Cholinergic differentiation of rat sympathetic neurons in culture: Effects of factors applied to distal neurites

LIF and the heart: just another brick in the wall?

Multiple studies have shown that the cytokine leukemia inhibitory factor (LIF) is protective of the myocardium in the acute stress of ischemia-reperfusion. All three major intracellular signaling pathways that are activated by LIF in cardiac myocytes have been linked to actions that protect against oxidative stress and cell death, either at the level of the mitochondrion or via nuclear transcription. In addition, LIF has been shown to contribute to post-myocardial infarction cardiac repair and regeneration, by stimulating the homing of bone marrow-derived cardiac progenitors to the injured myocardium, the differentiation of resident cardiac stem cells into endothelial cells, and neovascularization. Whether LIF offers protection to the heart under chronic stress such as hypertension-induced cardiac remodeling and heart failure is not known. However, mice with cardiac myocyte restricted knockout of STAT3, a principal transcription factor activated by LIF, develop heart failure with age, and cardiac STAT3 levels are reported to be decreased in heart failure patients. In addition, endogenously produced LIF has been implicated in the cholinergic transdiffrentiation that may serve to attenuate sympathetic overdrive in heart failure and in the peri-infarct region of the heart after myocardial infarction. Surprisingly, therapeutic strategies to exploit the beneficial actions of LIF on the injured myocardium have received scant attention. Nor is it established whether the purported so-called adverse effects of LIF observed in isolated cardiac myocytes have physiological relevance in vivo. Here we present an overview of the actions of LIF in the heart with the goal of stimulating further research into the translational potential of this pleiotropic cytokine.

Regulation of neurokine receptor signaling and trafficking

Leukemia inhibitory factor (LIF) and ciliary neurotrophic factor (CNTF) are neurally active cytokines, or neurokines. LIF signals through a receptor consisting of gp130 and the low affinity LIF receptor (LIFR), while the CNTF receptor consists of gp130, LIFR, and the low affinity CNTF receptor (CNTFR). Ser1044 of the LIFR is phosphorylated by Erk1/2 MAP kinase. Stimulation of neural cells with growth factors which strongly activate Erk1/2 decreases LIF-mediated signal transduction due to increased degradation of the LIFR as a consequence of Erk1/2-dependent phosphorylation of the receptor at Ser1044. The gp130 receptor subunit is phosphorylated, at least in part by calmodulin-dependent protein kinase II, at Ser782, which is adjacent to a dileucine internalization motif. Ser782 appears to negatively regulate cytokine receptor expression, as mutagenesis of Ser782 results in increased gp130 expression and cytokine-induced neuropeptide gene transcription. The LIFR and gp130 are transmembrane proteins, while CNTFR is a peripheral membrane protein attached to the cell surface via a glycosylphosphatidylinositol tail. In unstimulated cells, CNTFR but not LIFR and gp130 is localized to detergent-resistant lipid rafts. Stimulation of cells with CNTFR causes translocation of LIFR and gp130 into the lipid rafts, while stimulation with LIF does not induce receptor translocation, raising the possibility that CNTF could induce different patterns of signaling and/or receptor trafficking than caused by LIF. We used a compartmentalized culture system to examine the mechanisms for retrograde signaling by LIF and CNTF from distal neurites to the cell bodies of mouse sympathetic neurons. Stimulation with neurokines of the distal neurites of sympathetic neurons grown in a compartmentalized culture system resulted in the activation and nuclear translocation of the transcription factor Stat3. Retrograde signaling required Jak kinase activity in the cell body but not the distal neurites, and could be blocked by inhibitors of microtubule but not microfilament function. The results are consistent with a signaling endosomes model in which the ctyokine/receptor complex is transported back to the cell body where Stat3 is activated. While both LIF and CNTF mediate retrograde activation of Stat3, the kinetics for retrograde signaling differ for the two neurokines.

NGF uptake and retrograde signaling mechanisms in sympathetic neurons in compartmented cultures

Many neurons depend for their survival on retrograde signals to their cell bodies generated by nerve growth factor (NGF) or other neurotrophins at their axon terminals. Apoptosis resulting from the loss of retrograde NGF signaling contributes to the elimination of excess and misconnected neurons during development and to the death of neurons during the course of neurodegenerative diseases. Possible mechanisms of retrograde signaling include (1) retrograde transport of signaling endosomes, carrying NGF bound to activated TrkA, (2) retrograde transport of signaling molecules downstream of TrkA, and (3) retrograde propagation of a phosphorylation signal without transport of signaling molecules. Evidence is also described, which indicates that two or more retrograde signaling mechanisms exist to regulate neuronal survival, including recent evidence that withdrawal of NGF from distal axons produces a retrograde apoptotic signal, which is transported to the cell bodies, where it initiates the apoptotic program, leading to the death of the neuron.

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.

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.

Spatial considerations for stimulus-dependent transcription in neurons

Most neurons have elaborate dendrites as well as an axon emanating from the cell body that form synaptic connections with one or many target cells, which may be located a considerable distance from the cell body. Such complex and impressive morphologies allow some types of neurons to integrate inputs from one to many thousands of pre-synaptic partners and to rapidly propagate electrical signals, often over long distances, to post-synaptic target cells. Much slower, non-electrical signals also propagate from dendrites and distal axons to neuronal nuclei that influence survival, growth, and plasticity. The distances between distal dendrites and/or distal axons and cell bodies of neurons can be hundreds of microns to more than one meter. This long-range biochemical signal propagation from distal dendrites and distal axons to neuronal nuclei is entirely unique to neurons. This review is focused on excitatory neurotransmitter signaling from dendritic synapses to neuronal nuclei as well as on retrograde growth factor signaling from distal axons to neuronal nuclei.

Efficacy and safety analyses of a recombinant human immunodeficiency virus type 1 derived vector system

Lentiviruses infect both dividing and nondividing cells. In this study we characterized a lentiviral vector system consisting of a packaging vector (pHP) and a transducing vector (pTV) derived from a recombinant human immunodeficiency virus type 1 (HIV-1). In pHP, the long terminal repeats (LTRs), the 5' untranslated leader and portions of the env and nef genes were deleted. The leader sequence of pHP was substituted with a modified Rous sarcoma virus (RSV) 59 bp leader containing a mutated RSV gag AUG and a functional 5' splice site. The pHP construct was found to direct Gag-Pol synthesis as efficiently as wild-type HIV-1. The pTV construct contains sequences required for RNA packaging, reverse transcription and integration, but lacks viral genes. Co-transfection of pHP, pTV and a vesicular stomatitis virus G (VSV-G) envelope plasmid produced vectors at titers of 10(5)-10(6) transducing units per milliliter in 48 h. Replication-competent virus (RCV) was not detected when deletions were made in the env gene in pHP. The ability of this vector system to transduce dividing and nondividing cell in vitro and in vivo was also demonstrated. Compared with a Moloney murine leukemia virus (MLV) vector, the HP/TV vectors transduced human muscle-, kidney-, liver-derived cell lines and CD34+ primary hematopoietic progenitor cells more efficiently. Although the levels of the pTV transgene expression were high soon after transduction, the expression tended to decrease with time due either to the loss of proviral DNA or to the inactivation of promoter activity, which was found to be cell type-dependent. Analyses of extrachromosomal DNA showed that the unintegrated proviral DNA of lentiviral vectors survived much longer than that of the retroviral vectors. We demonstrate that the HP/TV vector is capable of high efficiency transduction and that long-term expression of lentiviral vectors is dependent on target cell type, the internal promoter and the transgene itself in the transducing vector.

FGF signaling and target recognition in the developing Xenopus visual system

We report that the growth cones of Xenopus retinal ganglion cells express fibroblast growth factor receptors (FGFRs) and that bFGF stimulates neurite extension from cultured retinal neurons. Furthermore, bFGF is abundant in the developing optic tract but is reduced in the optic tectum. To test whether FGF signaling plays a role in axonal guidance in vivo, bFGF was exogenously applied to the developing optic pathway in "exposed brain" preparations. FGF-treated retinal axons navigate normally through the optic tract, but the majority veer aberrantly at the tectal border and bypass the target. Our results implicate FGF signaling in target recognition and suggest that diminished levels of bFGF in the tectum cause arriving axons to slow their growth.

Activins as candidate cholinergic differentiation factors in vivo

A number of cytokine families have been implicated in shaping neuronal survival, growth and gene expression. The neuropoietic and transforming growth factor-beta (TGF-beta) cytokines, in particular, have emerged as candidates for regulating the phenotype of sympathetic neurons. Culture studies have shown that neuropoietic cytokines (such as leukemia inhibitory factor, ciliary neurotrophic factor, oncostatin M, growth promoting activity) can induce the cholinergic enzyme, choline acetyltransferase (ChAT) and several neuropeptides, whereas certain members of the TGF-beta family (activin A, bone morphogenetic proteins-2 and -6) induce partially overlapping but distinct sets of transmitter and neuropeptide genes in sympathetic neurons. Since activins can induce ChAT in cultured neurons, we have investigated whether these cytokines are expressed by the appropriate cells and tissues to make them candidates for the cholinergic differentiation factor that is known to alter the phenotype of sympathetic neurons that innervate the sweat gland in the footpad in vivo. In-situ hybridization with the anti-sense probe for activin beta B specifically labels the sweat glands but not other tissues in the footpads of developing rats. Ribonuclease protection assays indicate that beta B as well as the other activin and inhibin subunit mRNAs are expressed by a number of tissues, including footpad, hairy skin and submaxillary gland. Homogenates of developing rat footpads, however, failed to induce the set of neuropeptide genes in cultured sympathetic neurons that is characteristic for activins, although neuropoietic cytokine activity was readily detectable in this assay. Thus, while activin beta B mRNA is expressed in the sweat gland, this tissue does not contain detectable activin protein as assayed by its ability to regulate neuronal gene expression. Moreover, activin subunit mRNAs are expressed by targets of noradrenergic sympathetic neurons in vivo, indicating that activin expression is not limited to targets of cholinergic neurons.

NGF and the local control of nerve terminal growth

It is generally believed that the mechanism of action of neurotrophic factors involves uptake of neurotrophic factor by nerve terminals and retrograde transport through the axon and back to the cell body where the factor exerts its neurotrophic effect. This view originated with the observation almost 20 years ago that nerve growth factor (NGF) is retrogradely transported by sympathetic axons, arriving intact at the neuronal cell bodies in sympathetic ganglia. However, experiments using compartmented cultures of rat sympathetic neurons have shown that neurite growth is a local response of neurites to NGF locally applied to them which does not directly involve mechanisms in the cell body. Recently, several NGF-related neurotrophins have been identified, and several unrelated molecules have been shown to act as neurotrophic or differentiation factors for a variety of types of neurons in the peripheral and central nervous systems. It has become clear that knowledge of the mechanisms of action of these factors will be crucial to understanding neurodegenerative diseases and the development of treatments as well as the means to repair or minimize neuronal damage after spinal injury. The concepts derived from work with NGF suggest that the site of exposure of a neuron to a neurotrophic factor is important in determining its response.

Postnatal development of survival responsiveness in rat sympathetic neurons to leukemia inhibitory factor and ciliary neurotrophic factor

Embryonic rat sympathetic neurons undergo programmed cell death upon NGF deprivation. We show that during postnatal development, these neurons acquire the ability to be supported in vitro by LIF and CNTF as well as NGF. LIF and CNTF do not promote the long-term survival of embryonic day 21 sympathetic neurons in vitro. However, after 5 days of culture in the presence of NGF, the majority of embryonic day 21 sympathetic neurons can be supported by either of these factors. Furthermore, postnatal day 6 sympathetic neurons can be immediately supported by LIF and CNTF, indicating that acquisition of survival responsiveness occurs in vivo as well as in vitro. During this period, neuronal expression of LIF and CNTF receptor mRNAs remains constant, suggesting that sympathetic neurons alter their responsiveness to LIF and CNTF by allowing additional intracellular signaling pathways to promote survival.