Ciliary neurotrophic factor enhances the expression of NGF and p75 low-affinity NGF receptor in astrocytes

Inhibition of the Jak-STAT pathway prevents CNTF-mediated survival of axotomized oxytocinergic magnocellular neurons in organotypic cultures of the rat supraoptic nucleus

Previous studies have demonstrated that ciliary neurotrophic factor (CNTF) enhances survival and process outgrowth from magnocellular neurons in the paraventricular (PVN) and the supraoptic (SON) nuclei. However, the mechanisms by which CNTF facilitates these processes remain to be determined. Therefore, the aim of this study was to identify the immediate signal transduction events that occur within the rat SON following administration of exogenous rat recombinant CNTF (rrCNTF) and to determine the contribution of those intracellular signaling pathway(s) to neuronal survival and process outgrowth, respectively. Immunohistochemical and Western blot analyses demonstrated that axonal injury and acute unilateral pressure injection of 100 ng/μl of rrCNTF directly over the rat SON resulted in a rapid and transient increase in phosphorylated-STAT3 (pSTAT3) in astrocytes but not neurons in the SON in vivo. Utilizing rat hypothalamic organotypic explant cultures, we then demonstrated that administration of 25 ng/ml rrCNTF for 14days significantly increased the survival and process outgrowth of OT magnocellular neurons. In addition, pharmacological inhibition of the Jak-STAT pathway via AG490 and cucurbitacin I significantly reduced the survival of OT magnocellular neurons in the SON and PVN; however, the contribution of the Jak-STAT pathway to CNTF-mediated process outgrowth remains to be determined. Together, these data indicate that CNTF-induced survival of OT magnocellular neurons is mediated indirectly through astrocytes via the Jak-STAT signaling pathway.

Nerve growth factor induces cell cycle arrest of astrocytes

Neurotrophins can influence multiple cellular functions depending on the cellular context and the specific receptors they interact with. These neurotrophic factors have been extensively studied for their ability to support neuronal survival via Trk receptors and to induce apoptosis via the p75(NTR). However, the p75(NTR) is also detected on cell populations that do not undergo apoptosis in response to neurotrophins. In particular, the authors have detected p75(NTR) expression on astrocytes during development and after seizure-induced injury. In this study, the authors investigated the role of Nerve growth factor (NGF) in regulating astrocyte proliferation and in influencing specific aspects of the cell cycle. The authors have demonstrated that NGF prevents the induction of cyclins and their association with specific cyclin-dependent kinases, and thereby prevents progression through the G1 phase of the cell cycle. Since the authors have previously shown that p75(NTR) but not TrkA, is expressed in astrocytes, these data suggest that activation of p75(NTR) promotes withdrawal of astrocytes from the cell cycle, which may have important consequences during development and after injury.

Neurotrophin production in brain pericytes during hypoxia: a role of pericytes for neuroprotection

Neurotrophins are crucial regulators of neuronal survival and death. Evidence suggests that cells comprising the neurovascular unit (NVU) cooperatively mediate neuronal development, survival and regeneration. The aim of this study was to test whether cerebrovascular cells, endothelial cells and pericytes, produce neurotrophins and play neuroprotective roles during hypoxic insults. We examined the expression of neurotrophins and their receptors in cultured human cerebral microvascular endothelial cells and pericytes, astrocytes and the rat neuronal cell line PC12. Differentiated PC12 cells expressed TrkA, the NGF receptor, which was significantly upregulated by hypoxia at 1% O(2) and regulated neuronal survival. Both pericytes and astrocytes expressed three neurotrophins, i.e. NGF, BDNF and NT-3, while TrkB and TrkC, specific receptors for BDNF and NT-3, were expressed in astrocytes, but not pericytes. In response to hypoxia, among the neurotrophins expressed in pericytes and astrocytes only NT-3 expression was significantly upregulated in pericytes. Treatment of astrocytes with NT-3 significantly activated Erk1/2 and increased the expression of NGF both at mRNA and protein levels. The MEK1 inhibitor U0126 or siRNA-mediated knockdown of TrkC abolished the NT-3-induced upregulation of NGF in astrocytes. Taken together, cerebral microvascular pericytes and astrocytes are potent producers of neurotrophins in the NVU. In response to hypoxia, pericytes increase NT-3 production, which induces astrocytes to increase NGF production through the TrkC-Erk1/2 pathway. The interplay between pericytes and astrocytes through neurotrophins in the NVU may play an important role in neuronal survival under hypoxic conditions.

Neuronal activity and axonal sprouting differentially regulate CNTF and CNTF receptor complex in the rat supraoptic nucleus

We demonstrated previously that the hypothalamic supraoptic nucleus (SON) undergoes a robust axonal sprouting response following unilateral transection of the hypothalamo-neurohypophysial tract. Concomitant with this response is an increase in ciliary neurotrophic factor (CNTF) and CNTF receptor alpha (CNTFRα) expression in the contralateral non-uninjured SON from which the axonal outgrowth occurs. While these findings suggest that CNTF may act as a growth factor in support of neuronal plasticity in the SON, it remained to be determined if the observed increase in neurotrophin expression was related to the sprouting response per se or more generally to the increased neurosecretory activity associated with the post-lesion response. Therefore we used immunocytochemistry and Western blot analysis to examine the expression of CNTF and the components of the CNTF receptor complex in sprouting versus osmotically-stimulated SON. Western blot analysis revealed a significant increase in CNTF, CNTFRα, and gp130, but not LIFRß, protein levels in the sprouting SON at 10days post lesion in the absence of neuronal loss. In contrast, osmotic stimulation of neurosecretory activity in the absence of injury resulted in a significant decrease in CNTF protein levels with no change in CNTFRα, gp130, or LIFRß protein levels. Immunocytochemical analysis further demonstrated gp130 localization on magnocellular neurons and astrocytes while the LIFRß receptor was found only on astrocytes in the SON. These results are consistent with the hypothesis that increased CNTF and CNTFR complex in the sprouting, metabolically active SON are related directly to the sprouting response and not the increase in neurosecretory activity.

BMP9 (bone morphogenetic protein 9) induces NGF as an autocrine/paracrine cholinergic trophic factor in developing basal forebrain neurons

Acetylcholine (ACh) synthesis and release from basal forebrain cholinergic neurons (BFCN) innervating the cerebral cortex and hippocampus are essential processes for normal learning, memory and attention. Bone morphogenetic protein (BMP) 9 is a cholinergic differentiation factor in the developing septum that increases ACh synthesis and choline acetyltransferase (Chat) gene expression both in vivo and in vitro. We investigated the possible induction of cholinergic trophic factors by BMP9 in murine septal cells. Nerve growth factor (NGF) protein expression and secretion into the medium was increased in cultured embryonic septal cells treated with BMP9, and partially mediated BMP9-induced acetylcholine production and Chat gene expression. BMP9-induced Ngf gene expression was detected in postmitotic cells, required new protein synthesis and was blocked by BMP type I receptor inhibition. Cholinergic neurons were isolated by fluorescence-activated cell sorting based on either transgenic expression of green fluorescent protein driven by the Chat promoter or NGF receptor (p75) immunostaining. Although both noncholinergic and cholinergic neurons in untreated cultures expressed similar low levels of Ngf, increased Ngf gene expression was restricted to Chat-positive neurons in BMP9-treated cultures. Likewise, similar levels of Ngf mRNA were detected in p75-negative and p75-positive septal cells, yet only p75-positive BFCN increased their Ngf gene expression when treated with BMP9, and only these cells expressed the Alk1 BMP receptor. The data suggest an autocrine/paracrine role for NGF in the development and/or maintenance of BFCN and imply that the stimulation of NGF production and release contributes to the cholinergic-supportive properties of BMP9.

Nerve growth factor attenuates proliferation of astrocytes via the p75 neurotrophin receptor

The p75 neurotrophin receptor has been implicated in the regulation of multiple cellular functions that differ depending on the cell context. We have observed that p75(NTR) is strongly induced on astrocytes as well as neurons in the hippocampal CA3 region after seizures; however, the function of this receptor on these glial cells has not been defined. We have employed a primary culture system to investigate the effects of neurotrophins on astrocytes. Treatment of hippocampal astrocytes with nerve growth factor (NGF) caused a reduction in cell number, but did not elicit an apoptotic response, in contrast to hippocampal neurons. Instead, activation of p75(NTR) by NGF attenuated proliferation induced by mitogens such as EGF or serum. These studies demonstrate the cell type specificity of neurotrophin functions in the brain.

Effects of electro-acupuncture on CNTF expression in spared dorsal root ganglion and the associated spinal lamina II and nucleus dorsalis following adjacent dorsal root ganglionectomies in cats

It is well known that plasticity occurs in deafferented spinal cord, and that electro-acupuncture (EA) could promote functional restoration. The underlying mechanism is, however, unknown. Ciliary neurotrophic factor (CNTF) plays a crucial role in neurite outgrowth and neuronal survival both in vivo and in vitro, and its expression might explain some of the mechanism. In this study, we investigated the effects of EA on CNTF expression in the spared L(6) dorsal root ganglion (DRG), and spinal lamina II at spinal segments L(3) and L(6) as well as nucleus dorsalis (ND) of L(3) spinal segment following removal of L(1)-L(5) and L(7)-S(2) (DRG) in the cat. After ganglionectomies, the total and small-to-medium-sized numbers of immunoreactive neurons decreased at 3 dpo, and returned to the sham-operated level as early as 7 dpo. After EA, immunoreactive neurons in L(6) DRG noticeably increased at 7 dpo, compared with the non-acupunctured group. Notable increase in the large neurons was seen at 14 dpo, while their numbers in L(3) and L(6) spinal cord segments significantly declined at 3 dpo. Those in L(3) segment did not reach the sham-operated level until 14 dpo, but their numbers in L(6) segment returned to the sham-operated level as early as 7 dpo. CNTF immunopositive neurons in the ND of L(3) segment returned to the sham-operated level at 14 dpo. After EA, their number significantly increased as early as 7 dpo in lamina II of L(6) segment, and as late as 14 dpo in ND of L(3) segment. Western blot analysis showed CNTF changes corresponding to those shown in immunohistochemical staining. It is concluded that CNTF expression was involved in the EA promoted plastic changes in L(6) DRG and the associated deafferented spinal lamina and ND.

The effects of ciliary neurotrophic factor on neurological function and glial activity following contusive spinal cord injury in the rats

Ciliary neurotrophic factor (CNTF) has been implicated in the pathophysiology of injury to the central nervous system. The rapid increase in CNTF production following spinal cord injury (SCI) in rats is thought to serve a role in the neuronal survival and functional recovery. In this study, 40 SD rats were divided into four groups: sham-operated group, saline-treated group, 5- and 10-microg CNTF group. Saline and CNTF were given through lumbar intrathecal catheter for 10 days after T10 segment of spinal cord were injured by modified Allen contusion method. Animals were behaviorally tested for 6 weeks using the Basso, Beattie, Bresnahan locomotor rating scale and inclined plane test. At the end of 6 week, rubrospinal neurons of five rats in each group were labeled by retrograde transport of the horseradish peroxidase (HRP) from the lesion site, and then the labeled red nucleus neuron (RN) numbers were counted. Additional rats were histologically assessed for tissue sparing and neuronal loss and reactive gliosis at the injury site and adjacent areas. Rats treated with CNTF regained greater improvements in hindlimb function than controls. The amount of spared tissue was significantly higher in CNTF-treated animals than in controls. After CNTF treatment, the number of HRP-labeled RN neurons were significantly increased. Astrocytes and microglia reactivity was more pronounced in CNTF-treated animals than in controls. These results indicate that intrathecal infusion of exogenous CNTF following SCI may significantly reduce tissue damage and protect the rubrospinal descending tracks and enhances functional recovery, and may also induce more gliosis.

Rho activation patterns after spinal cord injury and the role of activated Rho in apoptosis in the central nervous system

Growth inhibitory proteins in the central nervous system (CNS) block axon growth and regeneration by signaling to Rho, an intracellular GTPase. It is not known how CNS trauma affects the expression and activation of RhoA. Here we detect GTP-bound RhoA in spinal cord homogenates and report that spinal cord injury (SCI) in both rats and mice activates RhoA over 10-fold in the absence of changes in RhoA expression. In situ Rho-GTP detection revealed that both neurons and glial cells showed Rho activation at SCI lesion sites. Application of a Rho antagonist (C3-05) reversed Rho activation and reduced the number of TUNEL-labeled cells by approximately 50% in both injured mouse and rat, showing a role for activated Rho in cell death after CNS injury. Next, we examined the role of the p75 neurotrophin receptor (p75NTR) in Rho signaling. After SCI, an up-regulation of p75NTR was detected by Western blot and observed in both neurons and glia. Treatment with C3-05 blocked the increase in p75NTR expression. Experiments with p75NTR-null mutant mice showed that immediate Rho activation after SCI is p75NTR dependent. Our results indicate that blocking overactivation of Rho after SCI protects cells from p75NTR-dependent apoptosis.

Nerve growth factor signaling, neuroprotection, and neural repair

Nerve growth factor (NGF) was discovered 50 years ago as a molecule that promoted the survival and differentiation of sensory and sympathetic neurons. Its roles in neural development have been characterized extensively, but recent findings point to an unexpected diversity of NGF actions and indicate that developmental effects are only one aspect of the biology of NGF. This article considers expanded roles for NGF that are associated with the dynamically regulated production of NGF and its receptors that begins in development, extends throughout adult life and aging, and involves a surprising variety of neurons, glia, and nonneural cells. Particular attention is given to a growing body of evidence that suggests that among other roles, endogenous NGF signaling subserves neuroprotective and repair functions. The analysis points to many interesting unanswered questions and to the potential for continuing research on NGF to substantially enhance our understanding of the mechanisms and treatment of neurological disorders.

Cytokines and the central nervous system

Cytokines are involved both in the immune response and in controlling various events in the central nervous system, that is, they are equally immunoregulators and modulators of neural functions and neuronal survival. On the other hand, cytokine production is under the tonic control of the peripheral and the central nervous system and the cytokine balance can be modulated by the action of neurotransmitters released from nonsynaptic varicosities [131]. The neuroimmune interactions are therefore bidirectional-cytokines and other products of the immune cells can modulate the action, differentiation, and survival of neuronal cells, while the neurotransmitter and neuropeptide release play a pivotal role in influencing the immune response. Cytokines and their receptors are constitutively expressed by and act on neurons in the central nervous system, in both its normal and its pathological state, but cytokine overexpression in the brain is an important factor in the pathogenesis of neurotoxic and neurodegenerative disorders. Accordingly, it can be accepted that the peripheral and central cytokine compartments appear to be integrated, and their effects might synergize or inhibit each other; however, it should always be taken into account that they are spatiotemporally differentially regulated. New concepts are reviewed in the regulation of relations between cytokine balance and neurodegeneration, including intracellular receptor-receptor, cell-cell, and systemic neuroimmune interactions that promote the further elucidation of the complexities and cascade of the possible interactions between cytokines and the central nervous system.

Expression and regulation of CNTF receptor-alpha in the in situ and in oculo grafted adult rat adrenal medulla

Cultured and transplanted adrenal medullary cells respond to ciliary neurotrophic factor (CNTF) with neurite formation and improved cell survival although the presence of the CNTF receptor-alpha (CNTFRalpha) has been unclear. This study show that CNTFRalpha mRNA was expressed in the postnatal day 1 as well as in the adult rat adrenal medulla. The highest CNTFRalpha mRNA signal was found in the ganglion cells of the adrenal medulla. After transplantation of adrenal medullary tissue the CNTFRalpha mRNA levels were down-regulated in the chromaffin cells. CNTF treatment of grafts did not normalize the receptor levels, but treatment with nerve growth factor (NGF) did. Thus, we demonstrate that CNTFRalpha mRNA is expressed in adrenal medulla, the levels becomes down-regulated after transplantation, but normalized after treatment with NGF.