Bone morphogenetic proteins: neurotrophic roles for midbrain dopaminergic neurons and implications of astroglial cells

Patterning of dopaminergic neurotransmitter identity among Caenorhabditis elegans ray sensory neurons by a TGFbeta family signaling pathway and a Hox gene

We have investigated the mechanism that patterns dopamine expression among Caenorhabditis elegans male ray sensory neurons. Dopamine is expressed by the A-type sensory neurons in three out of the nine pairs of rays. We used expression of a tyrosine hydroxylase reporter transgene as well as direct assays for dopamine to study the genetic requirements for adoption of the dopaminergic cell fate. In loss-of-function mutants affecting a TGFbeta family signaling pathway, the DBL-1 pathway, dopaminergic identity is adopted irregularly by a wider subset of the rays. Ectopic expression of the pathway ligand, DBL-1, from a heat-shock-driven transgene results in adoption of dopaminergic identity by rays 3–9; rays 1 and 2 are refractory. The rays are therefore prepatterned with respect to their competence to be induced by a DBL-1 pathway signal. Temperature-shift experiments with a temperature-sensitive type II receptor mutant, as well as heat-shock induction experiments, show that the DBL-1 pathway acts during an interval that extends from two to one cell generation before ray neurons are born and begin to differentiate. In a mutant of the AbdominalB class Hox gene egl-5, rays that normally express EGL-5 do not adopt dopaminergic fate and cannot be induced to express DA when DBL-1 is provided by a heat-shock-driven dbl-1 transgene. Therefore, egl-5 is required for making a subset of rays capable of adopting dopaminergic identity, while the function of the DBL-1 pathway signal is to pattern the realization of this capability.

Exposure of foetal mesencephalic cells to bone morphogenetic protein-2 enhances the survival of dopaminergic neurones in rat striatal grafts

The transplantation of foetal mesencephalic cells (FMC) into the brain striatal system is an emerging treatment for Parkinson's disease, despite of the relatively poor survival of implanted cells. The ability of neurotrophic factors to regulate neurone survival and differentiation suggests they could be used to enhance the success of cerebral grafts. We analyzed the effect of pre-treatment of FMC suspensions with bone morphogenetic protein-2 (BMP-2) (50 ng/ml) prior to grafting into the striatum of 6-hydroxydopamine lesioned rats. The viability of a FMC suspension was enhanced in vitro by BMP-2. Four weeks after transplantation, the number of dopaminergic neurones was higher and their morphology more developed in grafts pre-treated with BMP-2, compared with non-pre-treated grafts and rats showed a significant reduction in the turning behaviour test. Thus, the pre-treatment of FMCs with BMP-2 should be considered, together with other neurotrophic factors, as a procedure for transplantational treatment of Parkinson's disease.

Characterization of bone morphogenetic protein family members as neurotrophic factors for cultured sensory neurons

The bone morphogenetic proteins have been implicated in several inductive processes throughout vertebrate development including nervous system patterning. Recently, these proteins have also emerged as candidates for regulating survival of mesencephalic dopaminergic and sympathetic neurons. Interestingly, we have found that several bone morphogenetic proteins can be detected in developing embryonic day 14 rat dorsal root ganglia by means of reverse transcription-polymerase chain reaction and immunocytochemistry. To further elucidate their potential role during the period of ontogenetic neuron death, serum-free cultures of dorsal root sensory neurons from developing chick and rat embryos were treated with distinct bone morphogenetic proteins with or without simultaneous addition of other "established" neurotrophic factors. Our results show that bone morphogenetic proteins exert survival promoting effects on their own, and that they can positively modulate the effects of neurotrophins on sensory neurons. In particular, growth/differentiation factor-5, bone morphogenetic protein-2, -4, -7 and -12 significantly increased the survival promoting effects of neurotrophin-3 and nerve growth factor on cultured dorsal root ganglion neurons. These results fit well into the current concept that neurotrophic factors may act synergistically in ensuring neuronal survival. Moreover, these data suggest potential instructive interactions of bone morphogentic proteins and neurotrophins during sensory neuron development. Finally, the documented neurotrophic capacity of bone morphogenetic protein family members may have potential relevance for the treatment of peripheral neuropathies.

Differential regulation of distinct phenotypic features of serotonergic neurons by bone morphogenetic proteins

Bone morphogenetic proteins (BMPs), growth and differentiation factor 5 (GDF5) and glial cell line-derived neurotrophic factor (GDNF) are members of the transforming growth factor-beta superfamily that have been implicated in tissue growth and differentiation. Several BMPs are expressed in embryonic and adult brain. We show now that BMP-2, -6 and -7 and GDF5 are expressed in the embryonic rat hindbrain raphe. To start to define roles for BMPs in the regulation of serotonergic (5-HT) neuron development, we have generated serum-free cultures of 5-HT neurons isolated from the embryonic (E14) rat raphe. Addition of saturating concentrations (10 ng/mL) of BMP-6 and GDF5 augmented numbers of tryptophan hydroxylase (TpOH) -immunoreactive neurons and cells specifically taking up 5, 7-dihydroxytryptamine (5,7-DHT) by about two-fold. Alterations in 5-HT neuron numbers were due to the induction of serotonergic markers rather than increased survival, as shown by the efficacy of short-term treatments. Importantly, BMP-7 selectively induced 5, 7-DHT uptake without affecting TpOH immunoreactivity. BMP-6 and -7 also promoted DNA synthesis and increased numbers of cells immunoreactive for vimentin and glial fibrillary acidic protein (GFAP). Pharmacological suppression of cell proliferation or glial development abolished the induction of serotonergic markers by BMP-6 and -7, suggesting that BMPs act indirectly by stimulating synthesis or release of glial-derived serotonergic differentiation factors. Receptor bodies for the neurotrophin receptor trkB, but not trkC, abolished the BMP-mediated effects on serotonergic development, suggesting that the glia-derived factor is probably brain-derived neurotrophic factor (BDNF) or neurotrophin-4. In support of this notion, we detected increased levels of BDNF mRNA in BMP-treated cultures. Together, these data suggest both distinct and overlapping roles of several BMPs in regulating 5-HT neuron development.

Bone morphogenetic protein-2 promotes survival and differentiation of striatal GABAergic neurons in the absence of glial cell proliferation

We examined the potential neurotrophic effects of bone morphogenetic protein (BMP)-2 on the survival and differentiation of neurons cultured from the rat developing striatum at embryonic day 16, a period during which the mRNAs for BMP-2 and its receptor subunits (types IA, IB, and II) were detected. BMP-2 exerted potent activity to promote the survival of striatal neurons and increased the number of surviving microtubule-associated protein-2-positive cells by 2.4-fold as compared with the control cultures after 4 days in vitro. Although basic fibroblast growth factor (bFGF) also showed relatively high activity to promote the survival of striatal neurons, transforming growth factor-beta1, -beta2, and -beta3, glial cell line-derived neurotrophic factor, or brain-derived neurotrophic factor promoted their survival weakly. Striatal neurons cultured in the presence of BMP-2 or bFGF possessed extensive neurite outgrowths, the majority of which were GABA-immunoreactive. Inhibition of glial cell proliferation by 5-fluorodeoxyuridine did not affect the capacity of BMP-2 to promote the survival of striatal GABAergic neurons. In contrast, the ability of bFGF to promote the survival of striatal neurons was inhibited significantly by the treatment of cells with 5-fluorodeoxyuridine. All these results suggest that BMP-2 exerts potent neurotrophic effects on the striatal GABAergic neurons in a glial cell-independent manner.

Therapeutic potential of nerve growth factors in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative syndrome which primarily affects dopamine-producing neurons of the substantia nigra, resulting in poverty and slowness of movement, instability of gait and posture, and tremor at rest in individuals with the disease. While symptoms of the disease can be effectively managed for several years with available drugs, the syndrome is progressive and the efficacy of standard drugs wanes with time. One experimental approach to therapy is to use natural and synthetic molecules which promote survival and growth of dopaminergic neurons, so-called 'neurotrophic factors', to stabilise the diminishing population of dopaminergic neurons and stimulate compensation and growth in these cells. In this review, we examine the available evidence on 29 molecules with neurotrophic properties for dopaminergic neurons. The properties of these molecules provide ample reasons for optimism that a neurotrophic strategy can be developed that would provide a significant treatment option for patients with PD. While the search continues for even more specific, potent and long lasting agents, the single greatest challenge is the development of techniques for targeted delivery of these molecules.

Osteogenic protein-1 protects against cerebral infarction induced by MCA ligation in adult rats

BACKGROUND AND PURPOSE

Osteogenic protein-1 (OP1) not only possesses trophic activity on bone tissue but also influences neuronal survival and differentiation in vitro. Specific receptors for OP1 are present in brain and spinal cord and can be upregulated during cerebral contusion. OP1 is a member of the transforming growth factor-beta superfamily, several of whose members possess neuroprotective activity. In this study, the neuroprotective effect of OP1 in cerebral ischemia was evaluated in adult animals.

METHODS

Adult male Sprague-Dawley rats were anesthetized with chloral hydrate. OP1 or vehicle was administered intracortically or intracerebroventricularly to the rats. Thirty minutes, 24 hours, or 72 hours after OP1 injection, the right middle cerebral artery (MCA) was ligated for 90 minutes. Twenty-four hours after reperfusion, animals were tested for motor behavior. The animals were subsequently anesthetized with urethane and perfused intracardially with saline. Brain tissue was removed, sliced, and incubated with 2% triphenyltetrazolium chloride to localize the area of infarction.

RESULTS

Only animals pretreated with OP1 24 hours before MCA ligation showed a reduction in motor impairment. OP1, given 30 minutes or 72 hours before MCA ligation, did not reduce cortical infarction. In contrast, pretreatment with OP1 24 hours before MCA ligation significantly attenuated the volume of infarction in the cortex, in agreement with the behavioral findings.

CONCLUSIONS

Intracerebral administration of OP1 24 hours before MCA ligation reduces ischemia-induced injury in the cerebral cortex.

Cell lineage in the developing neural tube

Acquisition of cell type specific properties in the spinal cord is a process of sequential restriction in developmental potential. A multipotent stem cell of the nervous system, the neuroepithelial cell, generates central nervous system and peripheral nervous system derivatives via the generation of intermediate lineage restricted precursors that differ from each other and from neuroepithelial cells. Intermediate lineage restricted neuronal and glial precursors termed neuronal restricted precursors and glial restricted precursors, respectively, have been identified. Differentiation is influenced by extrinsic environmental signals that are stage and cell type specific. Analysis in multiple species illustrates similarities between chick, rat, mouse, and human cell differentiation. The utility of obtaining these precursor cell types for gene discovery, drug screening, and therapeutic applications is discussed.Key words: stem cells, oligodendrocytes, astrocytes, neurons, spinal cord.

A role for tectal midline glia in the unilateral containment of retinocollicular axons

Retinal fibers approach close to the tectal midline but do not encroach on the other side. Just before the entry of retinal axons into the superior colliculus (SC), a group of radial glia differentiates at the tectal midline; the spatiotemporal deployment of these cells points to their involvement in the unilateral containment of retinotectal axons. To test for such a barrier function of the tectal midline cells, we used two lesion paradigms for disrupting their radial processes in the neonatal hamster: (1) a heat lesion was used to destroy the superficial layers of the right SC, including the midline region, and (2) a horizontally oriented hooked wire was inserted from the lateral edge of the left SC toward the midline and was used to undercut the midline cells, leaving intact the retinorecipient layers in the right SC. In both cases, the left SC was denervated by removing its contralateral retinal input. Animals were killed 12 hr to 2 weeks later, after intraocular injections of anterograde tracers to label the axons from the remaining eye. Both lesions resulted in degeneration of the distal processes of the tectal raphe glia and in an abnormal crossing of the tectal midline by retinal axons, leading to an innervation of the opposite ("wrong") tectum. The crossover occurred only where glial cell attachments were disrupted. These results document that during normal development, the integrity of the midline septum is critical in compartmentalizing retinal axons and in retaining the laterality of the retinotectal projection.

Neurturin exerts potent actions on survival and function of midbrain dopaminergic neurons

Glial cell line-derived neurotrophic factor (GDNF) exhibits potent effects on survival and function of midbrain dopaminergic (DA) neurons in a variety of models. Although other growth factors expressed in the vicinity of developing DA neurons have been reported to support survival of DA neurons in vitro, to date none of these factors duplicate the potent and selective actions of GDNF in vivo. We report here that neurturin (NTN), a homolog of GDNF, is expressed in the nigrostriatal system, and that NTN exerts potent effects on survival and function of midbrain DA neurons. Our findings indicate that NTN mRNA is sequentially expressed in the ventral midbrain and striatum during development and that NTN exhibits survival-promoting actions on both developing and mature DA neurons. In vitro, NTN supports survival of embryonic DA neurons, and in vivo, direct injection of NTN into the substantia nigra protects mature DA neurons from cell death induced by 6-OHDA. Furthermore, administration of NTN into the striatum of intact adult animals induces behavioral and biochemical changes associated with functional upregulation of nigral DA neurons. The similarity in potency and efficacy of NTN and GDNF on DA neurons in several paradigms stands in contrast to the differential distribution of the receptor components GDNF Family Receptor alpha1 (GFRalpha1) and GFRalpha2 within the ventral mesencephalon. These results suggest that NTN is an endogenous trophic factor for midbrain DA neurons and point to the possibility that GDNF and NTN may exert redundant trophic influences on nigral DA neurons acting via a receptor complex that includes GFRalpha1.

Characterization of growth/differentiation factor 5 (GDF-5) as a neurotrophic factor for cultured neurons from chicken dorsal root ganglia

Growth/differentiation factor-5 (GDF-5), a morphogenetic protein, has previously been shown to act as a neurotrophic factor for midbrain dopaminergic neurons. To further elucidate the neurotrophic potential of GDF-5, serum free cultures of dorsal root ganglionic (DRG) neurons from developing chick embryos were treated with GDF-5 with or without the simultaneous addition of other trophic factors. Our results show that GDF-5 has a minor promoting effect on its own, but it can enhance the survival promoting effect of neurotrophin-3 (NT-3) and nerve growth factor (NGF) on cultured DRG neurons. Our finding fits well into the concept that neurotrophic factors may act synergistically in ensuring survival of different neuronal populations. The capacity of GDF-5 to reduce the requirement of a subpopulation of sensory neurons for NT-3 may have implications for the treatment of peripheral neuropathies.