Rapamycin is a neuroprotective treatment for traumatic brain injury

The mammalian target of rapamycin, commonly known as mTOR, is a serine/threonine kinase that regulates translation and cell division. mTOR integrates input from multiple upstream signals, including growth factors and nutrients to regulate protein synthesis. Inhibition of mTOR leads to cell cycle arrest, inhibition of cell proliferation, immunosuppression and induction of autophagy. Autophagy, a bulk degradation of sub-cellular constituents, is a process that keeps the balance between protein synthesis and protein degradation and is induced upon amino acids deprivation. Rapamycin, mTOR signaling inhibitor, mimics amino acid and, to some extent, growth factor deprivation. In the present study we examined the effect of rapamycin, on the outcome of mice after brain injury. Our results demonstrate that rapamycin injection 4 h following closed head injury significantly improved functional recovery as manifested by changes in the Neurological Severity Score, a neurobehavioral testing. To verify the activity of the injected rapamycin, we demonstrated that it inhibits p70S6K phosphorylation, reduces microglia/macrophages activation and increases the number of surviving neurons at the site of injury. We therefore suggest that rapamycin is neuroprotective following traumatic brain injury and as a drug used in the clinic for other indications, we propose that further studies on rapamycin should be conducted in order to consider it as a novel therapy for traumatic brain injury.

ErbB-4 activation inhibits apoptosis in PC12 cells

Neuregulins, a large family of polypeptide growth factors, exert various distinctive effects in the nervous system. neuregulins and their receptors are widely expressed in neurons implying important roles in neuronal cell functions. Recently, we have shown that ErbB-4 receptors expressed in PC12 cells mediate neuregulin-induced differentiation. In the present study we demonstrate that in the PC12-ErbB-4 cells, neuregulin rescues cells from apoptosis induced by serum deprivation or tumor necrosis factor (TNF)alpha treatment. The neuregulin-induced survival is comparable to the effect mediated by the neurotrophic factor nerve growth factor (NGF). Both neuregulin and NGF protect cells from apoptosis induced by serum deprivation and TNF alpha treatment. Moreover, neuregulin like NGF induces the survival of neuronal differentiated PC12-ErbB-4 cells. The survival effect of neuregulin is probably mediated by the phosphoinositide 3-kinase (PI3K) and protein kinase B/Akt signaling pathways. Neuregulin induces the activation of PI3K and prolonged activation of protein kinase B/Akt. In addition, inhibition of the PI3K activity prevented the neuregulin-induced survival effect. Taken together, these results indicate that survival induced by neuregulin in PC12-ErbB-4 cells requires PI3K signaling networks.

Neuregulin rescues PC12-ErbB4 cells from cell death induced by H(2)O(2). Regulation of reactive oxygen species levels by phosphatidylinositol 3-kinase

Neuregulins (NRGs), a large family of transmembrane polypeptide growth factors, mediate various cellular responses depending on the cell type and receptor expression. We previously showed that NRG mediates survival of PC12-ErbB4 cells from apoptosis induced by serum deprivation or tumor necrosis factor-alpha treatment. In the present study we show that NRG induces a significant protective effect from H(2)O(2)-induced death. This effect of NRG is mediated by the phosphatidylinositol 3-kinase (PI3K)-signaling pathway since NRG failed to rescue cells from H(2)O(2) insult in the presence of the PI3K inhibitor, LY294002. Furthermore, the downstream effector of PI3K, protein kinase B/AKT, is activated by NRG in the presence of H(2)O(2), and protein kinase B/AKT activation is inhibited by LY294002. In addition, our results demonstrate that reactive oxygen species (ROS) elevation induced by H(2)O(2) is inhibited by NRG. LY294002, which blocks NRG-mediated rescue, increases ROS levels. Moreover, both H(2)O(2)-induced ROS elevation and cell death are reduced by expression of activated PI3K. These results suggest that PI3K-dependent pathways may regulate toxic levels of ROS generated by oxidative stress.

Neuregulin induces sustained reactive oxygen species generation to mediate neuronal differentiation

Neuregulins (NRGs), which are highly expressed in the nervous system, bind and activate two receptor tyrosine kinases, ErbB-3 and ErbB-4. Recently, we have shown that ErbB-4 receptors expressed in PC12 cells mediate NRG-induced differentiation through the MAPK signaling pathway. Here we demonstrate that NRG induces an increase in the intracellular concentration of reactive oxygen species (ROS). N-acetylcysteine, a ROS scavenger, inhibited NRG-induced activation of Ras and Erk and PC12-ErbB-4 cell differentiation. These results suggest that ROS production is involved in NRG-mediated neuronal differentiation and that ROS can regulate activation of Ras and Erk. Constitutively active Ras enhanced ROS production and dominant negative Ras inhibited NRG-induced ROS production, suggesting, a positive regulatory loop between Ras and ROS. The mitogen, EGF, induced short-term ROS production whereas NRG and NGF, which induce cell differentiation, induced prolonged ROS production. These results strongly suggest that the kinetics of ROS production may determine whether the cells will differentiate or proliferate.

Closed head injury induces up-regulation of ErbB-4 receptor at the site of injury

ErbB-4 receptor tyrosine kinase and its ligand neu differentiation factor (NDF/neuregulin) are widely expressed in the brain. The closed head injury model was used to investigate the possible role of ErbB-4 receptor in neurodegeneration. It is demonstrated that levels of ErbB-4 are dramatically increased at the site of injury. Activated microglia/macrophages constitute the major population of cells with the highest receptor levels at the site of injury. In addition ErbB-4 expression after injury is elevated also in neurons but not in astrocytes. Confocal microscopy analysis suggests that the high level of ErbB-4 protein in activated microglia/macrophages is probably due to phagocytosis of neuronal cells. These findings show for the first time that ErbB-4 receptors play a role in brain responses to head trauma. Overexpression of ErbB-4 receptors may be important for directing activated microglia/macrophages to the lesion site.

ErbB-4 activation promotes neurite outgrowth in PC12 cells

Neu differentiation factor (NDF; also known as neuregulin) induces a pleiotropic cellular response that is cell type-dependent. NDF and its receptor ErbB-4 are highly expressed in neurons, implying important roles in neuronal cell functions. In the present study we demonstrate that ErbB-4 receptors expressed in PC12 cells mediate NDF-induced signals and neurite outgrowth that are indistinguishable from those mediated by the nerve growth factor-activated Trk receptors. In PC12-ErbB-4 cells but not in PC12 cells, NDF induced an initial weak mitogenic signal and subsequently neurite outgrowth. The NDF-induced differentiation in PC12-ErbB-4 cells was mimicked by the pan-ErbB ligand betacellulin but not by other epidermal growth factor-like ligands. Thus, NDF and betacellulin mediate similar activities through the ErbB-4 receptor. Indeed, only these ligands induced strong phosphorylation of the ErbB-4 receptors. Neurite outgrowth induced by NDF in PC12-ErbB-4 cells was accompanied by sustained activation of mitogen-activated protein kinase (MAPK) and induction of the neural differentiation marker GAP-43. Inhibition of the MAPK kinase MEK or of protein kinase C (PKC) blocked NDF-induced differentiation, whereas elevation of cyclic AMP levels enhanced the response. Taken together, these results indicate that neurite outgrowth induced by ErbB-4 in PC12 cells requires MAPK and PKC signaling networks.

Pathogenic poxviruses reveal viral strategies to exploit the ErbB signaling network

Virulence of poxviruses, the causative agents of smallpox, depends on virus-encoded growth factors related to the mammalian epidermal growth factor (EGF). Here we report that the growth factors of Shope fibroma virus, Myxoma virus and vaccinia virus (SFGF, MGF and VGF) display unique patterns of specificity to ErbB receptor tyrosine kinases; whereas SFGF is a broad-specificity ligand, VGF binds primarily to ErbB-1 homodimers, and the exclusive receptor for MGF is a heterodimer comprised of ErbB-2 and ErbB-3. In spite of 10- to 1000-fold lower binding affinity to their respective receptors, the viral ligands are mitogenically equivalent or even more potent than their mammalian counterparts. This remarkable enhancement of cell growth is due to attenuation of receptor degradation and ubiquitination, which leads to sustained signal transduction. Our results imply that signal potentiation and precise targeting to specific receptor combinations contribute to cell transformation at sites of poxvirus infection, and they underscore the importance of the often ignored low-affinity ligand-receptor interactions.

ErbB tyrosine kinases and the two neuregulin families constitute a ligand-receptor network

The recently isolated second family of neuregulins, NRG2, shares its primary receptors, ErbB-3 and ErbB-4, and induction of mammary cell differentiation with NRG1 isoforms, suggesting functional redundancy of the two growth factor families. To address this possibility, we analyzed receptor specificity of NRGs by using an engineered cellular system. The activity of isoform-specific but partly overlapping patterns of specificities that collectively activate all eight ligand-stimulatable ErbB dimers was revealed. Specifically, NRG2-alpha [corrected], like NRG1-beta [corrected], emerges as a narrow-specificity ligand, whereas NRG2-beta [corrected] is a pan-ErbB ligand that binds with different affinities to all receptor combinations, including those containing ErbB-1, but excluding homodimers of ErbB-2. The latter protein, however, displayed cooperativity with the direct NRG receptors. Apparently, signaling by all NRGs is funneled through the mitogen-activated protein kinase (MAPK). However, the duration and potency of MAPK activation depend on the identity of the stimulatory ligand-receptor ternary complex. We conclude that the NRG-ErbB network represents a complex and nonredundant machinery developed for fine-tuning of signal transduction.

Differential endocytic routing of homo- and hetero-dimeric ErbB tyrosine kinases confers signaling superiority to receptor heterodimers

Both homo- and hetero-dimers of ErbB receptor tyrosine kinases mediate signaling by a large group of epidermal growth factor (EGF)-like ligands. However, some ligands are more potent than others, although they bind to the same direct receptor. In addition, signaling by receptor heterodimers is superior to homodimers. We addressed the mechanism underlying these two features of signal tuning by using three ligands: EGF; transforming growth factor alpha (TGFalpha); and their chimera, denoted E4T, which act on cells singly expressing ErbB-1 as a weak, a strong, and a very strong agonist, respectively. Co-expression of ErbB-2, a developmentally important co-receptor whose expression is frequently elevated in human cancers, specifically potentiated EGF signaling to the level achieved by TGFalpha, an effect that was partially mimicked by ErbB-3. Analysis of the mechanism underlying this trans-potentiation implied that EGF-driven homodimers of ErbB-1 are destined for intracellular degradation, whereas the corresponding heterodimers with ErbB-2 or with ErbB-3, dissociate in the early endosome. As a consequence, in the presence of either co-receptor, ErbB-1 is recycled to the cell surface and its signaling is enhanced. This latter route is followed by TGFalpha-driven homodimers of ErbB-1, and also by E4T-bound receptors, whose signaling is further enhanced by repeated cycles of binding and dissociation from the receptors. We conclude that alternative endocytic routes of homo- and hetero-dimeric receptor complexes may contribute to tuning and diversification of signal transduction. In addition, the ability of ErbB-2 to shunt ligand-activated receptors to recycling may explain, in part, its oncogenic potential.

Epiregulin is a potent pan-ErbB ligand that preferentially activates heterodimeric receptor complexes

The ErbB signaling network consists of four transmembrane receptor tyrosine kinases and more than a dozen ligands sharing an epidermal growth factor (EGF) motif. The multiplicity of ErbB-specific ligands is incompletely understood in terms of signal specificity because all ErbB molecules signal through partially overlapping pathways. Here we addressed the action of epiregulin, a recently isolated ligand of ErbB-1. By employing a set of factor-dependent cell lines engineered to express individual ErbBs or their combinations, we found that epiregulin is the broadest specificity EGF-like ligand so far characterized: not only does it stimulate homodimers of both ErbB-1 and ErbB-4, it also activates all possible heterodimeric ErbB complexes. Consistent with its relaxed selectivity, epiregulin binds the various receptor combinations with an affinity that is approximately 100-fold lower than the affinity of ligands with more stringent selectivity, including EGF. Nevertheless, epiregulin's action upon most receptor combinations transmits a more potent mitogenic signal than does EGF. This remarkable discrepancy between binding affinity and bioactivity is permitted by a mechanism that prevents receptor down-regulation, and results in a weak, but prolonged, state of receptor activation.

The oncogenic ErbB-2/ErbB-3 heterodimer is a surrogate receptor of the epidermal growth factor and betacellulin

The ErbB-1 receptor tyrosine kinase binds to six different growth factors, whose prototype is the epidermal growth factor (EGF). Two homologous epithelial receptors, ErbB-3 and ErbB-4, bind all isoforms of another family of growth factors, the Neu differentiation factors (NDFs/neuregulins). The fourth member of the ErbB family, ErbB-2, acts as the preferred heterodimeric partner of ligand-occupied complexes of the three other ErbB proteins. Here we report that at high concentrations, EGF can induce cell growth and differentiation in the absence of ErbB-1. This function is shared by betacellulin, but not by three other ligands, including the transforming growth factor alpha (TGFalpha). The functional receptor was identified as a heterodimer between ErbB-3 and ErbB-2, a previously identified oncogenic complex. When singly expressed, neither ErbB-3 nor ErbB-2 can mediate signaling by EGF. In addition, when co-expressed, blocking either receptor by using site-specific antibodies inhibited EGF and betacellulin activities, indicating strict cooperativity between ErbB-3 and ErbB-2. Through analysis of chimeras between EGF and TGFalpha, we identified the middle portion of EGF (loop B) as the site that enables activation of ErbB-2/ErbB-3. In conclusion, cooperative and promiscuous binding of stroma-derived growth factors by the epithelium-expressed ErbB-2/ErbB-3 heterodimer may be significant to cancer development. The mechanistic implications of our results for a model that attributes receptor dimerization to ligand bivalency, as well as to a recently proposed mechanism of secondary dimerization, are discussed.

Activity-dependent regulation of Neu differentiation factor/neuregulin expression in rat brain

Neu differentiation factor (NDF/neuregulin) is widely expressed in the central and peripheral nervous systems, where it functions as a mediator of the interactions between nerve cells and Schwann, glia, oligodendrocyte, and muscle cells, to control cellular proliferation, differentiation, and migration. NDF binds to two receptor tyrosine kinases, ErbB-3 and ErbB-4. Here we demonstrate that NDF and its ErbB-4 receptor are highly reactive to changes in ambient neuronal activity in the rodent brain in a region-selective manner. Generation of epileptic seizures by using kainic acid, a potent glutamate analog, elevated levels of NDF transcripts in limbic cortical areas, hippocampus, and amygdala. Concomitantly, ErbB-4 mRNA was increased with a similar spatial distribution, but transcription of the other NDF receptor, ErbB-3, did not change. A more moderate stimulation, forced locomotion, was accompanied by an increase in NDF transcripts and protein in the hippocampus and in the motor cortex. Similar changes were found with ErbB-4, but not ErbB-3. Last, a pathway-specific tetanic stimulation of the perforant path, which produced long-term potentiation, was followed by induction of NDF expression in the ipsilateral dentate gyrus and CA3 area of the hippocampus. Taken together, these results indicate that NDF is regulated by physiological activity and may play a role in neural plasticity.

Differential expression of NDF/neuregulin receptors ErbB-3 and ErbB-4 and involvement in inhibition of neuronal differentiation

Two receptor tyrosine kinases, ErB-3 and ErbB-4, mediate signaling by Neu differentiation factors (NDFs, also called neuregulins), while ErbB-1 and ErbB-2 serve as co-receptors. We show that the two NDF/neuregulin receptors differ in spatial and temporal expression patterns: The kinase-defective receptor, ErbB-3, is expressed primarily in epithelial layers of various organs, in the peripheral nervous system, and in adult brain, whereas ErbB-4 is restricted to the developing central nervous system and to the embryonic heart. An example of alternating expression of the two receptors is provided by the developing cerebellum: During postnatal cerebellar development, ErbB-4 expression slightly decreases along with a decline in NDF transcription, whereas ErbB-3 expression commences after the peak of neurogenesis. To study functional differences, we established primary brain cultures and found that ErbB-3 was expressed only in oligodendrocytes, whereas ErbB-4 expression was shared by oligodendrocytes, astrocytes and neurons. Blocking the action of endogenous NDF in vitro, by using a soluble form of ErbB-4, accelerated neurite outgrowth in both primary cultures and in neuronal-type cultures of the P19 teratocarcinoma, suggesting an inhibitory effect of NDF on neural differentiation. Apparently, ErbB-3 is associated with proliferation of P19 cells, whereas ErbB-4 correlates with a differentiated phenotype. We conclude that the two NDF receptors play distinct, rather than redundant, developmental and physiological roles.

Bivalence of EGF-like ligands drives the ErbB signaling network

Signaling by epidermal growth factor (EGF)-like ligands is mediated by an interactive network of four ErbB receptor tyrosine kinases, whose mechanism of ligand-induced dimerization is unknown. We contrasted two existing models: a conformation-driven activation of a receptor-intrinsic dimerization site and a ligand bivalence model. Analysis of a Neu differentiation factor (NDF)-induced heterodimer between ErbB-3 and ErbB-2 favors a bivalence model; the ligand simultaneously binds both ErbB-3 and ErbB-2, but, due to low-affinity of the second binding event, ligand bivalence drives dimerization only when the receptors are membrane anchored. Results obtained with a chimera and isoforms of NDF/neuregulin predict that each terminus of the ligand molecule contains a distinct binding site. The C-terminal low-affinity site has broad specificity, but it prefers interaction with ErbB-2, an oncogenic protein acting as a promiscuous low-affinity subunit of the three primary receptors. Thus, ligand bivalence enables signal diversification through selective recruitment of homo- and heterodimers of ErbB receptors, and it may explain oncogenicity of erbB-2/HER2.

A subclass of tumor-inhibitory monoclonal antibodies to ErbB-2/HER2 blocks crosstalk with growth factor receptors

ErbB-2 is an orphan receptor that belongs to a family of tyrosine kinase receptors for either epidermal growth factor (EGF) or Neu differentiation factor (NDF/neuregulin). Because overexpression of the erbB-2 proto-oncogene is frequently associated with an aggressive clinical course of certain human adenocarcinomas, the encoded protein is an attractive target for immunotherapy. Indeed, certain monoclonal antibodies (mAbs) to ErbB-2 effectively inhibit tumor growth in animal models and in clinical trials, but the underlying mechanism is incompletely understood. To study this question, we generated a large battery of mAbs to ErbB-2, that were classified epitopically. Whereas most antibodies stimulated tyrosine phosphorylation of ErbB-2, their anti-tumor effect correlated with its accelerated endocytic degradation. One group of tumor-inhibitory mAbs (Class II mAbs) was elicited by the most antigenic site of ErbB-2, and inhibited in trans binding of NDF and EGF to their direct receptors. The inhibitory effect was due to acceleration of ligand dissociation, and it resulted in the reduction of the ability of ErbB-2 to transactivate the mitogenic signals of NDF and EGF. These results identify two potential mechanisms of antibody-induced therapy: acceleration of ErbB-2 endocytosis by homodimerization and blocking of heterodimerization between ErbB-2 and growth factor receptors.

ErbB receptors and EGF-like ligands: cell lineage determination and oncogenesis through combinatorial signaling

The ErbB/HER family of transmembrane receptor tyrosine kinases includes four members that bind more than two dozens ligands sharing an epidermal growth factor- (EGF)3 like motif. This family plays a pivotal role in cell lineage determination in a variety of tissues, including mesenchyme-epithelial inductive processes and the interactions between neurons and muscle, glia and Schwann cells. Certain ligands and receptors of the family, especially the ErbB-2 receptor tyrosine kinase, contribute to a relatively virulent phenotype of some human tumors; most notable are carcinomas of secretory epithelia. This large variety of biological signals is generated through a combinatorial network of signal transduction in which different ErbB ligands are apparently capable of stabilizing discrete homo- and heterodimeric receptor complexes, each coupled to a specific set of cytoplasmic signaling proteins. Because each receptor is unique in terms of catalytic activity, cellular routing and transmodulation, the resulting network allows not only an enormous potential for signal diversification but also fine tuning and stringent control of cellular functions. ErbB-2 emerges as a master coordinator of the network, prolonging and amplifying signaling by decelerating the dissociation rates of its heterologous ligands. Thus, the tumorigenic action of ErbB-2 may be attributed to its ability to act as a shared signaling subunit, rather than by functioning as a bone fide receptor.

Neu differentiation factor/neuregulin isoforms activate distinct receptor combinations

The multiple isoforms of Neu differentiation factor (NDF/neuregulin) induce a pleiotropic cellular response that is isoform-specific and cell type-dependent. The molecular basis of this heterogeneity was addressed by comparing the two major groups of isoforms, alpha and beta. Both groups bind to the catalytically impaired receptor tyrosine kinase ErbB-3, whose mitogenic stimulation by NDF requires transactivation by other ErbB proteins, either ErbB-1 or ErbB-2. By expressing each pair of receptors in interleukin 3-dependent myeloid cells, we found that both isoforms induced mitogenic signals in cells co-expressing the combination of ErbB-3 with ErbB-2. However, only the beta isoform stimulated cells that expressed both ErbB-3 and ErbB-1, and neither isoform was active on cells expressing ErbB-3 alone. Both isoforms bind to all ErbB-3-expressing cells, albeit with different affinities, but the co-stimulatory mitogenic effect is correlated with the ability of each auxiliary receptor to transphosphorylate ErbB-3. These results imply that NDF isoforms differ in their ability to induce receptor heterodimers; whereas both types of isoforms signal through ErbB-3/ErbB-2 heterodimers, only beta isoforms are able to stabilize ErbB-3/ErbB-1 heterodimers.

Diversification of Neu differentiation factor and epidermal growth factor signaling by combinatorial receptor interactions

The ErbB family includes two receptors, ErbB-1 and ErbB-3, that respectively bind to epidermal growth factor and Neu differentiation factor, and an orphan receptor, ErbB-2. Unlike ErbB-1 and ErbB-2, the intrinsic tyrosine kinase of ErbB-3 is catalytically impaired. By using interleukin-3-dependent cells that ectopically express the three ErbB proteins or their combinations, we found that ErbB-3 is devoid of any biological activity but both ErbB-1 and ErbB-2 can reconstitute its extremely potent mitogenic activity. Transactivation of ErbB-3 correlates with heterodimer formation and is reflected in receptor phosphorylation and the transregulation of ligand affinity. Inter-receptor interactions enable graded proliferative and survival signals: heterodimers are more potent than homodimers, and ErbB-3-containing complexes, especially the ErbB-2/ErbB-3 heterodimer, are more active than ErbB-1 complexes. Nevertheless, ErbB-1 signaling displays dominance over ErbB-3 when the two receptors are coexpressed. Although all receptor combinations activate the mitogen-activated protein kinases ERK and c-Jun kinase, they differ in their rate of endocytosis and in coupling to intervening signaling proteins. It is conceivable that combinatorial receptor interactions diversify signal transduction and confer double regulation, in cis and in trans, of the superior mitogenic activity of the kinase-defective ErbB-3.

M1 agonists for the treatment of Alzheimer's disease. Novel properties and clinical update

The AF series compounds, AF102B and congeners of AF150(S), are functionally selective agonists for m1 muscarinic receptors (m1AChRs). This is shown in stable transfected CHO and PC12 cells (PC12M1) with m1m5AChRs and m1AChRs, respectively. AF102B and AF150(S) are partial agonists, but AF150, AF151, and AF151 (S) are full agonists in stimulating phosphoinositides hydrolysis or arachidonic acid release in these cells. Yet, all these compounds behave as antagonists when compared with carbachol in elevating cAMP levels. In PC12M1 cells, unlike carbachol, the AF series compounds induce only minimal to moderate neurite outgrowth. Yet, these agonists synergize strongly with NGF, which by itself mediates only a mild response. Stimulation of m1AChRs by AF102B, AF150(S) and AF151(S) in PC12M1 cells enhances secretion of beta/A4 amyloid precursor protein derivatives (APPs). The enhanced APPs secretion induced by AF102B is potentiated by NGF. AF102B also stimulates APPs secretion from rat cortical slices. Stimulation of m1AChR in PC12M1 cells with carbachol or AF102B decreases tau phosphorylation as indicated by specific tau-1 mAb and alkaline phosphatase treatment. Due to the above mentioned properties m1 agonists may be of unique value in delaying the progression of Alzheimer's disease (AD). The AF series compounds show a wide safety margin and improve memory and learning deficits in animal models for AD. There is a dearth of clinical reports on m1 agonists. These include studies on AF102B and xanomeline, another m1 selective agonist. We tested AF102B in escalating doses of 20, 40, 60 mg, tid, po, (each dose for 2 weeks) for a total of 10 weeks. This was a single-blind placebo-controlled, parallel-group study in patients with probable AD. AF102B was significantly effective at 40 and 60 mg, tid in the ADAS, ADAS-cognitive and ADAS-word recognition scales.

NGF promotes amyloid precursor protein secretion via muscarinic receptor activation

Processing of beta-amyloid precursor protein (APP) is coupled to several neurotransmitter receptors, including m1 muscarinic (m1AChR), and is associated with decreased amyloid deposition. Muscarinic agonist-stimulated APP secretion and membrane APP were measured in control and in NGF-differentiated PC12 cells stably transfected with m1AChR. This secretion was markedly enhanced following treatment with 50 ng/ml NGF for 3 days, and was observed using either carbachol or the M1-selective agonist AF102B. The effects of NGF were reflected by larger reductions in membrane-associated APP levels following muscarinic stimulation. These observations imply that M1 muscarinic receptors may act in concert with NGF to boost APP processing, and M1-selective agonists may thus be beneficial for reducing amyloid deposition by NGF-responsive neurons.

Activation of muscarinic receptors inhibits apoptosis in PC12M1 cells

Previous studies have shown that PC12 cells depend on growth factors for their survival. When deprived of growth factors, the cells undergo a dying process termed "apoptosis" (programed cell death). We show here that muscarinic agonists inhibited the apoptotic death of growth factor-deprived PC12M1 cells (PC12 cells stably expressing cloned m1 muscarinic acetylcholine receptors). This protective effect of the muscarinic agonists was observed in both proliferating and neuronal PC12M1 cells, was blocked by the muscarinic antagonist atropine, and was not observed in PC12 cells lacking m1 receptors. Muscarinic receptors therefore mediate inhibition of apoptosis in these cells. In addition to its effect on survival, the muscarinic agonist oxotremorine induced inhibition of DNA synthesis as well as growth arrest of exponentially growing PC12M1 cells at the S and G2/M phases of the cell cycle. Muscarinic receptors in these cells may therefore mediate inhibition of cell cycle progression.

ErbB-3 mediates differential mitogenic effects of NDF/heregulin isoforms on mouse keratinocytes

The family of Neu differentiation factors (NDFs, or heregulins) includes a dozen secreted glycoproteins, whose receptor binding domain displays two variants, alpha and beta, and they bind to two receptor tyrosine kinases, ErbB-3 and ErbB-4. Certain isoforms were reported to induce growth-arrest and differentiation of mammary tumor cells, while other breast cancer cell lines responded mitogenically. The present study addressed the biologic effects of various NDF isoforms on normal EGF-dependent epithelial cells, Balb/MK keratinocytes, that can undergo either proliferation or differentiation. We found that beta isoforms of NDF induced a mitogenic effect, that was significantly smaller than the maximal response to EGF. By contrast with NDF-beta, NDF-alpha isoforms exerted almost no mitogenic effect, but they were sufficient to maintain keratinocytes in culture. Consistent with their higher mitogenic potency, NDF-beta isoforms bound to Balb/MK cells with higher affinity (Kd = 2.2 nM) than alpha isoforms, however both groups shared their receptor, that we identified as ErbB-3. No transcript of ErbB-4 was detectable in the keratinocytes, but these cells express multiple NDF mRNAs and also ErbB-2. We conclude that different isoforms of NDF induce distinct growth regulatory effects on cultured keratinocytes, through direct interaction with ErbB-3.

NGF-dependent neurotrophic-like effects of AF102B, an M1 muscarinic agonist, in PC12M1 cells

The non-selective muscarinic agonist oxotremorine induces atropine-sensitive neurite outgrowth in PC12 cells stably transfected with m1 muscarinic receptors. In contrast, AF102B, an M1-selective muscarinic agonist, mediated minimal neurite outgrowth in these cells. In the presence of nerve growth factor (NGF) however, it induced atropine-sensitive neurite outgrowth in almost half the cell population. AF102B mediated phosphoinositide hydrolysis, but unlike carbachol, it did not stimulate cyclic AMP accumulation in these cells. These signals were not affected by NGF, indicating that they were not directly responsible for the cholinergic neurotrophic-like response. Our observations suggest that AF102B may improve neuronal responsiveness to neurotrophic factors, and thus may provide another beneficial aspect for treating Alzheimer's disease.

Brain neurons and glial cells express Neu differentiation factor/heregulin: a survival factor for astrocytes

Neu differentiation factor (NDF, also called heregulin) was isolated from mesenchymal cells on the basis of its ability to elevate phosphorylation of ErbB proteins. Earlier in situ hybridization analysis showed that NDF was transcribed predominantly in the central nervous system during embryonic development. To gain insights into the role of NDF in brain we analyzed its distribution by immunohistochemistry and in situ hybridization. Late-gestation (day 17) rat embryos displayed high NDF immunoreactivity in both motor (e.g., putamen) and limbic (e.g., septum) regions. Lower levels of the factor were exhibited by adult brain, except for the cerebellum, where NDF expression was increased postnatally. Both neurons and glial cells were identified by immunohistochemistry as NDF-producing cells (e.g., pyramidal neurons in the cerebral cortex and glial cells in the corpus callosum). By establishment of primary cultures of rat brain cells we confirmed that NDF was expressed in neurons as well as in astrocytes. In addition, by using such primary cultures we observed that NDF treatment exerted only a limited mitogenic effect, which was accompanied by significant acceleration of astrocyte maturation. Furthermore, long-term incubation with the factor specifically protected astrocytes from apoptosis, implying that NDF functions in brain as a survival and maturation factor for astrocytes.

Amyloid precursor protein secretion via muscarinic receptors: reduced desensitization using the M1-selective agonist AF102B

Secretion of amyloid precursor protein (APP) by cultured cells is coupled to several receptors, including m1 muscarinic (m1AChR), and is associated with decreased production of beta A4 amyloid. Secreted and cell-associated APP levels were measured in m1AChR-transfected PC12 cells stimulated with the non-selective agonist carbachol or the M1-selective agonist, AF102B. Secreted APP levels following stimulation with AF102B (5-60 min) were about half compared with carbachol. Yet, following 24 h stimulation with carbachol or AF102B, cell-associated APP levels were similarly decreased. This may be associated with a smaller reduction in APP secretion following 24 h stimulation with AF102B as compared with carbachol. AF102B may therefore have an advantage over non-selective muscarinic ligands for sustained decrease of cell-associated APP.

Cholinergic agonists and interleukin 1 regulate processing and secretion of the Alzheimer beta/A4 amyloid protein precursor

Activation of protein kinase C by phorbol esters is known to accelerate the processing and secretion of the beta/A4 amyloid protein precursor. We have now examined various first messengers that increase protein kinase C activity of target cells for their ability to affect beta/A4 amyloid protein precursor metabolism. Acetylcholine and interleukin 1, which are altered in Alzheimer disease, were shown to increase processing of the beta/A4 amyloid protein precursor via the secretory cleavage pathway. Cholinergic agonists stimulated secretion in human glioma and neuroblastoma cells as well as in PC12 cells transfected with the M1 receptor, while interleukin 1 stimulated secretion in human endothelial and glioma cells.

Growth factor-like effects mediated by muscarinic receptors in PC12M1 cells

Rat pheochromocytoma (PC12) cells stably expressing cloned m1 muscarinic acetylcholine receptors (PC12M1) undergo morphological changes when stimulated by muscarinic agonists. These changes, which include the outgrowth of neurite-like processes, are blocked by the muscarinic antagonist atropine and are not observed in PC12 cells. The observed morphological changes, which are independent of RNA and protein synthesis, are blocked by the methylation inhibitor 5'-deoxy-5'-methylthioadenosine, suggesting that methylation plays a role in this process. Analysis of cyclic AMP accumulation and phosphoinositide turnover reveals that both processes are enhanced on activation by muscarinic agonist. Our data suggest, however, that the muscarinic-dependent neurite-like outgrowth processes are not mediated by cyclic AMP, Ca2+, or protein kinase C pathways. The muscarinic-dependent neurite outgrowth effect is enhanced by nerve growth factor, with a resulting increase in both the number of neurite-extending cells and the length of the neurite. In addition, activation of muscarinic receptors in PC12M1 cells stimulates the induction of marker genes for neuronal differentiation. Muscarinic receptors may therefore mediate growth factor-like effects in these cells.

Indications for selective coupling to phosphoinositide hydrolysis or to adenylate cyclase inhibition by endogenous muscarinic receptor subtypes M3 and M4 but not by M2 in tumor cell lines

The muscarinic receptor subtype mRNAs expressed in cell lines were determined by Northern blot analysis. The biochemical responses of the muscarinic receptors in these cell lines (phosphoinositide hydrolysis and cAMP levels) were studied and correlated to the corresponding muscarinic receptor subtype as determined by mRNA expression. PC12 cells that expressed M4 subtype mRNA exhibited muscarinically dependent adenylate cyclase inhibition, whereas C6 and SK-N-SH cells expressing M3 subtype mRNA exhibited muscarinically dependent phosphoinositide hydrolysis. IMR-32 cells (M2 subtype mRNA) exhibited both muscarinically dependent phosphoinositide hydrolysis and adenylate cyclase inhibition. These results suggest that endogenous M3 and M4 receptor subtypes are selectively coupled to phosphoinositide hydrolysis and adenylate cyclase inhibition, respectively, whereas the M2 receptor subtype is coupled to both responses.

Cloned rat M3 muscarinic receptors mediate phosphoinositide hydrolysis but not adenylate cyclase inhibition

Rat M3 mAChR subtype was stably expressed in RAT 1 cells. Investigation of the pharmacological and biochemical properties of the cloned M3 receptors revealed that they mediate phosphoinositide hydrolysis but not adenylate cyclase inhibition. The similarities and differences between the properties of cloned rat M1 and M3 receptors are discussed.

Cloned M1 muscarinic receptors mediate both adenylate cyclase inhibition and phosphoinositide turnover

The rat M1 muscarinic receptor gene was cloned and expressed in a rat cell line lacking endogenous muscarinic receptors. Assignment of the cloned receptors to the M1 class was pharmacologically confirmed by their high affinity for the M1-selective muscarinic antagonist pirenzepine and low affinity for the M2-selective antagonist AF-DX-116. Guanylyl imidodiphosphate [Gpp(NH)p] converted agonist binding sites on the receptor, from high-affinity to the low-affinity state, thus indicating that the cloned receptors couple to endogenous G-proteins. The cloned receptors mediated both adenylate cyclase inhibition and phosphoinositide hydrolysis, but by different mechanisms. Pertussis toxin blocked the inhibition of adenylate cyclase (indicating coupling of the receptor to inhibitory G-protein), but did not affect phosphoinositide turnover. Furthermore, the stimulation of phosphoinositide hydrolysis was less efficient than the inhibition of adenylate cyclase. These findings demonstrate that cloned M1 receptors are capable of mediating multiple responses in the cell by coupling to different effectors, possibly to different G-proteins.