Leukemia inhibitory factor mediates an injury response but not a target-directed developmental transmitter switch in sympathetic neurons

A comparative view of human and mouse telencephalon inhibitory neuron development

Human GABAergic inhibitory neurons (INs) in the telencephalon play crucial roles in modulating neural circuits, generating cortical oscillations, and maintaining the balance between excitation and inhibition. The major IN subtypes are based on their gene expression profiles, morphological diversity and circuit-specific functions. Although previous foundational work has established that INs originate in the ganglionic eminence regions in mice, recent studies have questioned origins in humans and non-human primates. We review the origins of INs in mice and compare with recent findings from primary human prenatal brain tissue culture experiments and lineage analysis from somatic variants in neurotypical human cadavers and human brain organoids. Together, these studies suggest potential primate- or human-specific processes that may have been overlooked in mouse models and could have implications for brain disorders.

Untangling Peripheral Sympathetic Neurocircuits

The sympathetic nervous system plays a critical role in regulating many autonomic functions, including cardiac rhythm. The postganglionic neurons in the sympathetic chain ganglia are essential components that relay sympathetic signals to target tissues and disruption of their activity leads to poor health outcomes. Despite this importance, the neurocircuitry within sympathetic ganglia is poorly understood. Canonically, postganglionic sympathetic neurons are thought to simply be activated by monosynaptic inputs from preganglionic cholinergic neurons of the intermediolateral cell columns of the spinal cord. Early electrophysiological studies of sympathetic ganglia where the peripheral nerve trunks were electrically stimulated identified excitatory cholinergic synaptic events in addition to retrograde action potentials, leading some to speculate that excitatory collateral projections are present. However, this seemed unlikely since sympathetic postganglionic neurons were known to synthesize and release norepinephrine and expression of dual neurochemical phenotypes had not been well recognized. In vitro studies clearly established the capacity of cultured sympathetic neurons to express and release acetylcholine and norepinephrine throughout development and even in pathophysiological conditions. Given this insight, we believe that the canonical view of ganglionic transmission needs to be reevaluated and may provide a mechanistic understanding of autonomic imbalance in disease. Further studies likely will require genetic models manipulating neurochemical phenotypes within sympathetic ganglia to resolve the function of cholinergic collateral projections between postganglionic neurons. In this perspective article, we will discuss the evidence for collateral projections in sympathetic ganglia, determine if current laboratory techniques could address these questions, and discuss potential obstacles and caveats.

The sympathetic nervous system in development and disease

The sympathetic nervous system prepares the body for 'fight or flight' responses and maintains homeostasis during daily activities such as exercise, eating a meal or regulation of body temperature. Sympathetic regulation of bodily functions requires the establishment and refinement of anatomically and functionally precise connections between postganglionic sympathetic neurons and peripheral organs distributed widely throughout the body. Mechanistic studies of key events in the formation of postganglionic sympathetic neurons during embryonic and early postnatal life, including axon growth, target innervation, neuron survival, and dendrite growth and synapse formation, have advanced the understanding of how neuronal development is shaped by interactions with peripheral tissues and organs. Recent progress has also been made in identifying how the cellular and molecular diversity of sympathetic neurons is established to meet the functional demands of peripheral organs. In this Review, we summarize current knowledge of signalling pathways underlying the development of the sympathetic nervous system. These findings have implications for unravelling the contribution of sympathetic dysfunction stemming, in part, from developmental perturbations to the pathophysiology of peripheral neuropathies and cardiovascular and metabolic disorders.

Effects of trigeminal nerve injury on the expression of galanin and its receptors in the rat trigeminal ganglion

In the spinal nervous system, the expression of galanin (GAL) and galanin receptors (GALRs) that play important roles in the transmission and modulation of nociceptive information can be affected by nerve injury. However, in the trigeminal nervous system, the effects of trigeminal nerve injury on the expression of GAL are controversy in the previous studies. Besides, little is known about the effects of trigeminal nerve injury on the expression of GALRs. In the present study, the effects of trigeminal nerve injury on the expression of GAL and GALRs in the rat trigeminal ganglion (TG) were investigated by using quantitative real-time reverse transcription-polymerase chain reaction and immunohistochemistry. To identify the nerve-injured and nerve-uninjured TG neurons, activating transcription factor 3 (ATF3, the nerve-injured neuron marker) was stained by immunofluorescence. The levels of GAL mRNA in the rostral half and caudal half of the TG dramatically increased after transection of infraorbital nerve (ION) and inferior alveolar nerve (IAN), respectively. Immunohistochemical labeling of GAL and ATF3 revealed that GAL level was elevated in both injured and adjacent uninjured small and medium-sized TG neurons after ION/IAN transection. In addition, the levels of GAL₂R-like immunoreactivity were reduced in both injured and adjacent uninjured TG neurons after ION/IAN transection, while levels of GAL₁R and GAL₃R-like immunoreactivity remained unchanged. Furthermore, the number of small to medium-sized TG neurons co-expressing GAL- and GAL₁R/GAL₂R/GAL₃R-like immunoreactivity was significantly increased after ION/IAN transection. In line with previous studies in other spinal neuron systems, these results suggest that GAL and GALRs play functional roles in orofacial neuropathic pain and trigeminal nerve regeneration after trigeminal nerve injury.

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.

Leukemia inhibitory factor promotes extracellular matrix synthesis in degenerative nucleus pulposus cells via MAPK-ERK1/2 signaling pathway

Extracellular matrix (ECM) anabolism and catabolism imbalance is key feature of chondrocyte and intervertebral disc nucleus pulposus (NP) cell degeneration. The role of LIF as a multifunctional cytokine in the ECM metabolism of chondrocytes is controversial, but no relevant research in the ECM metabolism of NP cells. This study aimed to explore the biofunction and related mechanisms of LIF in the degenerative NP cells. We obtained an increase in the expression of LIF in the human degenerated NP specimens. The addition of recombinant human leukemia inhibitory factor (rhLIF) to the degenerated NP cells cultured in vitro was found to stimulate the synthesis of ECM, and rhLIF could activate the ERK1/2 signaling pathway. However, coculture with PD98059, a signal inhibitor of ERK1/2, blocked the effect of rhLIF on the synthesis of ECM. To furtherly clarify the role of LIF, we carried out animal experiments and found that rhLIF treatment could successfully delay the degree of degeneration of the intervertebral disc in a rabbit model; but with the addition of PD98059, the function of rhLIF for degeneration protection disappeared. In summary, this study demonstrates that LIF plays a role in promoting ECM synthesis in the degenerated NP cells as a protective role in intervertebral disc degeneration (IDD), which is related to the activation of ERK1/2 signaling pathway.

Fibrin/Schwann Cell Matrix in Poly-Epsilon-Caprolactone Conduits Enhances Guided Nerve Regeneration

The goal of this study was to investigate if a three dimensional matrix, loaded homogeneously with Schwann cells and the neurotrophic factor LIF (leukemia inhibitory factor), enhances regeneration in a biodegradable nerve guidance channel as compared to non-structured cell suspensions. Therefore a 10 mm nerve gap in the buccal branch of the rat's facial nerve was bridged with tubular PCL (poly-epsilon-caprolactone) conduits filled with no matrix, Schwann cells, the three dimensional fibrin/Schwann cell matrix or the fibrin/Schwann cell matrix added with LIF. Four weeks after the nerve defects were bridged histological and morphometric analyses of the implants were performed. In conclusion, the three dimensional fibrin/Schwann cells matrix enhanced the quantity and the quality of peripheral nerve regeneration through PCL conduits. The application of LIF prevented hyperneurotization. Therefore, tissue engineered fibrin/Schwann cells matrices are new invented biocompatible and biodegradable devices for enhancing peripheral nerve regeneration as compared to non-structured cell suspensions without neurotrophic factors.

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.

Neurotransmitter Switching in the Developing and Adult Brain

Neurotransmitter switching is the gain of one neurotransmitter and the loss of another in the same neuron in response to chronic stimulation. Neurotransmitter receptors on postsynaptic cells change to match the identity of the newly expressed neurotransmitter. Neurotransmitter switching often appears to change the sign of the synapse from excitatory to inhibitory or from inhibitory to excitatory. In these cases, neurotransmitter switching and receptor matching thus change the polarity of the circuit in which they take place. Neurotransmitter switching produces up or down reversals of behavior. It is also observed in response to disease. These findings raise the possibility that neurotransmitter switching contributes to depression, schizophrenia, and other illnesses. Many early discoveries of the single gain or loss of a neurotransmitter may have been harbingers of neurotransmitter switching.

■ REVIEW : LIF, NGF, and the Cell Body Response to Axotomy

Adult peripheral neurons can regenerate after axonal damage. Large changes in gene expression occur in the cell bodies of these axotomized neurons, including decreases in expression of a number of proteins used for synaptic transmission and increases in expression of a number of proteins involved in regeneration. The signals that trigger these changes are just beginning to be elucidated. One characteristic of axotomized sympathetic, sensory, and motor neurons is that they increase expression of two neuropeptides, vasoactive intestinal peptide and galanin. These peptides may play important roles in the survival and regeneration of axotomized neurons deprived of their target-derived trophic factors. Recent studies have demonstrated two important signals in the induction of these peptides in sympathetic neurons: one is the release of leukemia inhibitory factor (LIF) by non-neuronal cells in the vicinity of the injured neurons and the other, the removal of target-derived nerve growth factor (NGF). Furthermore, there is a synergistic interaction between these two events whereby the removal of NGF alters the responsiveness of neurons to LIF. Future efforts will hopefully determine the extent to which LIF and NGF signal other aspects of the cell body response and the mechanisms that underlie these actions. NEUROSCIENTIST 3:176-185, 1997

■ REVIEW : Gene Expression in Nerve Regeneration

Injury of peripheral nerve in mammals leads to a complex but stereotypical pattern of histological events that comprise a highly reproducible sequence of degenerative reactions (Wallerian degeneration) succeeded by regenerative responses. These reactions are based on a corresponding sequence of cellular and mo lecular interactions that, in turn, reflect the differential expression of specific genes with functions in nerve degeneration and repair. We report on more than 60 genes and their products that show a specific pattern of regulation following peripheral nerve lesion. The group of regulated genes encoding, e.g., transcription factors, growth factors and their receptors, cytokines, neuropeptides, myelin proteins and lipid carriers, and cytoskeletal proteins as well as extracellular matrix and cell adhesion molecules. We describe and compare the distinct time-courses and cellular origin of expression and further discuss established or putative mo lecular interrelationships and functions with respect to the contribution of these genes/gene products to the molecular regeneration program of the PNS. NEUROSCIENTIST 3:112-122, 1997

Myocardial Infarction Causes Transient Cholinergic Transdifferentiation of Cardiac Sympathetic Nerves via gp130

Sympathetic and parasympathetic control of the heart is a classic example of norepinephrine (NE) and acetylcholine (ACh) triggering opposing actions. Sympathetic NE increases heart rate and contractility through activation of β receptors, whereas parasympathetic ACh slows the heart through muscarinic receptors. Sympathetic neurons can undergo a developmental transition from production of NE to ACh and we provide evidence that mouse cardiac sympathetic nerves transiently produce ACh after myocardial infarction (MI). ACh levels increased in viable heart tissue 10-14 d after MI, returning to control levels at 21 d, whereas NE levels were stable. At the same time, the genes required for ACh synthesis increased in stellate ganglia, which contain most of the sympathetic neurons projecting to the heart. Immunohistochemistry 14 d after MI revealed choline acetyltransferase (ChAT) in stellate sympathetic neurons and vesicular ACh transporter immunoreactivity in tyrosine hydroxylase-positive cardiac sympathetic fibers. Finally, selective deletion of the ChAT gene from adult sympathetic neurons prevented the infarction-induced increase in cardiac ACh. Deletion of the gp130 cytokine receptor from sympathetic neurons prevented the induction of cholinergic genes after MI, suggesting that inflammatory cytokines induce the transient acquisition of a cholinergic phenotype in cardiac sympathetic neurons. Ex vivo experiments examining the effect of NE and ACh on rabbit cardiac action potential duration revealed that ACh blunted both the NE-stimulated decrease in cardiac action potential duration and increase in myocyte calcium transients. This raises the possibility that sympathetic co-release of ACh and NE may impair adaptation to high heart rates and increase arrhythmia susceptibility.

SIGNIFICANCE STATEMENT

Sympathetic neurons normally make norepinephrine (NE), which increases heart rate and the contractility of cardiac myocytes. We found that, after myocardial infarction, the sympathetic neurons innervating the heart begin to make acetylcholine (ACh), which slows heart rate and decreases contractility. Several lines of evidence confirmed that the source of ACh was sympathetic nerves rather than parasympathetic nerves that are the normal source of ACh in the heart. Global application of NE with or without ACh to ex vivo hearts showed that ACh partially reversed the NE-stimulated decrease in cardiac action potential duration and increase in myocyte calcium transients. That suggests that sympathetic co-release of ACh and NE may impair adaptation to high heart rates and increase arrhythmia susceptibility.

Overexpression of the monocyte chemokine CCL2 in dorsal root ganglion neurons causes a conditioning-like increase in neurite outgrowth and does so via a STAT3 dependent mechanism

Neuroinflammation plays a critical role in the regeneration of peripheral nerves following axotomy. An injury to the sciatic nerve leads to significant macrophage accumulation in the L5 DRG, an effect not seen when the dorsal root is injured. We recently demonstrated that this accumulation around axotomized cell bodies is necessary for a peripheral conditioning lesion response to occur. Here we asked whether overexpression of the monocyte chemokine CCL2 specifically in DRG neurons of uninjured mice is sufficient to cause macrophage accumulation and to enhance regeneration or whether other injury-derived signals are required. AAV5-EF1α-CCL2 was injected intrathecally, and this injection led to a time-dependent increase in CCL2 mRNA expression and macrophage accumulation in L5 DRG, with a maximal response at 3 weeks post-injection. These changes led to a conditioning-like increase in neurite outgrowth in DRG explant and dissociated cell cultures. This increase in regeneration was dependent upon CCL2 acting through its primary receptor CCR2. When CCL2 was overexpressed in CCR2-/- mice, macrophage accumulation and enhanced regeneration were not observed. To address the mechanism by which CCL2 overexpression enhances regeneration, we tested for elevated expression of regeneration-associated genes in these animals. Surprisingly, we found that CCL2 overexpression led to a selective increase in LIF mRNA and neuronal phosphorylated STAT3 (pSTAT3) in L5 DRGs, with no change in expression seen in other RAGs such as GAP-43. Blockade of STAT3 phosphorylation by each of two different inhibitors prevented the increase in neurite outgrowth. Thus, CCL2 overexpression is sufficient to induce macrophage accumulation in uninjured L5 DRGs and increase the regenerative capacity of DRG neurons via a STAT3-dependent mechanism.

Induction of dissociated cytokine profiles by ISRAA with selective critical involvement of ERK1/2 in its signaling functions

The immune system-released activating agent (ISRAA) is an immune mediator activated as a result of a nerve stimulus initiated by immune challenge. We have previously demonstrated that ISRAA and tumor necrosis factor (TNF) receptor 1 (TNFR1) share an interspecies-conserved motif (72% homology) that induces the apoptosis and proliferation of human peripheral blood mononuclear cells (hPBMCs) in a dose-dependent manner. In the present study, cytokine profiles were examined in response to the stimulation of hPBMCs with ISRAA. Furthermore, the signaling pathways induced by ISRAA were mapped. The results revealed high measurable levels of TNF-α, interleukin (IL)-6, IL-8, IL-10 and interferon (IFN)-γ, but not IL-4, IL-17 (IL-17A) or transforming growth factor (TGF)-β. The analysis of signaling pathways revealed the activation of extracellular-regulated protein kinase (ERK)1/2 as a downstream signal in the mitogen‑activated protein kinase (MAPK) pathway during TNF‑α and IL-6 production and apoptosis, but not during proliferation following stimulation with ISRAA by triggering the Fas-associated protein with death domain (FADD). STAT3 was found to be unphosphorylated in the ISRAA‑stimulated hPBMCs, and STAT3 was ubiquitously expressed in unstimulated cells, suggesting that ISRAA has a protein inhibitor of activated STAT (PIAS)-like activity, by functioning as a negative regulator of the effects of STAT3 on the Janus kinase (JAK)/STAT pathway. The determination of the nature of cytokine responses together with the signaling pathways of cellular activity induced by ISRAA paves the way for the investigation of a potential target of ISRAA and for the development of novel therapeutic approaches for the treatment of immune-regulated disorders.

Leukemia inhibitory factor (LIF)

Leukemia inhibitory factor (LIF) is the most pleiotropic member of the interleukin-6 family of cytokines. It utilises a receptor that consists of the LIF receptor β and gp130 and this receptor complex is also used by ciliary neurotrophic growth factor (CNTF), oncostatin M, cardiotrophin1 (CT1) and cardiotrophin-like cytokine (CLC). Despite common signal transduction mechanisms (JAK/STAT, MAPK and PI3K) LIF can have paradoxically opposite effects in different cell types including stimulating or inhibiting each of cell proliferation, differentiation and survival. While LIF can act on a wide range of cell types, LIF knockout mice have revealed that many of these actions are not apparent during ordinary development and that they may be the result of induced LIF expression during tissue damage or injury. Nevertheless LIF does appear to have non-redundant actions in maternal receptivity to blastocyst implantation, placental formation and in the development of the nervous system. LIF has also found practical use in the maintenance of self-renewal and totipotency of embryonic stem cells and induced pluripotent stem cells.

The neuroimmunology of degeneration and regeneration in the peripheral nervous system

Peripheral nerves regenerate following injury due to the effective activation of the intrinsic growth capacity of the neurons and the formation of a permissive pathway for outgrowth due to Wallerian degeneration (WD). WD and subsequent regeneration are significantly influenced by various immune cells and the cytokines they secrete. Although macrophages have long been known to play a vital role in the degenerative process, recent work has pointed to their importance in influencing the regenerative capacity of peripheral neurons. In this review, we focus on the various immune cells, cytokines, and chemokines that make regeneration possible in the peripheral nervous system, with specific attention placed on the role macrophages play in this process.

Neurotransmitter Switching? No Surprise

Among the many forms of brain plasticity, changes in synaptic strength and changes in synapse number are particularly prominent. However, evidence for neurotransmitter respecification or switching has been accumulating steadily, both in the developing nervous system and in the adult brain, with observations of transmitter addition, loss, or replacement of one transmitter with another. Natural stimuli can drive these changes in transmitter identity, with matching changes in postsynaptic transmitter receptors. Strikingly, they often convert the synapse from excitatory to inhibitory or vice versa, providing a basis for changes in behavior in those cases in which it has been examined. Progress has been made in identifying the factors that induce transmitter switching and in understanding the molecular mechanisms by which it is achieved. There are many intriguing questions to be addressed.

STAT3 integrates cytokine and neurotrophin signals to promote sympathetic axon regeneration

The transcription factor STAT3 has been implicated in axon regeneration. Here we investigate a role for STAT3 in sympathetic nerve sprouting after myocardial infarction (MI) - a common injury in humans. We show that NGF stimulates serine phosphorylation (S727) of STAT3 in sympathetic neurons via ERK1/2, in contrast to cytokine phosphorylation of Y705. Maximal sympathetic axon regeneration in vitro requires phosphorylation of both S727 and Y705. Furthermore, cytokine signaling is necessary for NGF-induced sympathetic nerve sprouting in the heart after MI. Transfection studies in neurons lacking STAT3 suggest two independent pools of STAT3, phosphorylated on either S727 or Y705, that regulate sympathetic regeneration via both transcriptional and non-transcriptional means. Additional data identify STAT3-microtubule interactions that may complement the well-characterized role of STAT3 stimulating regeneration associated genes. These data show that STAT3 is critical for sympathetic axon regeneration in vitro and in vivo, and identify a novel non-transcriptional mode of action.

Neuroprotective effect of leukemia inhibitory factor on antimycin A-induced oxidative injury in differentiated PC12 cells

As a neurotrophic cytokine, leukemia inhibitory factor (LIF) has neuroendocrine effects and exerts neuroprotective effects on various neuron injuries both in vitro and in vivo. The aim of the present study was to investigate whether LIF can protect PC12 cells from antimycin A (AMA)-induced oxidative stress. LIF (0.5 and 1 ng/ml) increased PC12 cell viability and significantly attenuated AMA-induced cell death as demonstrated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Results from Hoechst 33342 staining and flow cytometry assay showed that AMA induced apoptosis significantly in PC12 cells, while pretreatment with LIF (0.5 and 1 ng/ml) can attenuate this injury. The presence of LIF partly prevented AMA-induced elevated reactive oxygen species level and decreased superoxide dismutase level, which indicated the antioxidative effects of LIF on the neuron oxidative injury. In conclusion, LIF might protect PC12 cells from the injury induced by AMA through the downregulation of oxidative stress, which may provide basic information of using LIF as a potential targeted therapy for oxidative injury in neurons.

Low-level vagus nerve stimulation upregulates small conductance calcium-activated potassium channels in the stellate ganglion

BACKGROUND

Small conductance calcium-activated potassium (SK) channels are responsible for afterhyperpolarization that suppresses nerve discharges.

OBJECTIVES

To test the hypothesis that low-level vagus nerve stimulation (LL-VNS) leads to the upregulation of SK2 proteins in the left stellate ganglion.

METHODS

Six dogs (group 1) underwent 1-week LL-VNS of the left cervical vagus nerve. Five normal dogs (group 2) were used as controls. SK2 protein levels were examined by using Western blotting. The ratio between SK2 and glyceraldehydes-3-phosphate-dehydrogenase levels was used as an arbitrary unit (AU).

RESULTS

We found higher SK2 expression in group 1 (0.124 ± 0.049 AU) than in group 2 (0.085 ± 0.031 AU; P<.05). Immunostaining showed that the density of nerve structures stained with SK2 antibody was also higher in group 1 (11,546 ± 7,271 μm(2)/mm(2)) than in group 2 (5321 ± 3164 μm(2)/mm(2); P<.05). There were significantly more ganglion cells without immunoreactivity to tyrosine hydroxylase (TH) in group 1 (11.4%±2.3%) than in group 2 (4.9% ± 0.7%; P<.05). The TH-negative ganglion cells mostly stained positive for choline acetyltransferase (95.9% ± 2.8% in group 1 and 86.1% ± 4.4% in group 2; P = .10). Immunofluorescence confocal microscopy revealed a significant decrease in the SK2 staining in the cytosol but an increase in the SK2 staining on the membrane of the ganglion cells in group 1 compared to group 2.

CONCLUSIONS

Left LL-VNS results in the upregulation of SK2 proteins, increased SK2 protein expression in the cell membrane, and increased TH-negative (mostly choline acetyltransferase-positive) ganglion cells in the left stellate ganglion. These changes may underlie the antiarrhythmic efficacy of LL-VNS in ambulatory dogs.

Specificity of peripheral nerve regeneration: interactions at the axon level

Peripheral nerves injuries result in paralysis, anesthesia and lack of autonomic control of the affected body areas. After injury, axons distal to the lesion are disconnected from the neuronal body and degenerate, leading to denervation of the peripheral organs. Wallerian degeneration creates a microenvironment distal to the injury site that supports axonal regrowth, while the neuron body changes in phenotype to promote axonal regeneration. The significance of axonal regeneration is to replace the degenerated distal nerve segment, and achieve reinnervation of target organs and restitution of their functions. However, axonal regeneration does not always allows for adequate functional recovery, so that after a peripheral nerve injury, patients do not recover normal motor control and fine sensibility. The lack of specificity of nerve regeneration, in terms of motor and sensory axons regrowth, pathfinding and target reinnervation, is one the main shortcomings for recovery. Key factors for successful axonal regeneration include the intrinsic changes that neurons suffer to switch their transmitter state to a pro-regenerative state and the environment that the axons find distal to the lesion site. The molecular mechanisms implicated in axonal regeneration and pathfinding after injury are complex, and take into account the cross-talk between axons and glial cells, neurotrophic factors, extracellular matrix molecules and their receptors. The aim of this review is to look at those interactions, trying to understand if some of these molecular factors are specific for motor and sensory neuron growth, and provide the basic knowledge for potential strategies to enhance and guide axonal regeneration and reinnervation of adequate target organs.

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.

gp130 cytokines stimulate proteasomal degradation of tyrosine hydroxylase via extracellular signal regulated kinases 1 and 2

Injury-induced cytokines act through gp130 in sympathetic neurons to suppress expression of tyrosine hydroxylase (TH) and other genes associated with noradrenergic transmission. These cytokines also trigger the local loss of TH in peri-infarct sympathetic axons after myocardial infarction, but altered gene expression cannot explain the selective loss of TH enzyme in one region of the heart. We hypothesized that inflammatory cytokines, which are highest near the infarct, stimulated local degradation of TH protein. We used cultured sympathetic neurons and neuroblastoma cells to test this hypothesis. The cytokines ciliary neurotrophic factor (CNTF) and leukemia inhibitory factor (LIF) suppressed TH content in both neurons and neuroblastoma cells. CNTF suppressed TH in a gp130-dependent manner, and decreased the half-life of TH protein by approximately 50%. CNTF stimulated the ubiquitination of TH in both neurons and neuroblastoma cells, and the proteasome inhibitors MG-132 and lactacystin prevented the CNTF-induced loss of TH protein. Inhibiting activation of extracellular signal regulated kinases 1&2 (ERK1/2) with U0126 prevented the CNTF-induced ubiquitination of TH and the associated decrease in protein half-life. Likewise, inhibiting ERK1/2 activation blunted the cytokine-stimulated loss of TH protein in sympathetic neurons, despite enhancing the loss of TH mRNA. These data suggest that gp130 cytokines stimulate proteasomal degradation of TH through an ERK1/2 dependent pathway, and may have important implications for local regulation of neurotransmission at sites of inflammation.

STATus and Context within the Mammalian Nervous System

Effective manipulation of human disease processes may be achieved by understanding transcriptional, posttranscriptional and epigenetic events that orchestrate cellular events. The levels of activation of specific molecules, spatial distribution and concentrations of relevant networks of signaling molecules along with the receptiveness of the chromatin to these signals are some of the parameters which dictate context. Effects elicited by the transcription factor signal transducers and activator of transcription 3 (Stat3) are discussed with respect to the context within which Stat3-mediated effects are elicited within the developing and adult mammalian nervous system. Stat3 signals are pivotal to the proliferation and differentiation of neural stem cells. They also participate in neuronal regeneration and cancers of the nervous system. An analysis of the context in which Stat3 activation occurs in these processes provides a potential predictive paradigm with which novel methods for intervention may be designed.

Transcribing the path to neurological recovery-From early signals through transcription factors to downstream effectors of successful regeneration

The peripheral nervous system is known to regenerate comparatively well and this ability is mirrored in the de novo expression or upregulation of a wide variety of molecules involved in axonal outgrowth starting with transcription factors, but also including growth-stimulating substances, guidance and cell adhesion molecules, intracellular signaling enzymes and proteins involved in regulating cell-surface cytoskeletal interactions. Recent studies using pharmacological agents, and global as well as neuron-selective gene inactivation techniques have shed light on those endogenous molecules that play a non-redundant role in mediating regenerative axonal outgrowth in vivo. The aim of the current review is to sketch the sequence of molecular events from early sensors of injury to transcription factors to downstream effectors that cooperate in successful regeneration and functional recovery.

Cytokines inhibit norepinephrine transporter expression by decreasing Hand2

Functional noradrenergic transmission requires the coordinate expression of enzymes involved in norepinephrine (NE) synthesis, as well as the norepinephrine transporter (NET) which removes NE from the synapse. Inflammatory cytokines acting through gp130 can suppress the noradrenergic phenotype in sympathetic neurons. This occurs in a subset of sympathetic neurons during development and also occurs in adult neurons after injury. For example, cytokines suppress noradrenergic function in sympathetic neurons after axotomy and during heart failure. The molecular basis for suppression of noradrenergic genes is not well understood, but previous studies implicated a reduction of Phox2a in cytokine suppression of dopamine beta hydroxylase. We used sympathetic neurons and neuroblastoma cells to investigate the role of Phox2a in cytokine suppression of NET transcription. Chromatin immunoprecipitation experiments revealed that Phox2a did not bind the NET promoter, and overexpression of Phox2a did not prevent cytokine suppression of NET transcription. Hand2 and Gata3 are transcription factors that induce noradrenergic genes during development and are present in mature sympathetic neurons. Both Hand2 and Gata3 were decreased by cytokines in sympathetic neurons and neuroblastoma cells. Overexpression of either Hand2 or Gata3 was sufficient to rescue NET transcription following suppression by cytokines. We examined expression of these genes following axotomy to determine if their expression was altered following nerve injury. NET and Hand2 mRNAs decreased significantly in sympathetic neurons 48 h after axotomy, but Gata3 mRNA was unchanged. These data suggest that cytokines can inhibit NET expression through downregulation of Hand2 or Gata3 in cultured sympathetic neurons, but axotomy in adult animals selectively suppresses Hand2 expression.

Galanin and spinal pain mechanisms: past, present, and future

Since the discovery of galanin in 1983, one of the most frequently suggested physiological function for this peptide is pain modulation at the level of the spinal cord. This notion, initially based on the preferential distribution of galanin in dorsal spinal cord, has been supported by results from a large number of morphological, molecular, and functional studies. It is generally agreed that spinally applied galanin produces a biphasic, dose-dependent effect on spinal nociception through activation of GalR1 (inhibitory) or GalR2 (excitatory) receptors. Galanin also appears to have an endogenous inhibitory role, particularly after peripheral nerve injury when the synthesis of galanin is increased in sensory neurons. In recent years, small molecule ligands of galanin receptors have been developed, which may lead to the development of analgesic drugs, which affects the galanin system at the spinal cord level.

Increased expression of interleukin-6 family members and receptors in urinary bladder with cyclophosphamide-induced bladder inflammation in female rats

Recent studies suggest that janus-activated kinases-signal transducer and activator of transcription signaling pathways contribute to increased voiding frequency and referred pain of cyclophosphamide (CYP)-induced cystitis in rats. Potential upstream chemical mediator(s) that may be activated by CYP-induced cystitis to stimulate JAK/STAT signaling are not known in detail. In these studies, members of the interleukin (IL)-6 family of cytokines including, leukemia inhibitory factor (LIF), IL-6, and ciliary neurotrophic factor (CNTF) and associated receptors, IL-6 receptor (R) α, LIFR, and gp130 were examined in the urinary bladder in control and CYP-treated rats. Cytokine and receptor transcript and protein expression and distribution were determined in urinary bladder after CYP-induced cystitis using quantitative, real-time polymerase chain reaction (Q-PCR), western blotting, and immunohistochemistry. Acute (4 h; 150 mg/kg; i.p.), intermediate (48 h; 150 mg/kg; i.p.), or chronic (75 mg/kg; i.p., once every 3 days for 10 days) cystitis was induced in adult, female Wistar rats with CYP treatment. Q-PCR analyses revealed significant (p ≤ 0.01) CYP duration- and tissue- (e.g., urothelium, detrusor) dependent increases in LIF, IL-6, IL-6Rα, LIFR, and gp130 mRNA expression. Western blotting demonstrated significant (p ≤ 0.01) increases in IL-6, LIF, and gp130 protein expression in whole urinary bladder with CYP treatment. CYP-induced cystitis significantly (p ≤ 0.01) increased LIF-immunoreactivity (IR) in urothelium, detrusor, and suburothelial plexus whereas increased gp130-IR was only observed in urothelium and detrusor. These studies suggest that IL-6 and LIF may be potential upstream chemical mediators that activate JAK/STAT signaling in urinary bladder pathways.

Effects of intracerebroventricular galanin or a galanin receptor 2/3 agonist on the lesion induced by transient occlusion of the middle cerebral artery in female rats

Several studies have shown that injury to the central and peripheral nervous system can increase expression of galanin, a 29 amino acid neuropeptide. Moreover, there is evidence that galanin, especially through its galanin receptor 2 (GalR2) receptor, plays a neuroprotective role in different injury models. However, direct studies of a possible neuroprotective effect of galanin in experimental stroke models are lacking. Galanin, a GalR2/3 agonist or artificial CSF was continuously infused intracerebroventricularly (i.c.v.) in naïve female rats after a 60min transient and focal occlusion of the middle cerebral artery. The animals were sacrificed, and the ischemic lesion was visualized using 2,3,5-triphenyltetrazolium hydrochloride (TTC) staining. The lesion was 98% larger after i.c.v. administration of the GalR2/3 agonist (2.4nmol/day) seven days after occlusion compared to artificial CSF (p=0.023). No statistically significant differences were found after seven days in the groups treated with galanin in three different concentrations (0.24, 2.4 and 24nmol/day; p=0.939, 0.715 and 0.977, respectively). There was no difference in the size of the ischemic lesions measured after three days in the galanin-treated group (2.4nmol/d) compared to artificial CSF (p=0.925). The present results show, surprisingly, that a GalR2/3 agonist doubled the size of the ischemic lesion. Whether this effect primarily reflects the properties of the current model, species, gender and/or the mode of galanin administration, e.g. causing desensitization, or whether galanin indeed lacks neuroprotective effect of its own, remains to be corroborated.

Cardiac ischemia-reperfusion regulates sympathetic neuropeptide expression through gp130-dependent and independent mechanisms

Cardiac function is regulated by a balance of sympathetic and parasympathetic transmission. Neuropeptide Y (NPY) and galanin (GAL) released from cardiac sympathetic neurons inhibits parasympathetic transmission in the heart. Sympathetic peptides may contribute to autonomic imbalance, which is characterized by increased sympathetic and decreased parasympathetic transmission and contributes to life threatening cardiovascular pathologies. Several gp130 cytokines are increased in the heart after myocardial infarction (MI), and these cytokines stimulate neuropeptide expression in sympathetic neurons. We used mice whose sympathetic neurons lack the gp130 receptor (gp130(DBH-Cre/lox) mice) to ask if cytokine activation of gp130 regulated neuropeptide expression in cardiac sympathetic nerves after MI. Myocardial infarction decreased NPY mRNA through a gp130 independent mechanism and increased VIP and PACAP mRNA via gp130, while GAL mRNA was unchanged. Immunohistochemistry revealed a gp130-dependent increase in PACAP38 in cells of the stellate ganglion after MI, and PACAP was detected in pre-ganglionic fibers of all genotypes and surgical groups. VIP was identified in a few sympathetic nerve fibers in all genotypes and surgical groups. GAL and PACAP38 were not detected in sham hearts, but peptide immunoreactivity was high in the infarct three days after MI. Surprisingly, peptides were abundant in cells that co-labeled with macrophage markers F4/80 and MAC2, but were not detected in sympathetic axons. PACAP protects cardiac myocytes from apoptosis, and GAL stimulates axon regeneration in addition to inhibiting parasympathetic transmission. Thus, these peptides may play an important role in cardiac and neuronal remodeling after ischemia-reperfusion.

The conditioning lesion effect on sympathetic neurite outgrowth is dependent on gp130 cytokines

Sympathetic neurons, like sensory neurons, increase neurite outgrowth after a conditioning lesion. Studies in leukemia inhibitory factor (LIF) knockout animals showed that the conditioning lesion effect in sensory neurons is dependent in part on this cytokine; however, similar studies on sympathetic neurons revealed no such effect. Comparable studies with sensory neurons taken from mice lacking the related cytokine interleukin-6 (IL-6) have yielded conflicting results. LIF and IL-6 belong to a family of cytokines known as the gp130 family because they act on receptors containing the subunit gp130. In sympathetic ganglia, axotomy leads to increases in mRNA for four of these cytokines (LIF, IL-6, IL-11, and oncostatin M). To test the role of this family of cytokines as a whole in the conditioning lesion response in sympathetic neurons, mice in which gp130 was selectively eliminated in noradrenergic neurons were studied. The postganglionic axons of the SCG were transected, and 7days later the ganglia were removed and neurite outgrowth was measured in explant and dissociated cell cultures. In both systems, neurons from wild type animals showed enhanced growth after a conditioning lesion. In contrast, no enhancement occurred in neurons from mutant animals. This lack of stimulation of outgrowth occurred despite an increase in expression of activating transcription factor 3 (ATF3) in the mutant mice. These studies demonstrate that stimulation of enhanced growth of sympathetic neurons after a conditioning lesion is dependent on gp130 cytokine signaling and is blocked in the absence of signaling by these cytokines in spite of an increase in ATF3.

Infarction-induced cytokines cause local depletion of tyrosine hydroxylase in cardiac sympathetic nerves

Myocardial infarction causes a heterogeneity of noradrenergic transmission that contributes to the development of ventricular arrhythmias and sudden cardiac death. Ischaemia-induced alterations in sympathetic transmission include regional variations in cardiac noradrenaline (NA) and in tyrosine hydroxylase, the rate-limiting enzyme in NA synthesis. Inflammatory cytokines that act through gp130 are elevated in the heart after myocardial infarction. These cytokines decrease expression of tyrosine hydroxylase in sympathetic neurons, and indirect evidence suggests that they contribute to the local depletion of tyrosine hydroxylase in the damaged left ventricle. However, gp130 cytokines are also important for the survival of cardiac myocytes following damage to the heart. To examine the effect of cytokines on tyrosine hydroxylase and NA content in cardiac nerves we used gp130(DBH-Cre/lox) mice, which have a deletion of the gp130 receptor in neurons expressing dopamine beta-hydroxylase. The absence of neuronal gp130 prevented the loss of tyrosine hydroxylase in cardiac sympathetic nerves innervating the left ventricle 1 week after ischaemia-reperfusion compared with wild-type C57BL/6J mice. Surprisingly, restoration of tyrosine hydroxylase in the damaged ventricle did not return neuronal NA content to normal levels. Noradrenaline uptake into cardiac nerves was significantly lower in gp130 knockout mice, contributing to the lack of neuronal NA stores. There were no significant differences in left ventricular peak systolic pressure, dP/dt(max) or dP/dt(min) between the two genotypes after myocardial infarction, but ganglionic blockade revealed differences in autonomic tone between the genotypes. Stimulation of the heart with dobutamine or release of endogenous NA with tyramine generated similar responses in both genotypes. Thus, the removal of gp130 from sympathetic neurons prevents the post-infarct depletion of tyrosine hydroxylase in the left ventricle, but does not alter NA content or cardiac function.

Heart failure causes cholinergic transdifferentiation of cardiac sympathetic nerves via gp130-signaling cytokines in rodents

Although several cytokines and neurotrophic factors induce sympathetic neurons to transdifferentiate into cholinergic neurons in vitro, the physiological and pathophysiological roles of this remain unknown. During congestive heart failure (CHF), sympathetic neural tone is upregulated, but there is a paradoxical reduction in norepinephrine synthesis and reuptake in the cardiac sympathetic nervous system (SNS). Here we examined whether cholinergic transdifferentiation can occur in the cardiac SNS in rodent models of CHF and investigated the underlying molecular mechanism(s) using genetically modified mice. We used Dahl salt-sensitive rats to model CHF and found that, upon CHF induction, the cardiac SNS clearly acquired cholinergic characteristics. Of the various cholinergic differentiation factors, leukemia inhibitory factor (LIF) and cardiotrophin-1 were strongly upregulated in the ventricles of rats with CHF. Further, LIF and cardiotrophin-1 secreted from cultured failing rat cardiomyocytes induced cholinergic transdifferentiation in cultured sympathetic neurons, and this process was reversed by siRNAs targeting Lif and cardiotrophin-1. Consistent with the data in rats, heart-specific overexpression of LIF in mice caused cholinergic transdifferentiation in the cardiac SNS. Further, SNS-specific targeting of the gene encoding the gp130 subunit of the receptor for LIF and cardiotrophin-1 in mice prevented CHF-induced cholinergic transdifferentiation. Cholinergic transdifferentiation was also observed in the cardiac SNS of autopsied patients with CHF. Thus, CHF causes target-dependent cholinergic transdifferentiation of the cardiac SNS via gp130-signaling cytokines secreted from the failing myocardium.

Absence of gp130 in dopamine beta-hydroxylase-expressing neurons leads to autonomic imbalance and increased reperfusion arrhythmias

Inflammatory cytokines that act through glycoprotein (gp)130 are elevated in the heart after myocardial infarction and in heart failure. These cytokines are potent regulators of neurotransmitter and neuropeptide production in sympathetic neurons but are also important for the survival of cardiac myocytes after damage to the heart. To examine the effect of gp130 cytokines on cardiac nerves, we used gp130(DBH-Cre/lox) mice, which have a selective deletion of the gp130 cytokine receptor in neurons expressing dopamine beta-hydroxylase (DBH). Basal sympathetic parameters, including norepinephrine (NE) content, tyrosine hydroxylase expression, NE transporter expression, and sympathetic innervation density, appeared normal in gp130(DBH-Cre/lox) compared with wild-type mice. Likewise, basal cardiovascular parameters measured under isoflurane anesthesia were similar in both genotypes, including mean arterial pressure, left ventricular peak systolic pressure, dP/dt(max), and dP/dt(min). However, pharmacological interventions revealed an autonomic imbalance in gp130(DBH-Cre/lox) mice that was correlated with an increased incidence of premature ventricular complexes after reperfusion. Stimulation of NE release with tyramine and infusion of the beta-agonist dobutamine revealed blunted adrenergic transmission that correlated with decreased beta-receptor expression in gp130(DBH-Cre/lox) hearts. Due to the developmental expression of the DBH-Cre transgene in parasympathetic ganglia, gp130 was eliminated. Cholinergic transmission was impaired in gp130(DBH-Cre/lox) hearts due to decreased parasympathetic drive, but tyrosine hydroxylase immunohistochemistry in the brain stem revealed that catecholaminergic nuclei appeared grossly normal. Thus, the apparently normal basal parameters in gp130(DBH-Cre/lox) mice mask an autonomic imbalance that includes alterations in sympathetic and parasympathetic transmission.

The dependence on gp130 cytokines of axotomy induced neuropeptide expression in adult sympathetic neurons

Adult peripheral neurons exhibit dramatic changes in gene expression after axonal injury, including changes in neuropeptide phenotype. For example, sympathetic neurons in the superior cervical ganglion (SCG) begin to express vasoactive intestinal peptide (VIP), galanin, pituitary adenylate cyclase activating polypeptide (PACAP), and cholecystokinin after axotomy. Before these changes, nonneuronal cells in the SCG begin to express leukemia inhibitory factor (LIF). When the effects of axotomy were compared in LIF-/- and wild-type mice, the increases in VIP and galanin expression were less in the former, though significant increases still occurred. LIF belongs to a family of cytokines with overlapping physiological effects and multimeric receptors containing the subunit gp130. Real-time PCR revealed large increases in the SCG after axotomy in mRNA for three members of this cytokine family, interleukin (IL)-6, IL-11, and LIF, with modest increases in oncostatin M, no changes in ciliary neurotrophic factor, and decreases in cardiotrophin-1. To explore the role of these cytokines, animals with selective elimination of the gp130 receptor in noradrenergic neurons were studied. No significant changes in mRNA levels for VIP, galanin, and PACAP were seen in axotomized ganglia from these mutant mice, while the increase in cholecystokinin was as large as that seen in wild-type mice. The data indicate that the inductions of VIP, galanin, and PACAP after axotomy are completely dependent on gp130 cytokines and that a second cytokine, in addition to LIF, is involved. The increase in cholecystokinin after axotomy, however, does not require the action of these cytokines.

Development of macrophages of cyprinid fish

The innate immune responses of early vertebrates, such as bony fishes, play a central role in host defence against infectious diseases and one of the most important effector cells of innate immunity are macrophages. In order for macrophages to be effective in host defence they must be present at all times in the tissues of their host and importantly, the host must be capable of rapidly increasing macrophage numbers during times of need. Hematopoiesis is a process of formation and development of mature blood cells, including macrophages. Hematopoiesis is controlled by soluble factors known as cytokines, that influence changes in transcription factors within the target cells, resulting in cell fate changes and the final development of specific effector cells. The processes involved in macrophage development have been largely derived from mammalian model organisms. However, recent advancements have been made in the understanding of macrophage development in bony fish, a group of organisms that rely heavily on their innate immune defences. Our understanding of the growth factors involved in teleost macrophage development, as well as the receptors and regulatory mechanisms in place to control them has increased substantially. Furthermore, model organisms such as the zebrafish have emerged as important instruments in furthering our understanding of the transcriptional control of cell development in fish as well as in mammals. This review highlights the recent advancements in our understanding of teleost macrophage development. We focused on the growth factors identified to be important in the regulation of macrophage development from a progenitor cell into a functional macrophage and discuss the important transcription factors that have been identified to function in teleost hematopoiesis. We also describe the findings of in vivo studies that have reinforced observations made in vitro and have greatly improved the relevance and importance of using teleost fish as model organisms for studying developmental processes.

Post-infarct cardiac sympathetic hyperactivity regulates galanin expression

The neuropeptide galanin is elevated in the cardiac sympathetic innervation after myocardial infarction (MI). Galanin inhibits vagal transmission and may support the regeneration of sympathetic nerves, thereby contributing to the development of arrhythmia and sudden cardiac death after MI. The reason for increased galanin production in sympathetic neurons after myocardial infarction is not known. Cardiac sympathetic neurons are activated chronically after cardiac ischemia-reperfusion, and activation of sympathetic neurons in culture stimulates galanin expression. Therefore, we tested the hypothesis that increased sympathetic nerve activity stimulates galanin expression in cardiac sympathetic neurons after myocardial infarction. To test this hypothesis we used TGR(ASrAOGEN) transgenic rats, which lack brain angiotensinogen and do not exhibit post-infarct sympathetic hyperactivity. Hearts and stellate ganglia were collected 1 week after ischemia-reperfusion. Galanin mRNA was quantified by real-time PCR and peptide content was assayed by enzyme-linked immunosorbent assay. Galanin mRNA increased approximately 3-fold after MI in cardiac sympathetic neurons of both genotypes compared to unoperated and sham controls. Left ventricular galanin content, however, increased after MI only in Sprague-Dawley rats and not in AOGEN rats. These data suggest that post-infarct cardiac sympathetic hyperactivity stimulates galanin peptide production but is not required for increased galanin mRNA expression.

TH and NPY in sympathetic neurovascular cultures: role of LIF and NT-3

The sympathetic nervous system is an important determinant of vascular function. The effects of the sympathetic nervous system are mediated via release of neurotransmitters and neuropeptides from postganglionic sympathetic neurons. The present study tests the hypothesis that vascular smooth muscle cells (VSM) maintain adrenergic neurotransmitter/neuropeptide expression in the postganglionic sympathetic neurons that innervate them. The effects of rat aortic and tail artery VSM (AVSM and TAVSM, respectively) on neuropeptide Y (NPY) and tyrosine hydroxylase (TH) were assessed in cultures of dissociated sympathetic neurons. AVSM decreased TH (39 +/- 12% of control) but did not affect NPY. TAVSM decreased TH (76 +/- 10% of control) but increased NPY (153 +/- 20% of control). VSM expressed leukemia inhibitory factor (LIF) and neurotrophin-3 (NT-3), which are known to modulate NPY and TH expression. Sympathetic neurons innervating blood vessels expressed LIF and NT-3 receptors. Inhibition of LIF inhibited the effect of AVSM on TH. Inhibition of neurotrophin-3 (NT-3) decreased TH and NPY in neurons grown in the presence of TAVSM. These data suggest that vascular-derived LIF decreases TH and vascular-derived NT-3 increases or maintains NPY and TH expression in postganglionic sympathetic neurons. NPY and TH in vascular sympathetic nerves are likely to modulate NPY and/or norepinephrine release from these nerves and are thus likely to affect blood flow and blood pressure. The present studies suggest a novel mechanism whereby VSM would modulate sympathetic control of vascular function.

Activating transcription factor 3 induction in sympathetic neurons after axotomy: response to decreased neurotrophin availability

Activating transcription factor 3 (ATF3) is induced in a high proportion of axotomized sensory and motor neurons after sciatic nerve transection. In the present study, we looked at the expression of this factor in the superior cervical ganglion (SCG) after axotomy and after other manipulations that induce certain aspects of the cell body response to axotomy. Sympathetic ganglia from intact rats and mice exhibit only a very occasional neuronal nucleus with activating transcription factor 3-like immunoreactivity (ATF3-IR); however, as early as 6 h and as late as 3 weeks postaxotomy, many of the neurons showed intense ATF3-IR. A second population of cells had smaller and generally less intensely stained nuclei, and at least some of these cells were satellite cells. Lesions distal to the SCG induced by administration of 6-hydroxydopamine or unilateral removal of the salivary glands produced increases in ATF3-IR similar to those seen after proximal axotomy, indicating that this response is not strictly dependent on the distance of the lesion from the cell body. Two proposed signals for triggering ATF3 expression were examined: reduction in nerve growth factor (NGF) availability and induction of the cytokine leukemia inhibitory factor (LIF). While administration of an antiserum raised against NGF to intact animals induced ATF3-IR, induction of ATF3-IR after axotomy was not reduced in LIF null mutant mice. Since axotomy, 6-hydroxydopamine, and sialectomy are known to decrease the concentration of NGF in the SCG, our data suggest that these decreases in NGF lead to increases in ATF3-IR. Furthermore, since the number of neurons in the SCG expressing ATF3-IR was greater after axotomy than after antiserum against NGF treatment, this raises the possibility that decreased NGF is not the only process regulating ATF3 expression after axotomy.

LIF promotes neurogenesis and maintains neural precursors in cell populations derived from spiral ganglion stem cells

Background

Stem cells with the ability to form clonal floating colonies (spheres) were recently isolated from the neonatal murine spiral ganglion. To further examine the features of inner ear-derived neural stem cells and their derivatives, we investigated the effects of leukemia inhibitory factor (LIF), a neurokine that has been shown to promote self-renewal of other neural stem cells and to affect neural and glial cell differentiation.

Results

LIF-treatment led to a dose-dependent increase of the number of neurons and glial cells in cultures of sphere-derived cells. Based on the detection of developmental and progenitor cell markers that are maintained in LIF-treated cultures and the increase of cycling nestin-positive progenitors, we propose that LIF maintains a pool of neural progenitor cells. We further provide evidence that LIF increases the number of nestin-positive progenitor cells directly in a cell cycle-independent fashion, which we interpret as an acceleration of neurogenesis in sphere-derived progenitors. This effect is further enhanced by an anti-apoptotic action of LIF. Finally, LIF and the neurotrophins BDNF and NT3 additively promote survival of stem cell-derived neurons.

Conclusion

Our results implicate LIF as a powerful tool to control neural differentiation and maintenance of stem cell-derived murine spiral ganglion neuron precursors. This finding could be relevant in cell replacement studies with animal models featuring spiral ganglion neuron degeneration. The additive effect of the combination of LIF and BDNF/NT3 on stem cell-derived neuronal survival is similar to their effect on primary spiral ganglion neurons, which puts forward spiral ganglion-derived neurospheres as an in vitro model system to study aspects of auditory neuron development.

Involvement of VIP and PACAP in neonatal brain lesions generated by a combined excitotoxic/inflammatory challenge

Several reports have highlighted the potential roles for the VIP-related neuropeptides in regeneration/neuroprotection after brain or nerve injuries. We previously reported that peripheral inflammation worsened ibotenate-induced cystic white matter lesions. Because VIP is also known as an immunomodulator, we wonder if VIP could also limit the deleterious effects of local inflammation. Therefore, we first tested the effects of peripheral IL-1beta on VIP and PACAP central production. Second, we observed that cox-2 activation by IL-1beta was essential to generate changes in ligand/receptor gene expression. We further tested whether the intraperitoneal injection of IL-1beta, known to aggravate the ibotenate-induced lesions, could modify the expression pattern of VIP-related genes. Finally, we concluded using histological analysis that VIP[ala(11,22,28)], a synthetic VPAC(1) agonist completely reversed the aggravating effects of IL-1beta on ibotenate-induced lesions of the periventricular white matter. Conversely, VIP-neurotensin hybrid, a nonselective VIP receptor antagonist, worsened the lesions. All together, our results suggest that an activation of VIP/VPAC(1) signaling cascade in the vicinity of the injury site could circumvent the synergizing degenerative effects of ibotenate and pro-inflammatory cytokines. Therefore, development of therapeutic tools inducing/sustaining the activation of VIP/VPAC(1) signaling cascade may lead to future preventive treatments for inflammatory conditions during pregnancy.

Retrograde activation of STAT3 by leukemia inhibitory factor in sympathetic neurons

Leukemia inhibitory factor (LIF) is a member of the interleukin-6 family of cytokines and signals through the glycoprotein 130 and LIF receptor beta subunits. Binding of cytokines to these subunits activates multiple signaling cascades, including the Janus kinase (Jak)/signal transducers and activators of transcription (STAT) pathway. We used compartmentalized cultures of sympathetic neurons and immunocytochemical analyses of STAT3 to examine the mechanisms involved in retrograde signaling of LIF from distal neurites (DN) to cell bodies. Addition of LIF to the DN of these neurons triggers the activation and nuclear translocation of STAT3. Inhibition of Jak activity in the cell bodies prevented LIF-induced retrograde activation of STAT3, while block of Jak activity in the DN had no effect on the appearance of activated STAT3 in the nucleus. These results show that the transport of activated Jak is not the main mechanism mediating retrograde signaling. Although there is an increase in phosphorylated STAT3 in the neurites after distal stimulation, the transport of activated STAT3 is not necessary for retrograde signaling. Our results are consistent with a signaling endosome model for retrograde signaling, in which the LIF/glycoprotein 130/LIF receptor/Jak complex is internalized and transported to activate STAT3 in the cell body.

The neuropoietic cytokine family in development, plasticity, disease and injury

Neuropoietic cytokines are well known for their role in the control of neuronal, glial and immune responses to injury or disease. Since this discovery, it has emerged that several of these proteins are also involved in nervous system development, in particular in the regulation of neurogenesis and stem cell fate. Recent data indicate that these proteins have yet more functions, as key modulators of synaptic plasticity and of various behaviours. In addition, neuropoietic cytokines might be a factor in the aetiology of psychiatric disorders.

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 axotomy-induced neuropeptides galanin and pituitary adenylate cyclase-activating peptide promote axonal sprouting of primary afferent and cranial motor neurones

The neuropeptides galanin and pituitary adenylate cyclase-activating peptide (PACAP) are markedly up-regulated in response to peripheral nerve lesion. Both peptides are involved in neuronal differentiation and neurite outgrowth during development. In this study, we investigated the effects of galanin and PACAP on axonal elongation and sprouting by adult rat sensory neurones in vitro and facial motor neurones in vivo. Dissociated rat dorsal root ganglion neurones were plated on laminin substrate and analysed morphometrically. Both the mean axonal length and the number of branch points significantly increased in the presence of galanin or PACAP (2-5 microm). Effects on axonal collateralization were investigated in the rat facial nerve lesion model by direct application of the peptides to collagen-filled conduits entubulating the transected facial nerve stumps. Triple retrograde labelling of brainstem neurones confirmed that the peptides potently induce axonal sprouting of cranial motor neurones. The number of neurones regenerating into identified rami of the facial nerve increased up to fivefold. Biometrical analysis of whisking behaviour revealed that galanin and PACAP impaired the functional outcome when compared with vehicle-treated animals 8 weeks after surgery. In conclusion, although galanin and PACAP have been established as neurotrophic molecules with respect to axonal development and regeneration, their potential as treatments for peripheral nerve lesions appears limited because of the extensive stimulation of collateral axon branching. These branches are misrouted towards incorrect muscles and cause impairment in their coordinated activity.

Leukemia inhibitory factor promotes neural stem cell self-renewal in the adult brain

Although neural stem cells (NSCs) persist in various areas of the adult brain, their contribution to brain repair after injury is very limited. Treatment with exogenous growth factors can mitigate this limitation, suggesting that the brain environment is normally deficient in permissive cues and that it may be possible to stimulate the latent regenerative potential of endogenous progenitors with appropriate signals. We analyzed the effects of overexpressing the cytokine leukemia inhibitory factor (LIF) on adult neurogenesis in the normal brain. We found that LIF reduces neurogenesis in the olfactory bulb and subventricular zone by acting directly on NSCs. LIF appears to promote NSC self-renewal, preventing the emergence of more differentiated cell types. This ultimately leads to an expansion of the NSC pool. Our results have implications for the development of therapeutic strategies for brain repair and suggest that LIF may be useful, in combination with other factors, in promoting regeneration in the adult brain.

Reduction in nerve growth factor availability leads to a conditioning lesion-like effect in sympathetic neurons

Axotomized peripheral neurons are capable of regeneration, and the rate of regeneration can be enhanced by a conditioning lesion (i.e., a lesion prior to the lesion after which neurite outgrowth is measured). A possible signal that could trigger the conditioning lesion effect is the reduction in availability of a target-derived factor resulting from the disconnection of a neuron from its target tissue. We tested this hypothesis with respect to nerve growth factor (NGF) and sympathetic neurons by administering an antiserum to NGF to adult mice for 7 days prior to explantation or dissociation of the superior cervical ganglion (SCG) and subsequently measuring neurite outgrowth. The antiserum treatment dramatically lowered the concentration of NGF in the SCG and increased the rate of neurite outgrowth in both explants and cell cultures. The increase in neurite outgrowth was similar in magnitude to that seen after a conditioning lesion. To determine if exogenous NGF could block the effect of a conditioning lesion, mice were injected with NGF or cytochrome C immediately prior to unilateral axotomy of the SCG, and for 7 days thereafter. A conditioning lesion effect of similar magnitude was seen in NGF-treated and control animals. While NGF treatment increased NGF levels in the contralateral control ganglion, it did not significantly elevate levels in the axotomized ganglion. The results suggest that the decreased availability of NGF after axotomy is a sufficient stimulus to induce the conditioning lesion effect in sympathetic neurons. While NGF administration did not prevent the conditioning lesion effect, this may be due to the markedly decreased ability of sympathetic neurons to accumulate the growth factor after axotomy.