Constitutive phosphorylation of TrkC receptors in cultured cerebellar granule neurons might be responsible for the inability of NT-3 to increase neuronal survival and to activate p21 Ras

A new in vitro injury model of mouse neurons induced by mechanical scratching

The mixed culture of neurons and glial cells has been widely used as a mechanical insult model for the study of neuron injury in vitro. However, a better model is desirable to eliminate the interference of glial cells during the study. Here we report a new model with exclusive cerebellar granule neurons (CGNs), which can be used for the study of in vitro neuron injury without involvement of glial cells. We found that after scratching insult, there was a decrease in both the survival rate and vitality of injured CGNs. Meanwhile, pathological changes were observed by electron microscopy. With this new model, we also tested the effects of neurotrophin-3 (NT-3) on neuroprotection. The result showed that the vitality of injured CGNs was enhanced by the administration of NT-3. These findings demonstrate that this new model is useful for investigation of the precise effect of mechanical damage on neurons excluding other factors, and to detect the neuroprotective effect of certain factors on mechanically injured neurons.

Neurotrophin-induced upregulation of p75NTR via a protein kinase C-delta-dependent mechanism

Neurotrophins exert their biological effects via p75NTR and Trk receptors. Functional interplay between these two receptors has been widely explored with respect to p75NTR enhancing the activation and signalling of Trk, but few studies address the bidirectional aspects. We have previously demonstrated that the expression of p75NTR can be differentially modulated by different Trk receptor mutations. Here we investigate the mechanism of Nerve Growth Factor (NGF)-induced upregulation of p75NTR expression. We utilize pharmacological inhibition to investigate the role of various TrkA-associated signalling intermediates in this regulatory cascade. Notably, the inhibition of phospholipase C-gamma (PLC-gamma) using U73122, prevented the NGF-induced upregulation of p75NTR protein and mRNA. The inhibition of protein kinase C-delta (PKC-delta) activation by rottlerin, a selective PKC-delta inhibitor, and by small interfering RNA (siRNA) directed against PKC-delta also inhibited this NGF-induced upregulation. Finally, we also show that in cerebellar granule neurons, BDNF acting via TrkB increases p75NTR expression in a PKC-delta dependent manner. These results indicate the importance of Trk-dependent PLC-gamma and PKC-delta activation for downstream regulation of p75NTR protein expression in response to neurotrophin stimulation, a process that has implications to the survival and growth of the developing nervous system.

Hyper immunoglobulin M syndrome due to CD40 deficiency: clinical, molecular, and immunological features

CD40 is a member of the tumor necrosis factor receptor family, which is expressed by a variety of cells including B cells, macrophages, dendritic cells, and other nonimmune cell types. CD40 activation is critical for B-cell proliferation, immunoglobulin (Ig)-isotype switching, and germinal center formation. In physiological conditions, the activation of CD40 occurs by binding to its natural ligand, CD154, which is expressed on activated T cells. The in vivo critical role of CD40-CD154 interaction on B-cell differentiation and isotype switching is provided by the discovery that mutations in either CD40 or CD154 gene cause the hyper IgM syndrome, termed HIGM3 or HIGM1, respectively, characterized by very low levels of serum IgG, IgA, and IgE, with normal or elevated IgM, associated with a defective germinal center formation. Originally considered humoral primary immunodeficiencies, the clinical features and the defect of T-cell priming, resulting from a defective T-B cell or dendritic cell interaction, is now considered as combined immunodeficiencies. In this article, we present a comprehensive overview of the clinical, genetic, and immunological features of patients with hyper IgM syndrome due to CD40 mutations.

CD40 is expressed and functional on neuronal cells

We show here that CD40 mRNA and protein are expressed by neuronal cells, and are increased in differentiated versus undifferentiated N2a and PC12 cells as measured by RT-PCR, western blotting and immunofluorescence staining. Additionally, immunohistochemistry reveals that neurons from adult mouse and human brain also express CD40 in situ. CD40 ligation results in a time-dependent increase in p44/42 MAPK activation in neuronal cells. Furthermore, ligation of CD40 opposes JNK phosphorylation and activity induced by NGF-beta removal from differentiated PC12 cells or serum withdrawal from primary cultured neurons. Importantly, CD40 ligation also protects neuronal cells from NGF-beta or serum withdrawal-induced injury and affects neuronal differentiation. Finally, adult mice deficient for the CD40 receptor demonstrate neuronal dysfunction as evidenced by decreased neurofilament isoforms, reduced Bcl-x(L):Bax ratio, neuronal morphological change, increased DNA fragmentation, and gross brain abnormality. These changes occur with age, and are clearly evident at 16 months. Taken together, these data demonstrate a role of CD40 in neuronal development, maintenance and protection in vitro and in vivo.

Signal transduction pathways through TRK-A and TRK-B receptors in human neuroblastoma cells

Little is known about the signal transduction pathways of TRK family receptors in neuroblastoma (NB) cells. In this study, an NB cell line, designated MP-N-TS, was established from an adrenal tumor taken from a 2-year-old boy. This cell line expressed both TRK-A and TRK-B receptors, which is rare in a single NB cell line. Therefore, the MP-N-TS cell line was used to determine whether the signal transduction through these constitutive receptors is functional. Three neurotrophins, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-4 / 5 (NT-4 / 5), induced tyrosine phosphorylation of panTRK, and BDNF and NT-4 / 5 induced tyrosine phosphorylation of TRK-B. Tyrosine phosphorylation of panTRK and / or TRK-B by the neurotrophins was inhibited in the presence of a tyrosine kinase inhibitor K252a. Tyrosine phosphorylation of Src homologous and collagen (Shc), extracellular signal-regulated kinase (ERK)-1 and ERK-2, and phospholipase C-gamma1 (PLC-gamma1) was increased by the three neurotrophins and the increase was inhibited in the presence of K252a. Activation of Ras, detected as the GTP-bound form of Ras, was induced by the three neurotrophins. The neurotrophins did not modulate the expressions of TRK-A or TRK-B mRNA, but they did induce the expression of c-fos mRNA. Exogenous NGF induced weak neurite outgrowth, whereas exogenous BDNF and NT-4 / 5 induced distinct neurite outgrowth. Exogenous BDNF and NT-4 / 5 increased the number of viable cells, while NGF did not. Our results demonstrate that the signal transduction pathways through TRK-A and TRK-B in MP-N-TS cells are functional and similar, and the main downstream signaling pathways from the three neurotrophins are mitogen-activated protein kinase (MAPK) cascades through Shc, activated Ras, ERK-1 and ERK-2, and the transduction pathway through PLC-gamma1. Further, BDNF and NT-4 / 5 increased cell viability. The MP-N-TS cell line should be useful for clarifying the TRK-A and TRK-B signaling pathways responsible for the different prognoses in patients with NB.

Regulation of Trk receptors following contusion of the rat spinal cord

Neurotrophins function through high-affinity tyrosine kinase (Trk) receptors to promote growth and survival of cells in the injured nervous system. To investigate the role of Trk receptors in the adult nervous system, we examined TrkA, TrkB, and TrkC mRNA expression in spinal cord and brain after spinal contusion. At 1 day postinjury, all Trk receptor transcripts were down regulated at and around the site of injury, a situation that persisted through the first week. By 42 days, Trk expression was absent only within the cavity. In addition, truncated TrkB expression was substantially increased in ependymal cells and astrocytes surrounding the lesion cavity of chronically injured spinal cords. Rostral and caudal to the injury site, TrkA, TrkB, and TrkC mRNA expression did not differ from that of uninjured control spinal cords. Furthermore, no changes were observed in TrkB or TrkC expression in the axotomized corticospinal and rubrospinal neurons. These studies suggest that loss of Trk receptors at the injury site may contribute to the early progressive cellular loss in injured spinal cords, while increased presence of truncated TrkB receptors in the chronic injured spinal cord may sequester and restrict BDNF availability to support axonal regeneration and neuronal survival. The persistence of Trk receptors on supraspinal neurons suggests that neurotrophin application can support growth and survival in the acute and chronic injury states.

Extracellular-regulated kinases and phosphatidylinositol 3-kinase are involved in brain-derived neurotrophic factor-mediated survival and neuritogenesis of the neuroblastoma cell line SH-SY5Y

Retinoic acid (RA) induces the differentiation of many cell lines, including those derived from neuroblastoma. RA treatment of SH-SY5Y cells induces the appearance of functional Trk B and Trk C receptors. Acute stimulation of RA-predifferentiated SH-SY5Y cells with brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), or neurotrophin 4/5 (NT-4/5), but not nerve growth factor (NGF), induces Trk autophosphorylation, followed by phosphorylation of Akt and the extracellular signal-regulated kinases (ERKs) 1 and 2. In addition, BDNF, NT-3, or NT-4/5, but not NGF, promotes cell survival and neurite outgrowth in serum-free medium. The mitogen-activated protein kinase and ERK kinase (MEK) inhibitor PD98059 blocks BDNF-induced neurite outgrowth and growth-associated protein-43 expression but has no effects on cell survival. On the other hand, the phosphatidylinositol 3-kinase inhibitor LY249002 reverses the survival response elicited by BDNF, leading to a cell death with morphological features of apoptosis.

Bcl-2 regulates the levels of the cysteine proteases ICH and CPP32/Yama in human neuronal precursor cells

Members of the Bcl-2 family are major regulators of cell death and survival. Bcl-2 has been shown to heterodimerize with the death-inducing protein Bax, but the mechanism of action of Bcl-2 is not fully understood. Here we show, using the human NT-2 neuronal cell line, that overexpression of Bcl-2 leads to dramatic down-regulation of the cysteine proteases ICH and CPP32/Yama, which are directly involved in cell death. In addition, the nuclear enzyme poly(ADP-ribose) polymerase was cleaved in control cells but not in cells overexpressing Bcl-2 following induction of apoptosis. The mRNA levels of ICH and CPP32/Yama were differentially affected by Bcl-2 overexpression, suggesting both transcriptional and post-transcriptional effects of the protein. These results demonstrate novel mechanisms of action of Bcl-2 in influencing the expression of death effectors such as the cysteine proteases. The relative levels of Bcl-2 and of various cysteine proteases ultimately determine survival and death of different cells, including neurons.