Neurogenesis in postnatal mouse dorsal root ganglia

The dorsal root ganglion as a target for neurorestoration in neuropathic pain

Neuropathic pain is a severe and chronic condition widely found in the general population. The reason for this is the extensive variety of damage or diseases that can spark this unpleasant constant feeling in patients. During the processing of pain, the dorsal root ganglia constitute an important region where dorsal root ganglion neurons play a crucial role in the transmission and propagation of sensory electrical stimulation. Furthermore, the dorsal root ganglia have recently exhibited a regenerative capacity that should not be neglected in the understanding of the development and resolution of neuropathic pain and in the elucidation of innovative therapies. Here, we will review the complex interplay between cells (satellite glial cells and inflammatory cells) and factors (cytokines, neurotrophic factors and genetic factors) that takes place within the dorsal root ganglia and accounts for the generation of the aberrant excitation of primary sensory neurons occurring in neuropathic pain. More importantly, we will summarize an updated view of the current pharmacologic and nonpharmacologic therapies targeting the dorsal root ganglia for the treatment of neuropathic pain.

Satellite glia of the adult dorsal root ganglia harbor stem cells that yield glia under physiological conditions and neurons in response to injury

The presence of putative stem/progenitor cells has been suggested in adult peripheral nervous system (PNS) tissue, including the dorsal root ganglion (DRG). To date, their identification and fate in pathophysiological conditions have not been addressed. Combining multiple in vitro and in vivo approaches, we identified the presence of stem cells in the adult DRG satellite glial population, and progenitors were present in the DRGs and sciatic nerve. Cell-specific transgenic mouse lines highlighted the proliferative potential of DRG stem cells and progenitors in vitro. DRG stem cells had gliogenic and neurogenic potentials, whereas progenitors were essentially gliogenic. Lineage tracing showed that, under physiological conditions, adult DRG stem cells maintained DRG homeostasis by supplying satellite glia. Under pathological conditions, adult DRG stem cells replaced DRG neurons lost to injury in addition of renewing the satellite glial pool. These novel findings open new avenues for development of therapeutic strategies targeting DRG stem cells for PNS disorders.

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Adult murine DRGs contain slowly proliferating putative stem cells

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The putative stem cells are a subpopulation of adult DRG satellite cells

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Purified adult DRG putative stem cells generate neurons and glia in vitro

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They are gliogenic in vivo and generate neurons in response to injury

Neurotrophic effects of GM1 ganglioside, NGF, and FGF2 on canine dorsal root ganglia neurons in vitro

Dogs share many chronic morbidities with humans and thus represent a powerful model for translational research. In comparison to rodents, the canine ganglioside metabolism more closely resembles the human one. Gangliosides are components of the cell plasma membrane playing a role in neuronal development, intercellular communication and cellular differentiation. The present in vitro study aimed to characterize structural and functional changes induced by G_M1 ganglioside (G_M1) in canine dorsal root ganglia (DRG) neurons and interactions of G_M1 with nerve growth factor (NGF) and fibroblast growth factor (FGF2) using immunofluorescence for several cellular proteins including neurofilaments, synaptophysin, and cleaved caspase 3, transmission electron microscopy, and electrophysiology. G_M1 supplementation resulted in increased neurite outgrowth and neuronal survival. This was also observed in DRG neurons challenged with hypoxia mimicking neurodegenerative conditions due to disruptions of energy homeostasis. Immunofluorescence indicated an impact of G_M1 on neurofilament phosphorylation, axonal transport, and synaptogenesis. An increased number of multivesicular bodies in G_M1 treated neurons suggested metabolic changes. Electrophysiological changes induced by G_M1 indicated an increased neuronal excitability. Summarized, G_M1 has neurotrophic and neuroprotective effects on canine DRG neurons and induces functional changes. However, further studies are needed to clarify the therapeutic value of gangliosides in neurodegenerative diseases.

Safe and Sustained Expression of Human Iduronidase After Intrathecal Administration of Adeno-Associated Virus Serotype 9 in Infant Rhesus Monkeys

Many neuropathic diseases cause early, irreversible neurologic deterioration, which warrants therapeutic intervention during the first months of life. In the case of mucopolysaccharidosis type I, a recessive lysosomal storage disorder that results from a deficiency of the lysosomal enzyme α-l-iduronidase (IDUA), one of the most promising treatment approaches is to restore enzyme expression through gene therapy. Specifically, administering pantropic adeno-associated virus (AAV) encoding IDUA into the cerebrospinal fluid (CSF) via suboccipital administration has demonstrated remarkable efficacy in large animals. Preclinical safety studies conducted in adult nonhuman primates supported a positive risk-benefit profile of the procedure while highlighting potential subclinical toxicity to primary sensory neurons located in the dorsal root ganglia (DRG). This study investigated the long-term performance of intrathecal cervical AAV serotype 9 gene transfer of human IDUA administered to 1-month-old rhesus monkeys (N = 4) with half of the animals tolerized to the human transgene at birth via systemic administration of an AAV serotype 8 vector expressing human IDUA from the liver. Sustained expression of the transgene for almost 4 years is reported in all animals. Transduced cells were primarily pyramidal neurons in the cortex and hippocampus, Purkinje cells in the cerebellum, lower motor neurons, and DRG neurons. Both tolerized and non-tolerized animals were robust and maintained transgene expression as measured by immunohistochemical analysis of brain tissue. However, the presence of antibodies in the non-tolerized animals led to a loss of measurable levels of secreted enzyme in the CSF. These results support the safety and efficiency of treating neonatal rhesus monkeys with AAV serotype 9 gene therapy delivered into the CSF.

Beyond Trophic Factors: Exploiting the Intrinsic Regenerative Properties of Adult Neurons

Injuries and diseases of the peripheral nervous system (PNS) are common but frequently irreversible. It is often but mistakenly assumed that peripheral neuron regeneration is robust without a need to be improved or supported. However, axonal lesions, especially those involving proximal nerves rarely recover fully and injuries generally are complicated by slow and incomplete regeneration. Strategies to enhance the intrinsic growth properties of reluctant adult neurons offer an alternative approach to consider during regeneration. Since axons rarely regrow without an intimately partnered Schwann cell (SC), approaches to enhance SC plasticity carry along benefits to their axon partners. Direct targeting of molecules that inhibit growth cone plasticity can inform important regenerative strategies. A newer approach, a focus of our laboratory, exploits tumor suppressor molecules that normally dampen unconstrained growth. However several are also prominently expressed in stable adult neurons. During regeneration their ongoing expression “brakes” growth, whereas their inhibition and knockdown may enhance regrowth. Examples have included phosphatase and tensin homolog deleted on chromosome ten (PTEN), a tumor suppressor that inhibits PI3K/pAkt signaling, Rb1, the protein involved in retinoblastoma development, and adenomatous polyposis coli (APC), a tumor suppressor that inhibits β-Catenin transcriptional signaling and its translocation to the nucleus. The identification of several new targets to manipulate the plasticity of regenerating adult peripheral neurons is exciting. How they fit with canonical regeneration strategies and their feasibility require additional work. Newer forms of nonviral siRNA delivery may be approaches for molecular manipulation to improve regeneration.

Neuronal Subtype and Satellite Cell Tropism Are Determinants of Varicella-Zoster Virus Virulence in Human Dorsal Root Ganglia Xenografts In Vivo

Varicella zoster virus (VZV), a human alphaherpesvirus, causes varicella during primary infection. VZV reactivation from neuronal latency may cause herpes zoster, post herpetic neuralgia (PHN) and other neurologic syndromes. To investigate VZV neuropathogenesis, we developed a model using human dorsal root ganglia (DRG) xenografts in immunodeficient (SCID) mice. The SCID DRG model provides an opportunity to examine characteristics of VZV infection that occur in the context of the specialized architecture of DRG, in which nerve cell bodies are ensheathed by satellite glial cells (SGC) which support neuronal homeostasis. We hypothesized that VZV exhibits neuron-subtype specific tropism and that VZV tropism for SGC contributes to VZV-related ganglionopathy. Based on quantitative analyses of viral and cell protein expression in DRG tissue sections, we demonstrated that, whereas DRG neurons had an immature neuronal phenotype prior to implantation, subtype heterogeneity was observed within 20 weeks and SGC retained the capacity to maintain neuronal homeostasis longterm. Profiling VZV protein expression in DRG neurons showed that VZV enters peripherin+ nociceptive and RT97+ mechanoreceptive neurons by both axonal transport and contiguous spread from SGC, but replication in RT97+ neurons is blocked. Restriction occurs even when the SGC surrounding the neuronal cell body were infected and after entry and ORF61 expression, but before IE62 or IE63 protein expression. Notably, although contiguous VZV spread with loss of SGC support would be predicted to affect survival of both nociceptive and mechanoreceptive neurons, RT97+ neurons showed selective loss relative to peripherin+ neurons at later times in DRG infection. Profiling cell factors that were upregulated in VZV-infected DRG indicated that VZV infection induced marked pro-inflammatory responses, as well as proteins of the interferon pathway and neuroprotective responses. These neuropathologic changes observed in sensory ganglia infected with VZV may help to explain the neurologic sequelae often associated with zoster and PHN.

Varicella zoster virus (VZV) causes varicella; herpes zoster results from VZV reactivation and is associated with post herpetic neuralgia (PHN). We hypothesized that VZV exhibits neuron-subtype specific tropism and that VZV tropism for satellite glial cells (SGC) results in loss of SGC functions that support neurons and contributes to VZV-related ganglionopathy. Using human DRG xenografts in SCID mice, we demonstrated that initial VZV access to neuronal cell bodies occurs by the axonal route, followed by axonal and contiguous spread between neuron-satellite cell complexes. VZV replication is restricted in mechanoreceptive neurons compared to nociceptive neurons. Despite restricted infection, mechanoreceptive neurons were selectively depleted in association with SGC loss following acute DRG infection. VZV infection of DRG triggers release of pro-inflammatory cytokines that cause neuronal damage. These observations may help to explain the neurologic sequelae often associated with herpes zoster and PHN.

Dorsal root ganglion progenitors differentiate to gamma-aminobutyric acid- and choline acetyltransferase-positive neurons

This study examined the isolation and differentiation of dorsal root ganglion progenitor cells for therapeutic use in neurodegenerative diseases. Rat embryonic dorsal root ganglia progenitors were isolated and purified using the differential adhesion method combined with cytosine arabinoside treatment. After culture in serum-free medium supplemented with B27, basic fibroblast growth factor and epidermal growth factor, these cells remained viable and survived for more than 18 months in vitro. Most cells differentiated to neurons that were immunoreactive for gamma-aminobutyric acid and choline acetyltransferase as detected by immunohistochemical staining. In addition, nerve growth factor and neurotrophic tyrosine kinase receptor expression were also observed in dorsal root ganglion progenitors and differentiated cells. K252a, an inhibitor that blocks nerve growth factor-induced signaling, inhibited cell survival, suggesting the possible existence of a nerve growth factor autocrine loop in these proliferating cells.

Neurogenesis in the adult peripheral nervous system

Most researchers believe that neurogenesis in mature mammals is restricted only to the subgranular zone of the dentate gyrus and the subventricular zone of the lateral ventricle in the central nervous system. In the peripheral nervous system, neurogenesis is thought to be active only during prenatal development, with the exception of the olfactory neuroepithelium. However, sensory ganglia in the adult peripheral nervous system have been reported to contain precursor cells that can proliferate in vitro and be induced to differentiate into neurons. The occurrence of insult-induced neurogenesis, which has been reported by several investigators in the brain, is limited to a few recent reports for the peripheral nervous system. These reports suggest that damage to the adult nervous system induces mechanisms similar to those that control the generation of new neurons during prenatal development. Understanding conditions under which neurogenesis can be induced in physiologically non-neurogenic regions in adults is one of the major challenges for developing therapeutic strategies to repair neurological damage. However, the induced neurogenesis in the peripheral nervous system is still largely unexplored. This review presents the history of research on adult neurogenesis in the peripheral nervous system, which dates back more than 100 years and reveals the evidence on the under estimated potential for generation of new neurons in the adult peripheral nervous system.

Generation of new neurons in dorsal root Ganglia in adult rats after peripheral nerve crush injury

The evidence of neurons generated ex novo in sensory ganglia of adult animals is still debated. In the present study, we investigated, using high resolution light microscopy and stereological analysis, the changes in the number of neurons in dorsal root ganglia after 30 days from a crush lesion of the rat brachial plexus terminal branches. Results showed, as expected, a relevant hypertrophy of dorsal root ganglion neurons. In addition, we reported, for the first time in the literature, that neuronal hypertrophy was accompanied by massive neuronal hyperplasia leading to a 42% increase of the number of primary sensory neurons. Moreover, ultrastructural analyses on sensory neurons showed that there was not a relevant neuronal loss as a consequence of the nerve injury. The evidence of BrdU-immunopositive neurons and neural progenitors labeled with Ki67, nanog, nestin, and sox-2 confirmed the stereological evidence of posttraumatic neurogenesis in dorsal root ganglia. Analysis of morphological changes following axonal damage in addition to immunofluorescence characterization of cell phenotype suggested that the neuronal precursors which give rise to the newly generated neurons could be represented by satellite glial cells that actively proliferate after the lesion and are able to differentiate toward the neuronal lineage.

Neural proliferation in the dorsal root ganglia of the adult rat following capsaicin-induced neuronal death

Glial proliferation is a major component of the nervous system's response to injury. In addition to glial proliferation, injury may induce neuronal proliferation in areas of the adult nervous system not considered neurogenic. We have previously reported increased neural proliferation within adult nodose ganglia following capsaicin-induced neuronal death. However, proliferation within the dorsal root ganglia (DRG) remains to be characterized. We hypothesized that capsaicin-induced neuronal death would increase proliferation of satellite glial cells (SGCs) within the DRG. To test this hypothesis, 6-week-old Sprague-Dawley rats received a neurotoxic dose of capsaicin, and proliferation was quantified and characterized at multiple time points thereafter. Proliferation of satellite glial cells expressing the progenitor cell marker nestin was increased at 1 and 3 days following capsaicin administration as shown by BrdU incorporation. In addition to SGCs was a large population of proliferating resident macrophages, as shown by retrovirally mediated expression of GFP. SGC proliferation at these early time points was followed by recovery of neuronal numbers after a loss of 40% of the neuronal population in the DRG. This recovery in neuronal number correlated with recovery of function as shown by paw withdrawal from a noxious heat source. Further understanding of the role that glial proliferation plays in the recovery of neuronal numbers and function may lead to the development of therapeutic treatments for neurodegenerative conditions.

Intracerebroventricular administration of nerve growth factor induces gliogenesis in sensory ganglia, dorsal root, and within the dorsal root entry zone

Previous studies indicated that intracerebroventricular administration of nerve growth factor (NGF) leads to massive Schwann cell hyperplasia surrounding the medulla oblongata and spinal cord. This study was designed to characterize the proliferation of peripheral glial cells, that is, Schwann and satellite cells, in the trigeminal ganglia and dorsal root ganglia (DRG) of adult rats during two weeks of NGF infusion using bromodeoxyuridine (BrdU) to label dividing cells. The trigeminal ganglia as well as the cervical and lumbar DRG were analyzed. Along the entire neuraxis a small number of dividing cells were observed within these regions under physiological condition. NGF infusion has dramatically increased the generation of new cells in the neuronal soma and axonal compartments of sensory ganglia and along the dorsal root and the dorsal root entry zone. Quantification of BrdU positive cells within sensory ganglia revealed a 2.3- to 3-fold increase in glial cells compared to controls with a similar response to NGF for the different peripheral ganglia examined. Immunofluorescent labeling with S100β revealed that Schwann and satellite cells underwent mitosis after NGF administration. These data indicate that intracerebroventricular NGF infusion significantly induces gliogenesis in trigeminal ganglia and the spinal sensory ganglia and along the dorsal root entry zone as well as the dorsal root.

Sox2 in the adult rat sensory nervous system

Sex-determining region Y (SRY)-box 2 (Sox2) is a member of the Sox family transcription factors. In the central nervous system, Sox2 is expressed in neural stem cells from neurogenic regions, and regulates stem cell proliferation and differentiation. In the peripheral nervous system, Sox2 is found only in the immature and dedifferentiated Schwann cells, and is involved in myelination inhibition or N-cadherin redistribution. In the present immunohistochemical study, we found that Sox2 is also expressed in other cells of the adult rat peripheral nervous system. Nuclear Sox2 was observed in all satellite glial cells, non-myelinating Schwann cells, and the majority of terminal Schwann cells that form lamellar corpuscles and longitudinal lanceolate endings. Sox2 was not found in myelinating Schwann cells and terminal Schwann cells of subepidermal free nerve endings. Satellite glial cells exhibit strong Sox2 immunoreactivity, whereas non-myelinating Schwann cells show weak immunoreactivity. RT-PCR confirmed the presence of Sox2 mRNA, indicating that the cells are likely Sox2 expressors. Our findings suggest that the role of Sox2 in the peripheral nervous system may be cell-type-dependent.

Isolation, characterization, and differentiation of progenitor cells from human adult adrenal medulla

Chromaffin cells, sympathetic neurons of the dorsal ganglia, and the intermediate small intensely fluorescent cells derive from a common neural crest progenitor cell. Contrary to the closely related sympathetic nervous system, within the adult adrenal medulla a subpopulation of undifferentiated progenitor cells persists, and recently, we established a method to isolate and differentiate these progenitor cells from adult bovine adrenals. However, no studies have elucidated the existence of adrenal progenitor cells within the human adrenal medulla. Here we describe the isolation, characterization, and differentiation of chromaffin progenitor cells obtained from adult human adrenals. Human chromaffin progenitor cells were cultured in low-attachment conditions for 10-12 days as free-floating spheres in the presence of fibroblast growth factor-2 (FGF-2) and epidermal growth factor. These primary human chromosphere cultures were characterized by the expression of several progenitor markers, including nestin, CD133, Notch1, nerve growth factor receptor, Snai2, Sox9, Sox10, Phox2b, and Ascl1 on the molecular level and of Sox9 on the immunohistochemical level. In opposition, phenylethanolamine N-methyltransferase (PNMT), a marker for differentiated chromaffin cells, significantly decreased after 12 days in culture. Moreover, when plated on poly-l-lysine/laminin-coated slides in the presence of FGF-2, human chromaffin progenitor cells were able to differentiate into two distinct neuron-like cell types, tyrosine hydroxylase (TH)(+)/β-3-tubulin(+) cells and TH(-)/β-3-tubulin(+) cells, and into chromaffin cells (TH(+)/PNMT(+)). This study demonstrates the presence of progenitor cells in the human adrenal medulla and reveals their potential use in regenerative medicine, especially in the treatment of neuroendocrine and neurodegenerative diseases.

Adult neurogenesis in the central and peripheral nervous systems

Neurogenesis occurs during embryonic development and continues throughout adulthood. Although spontaneous adult neurogenesis is restricted to selective germinal regions, including the subventricular zone and dentate gyrus of the hippocampus, neural stem cells (NSCs) are widely distributed in the central and peripheral nervous systems. Besides their ability to integrate into existing neural networks during physiological conditions, NSCs also proliferate and differentiate in response to injury, thus promising the potential use of endogenous NSCs for the treatment of neuropathological conditions. In this review, we discuss recent progress in the understanding of adult neurogenesis in the brain and peripheral nervous system.

SCG postnatal remodelling--hypertrophy and neuron number stability--in Spix's yellow-toothed cavies (Galea spixii)

Whilst a fall in neuron numbers seems a common pattern during postnatal development, several authors have nonetheless reported an increase in neuron number, which may be associated with any one of a number of possible processes encapsulating either neurogenesis or late maturation and incomplete differentiation. Recent publications have thus added further fuel to the notion that a postnatal neurogenesis may indeed exist in sympathetic ganglia. In the light of these uncertainties surrounding the effects exerted by postnatal development on the number of superior cervical ganglion (SCG) neurons, we have used state-of-the-art design-based stereology to investigate the quantitative structure of SCG at four distinct timepoints after birth, viz., 1-3 days, 1 month, 12 months and 36 months. The main effects exerted by ageing on the SCG structure were: (i) a 77% increase in ganglion volume; (ii) stability in the total number of the whole population of SCG nerve cells (no change--either increase or decrease) during post-natal development; (iii) a higher proportion of uninucleate neurons to binucleate neurons only in newborn animals; (iv) a 130% increase in the volume of uninucleate cell bodies; and (v) the presence of BrdU positive neurons in animals at all ages. At the time of writing our results support the idea that neurogenesis takes place in the SCG of preás, albeit it warrants confirmation by further markers. We also hypothesise that a portfolio of other mechanisms: cell repair, maturation, differentiation and death may be equally intertwined and implicated in the numerical stability of SCG neurons during postnatal development.

The generation of rat dorsal root ganglion cell lines to identify the target of KW-7158, a novel treatment for overactive bladder

KW-7158 is a drug candidate for the treatment of overactive bladder. Although pharmacological studies have suggested that it suppresses afferent nerve conduction, its molecular target is unknown. We herein report the establishment of dorsal root ganglion (DRG) cell lines useful for identification of the target of this compound. First, we confirmed that the target exists in rat primary DRG by [(3)H]KW-7158 binding. To establish DRG cell lines, we used DRG from transgenic rats harboring the temperature-sensitive large T-antigen. The immortalized cells were initially screened for their expression of neuronal markers, and 72 positive clones were obtained (designated as TRD cells). Next, in order to select TRD cells expressing the target of KW-7158, we measured the binding affinity and amount of the binding sites present in each clone. Most clones expressed two binding sites, one with low affinity and one with high affinity. Differential binding of KW-7158 derivatives to each site revealed that the high affinity site is pharmacologically relevant. Therefore, we successfully identified "TRD-10" which express the largest amount of the high affinity site. These cell lines will therefore be useful tools to identify the target of KW-7158.

Neural proliferation and restoration of neurochemical phenotypes and compromised functions following capsaicin-induced neuronal damage in the nodose ganglion of the adult rat

We previously reported that neuronal numbers within adult nodose ganglia (NG) were restored to normal levels 60 days following the capsaicin-induced destruction of nearly half of the neuronal population. However, the nature of this neuronal replacement is not known. Therefore, we aimed to characterize neural proliferation, neurochemical phenotypes, and functional recovery within adult rat NG neurons following capsaicin-induced damage. Sprague-Dawley rats received intraperitoneal injections of capsaicin or vehicle solution, followed by 5-bromo-2-deoxyuridine (BrdU) injections to reveal cellular proliferation. NG were collected at multiple times post-treatment (up to 300 days) and processed for immunofluorescence, RT-PCR, and dispersed cell cultures. Capsaicin-induced cellular proliferation, indicated by BrdU/Ki-67-labeled cells, suggests that lost neurons were replaced through cell division. NG cells expressed the stem cell marker, nestin, indicating that these ganglia have the capacity to generate new neurons. BrdU-incorporation within β-III tubulin-positive neuronal profiles following capsaicin suggests that proliferating cells matured to become neurons. NG neurons displayed decreased NMDAR expression up to 180-days post-capsaicin. However, both NMDAR expression within the NG and synaptophysin expression within the central target of NG neurons, the NTS, were restored to pre-injury levels by 300 days. NG cultures from capsaicin-treated rats contained bipolar neurons, normally found only during development. To test the functional recovery of NG neurons, we injected the satiety molecule, CCK. The effect of CCK on food intake was restored by 300-days post-capsaicin. This restoration may be due to the regeneration of damaged NG neurons or generation of functional neurons that replaced lost connections.

Differentiation of nonhuman primate embryonic stem cells along neural lineages

The differentiation of embryonic stem cells (ESCs) into neurons and glial cells represents a promising cell-based therapy for neurodegenerative diseases. Because the rhesus macaque is physiologically and phylogenetically similar to humans, it is a clinically relevant animal model for ESC research. In this study, the pluripotency and neural differentiation potential of a rhesus monkey ESC line (ORMES6) was investigated. ORMES6 was derived from an in vitro produced blastocyst, which is the same way human ESCs have been derived. ORMES6 stably expressed the embryonic transcription factors POU5F1 (Oct4), Sox2 and NANOG. Stage-specific embryonic antigen 4 (SSEA 4) and the glycoproteins TRA-1-60 and TRA-1-81 were also expressed. The embryoid bodies (EBs) formed from ORMES6 ESCs spontaneously gave rise to cells of three germ layers. After exposure to basic fibroblast growth factor (bFGF) for 14-16 days, columnar rosette cells formed in the EB outgrowths. Sox2, microtubule-associated protein (MAP2), beta-tublinIII and glial fibrillary acidic protein (GFAP) genes and Nestin, FoxD3, Pax6 and beta-tublinIII antigens were expressed in the rosette cells. Oct4 and NANOG expression were remarkably down-regulated in these cells. After removal of bFGF from the medium, the rosette cells differentiated along neural lineages. The differentiated cells expressed MAP2, beta-tublinIII, Neuro D and GFAP genes. Most differentiated cells expressed early neuron-specific antigen beta-tublinIII (73+/-4.7%) and some expressed intermediate neuron antigen MAP2 (18+/-7.2%). However, some differentiated cells expressed the glial cell antigens A2B5 (7.17%+/-1.2%), GFAP (4.93+/-1.9%), S100 (7+/-3.5%) and O4 (0.27+/-0.2%). The rosette cells were transplanted into the striatum of immune-deficient NIHIII mice. The cells persisted for approximately 2 weeks and expressed Ki67, NeuN, MAP2 and GFAP. These results demonstrate that the rhesus monkey ESC line ORMES6 retains the pluripotent characteristics of ESCs and can be efficiently induced to differentiate along neural lineages.

Axotomy-induced up-regulation of tumor necrosis factor-alpha in the dorsal root ganglia

OBJECTIVES

Neuropathic pain is a chronic pain syndrome associated with drug, injury or disease-induced damage or destruction of sensory afferent fibers of the dorsal root ganglia (DRG). Although the exact underlying pathologic mechanisms are not known, pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha) are recognized as potential modulators of peripheral and central nervous system inflammatory responses. They play a crucial role in injury and the pathologic development of chronic pain syndromes such as neuropathic pain.

METHODS

Twenty-four rats were divided into a naive control (n=6), sham (surgery exposing sciatic nerve, n=6), and peripheral nerve lesion group (unilateral axotomy of sciatic nerve, n=12).

RESULTS

The results of this study demonstrate a transient up-regulation of TNF-alpha expression within ipsi- and contralateral DRG following complete unilateral sciatic nerve axotomy as confirmed by immunohistochemistry, reverse transcriptase-polymerase chain reaction (RT-PCR) and real-time PCR. Elevated expression of TNF-alpha was noted to occur within the first 7 days post-axotomy, which subsequently normalized to baseline levels by day 14. This transient up-regulation was also associated with a switch in cellular source from predominant satellite cell expression at baseline to that involving satellite cells and abundant numbers of sensory neurons.

DISCUSSION

These results support the role of TNF-alpha in the upstream cascade of cellular events involved in the underlying pathogenesis of neuropathic pain. Strategies targeting the early attenuation of TNF-alpha within the DRG during the first week post-injury may have significant clinical impact in preventing the downstream cascade of events involved in the underlying cellular pathology of neuropathic pain.

Implanted embryonic sensory neurons project axons toward adult auditory brainstem neurons in roller drum and Stoppini co-cultures

Previously we have shown in vivo the survival, migration and integration of embryonic dorsal root ganglion (DRG) neurons that were grafted into the inner ear and peripheral auditory nervous system. In order to evaluate relevant factors determining integration of sensory neurons further into the central auditory nervous system, complementary in vitro techniques are necessary. The advantages of in vitro systems are that a large number of factors including various grafts and different conditions can be efficiently examined for. Hence, we co-cultured 300 microm thick postnatal rat brainstem slices containing the cochlear nucleus including the central part of the 8th cranial nerve with mouse embryonic DRG neurons. The organotypic co-cultures were either grown on coverslips using the roller drum method described by Gähwiler or on membranes according to the interface method described by Stoppini. Neurons in the cochlear nucleus were labeled with DiI. The results demonstrate that (1) brainstem slices survive for up to 5 weeks in culture, and that (2) co-cultures of embryonic sensory neurons and brainstem show a high degree of neuronal survival, and that (3) survival and axonal outgrowth from the implanted embryonic neurons are dependent on the presence of the brainstem slice rather than on exogenous NGF and that (4) implanted embryonic neurons send axons toward neurons in the cochlear nucleus.

Neurotrophin-3 administration alters neurotrophin, neurotrophin receptor and nestin mRNA expression in rat dorsal root ganglia following axotomy

In the months following transection of adult rat peripheral nerve some sensory neurons undergo apoptosis. Two weeks after sciatic nerve transection some neurons in the L4 and L5 dorsal root ganglia begin to show immunoreactivity for nestin, a filament protein expressed by neuronal precursors and immature neurons, which is stimulated by neurotrophin-3 (NT-3) administration. The aim of this study was to examine whether NT-3 administration could be compensating for decreased production of neurotrophins or their receptors after axotomy, and to determine the effect on nestin synthesis. The levels of mRNA in the ipsilateral and contralateral L4 and L5 dorsal root ganglia were analyzed using real-time polymerase chain reaction, 1 day, 1, 2 and 4 weeks after unilateral sciatic nerve transection and NT-3 or vehicle administration via s.c. micro-osmotic pumps. In situ hybridization was used to identify which cells and neurons expressed mRNAs of interest, and the expression of full-length trkC and p75NTR protein was investigated using immunohistochemistry. Systemic NT-3 treatment increased the expression of brain-derived neurotrophic factor, nestin, trkA, trkB and trkC mRNA in ipsilateral ganglia compared with vehicle-treated animals. Some satellite cells surrounding neurons expressed trkA and trkC mRNA and trkC immunoreactivity. NT-3 administration did not affect neurotrophin mRNA levels in the contralateral ganglia, but decreased the expression of trkA mRNA and increased the expression of trkB mRNA and p75NTR mRNA and protein. These data suggest that systemically administered NT-3 may counteract the decrease, or even increase, neurotrophin responsiveness in both ipsi- and contralateral ganglia after nerve injury.

Isolation and Characterization of Neural Crest Progenitors from Adult Dorsal Root Ganglia

After peripheral nerve injury, the number of sensory neurons in the adult dorsal root ganglia (DRG) is initially reduced but recovers to a normal level several months later. The mechanisms underlying the neuronal recovery after injury are not clear. Here, we showed that in the DRG explant culture, a subpopulation of cells that emigrated out from adult rat DRG expressed nestin and p75 neurotrophin receptor and formed clusters and spheres. They differentiated into neurons, glia, and smooth muscle cells in the presence or absence of serum and formed secondary and tertiary neurospheres in cloning assays. Molecular expression analysis demonstrated the characteristics of neural crest progenitors and their potential for neuronal differentiation by expressing a set of well-defined genes related to adult stem cells niches and neuronal fate decision. Under the influence of neurotrophic factors, some of these progenitors gave rise to neuropeptide-expressing cells and protein zero-expressing Schwann cells. In a 5-bromo-2'-deoxyuridine chasing study, we showed that these progenitors likely originate from satellite glial cells. Our study suggests that a subpopulation of glia in adult DRG is likely to be progenitors for neurons and glia and may play a role in neurogenesis after nerve injury. Disclosure of potential conflicts of interest is found at the end of this article.

Evidence of Postnatal Neurogenesis in Dorsal Root Ganglion: Role of Nitric Oxide and Neuronal Restrictive Silencer Transcription Factor

The various mechanisms underlying postnatal neurogenesis from discrete CNS regions have emerged recently. However, little is known about postnatal neurogenesis in dorsal root ganglion (DRG). BrdU incorporation and subsequent immunostaining for BrdU, neural stem cell marker, nestin and neuronal marker, PGP 9.5 have provided evidence for postnatal neurogenesis in DRG. We further demonstrate, in vivo and in vitro, that nitric oxide (NO) regulates neural stem cells (nestin+) proliferation and, possibly, differentiation into neurons. Surprisingly, nerve growth factor (NGF) had no effect on nestin+ cells proliferation. Axotomy or NGF-deprivation of DRG neurons-satellite glia co-culture increases NO production by neurons and treating with a NO synthase (NOS) inhibitor, N G-nitro-L-arginine methylester (L-NAME) in vitro or 7-nitroindazole (7NI) in vivo, causes a significant increase in nestin+ cell numbers. However, a soluble guanylyl cyclase (sGC) blocker, 1H-[1, 2, 4] oxadiazolo [4, 3-a] quinoxalin-1-one (ODQ) treatment of NGF-deprived DRG neurons-satellite glia co-culture had no significant effect on nestin+ cell numbers. This implies NO regulates nestin+ cell proliferation independent of cGMP. We hypothesised that the neuronal-restrictive silencer transcription factor (NRSF, also termed REST), a master regulator of neuronal genes in non-neuronal cells, may be modulated by NO in satellite glia cultures. A NO donor, dimethyl-triamino-benzidine (DETA)-NO treatment of satellite glia cell cultures results in a significant increase in the NRSF/REST mRNA expression. The majority of cultured satellite glia cells express nestin, and also show increased levels of NOS, thus L-NAME treatment of these cultures causes a dramatic reduction in NRSF/REST mRNA. Overall these results suggest that NO inhibits neurogenesis in DRG and this is correlated with modulation of NRSF, a known modulator of differentiation.

NO-cGMP mediated galanin expression in NGF-deprived or axotomized sensory neurons

Leukaemia inhibitory factor (LIF) and nerve growth factor (NGF) are well characterized regulators of galanin expression. However, LIF knockout mice containing the rat galanin 5' proximal promoter fragment (- 2546 to + 15 bp) driving luciferase responded to axotomy in the same way as control mice. Also, LIF had no effect on reporter gene expression in vitro, neither in the presence or absence of NGF, suggesting that other factors mediate an axotomy response from the galanin promoter. We then addressed the role of nitric oxide (NO) using NGF-deprived rat dorsal root ganglion (DRG) neuron cultures infected with viral vectors containing the above-mentioned construct, and also studied endogenous galanin expression in axotomized DRG in vivo. Blocking endogenous NO in NGF-deprived DRG cultures suppressed galanin promoter activity. Consistent with this, axotomized/NGF-deprived DRG neurons expressed high levels of neuronal NO synthase (nNOS) and galanin. Further, using pharmacological NOS blockers, or adenoviral vectors expressing dominant-negative either for nNOS or soluble guanylate cyclase in vivo and in vitro, we show that the NO-cGMP pathway induces endogenous galanin in DRG neurons. We propose that both LIF and NO, acting at different promoter regions, are important for the up-regulation of galanin, and for DRG neuron survival and regeneration after axotomy.

The chemokine stromal cell-derived factor-1 regulates the migration of sensory neuron progenitors

Chemokines and their receptors are essential for the development and organization of the hematopoietic/lymphopoietic system and have now been shown to be expressed by different types of cells in the nervous system. In mouse embryos, we observed expression of the chemokine (CXC motif) receptor 4 (CXCR4) by neural crest cells migrating from the dorsal neural tube and in the dorsal root ganglia (DRGs). Stromal cell-derived factor-1 (SDF-1), the unique agonist for CXCR4, was expressed along the path taken by crest cells to the DRGs, suggesting that SDF-1/CXCR4 signaling is needed for their migration. CXCR4 null mice exhibited small and malformed DRGs. Delayed migration to the DRGs was suggested by ectopic cells expressing tyrosine receptor kinase A (TrkA) and TrkC, neurotrophin receptors required by DRG sensory neuron development. In vitro, the CXCR4 chemokine receptor was upregulated by migratory progenitor cells just as they exited mouse neural tube explants, and SDF-1 acted as a chemoattractant for these cells. Most CXCR4-expressing progenitors differentiated to form sensory neurons with the properties of polymodal nociceptors. Furthermore, DRGs contained a population of progenitor cells that expressed CXCR4 receptors in vitro and differentiated into neurons with a similar phenotype. Our findings indicate an important role for SDF-1/CXCR4 signaling in directing the migration of sensory neuron progenitors to the DRG and potentially in other aspects of development once the DRGs have coalesced.

Effects of systemically administered NT-3 on sensory neuron loss and nestin expression following axotomy

Previous work has shown that administration of the neurotrophin NT-3 intrathecally or to the proximal stump can prevent axotomy-induced sensory neuron loss and that NT-3 can stimulate sensory neuron differentiation in vitro. We have examined the effect of axotomy and systemic NT-3 administration on neuronal loss, apoptosis (defined by morphology and activated caspase-3 immunoreactivity), and nestin expression (a protein expressed by neuronal precursor cells) in dorsal root ganglia (DRG) following axotomy of the adult rat sciatic nerve. Systemic administration of 1.25 or 5 mg of NT-3 over 1 month had no effect on the incidence of apoptotic neurons but prevented the overall loss of neurons seen at 4 weeks in vehicle-treated animals. Nestin-immunoreactive neurons began to appear 2 weeks after sciatic transection in untreated animals and steadily increased in incidence over the next 6 weeks. NT-3 administration increased the number of nestin-immunoreactive neurons at 1 month by two- to threefold. Nestin-IR neurons had a mean diameter of 20.78 +/- 2.5 microm and expressed the neuronal markers neurofilament 200, betaIII-tubulin, protein gene product 9.5, growth associated protein 43, trkA, and calcitonin gene-related peptide. Our results suggest that the presence of nestin in DRG neurons after nerve injury is due to recent differentiation and that exogenous NT-3 may prevent neuron loss by stimulating this process, rather than preventing neuron death.

Adult human olfactory neural progenitors cultured in defined medium

Neurosphere-forming cells (NSFCs) derived from primary cultures of adult human olfactory epithelium were established in minimum essential medium (MEM) with Hanks balanced salts and 10% heat-inactivated fetal bovine serum (FBS). A totally defined medium (DM) was employed to examine their proliferation, lineage restriction and differentiation. DMEM/F12 (DF) was found to support NSFCs and served as the base medium for this study. NSFCs were adapted to the DM through serial serum reductions at successive feedings. NSFCs in DF supplemented with N2, B27 or insulin attained saturation density and formed extensive processes. Immunolocalization of lineage specific markers [i.e., nestin, beta-tubulin III, peripherin, neural cell adhesion molecule, A2B5, O4, microtubule-associated-protein-2 (MAP2) and glial fibrillary acidic protein], as well as 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide and ornithine decarboxylase assays were employed to characterize the NSFCs. The effects of trophic factors including epidermal growth factor (EGF), nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), neurotrophic factors (NT-3), and basic fibroblast growth factor (bFGF) were evaluated. With the reduction of serum and the addition N2, B27, and other nutrients, there was a change in lineage restriction including an increase the expression of A2B5 and other glial markers as well as the expression of mature neuronal markers with a simultaneous reduction of nestin reactivity. NSFCs proliferated and maintained their pluripotency for over a year in the DM. Further studies will determine the utility of NSFCs for cell replacement therapy.

Pharmacologic suppression of neuronal oxidative damage and dendritic degeneration following direct activation of glial innate immunity in mouse cerebrum

Activation of glial innate immunity is widely proposed to contribute to a number of degenerative and destructive diseases of brain. However, the precise role of activated innate immunity has been difficult to define in vivo because of multiple simultaneous pathogenic processes and responses to injury that confound interpretation of results from complex models of disease. Here, we used the model of intracerebroventricular (ICV) injection of lipopolysaccharide (LPS) to test the hypothesis that directly activated glial innate immunity leads to neurodegeneration in cerebrum and to establish the molecular determinants of and neuroprotectants from such innate immunity-mediated neuronal damage. Our results showed that ICV LPS induced delayed, reversible oxidative damage to cerebral neuronal membranes as measured by F4-neuroprostanes that was coincident with degeneration of the hippocampal pyramidal neuron dendritic system, but not neuron death, in adult mice. Both neuronal oxidative damage and dendritic degeneration were NF-kappaB and iNOS dependent and were completely suppressed by ibuprofen and alpha-tocopherol, but not naproxen or gamma-tocopherol. These results prove that activation of glial innate immunity can lead to neurodegeneration independent of other pathologic processes, closely associate oxidative damage to neuronal membranes with degeneration of the dendritic system, and provide a possible explanation for the varying efficacy of neuroprotectants that have been suggested in observational studies of dementia.

Expression of basic fibroblast growth factor isoforms in postmitotic sympathetic neurons: synthesis, intracellular localization and involvement in karyokinesis

Low and high molecular weight isoforms of the mitogen and multifunctional cytokine basic fibroblast growth factor (FGF-2) are up-regulated in neurons and glial cells in response to peripheral nerve lesion. While synthesis, regulation and functions of FGF-2 in non-neuronal cells are well established, the significance of neuronal FGF-2 remains to be investigated in the peripheral nervous system. Therefore, the expression, intracellular localization and possible effects of FGF-2 isoforms were analyzed in primary sympathetic neurons derived from the rat superior cervical ganglion. FGF-2 is detected in the nucleus and in perinuclear Golgi fields of early postnatal neurons which also express mRNA and protein for the FGF receptor type 1. Biolistic transfection of plasmids encoding FGF-2 isoforms fused to fluorescent proteins demonstrates nuclear targeting of 18 kDa FGF-2 and 23 kDa FGF-2 with prominent accumulation in the nucleolus of neurons. Neither overexpression nor treatment with FGF-2 isoforms promotes survival of sympathetic neurons deprived of nerve growth factor; however, neuronal transfection of the high molecular weight FGF-2 isoform in dissociated and slice cultures results in a bi- or multinuclear phenotype. The present study provides evidence for neuronal synthesis and targeting of FGF-2 to the nucleus and Golgi apparatus supporting a dual role of FGF-2 in the nucleus and secretory pathway of sympathetic neurons.

Late differentiation contributes to the apparent increase in sensory neuron number in juvenile rat

Using both profile counts and unbiased stereological methods, estimates of neuron number in the lumbar dorsal root ganglia of the rat have been shown to increases approximately 35% during postnatal life [J. Comp. Neurol. 386 (1997) 8-15; J. Comp. Neurol. 449 (2002) 158-165]. The mechanism underlying this addition of neurons was investigated. No evidence of incorporation of (BrdU), a mitotic marker, was found. Similarly, counts of myelinated and unmyelinated axons in the sural nerve were the same in neonates and adults. These results are not consistent with the possibility that neurogenesis accounts for neuron addition. A population of neurons that stains with TuJ1, an antibody against neuronal class III beta tubulin, but not with an antibody against the phosphorylated and non-phosphorylated forms of heavy chain neurofilament protein (NF-H) was found in neonates, but not adults. These less-differentiated (type-L) neurons are not detected by either profile counts or unbiased stereology and do not transport HRP retrogradely. Maturation of this pool of incompletely differentiated neurons appears to be one mechanism whereby neuron number is augmented during postnatal life.

Expression of fibroblast growth factors in rat dorsal root ganglion neurons and regulation after peripheral nerve injury

Using cDNA array, we observed the expression of eight members of the fibroblast growth factor (FGF) family, FGF 2, 5, 7, 9, 10, 13 and 14, in rat lumbar 4 and 5 dorsal root ganglia (DRGs). Over a period of 28 days after sciatic nerve transection, the array signals for FGF 2 and 7 were significantly increased in the DRGs, while FGF 13 decreased. Using the reverse transcription polymerase chain reaction (RT-PCR), we confirmed the axotomy-induced changes in the expression of FGF 7 and 13. hybridization showed that FGF 13 was expressed in 60% of DRG neurons under normal circumstance. Seven days after axotomy the number of FGF 13-positive neurons was decreased to 18%, but partially recovered to 40% after 28 days. FGF 13 immunoreactivity was also decreased. These data indicate that FGFs are important for DRG neurons under normal circumstance and after nerve injury.

Postnatal histogenesis in the peripheral nervous system

The issue of postnatal neurogenesis has gained great importance over the last few years and the recent amazing scientific advancements, changing our viewpoint on the long-lasting "no new neurons" dogma, have opened promising new perspectives on the treatment of the damaged nervous system. While most of the researchers have focused on the central nervous system, the peripheral nervous system has received little attention so far with respect to postnatal histogenesis. To attract scientific attention on this issue, the present article was written with the aim of reviewing the body of literature on postnatal histogenesis in the various districts of the peripheral nervous system, from the historical roots to the most recent reports.

Sensory neuron addition in juvenile rat: time course and specificity

The time course and specificity of neuron addition to lumbar dorsal root ganglia (DRGs) L(4)-L(6) of rats was investigated. By using methods validated by three-dimensional reconstructions, profile counts in paraffin sections of nucleoli within a nucleus were 36% greater in 100-day-old (P100) rats than in 1-day-old (P1) rats. Adult values were reached by P50. Added neurons fell disproportionately into the population of neurons whose size was below that of the mean size within the ganglion. The biochemical characteristics of small neurons were used to determine whether added neurons fall into particular subpopulations. In DRGs, L(3) and L(4), the number of neurons immunoreactive to substance P (SP) or calcitonin-gene-related peptide (CGRP) or that bound the lectin isolectin B4 (IB4) was determined. Between P5 and P100, the number of SP-stained neurons increased by 2,280 (40% increase), CGRP-stained neurons increased by 6,080 (70% increase), and IB4-stained neurons increased by 6,900 (90% increase). The increase in the number of neurons stained for CGRP or IB4 was more than twice the number of neurons found to be added to these ganglia, indicating that coexpression of these markers as well as neuron number may be developmentally regulated during postnatal life.