Blocked MAP kinase activity selectively enhances neurotrophic growth responses

SP600125 Enhances Temperature-Controlled Repeated Thermal Stimulation-Induced Neurite Outgrowth in PC12-P1F1 Cells

This study evaluated the mechanism of temperature-controlled repeated thermal stimulation (TRTS)-mediated neuronal differentiation. We assessed the effect of SP600125, a c-Jun N-terminal kinase (JNK) inhibitor, on neuronal differentiation of rat PC12-P1F1 cells, which can differentiate into neuron-like cells by exposure to TRTS or neurotrophic factors, including bone morphogenetic protein (BMP) 4. We evaluated neuritogenesis by incubating the cells under conditions of TRTS and/or SP600125. Cotreatment with SP600125 significantly enhanced TRTS-mediated neuritogenesis, whereas that with other selective mitogen-activated protein kinase (MAPK) inhibitors did not-e.g., extracellular signal-regulated kinase (ERK)1/2 inhibitor U0126, and p38 MAPK inhibitor SB203580. We tried to clarify the mechanism of SP600125 action by testing the effect of U0126 and the BMP receptor inhibitor LDN193189 on the SP600125-mediated enhancement of intracellular signaling. SP600125-enhanced TRTS-induced neuritogenesis was significantly inhibited by U0126 or LDN193189. Gene expression analysis revealed that TRTS significantly increased β3-Tubulin, MKK3, and Smad7 gene expressions. Additionally, Smad6 and Smad7 gene expressions were substantially attenuated through SP600125 co-treatment during TRTS. Therefore, SP600125 may partly enhance TRTS-induced neuritogenesis by attenuating the negative feedback loop of BMP signaling. Further investigation of the mechanisms underlying the effect of SP600125 during TRTS-mediated neuritogenesis may contribute to the future development of regenerative neuromedicine.

Chordin and noggin expression in the adult rat trigeminal nuclei

Bone morphogenetic proteins (BMP) exert its biological functions by interacting with membrane bound receptors. However, functions of BMPs are also regulated in the extracellular space by secreted antagonistic regulators, such as chordin and noggin. Although the deep involvement of BMP signaling in the development and functions of the trigeminal nuclei has been postulated, little information is available for its expression in the trigeminal nuclei. We, thus, investigated chordin and noggin expression in the adult rat trigeminal nuclei using immunohistochemistry. Chordin and noggin were intensely expressed throughout the trigeminal nuclei. In addition, interesting differences are observed between chordin expression and noggin expression. For example, chordin prefers dendritic expression than noggin, suggesting that chordin is involved in the regulation of dendritic morphology and synaptic homeostasis. Furthermore, chordin and noggin were differentially expressed in the neuropil of the trigeminal nuclei. Since BMP signaling is known to play a pivotal role to make precise neural network, theses differences might be important to keep precise interneuronal connections by regulating local BMP signaling intensity in each region. Interestingly, we also detected chordin and noggin expression in axons of the trigeminal nerves. These data indicate that chordin and noggin play pivotal roles also in the adult trigeminal system.

A small molecule TrkB/TrkC neurotrophin receptor co-activator with distinctive effects on neuronal survival and process outgrowth

Neurotrophin (NT) receptors are coupled to numerous signaling networks that play critical roles in neuronal survival and plasticity. Several non-peptide small molecule ligands have recently been reported that bind to and activate specific tropomyosin-receptor kinase (Trk) NT receptors, stimulate their downstream signaling, and cause biologic effects similar to, though not completely overlapping, those of the native NT ligands. Here, in silico screening, coupled with low-throughput neuronal survival screening, identified a compound, LM22B-10, that, unlike prior small molecule Trk ligands, binds to and activates TrkB as well as TrkC. LM22B-10 increased cell survival and strongly accelerated neurite outgrowth, superseding the effects of brain-derived neurotrophic factor (BDNF), NT-3 or the two combined. Additionally, unlike the NTs, LM22B-10 supported substantial early neurite outgrowth in the presence of inhibiting glycoproteins. Examination of the mechanisms of these actions suggested contributions of the activation of both Trks and differential interactions with p75(NTR), as well as a requirement for involvement of the Trk extracellular domain. In aged mice, LM22B-10 activated hippocampal and striatal TrkB and TrkC, and their downstream signaling, and increased hippocampal dendritic spine density. Thus, LM22B-10 may constitute a new tool for the study of TrkB and TrkC signaling and their interactions with p75(NTR), and provides groundwork for the development of ligands that stimulate unique combinations of Trk receptors and activity patterns for application to selected neuronal populations and deficits present in various disease states.

BMP5 expression in the adult rat brain

Bone morphogenetic protein-5 (BMP5), a member of the transforming growth factor-β (TGF-β) superfamily, has many effects in several biological events. Although BMP5 expression has been well reported in the early development of the central nervous system (CNS), there is little information about its expression in the adult CNS. Thus, we analyzed BMP5 expression in the adult rat CNS by immunohistochemistry. Abundant BMP5 expression was observed in most neurons, and their dendrites and axons. Furthermore, strong BMP5 expression was also detected in the neuropil of the gray matters with high plasticity, such as the molecular layer of the cerebellum, locus coeruleus, and nucleus of the solitary tract. In addition, we showed BMP5 expression also in astrocytes, ependymal cells and meninges. Our data suggest that BMP5 is widely expressed throughout the adult CNS, and this abundant expression in the adult brain strongly supports the idea that BMP5 plays important roles not only in the developing brain but also in the adult brain.

BMP signaling in axon regeneration

Neuronal competence to re-extend axons and a permissive environment that allows growth cone navigation are two major determinants for successful axon regeneration. Here, we review the roles of bone morphogenetic protein (BMP) signaling in mediating both neuronal and glial injury responses after CNS injury. BMPs can activate a pro-regenerative transcription program in neurons through Smad-mediated canonical pathway, or act locally on cytoskeleton assembly at distal axons via non-canonical pathways. Emerging evidence implicates retrograde BMP signalosomes in connecting the cytoskeletal and nuclear responses. In addition, BMP/Smad signaling modulates neurotrophin-mediated axonal outgrowth, and interacts with the epigenetic machinery to initiate epigenetic reprogramming for axon regeneration. Besides their influences on neurons, BMPs also regulate astrogliosis, inflammatory processes, and neural progenitor cell differentiation at the injury site, all of which can either positively or negatively modify the injury microenvironment. Lastly, an increasing number of BMP signaling partners, sensitizers, and downstream effectors collectively fine-tune the signaling intensity and spatiotemporal dynamics of BMP activity in an integrated signaling network during axon regeneration.

Chordin expression in the adult rat brain

Bone morphogenetic proteins (BMPs) exert its biological functions by interacting with membrane bound receptors. However, functions of BMPs are also regulated in the extracellular space by secreted antagonistic regulators. Chordin is an extracellular BMP antagonist that binds BMP-2, 4, and 7 with high affinity and thus interferes with binding to BMP receptors. Although chordin expression has been well described in the early development of the CNS, little information is available for its expression in the adult CNS. We, thus, investigated chordin expression in the adult rat CNS using immunohistochemistry. Chordin was intensely expressed in most neurons, and their dendrites and axons. In addition, abundant chordin expression was also observed in the neuropil of the gray matters where high plasticity is reported, such as the molecular layer of the cerebellum and the superficial layer of the superior colliculus. Furthermore, we found that astrocytes and ependymal cells also express chordin protein. These data indicate that chordin is more widely expressed throughout the adult CNS than previously reported, and its continued abundant expression in the adult brain strongly supports the idea that chordin plays pivotal roles also in the adult brain.

Ret-dependent and Ret-independent mechanisms of Gfl-induced sensitization

BACKGROUND

The GDNF family ligands (GFLs) are regulators of neurogenic inflammation and pain. We have previously shown that GFLs increase the release of the sensory neuron neuropeptide, calcitonin gene-related peptide (CGRP) from isolated mouse DRG.

RESULTS

Inhibitors of the mitogen-activated protein kinase (MAPK) pathway abolished the enhancement of CGRP release by GDNF. Neurturin-induced enhancement in the stimulated release of CGRP, used as an indication of sensory neuronal sensitization, was abolished by inhibition of the phosphatidylinositol-3 kinase (PI-3K) pathway. Reduction in Ret expression abolished the GDNF-induced sensitization, but did not fully inhibit the increase in stimulus-evoked release of CGRP caused by neurturin or artemin, indicating the presence of Ret-independent GFL-induced signaling in sensory neurons. Integrin β-1 and NCAM are involved in a component of Ret-independent GFL signaling in sensory neurons.

CONCLUSIONS

These data demonstrate the distinct and variable Ret-dependent and Ret-independent signaling mechanisms by which GFLs induce sensitization of sensory neurons. Additionally, there is a clear disconnect between intracellular signaling pathway activation and changes in sensory neuronal function.

BMP2 expression in the adult rat brain

Bone morphogenetic protein-2 (BMP2) is a member of the transforming growth factor-β (TGF-β) superfamily and plays important roles in multiple biological events. Although BMP2 expression has been well described in the early development of the central nervous system (CNS), little information is available on its expression in the adult CNS. We thus investigated BMP2 expression in the adult rat CNS by using immunohistochemistry. Here we show that BMP2 is widely expressed throughout the adult CNS. In addition, besides intense BMP2 expression in almost all neurons, we found BMP2 expression in astrocytes and ependymal cells. Interestingly, we found that the axons of olfactory sensory neurons express BMP2. In addition, in the glomerular layer, BMP2 was very strongly expressed in some glomeruli, whereas the other glomeruli were weakly stained, suggesting that the variations in BMP2 expression level in each glomerus might be cues for each axon to find its adequate target and to keep its identity. Furthermore, we compared the expression patterns of BMP2 and BMP4. Interestingly, BMP4 was preferentially expressed in the dendrites of several neurons, whereas BMP2 was basically not expressed in the dendrites; however, it was detected in the axons. This means that in a single neuron the localizations of BMP2 and BMP4 are differentially regulated. These data indicate that BMP2 is more widely expressed throughout the adult CNS than previously reported, and its continued abundant expression in the adult brain strongly supports the idea that BMP2 also plays pivotal roles in the adult brain.

Bone morphogenetic protein receptor expressions in the adult rat brain

Bone morphogenetic proteins (BMP) are members of the transforming growth factor β (TGF-β) superfamily. BMPs exert its biological functions by interacting with membrane bound receptors belonging to the serine/threonine kinase family including bone morphogenetic protein receptor I (BMPRIA, BMPRIB) and type II (BMPRII). Although BMPR expressions have been well described in the early development of the CNS, little information is available for their expressions in the adult CNS. We, thus, investigated BMPR expressions in the adult rat CNS using immunohistochemistry. Here, we show that BMPRIA, IB and II proteins are widely expressed throughout the adult CNS. Interestingly, we observed that BMPRIA, IB and II proteins are abundantly expressed in many kinds of axons. In addition, we found that BAMRIB-IR was preferentially expressed in dendrites of many neurons throughout the CNS, while BMPRIA was mainly expressed in cell bodies, showing that BMPRIA and BMPRIB are differentially targeted in a single neuron. In addition, besides abundant BMPR expressions in neurons, we exhibited BMPR expressions in astrocytes and ependymal cells. These data indicate that BMPRs are more widely expressed throughout the adult CNS than previously reported, and their continued abundant expressions in the adult brain strongly support the idea that BMPRs play pivotal roles also in the adult brain.

Multiple signaling pathways converge to regulate bone-morphogenetic-protein-dependent glial gene expression

A fundamental problem in developmental neuroscience is understanding how extracellular cues link to complex intracellular signaling pathways to drive stage-specific developmental decisions. During the formation of the mammalian peripheral nervous system, bone morphogenetic proteins (BMPs) promote neuronal differentiation. BMPs also maintain the expression of early glial genes such as GFAP, while blocking the acquisition of a mature, myelinating Schwann cell phenotype. We investigated the BMP-activated signaling pathways that contribute to early glial gene expression to address the question of how specific signaling interactions contribute to cell fate decisions in neural crest lineages. Using a neural-crest-derived cell line that exhibits the characteristics of immature Schwann cells, we found that BMP2 promotes GFAP expression using Smad signaling as well as the phosphoinositide-3 kinase (PI3K) and mitogen-activated protein kinase1/2extracellular signal-regulated kinase- (MEK1/2/ERK) pathways. The GFAP promoter does not contain known Smad consensus sites, suggesting that Smads may act indirectly to promote GFAP expression. We provide evidence that this indirect effect may be mediated via induction of immediate early genes and the transcription factor Sp1 by demonstrating that these transcriptional regulators are induced by BMP2 and contribute to GFAP promoter activity. These findings demonstrate new roles for intracellular kinase pathways in mediating the effects of BMPs during the early stages of glial differentiation and suggest that differential contributions by signaling and transcriptional networks may contribute to the range of effects of BMPs on neuronal and glial development during the formation of the peripheral nervous system.

Casein kinase II contributes to the synergistic effects of BMP7 and BDNF on Smad 1/5/8 phosphorylation in septal neurons under hypoglycemic stress

The combination of bone morphogenetic protein 7 (BMP7) and neurotrophins (e.g. brain-derived neurotrophic factor, BDNF) protects septal neurons during hypoglycemic stress. We investigated the signaling mechanisms underlying this synergistic protection. BMP7 (5 nM) increased phosphorylation and nuclear translocation of BMP-responsive Smads 1/5/8 within 30 min in cultures of rat embryonic septal neurons. BDNF (100 ng/mL) enhanced the BMP7-induced increase in phospho-Smad levels in both nucleus and cytoplasm; this effect was more pronounced after a hypoglycemic stress. BDNF increased both Akt and Erk phosphorylation, but pharmacological blockade of these kinase pathways (with wortmannin and U0126, respectively) did not reduce the Smad phosphorylation produced by the BMP7 + BDNF combination. Inhibitors of casein kinase II (CK2) activity reduced the (BMP7 + BDNF)-induced Smad phosphorylation, and this trophic factor combination increased CK2 activity in hypoglycemic cultures. These findings suggest that BDNF can increase BMP-dependent Smad phosphorylation via a mechanism requiring CK2.

The role of bone morphogenetic protein-2 in vivo in regeneration of peripheral nerves

We investigated the effects of bone morphogenetic protein-2 (BMP-2) and some other BMPs on regeneration of peripheral motor nerves in vivo. The facial nerves of 24 New Zealand rabbits were crushed to examine a series of retrograde changes in the facial nuclei and axons, in what has been called the "axon reaction". The facial nerves of the experimental group were treated with epineurial coaptation and BMP-2 after the injury. Nerves not treated with BMP-2 were regarded as controls. The expression of BMP-2 was investigated by in situ hybridisation in the neurons of facial nuclei. The electrophysiology, image analysis and transmission electron microscopy were used to evaluate the level of the recovery of facial nerves. The results showed that the axons in the experimental group were thicker and denser than those in the control group four weeks later. The expression of BMP-2 in the neurons of facial nuclei increased after injury. The electron microscopic observations showed that the axons' degeneration in the experimental group was less than that in the control group. Despite the morphological difference between the two groups, there was no apparent difference between them in nerve conduction velocity. These findings suggest that BMP-2 might be involved in the regeneration of facial nerves, and might function as a potential neurotrophic factor.

Raf kinase signaling functions in sensory neuron differentiation and axon growth in vivo

To define the role of the Raf serine/threonine kinases in nervous system development, we conditionally targeted B-Raf and C-Raf, two of the three known mammalian Raf homologs, using a mouse line expressing Cre recombinase driven by a nestin promoter. Targeting of B-Raf, but not C-Raf, markedly attenuated baseline phosphorylation of Erk in neural tissues and led to growth retardation. Conditional elimination of B-Raf in dorsal root ganglion (DRG) neurons did not interfere with survival, but instead caused marked reduction in expression of the glial cell line-derived neurotrophic factor receptor Ret at postnatal stages, associated with a profound reduction in levels of transcription factor CBF-beta. Elimination of both alleles of Braf, which encodes B-Raf, and one allele of Raf1, which encodes C-Raf, affected DRG neuron maturation as well as proprioceptive axon projection toward the ventral horn in the spinal cord. Finally, conditional elimination of all Braf and Raf1 alleles strongly reduced neurotrophin-dependent axon growth in vitro as well as cutaneous axon terminal arborization in vivo. We conclude that Raf function is crucial for several aspects of DRG neuron development, including differentiation and axon growth.

Bone morphogenetic protein-4 expression in the adult rat brain

Bone morphogenetic protein-4 (BMP4) is a member of the transforming growth factor-beta (TGF-beta) superfamily and plays important roles in multiple biological events. Although BMP4 expression has been well described in the early development of the central nervous system (CNS), little information is available on its expression in the adult CNS. Therefore, we investigated BMP4 expression in the adult rat CNS by using immunohistochemistry. BMP4 is intensely expressed in most neurons and their dendrites. In addition, intense BMP4 expression was also observed in the neuropil of the gray matters where high plasticity is reported, such as the molecular layer of the cerebellum and the superficial layer of the superior colliculus. Furthermore, we found that astrocytes also express BMP4 protein. These data indicate that BMP4 is more widely expressed throughout the adult CNS than previously reported, and its continued abundant expression in the adult brain strongly supports the idea that BMP4 plays pivotal roles also in the adult brain.

BMP enhances transcriptional responses to NGF during PC12 cell differentiation

Bone morphogenetic proteins (BMPs) enhance neurite outgrowth in nerve growth factor (NGF)-stimulated PC12 cells. To investigate the mechanism of this potentiating effect, real-time PCR was used to analyze the expression of 45 selected genes. A robust increase in expression of 10 immediate early genes including Egr1-4, Hes1, Junb, Jun and Fos was observed already after 1 h treatment with NGF alone. NGF plus BMP4 further increased these transcripts at 1 h and activated 18 additional genes. BMP4 alone induced Smad6, Mtap1b and Hes1. Egr3 was the gene most strongly upregulated by NGF and BMP4. However, luciferase assays showed that the cloned Egr3 proximal promoter was not involved in the BMP4 potentiation. Blocking Egr3 and Junb function by dominant-negative constructs reduced neurite outgrowth under stimulating conditions, proving that activation of members of both the Egr and Jun families is necessary for maximal PC12 cell response to NGF and BMP4.

Genetically modified bone morphogenetic protein signalling Alters traumatic brain injury-induced gene expression responses in the adult mouse

Three genetic mouse models were examined to define effects of bone morphogenetic protein (BMP) signalling on gene expression in normal and injured adult brain. CaMKII-Cre eliminated the BMP receptor Acvr1 (Alk2) and the common TGFbeta superfamily signal mediator Smad4 or activated a constitutively active Acvr1 in postnatal forebrain neurons. All mutants followed mendelian ratios, with no overt phenotypic changes. In situ hybridization demonstrated normal patterns of the dendritic marker MAP2 (Mtap2) throughout cortex despite neuron-specific losses of Acvr1 or Smad4. However, strong up-regulation of Mtap2 transcript in these mice was found by quantitative RT-PCR (qRT-PCR), indicating that Mtap2 is normally suppressed by BMP. Traumatic brain injury (TBI) resulted in increases of histone-associated DNA fragments in both control and Smad4-deficient cortex. Several cell-type-specific transcripts known to be involved in injury-related responses were measured by qRT-PCR. Gfap mRNA was strongly up-regulated in controls as well as in the loss-of-BMP-signalling mutants. Notably, activated Acvr1 signalling gave significantly lower TBI-induced up-regulations of Gfap and Phox2a mRNA levels, indicating reductions in astroglial and neuronal reactions to injury. Strong impairment in injury-induced Timp1 transcript up-regulation was also seen in these mice. In contrast, osteopontin (Spp1) transcript levels in activated microglia were not reduced by Acvr1 signalling. Altogether, the data suggest that BMP signalling is dispensable in adult cortical neurons but that augmented BMP signalling affects molecular changes associated with neuronal lesions.

Gene expression pathways induced by axotomy and decentralization of rat superior cervical ganglion neurons

To identify genes potentially involved in remodelling synaptic connections, we induced the temporary detachment of pre- and post-synaptic elements by axotomy or denervation of rat superior cervical ganglion neurons. cDNA microarray analysis followed by stringent selection criteria allowed the identification of a panel of genes whose expression was modulated by axotomy at various time points after injury. Among these genes, 11 were validated by real-time reverse transcriptase-polymerase chain reaction on independently prepared samples after superior cervical ganglion neuron axotomy (1, 3 and 6 days) and compared with the effect of decentralization (8 h, 1 and 3 days). These genes code for extracellular matrix/space [apolipoprotein D (apoD), decorin, collagen alpha1 type I, collagen alpha1 type III] and intermediate filament (vimentin) proteins, for modulators of neurite outgrowth (thrombin receptor, plasminogen activator inhibitor-1, bone morphogenetic protein 4, annexin II and S-100-related protein, clone 42C) and for a nerve cell transcription factor (brain finger protein). Eight of these 11 genes showed significant and persistent modulations after both types of injury. Finally, protein levels of apoD were shown to increase in superior cervical ganglion after axotomy. Our results identify hitherto unrecorded genes responsive to axotomy and decentralization of superior cervical ganglion neurons, and probably involved in synapse formation, remodelling and elimination.

Differential expression of cell fate determinants in neurons and glial cells of adult mouse spinal cord after compression injury

Cellular responses after spinal cord injury include activation of astrocytes, degeneration of neurons and oligodendrocytes, and reactions of the ependymal layer and meningeal cells. Because it has been suggested that tissue repair partially recapitulates morphogenesis, we have investigated the expression of several developmentally prominent molecules after spinal cord injury of adult mice where neurogenesis does not occur after injury. Cell fate determinants Numb, Notch-1, Shh and BMPs are abundantly expressed during development but mostly decline in the adult. In the present study, we investigated whether these genes are triggered by spinal cord injury as a sign of attempted recapitulation of development. Expression of Numb, Notch, Shh, BMP2/4 and Msx1/2 was analysed in the adult mouse spinal cord after compression injury by in situ hybridization up to 1 month after injury. The mRNA expression levels of Notch-1, Numb, Shh, BMP4 and Msx2 increased in the grey matter and/or white matter and in the ependyma rostral and caudal to the lesion site after injury. However, BMP2 and Msx1 were not up-regulated. Combining immunohistochemistry of cell type-specific markers with in situ hybridization we found that all the up-regulated genes were expressed in neurons. Moreover, Numb, BMP4 and Msx2 were also expressed by GFAP-positive astrocytes, while Shh was expressed by MBP-positive oligodendrocytes. In conclusion, the cell fate determinants Notch-1, Numb, Shh, BMP4 and Msx2 are expressed in neurons and/or glial cells after injury in a time-dependent manner, suggesting that these genes reflect to some extent an endogenous self-repair potential by recapitulating some features of development.

BARHL1 homeogene, the human ortholog of the mouse Barhl1 involved in cerebellum development, shows regional and cellular specificities in restricted domains of developing human central nervous system

The mouse homeobox gene Barhl1 plays a central role in cerebellum development and its expression is activated by the transcription factor Math1 which is involved in bone morphogenetic protein response pathways. We studied the human ortholog BARHL1 and we found that human, mouse, monkey, rat, and zebrafish orthologs were highly conserved and are members of the BarH homeogene family, containing Drosophila BarH1 and BarH2. The N-terminus of BARHL1 protein presents two FIL domains and an acidic domain rich in serine/threonine and proline, while the C-terminus contains a canonical proline-rich domain. Secondary structure analysis showed that outside the three helixes of the homeodomain, BARHL1 protein has essentially random coil structure. We isolated BARHL1 and defined its expression pattern in human embryonic and fetal central nervous system (CNS) and compared it to the mouse Barhl1 transcription. BARHL1 mRNA was found exclusively in the CNS restricted to p1-p4 prosomeres of the diencephalon, to the dorsal cells of the mesencephalon, to the dorsal dl1 sensory neurons of the spinal cord, and to the rhombic lips yielding the cerebellar anlage. Detailed analysis of BARHL1 expression in fetal cerebellar cell layers using our new optic microscopy technology showed BARHL1 expression in external and internal granular cells and also in mouse adult granular cells, in agreement to Barhl1 null mouse phenotype affecting the differentiation and migration of granular cells. These findings indicate that the regional and cellular specificities of BARHL1 transcriptional control well correspond to the mouse Barhl1 transcription and suggest a potential role of this gene in the differentiation of BARHL1-expressing neuronal progenitors involved in the pattern formation of human cerebral and cerebellar structures.

Bone morphogenetic protein signalling in NGF-stimulated PC12 cells

Bone morphogenetic proteins (BMPs) are shown to potentiate NGF-induced neuronal differentiation in PC12 phaeochromocytoma cells grown on collagen under low-serum conditions. Whereas, cell bodies remained rounded in control medium or with only BMPs present, addition of BMP4 or BMP6 robustly increased the neuritogenic effect of NGF within 2 days. NGF-increased phosphorylation of p44(Erk1) and p42(Erk2) between 2 and 24h was unaffected by addition of BMP6. PC12 cells transfected with the SBE(4x)-luc reporter showed that BMP4 significantly increased receptor-activated Smad activity. Expression of constitutively active BMP receptor ALK2 activating Smad1 and Smad5 resulted in a strong increase in the SBE(4x)-luc reporter response. Adding the inhibitory Smad7 drastically reduced this signal. In contrast to wild-type (wt) Smad5, a Smad5 variant lacking five Erk phosphorylation sites in the linker region (designated Smad5/5SA) showed a strong background transcriptional activity. A fusion construct (Gal4-Smad5/5SA) was also highly transcriptionally active. Addition of the MEK inhibitor U0126 to PC12 cells expressing Gal4-Smad5/wt did not increase background transcriptional activity. However, upon activation by constitutively active ALK2 both Gal4-Smad5/wt and Gal4-Smad5/5SA strongly stimulated transcription. The data show that serine residues of the linker region of Smad5 reduce spontaneous transcriptional activity and that NGF-activated Erk does not antagonise BMP signalling at this site. Hence, NGF and BMP signals are likely to interact further downstream at the transcriptional level in neuronal differentiation of the PC12 cells.