Dissection, plating, and maintenance of dorsal horn neuron cultures

The EGF-LIKE domain of thrombospondin-4 is a key determinant in the development of pain states due to increased excitatory synaptogenesis

Up-regulation of thrombospondin-4 (TSP4) or voltage-gated calcium channel subunit α₂δ₁ (Ca_vα₂δ₁) proteins in the spinal cord contributes to neuropathic pain development through an unidentified mechanism. We have previously shown that TSP4 interacts with Ca_vα₂δ₁ to promote excitatory synaptogenesis and the development of chronic pain states. However, the TSP4 determinants responsible for these changes are not known. Here, we tested the hypothesis that the Ca_vα₂δ₁-binding domains of TSP4 are synaptogenic and pronociceptive. We mapped the major Ca_vα₂δ₁-binding domains of TSP4 within the coiled-coil and epidermal growth factor (EGF)-like domains in vitro Intrathecal injection of TSP4 fragment proteins containing the EGF-like domain (EGF-LIKE) into naïve rodents was sufficient for inducing behavioral hypersensitivity similar to that produced by an equal molar dose of full-length TSP4. Gabapentin, a drug that binds to Ca_vα₂δ₁, blocked EGF-LIKE-induced behavioral hypersensitivity in a dose-dependent manner, supporting the notion that EGF-LIKE interacts with Ca_vα₂δ₁ and thereby mediates behavioral hypersensitivity. This notion was further supported by our findings that a peptide within EGF-LIKE (EGFD355-369) could block TSP4- or Ca_vα₂δ₁-induced behavioral hypersensitivity after intrathecal injections. Furthermore, only TSP4 proteins that contained EGF-LIKE could promote excitatory synaptogenesis between sensory and spinal cord neurons, which could be blocked by peptide EGFD355-369. Together, these findings indicate that EGF-LIKE is the molecular determinant that mediates aberrant excitatory synaptogenesis and chronic pain development. Blocking interactions between EGF-LIKE and Ca_vα₂δ₁ could be an alternative approach in designing target-specific pain medications.

Gabapentin prevents synaptogenesis between sensory and spinal cord neurons induced by thrombospondin-4 acting on pre-synaptic Cav α2 δ1 subunits and involving T-type Ca2+ channels

BACKGROUND AND PURPOSE

Nerve injury induces concurrent up-regulation of the voltage-gated calcium channel subunit Cav α2 δ1 and the extracellular matrix protein thrombospondin-4 (TSP4) in dorsal root ganglia and dorsal spinal cord, leading to the development of a neuropathic pain state. Interactions of these proteins promote aberrant excitatory synaptogenesis that contributes to neuropathic pain state development through unknown mechanisms. We investigated the contributions of Cav α2 δ1 subunits and TSP4 to synaptogenesis, and the pathways involved in vitro, and whether treatment with gabapentin could block this process and pain development in vivo.

EXPERIMENTAL APPROACH

A co-culture system of sensory and spinal cord neurons was used to study the contribution from each protein to synaptogenesis and the pathway(s) involved. Anti-synaptogenic actions of gabapentin were studied in TSP4-injected mice.

KEY RESULTS

Only presynaptic, but not postsynaptic, Cav α2 δ1 subunits interacted with TSP4 to initiate excitatory synaptogenesis through a pathway modulated by T-type calcium channels. Cav α2 δ1 /TSP4 interactions were not required for maintenance of already formed synapses. In vivo, early, but not delayed, treatment with low-dose gabapentin blocked this pathway and the development of the pain state.

CONCLUSIONS AND IMPLICATIONS

Cav α2 δ1 /TSP4 interactions were critical for the initiation, but not for the maintenance, of abnormal synapse formation between sensory and spinal cord neurons. This process was blocked by early, but was not reversed by delayed, treatment with gabapentin. Early intervention with gabapentin may prevent the development of injury-induced chronic pain, resulting from Cav α2 δ1 /TSP4-initiated abnormal synapse formation.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Rapid genotyping of animals followed by establishing primary cultures of brain neurons

High-resolution analysis of the morphology and function of mammalian neurons often requires the genotyping of individual animals followed by the analysis of primary cultures of neurons. We describe a set of procedures for: labeling newborn mice to be genotyped, rapid genotyping, and establishing low-density cultures of brain neurons from these mice. Individual mice are labeled by tattooing, which allows for long-term identification lasting into adulthood. Genotyping by the described protocol is fast and efficient, and allows for automated extraction of nucleic acid with good reliability. This is useful under circumstances where sufficient time for conventional genotyping is not available, e.g., in mice that suffer from neonatal lethality. Primary neuronal cultures are generated at low density, which enables imaging experiments at high spatial resolution. This culture method requires the preparation of glial feeder layers prior to neuronal plating. The protocol is applied in its entirety to a mouse model of the movement disorder DYT1 dystonia (ΔE-torsinA knock-in mice), and neuronal cultures are prepared from the hippocampus, cerebral cortex and striatum of these mice. This protocol can be applied to mice with other genetic mutations, as well as to animals of other species. Furthermore, individual components of the protocol can be used for isolated sub-projects. Thus this protocol will have wide applications, not only in neuroscience but also in other fields of biological and medical sciences.

Modulation of neurite outgrowth by activation of calcium-permeable kainate receptors expressed by rat nociceptive-like dorsal root ganglion neurons

Neurite outgrowth is a fundamental step in establishing proper neuronal connections in the developing central nervous system. Dynamic control of outgrowth has been attributed to changes in growth cone Ca2+ levels in response to extracellular cues. Here we have investigated a possible role for Ca2+ permeable kainate (KA) receptors in regulating neurite outgrowth of nociceptive-like dorsal root ganglion (DRG) neurons. To identify KA receptor subunits likely to be involved, we used quantitative RT-PCR on acutely dissociated DRG and dorsal horn neurons. DRG neurons expressed more GluK1, particularly the GluK1b spice variant, than dorsal horn neurons. Conversely, dorsal horn neurons expressed more GluK2, particularly GluK2a, than DRG neurons. Further, an RNA editing assay indicated that the majority of GluK1 and GluK2 mRNA transcripts in DRG were unedited. Imaging Ca2+ transients following application of a KA receptor agonist to DRG and dorsal horn co-cultures revealed increases in intracellular Ca2+ in the growth cones of DRG neurons. In the majority of cases, this increase in Ca2+ was partly or completely blocked by Joro spider toxin (JSTX), an antagonist for Ca2+-permeable AMPA and KA receptors. Treatment of DRG/dorsal horn co-cultures with KA for 18 hours suppressed neurite outgrowth while application of the rapidly desensitizing KA receptor agonist SYM 2081, the competitive AMPA/KA receptor antagonist, CNQX, and JSTX or philanthotoxin enhanced neurite outgrowth and prevented KA effects on neurite outgrowth. Thus, Ca2+ entry through KA receptors at the growth cone of DRG neurons may be an important regulator of neurite outgrowth.

An in vitro assay system for studying synapse formation between nociceptive dorsal root ganglion and dorsal horn neurons

Synapses between nociceptive dorsal root ganglion (DRG) neurons and spinal cord dorsal horn neurons represent the first loci for transmission of painful stimuli. Our knowledge of the molecular organization and development of these synapses is sparse due, partly, to a lack of a reliable model system that reconstitutes synaptogenesis between these two neuronal populations. To address this issue, we have established an in vitro assay system consisting of separately purified DRG neurons and dorsal horn neurons on astrocyte microislands. Using immunocytochemistry, we have found that 97%, 93%, 98%, 96%, and 94% of DRG neurons on these microislands express markers often associated with nociceptive neurons including Substance P, TRPV1, calcitonin-gene related peptide (CGRP), TrKA, and peripherin, respectively. Triple labeling with these nociceptive-like markers, synaptic vesicle marker Vglut2 and using MAP2 as a dendritic marker revealed the presence of nociceptive-like markers at synaptic terminals. Using this immunocytochemical approach, we counted contact points as overlapping MAP2/Vglut2 puncta and showed that they increased with time in culture. Single and dual patch-clamp recordings showed that overlapping Vglut2/MAP2 puncta observed after a few days in culture are likely to be functional synapses between DRG and dorsal horn neurons in our in vitro assay system. Taken together, these data suggest our co-culture microisland model system consists of mostly nociceptive-like DRG neurons that express presynaptic markers and form functional synapses with their dorsal horn partners. Thus, this model system may have direct application for studies on factors regulating development of nociceptive DRG/dorsal horn synapses.