Non-neuronal cells inhibit catecholaminergic differentiation of primary sensory neurons: role of leukemia inhibitory factor

The role of the leukemia inhibitory factor receptor in neuroprotective signaling

Several neurotropic cytokines relay their signaling through the leukemia inhibitory factor receptor. This 190kDa subunit couples with the 130kDa gp130 subunit to transduce intracellular signaling in neurons and oligodendrocytes that leads to expression of genes associated with neurosurvival. Moreover, activation of this receptor alters the phenotype of immune cells to an anti-inflammatory one. Although cytokines that activate the leukemia inhibitory factor receptor have been studied in the context of neurodegenerative disease, therapeutic targeting of the specific receptor subunit has been understudied in by comparison. This review examines the role of this receptor in the CNS and immune system, and its application in the treatment in stroke and other brain pathologies.

A novel role for CD4+ T cells in the control of cachexia

Cachexia is the dramatic weight loss and muscle atrophy seen in chronic disease states, including autoimmunity, cancer, and infection, and is often associated with lymphopenia. We have previously shown that CD4(+) T cells that express the lowest density of CD44 (CD4(+)CD44(v.low)) are significantly reduced in diabetic NOD mice that are cachexic compared with diabetic mice that are not cachexic. Using this model, and a model of cancer cachexia, we test the hypothesis that CD4(+)CD44(v.low) cells play an active role in protecting the host from cachexia. CD4(+)CD44(v.low) cells, but not CD4(+) cells depleted of CD44(v.low) cells, delay the onset of wasting when infused into either diabetic or prediabetic NOD recipients. However, no significant effect on the severity of diabetes was detected. In a model of cancer cachexia, they significantly reduce muscle atrophy, and inhibit muscle protein loss and DNA loss, even when given after the onset of cachexia. Protection from wasting and muscle atrophy by CD4(+)CD44(v.low) cells is associated with protection from lymphopenia. These data suggest, for the first time, a role for an immune cell subset in protection from cachexia, and further suggest that the mechanism of protection is independent of protection from autoimmunity.

Retrograde Axonal Transport of Dopamine Beta Hydroxylase Antibodies by Neurons in the Trigeminal Ganglion

In this study we describe a population of neurons in the adult rat trigeminal ganglion (TG) that express dopamine beta-hydroxylase (DBH) and tyrosine hydroxylase (TH), and transport anti-DBH from their terminals. We have used NGF and NT3 labeled with biotin and anti-p75NTR labeled with FITC to examine the transport of neurotrophins and their receptors by these cells. In both the superior cervical ganglion (SCG) and the TG all neurons that transported anti-DBH transported NGF. While 100% of the DBH positive neurons in the TG also transported NT3, approximately 25% of these neurons in the SCG failed to transport NT3. In the SCG virtually all the neurons transported anti-p75NTR with the neurotrophins while in the TG more than 25% of these neurons failed to transport anti-p75NTR with the neurotrophins. These findings suggest that DBH positive neurons in the TG depend upon target-derived NGF and NT3 for their noradrenergic phenotype.

Dynamic expression of de novo DNA methyltransferases Dnmt3a and Dnmt3b in the central nervous system

To explore the role of DNA methylation in the brain, we examined the expression pattern of de novo DNA methyltransferases Dnmt3a and Dnmt3b in the mouse central nervous system (CNS). By comparing the levels of Dnmt3a and Dnmt3b mRNAs and proteins in the CNS, we showed that Dnmt3b is detected within a narrow window during early neurogenesis, whereas Dnmt3a is present in both embryonic and postnatal CNS tissues. To determine the precise pattern of Dnmt3a and Dnmt3b gene expression, we carried out X-gal histochemistry in transgenic mice in which the lacZ marker gene is knocked into the endogenous Dnmt3a or Dnmt3b gene locus (Okano et al. [1999] Cell 99:247-257). In Dnmt3b-lacZ transgenic mice, X-gal-positive cells are dispersed across the ventricular zone of the CNS between embryonic days (E) 10.5 and 13.5 but become virtually undetectable in the CNS after E15.5. In Dnmt3a-lacZ mice, X-gal signal is initially observed primarily in neural precursor cells within the ventricular and subventricular zones between E10.5 and E17.5. However, from the newborn stage to adulthood, Dnmt3a X-gal signal was detected predominantly in postmitotic CNS neurons across all the regions examined, including olfactory bulb, cortex, hippocampus, striatum, and cerebellum. Furthermore, Dnmt3a signals in CNS neurons increase during the first 3 weeks of postnatal development and then decline to a relatively low level in adulthood, suggesting that Dnmt3a may be of critical importance for CNS maturation. Immunocytochemistry experiments confirmed that Dnmt3a protein is strongly expressed in neural precursor cells, postmitotic CNS neurons, and oligodendrocytes. In contrast, glial fibrillary acidic protein-positive astrocytes exhibit relatively weak or no Dnmt3a immunoreactivity in vitro and in vivo. Our data suggest that whereas Dnmt3b may be important for the early phase of neurogenesis, Dnmt3a likely plays a dual role in regulating neurogenesis prenatally and CNS maturation and function postnatally.

Identification of a novel gp130-responsive site in the vasoactive intestinal peptide cytokine response element

The neuropoietic cytokine ciliary neurotrophic factor (CNTF) potently induces transcription of the vasoactive intestinal peptide (VIP) gene through a 180-base pair (bp) cytokine response element (CyRE) in the VIP promoter. We have previously shown that CNTF induction of STAT and AP-1 protein binding within the CyRE is necessary to mediate CNTF induction of VIP gene transcription. We now show that a third, previously uncharacterized site at the 3'-end of the CyRE is also critical to CNTF induction of CyRE transcription. A 4-bp mutation in this 3'-region reduced CNTF-mediated induction of transcription approximately 80%. Whereas mutations in both the STAT and AP-1 sites substantially reduced CNTF induction of transcription, mutations in these sites together with the novel 3'-site completely abolished the ability of CNTF to induce CyRE-mediated transcription. Gel shift analysis indicated that a complex in neuroblastoma cells bound specifically to this 3'-site. This complex was not altered by CNTF treatment. Mutations in an 8-bp sequence (TTACTGGA) eliminated binding of this protein complex and markedly reduced transcriptional activation of the CyRE by CNTF. Thus, we have identified a protein complex binding to a novel DNA sequence that is necessary for full CNTF induction of VIP gene transcription.

Cytokines that signal through the leukemia inhibitory factor receptor-beta complex in the nervous system

Cytokines that signal through the leukemia inhibitory factor (LIF) receptor, such as LIF and ciliary neuronotrophic factor, have a wide range of roles within both the developing and mature nervous system. They play a vital role in the differentiation of neural precursor cells into astrocytes and can prevent or promote neuronal differentiation. One of the conundrums regarding signalling through the LIF receptor is how it can have multiple, often conflicting roles in different cell types, such as enhancing the differentiation of astrocytes while inhibiting the differentiation of some neuronal cells. Factors that can modulate signal transduction downstream of cytokine signalling, such as "suppressor of cytokine signalling" proteins, which inhibit the JAK/STAT but not the mitogen-activated protein kinase pathway, may therefore play an important role in determining how a given cell will respond to cytokine signalling. This review discusses the general effects of cytokine signalling within the nervous system. Special emphasis is placed on differentiation of neural precursor cells and the role that regulation of cytokine signalling may play in how a given precursor cell responds to cytokine stimulation.

The levels of leukemia inhibitory factor mRNA in a Schwann cell line are regulated by multiple second messenger pathways

Axotomy of sympathetic and sensory neurons leads to changes in their neuropeptide phenotypes. These changes are mediated in part by the induction of leukemia inhibitory factor (LIF) by nonneuronal cells. In the present study, we identified satellite/Schwann cells as a possible source of the injury-induced LIF. Using a Schwann cell line, SC-1 cells, we examined mechanisms of LIF induction. LIF mRNA levels increased rapidly when the cells were treated with 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate, phorbol 12-myristate 13-acetate (PMA), or A23187. Among these reagents, PMA was the most efficacious. Inhibition of protein kinase C (PKC) by GF-1 09203X significantly reduced the PMA-induced LIF mRNA levels. As PKC is known to activate the extracellular signal-regulated kinase (ERK) signaling pathway, the involvement of this pathway in the PMA-stimulated induction of LIF mRNA was examined. Phosphorylation of ERKs was increased following PMA treatment in SC-1 cells. Moreover, inhibition of ERK kinase activity by PD98059 dramatically reduced PMA-stimulated phosphorylation of ERKs and induction of LIF mRNA. These results indicate that LIF mRNA levels can be regulated by ERK activation via stimulation of PKC in Schwann cells.

Analysis of neuronal and glial phenotypes in brains of mice deficient in leukemia inhibitory factor

Leukemia inhibitory factor (LIF) can regulate the survival and differentiation of certain neurons and glial cells in culture. To determine the role of this cytokine in the central nervous system in vivo, we examined the brains of young and adult mice in which the LIF gene was disrupted. Immunohistochemical staining of neurons for choline acetyltransferase, tyrosine hydroxylase, serotonin, parvalbumin, calbindin, neuropeptide Y, vasoactive intestinal polypeptide, and calcitonin gene-related peptide revealed no significant differences between null mutant and wild-type (WT) brains. In contrast, analysis of glial phenotypes demonstrated striking deficits in the LIF-knockout brain. Staining with several anti-glial fibrillary acidic protein (GFAP) antibodies showed that the number of GFAP-positive cells in various regions of the hippocampus in the female mutant is much lower than in the WT. The null male hippocampus also displays a significant, though less marked deficit. The number of astrocytes in the mutant hippocampus, as determined by S-100 staining, is not, however, significantly different from WT. In addition, quantification of immunohistochemical staining of female, but not male, mutants reveals a significant deficit in myelin basic protein content in three brain regions, suggesting alterations in oligodendrocytes as well. Thus, while overall brain histology appears normal, the absence of LIF in vivo leads to specific, sexually dimorphic alterations in glial phenotype.

NT-3 regulates expression of Brn3a but not Brn3b in developing mouse trigeminal sensory neurons

We have used a quantitative RT-PCR approach to determine the levels of Brn3a and Brn3b POU domain transcription factor mRNAs in the developing mouse trigeminal ganglion from E10 to E18. Using low density neuronal cultures, we have shown that NT-3 can regulate the expression of Brn3a mRNA in trigeminal neurons during the periods that they are differentiating and innervating their peripheral and central targets. In contrast to Brn3a, Brn3b mRNA is expressed at extremely low levels in the early trigeminal ganglion. Trigeminal neurons from early ganglia express low levels of Brn3b mRNA in culture and do not up-regulate Brn3b mRNA in response to a number of growth factors and experimental conditions. However, at later ages, when in vivo levels of Brn3b mRNA are high, FGF2, TGFbeta1 and retinoic acid all up-regulate Brn3b mRNA expression in cultured trigeminal neurons. Since NT-3 regulates the developmental expression of Brn3a, Brn3a may mediate some of the effects that NT-3 exerts on sensory neurons and their progenitors. Similarly, Brn3b may mediate some of the effects that FGF2, TGFbeta1 and retinoic acid have on neurons.

Cytokines promote the survival of mouse cranial sensory neurones at different developmental stages

To investigate when the neurotrophic cytokines ciliary neurotrophic factor (CNTF), leukaemia inhibitory factor (LIF), oncostatin-M (OSM), interleukin-6 (IL-6) and cardiotrophin-1 (CT-1) act on developing sensory neurones and whether they co-operate with neurotrophins in regulating neuronal survival, we studied the in vitro trophic effects of these factors on two well-characterized populations of cranial sensory neurones at closely staged intervals throughout embryonic development. The cutaneous sensory neurones of the trigeminal ganglion, which show an early, transient survival response to BDNF and NT3 before becoming NGF-dependent, were supported by CNTF, LIF, OSM and CT-1 during the late fetal period, several days after the neurones become NGF-dependent. At this stage of development, these cytokines promoted the survival of a subset of NGF-responsive neurones. The enteroceptive neurones of the nodose ganglion, which retain dependence on BDNF throughout fetal development, were supported throughout their development by CNTF, LIF, OSM and CT-1, and displayed an additional survival response to IL-6 in the late fetal period. These findings indicate that populations of sensory neurones display different developmental patterns of cytokine responsiveness and show that embryonic trigeminal neurones pass through several phases of differing neurotrophic factor survival requirements.

Cytokines which signal through the LIF receptor and their actions in the nervous system

A number of different cytokines, each initially characterized on the basis of very different biological activities, all have very similar signalling pathways and share a similar tertiary structure. These cytokines include leukaemia inhibitory factor, ciliary neuronotrophic factor, oncostatin M, growth-promoting activity and cardiotrophin 1. They all have been found to regulate a number of properties of cells of the developing and mature nervous system in vitro and thus are neuroregulatory cytokines. The actions of these cytokines include regulation of neurotransmitter phenotype, differentiation of neuronal precursor cells both in the peripheral nervous system and in the spinal cord, survival of differentiated neurons, and regulation of development of both astrocytes and oligodendrocytes. In addition, studies in animal models show that these factors can rescue sensory and motor neurons from axotomy-induced cell death, which suggests that they can act as trauma factors for injured neurons. Analysis of the expression patterns of the different neuroregulatory cytokines and their receptors reveals that the receptors are expressed throughout nervous system development and following trauma, whereas the cytokines show temporal and spatial specific expression patterns. This is consistent with the idea that specific cytokines have specific roles in neural development and repair, but that their signalling pathways are shared. The phenotypes of the receptor knockouts show clear deficits in nervous system development, indicating a crucial role for LIF receptor signalling. Knockouts of individual cytokines are less dramatic, but LIF and CNTF knockouts do reveal deficits in maintenance of motor neurons or following trauma. Thus, whereas LIF and CNTF have clear roles in maintenance and following trauma, it is unclear which of the cytokines is involved in nervous system development. In clinical terms, these findings add further support to the use of these cytokines in nervous system trauma and disease.

Leukaemia inhibitory factor is retrogradely transported by a distinct population of adult rat sensory neurons: co-localization with trkA and other neurochemical markers

Sciatic sensory afferents that retrogradely transport and accumulate leukaemia inhibitory factor (LIF) within their soma were characterized in the adult rat in vivo. Twenty-four percent of neurons within the L4 and L5 dorsal root ganglia accumulated biotinylated LIF following intraneural injection of the cytokine into the sciatic nerve. Labelled cell bodies were predominantly of small diameter (20.1 +/- 0.5 microm). Retrograde transport was eliminated by excess unlabelled LIF but not by the related cytokines, ciliary-derived neurotrophic factor (CNTF) and interleukin-6 (IL-6). Double labelling revealed that the majority (81%) of LIF-accumulating neurons were immunopositive for CGRP and 34% were immunopositive for the cell surface glycoconjugate IB4. Sixty-two percent of LIF-accumulating neurons were immunopositive for trkA. Our results demonstrate a group of small-diameter sensory neurons that retrogradely transport LIF, comprising cells that constitutively express neuropeptides and those likely to be peptide-deficient. LIF-accumulating neurons expressing trkA are also potentially responsive to nerve growth factor. It is likely that the LIF-accumulating neurons described in this study are nociceptive in function.

Developmental regulation of two distinct neuronal phenotypes in rat dorsal root ganglia

In a previous study we described two distinct neuronal phenotypes in rat dorsal root ganglia based on immunocytochemical assays for the neuronal intermediate filament proteins, peripherin and low-molecular-weight neurofilaments [Goldstein M. E. et al. (1991) J. Neurosci. Res. 30, 92-104]. In this paper we have extended this classification by using in situ hybridization to localize and evaluate the levels of various cytoskeletal and neuropeptide messenger RNAs within the peripherin-immunoreactive and peripherin-immunoreactive-negative neurons found in embryonic day 15 and 20, postnatal day 2 and adult dorsal root ganglia. We found in postnatal and adult dorsal root ganglia in vivo that the large, peripherin-immunoreactive-negative neurons, which are intensely stained by low-molecular-weight neurofilament antibodies, also contain high levels of low, medium and high-molecular-weight neurofilament messenger RNAs, whereas the smaller peripherin-immunoreactive neurons do not. On the other hand, both cell types contained comparable levels of peripherin and alpha-tubulin messenger RNA. The presence of peripherin messenger RNA but no peripherin immunoreactivity in the large cells suggested either a translational or post-translational regulation of this polypeptide, or rapid clearance of this protein from the perikaryon into the axon. In adult dorsal root ganglia, more than 50% of the peripherin-immunoreactive neurons also contained high levels of substance P and/or calcitonin gene-related peptide messenger RNAs, while less than 20% of the large peripherin-immunoreactive-negative neurons did. The attainment of these phenotypic characteristics during development in vivo was studied by northern blot and in situ hybridization histochemistry. In early embryonic stages (embryonic days 15-16), virtually all neurons were peripherin-immunoreactive and were positive for peripherin, alpha-tubulin and low-molecular-weight neuro-filament messenger RNAs, suggesting a homogeneous population. By embryonic day 20, the two adult phenotypes became clearly evident, and were fully established by postnatal day 2. In cultures of embryonic day 15 dorsal root ganglion neurons grown in the presence of nerve growth factor, peripherin and low-molecular-weight neurofilament messenger RNAs were expressed in all neurons, even after nine days in vitro, similar to embryonic dorsal root ganglia in vivo. Nerve growth factor supplemented by skeletal and heart muscle extracts did up-regulate neurofilament gene expression, but not to the extent characteristic of the peripherin-immunoreactive-negative adult phenotype. These results suggest that development of the mature phenotype of dorsal root ganglion neurons occurs by postnatal day 2 in vivo and is dependent upon target contact and/or target-derived factors.

The role of signal transduction systems in mediating cell density dependent changes in tyrosine hydroxylase gene expression

Cell density has been implicated in the regulation of neuronal gene phenotype. This study tested the interaction of signal transduction pathways and the expression of tyrosine hydroxylase (TH) mRNA with varying cell density. Increasing cell density in a parental, wild type PC12 cell line elevated steady state levels of TH mRNA. Three observations suggested that this induction is not related to the cyclic AMP dependent signalling pathway: (1) Forskolin stimulated the level of TH mRNA similarly at multiple densities. (2) PKA deficient mutant PC12 cell lines that have either one third (A123.7, AB11) or 3% (A126-1B2) of normal basal expression of TH mRNA still exhibit the same density induced elevation of TH mRNA levels as the wild type. (3) Different cell densities did not change cyclic AMP concentrations in the basal or in the receptor stimulated state. Increasing cell density did not change basal levels of inositol triphosphate (IP3) levels, which suggests that the phosphatidylinositol cascade (PI) is not responsible for density dependent changes in TH expression. Increasing confluence was highly correlated to [Ca2+]i in control (r = 0.70; P < 0.0001), A123.7 (r = 0.92; P < 0.001), AB11 (r = 0.72; P < 0.0001) and A126 (r = 0.42; P < 0.07). Taken together, the results show that neither cyclic AMP nor the PI cascade is involved in cell density induced changes in TH mRNA and suggest that altered [Ca2+]i may have a role.

Multiple mechanisms regulate sympathetic neuronal phenotype

Adult rat sympathetic neurons can possess specific neuropeptides utilized as cotransmitters along with norepinephrine, but the factors that regulate their expression remain unknown. 60% of adult rat superior cervical ganglion (SCG) neurons express neuropeptide Y (NPY) in vivo. To determine whether the restricted expression was an intrinsic property of sympathetic ganglia, we examined if embryonic sympathetic precursors gave rise to NPY immunoreactive (-IR) neurons in vitro. After one week in culture, 60% of neurons derived from the E14.5 rat SCG were NPY-IR. Thus, ganglia isolated before peripheral target contact or preganglionic innervation were capable of regulating NPY expression both in the number of neurons with NPY and in the developmental timing of NPY expression. To determine if the restricted expression of NPY was a reflection of neuroblasts committed to an NPY fate, SCG precursors were labeled with a replication incompetent retrovirus carrying lacZ, and NPY expression in lacZ-labeled clones examined after one week. Two thirds of neuronal clones obtained were uniformly NPY-IR; that is, all neurons in a clone either possessed or lacked NPY. One-third of the neuronal clones were mixed and contained both neurons with and without NPY. We provide a novel demonstration that both lineage and environmental cues contribute to neuropeptide phenotype.

Targeted disruption of the low-affinity leukemia inhibitory factor receptor gene causes placental, skeletal, neural and metabolic defects and results in perinatal death

The low-affinity receptor for leukemia inhibitory factor (LIFR) interacts with gp130 to induce an intracellular signal cascade. The LIFR-gp130 heterodimer is implicated in the function of diverse systems. Normal placentation is disrupted in LIFR mutant animals, which leads to poor intrauterine nutrition but allows fetuses to continue to term. Fetal bone volume is reduced greater than three-fold and the number of osteoclasts is increased six-fold, resulting in severe osteopenia of perinatal bone. Astrocyte numbers are reduced in the spinal cord and brain stem. Late gestation fetal livers contain relatively high stores of glycogen, indicating a metabolic disorder. Hematologic and primordial germ cell compartments appear normal. Pleiotropic defects in the mutant animals preclude survival beyond the day of birth.

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

Leukemia inhibitory factor (LIF; also known as cholinergic differentiation factor) is a multifunctional cytokine that affects neurons, as well as many other cell types. To examine its neuronal functions in vivo, we have used LIF-deficient mice. In culture, LIF alters the transmitter phenotype of sympathetic neurons, inducing cholinergic function, reducing noradrenergic function, and altering neuropeptide expression. In vivo, a noradrenergic to cholinergic switch occurs in the developing sweat gland innervation, and changes in neuropeptide phenotype occur in axotomized adult ganglia. We find that the gland innervation of LIF-deficient mice is indistinguishable from normal. In contrast, neuropeptide induction in ganglia cultured as explants or axotomized in situ is significantly suppressed in LIF-deficient mice. Thus, LIF plays a role in transmitter changes induced by axotomy but not by developmental interactions with sweat glands.

Plasticity in neural crest cell differentiation

The neural crest is a pluripotent population of cells that are endowed with migratory capacities. It has long been known that the differentiation pathway taken by cells derived from the neural crest is largely controlled by the microenvironment to which they home after their migration phase, indicating a high degree of plasticity in their developmental fate. Recent progress has been made concerning the factors which influence survival, growth and differentiation of selected sets of precursors in each embryonic site colonised by derivatives of the neural crest.