Biomedical subjects

M V Chao

Publications and source records attributed to M V Chao.

At least 37 records · Page 2Linked to original sources

PubMed2000

Trophic factors: An evolutionary cul-de-sac or door into higher neuronal function?

Trophic factors, such as the neurotrophins, CNTF, and GDNF, represent unique families of proteins that are essential for the development of the vertebrate nervous system. Surprisingly, there is little evidence to date that these proteins exist in Drosophila melanogaster and Caenorhabditis elegans, even though other polypeptide growth factors, such as EGF, FGF, and insulin, are conserved in these species. For the neurotrophins, the evolution of NGF, BDNF, NT-3, and NT-4 as a family implies that these signaling molecules may be involved in mediating additional neural activities, such as learning, memory, and behavior. Indeed, there is abundant evidence now that BDNF is involved in synapse modification, neurotransmitter release, long-term potentiation, and mechanosensation. The widening scope of neurotrophin action will require more physiological, genomic, and integrative approaches to understand fully the mechanisms by which survival factors can mediate so many diverse effects.

Animals↗

PubMed2000

Neurotrophin receptor structure and interactions.

Although ligand-induced dimerization or oligomerization of receptors is a well established mechanism of growth factor signaling, increasing evidence indicates that biological responses are often mediated by receptor trans-signaling mechanisms involving two or more receptor systems. These include G protein-coupled receptors, cytokine, growth factor and trophic factor receptors. Greater flexibility is provided when different signaling pathways are merged through multiple receptor signaling systems. Trophic factors exemplified by NGF and its family members, ciliary neurotrophic factor (CNTF) and glial derived neurotrophic factor (GDNF) all utilize increased tyrosine phosphorylation of cellular substrates to mediate neuronal cell survival. Actions of the NGF family of neurotrophins are not only dictated by ras activation through the Trk family of receptor tyrosine kinases, but also a survival pathway defined by phosphatidylinositol-3-kinase activity (Yao and Cooper, 1995), which gives rise to phosphoinositide intermediates that activate the serine/threonine kinase Akt/PKB (Dudek et al., 1997). Induction of the serine-threonine kinase activity is critical for cell survival, as well as cell proliferation. Hence, for many trophic factors, multiple proteins constitute a functional multisubunit receptor complex that activates ras-dependent and ras-independent intracellular signaling. The NGF receptors provide an example of bidirectional crosstalk. In the presence of TrkA receptors, p75 can participate in the formation of high affinity binding sites and enhanced neurotrophin responsiveness leading to a survival or differentiation signal. In the absence of TrkA receptors, p75 can generate, in only specific cell populations, a death signal. These activities include the induction of NF kappa B (Carter et al., 1996); the hydrolysis of sphingomyelin to ceramide (Dobrowsky et al., 1995); and the pro-apoptotic functions attributed to p75. Receptors are generally drawn and viewed as isolated integral membrane proteins which span the lipid bilayer, with signal transduction proceeding in a linear step-wise fashion. There are now numerous examples which indicate that each receptor acts not only in a linear, independent manner, but can also influence the activity of other cell surface receptors, either directly or through signaling intermediates. Which step and which intermediates are utilized for crosstalk between the receptors is a critical question. For neurotrophins, their primary function in sustaining the viability of neurons is counterbalanced by a receptor mechanism to eliminate cells by an apoptotic mechanism. It is conceivable that this bidirectional system may be utilized selectively during development and in neurodegenerative diseases.

Animals↗

PubMed2000

TrkA immunoglobulin-like ligand binding domains inhibit spontaneous activation of the receptor.

The extracellular region of the nerve growth factor (NGF) receptor, TrkA, contains two immunoglobulin (Ig)-like domains that are required for specific ligand binding. We have investigated the possible role of these two Ig-like domains in receptor dimerization and activation by using different mutants of the TrkA extracellular region. Deletions of each Ig-like domain, of both, and of the entire extracellular region were made. To probe the structural constraints on ligand-independent receptor dimerization, chimeric receptors were generated by swapping the Ig-like domains of the TrkA receptor for the third or fourth Ig-like domain of c-Kit. We also introduced single-amino-acid changes in conserved residues within the Ig-like domains of TrkA. Most of these TrkA variants did not bind NGF, and their expression in PC12nnr5 cells, which lack endogenous TrkA, promoted ligand-independent neurite outgrowth. Some TrkA mutant receptors induced malignant transformation of Rat-1 cells, as assessed by measuring proliferation in the absence of serum, anchorage-independent growth, and tumorigenesis in nude mice. These mutants exhibited constitutive phosphorylation and spontaneous dimerization consistent with their biological activities. Our data suggest that spontaneous dimerization of TrkA occurs when the structure of the Ig-like domains is altered, implying that the intact domains inhibit receptor dimerization in the absence of NGF.

Amino Acid Substitution↗

PubMed1999

Identification of a zinc finger protein whose subcellular distribution is regulated by serum and nerve growth factor.

A subclass of zinc finger proteins containing a unique protein motif called the positive regulatory (PR) domain has been described. The members include the PRDI-BF1/Blimp-1 protein, the Caenorhabditis elegans egl-43 and EVI1 gene products, and the retinoblastoma interacting protein RIZ. Here we describe a member of this family, SC-1, that exhibits several distinctive features. First, SC-1 interacts with the p75 neurotrophin receptor and is redistributed from the cytoplasm to the nucleus after nerve growth factor (NGF) treatment of transfected COS cells. The translocation of SC-1 to the nucleus was specific for p75, as NGF binding to the TrkA receptor did not lead to nuclear localization of SC-1. Thus, SC-1 provides a downstream transducer for the effects of NGF through the p75 neurotrophin receptor. Under normal growth conditions, SC-1 was found predominantly in the cytoplasm. On serum-starvation, SC-1 also translocated into the nucleus. A direct correlation between nuclear expression of SC-1 with the loss of BrdUrd incorporation was observed. These results imply that SC-1 may be involved in events associated with growth arrest.

Amino Acid Sequence↗

PubMed1999

Morphological differentiation of oligodendrocytes requires activation of Fyn tyrosine kinase.

In the central nervous system, myelination of axons occurs when oligodendrocyte progenitors undergo terminal differentiation and initiate process formation and axonal ensheathment. Although it is hypothesized that neuron-oligodendrocyte contact initiates this process, the molecular signals are not known. Here we find that Fyn tyrosine kinase activity is upregulated very early during oligodendrocyte progenitor cell differentiation. Concomitant with this increase is the appearance of several tyrosine phosphorylated proteins present only in differentiated cells. The increased tyrosine kinase activity is specific to Fyn, as other Src family members are not active in oligodendrocytes. To investigate the function of Fyn activation on differentiation, we used Src family tyrosine kinase inhibitors, PP1 and PP2, in cultures of differentiating oligodendrocyte progenitors. Treatment of progenitors with these compounds prevented activation of Fyn and reduced process extension and myelin membrane formation. This inhibition was reversible and not observed with related inactive analogues. A similar effect was observed when a dominant negative Fyn was introduced in progenitor cells. These findings strongly suggest that activation of Fyn is an essential signaling component for the morphological differentiation of oligodendrocytes.

Animals↗

PubMed1999

Oligodendrocyte apoptosis mediated by caspase activation.

Treatment with NGF causes long-term cultures of oligodendrocytes to die via a yet undefined mechanism mediated by the p75 neurotrophin receptor. The p75 receptor belongs to the TNF receptor superfamily of molecules, which includes Fas and p55 TNF receptors. The Fas and TNF receptors use adaptor molecules to recruit and activate caspase-8 to the receptor. Using a combination of immunohistochemical and Western blotting assays, we have examined caspase activity during NGF-induced apoptosis. Interestingly, although caspase-1 [interleukin-1beta-converting enzyme (ICE)], caspase-2, caspase-3, and caspase-8 were expressed in oligodendrocytes, only caspase-1, -2, and -3 were activated after NGF treatment, whereas caspase-8 was not. These data suggest that the mechanism of apoptosis by NGF through the p75 receptor is different from TNF and Fas-mediated killing. gamma Radiation of oligodendrocytes also activated a similar subset of caspases as NGF, indicating that NGF-induced oligodendrocyte apoptosis uses a similar cell death execution mechanism as injury models. This consolidates a potential role of the p75 neurotrophin receptor during stress and inflammatory conditions.

Animals↗

PubMed1999

Association of the p75 neurotrophin receptor with TRAF6.

In addition to the Trk tyrosine kinase receptors, neurotrophins also bind to a second receptor, p75, a member of the tumor necrosis factor receptor superfamily. Several signaling pathways have been implicated for p75 in the absence of Trk receptors, including induction of NF-kappaB and c-Jun kinase activities and increased production of ceramide. However, to date, the mechanisms by which the p75 receptor initiates intracellular signal transduction have not been defined. Here we report a specific interaction between p75 and TRAF6 (tumor necrosis factor receptor-associated factor-6) after transient transfection in HEK293T cells. The interaction was ligand-dependent and maximal at 100 ng/ml of nerve growth factor (NGF). Other neurotrophins also promoted the association of TRAF6 with p75 but to a lesser extent. The binding of TRAF6 was localized to the juxtamembrane region of p75 by co-immunoprecipitation and Western blotting. To assess the functional significance of this interaction, we have tested responses in cultured Schwann cells that express p75 and TRAF6. An NGF-mediated increase in the nuclear localization of the p65 subunit of NF-kappaB could be blocked by the introduction of a dominant negative form of TRAF6 in Schwann cells. These results indicate that TRAF6 can potentially function as a signal transducer for NGF actions through the p75 receptor.

Cell Line↗

PubMed1999

Neurotrophins in cell survival/death decisions.

Neurotrophins are target-derived soluble factors required for neuronal survival. Nerve growth factor (NGF) the founding member of the neurotrophin family, binds to two types of receptors: Trk tyrosine kinase and the p75 neurotrophin receptor, which belongs to the Fas-tumor necrosis factor (TNF) receptor superfamily. Binding of neurotrophins to Trk receptor tyrosine kinases initiate signaling cascades that promote cell survival sand differentiation. In contrast, p75 NGFR has been shown to modulate the susceptibility to death of selective cellular populations--including differentiated rat oligodendrocytes--in specific conditions. Notably, NGF effect on viability was only observed in fully differentiated oligodendrocytes and not in oligodendrocyte progenitor cells. The effect of p75 activation on oligodendrocyte survival correlates with increased activity of the stress related kinase JNK-1 and cleavage of specific caspases. Indeed, activation of additional stress pathways or impairment of survival signals may be required for p75 mediated activation of cell death execution programs. Interestingly, co-expression of the TrkA receptor in the same cell type abolishes the JNK-1 mediated death signal and induces MAP kinase activity, resulting in cell survival. This suggests that glial cell survival results from a balance between positive and negative regulators modulated by selective signalling pathways by tyrosine kinases and cytokine receptors.

Animals↗

PubMed1999

Differential association of phosphatidylinositol-5-phosphate 4-kinase with the EGF/ErbB family of receptors.

Phosphatidylinositol-5-phosphate 4-kinase (PIP4K) is required for the production of phosphoinositol-4,5-hisphosphate (PIP2), which has been closely associated with growth factor signalling. Here we have tested the possibility that phosphoinositide kinases may be take part in signal transduction through interactions with the epidermal growth factor (EGF) receptor and the ErbB family of tyrosine kinase receptors. Interactions of the Type IIbeta isoform of PIP4K were observed with the EGF receptor family members in a number of diverse cell lines, including A431, PC12 and MCF7 cells but not with the N6F TrkA receptor. Co-immunoprecipitation experiments indicate that PIP4K interacts with not only the EGF receptor, but also selectively with members of the ErbB tyrosine kinase family. These results demonstrate another enzyme substrate for EGF receptors that facilitates the production of phosphoinositides at the cell membrane.

Animals↗

PubMed1999

A comparison of the cytoplasmic domains of the Fas receptor and the p75 neurotrophin receptor.

The p75 neurotrophic receptor (p75) shares structural features with the Fas receptor (FasR). Both receptors contain extracellular cysteine-rich repeats, a single transmembrane domain, and intracellular death domains. However, it has not been clearly established whether their death domains are equivalent in their ability to mediate apoptosis. To understand better the role of p75 during apoptosis, we constructed chimeric receptors that contained the extracellular portion of the FasR and the intracellular portion of p75. These chimeric receptors, one containing the p75 transmembrane domain and the other containing the FasR transmembrane portion, as well as wild-type p75 and Fas receptors, were transiently transfected into human U373 glioma cells and human embryonic kidney 293 cells (293 cells), which are both responsive to Fas-mediated apoptosis. Whereas expression of FasR was sufficient to induce apoptosis in U373 and 293 cells, expression of p75 and the chimeric receptors induced only minimal levels of cell death compared to FasR. The results indicate that the magnitudes of FasR- and p75-induced killing are different and suggest that the death domain of p75 does not function in the same manner as the FasR death domain.

Animals↗

PubMed1999

The molecular basis for apoptotic defects in patients with CD95 (Fas/Apo-1) mutations.

Heterozygous mutations of the receptor CD95 (Fas/Apo-1) are associated with defective lymphocyte apoptosis and a clinical disease characterized by lymphadenopathy, splenomegaly, and systemic autoimmunity. From our cohort of 11 families, we studied eight patients to define the mechanisms responsible for defective CD95-mediated apoptosis. Mutations in and around the death domain of CD95 had a dominant-negative effect that was explained by interference with the recruitment of the signal adapter protein, FADD, to the death domain. The intracellular domain (ICD) mutations were associated with a highly penetrant Canale-Smith syndrome (CSS) phenotype and an autosomal dominant inheritance pattern. In contrast, mutations affecting the CD95 extracellular domain (ECD) resulted in failure of extracellular expression of the mutant protein or impaired binding to CD95 ligand. They did not have a dominant-negative effect. In each of the families with an ECD mutation, only a single individual was affected. These observations were consistent with differing mechanisms of action and modes of inheritance of ICD and ECD mutations, suggesting that individuals with an ECD mutation may require additional defect(s) for expression of CSS.

Apoptosis↗

PubMed1999

Loss of p27Kip1 function results in increased proliferative capacity of oligodendrocyte progenitors but unaltered timing of differentiation.

In many tissues, progenitor cells permanently withdraw from the cell cycle prior to commitment towards a differentiated phenotype. In the oligodendrocyte lineage a counting mechanism has been proposed, linking the number of cell divisions to growth arrest and differentiation. A direct prediction of this model is that an increase in the number of cell divisions would result in a delayed onset of differentiation. Since the cell cycle inhibitor p27Kip1 is an essential component of the machinery leading to oligodendrocyte progenitor growth arrest, we examined the temporal relationship between cell cycle withdrawal and expression of late differentiation markers in vivo, in mice carrying a targeted deletion in the p27Kip1 gene. Using bromodeoxyuridine to label proliferating cells, quaking (QKI) to identify embryonic glial progenitors, NG2 to identify neonatal oligodendrocyte progenitors, and myelin basic protein to label differentiated oligodendrocytes, we found an increased number of proliferating QKI- and NG2-positive cells in germinal zones of p27Kip1(-/-) mice at the peak of gliogenesis. However, no delay was observed in these mice in the appearance of the late differentiation marker myelin basic protein in the developing corpus callosum and cerebellum. Significantly, a decrease in cyclin E levels was observed in the brain of p27Kip1 null mice coincident with oligodendrocyte growth arrest. We conclude that two distinct modalities of growth arrest occur in the oligodendrocyte lineage: a p27Kip1-dependent mechanism of growth arrest affecting proliferation in early phases of gliogenesis, and a p27Kip1-independent event leading to withdrawal from the cell cycle and differentiation.

Animals↗

PubMed1998

Functional expression of TrkA receptors in hippocampal neurons.

Nerve growth factor (NGF) initiates its biological effects by promoting the dimerization and activation of the tyrosine kinase receptor TrkA. The requirements for NGF signaling through the TrkA receptor have been defined extensively from studies in immortalized cells, involving transfection of NIH 3T3, COS, and PC12 cells. In the present study, we tested the effects of extracellular and intracellular mutations of TrkA after DNA-mediated transfection in primary cultures of embryonic day 17 hippocampal neurons. We found that the action of the TrkA receptor on neuronal differentiation depends on specific motifs in the extracellular domain and on tyrosine 490 (Y490), the site for SHC protein binding. In contrast with previous observations in a PC12 background, a mutation in the SHC Y490 binding site in TrkA resulted in a loss of NGF-dependent process formation. These results indicate that tyrosine 490 is necessary for neurite outgrowth in hippocampal neurons. Moreover, a constitutively active form of TrkA did not give enhanced responsiveness in hippocampal neurons, indicating that the behavior of TrkA receptors in primary neuronal cells is distinct from that of other cell types.

Adaptor Proteins, Signal Transducing↗

PubMed1998

Ectopic expression of p27Kip1 in oligodendrocyte progenitor cells results in cell-cycle growth arrest.

Oligodendrocyte differentiation is a complex process believed to be controlled by an intrinsic mechanism associated with cell-cycle arrest. Recently, the cell-cycle inhibitor protein p27 Kip1 has been proposed as a key element in causing growth arrest of oligodendrocyte precursor cells. To investigate the effects of p27 upon oligodendrocyte cell development, we have introduced the p27 cDNA in oligodendrocyte progenitor cells using an adenovirus vector. Progenitor cells normally express low levels of p27. After adenoviral infection and p27 overexpression, progenitor cells were able to undergo cell-cycle arrest, even in the presence of strong mitogens. The effects of p27 were shown to be directly upon cyclin-dependent kinase-2 (CDK2), the protein kinase complex responsible for G1/S transition, as immunodepletion of oligodendrocyte extracts of p27 protein resulted in the activation of CDK2 activity. However, cells that became growth arrested owing to infection with p27 adenovirus did not display conventional oligodendrocyte differentiation markers, such as O4 or O1. Taken together, these data provide mechanistic evidence indicating that p27 is primarily involved in oligodendroglial progenitor proliferation by inhibiting CDK2 activity and inducing oligodendrocyte cell-cycle arrest.

Adenoviridae↗

PubMed1998

Competitive signaling between TrkA and p75 nerve growth factor receptors determines cell survival.

In addition to its role as a survival factor, nerve growth factor (NGF) has been implicated in initiating apoptosis in restricted cell types both during development and after terminal cell differentiation. NGF binds to the TrkA tyrosine kinase and the p75 neurotrophin receptor, a member of the tumor necrosis factor cytokine family. To understand the mechanisms underlying survival versus death decisions, the TrkA receptor was introduced into oligodendrocyte cell cultures that undergo apoptosis in a p75-dependent manner. Here we report that activation of the TrkA NGF receptor in oligodendrocytes negates cell death by the p75 receptor. TrkA-mediated rescue from apoptosis correlated with mitogen-activated protein kinase activation. Concurrently, activation of TrkA in oligodendrocytes resulted in suppression of c-jun kinase activity initiated by p75, whereas induction of NFkappaB activity by p75 was unaffected. These results indicate that TrkA-mediated rescue involves not only activation of survival signals but also simultaneous suppression of a death signal by p75. The selective interplay between tyrosine kinase and cytokine receptors provides a novel mechanism that achieves alternative cellular responses by merging signals from different ligand-receptor systems.

Animals↗

PubMed1998

TNF-related ligands and their receptors.

Multicellular organisms have the challenging task of coordinating the activities of many distinct cell types. This coordination is accomplished largely by cell-associated and soluble signalling molecules that act locally or distantly to alter target-cell physiology. The tumour necrosis factor family of cytokines are type II transmembrane proteins that are important regulators of homeostasis and have been implicated as mediators of disease. These molecules serve as ligands for a family of cell-surface receptors termed the tumour necrosis factor/nerve growth factor (TNF/NGF) receptor family. The receptors are type I transmembrane proteins capable of mediating a wide range of responses in vitro and in vivo. Signal transduction is mediated by several newly discovered cytoplasmic proteins that couple these receptors to downstream signalling events. The elucidation and use of spontaneously occurring mutants in TNF-related ligands and receptors in addition to gene-targeting experiments have begun to clarify the diverse biological effects mediated by this superfamily of cytokines.

Animals↗

PubMed1998

Neurotrophins: the biological paradox of survival factors eliciting apoptosis.

Neurotrophins are target-derived soluble polypeptides required for neuronal survival. Binding of neurotrophins to Trk receptor tyrosine kinases initiate signaling cascades that promote cell survival and differentiation. All family members bind to another receptor (p75NTR), which belongs to the tumor necrosis factor superfamily. Hence, nerve growth factor (NGF) and related trophic factors are unique in that two separate receptor types are utilized. Although the biological function of p75NTR has been elusive, it has been suggested to mediate apoptosis of developing neurons in the absence of Trk receptors. This presents a tantalizing paradigm, in which life-death decisions of cells are dependent upon the expression and action of two different receptors with distinctive signaling mechanisms. In the presence of TrkA receptors, p75 can participate in the formation of high affinity binding sites and enhanced NGF responsiveness leading to a survival signal. In the absence of TrkA receptors, p75 can generate, in only specific cell populations, a death signal. Here we discuss the unique features and implications of this unusual signal transduction system.

Animals↗

PubMed1998

BRE: a modulator of TNF-alpha action.

A stress-responsive gene highly expressed in brain and reproductive organs (BRE) is down-regulated after UV irradiation, DNA damaging agents, or retinoic acid treatment. The human BRE gene encodes a mRNA of 1.9 kb, which gives rise to a protein of 383 amino acids with a molecular size of 44 kilodaltons. BRE is not homologous to any known gene and its function has not been defined. Here we report that BRE was identified multiple times in a yeast two-hybrid screen of a murine cerebellar cDNA library, using the juxtamembrane domain of the p55 tumor necrosis factor alpha (TNF) receptor. The interaction between the p55 receptor and BRE was verified by an in vitro biochemical assay by using recombinant fusion proteins and by co-immunoprecipitation of transfected mammalian cells. In the yeast two-hybrid assay, BRE specifically interacted with p55 TNF receptor but not with other TNF family members such as the Fas receptor, the p75 TNF receptor, and p75 neurotrophin receptor. Overexpression of BRE inhibited TNF-induced NFkappaB activation, indicating that the interaction of BRE protein with the cytoplasmic region of p55 TNF receptor may modulate signal transduction by TNF-alpha.

Animals↗