Targeted disruption of the galanin gene reduces the number of sensory neurons and their regenerative capacity

Investigating the potential of GalR2 as a drug target for neuropathic pain

Neuropathic pain is a chronic and debilitating condition characterised by episodes of hyperalgesia and allodynia. It occurs following nerve damage from disease, inflammation or injury and currently impacts up to 17% of the UK population. Existing therapies lack efficacy and have deleterious side effects that can be severely limiting. Galanin receptor 2 (GalR2) is a G-protein coupled receptor (GPCR) implicated in the control and processing of painful stimuli. Within the nervous system it is expressed in key tissues involved in these actions such as dorsal root ganglia (DRG) and the dorsal horn of the spinal cord. Stimulation of GalR2 is widely reported to have a role in the attenuation of inflammatory and neuropathic pain. Several studies have indicated GalR2 as a possible drug target, highlighting the potential of specific GalR2 agonists to both provide efficacy and to address the side-effect profiles of current pain therapies in clinical use. A strong biological target for drug discovery will be well validated with regards to its role in the relevant disease pathology. Ideally there will be good translational models, sensitive probes, selective and appropriate molecular tools, translational biomarkers, a clearly defined patient population and strong opportunities for commercialisation. Before GalR2 can be considered as a drug target suitable for investment, key questions need to be asked regarding its expression profile, receptor signalling and ligand interactions. This article aims to critically review the available literature and determine the current strength of hypothesis of GalR2 as a target for the treatment of neuropathic pain.

Nanogel-mediated delivery of oncomodulin secreted from regeneration-associated macrophages promotes sensory axon regeneration in the spinal cord

Preconditioning nerve injury enhances axonal regeneration of dorsal root ganglia (DRG) neurons in part by driving pro-regenerative perineuronal macrophage activation. How these macrophages influence the neuronal capacity of axon regeneration remains elusive. We report that oncomodulin (ONCM) is produced from the regeneration-associated macrophages and strongly influences regeneration of DRG sensory axons. We also attempted to promote sensory axon regeneration by nanogel-mediated delivery of ONCM to DRGs.

Methods:In vitro neuron-macrophage interaction model and preconditioning sciatic nerve injury were used to verify the necessity of ONCM in preconditioning injury-induced neurite outgrowth. We developed a nanogel-mediated delivery system in which electrostatic encapsulation of ONCM by a reducible epsilon-poly(_L-lysine)-nanogel (REPL-NG) enabled a controlled release of ONCM.

Results: Sciatic nerve injury upregulated ONCM in DRG macrophages. ONCM in macrophages was necessary to produce pro-regenerative macrophages in the in vitro model of neuron-macrophage interaction and played an essential role in preconditioning-induced neurite outgrowth. ONCM increased neurite outgrowth in cultured DRG neurons by activating a distinct gene set, particularly neuropeptide-related genes. Increasing extracellularly secreted ONCM in DRGs sufficiently enhanced the capacity of neurite outgrowth. Intraganglionic injection of REPL-NG/ONCM complex allowed sustained ONCM activity in DRG tissue and achieved a remarkable long-range regeneration of dorsal column sensory axons beyond spinal cord lesion.

Conclusion: NG-mediated ONCM delivery could be exploited as a therapeutic strategy for promoting sensory axon regeneration following spinal cord injury.

Overlapping transcriptional programs promote survival and axonal regeneration of injured retinal ganglion cells

Injured neurons in the adult mammalian central nervous system often die and seldom regenerate axons. To uncover transcriptional pathways that could ameliorate these disappointing responses, we analyzed three interventions that increase survival and regeneration of mouse retinal ganglion cells (RGCs) following optic nerve crush (ONC) injury, albeit not to a clinically useful extent. We assessed gene expression in each of 46 RGC types by single-cell transcriptomics following ONC and treatment. We also compared RGCs that regenerated with those that survived but did not regenerate. Each intervention enhanced survival of most RGC types, but type-independent axon regeneration required manipulation of multiple pathways. Distinct computational methods converged on separate sets of genes selectively expressed by RGCs likely to be dying, surviving, or regenerating. Overexpression of genes associated with the regeneration program enhanced both survival and axon regeneration in vivo, indicating that mechanistic analysis can be used to identify novel therapeutic strategies.

Minimal Increments of Hydrophobic Collapse within the N-Terminus of the Neuropeptide Galanin

The neuropeptide galanin has a 35-year history as an intriguing target in drug design owing to its implication as a potential anticonvulsant and neuronal trophic factor among many other therapeutically interesting functions including analgesia and mood alteration. In this study, we report the structural characterization of three synthetic fragments of the galanin N-terminus in buffered aqueous solution: hGal(2-12)KK, hGal(1-12)KK, and hGal(1-17)KK. High-field two-dimensional ¹H-¹H nuclear magnetic resonance (NMR) data were acquired for these fragments and used to derive distance restraints. We further utilized modified hydrogen bonding and dihedral restraints to reflect chemical shift patterns in the data, which revealed the signature of a weakly folded helix. Together, these sets of restraints were used to generate NMR structures of all three fragments, which depict a core of hydrophobic residues that cluster together regardless of the presence of a helical structure, and correspond to residues in the N-terminus of galanin that have been previously shown to be critical for binding its receptors. The helical structure only appears following the inclusion of Gly(1) in the sequence, and at longer sequence lengths, unlike many other peptides, the helix does not propagate. Rather, a few turns of poorly ordered helix appear to be a secondary consequence of clusters of hydrophobic sidechains that are conserved across all of the peptides in this study; the helices themselves appear ordered as a consequence of this clustering, and these clusters compare directly to those observed recently to make contacts between galanin and two of its receptor subtypes. Collapsed hydrophobic residues therefore organize and compose the functional core of human galanin and raise interesting questions about the nature of the conformational order in ligands that bind cell surface receptors.

Galanin family peptides: Molecular structure, expression and roles in the neuroendocrine axis and in the spinal cord

Galanin is a neurohormone as well as a neurotransmitter and plays versatile physiological roles for the neuroendocrine axis, such as regulating food intake, insulin level and somatostatin release. It is expressed in the central nervous system, including hypothalamus, pituitary, and the spinal cord, and colocalises with other neuronal peptides within neurons. Structural analyses reveal that the human galanin precursor is 104 amino acid (aa) residues in length, consisting of a mature galanin peptide (aa 33-62), and galanin message-associated peptide (GMAP; aa 63-104) at the C-terminus. GMAP appears to exhibit distinctive biological effects on anti-fungal activity and the spinal flexor reflex. Galanin-like peptide (GALP) has a similar structure to galanin and acts as a hypothalamic neuropeptide to mediate metabolism and reproduction, food intake, and body weight. Alarin, a differentially spliced variant of GALP, is specifically involved in vasoactive effect in the skin and ganglionic differentiation in neuroblastic tumors. Dysregulation of galanin, GALP and alarin has been implicated in various neuroendocrine conditions such as nociception, Alzheimer's disease, seizures, eating disorders, alcoholism, diabetes, and spinal cord conditions. Further delineation of the common and distinctive effects and mechanisms of various types of galanin family proteins could facilitate the design of therapeutic approaches for neuroendocrine diseases and spinal cord injury.

The Role of Galanin in Cerebellar Granule Cell Migration in the Early Postnatal Mouse during Normal Development and after Injury

Galanin, one of the most inducible neuropeptides, is widely present in developing brains, and its expression is altered by pathologic events (e.g., epilepsy, ischemia, and axotomy). The roles of galanin in brain development under both normal and pathologic conditions have been hypothesized, but the question of how galanin is involved in fetal and early postnatal brain development remains largely unanswered. In this study, using granule cell migration in the cerebellum of early postnatal mice (both sexes) as a model system, we examined the role of galanin in neuronal cell migration during normal development and after brain injury. Here we show that, during normal development, endogenous galanin participates in accelerating granule cell migration via altering the Ca2+ and cAMP signaling pathways. Upon brain injury induced by the application of cold insults, galanin levels decrease at the lesion sites, but increase in the surroundings of lesion sites. Granule cells exhibit the following corresponding changes in migration: (1) slowing down migration at the lesion sites; and (2) accelerating migration in the surroundings of lesion sites. Experimental manipulations of galanin signaling reduce the lesion site-specific changes in granule cell migration, indicating that galanin plays a role in such deficits in neuronal cell migration. The present study suggests that manipulating galanin signaling may be a potential therapeutic target for acutely injured brains during development.SIGNIFICANCE STATEMENT Deficits in neuronal cell migration caused by brain injury result in abnormal development of cortical layers, but the underlying mechanisms remain to be determined. Here, we report that on brain injury, endogenous levels of galanin, a neuropeptide, are altered in a lesion site-specific manner, decreasing at the lesion sites but increasing in the surroundings of lesion sites. The changes in galanin levels positively correlate with the migration rate of immature neurons. Manipulations of galanin signaling ameliorate the effects of injury on neuronal migration and cortical layer development. These results shed a light on galanin as a potential therapeutic target for acutely injured brains during development.

Involvement of Neuropeptide Galanin Receptors 2 and 3 in Learning, Memory and Anxiety in Aging Mice

The neuropeptide galanin (GAL), which is expressed in limbic brain structures, has a strong impact on the regulation of mood and behavior. GAL exerts its effects via three G protein-coupled receptors (GAL_1-3-R). Little is known about the effects of aging and loss of GAL-Rs on hippocampal-mediated processes connected to neurogenesis, such as learning, memory recall and anxiety, and cell proliferation and survival in the dorsal dentate gyrus (dDG) in mice. Our results demonstrate that loss of GAL₃-R, but not GAL₂-R, slowed learning and induced anxiety in older (12-14-month-old) mice. Lack of GAL₂-R increased cell survival (BrdU incorporation) in the dDG of young mice. However, normal neurogenesis was observed in vitro using neural stem and precursor cells obtained from GAL₂-R and GAL₃-R knockouts upon GAL treatment. Interestingly, we found sub-strain differences between C57BL/6J and C57BL/6N mice, the latter showing faster learning, less anxiety and lower cell survival in the dDG. We conclude that GAL-R signaling is involved in cognitive functions and can modulate the survival of cells in the neurogenic niche, which might lead to new therapeutic applications. Furthermore, we observed that the mouse sub-strain had a profound impact on the behavioral parameters analyzed and should therefore be carefully considered in future studies.

Atoh7-independent specification of retinal ganglion cell identity

Retinal ganglion cells (RGCs) relay visual information from the eye to the brain. RGCs are the first cell type generated during retinal neurogenesis. Loss of function of the transcription factor Atoh7, expressed in multipotent early neurogenic retinal progenitors leads to a selective and essentially complete loss of RGCs. Therefore, Atoh7 is considered essential for conferring competence on progenitors to generate RGCs. Despite the importance of Atoh7 in RGC specification, we find that inhibiting apoptosis in Atoh7-deficient mice by loss of function of Bax only modestly reduces RGC numbers. Single-cell RNA sequencing of Atoh7;Bax-deficient retinas shows that RGC differentiation is delayed but that the gene expression profile of RGC precursors is grossly normal. Atoh7;Bax-deficient RGCs eventually mature, fire action potentials, and incorporate into retinal circuitry but exhibit severe axonal guidance defects. This study reveals an essential role for Atoh7 in RGC survival and demonstrates Atoh7-dependent and Atoh7-independent mechanisms for RGC specification.

Role of Myc Proto-Oncogene as a Transcriptional Hub to Regulate the Expression of Regeneration-Associated Genes following Preconditioning Peripheral Nerve Injury

Preconditioning peripheral nerve injury enhances the intrinsic growth capacity of DRGs sensory axons by inducing transcriptional upregulation of the regeneration-associated genes (RAGs). However, it is still unclear how preconditioning injury leads to the orchestrated induction of many RAGs. The present study identified Myc proto-oncogene as a transcriptional hub gene to regulate the expression of a distinct subset of RAGs in DRGs following the preconditioning injury. We demonstrated that c-MYC bound to the promoters of certain RAGs, such as Jun, Atf3, and Sprr1a, and that Myc upregulation following SNI preceded that of the RAGs bound by c-MYC. Marked DNA methylation of the Myc exon 3 sequences was implicated in the early transcriptional activation and accompanied by open histone marks. Myc deletion led to a decrease in the injury-induced expression of a distinct subset of RAGs, which were highly overlapped with the list of RAGs that were upregulated by Myc overexpression. Following dorsal hemisection spinal cord injury in female rats, Myc overexpression in DRGs significantly prevented the retraction of the sensory axons in a manner dependent on its downstream RAG, June Our results suggest that Myc plays a critical role in axon regeneration via its transcriptional activity to regulate the expression of a spectrum of downstream RAGs and subsequent effector molecules. Identification of more upstream hub transcription factors and the epigenetic mechanisms specific for individual hub transcription factors would advance our understanding of how the preconditioning injury induces orchestrated upregulation of RAGs.

Morphology and distribution of hypothalamic peptidergic systems

Neuropeptides participate in the regulation of numerous hypothalamic functions that are aimed for sustaining the homeostasis of the organism. These neuropeptides can act in two different levels. They can influence the release of hormones from the adenohypophysis via the portal circulation; in addition, they can act as neurotransmitters/neuromodulators modulating the functioning of numerous hypothalamic neurotransmitter systems. Indeed, most of these peptidergic systems form a complex network in the infundibular and periventricular nuclei of the human hypothalamus, communicating with each other by synaptic connections that may control fundamental physiologic functions. In the present chapter, we provide an overview of the distribution of neuropeptides in the human hypothalamus using immunohistochemistry and high-resolution, three-dimensional mapping.

Elimination of galanin synthesis in noradrenergic neurons reduces galanin in select brain areas and promotes active coping behaviors

Accumulating evidence indicates that disruption of galanin signaling is associated with neuropsychiatric disease, but the precise functions of this neuropeptide remain largely unresolved due to lack of tools for experimentally disrupting its transmission in a cell type-specific manner. To examine the function of galanin in the noradrenergic system, we generated and crossed two novel knock-in mouse lines to create animals lacking galanin specifically in noradrenergic neurons (Gal^cKO-Dbh). We observed reduced levels of galanin peptide in pons, hippocampus, and prefrontal cortex of Gal^cKO-Dbh mice, indicating that noradrenergic neurons are a significant source of galanin to those brain regions, while midbrain and hypothalamic galanin levels were comparable to littermate controls. In these same brain regions, we observed no change in levels of norepinephrine or its major metabolite at baseline or after an acute stressor, suggesting that loss of galanin does not affect noradrenergic synthesis or turnover. Gal^cKO-Dbh mice had normal performance in tests of depression, learning, and motor-related behavior, but had an altered response in some anxiety-related tasks. Specifically, Gal^cKO-Dbh mice showed increased marble and shock probe burying and had a reduced latency to eat in a novel environment, indicative of a more proactive coping strategy. Together, these findings indicate that noradrenergic neurons provide a significant source of galanin to discrete brain areas, and noradrenergic-specific galanin opposes adaptive coping responses.

Changes in the transcriptional fingerprint of satellite glial cells following peripheral nerve injury

Satellite glial cells (SGCs) are homeostatic cells enveloping the somata of peripheral sensory and autonomic neurons. A wide variety of neuronal stressors trigger activation of SGCs, contributing to, for example, neuropathic pain through modulation of neuronal activity. However, compared to neurons and other glial cells of the nervous system, SGCs have received modest scientific attention and very little is known about SGC biology, possibly due to the experimental challenges associated with studying them in vivo and in vitro. Utilizing a recently developed method to obtain SGC RNA from dorsal root ganglia (DRG), we took a systematic approach to characterize the SGC transcriptional fingerprint by using next-generation sequencing and, for the first time, obtain an overview of the SGC injury response. Our RNA sequencing data are easily accessible in supporting information in Excel format. They reveal that SGCs are enriched in genes related to the immune system and cell-to-cell communication. Analysis of SGC transcriptional changes in a nerve injury-paradigm reveal a differential response at 3 days versus 14 days postinjury, suggesting dynamic modulation of SGC function over time. Significant downregulation of several genes linked to cholesterol synthesis was observed at both time points. In contrast, regulation of gene clusters linked to the immune system (MHC protein complex and leukocyte migration) was mainly observed after 14 days. Finally, we demonstrate that, after nerve injury, macrophages are in closer physical proximity to both small and large DRG neurons, and that previously reported injury-induced proliferation of SGCs may, in fact, be proliferating macrophages.

Comprehensive single-cell transcriptome lineages of a proto-vertebrate

Ascidian embryos highlight the importance of cell lineages in animal development. As simple proto-vertebrates they also provide insights into the evolutionary origins of novel cell types, such as cranial placodes and neural crest. To build upon these efforts we have determined single cell transcriptomes for more than 90,000 cells spanning the entirety of Ciona intestinalis development, from the onset of gastrulation to swimming tadpoles. This represents an average of over 12-fold coverage for every cell at every stage of development, owing to the small cell numbers of ascidian embryos. Single cell transcriptome trajectories were used to construct “virtual” cell lineage maps and provisional gene networks for nearly 40 different neuronal subtypes comprising the larval nervous system. We summarize several applications of these datasets, including annotating the synaptome of swimming tadpoles and tracing the evolutionary origin of novel cell types such as the vertebrate telencephalon.

Subclinical lipopolysaccharide from Salmonella Enteritidis induces neuropeptide dysregulation in the spinal cord and the dorsal root ganglia

Background

Despite increasing evidence that lipopolysaccharide (LPS) affects the biological active substances of dorsal root ganglia (DRG) we have limited knowledge of the influence of a single low dose of LPS, which does not result in any clinical symptoms of disease (subclinical LPS) on neuropeptides connected with the sensory pathway. Accordingly, in this work, we investigated the influence of subclinical LPS from Salmonella Enteritidis on selected neuropeptides: substance P (SP), galanin (GAL), neuropeptide Y (NPY), vasoactive intestinal peptide (VIP) and somatostatin (SOM) in the cervical, thoracic, lumbar and sacral regions of the DRG and spinal cord.

Methods

This study was performed on immature female pigs of the Pietrain × Duroc breed. Seven days after the intravenous injection of saline solution for control animals (n = 5) and 5 μg/kg b.w. LPS from S. Enteritidis for the experimental group (n = 5), the DRG and the spinal cord were collected to extract the neuropeptides using solid-phase extraction technology.

Results

Our results demonstrated that subclinical LPS in DRG was able to change the levels of all studied neuropeptides except SOM, whereas in the spinal cord it down-regulated all studied neuropeptides in the sacral spinal cord, maintaining the concentration of all studied neuropeptides in other regions similar to that observed in the control animals. The significant differences in the intensity and character of observed changes between particular regions of the DRG suggest that the exact functions of the studied neuropeptides and mechanisms of responses to subclinical LPS action depend on specific characteristics and functions of each examination region of DRG.

Conclusions

The mechanisms of observed changes are not fully understood and require further study of the molecular interactions between subclinical LPS from S. Enteritidis and neuronal and non-neuronal cells of DRG and spinal cord. The peripheral and central pain pathways must be analysed with the aspect of unknown long-term consequences of the influence of subclinical LPS from S. Enteritidis on neuropeptides in the spinal cord and the dorsal root ganglia.

Macrophage biology in the peripheral nervous system after injury

Neuroinflammation has positive and negative effects. This review focuses on the roles of macrophage in the PNS. Transection of PNS axons leads to degeneration and clearance of the distal nerve and to changes in the region of the axotomized cell bodies. In both locations, resident and infiltrating macrophages are found. Macrophages enter these areas in response to expression of the chemokine CCL2 acting on the macrophage receptor CCR2. In the distal nerve, macrophages and other phagocytes are involved in clearance of axonal debris, which removes molecules that inhibit nerve regeneration. In the cell body region, macrophage trigger the conditioning lesion response, a process in which neurons increase their regeneration after a prior lesion. In mice in which the genes for CCL2 or CCR2 are deleted, neither macrophage infiltration nor the conditioning lesion response occurs in dorsal root ganglia (DRG). Macrophages exist in different phenotypes depending on their environment. These phenotypes have different effects on axonal clearance and neurite outgrowth. The mechanism by which macrophages affect neuronal cell bodies is still under study. Overexpression of CCL2 in DRG in uninjured animals leads to macrophage accumulation in the ganglia and to an increase in the growth potential of DRG neurons. This increased growth requires activation of neuronal STAT3. In contrast, in acute demyelinating neuropathies, macrophages are involved in stripping myelin from peripheral axons. The molecular mechanisms that trigger macrophage action after trauma and in autoimmune disease are receiving increased attention and should lead to avenues to promote regeneration and protect axonal integrity.

Galanin Administration Partially Restores Erectile Function After Cavernous Nerve Injury and Mediates Endogenous Nitrergic Nerve Outgrowth In Vitro

BACKGROUND

Previously, we found that the neuropeptide galanin was strongly upregulated soon after bilateral cavernous nerve injury (BCNI) and that galanin and its receptors were expressed in nitrergic erectile innervation. Galanin has been observed to exert neuroregenerative effects in dorsal root ganglion neurons, but evidence for these effects in the major pelvic ganglion (MPG) after BCNI is lacking.

AIM

To evaluate the neurotropic effects of galanin receptor agonists and antagonists in vitro in nitrergic neurons and MPG and in vivo in rats after BCNI.

METHODS

Male Sprague-Dawley rats underwent BCNI and sham surgery. Organ culture and single-cell neuron culture of the MPG were performed. Osmotic pump treatment with the galanin agonist in vivo and measurement of erectile response to electrostimulation after BCNI, immunohistochemical localization of galanin and receptors in the human neurovascular bundle, and myographic analysis of rat corpus cavernosum smooth muscle relaxation to galanin receptor agonists were investigated.

OUTCOMES

Neurite outgrowth in vitro and erectile response to electrostimulation after BCNI in vivo, immunohistochemical localization of galanin and receptors, and penile muscle relaxation in vitro.

RESULTS

Galanin showed neurotrophic action in vitro and inhibition of endogenous galanin significantly impaired neurite outgrowth in nitrergic but not in sympathetic MPG neurons. In vivo administration of a selective galanin receptor-2 agonist, M1145, resulted in partial recovery of erectile function (EF) after BCNI. Galanin did not act as a direct vasodilator on corpus cavernosum muscle strips.

CLINICAL TRANSLATION

Endogenous neurotrophins such as galanin could be used as a strategy to improve EF for patients after BCNI from radical prostatectomy.

STRENGTHS AND LIMITATIONS

We evaluated the effect of galanin on nerve regeneration and EF recovery in vivo and in vitro. Limitations include the lack of washout period for the in vivo experiment and absence of differences in the expression of neuronal markers between treatment groups.

CONCLUSIONS

We identified galanin as a potential endogenous mechanism for nerve regeneration after BCNI, which could play a physiologic role in EF recovery after radical prostatectomy. In vivo treatment with exogenous galanin was beneficial in enhancing EF recovery after BCNI, but further research is necessary to understand the underlying mechanisms. Weyne E, Hannan JL, Gevaert T, et al. Galanin Administration Partially Restores Erectile Function After Cavernous Nerve Injury and Mediates Endogenous Nitrergic Nerve Outgrowth In Vitro. J Sex Med 2018;15:480-491.

Atf3 mutant mice show reduced axon regeneration and impaired regeneration-associated gene induction after peripheral nerve injury

Axon injury in the peripheral nervous system (PNS) induces a regeneration-associated gene (RAG) response. Atf3 (activating transcription factor 3) is such a RAG and ATF3's transcriptional activity might induce ‘effector’ RAGs (e.g. small proline rich protein 1a (Sprr1a), Galanin (Gal), growth-associated protein 43 (Gap43)) facilitating peripheral axon regeneration. We provide a first analysis of Atf3 mouse mutants in peripheral nerve regeneration. In Atf3 mutant mice, facial nerve regeneration and neurite outgrowth of adult ATF3-deficient primary dorsal root ganglia neurons was decreased. Using genome-wide transcriptomics, we identified a neuropeptide-encoding RAG cluster (vasoactive intestinal peptide (Vip), Ngf, Grp, Gal, Pacap) regulated by ATF3. Exogenous administration of neuropeptides enhanced neurite growth of Atf3 mutant mice suggesting that these molecules might be effector RAGs of ATF3's pro-regenerative function. In addition to the induction of growth-promoting molecules, we present data that ATF3 suppresses growth-inhibiting molecules such as chemokine (C-C motif) ligand 2. In summary, we show a pro-regenerative ATF3 function during PNS nerve regeneration involving transcriptional activation of a neuropeptide-encoding RAG cluster. ATF3 is a general injury-inducible factor, therefore ATF3-mediated mechanisms identified herein might apply to other cell and injury types.

Injury-induced gp130 cytokine signaling in peripheral ganglia is reduced in diabetes mellitus

Neuropathy is a major diabetic complication. While the mechanism of this neuropathy is not well understood, it is believed to result in part from deficient nerve regeneration. Work from our laboratory established that gp130 family of cytokines are induced in animals after axonal injury and are involved in the induction of regeneration-associated genes (RAGs) and in the conditioning lesion response. Here, we examine whether a reduction of cytokine signaling occurs in diabetes. Streptozotocin (STZ) was used to destroy pancreatic β cells, leading to chronic hyperglycemia. Mice were injected with either low doses of STZ (5×60mg/kg) or a single high dose (1×200mg/kg) and examined after three or one month, respectively. Both low and high dose STZ treatment resulted in sustained hyperglycemia and functional deficits associated with the presence of both sensory and autonomic neuropathy. Diabetic mice displayed significantly reduced intraepidermal nerve fiber density and sudomotor function. Furthermore, low and high dose diabetic mice showed significantly reduced tactile touch sensation measured with Von Frey monofilaments. To look at the regenerative and injury-induced responses in diabetic mice, neurons in both superior cervical ganglia (SCG) and the 4th and 5th lumbar dorsal root ganglia (DRG) were unilaterally axotomized. Both high and low dose diabetic mice displayed significantly less axonal regeneration in the sciatic nerve, when measured in vivo, 48h after crush injury. Significantly reduced induction of two gp130 cytokines, leukemia inhibitory factor and interleukin-6, occurred in diabetic animals in SCG 6h after injury compared to controls. Injury-induced expression of interleukin-6 was also found to be significantly reduced in the DRG at 6h after injury in low and high dose diabetic mice. These effects were accompanied by reduced phosphorylation of signal transducer and activator of transcription 3 (STAT3), a downstream effector of the gp130 signaling pathway. We also found decreased induction of several gp130-dependent RAGs, including galanin and vasoactive intestinal peptide. Together, these data suggest a novel mechanism for the decreased response of diabetic sympathetic and sensory neurons to injury.

Targeted disruption of the orphan receptor Gpr151 does not alter pain-related behaviour despite a strong induction in dorsal root ganglion expression in a model of neuropathic pain

BACKGROUND

Gpr151 is an orphan GPCR whose function is unknown. The restricted pattern of neuronal expression in the habenula, dorsal horn of the spinal cord and dorsal root ganglion plus homology with the galanin family of receptors imply a role in nociception.

RESULTS

Real-time quantitative RT-PCR demonstrated a 49.9±2.9 fold highly significant (P<0.001) increase in Gpr151 mRNA expression in the dorsal root ganglion 7days after the spared nerve injury model of neuropathic pain. Measures of acute, inflammatory and neuropathic pain behaviours were not significantly different using separate groups of Gpr151 loss-of-function mutant mice and wild-type controls. Galanin at concentrations between 100nM and 10μM did not induce calcium signalling responses in ND7/23 cells transfected with Gpr151.

CONCLUSIONS

Our results indicate that despite the very large upregulation in the DRG after a nerve injury model of neuropathic pain, the Gpr151 orphan receptor does not appear to be involved in the modulation of pain-related behaviours. Further, galanin is unlikely to be an endogenous ligand for Gpr151.

Methodological Approaches to In Vitro Evaluation of Transcription Activity of Nuclear Factor Kappa B (NF-κB) in Sensory Neurons

Transcription activity of NF-κB in sensory neurons was analyzed in vitro using classical immunocytochemical methods and transgenic technologies. Activation of NF-κB in NIH3T3 cells and in murine sensory neurons after in vitro stimulation with TNF-α was demonstrated by the immunocytochemical method; however, the expression of the reporter NF-κB/LacZ transgene was detected only after addition of histon deacetylase inhibitor. Hence, formally contradictory conclusions from the results of immunocytochemical analysis and reporter transgene expression were in line with the hypothesis on epigenetic repression of NF-κB activity in sensory neurons mediated by histon deacetylases.

Molecular and Cellular Mechanisms of Axonal Regeneration After Spinal Cord Injury

Following axotomy, a complex temporal and spatial coordination of molecular events enables regeneration of the peripheral nerve. In contrast, multiple intrinsic and extrinsic factors contribute to the general failure of axonal regeneration in the central nervous system. In this review, we examine the current understanding of differences in protein expression and post-translational modifications, activation of signaling networks, and environmental cues that may underlie the divergent regenerative capacity of central and peripheral axons. We also highlight key experimental strategies to enhance axonal regeneration via modulation of intraneuronal signaling networks and the extracellular milieu. Finally, we explore potential applications of proteomics to fill gaps in the current understanding of molecular mechanisms underlying regeneration, and to provide insight into the development of more effective approaches to promote axonal regeneration following injury to the nervous system.

Regulation of neurological and neuropsychiatric phenotypes by locus coeruleus-derived galanin

Decades of research confirm that noradrenergic locus coeruleus (LC) neurons are essential for arousal, attention, motivation, and stress responses. While most studies on LC transmission focused unsurprisingly on norepinephrine (NE), adrenergic signaling cannot account for all the consequences of LC activation. Galanin coexists with NE in the vast majority of LC neurons, yet the precise function of this neuropeptide has proved to be surprisingly elusive given our solid understanding of the LC system. To elucidate the contribution of galanin to LC physiology, here we briefly summarize the nature of stimuli that drive LC activity from a neuroanatomical perspective. We go on to describe the LC pathways in which galanin most likely exerts its effects on behavior, with a focus on addiction, depression, epilepsy, stress, and Alzheimer׳s disease. We propose a model in which LC-derived galanin has two distinct functions: as a neuromodulator, primarily acting via the galanin 1 receptor (GAL1), and as a trophic factor, primarily acting via galanin receptor 2 (GAL2). Finally, we discuss how the recent advances in neuropeptide detection, optogenetics and chemical genetics, and galanin receptor pharmacology can be harnessed to identify the roles of LC-derived galanin definitively. This article is part of a Special Issue entitled SI: Noradrenergic System.

Crosstalk between exercise and galanin system alleviates insulin resistance

Studies have demonstrated that aerobic exercise can enhance insulin sensitivity, however, the precise mechanism for this outcome is not entirely identified. Emerging evidences point out that exercise can upregulate galanin protein and mRNA expression, resulting in improvement of insulin sensitivity via an increase in translocation of glucose transporter 4 and subsequent glucose uptake in myocytes and adipocytes of healthy and type 2 diabetic rats, which may be blocked by galanin antagonist. In return, galanin can exert the exercise-protective roles to prevent excessive movement of skeletal muscle and to accelerate exercise trauma repair in exercise-relative tissues. Studies also implicated that combination of aerobic exercise and activation of galanin system may make more significant improvement in insulin sensitivity than that of either one did. These suggest that galanin system is essential for physical activity to alleviate insulin resistance, namely, the beneficial effect of physical activity on glucose uptake is at least partly mediated by galanin system. Besides, co-treatment with galanin and exercise is an effective therapeutic strategy for reducing insulin resistance.

What makes a RAG regeneration associated?

Regenerative failure remains a significant barrier for functional recovery after central nervous system (CNS) injury. As such, understanding the physiological processes that regulate axon regeneration is a central focus of regenerative medicine. Studying the gene transcription responses to axon injury of regeneration competent neurons, such as those of the peripheral nervous system (PNS), has provided insight into the genes associated with regeneration. Though several individual “regeneration-associated genes” (RAGs) have been identified from these studies, the response to injury likely regulates the expression of functionally coordinated and complementary gene groups. For instance, successful regeneration would require the induction of genes that drive the intrinsic growth capacity of neurons, while simultaneously downregulating the genes that convey environmental inhibitory cues. Thus, this view emphasizes the transcriptional regulation of gene “programs” that contribute to the overall goal of axonal regeneration. Here, we review the known RAGs, focusing on how their transcriptional regulation can reveal the underlying gene programs that drive a regenerative phenotype. Finally, we will discuss paradigms under which we can determine whether these genes are injury-associated, or indeed necessary for regeneration.

Physiology, signaling, and pharmacology of galanin peptides and receptors: three decades of emerging diversity

Galanin was first identified 30 years ago as a "classic neuropeptide," with actions primarily as a modulator of neurotransmission in the brain and peripheral nervous system. Other structurally-related peptides-galanin-like peptide and alarin-with diverse biologic actions in brain and other tissues have since been identified, although, unlike galanin, their cognate receptors are currently unknown. Over the last two decades, in addition to many neuronal actions, a number of nonneuronal actions of galanin and other galanin family peptides have been described. These include actions associated with neural stem cells, nonneuronal cells in the brain such as glia, endocrine functions, effects on metabolism, energy homeostasis, and paracrine effects in bone. Substantial new data also indicate an emerging role for galanin in innate immunity, inflammation, and cancer. Galanin has been shown to regulate its numerous physiologic and pathophysiological processes through interactions with three G protein-coupled receptors, GAL1, GAL2, and GAL3, and signaling via multiple transduction pathways, including inhibition of cAMP/PKA (GAL1, GAL3) and stimulation of phospholipase C (GAL2). In this review, we emphasize the importance of novel galanin receptor-specific agonists and antagonists. Also, other approaches, including new transgenic mouse lines (such as a recently characterized GAL3 knockout mouse) represent, in combination with viral-based techniques, critical tools required to better evaluate galanin system physiology. These in turn will help identify potential targets of the galanin/galanin-receptor systems in a diverse range of human diseases, including pain, mood disorders, epilepsy, neurodegenerative conditions, diabetes, and cancer.

Galanin modulates the neural niche to favour perineural invasion in head and neck cancer

Perineural invasion (PNI) is an indicator of poor survival in multiple cancers. Unfortunately, there is no targeted treatment for PNI since the molecular mechanisms are largely unknown. PNI is an active process, suggesting that cancer cells communicate with nerves. However, nerve-tumour crosstalk is understudied due to the lack of in vivo models to investigate the mechanisms. Here, we developed an in vivo model of PNI to characterise this interaction. We show that the neuropeptide galanin (GAL) initiates nerve-tumour crosstalk via activation of its G-protein-coupled receptor, GALR2. Our data reveal a novel mechanism by which GAL from nerves stimulates GALR2 on cancer cells to induce NFATC2-mediated transcription of cyclooxygenase-2 and GAL. Prostaglandin E2 promotes cancer invasion, and in a feedback mechanism, GAL released by cancer induces neuritogenesis, facilitating PNI. This study describes a novel in vivo model for PNI and reveals the dynamic interaction between nerve and cancer.

Selective Expression of Galanin in Neuronal-Like Cells of the Human Carotid Body

The carotid body is a neural-crest-derived organ devoted to respiratory homeostasis through sensing changes in blood oxygen levels. The sensory units are the glomeruli composed of clusters of neuronal-like (type I) cells surrounded by glial-like (type II) cells. During chronic hypoxia, the carotid body shows growth, with increasing neuronal-like cell numbers. We are interested in the signals involved in the mechanisms that underlie such response, because they are not well understood and described. Considering that, in literature, galanin is involved in neurotrophic or neuroprotective role in cell proliferation and is expressed in animal carotid body, we investigated its expression in human. Here, we have shown the expression and localisation of galanin in the human carotid body.

Galanin plays an important role in cancer invasiveness and is associated with poor prognosis in stage II colorectal cancer

Reliable predictors of tumor recurrence for patients with stage II colorectal cancer (CRC) are needed to select patients who should receive adjuvant chemotherapy. Although galanin (GAL) is expressed in several malignant tumors and is associated with cell proliferation and tumor growth, the prognostic value of GAL expression in CRC is poorly understood. We compared GAL expression between 56 patients with stage II and III CRC who developed tumor recurrences and 56 patients who did not. The clinical and prognostic significance of GAL expression was examined using our data and independent public datasets. We also analyzed the influence of GAL expression on the proliferation and invasive activity of CRC cells. Higher expression of GAL was associated with tumor recurrence among the CRC patients (P<0.001). Stage II CRC patients who presented with high expression levels of GAL had significantly poorer prognosis than those with low expression levels of GAL [5-year overall survival: hazard ratio (HR), 7.31; 95% confidence interval (CI), 2.38–24.04; P<0.001; 5-year recurrence-free survival: HR, 3.99; 95% CI, 1.61–9.44; P=0.004], but there was no association between GAL expression and survival in stage III CRC patients. These findings were supported by analysis of two public datasets. Functionally, siRNA-mediated silencing of GAL resulted in a significant decrease in the proliferative and invasive activities of CRC cells. In conclusion, high expression of GAL is associated with poor prognosis of stage II CRC patients and GAL expression may be related to the aggressive behavior of CRC.

Spinal cord injury and the neuron-intrinsic regeneration-associated gene program

Spinal cord injury (SCI) affects millions of people worldwide and causes a significant physical, emotional, social and economic burden. The main clinical hallmark of SCI is the permanent loss of motor, sensory and autonomic function below the level of injury. In general, neurons of the central nervous system (CNS) are incapable of regeneration, whereas injury to the peripheral nervous system is followed by axonal regeneration and usually results in some degree of functional recovery. The weak neuron-intrinsic regeneration-associated gene (RAG) response upon injury is an important reason for the failure of neurons in the CNS to regenerate an axon. This response consists of the expression of many RAGs, including regeneration-associated transcription factors (TFs). Regeneration-associated TFs are potential key regulators of the RAG program. The function of some regeneration-associated TFs has been studied in transgenic and knock-out mice and by adeno-associated viral vector-mediated overexpression in injured neurons. Here, we review these studies and propose that AAV-mediated gene delivery of combinations of regeneration-associated TFs is a potential strategy to activate the RAG program in injured CNS neurons and achieve long-distance axon regeneration.

Effect of antipsychotic drugs on gene expression in the prefrontal cortex and nucleus accumbens in the spontaneously hypertensive rat (SHR)

Antipsychotic drugs (APDs) are the standard treatment for schizophrenia. The therapeutic effect of these drugs is dependent upon the dopaminergic D2 blockade, but they also modulate other neurotransmitter pathways. The exact mechanisms underlying the clinical response to APDs are not fully understood. In this study, we compared three groups of animals for the expression of 84 neurotransmitter genes in the prefrontal cortex (PFC) and nucleus accumbens (NAcc). Each group was treated with a different APD (risperidone, clozapine or haloperidol), and with a non-treated group of spontaneously hypertensive rats (SHRs), which is an animal model for schizophrenia. This study also explored whether or not differential expression was regulated by DNA methylation in the promoter region (PR). In the clozapine group, we found that Chrng was downregulated in the NAcc and six genes were downregulated in the PFC. In the haloperidol group, Brs3 and Glra1 were downregulated, as was Drd2 in the clozapine group and Drd3, Galr3 and Gabrr1 in the clozapine and haloperidol groups. We also encountered four hypermethylated CG sites in the Glra1 PR, as well as three in the risperidone group and another in the haloperidol group, when compared to non-treated rats. Following the APD treatment, the gene expression results revealed the involvement of genes that had not previously been described, in addition to the activity of established genes. The investigation of the involvement of these novel genes can lead to better understanding about the specific mechanisms of action of the individual APDs studied.

Heterogeneous responses of dorsal root ganglion neurons in neuropathies induced by peripheral nerve trauma and the antiretroviral drug stavudine

Background

Heterogeneity is increasingly recognized in clinical presentation of neuropathic pain (NP), but less often recognized in animal models. Neurochemical dysregulation in rodent dorsal root ganglia (DRG) is associated with peripheral nerve trauma, but poorly studied in non-traumatic NP conditions.

Methods

This study aimed to investigate the temporal expressions of activating transcription factor-3 (ATF-3), growth-associated protein-43 (GAP-43), neuropeptide Y (NPY) and galanin in traumatic and non-traumatic rat models of neuropathies associated with NP. Expressions of these markers were examined in the DRG at different time points following tibial nerve transection (TNT) injury and antiretroviral drug stavudine (d4T) administration using immunohistochemistry. The development of sensory gain following these insults was assessed by measuring limb withdrawal to a punctate mechanical stimulus.

Results

Both TNT-injured and d4T-treated rats developed hindpaw mechanical hypersensitivity. Robust expressions of ATF-3, GAP-43, NPY and galanin in both small- and large-sized L5 DRG neurons were observed in the DRG from TNT-injured rats. In contrast, d4T-treated rats did not exhibit any significant neurochemical changes in the DRG.

Conclusions

Taken together, the results suggest that ATF-3, GAP-43, NPY and galanin are likely indicators of nerve trauma-associated processes and not generic markers for NP. These experiments also demonstrate distinct expression patterns of neurochemical markers in the DRG and emphasize the mechanistic difference between nerve trauma and antiretroviral drug-associated NP.

Galanin promotes neuronal differentiation from neural progenitor cells in vitro and contributes to the generation of new olfactory neurons in the adult mouse brain

Galanin is a pleiotropic neuropeptide widely expressed in the nervous system. It plays a role in many diverse physiological functions - including nociception, cognition and metabolism regulation - and acts as neurotrophic/neuroprotective factor for several neuronal populations. In this article we sought to determine the role of galanin on neural stem cell function and its contribution to the plasticity of the nervous system. Here we show that galanin and its receptors are expressed in neural progenitor cells (NPCs) isolated from the developing striatum. Stimulation with galanin results in upregulation of Bcl-Xl, Bcl-2, Mash-1 and Olig-2 that are part of well known pro-survival/pro-neuronal signalling pathways. Accordingly, treatment with galanin increases the number of neurons upon differentiation from these progenitors. We then show that these effects are recapitulated in NPCs isolated from the adult subventricular zone (SVZ), where galanin increases the total number of neurons and the number of newly-generated neurons upon differentiation in vitro. The significance of these findings is highlighted in the adult brain where loss of galanin leads to a marked decrease in the rate of adult SVZ neurogenesis and a reduction in the number of newly generated cells in the olfactory bulb. Interestingly, Gal-KO mice display normal performances in simple tasks of olfactory detection and discrimination, which points to the existence of a certain degree of redundancy in SVZ neurogenesis. Our findings establish the role of galanin as a modulator of neural stem cell function and support the importance of galanin for brain plasticity and repair.

Galanin stimulates neurite outgrowth from sensory neurons by inhibition of Cdc42 and Rho GTPases and activation of cofilin

We and others have previously shown that the neuropeptide galanin modulates neurite outgrowth from adult sensory neurons via activation of the second galanin receptor; however, the intracellular signalling pathways that mediate this neuritogenic effect have yet to be elucidated. Here, we demonstrate that galanin decreases the activation state in adult sensory neurons and PC12 cells of Rho and Cdc42 GTPases, both known regulators of filopodial and growth cone motility. Consistent with this, activated levels of Rho and Cdc42 levels are increased in the dorsal root ganglion of adult galanin knockout animals compared with wildtype controls. Furthermore, galanin markedly increases the activation state of cofilin, a downstream effector of many of the small GTPases, in the cell bodies and growth cones of sensory neurons and in PC12 cells. We also demonstrate a reduction in the activation of cofilin, and alteration in growth cone motility, in cultured galanin knockout neurons compared with wildtype controls. These data provide the first evidence that galanin regulates the Rho family of GTPases and cofilin to stimulate growth cone dynamics and neurite outgrowth in sensory neurons. These findings have important therapeutic implications for the treatment of peripheral sensory neuropathies.

VEGF-A165b is an endogenous neuroprotective splice isoform of vascular endothelial growth factor A in vivo and in vitro

Vascular endothelial growth factor (VEGF) A is generated as two isoform families by alternative RNA splicing, represented by VEGF-A165a and VEGF-A165b. These isoforms have opposing actions on vascular permeability, angiogenesis, and vasodilatation. The proangiogenic VEGF-A165a isoform is neuroprotective in hippocampal, dorsal root ganglia, and retinal neurons, but its propermeability, vasodilatatory, and angiogenic properties limit its therapeutic usefulness. In contrast, a neuroprotective effect of endogenous VEGF-A165b on neurons would be advantageous for neurodegenerative pathologies. Endogenous expression of human and rat VEGF-A165b was detected in hippocampal and cortical neurons. VEGF-A165b formed a significant proportion of total VEGF-A in rat brain. Recombinant human VEGF-A165b exerted neuroprotective effects in response to multiple insults, including glutamatergic excitotoxicity in hippocampal neurons, chemotherapy-induced cytotoxicity of dorsal root ganglion neurons, and retinal ganglion cells (RGCs) in rat retinal ischemia-reperfusion injury in vivo. Neuroprotection was dependent on VEGFR2 and MEK1/2 activation but not on p38 or phosphatidylinositol 3-kinase activation. Recombinant human VEGF-A165b is a neuroprotective agent that effectively protects both peripheral and central neurons in vivo and in vitro through VEGFR2, MEK1/2, and inhibition of caspase-3 induction. VEGF-A165b may be therapeutically useful for pathologies that involve neuronal damage, including hippocampal neurodegeneration, glaucoma diabetic retinopathy, and peripheral neuropathy. The endogenous nature of VEGF-A165b expression suggests that non-isoform-specific inhibition of VEGF-A (for antiangiogenic reasons) may be damaging to retinal and sensory neurons.

JUN regulates early transcriptional responses to axonal injury in retinal ganglion cells

The AP1 family transcription factor JUN is an important molecule in the neuronal response to injury. In retinal ganglion cells (RGCs), JUN is upregulated soon after axonal injury and disrupting JUN activity delays RGC death. JUN is known to participate in the control of many different injury response pathways in neurons, including pathways controlling cell death and axonal regeneration. The role of JUN in regulating genes involved in cell death, ER stress, and regeneration was tested to determine the overall importance of JUN in regulating RGC response to axonal injury. Genes from each of these pathways were transcriptionally controlled following axonal injury and Jun deficiency altered the expression of many of these genes. The differentially expressed genes included, Atf3, Ddit3, Ecel1, Gadd45α, Gal, Hrk, Pten, Socs3, and Sprr1a. Two of these genes, Hrk and Atf3, were tested for importance in RGC death using null alleles of each gene. Disruption of the prodeath Bcl2 family member Hrk did not affect the rate or amount of RGC death after axonal trauma. Deficiency in the ATF/CREB family transcription factor Atf3 did lessen the amount of RGC death after injury, though it did not provide long term protection to RGCs. Since JUN's dimerization partner determines its transcriptional targets, the expression of several candidate AP1 family members were examined. Multiple AP1 family members were induced by axonal injury and had a different expression profile in Jun deficient retinas compared to wildtype retinas (Fosl1, Fosl2 and Jund). Overall, JUN appears to play a multifaceted role in regulating RGC response to axonal injury.

Galanin gene expression and effects of its knock-down on the development of the nervous system in larval zebrafish

Despite the known importance of galanin in the nervous system of vertebrates, the galanin gene structure and expression and the consequences of galanin deficiency in developing zebrafish are unknown. We cloned the galanin gene and analyzed its expression by using in situ hybridization, PCR, and immunocytochemistry throughout the early development of zebrafish until the end of the first week of life. The single zebrafish galanin gene encoded for a single amidated galanin peptide and a galanin message-associated peptide. Two forms resulting from alternative processing were identified. Galanin mRNA was maternally expressed and found in developing fish throughout early development. In situ hybridization showed the first positive neurons in three groups in the brain at 28 hours postfertilization. At 2 days postfertilization, three prosencephalic neuron groups were seen in the preoptic area and in rostral and caudal periventricular hypothalamus. In addition, two other groups of weakly stained neurons were visible, one in the midbrain and another in the hindbrain. Translation inhibition of galanin mRNA with morpholino oligonucleotides caused complete disappearance of galanin immunoreactivity in the brain until 7 dpf and did not induce known cascades of nonspecific pathways or morphological abnormalities. A minor disturbance of sensory ganglia was found. Galanin knockdown did not alter the expression of tyrosine hydroxylases 1 and 2, choline acetyltransferase, histidine decarboxylase, or orexin mRNA. The results suggest that galanin does not regulate the development of these key markers of specific neurons, although galanin-expressing fibers were in a close spatial proximity to several neurons of these neuronal populations.

The pros and cons of growth factors and cytokines in peripheral axon regeneration

Injury to a peripheral nerve induces a complex cellular and molecular response required for successful axon regeneration. Proliferating Schwann cells organize into chains of cells bridging the lesion site, which is invaded by macrophages. Approximately half of the injured neuron population sends out axons that enter the glial guidance channels in response to secreted neurotrophic factors and neuropoietic cytokines. These lesion-associated polypeptides create an environment that is highly supportive for axon regrowth, particularly after acute injury, and ensure that the vast majority of regenerating axons are directed toward the distal nerve stump. Unfortunately, most neurotrophic factors and neuropoietic cytokines are also strong stimulators of axonal sprouting. Although some of the axonal branches will withdraw at later stages, the sprouting effect contributes to the misdirection of reinnervation that results in the lack of functional recovery observed in many patients with peripheral nerve injuries. Here, we critically review the role of neuronal growth factors and cytokines during axon regeneration in the peripheral nervous system. Their differential effects on axon elongation and sprouting were elucidated in various studies on intraneuronal signaling mechanisms following nerve lesion. The present data define a goal for future therapeutic strategies, namely, to selectively stimulate a Ras/Raf/ERK-mediated axon elongation program over an intrinsic PI3K-dependent axonal sprouting program in lesioned motor and sensory neurons. Instead of modulating growth factor or cytokine levels at the lesion site, targeting specific intraneuronal molecules, such as the negative feedback inhibitors of ERK signaling, has been shown to promote long-distance regeneration while avoiding sprouting of regenerating axons until they have reached their target areas.

Galanin-expression and galanin-dependent sensory neurons are not required for itch

Background

Galanin is a key modulator of nociception, and it is also required for the developmental survival of a subset of C-fibre sensory neurons which are critical to the mediation of neuropathic and inflammatory pain. However, the potential modulatory roles played by galanin, or the galanin-dependent neurons, in pruritoceptive mechanisms underlying the sensation of itch have not been investigated.

Findings

Here we report that mice carrying a loss-of-function mutation in the galanin gene (Gal-KO) show no differences in spontaneous behavioural itch responses compared to wild-type (WT) controls. Similarly, the responses to a range of pruritogens are not significantly different between the two genotypes.

Conclusions

These results suggest that neither galanin expression, nor the galanin-dependent subpopulation of sensory neurons is required for itch-related behaviours.

PPARγ agonists promote oligodendrocyte differentiation of neural stem cells by modulating stemness and differentiation genes

Neural stem cells (NSCs) are a small population of resident cells that can grow, migrate and differentiate into neuro-glial cells in the central nervous system (CNS). Peroxisome proliferator-activated receptor gamma (PPARγ) is a nuclear receptor transcription factor that regulates cell growth and differentiation. In this study we analyzed the influence of PPARγ agonists on neural stem cell growth and differentiation in culture. We found that in vitro culture of mouse NSCs in neurobasal medium with B27 in the presence of epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) induced their growth and expansion as neurospheres. Addition of all-trans retinoic acid (ATRA) and PPARγ agonist ciglitazone or 15-Deoxy-Δ^12,14-Prostaglandin J₂ (15d-PGJ2) resulted in a dose-dependent inhibition of cell viability and proliferation of NSCs in culture. Interestingly, NSCs cultured with PPARγ agonists, but not ATRA, showed significant increase in oligodendrocyte precursor-specific O4 and NG2 reactivity with a reduction in NSC marker nestin, in 3–7 days. In vitro treatment with PPARγ agonists and ATRA also induced modest increase in the expression of neuronal β-III tubulin and astrocyte-specific GFAP in NSCs in 3–7 days. Further analyses showed that PPARγ agonists and ATRA induced significant alterations in the expression of many stemness and differentiation genes associated with neuro-glial differentiation in NSCs. These findings highlight the influence of PPARγ agonists in promoting neuro-glial differentiation of NSCs and its significance in the treatment of neurodegenerative diseases.

Identification and characterization of the chicken galanin receptor GalR2 and a novel GalR2-like receptor (GalR2-L)

In mammals, the neuropeptide galanin exerts a variety of physiological roles in the neuroendocrine system through its interactions with three galanin receptor subtypes (GalR1, GalR2 and GalR3). However, little is known about the characteristics of galanin receptors in birds, and it is only recently that avian GalR1 and a novel GalR1-like receptor were first identified in chickens. In this study, we report the cDNA cloning and characterization of the other two chicken galanin receptors, the galanin type II receptor (cGalR2) and a novel GalR2-like receptor (GalR2-L), which share high degrees of similarity in sequence identity, gene structure and signaling properties. cGalR2 and cGalR2-L cDNAs encode two putative receptors of 371 and 370 amino acids, in which they show considerable amino acid sequence identities (65-67%, and 53-55%, respectively) with the mammalian GalR2. RT-PCR assays revealed that cGalR2 and cGalR2-L mRNA were widely expressed in the adult chicken tissues including the whole brain, intestine, lung, ovary, pituitary and different regions of the oviduct. As assayed with different luciferase reporter systems, chicken galanin (cGal 1-29) and human galanin-like peptide (hGALP 1-60) were demonstrated to stimulate the luciferase activities in Chinese hamster ovary cells expressing cGalR2 and cGalR2-L through the activations of cAMP/PKA, Ca(2+)/calcineurin and MAPK/ERK signaling pathways, hence suggesting that both receptors are functionally coupled to the G(s) and G(q) proteins. Furthermore, the previously identified cGalR1 and cGalR1-L were found to be solely coupled to the G(i/o) proteins, and the hGALP (1-60) exhibited only a low potency to cGalR1, cGalR1-L, cGalR2 and cGalR2-L activations.

Lack of the transcription factor C/EBPδ impairs the intrinsic capacity of peripheral neurons for regeneration

Adult neurons of the peripheral nervous system (PNS), in contrast to those of the central nervous system, have a remarkable capacity to repair themselves after injury, yet the mechanisms underlying this regenerative propensity of peripheral neurons are far from completely understood. Here we show that the transcription factor CCAAT enhancer binding protein delta (C/EBPδ) is necessary for the efficient axonal regeneration of dorsal root ganglia (DRG) neurons after sciatic nerve crush injury. Loss of C/EBPδ substantially impairs axonal growth in dissociated cultured DRG neurons. In addition, lack of C/EPBδ causes a major reduction in the regenerative response of DRG neurons to a conditioning lesion, which is a well known paradigm of injury that enhances axonal growth due to a transcription-dependent cell body response. C/EBPδ is required for the induction of selected regeneration-associated genes. For example, the expression of SPRR1A (small proline-rich repeat protein 1A) is greatly reduced in DRG neurons of C/EBPδ knockout mice during axonal regeneration compared to those in wild-type mice, while the expression of GAP-43 (growth associated protein-43) and galanin is not affected. Nevertheless, the expected prompt recovery of sciatic nerve function after injury is severely impaired in C/EBPδ knockout mice, having a delay time of approximately 1 month for reaching the full function of recovering wild-type mice, suggesting that a transcription mechanism mediated by C/EBPδ is required for efficient axonal regeneration. Taken together, our results identify C/EBPδ as a crucial component of the transcriptional regulatory machinery which underlies the intrinsic capacity of peripheral neurons for axonal regeneration.

Downstream effector molecules in successful peripheral nerve regeneration

The robust axon regeneration that occurs following peripheral nerve injury is driven by transcriptional activation of the regeneration program and by the expression of a wide range of downstream effector molecules from neuropeptides and neurotrophic factors to adhesion molecules and cytoskeletal adaptor proteins. These regeneration-associated effector molecules regulate the actin-tubulin machinery of growth-cones, integrate intracellular signalling and stimulatory and inhibitory signals from the local environment and translate them into axon elongation. In addition to the neuronally derived molecules, an important transcriptional component is found in locally activated Schwann cells and macrophages, which release a number of cytokines, growth factors and neurotrophins that support neuronal survival and axonal regeneration and that might provide directional guidance cues towards appropriate peripheral targets. This review aims to provide a comprehensive up-to-date account of the transcriptional regulation and functional role of these effector molecules and of the information that they can give us with regard to the organisation of the regeneration program.

Role of transcription factors in peripheral nerve regeneration

Following axotomy, the activation of multiple intracellular signaling cascades causes the expression of a cocktail of regeneration-associated transcription factors which interact with each other to determine the fate of the injured neurons. The nerve injury response is channeled through manifold and parallel pathways, integrating diverse inputs, and controlling a complex transcriptional output. Transcription factors form a vital link in the chain of regeneration, converting injury-induced stress signals into downstream protein expression via gene regulation. They can regulate the intrinsic ability of axons to grow, by controlling expression of whole cassettes of gene targets. In this review, we have investigated the functional roles of a number of different transcription factors - c-Jun, activating transcription factor 3, cAMP response element binding protein, signal transducer, and activator of transcription-3, CCAAT/enhancer binding proteins β and δ, Oct-6, Sox11, p53, nuclear factor kappa-light-chain-enhancer of activated B cell, and ELK3 - in peripheral nerve regeneration. Studies involving use of conditional mutants, microarrays, promoter region mapping, and different injury paradigms, have enabled us to understand their distinct as well as overlapping roles in achieving anatomical and functional regeneration after peripheral nerve injury.

Changes in galanin and GalR1 gene expression in discrete brain regions after transient occlusion of the middle cerebral artery in female rats

Injury to neurons results in up-regulation of galanin in some central and peripheral systems, and it has been suggested that this neuropeptide may play a protective and trophic role, primarily mediated by galanin receptor 2 (GalR2). The objective of the present study was to investigate galanin, GalR1, GalR2 and GalR3 gene expression in the female rat brain 7 days after a 60-min unilateral occlusion of the middle cerebral artery followed by reperfusion. Quantitative real-time PCR was employed in punch-biopsies from the locus coeruleus, somatosensory cortex and dorsal hippocampal formation, including sham-operated rats as controls. Galanin gene expression showed a ∼2.5-fold increase and GalR1 a ∼1.5-fold increase in the locus coeruleus of the ischemic hemisphere compared to the control side. Furthermore, the GalR1 mRNA levels decreased by 35% in somatosensory cortex of the ischemic hemisphere. Immunohistochemical analysis indicated a depletion of galanin from cell bodies and dendrites in the locus coeruleus after middle cerebral artery occlusion. The present results suggest that a stroke-induced forebrain lesion up-regulates synthesis of galanin and GalR1 in the locus coeruleus, a noradrenergic cell group projecting to many forebrain areas, including cortex and the hippocampal formation. These results support the notion that galanin may play a role in the response of the central nervous system to injury.

gp130 cytokines are positive signals triggering changes in gene expression and axon outgrowth in peripheral neurons following injury

Adult peripheral neurons, in contrast to adult central neurons, are capable of regeneration after axonal damage. Much attention has focused on the changes that accompany this regeneration in two places, the distal nerve segment (where phagocytosis of axonal debris, changes in the surface properties of Schwann cells, and induction of growth factors and cytokines occur) and the neuronal cell body (where dramatic changes in cell morphology and gene expression occur). The changes in the axotomized cell body are often referred to as the "cell body response." The focus of the current review is a family of cytokines, the glycoprotein 130 (gp130) cytokines, which produce their actions through a common gp130 signaling receptor and which function as injury signals for axotomized peripheral neurons, triggering changes in gene expression and in neurite outgrowth. These cytokines play important roles in the responses of sympathetic, sensory, and motor neurons to injury. The best studied of these cytokines in this context are leukemia inhibitory factor (LIF) and interleukin (IL)-6, but experiments with conditional gp130 knockout animals suggest that other members of this family, not yet determined, are also involved. The primary gp130 signaling pathway shown to be involved is the activation of Janus kinase (JAK) and the transcription factors Signal Transducers and Activators of Transcription (STAT), though other downstream pathways such as mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) may also play a role. gp130 signaling may involve paracrine, retrograde, and autocrine actions of these cytokines. Recent studies suggest that manipulation of this cytokine system can also stimulate regeneration by injured central neurons.