Collaboration of fibronectin matrix and neurotrophin in regulating spontaneous transmitter release at developing neuromuscular synapses in Xenopus cell cultures

BDNF and extracellular matrix regulate differentiation of mice neurosphere-derived cells into a GABAergic neuronal phenotype

Differentiation of neurosphere-derived cells is regulated by extracellular cues, namely, growth factors and proteins of the extracellular matrix (ECM). In this study we analyzed the influence of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), retinoic acid plus potassium chloride (RA-KCl), and the nonsynthetic ECMs laminin (LN) and fibronectin (FN) versus the synthetic adhesion substrate poly-L-lysine (PLL) in the in vitro differentiation of postnatal neurosphere cells. BDNF increased the number of differentiated neurons and decreased the number of neuronal precursors (nestin-positive cells) compared with NGF or RA-KCl. Moreover, cells treated with BDNF plus B27 supplement acquired a gamma-aminobutyric acid (GABA)-ergic phenotype and showed increased survival. No significant differences were found in the number of differentiated neurons in the presence of the ECMs alone. Nevertheless, FN or PLL in combination with BDNF promoted the acquisition of a GABAergic phenotype. The results obtained in this study highlight the importance of growth factors and ECM proteins for the potential of neurosphere cells to differentiate into neurons.

Brain-derived neurotrophic factor promotes long-term potentiation-related cytoskeletal changes in adult hippocampus

Brain-derived neurotrophic factor (BDNF) is an extremely potent, positive modulator of theta burst induced long-term potentiation (LTP) in the adult hippocampus. The present studies tested whether the neurotrophin exerts its effects by facilitating cytoskeletal changes in dendritic spines. BDNF caused no changes in phalloidin labeling of filamentous actin (F-actin) when applied alone to rat hippocampal slices but markedly enhanced the number of densely labeled spines produced by a threshold level of theta burst stimulation. Conversely, the BDNF scavenger TrkB-Fc completely blocked increases in spine F-actin produced by suprathreshold levels of theta stimulation. TrkB-Fc also blocked LTP consolidation when applied 1-2 min, but not 10 min, after theta trains. Additional experiments confirmed that p21 activated kinase and cofilin, two actin-regulatory proteins implicated in spine morphogenesis, are concentrated in spines in mature hippocampus and further showed that both undergo rapid, dose-dependent phosphorylation after infusion of BDNF. These results demonstrate that the influence of BDNF on the actin cytoskeleton is retained into adulthood in which it serves to positively modulate the time-dependent LTP consolidation process.

Functional changes in bladder tissue from type III collagen-deficient mice

OBJECTIVE

Collagen fibers impart tensile strength and transfer tension from bladder smooth muscle cells. We have previously shown that fibrotic bladders are characterized by an increased type III:type I collagen ratio. To determine the effect of decreased type III collagen on bladder function, type III collagen-deficient mice (COL3A1) were studied physiologically.

METHODS

Bladders from wild-type (+/+) and heterozygous (+/-) COL3A1 mice were biochemically characterized to determine total collagen (hydroxyproline analysis) and collagen subtype concentration (cyanogen bromide digestion and ELISA). Alterations in collagen fiber diameter were assessed by electron microscopy. Bladder muscle strips were used to assess physiologic function.

RESULTS

Hydroxyproline content decreased in heterozygous bladders, which had 50% less type III collagen. Wild-type bladders had a biphasic distribution of collagen fiber sizes, whereas heterozygous bladder collagen fibers spanned a broad range. Physiologically, there were no differences in contractile responses between wild-type and heterozygotes when stimulated with ATP, carbachol or KCl, indicating normal contraction via purinergic and muscarinic receptors, and in response to direct membrane depolarization. In contrast, tension generation in heterozygotes was decreased after field stimulation (FS), indicating decreased synaptic transmission. Length-tension studies showed that the heterozygote muscle strips generated less tension per unit length, indicating that they were more compliant than wild-type controls.

CONCLUSIONS

Critical levels of type III collagen appear to be a requirement for normal bladder tension development and contraction. Our data show that a decrease in the type III:type I collagen ratio, and altered fiber size, results in a more compliant bladder with altered neurotransmitter function.

Extracellular matrix and matrix metalloproteinases in sciatic nerve

Although matrix metalloproteinases (MMPs) are increasingly being implicated in several pathologies of the nervous system, it is not yet clear what role they play in normal neurobiological processes. We review the expression of extracellular matrix (ECM) components as well as MMPs and tissue inhibitors of metalloproteinases (TIMPs) in the peripheral nervous system. We explore the expression of certain MMPs and the four TIMPs at the mRNA level in the postnatal mouse sciatic nerve. In addition, we have used substrate gel and in situ zymography to determine levels of MMP-2 and -9 and TIMP activity in rat sciatic nerve after crush and during regeneration. A rapid and transient increase in MMP-9 localised at and immediately distal to the site of injury was observed, whereas an increase in MMP-2 activity was delayed, prolonged, and extended proximal and distal to the injury site. This activity coincides with periods of axonal elongation, suggesting that it could act to facilitate axonal extension along the nerve matrix. We also detected multiple species of gelatinolytic inhibitory activity, including TIMP-1 and -3 in control and injured nerve. These activities probably act to prevent uncontrolled gelatinolytic activity, maintaining nerve integrity at the level essential for axonal regrowth.

A novel BDNF gene promoter directs expression to skeletal muscle

BACKGROUND

Cell-specific expression of the gene that encodes brain-derived neurotrophic factor (BDNF) is required for the normal development of peripheral sensory neurons and efficient synaptic transmission in the mature central and peripheral nervous system. The control of BDNF gene expression involves multiple tissue and cell-specific promoters that are differentially regulated. The molecular mechanisms that are responsible for tissue and cell-specific expression of these promoters are still incompletely understood.

RESULTS

The cloning and analysis of three additional zebrafish (Danio rerio) BDNF gene exons and two associated promoters, is reported. Among them are two exons that generate a novel tripartite mature transcript. The exons were located on the transcription unit, whose overall organization was determined by cloning, Southern blot hybridization and sequence analysis, and compared with the pufferfish (Fugu rubripes) and mammalian BDNF loci, revealing a conserved but more compact organization. Structural and functional analysis of the exons, their adjacent promoters and 5' flanks, showed that they are expressed cell-specifically. The promoter associated with the 5' exon of the tripartite transcript is GC-rich, TATA-less and the 5' flank adjacent to it contains multiple Sp1, Mef2, and AP1 elements. A fusion gene containing the promoter and 1.5 KB of 5' flank is directed exclusively to skeletal muscle of transiently transfected embryos. The second promoter, whose associated 5' exon contains a 25-nucleotide segment of identity with a mammalian BDNF gene exon, was transiently expressed in yolk of the early embryo. RT-PCR analysis of total RNA from whole juvenile fish and adult female skeletal muscle revealed tissue-specific expression of the 5' exons but the novel exon could not be detected even after two rounds of nested PCR.

CONCLUSION

The zebrafish BDNF gene is as complex as the mammalian gene yet much more compact. Its exons are expressed in an independently regulated and cell-specific fashion. An initial structural and functional analysis has shown that the regions controlling zebrafish BDNF gene expression have been cloned and identified. They can now be subjected to detailed molecular and genetic analyses to identify the cellular mechanisms by which the transcription factors that act on these regions control BDNF gene expression.

Modulation of protein kinase A activation by fibronectin matrix proteins at developing neuromuscular synapses in Xenopus laevis cell cultures

Extracellular matrix proteins, such as fibronectin, laminin, and collagen, have been implicated in a wide variety of cellular properties, which include cell adhesion, migration, differentiation, and proliferation. In this study, we investigated the modulation of protein kinase A (PKA) activity by matrix proteins at developing motoneurons. The cultures of spinal neurons and myotomal cells were prepared from 1-day-old Xenopus laevis embryos. Spontaneous synaptic currents (SSC) were recorded from innervated myocytes of natural synapses by whole-cell voltage-clamped recordings (V(h) = -60 to approximately -65 mV). Bath application of agents, which directly or indirectly activate PKA, such as forskolin (20 microM), dibutyryl cAMP (DBcAMP) (1 mM), isoproterenol (10 microM), or albuterol (10 microM), significantly increased SSC frequency in cultures grown on fibronectin (FN)-coated substratum, but not on laminin- or collagen-coated glasses. The evoked synaptic currents increased in response to forskolin in neurons grown on FN substratum. Triflavin, an Arg-Gly-Asp-dependent disintegrin, inhibited potentiating action of isoproterenol in neurons grown on FN substratum, suggesting that integrin is involved in the potentiation of the PKA pathway in the regulation of acetylcholine (ACh) release. There is collaboration of neurotrophic factors and the FN matrix in regulating synaptic transmission in response to DBcAMP. Chronic treatment with neurotrophic factors, such as ciliary neurotrophic factor (150 ng/ml), glial cell line-derived neurotrophic factor (30 ng/ml), or neurotrophin-3 (50 ng/ml), enhanced the SSC-increasing action of DBcAMP in neurons grown on FN-coated glasses. These results suggest that the FN matrix potentiates synaptic transmission in response to PKA activation. Neurotrophic factors may collaborate with FN to regulate spontaneous ACh secretion at developing motoneurons, which may play an important role in the maturation of embryonic neuromuscular synapses.