Genetic Study of Axon Regeneration with Cultured Adult Dorsal Root Ganglion Neurons

It is well known that mature neurons in the central nervous system (CNS) cannot regenerate their axons after injuries due to diminished intrinsic ability to support axon growth and a hostile environment in the mature CNS^1,2. In contrast, mature neurons in the peripheral nervous system (PNS) regenerate readily after injuries³. Adult dorsal root ganglion (DRG) neurons are well known to regenerate robustly after peripheral nerve injuries. Each DRG neuron grows one axon from the cell soma, which branches into two axonal branches: a peripheral branch innervating peripheral targets and a central branch extending into the spinal cord. Injury of the DRG peripheral axons results in substantial axon regeneration, whereas central axons in the spinal cord regenerate poorly after the injury. However, if the peripheral axonal injury occurs prior to the spinal cord injury (a process called the conditioning lesion), regeneration of central axons is greatly improved⁴. Moreover, the central axons of DRG neurons share the same hostile environment as descending corticospinal axons in the spinal cord. Together, it is hypothesized that the molecular mechanisms controlling axon regeneration of adult DRG neurons can be harnessed to enhance CNS axon regeneration. As a result, adult DRG neurons are now widely used as a model system to study regenerative axon growth^5-7.Here we describe a method of adult DRG neuron culture that can be used for genetic study of axon regeneration in vitro. In this model adult DRG neurons are genetically manipulated via electroporation-mediated gene transfection^6,8. By transfecting neurons with DNA plasmid or si/shRNA, this approach enables both gain- and loss-of-function experiments to investigate the role of any gene-of-interest in axon growth from adult DRG neurons. When neurons are transfected with si/shRNA, the targeted endogenous protein is usually depleted after 3-4 days in culture, during which time robust axon growth has already occurred, making the loss-of-function studies less effective. To solve this problem, the method described here includes a re-suspension and re-plating step after transfection, which allows axons to re-grow from neurons in the absence of the targeted protein. Finally, we provide an example of using this in vitro model to study the role of an axon regeneration-associated gene, c-Jun, in mediating axon growth from adult DRG neurons⁹.     

Absence of neurogenesis of adult rat dorsal root ganglion cells

Recently, an age-related increase in the number of dorsal root ganglion (DRG) cells was reported in adult rats. This suggests neurogenesis of adult primary afferent neurons, which would be an extremely important phenomenon if it occurred. Other evidence is not compatible with this idea, however, so the issue is not settled. The primary point of contention concerns the counts of DRG cells in relation to age. In our opinion, these disagreements arise, at least in part, because different counting methods give different results for the same material. Thus, any method for determining DRG cell numbers should be calibrated. We previously calibrated some of the common methods used to count DRG cells and found that an empirical method gave accurate cell counts. In the present study, we have used this method and asked whether an age-related increase in the number of lumbar DRG cells can be demonstrated in adult rats. Our data indicate that DRG cell numbers remain essentially constant from 3 to 22 months of age. Most ancillary evidence is consistent with the hypothesis that mammalian DRG cell numbers do not change during adult life. Thus, we feel that the evidence does not support the hypothesis that there is neurogenesis of adult rat primary afferent neurons.     

Efficient delivery of RNA Interference to peripheral neurons in vivo using herpes simplex virus

Considerable interest has been focused on inducing RNA interference (RNAi) in neurons to study gene function and identify new targets for disease intervention. Although small interfering RNAs (siRNAs) have been used to silence genes in neurons, in vivo delivery of RNAi remains a major challenge limiting its applications. We have developed a highly efficient method for in vivo gene silencing in dorsal root ganglia (DRG) using replication-defective herpes simplex viral (HSV-1) vectors. HSV-mediated delivery of short-hairpin RNA (shRNA) targeting reporter genes resulted in highly effective and specific silencing in neuronal and non-neuronal cells in culture and in the DRG of mice in vivo including in a transgenic mouse model. We further establish proof of concept by demonstrating in vivo silencing of the endogenous trpv1 gene. These data are the first to show silencing in DRG neurons in vivo by vector-mediated delivery of shRNA. Our results support the utility of HSV vectors for gene silencing in peripheral neurons and the potential application of this technology to the study of nociceptive processes and in pain gene target validation studies.     

Dorsal Root Ganglia-Specific Expression of the β-Galactoside α1,2-Fucosyltransferase Genes in Rabbits

Abstract: The neurons of dorsal root ganglia (DRG) mediate several sensation modalities. The carbohydrate antigens on DRG neurons differ with the sensation modalities that subsets of neurons convey. Despite the important roles of gangliosides and glycoproteins in neuronal differentiation and neuritogenesis of the mammalian nervous system, little is known about the mechanisms underlying the regulation of glycosylation. We previously demonstrated the expression of H-blood type antigens (Fucα1, 2Galβ) on rabbit DRG neurons of small diameter and dramatic changes in H antigens during the perinatal period. To investigate the possible biological roles and regulatory mechanisms of H antigens, we recently cloned three types of rabbit α1,2-fucosyltransferase gene that catalyze the biosynthesis of H antigens. Here, we analyze the expression of these genes, RFT-I, II, and III, in rabbit DRG. The H-type α1,2-fucosyltransferase gene, RFT-I, was expressed in DRG in late embryos to adult rabbits, as detected on northern blotting. The other two secretor-type α1,2-fucosyltransferase genes, RFT-II and III, were observed to be expressed in late embryonic DRG on RTPCR analysis but were not detectable on northern blotting. The expression of the H-type α1,2-fucosyltransferase gene was analyzed by in situ hybridization and was found to be abundant in small-diameter DRG neurons. These results indicate that the H-type α1,2-fucosyltransferase gene plays a major role in the regulation of the H antigen expression in DRG during the perinatal period.     

Overexpression of DRG2 suppresses the growth of Jurkat T cells but does not induce apoptosis

Developmentally regulated GTP-binding protein (DRG) is a new subfamily within the superfamily of GTP-binding proteins. Its expression is regulated during embryonic development. To investigate the effect of the expression of DRG2 on cell growth, we constructed a human Jurkat-T-cell line that overexpresses DRG2. Overexpression of DRG2 suppressed the growth and the aggregation of Jurkat cells but did not induce apoptotic cell death. We used cDNA microarray analysis to examine the global changes in gene expression induced by an overexpression of DRG2. DNA array analyses identified genes that may suppress cell growth at a number of levels in multiple signaling cascades in Jurkat cells and also several prosurvival genes that may protect cells from apoptosis.     

Overexpression of DRG2 increases G2/M phase cells and decreases sensitivity to nocodazole-induced apoptosis

DRG2, a member of the DRG subfamily in the GTP-binding protein superfamily, was identified as a repressed gene product in fibroblasts transformed by SV40. The significance of this down-regulation and the cellular role of DRG2 has not been understood in the past. To investigate the function of DRG2 we made a Jurkat cell line, Jurkat-LNCX2-DRG2, stably transfected with pLNCX2-DRG2 to overexpress human DRG2. Cell cycle distribution analysis revealed an increased accumulation of G(2)/M phase cells in Jurkat-LNCX2-DRG2 cells, indicating a retardation of cell-cycle progression. In addition, an overexpression of DRG2 reduced the sensitivity of Jurkat cells to the mitotic poison nocodazole. Our data suggest that overexpression of DRG2 in Jurkat cells affects genes regulating cell-cycle arrest and apoptosis, and that these molecular changes may be important in the growth or differentiation of cells.     

Expression of peptidergic and nitrergic structures in dorsal root ganglia of the rabbit

In this study we have demonstrated the presence of neuropeptide substance P (SP)and nonpeptide neurotransmiter NO (nitric oxide) in the dorsal root ganglia (DRG) of rabbits. NADPH-diaphorase histochemical staining was used for detection of NO and an immunohistochemical method for detection of substance P. A number of DRG cells were stained by SP- and NADPH-d reactions. The presence of SP and NADPH-diaphorase positive cells varied depending upon the spinal level of the DRG. Positively stained neurons were only small and intermediate in size. Cells of large diameter profiles showed no staining. Substance P immunoreactive cells were of brown and dark brown colour, the intensity of NADPH-d staining varied from light to very dark blue. In some DRG cells, there was very significant neuronal co-localization of immunoreactivity for SP and reactivity for NADPH-d. In summary, DRG cells appear to express diaphorase and substance P activity, and some of them show the presence of both neurotransmitters. Recent studies on the participation of NO in the regulation of SP release in the spinal cord suggest, that also in the DRG neurons there may be a close interaction between NO and SP.     

Hedgehog signaling is directly required for the development of zebrafish dorsal root ganglia neurons

Hedgehog (Hh) signal transduction is directly required in zebrafish DRG precursors for proper development of DRG neurons. Zebrafish mutations in the Hh signaling pathway result in the absence of DRG neurons and the loss of expression of neurogenin1 (ngn1), a gene required for determination of DRG precursors. Cell transplantation experiments demonstrate that Hh acts directly on DRG neuron precursors. Blocking Hh pathway activation at later stages of embryogenesis with the steroidal alkaloid, cyclopamine, further reveals that the requirement for a Hh signal response in DRG precursors correlates with the onset of ngn1 expression. These results suggest that Hh signaling may normally promote DRG development by regulating expression of ngn1 in DRG precursors.     

The metalloproteinase inhibitor Reck is essential for zebrafish DRG development

The neural crest is a migratory, multipotent cell lineage that contributes to myriad tissues, including sensory neurons and glia of the dorsal root ganglia (DRG). To identify genes affecting cell fate specification in neural crest, we performed a forward genetic screen for mutations causing DRG deficiencies in zebrafish. This screen yielded a mutant lacking all DRG, which we named sensory deprived (sdp). We identified a total of four alleles of sdp, all of which possess lesions in the gene coding for reversion-inducing cysteine-rich protein containing Kazal motifs (Reck). Reck is an inhibitor of metalloproteinases previously shown to regulate cell motility. We found reck function to be both necessary for DRG formation and sufficient to rescue the sdp phenotype. reck is expressed in neural crest cells and is required in a cell-autonomous fashion for appropriate sensory neuron formation. In the absence of reck function, sensory neuron precursors fail to migrate to the position of the DRG, suggesting that this molecule is crucial for proper migration and differentiation.     

DRG represents a family of two closely related GTP-binding proteins

In a previous publication we identified a novel human GTP-binding protein that was related to DRG, a developmentally regulated GTP-binding protein from the central nervous system of mouse. Here we demonstrate that both the human and the mouse genome possess two closely related drg genes, termed drg1 and drg2. The two genes share 62% sequence identity at the nucleotide and 58% identity at the protein level. The corresponding proteins appear to constitute a separate family within the superfamily of the GTP-binding proteins. The DRG1 and the DRG2 mRNA are widely expressed in human and mouse tissues and show a very similar distribution pattern. The human drg1 gene is located on chromosome 22q12, the human drg2 gene on chromosome 17p12. Distantly related species including Caenorhabditis elegans, Schizosaccharomyces pombe and Saccharomyces cerevisiae also possess two drg genes. In contrast, the genomes of archaebacteria (Halobium, Methanococcus, Thermoplasma) harbor only one drg gene, while eubacteria do not seem to contain any. The high conservation of the polypeptide sequences between distantly related organisms indicates an important role for DRG1 and DRG2 in a fundamental pathway.     

Adult mouse dorsal root ganglia neurons in cell culture.

A method has been developed for the long-term culture of dissociated adult mouse dorsal root ganglia (DRG). Of critical importance to the success of this technique was a three-hour incubation in collagenase which softened the DRG and permitted gentle dissociation. The morphological and electrophysiological features of the dissociated adult DRG were similar to those observed in previous studies of immature (i.e., embryonic and newborn) DRG in culture and also to those of adult DRG in situ. With regard to electrophysiological work, the adult DRG neurons are superior to embryonic and newborn neurons because of their larger size and greatly increased survival in culture (no degeneration for first six days, and thereafter a relatively slow decrease). The adult neurons regenerated nerve fibers to an extent comparable to that of immature neurons. Therefore, the adult DRG cultures might be useful to study factors influencing regeneration in the adult mammalian nervous system. The adult cultures might also be useful to investigated factors influencing the aging process.     

Alterations in lipid homeostasis of mouse dorsal root ganglia induced by apolipoprotein E deficiency: a shotgun lipidomics study

One of the fundamental goals of lipidomics research is to identify the linkage of an individual gene with a given lipidome, thereby revealing the role of that gene in lipid metabolism, transport, and homeostasis. In this study, we have identified four apolipoprotein E (apoE)-induced alterations in the lipidome of mouse dorsal root ganglia (DRG) through utilizing the technology of shotgun lipidomics. First, apoE mediates sulfatide mass content in mouse DRG, which is comparable to its role in the CNS. Second, apoE contributes to galactosylceramide and ceramide homeostasis in mouse DRG. Third, apoE significantly modulates cholesterol levels in mouse DRG. The latter two functions of apoE are distinct from those in the CNS. Finally, mice null for apoE have dramatically less triacylglycerol mass content in DRG which are opposite to the effects observed in the peripheral organs and vascular system. Collectively, this study identifies the specific alterations in the DRG lipidome induced by apoE knockout and suggests the potential roles of apoE in lipid transport and homeostasis in a tissue specific manner, thereby providing insights into the biochemical mechanisms underlying the functions of apoE in the PNS.     

Adult mouse dorsal root ganglia neurons in cell culture

A method has been developed for the long-term culture of dissociated adult mouse dorsal root ganglia (DRG). Of critical importance to the success of this technique was a three-hour incubation in collagenase which softened the DRG and permitted gentle dissociation. The morphological and electrophysiological features of the dissociated adult DRG were similar to those observed in previous studies of immature (i.e., embryonic and newborn) DRG in culture and also to those of adult DRG in situ. With regard to electrophysiological work, the adult DRG neurons are superior to embryonic and newborn neurons because of their larger size and greatly increased survival in culture (no degeneration for first six days, and thereafter a relatively slow decrease). The adult neurons regenerated nerve fibers to an extent comparable to that of immature neurons. Therefore, the adult DRG cultures might be useful to study factors influencing regeneration in the adult mammalian nervous system. The adult cultures might also be useful to investigate factors influencing the aging process.     

一种适合膜片钳单通道记录的大鼠DRG神经元急性分离方法

目的：建立一种适合膜片钳单通道记录的脊髓背根神经节神经元急性分离方法。方法：用酶消化和机械分离相结合的方法急性分离大鼠DRG神经元。结果：用本方法分离的DRG细胞容易形成较高的封接电阻（〉5GΩ）,降低了噪音干扰,可记录到pA级的单通道电流。结论：本方法急性分离的DRG神经元适合单通道膜片钳实验研究。     

The cytochrome P450 inhibitor, ketoconazole, inhibits oxidized linoleic acid metabolite-mediated peripheral inflammatory pain

Background

Painful neuropathy is a common complication of diabetes. Previous studies have identified significant increases in the amount of voltage gated sodium channel isoforms Na_V1.7 and Na_V1.3 protein in the dorsal root ganglia (DRG) of rats with streptozotocin (STZ)-induced diabetes. We found that gene transfer-mediated release of the inhibitory neurotransmitters enkephalin or gamma amino butyric acid (GABA) from DRG neurons in diabetic animals reduced pain-related behaviors coincident with a reduction in Na_V1.7 protein levels in DRG in vivo. To further evaluate the role of Na_V?? subunit levels in DRG in the pathogenesis of pain in diabetic neuropathy, we constructed a non-replicating herpes simplex virus (HSV)-based vector expressing a microRNA (miRNA) against Na_V?? subunits.

Results

Subcutaneous inoculation of the miRNA-expressing HSV vector into the feet of diabetic rats to transduce DRG resulted in a reduction in Na_V?? subunit levels in DRG neurons, coincident with a reduction in cold allodynia, thermal hyperalgesia and mechanical hyperalgesia.

Conclusions

These data support the role of increased Na_V?? protein in DRG in the pathogenesis of pain in diabetic neuropathy, and provide a proof-of-principle demonstration for the development of a novel therapy that could be used to treat intractable pain in patients with diabetic neuropathy.     

An Efficient Method for Dorsal Root Ganglia Neurons Purification with a One-Time Anti-Mitotic Reagent Treatment

Background

The dorsal root ganglia (DRG) neuron is an invaluable tool in axon growth, growth factor regulation, myelin formation and myelin-relevant researches. The purification of DRG neurons is a key step in these studies. Traditionally, purified DRG neurons were obtained in two weeks after exposure to several rounds of anti-mitotic reagent.

Methods and Results

In this report, a novel, simple and efficient method for DRG purification is presented. DRG cultures were treated once with a high-dose anti-mitotic reagent cocktail for 72 hours. Using this new method, DRG neurons were obtained with 99% purification within 1 week. We confirmed that the neurite growth and the viability of the purified DRG neurons have no difference from the DRG neurons purified by traditional method. Furthermore, P0 and MBP expression was observed in myelin by immunocytochemistry in the DRG/SC co-culture system. The formation of mature node of Ranvier in DRG-Schwann cell co-culture system was observed using anti-Nav 1.6 and anti-caspr antibody.

Conclusion and Significance

The results indicate that this high dose single treatment did not compromise the capacity of DRG neurons for myelin formation in the DRG/SC co-culture system. In conclusion, a convenient approach for purifying DRG neurons was developed which is time-saving and high-efficiency.     

Effective neuropathic pain relief through sciatic nerve administration of GAD65-expressing rAAV2

Recently, we demonstrated that the administration of GAD65-expressing rAAV2 to DRG attenuates peripheral neuropathy by inducing GABA release in the spinal cord. However, the direct injection to DRG is invasive and may therefore cause nerve injury and other side effects.To circumvent this surgical intervention, we explored the potential of a much simpler and less invasive route of sciatic nerve administration. Using a neuropathic pain model, we introduced rAAV2-GAD65 through sciatic nerve and examined its therapeutic potency in pain-related behavior tests. Both GFP and GAD65 expression indicated that effective transgene delivery to the DRG can be accomplished via sciatic nerve administration. Equally importantly, the GABA concentration in the spinal cord increased significantly after GAD65 introduction, and pain symptoms were dramatically reduced and persistently controlled. The implication is that the sciatic nerve is a highly promising route for delivering rAAV2 to the DRG, and thus represents a much less invasive, clinically viable gene therapy option.     

Varicella-Zoster Virus Glycoprotein I Is Essential for Spread in Dorsal Root Ganglia and Facilitates Axonal Localization of Structural Virion Components in Neuronal Cultures

Neurons of the sensory ganglia are the major site of varicella-zoster virus (VZV) latency and may undergo productive infection during reactivation. Although the VZV glycoprotein E/glycoprotein I (gE/gI) complex is known to be critical for neurovirulence, few studies have assessed the roles of these proteins during infection of dorsal root ganglia (DRG) due to the high human specificity of the virus. Here, we show that the VZV glycoprotein I gene is an important neurotropic gene responsible for mediating the spread of virus in neuronal cultures and explanted DRG. Inoculation of differentiated SH-SY5Y neuronal cell cultures with a VZV gI gene deletion strain (VZV rOkaΔgI) showed a large reduction in the percentage of cells infected and significantly smaller plaque sizes in a comparison with cultures infected with the parental strain (VZV rOka). In contrast, VZV rOkaΔgI was not significantly attenuated in fibroblast cultures, demonstrating a cell type-specific role for VZV gI. Analysis of rOkaΔgI protein localization by immunofluorescent staining revealed aberrant localization of viral glycoprotein and capsid proteins, with little or no staining present in the axons of differentiated SH-SY5Y cells infected with rOkaΔgI, yet axonal vesicle trafficking was not impaired. Further studies utilizing explanted human DRG indicated that VZV gI is required for the spread of virus within DRG. These data demonstrate a role for VZV gI in the cell-to-cell spread of virus during productive replication in neuronal cells and a role in facilitating the access of virion components to axons.