TGF-beta rescues target-deprived preganglionic sympathetic neurons in the spinal cord

Smad3 deficiency increases cortical and hippocampal neuronal loss following traumatic brain injury

Transforming growth factor-β (TGF-β) signaling is involved in pathological processes following brain injury. TGF-β signaling through Smad3 contributes significantly to the immune response and glial scar formation after brain injury. However, TGF-β is also neuroprotective, suggesting that Smad3 signaling may also be involved in neuroprotection after injury. We found expression of the TGF-β type II receptor (TβRII) and Smad3 protein to be strongly and rapidly induced in neurons in the ipsilateral cortex and CA1 region of the hippocampus after stab wound injury. In contrast, astrocytic expression of TβRII and Smad3 was induced more slowly. Comparison of the response of wild-type and Smad3 null mice to cortical stab wound injury showed a more pronounced loss of neuronal viability in Smad3 null mice. Neuronal density was more strongly reduced in Smad3 null mice than in wild-type mice at 1 and 3days post lesion in both the ipsilateral cortex and hippocampal CA1 region. Fluoro-Jade B, TUNEL staining, and cleaved caspase-3 staining also demonstrated increased neuronal degeneration at early time points after injury in the ipsilateral hemisphere in Smad3 null mice. Taken together, our results suggest that TGF-β cytokine family signaling through Smad3 protects neurons in the damaged cortex and hippocampus at early time points after injury.

Critical factors in the design of growth factor releasing scaffolds for cartilage tissue engineering

BACKGROUND

Trauma or degenerative diseases of the joints are common clinical problems resulting in high morbidity. Although various orthopedic treatments have been developed and evaluated, the low repair capacities of articular cartilage renders functional results unsatisfactory in the long term. Over the last decade, a different approach (tissue engineering) has emerged that aims not only to repair impaired cartilage, but also to fully regenerate it, by combining cells, biomaterials mimicking extracellular matrix (scaffolds) and regulatory signals. The latter is of high importance as growth factors have the potency to induce, support or enhance the growth and differentiation of various cell types towards the chondrogenic lineage. Therefore, the controlled release of different growth factors from scaffolds appears to have great potential to orchestrate tissue repair effectively.

OBJECTIVE

This review aims to highlight considerations and limitations of the design, materials and processing methods available to create scaffolds, in relation to the suitability to incorporate and release growth factors in a safe and defined manner. Furthermore, the current state of the art of signalling molecules release from scaffolds and the impact on cartilage regeneration in vitro and in vivo is reported and critically discussed.

METHODS

The strict aspects of biomaterials, scaffolds and growth factor release from scaffolds for cartilage tissue engineering applications are considered.

CONCLUSION

Engineering defined scaffolds that deliver growth factors in a controlled way is a task seldom attained. If growth factor delivery appears to be beneficial overall, the optimal delivery conditions for cartilage reconstruction should be more thoroughly investigated.

Tailored release of TGF-β1 from porous scaffolds for cartilage tissue engineering

In view of cartilage tissue engineering, the possibility to prepare porous scaffolds releasing transforming growth factor-beta(1) (TGF-beta(1)) in a well controlled fashion was investigated by means of an emulsion-coating method. Poly(ether-ester) multiblock copolymers were used to prepare emulsions containing TGF-beta(1) which were subsequently applied onto prefabricated scaffolds. This approach resulted in defined porous structures (66%) with interconnected porosity, suitable to allow tissue ingrowth. The scaffolds were effectively associated with TGF-beta(1) and allowed to tailor precisely the release of the growth factor from 12 days to more than 50 days by varying the copolymer composition of the coating. An incomplete release was measured by ELISA, possibly linked to the rapid concentration decrease of the protein in solution. The released growth factor retained its biological activity as was assessed by a cell proliferation assay and by the ability of the released protein to induce chondrogenic differentiation of bone marrow-derived mesenchymal stem cells. However, exact bioactivity quantification was rendered difficult by the protein concentration decrease during storage. Therefore, this study confirms the interest of poly(ether-ester) multiblock copolymers for controlled release of growth factors, and indicates that emulsion-coated scaffolds are promising candidates for cartilage tissue engineering applications requiring precise TGF-beta(1) release rates.

GDNF applied to the MPTP-lesioned nigrostriatal system requires TGF-beta for its neuroprotective action

GDNF is a potent neurotrophic factor for nigrostriatal dopaminergic neurons in vitro and in animal models of Parkinson's disease (PD), but has largely failed when tested in therapeutic applications in human PD. We report here that GDNF requires transforming growth factor-beta (TGF-beta) to elicit its neurotrophic activity. Lesioning the mouse nigrostriatal system with MPTP significantly upregulates striatal TGF-beta2 mRNA levels. As expected, GDNF protects against the destructive effects of MPTP, including losses of TH-ir nigral neurons, striatal dopamine and TH-ir fibers. Application of antibodies neutralizing all three TGF-beta isoforms to the MPTP-lesioned striatum abolishes the neurotrophic effect of GDNF. We show that TGF-beta antibodies are not toxic and do not interfere with retrograde transport of iodinated GDNF, suggesting that TGF-beta antibodies do not impair internalization and retrograde trafficking of GDNF. We conclude that striatal TGF-beta may be essential for permitting exogenous GDNF to act as a neuroprotective factor.

Loss of leukemia inhibitory factor receptor beta or cardiotrophin-1 causes similar deficits in preganglionic sympathetic neurons and adrenal medulla

Leukemia inhibitory factor (LIF) receptor beta (LIFRbeta) is a receptor for a variety of neurotrophic cytokines, including LIF, ciliary neurotrophic factor (CNTF), and cardiotrophin-1 (CT-1). These cytokines play an essential role for the survival and maintenance of developing and postnatal somatic motoneurons. CNTF may also serve the maintenance of autonomic, preganglionic sympathetic neurons (PSNs) in the spinal cord, as suggested by its capacity to prevent their death after destruction of one of their major targets, the adrenal medulla. Although somatic motoneurons and PSNs share a common embryonic origin, they are distinct in several respects, including responses to lesions. We have studied PSNs in mice with targeted deletions of the LIFRbeta or CT-1 genes, respectively. We show that LIF, CNTF, and CT-1 are synthesized in embryonic adrenal gland and spinal cord and that PSNs express LIFRbeta. In embryonic day 18.5 LIFRbeta (-/-) and CT-1 (-/-) mice, PSNs were reduced by approximately 20%. PSNs projecting to the adrenal medulla were more severely affected (-55%). Although LIFRbeta (-/-) mice revealed normal numbers of adrenal chromaffin cells and axons terminating on chromaffin cells, levels of adrenaline and numbers of adrenaline-synthesizing cells were significantly reduced. We conclude that activation of LIFRbeta is required for normal development of PSNs and one of their prominent targets, the adrenal medulla. Thus, both somatic motoneurons and PSNs in the spinal cord depend on LIFRbeta signaling for their development and maintenance, although PSNs seem to be overall less affected than somatic motoneurons by LIFRbeta deprivation.

What is transforming growth factor-beta (TGF-β)?

The TGF-beta superfamily of proteins produces a wide range of frequently opposing effects in different cells and tissues in the body. However, its activation and mode of action are only partially understood because of its complexity in structure and functions and the variability in its downstream targets. Current work on these cytokines focuses on their receptors and the intercellular signalling pathways, comparing bioactivities between cell types and tracking their physiological and immunological effects in vivo. Future research will yield important therapeutic applications and the ability to manipulate these proteins in vivo.

TGF-betas and their roles in the regulation of neuron survival

Transforming growth factor-betas (TGF-betas) are a still growing superfamily of cytokines with widespread distribution and diverse biological functions. They fall into several subfamilies including the TGF-betas 1, 2, and 3, the bone morphogenetic proteins (BMPs), the growth/differentiation factors (GDFs), activins and inhibins, and the members of the glial cell line-derived neurotrophic factor family. Following a brief description of their general roles and signaling in development, maintenance of homeostasis, and disease, we shall focus on their distribution in the CNS and their involvement in regulating neuron survival and death.

Transforming growth factor-beta1 enhances expression of brain-derived neurotrophic factor and its receptor, TrkB, in neurons cultured from rat cerebral cortex

The effects of transforming growth factor (TGF)-beta1 on expression of brain-derived neurotrophic factor (BDNF) and its high-affinity receptor, TrkB, in neurons cultured from the cerebral cortex of 18-day-old embryonic rats were examined. BDNF mRNA was significantly increased from 24-48 hr after the TGF-beta1 treatment over 20 ng/ml. Accumulation of BDNF protein in the culture medium was also potentiated by TGF-beta1, although the intracellular content of BDNF was nearly unchanged. The enhancement of BDNF mRNA expression was suppressed by the co-presence of decorin, a small TGF-beta-binding proteoglycan that inhibits the biological activities of TGF-betas. mRNA expression of full-length TrkB, the bioactive high-affinity receptor for BDNF, was also upregulated after treatment with TGF-beta1. These observations suggest that: 1) TGF-beta1 potentiates BDNF/TrkB autocrine or local paracrine system; and 2) the neurotrophic activity of TGF-beta1 is partly responsible for the BDNF induced by TGF-beta1 itself. To test this latter possibility, we examined the neuronal survival activity of TGF-beta1 with or without K252a, a selective inhibitor of Trk family tyrosine kinases. TGF-beta1 significantly enhanced neuronal survival, but the co-presence of K252a completely suppressed the activity, demonstrating the involvement of Trk receptor signaling in TGF-beta1-mediated neuronal survival in cultured rat cortical neurons. These results seem to be in line with recent findings by other investigators that some neurotrophic factors including BDNF require TGF-betas as a cofactor to exert their neurotrophic activities.

Growth and neurotrophic factors regulating development and maintenance of sympathetic preganglionic neurons

The functional anatomy of sympathetic preganglionic neurons is described at molecular, cellular, and system levels. Preganglionic sympathetic neurons located in the intermediolateral column of the spinal cord connect the central nervous system with peripheral sympathetic ganglia and chromaffin cells inside and outside the adrenal gland. Current knowledge is reviewed of the development of these neurons, which share their origin with progenitor cells, giving rise to somatic motoneurons in the ventral horn. Their connectivities, transmitters involved, and growth factor receptors are described. Finally, we review the distribution and functions of trophic molecules that may have relevance for development and maintenance of preganglionic sympathetic neurons.

Fibroblast growth factor-5 is expressed in Schwann cells and is not essential for motoneurone survival

Fibroblast growth factor-5 (FGF-5) is a putative target-derived survival factor for motoneurones as it is concentrated in the synaptic portions of skeletal muscles and because it promotes the survival of embryonic motoneurones in vitro. A variety of experimental approaches have been used to examine this possibility. The expression of FGF-5 in the neuromuscular system was analysed using the reverse transcription-polymerase chain reaction (RT-PCR). Both splice variants of FGF-5 were detected in adult rat skeletal muscle, sciatic nerve, and spinal cord. The expression of FGF-5 in skeletal muscle was up-regulated after denervation. At first sight this appears to be consistent with FGF-5 being a target-derived factor. However, FGF-5 protein was detected in Schwann cells, macrophages, vascular smooth muscle and endothelial cells, but not in muscle fibres. The absence of FGF-5 in muscle fibres was confirmed by RT-PCR examination of isolated muscle fibres. Furthermore, FGF-5 protein was also not detected in denervated fibres, as would be expected for a neuronal survival factor. Denervation did however lead to up-regulation of FGF-5 in the Schwann cells of the distal nerve trunk. This may indicate that FGF-5 is either an autocrine regulator of Schwann cells or a Schwann cell-derived neurotrophic factor. The latter appears not to be the case for two reasons. First, the double-ligation technique was used to show that endogenous FGF-5 is not transported in motor axons. Second, stereological estimates of the number of motoneurones in an FGF-5 null mutant (Angora) mouse failed to reveal any loss of motoneurones. Collectively these experiments suggest that FGF-5 is not a physiological regulator of motoneurones, and therefore raise the possibility that it is an autocrine regulator of Schwann cells.

Increased expression of transforming growth factor-beta after cerebral ischemia in the baboon: an endogenous marker of neuronal stress?

There has been an increasing interest in recent years in the evaluation of the neuronal and glial responses to ischemic insult. Some cytokines, including transforming growth factor-beta (TGF-beta), that are overexpressed after experimental stroke in rodents are thought to be implicated in the neuronal processes that lead to necrosis. Thus, such cytokines could predict tissue fate after stroke in humans, although data are currently sparse for gyrencephalic species. The current study addressed the expression pattern of TGF-beta1 in a nonhuman primate model of middle cerebral artery occlusion. Focal permanent ischemia was induced for 1 or 7 days in 6 baboons and the following investigations were undertaken: cerebral oxygen metabolism (CMRO2) positron emission tomography studies, magnetic resonance imaging, postmortem histology, and reverse transcription-polymerase chain reaction. The aim of the current study was to correlate the expression of TGF-beta1 to the underlying metabolic and histologic state of the threatened cerebral parenchyma. The authors evidenced increased TGF-beta1 mRNA levels (up to 25-fold) in those regions displaying a moderate (20% to 49%) reduction in CMRO2. The current findings suggest that the greatly enhanced expression of TGF-beta1 in the penumbral zones that surround tissue destined to infarction may represent a robust index of potentially salvageable brain. The current investigation, in the nonhuman primate, strengthens the authors' hypothesis, derived from rodent models, that TGF-beta1 may be involved in the physiopathology of human stroke.

Transforming growth factor-beta and ciliary neurotrophic factor synergistically induce vasoactive intestinal peptide gene expression through the cooperation of Smad, STAT, and AP-1 sites

The cytokine ciliary neurotrophic factor (CNTF) and transforming growth factor-beta (TGF-beta) both induce transcription of the vasoactive intestinal peptide (VIP) gene through a 180-base pair cytokine response element (CyRE) in the VIP promoter. While CNTF induces STAT and AP-1 proteins to bind to cognate sites in the VIP CyRE, the mechanism through which TGF-beta acts to induce VIP gene transcription is not known. Here we show that Smad3 and Smad4 proteins can bind to two distinct sites within the VIP CyRE. These sites are absolutely required for the induction of VIP CyRE transcription by TGF-beta. TGF-beta induces endogenous Smad-containing complexes to bind to these sites in human neuroblastoma cells. CNTF and TGF-beta synergize to induce VIP mRNA expression and transcription through the VIP CyRE. This synergy is dependent on the Smad, STAT, and AP-1 sites, suggesting that these two independent cytokine pathways synergize through the cooperation of pathway-specific transcription factors binding to distinct sites within the VIP CyRE.

Lack of neurotrophin-4 causes selective structural and chemical deficits in sympathetic ganglia and their preganglionic innervation

Neurotrophin-4 (NT-4) is perhaps the still most enigmatic member of the neurotrophin family. We show here that NT-4 is expressed in neurons of paravertebral and prevertebral sympathetic ganglia, i.e., the superior cervical (SCG), stellate (SG), and celiac (CG) ganglion. Mice deficient for NT-4 showed a significant reduction (20-30%) of preganglionic sympathetic neurons in the intermediolateral column (IML) of the thoracic spinal cord. In contrast, neuron numbers in the SCG, SG, and CG were unchanged. Numbers of axons in the thoracic sympathetic trunk (TST) connecting the SG with lower paravertebral ganglia were also reduced, whereas axon numbers in the cervical sympathetic trunk (CST) were unaltered. Axon losses in the TST were paralleled by losses of synaptic terminals on SG neurons visualized by electron microscopy. Furthermore, immunoreactivity for the synaptic vesicle antigen SV2 was clearly reduced in the SG and CG. Levels of catecholamines and tyrosine hydroxylase immunoreactivity were dramatically reduced in the SG and the CG but not in the SCG. Despite this severe phenotype in the sympathetic system, blood pressure levels were not reduced and displayed a pattern more typical of deficits in baroreceptor afferents. Numbers of IML neurons were unaltered at postnatal day 4, suggesting a postnatal requirement for their maintenance. In light of these and previous data, we hypothesize that NT-4 provided by postganglionic sympathetic neurons is required for establishing and/or maintaining synapses of IML neurons on postganglionic cells. Impairment of synaptic connectivity may consequently reduce impulse flow, causing a reduction in transmitter synthesis in postganglionic neurons.

TGF-beta 2 attenuates the injury-induced death of mature motoneurons

The distributions of transforming growth factor-betas (TGF-betas) and their receptors suggest that the TGF-betas regulate motoneuron survival. This hypothesis was tested by avulsing the hypoglossal nerve of adult rats and perfusing either TGF-beta 2 or vehicle adjacent to the hypoglossal nucleus. By 4 weeks, half of the avulsed motoneurons had died. Infusion of 6 ng of TGF-beta 2 adjacent to the avulsed motor nucleus caused a significant attenuation of this death. This dose of TGF-beta 2 is low compared to that used with GDNF or BDNF in previous studies of avulsed motoneurons, indicating that TGF-beta 2 may be one of the more potent survival factors for adult motoneurons. TGF-beta 2 was, however, unable to prevent or reduce the axotomy-induced down regulation of choline acetyltransferase. Other motoneuron survival factors also have a narrow-spectrum of actions, suggesting that the homeostasis of motoneurons is regulated by a cocktail of growth factors with distinct but partially overlapping actions.

Molecular cues for the development of adrenal chromaffin cells and their preganglionic innervation

Based on recent evidence from in vitro and gene knockout/insertion studies, this short review summarizes the molecular scenario underlying the development of adrenal chromaffin cells and their preganglionic innervation. During migration of neural crest cells from the dorsal surface of the neural tube to their destinations in the sympathetic primordia and adrenal glands, precursors of the so-called sympathoadrenal (SA) cell lineage are exposed to signals from the notochord and ventral neural tube probably including the protein, Sonic hedgehog. These, and signals in the region of the dorsal aorta (members of the family of bone morphogentic proteins), where SA progenitor cells subsequently assemble, are essential for the induction of the adrenergic phenotype. SA progenitor cells subsequently differentiate into paravertebral and prevertebral sympathetic neurones, intra- and extra-adrenal chromaffin cells and intermediate SIF (small intensely fluorescent) cells. Based on in vitro studies with isolated SA and chromaffin progenitor cells, glucocortiocids have been claimed as essential for suppressing neuronal commitment and for channelling SA cells towards the chromaffin phenotype. However, mice deficient for a functional glucocorticoid receptor possess the full complement of adrenal chromaffin cells at birth, suggesting that signals other than glucocorticoid hormones may be important in triggering chromaffin cell differentiation. The cholinergic neurones that are preganglionic to adrenal chromaffin cells have their cell bodies located in the intermediolateral column (IML) of the spinal cord. For their normal development, these neurones require signals from the adrenal medulla, which include neurotrophin-4, a major neurotrophic factor of adrenal chromaffin cells. Taken together, these data provide a more complete picture of molecular signalling in the development of one of the most important neuroendocrine tissues in vertebrates.

Transforming growth factor-beta 2 is anterogradely and retrogradely transported in motoneurons and up-regulated after nerve injury

The survival of motoneurons is dependent on them receiving continual trophic support from muscle fibres and various other cell types. Numerous putative survival factors have been identified and a set of criteria established by which these candidates can be assessed. These criteria include the need for the factor and its receptors to be in appropriate locations and for the factor or its second message to be retrogradely transported. In this paper, we demonstrate that a multifunctional cytokine, transforming growth factor-beta 2, appears to meet these criteria. The locations of the transforming growth factor-beta 2 and its receptors in the neuromuscular system were determined by reverse transcriptase-polymerase chain reaction and immunohistochemistry. Motoneurons were shown to synthesize the three proteins involved in transforming growth factor-beta 2 signalling (types I and II transforming growth factor-beta receptor and betaglycan) and to transport them anterogradely, where they were inserted into the axonal membrane and nerve terminal. Transforming growth factor-beta 2 was detected in the synaptic portions of muscle fibres, motoneurons and in injured nerves, indicating that motoneurons may be exposed to multiple and potentially redundant sources of transforming growth factor-beta 2. Double-ligation experiments were used to demonstrate that motoneurons transport transforming growth factor-beta 2 up and down their axons. The anterograde transport of both transforming growth factor-beta 2 and its receptors, coupled with the fact that most of a motoneuron's mitochondria are located in the axon, raises the issue of whether the repression of the initiation of apoptosis is restricted to the cell body or occurs along the entire length of a neuron.

Glial cell line-derived neurotrophic factor rescues target-deprived sympathetic spinal cord neurons but requires transforming growth factor-beta as cofactor in vivo

Glial cell line-derived neurotrophic factor (GDNF) is a potent neurotrophic factor for several populations of CNS and peripheral neurons. Synthesis and storage of GDNF by the neuron-like adrenal medullary cells suggest roles in adrenal functions and/or in the maintenance of spinal cord neurons that innervate the adrenal medulla. We show that unilateral adrenomedullectomy causes degeneration of all sympathetic preganglionic neurons within the intermediolateral column (IML) of spinal cord segments T7-T10 that project to the adrenal medulla. In situ hybridization revealed that IML neurons express the glycosylphosphatidylinositol-linked alpha receptor 1 and c-Ret receptors, which are essential for GDNF signaling. IML neurons also display immunoreactivity for transforming growth factor-beta (TGF-beta) receptor II. Administration of GDNF (recombinant human, 1 microg) in Gelfoam implanted into the medullectomized adrenal gland rescued all Fluoro-Gold-labeled preganglionic neurons projecting to the adrenal medulla after four weeks. Cytochrome c applied as a control protein was not effective. The protective effect of GDNF was prevented by co-administration to the Gelfoam of neutralizing antibodies recognizing all three TGF-beta isoforms but not GDNF. This suggests that the presence of endogenous TGF-beta was essential for permitting a neurotrophic effect of GDNF. Our data indicate that GDNF has a capacity to protect a population of autonomic spinal cord neurons from target-deprived cell death. Furthermore, our results demonstrate for the first time that the previously reported requirement of TGF-beta for permitting trophic actions of GDNF in vitro (Kreiglstein et al., 1998) also applies to the in vivo situation.

Glial cell line-derived neurotrophic factor requires transforming growth factor-beta for exerting its full neurotrophic potential on peripheral and CNS neurons

Numerous studies have suggested that glial cell line-derived neurotrophic factor (GDNF) is a potent neurotrophic molecule. We show now on a variety of cultured neurons including peripheral autonomic, sensory, and CNS dopaminergic neurons that GDNF is not trophically active unless supplemented with TGF-beta. Immunoneutralization of endogenous TGF-beta provided by serum or TGF-beta-secreting cells, as e.g., neurons, in culture abolishes the neurotrophic effect of GDNF. The dose-response relationship required for the synergistic effect of GDNF and TGF-beta identifies 60 pg/ml of either factor combined with 2 ng/ml of the other factor as the EC50. GDNF/TGF-beta signaling employs activation of phosphatidylinositol-3 (PI-3) kinase as an intermediate step as shown by the effect of the specific PI-3 kinase inhibitor wortmannin. The synergistic action of GDNF and TGF-beta involves protection of glycosylphosphatidylinositol (GPI)-linked receptors as shown by the restoration of their trophic effects after phosphatidylinositol-specific phospholipase C-mediated hydrolysis of GPI-anchored GDNF family receptor alpha. The biological significance of the trophic synergism of GDNF and TGF-beta is underscored by colocalization of the receptors for TGF-beta and GDNF on all investigated GDNF-responsive neuron populations in vivo. Moreover, the in vivo relevance of the TGF-beta/GDNF synergism is highlighted by the co-storage of TGF-beta and GDNF in secretory vesicles of a model neuron, the chromaffin cell, and their activity-dependent release. Our results broaden the definition of a neurotrophic factor by incorporating the possibility that two factors that lack a neurotrophic activity when acting separately become neurotrophic when acting in concert. Moreover, our data may have a substantial impact on the treatment of neurodegenerative diseases.

The non-synaptic expression of transforming growth factor-beta 2 is neurally regulated and varies between skeletal muscle fibre types

In adult skeletal muscles, transforming growth factor-beta 2 is restricted to the postsynaptic domain of the neuromuscular junction. The various putative functions of this transforming growth factor-beta 2 predict different patterns of transforming growth factor-beta 2 expression in denervated muscles. We therefore denervated rat tibialis anterior, extensor digitorum longus and soleus muscles and examined the expression of transforming growth factor-beta 2 using semi-quantitative reverse-transcription polymerase chain reaction and immunohistochemistry. Denervation up-regulated transforming growth factor-beta 2 expression extrasynaptically with little or no effect on synaptic expression. The up-regulation was detectable by one day, had become significant by three days and remained elevated for at least two weeks. This proves that the transforming growth factor-beta 2 associated with the neuromuscular junction is not under neural control and is consistent with transforming growth factor-beta 2 being a trophic factor for motoneurons. This pattern of transforming growth factor-beta 2 expression is similar to that described for other proteins associated with the neuromuscular junction, notably the acetylcholine receptor subunit genes. However, in contrast to the acetylcholine receptor subunit genes, the extent of up-regulation of transforming growth factor-beta 2 varied between fibre types, with the glycolytic IIB fibres being less affected than other fibre types.

TrkB and neurotrophin-4 are important for development and maintenance of sympathetic preganglionic neurons innervating the adrenal medulla

The adrenal medulla receives its major presynaptic input from sympathetic preganglionic neurons that are located in the intermediolateral (IML) column of the thoracic spinal cord. The neurotrophic factor concept would predict that these IML neurons receive trophic support from chromaffin cells in the adrenal medulla. We show here that adrenal chromaffin cells in the adult rat store neurotrophin (NT)-4, but do not synthesize or store detectable levels of BDNF or NT-3, respectively. Preganglionic neurons to the adrenal medulla identified by retrograde tracing with fast blue or Fluoro-Gold (FG) express TrkB mRNA. After unilateral destruction of the adrenal medulla, 24% of IML neurons, i.e., all neurons that are preganglionic to the adrenal medulla in spinal cord segments T7-T10, disappear. Administration of NT-4 in gelfoams (6 microgram) implanted into the medullectomized adrenal gland rescued all preganglionic neurons as evidenced by their presence after 4 weeks. NT-3 and cytochrome C were not effective. The action of NT-4 is accompanied by massive sprouting of axons in the vicinity of the NT-4 source as monitored by staining for acetylcholinesterase and synaptophysin immunoreactivity, suggesting that NT-4 may enlarge the terminal field of preganglionic nerves and enhance their access to trophic factors. Analysis of TrkB-deficient mice revealed degenerative changes in axon terminals on chromaffin cells. Furthermore, numbers of FG-labeled IML neurons in spinal cord segments T7-T10 of NT-4-deficient adult mice were significantly reduced. These data are consistent with the notion that NT-4 from chromaffin cells operates through TrkB receptors to regulate development and maintenance of the preganglionic innervation of the adrenal medulla.

Reduced acetylcholinesterase (AChE) activity in adrenal medulla and loss of sympathetic preganglionic neurons in TrkA-deficient, but not TrkB-deficient, mice

TrkA high-affinity receptors are essential for the normal development of sympathetic paravertebral neurons and subpopulations of sensory neurons. Paravertebral sympathetic neurons and chromaffin cells of the adrenal medulla share an ontogenetic origin, responsiveness to NGF, and expression of TrkA. Which aspects of development of the adrenal medulla might be regulated via TrkA are unknown. In the present study we demonstrate that mice deficient for TrkA, but not the neurotrophin receptor TrkB, show an early postnatal progressive reduction of acetylcholinesterase (AChE) enzymatic activity in the adrenal medulla and in preganglionic sympathetic neurons within the thoracic spinal cord, which are also significantly reduced in number. Quantitative determinations of specific AChE activity revealed a massive decrease (-62%) in the adrenal gland and a lesser, but still pronounced, reduction in the thoracic spinal cord (-40%). Other markers of the adrenal medulla and its innervation, including various neuropeptides, chromogranin B, secretogranin II, amine transporters, the catecholamine-synthesizing enzymes tyrosine hydroxylase and PNMT, synaptophysin, and L1, essentially were unchanged. Interestingly, AChE immunoreactivity appeared unaltered, too. Preganglionic sympathetic neurons, in contrast to adrenal medullary cells, do not express TrkA. They must, therefore, be affected indirectly by the TrkA knock-out, possibly via a retrograde signal from chromaffin cells. Our results suggest that signaling via TrkA, but not TrkB, may be involved in the postnatal regulation of AChE activity in the adrenal medulla and its preganglionic nerves.

Localization, differential expression and retrograde axonal transport suggest physiological role of FGF-2 in spinal autonomic neurons of the rat

Fibroblast growth factor-2 (FGF-2) has marked pharmacological neurotrophic effects on lesioned spinal autonomic neurons following target removal of the adrenal medulla, yet expression and axonal transport in autonomic neurons remain to be shown. We show here FGF-2 and FGF receptor type 1 (FGFR1) protein and mRNA expression in preganglionic intermediolateral neurons of the rat thoracic spinal cord. While immunoreactivity of both FGF-2 and FGFR1 co-localize to intermediolateral neurons, mRNA transcripts of FGFR1, but not of FGF-2, are detectable in intermediolateral preparations by RNase protection analysis, suggesting protein translocation in vivo. Unilateral microinjection of 125iodinated FGF-2 into the adrenal medulla (a major target of intermediolateral neurons) results in significant accumulation of specific radioactivity in thoracic spinal cord tissue, including the intermediolateral neurons, and the ipsilateral splanchnic nerve. Emulsion autoradiography demonstrated labelling over ipsilateral intermediolateral neurons only. Neuronal co-localization of FGF-2/FGFR1 protein, differential mRNA expression, specific retrograde axonal transport and the known neurotrophic actions in vivo, strongly suggest unique physiological roles of FGF-2 in the autonomic nervous system.

Expression of neurotrophins and their receptors in the developing and adult rat adrenal gland

We have studied the postnatal expression of neurotrophins, their cognate high-affinity trk receptors and the low-affinity NGF receptor (p75LNGFR) in the rat adrenal gland using RT-PCR. Neurotrophin mRNAs were detectable during the whole postnatal period. Strongest signals were obtained for BDNF and NT4/5. Expression of trkA, trkB, trkC and p75LNGFR was found at all ages studied. Signals for trkA were highest in the adult adrenal medulla, whereas signals for p75LNGFR were highest in the adult adrenal cortex. Cur data suggest still largely enigmatic roles for neurotrophins in functions of the adrenal medulla and possibly also the cortex.

TGF-beta-induced apoptosis of cerebellar granule neurons is prevented by depolarization

The regulation of programmed cell death in the developing nervous system involves target-derived survival factors, afferent synaptic activity, and hormone- and cytokine-dependent signaling. Cultured immature cerebellar granule neurons die by apoptosis within several days in vitro unless maintained in depolarizing (high) concentrations of potassium (25 mM K+). Here we report that transforming growth factors (TGF)-beta1, -beta2, and -beta3 accelerate apoptosis of these neurons when maintained in physiological (low) K+ medium (5mM K+) as assessed by measures of viability, quantitative DNA fragmentation, and nuclear morphology. TGF-beta-induced apoptosis of these neurons is not blocked by CNTF and LIF, cytokines that enhance neuronal survival when applied alone, or by IGF-I, which prevents apoptosis upon potassium withdrawal. In contrast, neurons that differentiate in high K+ medium for several days in vitro acquire resistance to TGF-beta-mediated cell death. Granule neurons maintained in either low or high K+ medium produce latent, but not bioactive, TGF-beta1 and -beta2. Because neutralizing TGF-beta antibodies fail to augment survival of low K+ neurons, the cerebellar neurons are apparently unable to activate latent TGF-beta. Thus, apoptosis of low K+ neurons is not attributable to endogenous production of TGF-beta. Taken together, our data suggest that TGF-beta may limit the expansion of postmitotic neuronal precursor populations by promoting their apoptosis but may support survival of those neurons that have maturated, differentiated, and established supportive synaptic connectivity.