to human sensory neurons (Shi et al. 2008; Pan et al. 2012).

In summary, there clearly is a need to analyze human sensory neurons in more detail on molecular as well as functional levels for several reasons: (1) only when we know how human nociceptors operate, we will be able to find suitable means of interfering with pathways involved in the onset and perpetuation of maladaptive forms of pain; (2) in light of the growing interest to use human pluripotent stem cells to derive sensory neurons from human subjects including pain patients, we need to be able to compare the derived neurons with their in vivo equivalents.

2.3 Human Microneurography

In order to assess the functionality of human nerve fibers, the microneurography technique has been used for decades to record electrical activity from nerve fibers in living humans (Vallbo and Hagbarth 1968). In particular, peripheral nerves innervating the skin can be accessed by inserting a fine (100–200 μm) tungsten electrode in close vicinity to a human nerve. A well-placed electrode is able to discriminate impulses from a single nerve fiber of interest in relation to an adjacent reference electrode. As the method can be exerted in fully awake human beings and does not harm or destruct the nerve, it provides a valuable tool to study alterations in the electrical activity of specific single nerve fibers and their response to stimuli in their receptive field. By these means researchers were able to demonstrate that patients suffering from neuropathic pain show greater spontaneous activity in C-fibers (Kleggetveit et al. 2012; Serra et al. 2012). The method even allows to specify the type of C-fiber involved; in the case of neuropathic pain, silent nociceptors that are mechano-insensitive (CMi-nociceptors) are thought to be responsible for the increase in spontaneous activity (Kleggetveit et al. 2012). Additionally, a faster recovery of activity-dependent slowing of conduction could also be observed in chronic pain patients, which indicates alterations of axonal ion channels. Despite the possibilities offered by microneurography, such as the identification of fiber types involved in particular pathological pain condition, the technique only rarely allows to deduce molecular mechanisms underlying the observed electrical properties. To gain mechanistic insight, model systems are indispensable. Nevertheless, the method can be used to test defined (and “prescreened”) compounds for their ability to, for example, reduce or even stop spontaneous activity in diseased sensory neurons, thereby helping to identify analgesic substances (Serra 2010; Wehrfritz et al. 2011; Kankel et al. 2012; Schwarz et al. 2017).

Taken together, the already available data and the stagnancy regarding the successful implementation of preclinical data show the need to reevaluate the model systems currently used and to assess the possibility/feasibility of incorporating emerging new model systems.

With the isolation of human embryonic stem cells (hESCs) in 1998 (Thomson et al. 1998) and remarkable technological advances that allow the reprogramming of somatic cells into induced pluripotent stem cells [iPSCs (Takahashi et al. 2007)], we have now tools at hand that could help to invigorate pain research. A crucial step forward is the development of differentiation procedures by several researches to generate human sensory neuron-like cells from hESCs and hiPSCs in the dish (Valensi-Kurtz et al. 2010; Chambers et al. 2012; Blanchard et al. 2015; Schrenk-Siemens et al. 2015; Wainger et al. 2015). These cells may offer the possibility to help elucidating and understanding possible differences in the cellular makeup and functionality of human and rodent sensory neurons and thereby refine our way of developing means to interfere with pain and the maladaptation of it (Fig. 1).

Fig. 1
figure 1

Model system using human nociceptor-like cells. By obtaining a tissue sample from a pain patient, these cells can be reprogrammed into induced pluripotent stem cell (iPSCs). iPSCs can be differentiated into sensory neurons, which carry the genetic background of the patient. The pathophysiology of the nociceptors is analyzed using molecular and electrophysiological techniques, such as staining for markers, patch-clamp, and multielectrode recordings. Drugs can be tested on iPSC-derived sensory neurons, and personalized treatment may be identified which could reverse patient-specific pathophysiological changes.

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2.4 Human Pluripotent Stem Cell-Derived Nociceptors

Currently, there are two different strategies, how researchers succeeded in generating human nociceptor-like cells in a dish: One is by interfering with specific cell intrinsic pathways (Chambers et al. 2012), the other via overexpression of transcription factors relevant for sensory neuron development (Blanchard et al. 2015; Wainger et al. 2015).

In the first approach, Chambers et al. could show that the combination of five small-molecule pathway inhibitors was sufficient to efficiently trigger the differentiation of human pluripotent stem cells into neurons in a rather short time frame compared to classical multistep differentiation procedures. The resulting neurons showed functional and molecular hallmarks of nociceptive neurons such as the expression of TRKA. Also the upregulation of several ion channels such as SCN9A (Nav1.7), SCN10A (Nav1.8), and SCN11A (Nav1.9), the purinergic receptor P2RX3, and the TRP channels TRPV1 and TRPM8 indicated that the derived cells have a nociceptive-like phenotype. A later study compared the mRNA transcript profile of the differentiated sensory neurons after 30 days of culture with those from native human DRG cells and found them to be highly comparable. On the level of ion channel expression 84% of human ion channel genes were also expressed in human-differentiated sensory neurons (Young et al. 2014). Especially the presence of Navs in these model cells (Eberhardt et al. 2015) makes this method attractive to use as an in vitro model system for the analysis of the signaling pathways involved in pain perception and transduction. The universality of the protocol makes it possible to derive nociceptor-like cells also from hiPSCs that could be isolated from patients suffering from pain conditions triggered by mutations in the Nav genes such as Nav1.7-dependent inherited erythromelalgia (Cao et al. 2016).

Using overexpression of transcription factors, known to be involved in sensory neuron development of the mouse, has been the basis of two recently published differentiation strategies (Blanchard et al. 2015; Wainger et al. 2015). Both studies used virus-mediated infection of human fibroblasts, thereby introducing genes for reprogramming the cells into sensory-like neurons, including nociceptors. Consensus of both studies is that human fibroblasts are sufficient as starting material for nociceptor generation, and no reprogramming into iPSCs is needed.

Blanchard et al. (2015) used the basic helix-loop-helix transcription factors neurogenin 1 (NGN1) or neurogenin 2 (NGN2) in combination with BRN3A (POU4F1) to drive the generation of sensory neurons (Blanchard et al. 2015). They made use of an inducible expression system and could thereby regulate the transcription factor expression by doxycycline treatment. The conversion of fibroblasts resulted in the generation of neurons showing molecular and key electrophysiological features of all three main subtypes (mechano-, noci-, and proprioceptors) of sensory neurons. Investigating the derived nociceptor-like cells in more detail, the authors could show the expression of receptor ion channels TRPV1, TRPM8, and TRPA1 which are responsible for the detection of heat, cold, and noxious chemicals, respectively. Evaluating the electrophysiological properties of the derived nociceptive-like cells showed characteristics of functional neurons including the presence of TTX-resistant currents, which refers to the presence of Nav1.8 and/or Nav1.9.

Wainger et al. (2015) used a similar approach: also here virus-mediated overexpression of transcription factors was harnessed to induce the conversion of human fibroblast cells into sensory neuron-like cells (Wainger et al. 2015). In this case, five different transcription factors, some of which have been connected to nociceptor generation in vivo, were continuously overexpressed: ASCL1, MYT1L, ISL2, NGN1, and KLF7. The resulting neurons showed features of nociceptors on a functional level such as firing broad action potentials and the presence of TTX-resistant currents, which refers to the presence of Nav1.8 and/or Nav1.9.

As a result of the progress that was made during the last few years, it is now possible to generate human nociceptive-like cells from human iPSCs. Maturation of the so derived sensory neurons varies and expression of the TTX-resistant subtype Nav1.5 hints toward immaturity in some sensory neurons derived via the small-molecule approach (Eberhardt et al. 2015). Nevertheless, reliable expression of slowly gating TTXr currents which resemble Nav1.8 suggests that these neurons are still useful model systems to investigate the function of several sodium channels. Additionally, species differences may also occur here, and it is possible that expression of Nav1.5 in human DRGs is more pronounced compared to rodent sensory neurons. Although the published differentiation strategies differ in their outcome with regard to the homogeneity of the derived sensory neuron subtype as well as the level of maturity (Eberhardt et al. 2015) and comparability to the in vivo human situation, several research labs implemented the protocols already in their pain research, and first results have been published. One of these studies that used stem cell-derived nociceptive-like cells deals with IEM (Cao et al. 2016). Human iPSC lines were generated from four different IEM patients, harboring different Nav1.7 mutations as well as four different non-IEM patients without any Nav1.7 mutations as control group. The pluripotent stem cells were differentiated using the small-molecule inhibitor approach (Chambers et al. 2012) for a total of 9 weeks before electrophysiological experiments were performed. The main goal of the study was to test the contribution of Nav1.7 on the firing pattern of sensory neurons in IEM subjects and the usefulness of two selective Nav1.7 blockers (clinical compound PF-05089771 and the in vitro tool PF-05153462) in abrogating pathological firing patterns in Nav1.7 mutant cells.

Without any treatment, the IEM-derived nociceptive-like neurons showed a significant higher proportion of spontaneously active cells compared to the non-IEM donors, although one IEM donor and one non-IEM donor showed spontaneous firing to a very similar degree. This might point toward some intrinsic heterogeneity among the derived cell lines. Another difference could be observed regarding the rheobase (which indicates the minimal current injection that is required to evoke an action potential), which was on average lower in the IEM-derived neurons. The investigators tested both Nav1.7 blockers on the generated nociceptive-like cells and saw a reduction of spontaneous firing in those IEM cells that showed elevated spontaneous activity. Investigating the involvement of Nav1.7 on reducing rheobase, the scientists found that treatment with both blockers showed an increase in rheobase for mutated as well as control Nav1.7, while the magnitude of the increase was significantly greater in cells derived from IEM patients.

Heat can trigger pain attacks in IEM patients. A modest, innocuous elevation of temperature was already sufficient to increase the excitability of IEM-derived neurons in vitro. Application of PF-0515462 was able to reverse the effect of temperature on rheobase in IEM patient-derived nociceptive-like neurons, suggesting that mutated Nav1.7 channel contributes to the increased heat sensitivity in IEM patient-derived sensory neuron-like cells.

Although Cao et al. only had five IEM patients of which four allowed the isolation of hiPSCs, the researchers also conducted a small in vivo study by treating the patients themselves with the Nav1.7 blocker PF-05089771 or a placebo. Onset of pain was triggered by using a controlled heat stimulus, and the subjects rated their pain using a pain intensity numerical rating scale. A reduction in the magnitude of pain perception by some of the individual patients could be observed in the presence of the drug in comparison to subjects who received placebo, although only to a limited extent and in a very narrow time window.

Although it is quite obvious that the number of patients involved in this study is too small to draw a final conclusion about the usefulness of Nav1.7 channel blockers, the study demonstrates how the derivation of hiPSCs from IEM patients, their differentiation into nociceptive-like cells, in vitro characterization, and subsequent treatment of the patients themselves can help to understand the molecular base of a disease as well as testing compounds that can interfere with the pathological mechanisms.

3 Concluding Remarks

The long-lasting assumption that data generated in animal models in the field of pain research can be easily translated to human patients has been a misconception. One indicator for this is the strong imbalance between the high number of successfully conducted preclinical animal studies and the low number of successfully conducted clinical trials thereof.

With the sensational achievement of reprogramming human somatic cells into pluripotent stem cells in 2007 and the development of differentiation strategies to generate neurons showing hallmarks of peripheral sensory neurons, we have now the opportunity to implement some aspects of translational nociceptive research. However, we are still in need of data from native human DRG neurons concerning their molecular make up and their functional features for comparative purposes and to assess if the derived sensory neurons faithfully recapitulate features of their native counterparts. It is still early days, but some recent studies already illustrate the potential stem cell-derived nociceptors could have on drug research in the future.

Personalized medicine could become one of the avenues to pursue by generation of patient-derived iPSCs to study basic aspects of the disease, perform individual drug treatments on the cells, and translate the findings back to the patient itself, with the hope to find a better suitable, personalized therapy.