Investigation of TRPV1 loss-of-function phenotypes in transgenic shRNA expressing and knockout mice

Delivery of RNA interference triggers to sensory neurons in vivo using herpes simplex virus

IMPORTANCE OF THE FIELD

Pain is a hugely important area of research attracting considerable academic and commercial interest. However, the application of RNA interference (RNAi) to the study of nociceptive processes and the development of new analgesics has been limited by the specific challenges associated with the delivery of RNAi triggers to the cell bodies of sensory neurons in the dorsal root ganglia (DRG).

AREAS COVERED IN THIS REVIEW

In the past five years, delivery of small-interfering RNA (siRNA) to the DRG and spinal cord has achieved effective and specific silencing of targeted genes in various animal models of pain. However, delivery of short-hairpin RNA (shRNA) or artificial microRNA (miRNA) to sensory neurons in vivo has not been feasible using most delivery systems currently available.

WHAT THE READER WILL GAIN

Replication-defective vectors based on herpes simplex virus (HSV), which are particularly efficient at targeting DRG neurons, have been recently engineered to express shRNA and artificial miRNA. Whilst silencing induced by siRNA is transient and requires relatively high doses of silencing triggers, HSV-mediated expression of shRNA/miRNA in sensory neurons allows silencing of targeted genes for at least one week following a single injection.

TAKE HOME MESSAGE

The potential to use inducible or tissue-specific promoters and to simultaneously silence multiple gene targets, in addition to recent studies suggesting that artificial miRNAs may have improved safety profiles, hold clear advantages for the use of miRNA-based vectors for gene silencing in sensory neurons.

Novel insights on diagnosis, cause and treatment of diabetic neuropathy: focus on painful diabetic neuropathy

Diabetic neuropathy is common, under or misdiagnosed, and causes substantial morbidity with increased mortality. Defining and developing sensitive diagnostic tests for diabetic neuropathy is not only key to implementing earlier interventions but also to ensure that the most appropriate endpoints are employed in clinical intervention trials. This is critical as many potentially effective therapies may never progress to the clinic, not due to a lack of therapeutic effect, but because the endpoints were not sufficiently sensitive or robust to identify benefit. Apart from improving glycaemic control, there is no licensed treatment for diabetic neuropathy, however, a number of pathogenetic pathways remain under active study. Painful diabetic neuropathy is a cause of considerable morbidity and whilst many pharmacological and nonpharmacological interventions are currently used, only two are approved by the US Food and Drug Administration. We address the important issue of the 'placebo effect' and also consider potential new pharmacological therapies as well as nonpharmacological interventions in the treatment of painful diabetic neuropathy.

Transgenic RNAi applications in the mouse

Within the past 10 years, RNA interference has emerged as a powerful experimental tool as it allows rapid gene function analysis. Unique features such as reversibility of gene silencing and simultaneous targeting of several genes characterize the approach. In this chapter, transgenic RNAi techniques in reverse mouse genetics are discussed and protocols are provided.

The transient receptor potential vanilloid-1 channel in thermoregulation: a thermosensor it is not

The development of antagonists of the transient receptor potential vanilloid-1 (TRPV1) channel as pain therapeutics has revealed that these compounds cause hyperthermia in humans. This undesirable on-target side effect has triggered a surge of interest in the role of TRPV1 in thermoregulation and revived the hypothesis that TRPV1 channels serve as thermosensors. We review literature data on the distribution of TRPV1 channels in the body and on thermoregulatory responses to TRPV1 agonists and antagonists. We propose that two principal populations of TRPV1-expressing cells have connections with efferent thermoeffector pathways: 1) first-order sensory (polymodal), glutamatergic dorsal-root (and possibly nodose) ganglia neurons that innervate the abdominal viscera and 2) higher-order sensory, glutamatergic neurons presumably located in the median preoptic hypothalamic nucleus. We further hypothesize that all thermoregulatory responses to TRPV1 agonists and antagonists and thermoregulatory manifestations of TRPV1 desensitization stem from primary actions on these two neuronal populations. Agonists act primarily centrally on population 2; antagonists act primarily peripherally on population 1. We analyze what roles TRPV1 might play in thermoregulation and conclude that this channel does not serve as a thermosensor, at least not under physiological conditions. In the hypothalamus, TRPV1 channels are inactive at common brain temperatures. In the abdomen, TRPV1 channels are tonically activated, but not by temperature. However, tonic activation of visceral TRPV1 by nonthermal factors suppresses autonomic cold-defense effectors and, consequently, body temperature. Blockade of this activation by TRPV1 antagonists disinhibits thermoeffectors and causes hyperthermia. Strategies for creating hyperthermia-free TRPV1 antagonists are outlined. The potential physiological and pathological significance of TRPV1-mediated thermoregulatory effects is discussed.

Therapeutic potential of RNA interference in pain medicine

In recent years RNA interference (RNAi) has rapidly become the most widely used tool for gene knockdown due to its high specificity and potency. RNAi is an evolutionarily conserved mechanism for silencing gene expression by targeted degradation of mRNA. In the past decade, hundreds of molecular targets have been identified for their roles in pain modulation. But most molecular targets are not readily druggable with small molecules. RNAi represents a therapeutic approach applicable to these non-druggable targets. There is a rapid increase in the number of studies that use small interfering RNAs (siRNAs) to validate new targets for pain regulation. In this review, we will discuss these pain-related RNAi studies (Table 1). We will also compare the advantages and disadvantages of RNAi with antisense knockdown (Table 2), because antisense oligodeoxynucleotides have been extensively used for target validation in pain research. Although in vivo delivery of siRNA remains to be a challenge, RNAi has a great potential to become a major therapeutic tool for pain management.

Neuropathic pain: a maladaptive response of the nervous system to damage

Neuropathic pain is triggered by lesions to the somatosensory nervous system that alter its structure and function so that pain occurs spontaneously and responses to noxious and innocuous stimuli are pathologically amplified. The pain is an expression of maladaptive plasticity within the nociceptive system, a series of changes that constitute a neural disease state. Multiple alterations distributed widely across the nervous system contribute to complex pain phenotypes. These alterations include ectopic generation of action potentials, facilitation and disinhibition of synaptic transmission, loss of synaptic connectivity and formation of new synaptic circuits, and neuroimmune interactions. Although neural lesions are necessary, they are not sufficient to generate neuropathic pain; genetic polymorphisms, gender, and age all influence the risk of developing persistent pain. Treatment needs to move from merely suppressing symptoms to a disease-modifying strategy aimed at both preventing maladaptive plasticity and reducing intrinsic risk.

Transient receptor potential channels on sensory nerves

The somatosensory effects of natural products such as capsaicin, mustard oil, and menthol have been long recognized. Over the last decade, the identification of transient receptor potential (TRP) channels in primary sensory neurons as the targets for these agents has led to an explosion of research into the roles of "thermoTRPs" TRPV1, TRPV2, TRPV3, TRPV4, TRPA1, and TRPM8 in nociception. In concert, through the efforts of many industrial and academic teams, a number of agonists and antagonists of these channels have been discovered, paving the way for a better understanding of sensory biology and, potentially, for novel treatments for diseases.

RNA interference: from basic research to therapeutic applications

An efficient mechanism for the sequence-specific inhibition of gene expression is RNA interference. In this process, double-stranded RNA molecules induce cleavage of a selected target RNA (see picture). This technique has in recent years developed into a standard method of molecular biology. Successful applications in animal models have already led to the initiation of RNAi-based clinical trials as a new therapeutic option.Only ten years ago Andrew Fire and Craig Mello were able to show that double-stranded RNA molecules could inhibit the expression of homologous genes in eukaryotes. This process, termed RNA interference, has developed into a standard method of molecular biology. This Review provides an overview of the molecular processes involved, with a particular focus on the posttranscriptional inhibition of gene expression in mammalian cells, the possible applications in research, and the results of the first clinical studies.