A key issue in signal transduction is how signaling pathways common to many systems-so-called canonical signaling cassettes-integrate signals from molecules having a wide spectrum of activities, such as hormones and neurotrophins, to deliver distinct biological outcomes. The neuroendocrine cell line PC12, derived from rat pheochromocytoma, provides an example of how one canonical signaling cassette-the Raf --> mitogen-activated protein kinase kinase (MEK) --> extracellular signal-regulated kinase (ERK) pathway-can promote distinct outcomes, which in this case include neuritogenesis, gene induction, and proliferation. Two growth hormones, epidermal growth factor (EGF) and nerve growth factor (NGF), use the same pathway to cause PC12 proliferation and differentiation, respectively. In addition, pituitary adenylate cyclase-activating polypeptide (PACAP), a neurotransmitter that also causes differentiation, uses the same canonical cassette as NGF but in a different way. The Connections Map for PC12 Cell Differentiation brings into focus the complex array of specific cellular responses that rely on canonical signal transduction systems.

K-252a, a kinase inhibitor isolated from the culture broth of Nocardiopsis sp., selectively inhibits the actions of nerve growth factor (NGF) on PC 12 cells. At a concentration of 200 nM, K-252a prevents neurite generation initiated by NGF, but not neurite generation produced by fibroblast growth factor or outgrowth produced by dibutyryl cAMP. K-252a also inhibits the induction of ornithine decarboxylase by NGF, but stimulates ornithine decarboxylase induction by epidermal growth factor. Stimulation of phosphatidylinositol breakdown by NGF was similarly inhibited by K-252a, while stimulation by epidermal growth factor was enhanced. The NGF-induced decrease in the phosphorylation of a soluble protein, Nsp 100, was prevented by K- 252a. K-252a blocks the NGF-induced heterodown-regulation of the epidermal growth factor receptor, but not the epidermal growth factor- induced homodown-regulation of the epidermal growth factor receptor. K- 252a, then, provides a new tool for the dissection and study of NGF- requiring processes.

Nerve growth factor (NGF) has important functions during embryonic development and on various tissues and organs under normal and pathological conditions during the extrauterine life. RT-PCR analysis and immunological methods demonstrate that human umbilical vein endothelial cells (HUVECs) express the NGF receptors trkA(NGFR) and p75NTR. NGF treatment caused a rapid phosphorylation of trkA(NGFR) in HUVECs, determining a parallel increase of phosphorylated ERK1/2. Accordingly, NGF induced a significant increase in HUVEC proliferation that was abolished by the trkA(NGFR) inhibitor K252a. Also, HUVECs express significant levels of NGF under standard culture conditions that were up-regulated during serum starvation. Endogenous NGF was responsible for the basal levels of trkA(NGFR) and ERK1/2 phosphorylation observed in untreated HUVEC cultures. Finally, NGF exerted a potent, direct, angiogenic activity in vivo when delivered onto the chorioallantoic membrane of the chicken embryo. The data indicate that NGF may play an important role in blood vessel formation in the nervous system and in several pathological processes, including tumors and inflammatory diseases. Unraveling mechanisms of NGF-dependent angiogenesis could provide valuable tools for novel therapeutic approaches in antiangiogenic therapy.

Dopamine is a brain neurotransmitter involved in the pathology of schizophrenia. The dopamine hypothesis states that, in schizophrenia, dopaminergic signal transduction is hyperactive. The cAMP-response element binding protein (CREB) is an intracellular protein that regulates the expression of genes that are important in dopaminergic neurons. Dopamine affects the phosphorylation of CREB via G protein-coupled receptors. Neurotrophins, such as brain derived growth factor (BDNF), are critical regulators during neurodevelopment and synaptic plasticity. The CREB is one of the major regulators of neurotrophin responses since phosphorylated CREB binds to a specific sequence in the promoter of BDNF and regulates its transcription. Moreover, susceptibility genes associated with schizophrenia also target and stimulate the activity of CREB. Abnormalities of CREB expression is observed in the brain of individuals suffering from schizophrenia, and two variants (-933T to C and -413G to A) were found only in schizophrenic patients. The CREB was also involved in the therapy of animal models of schizophrenia. Collectively, these findings suggest a link between CREB and the pathophysiology of schizophrenia. This review provides an overview of CREB structure, expression, and biological functions in the brain and its interaction with dopamine signaling, neurotrophins, and susceptibility genes for schizophrenia. Animal models in which CREB function is modulated, by either overexpression of the protein or knocked down through gene deletion/mutation, implicating CREB in schizophrenia and antipsychotic drugs efficacy are also discussed. Targeting research and drug development on CREB could potentially accelerate the development of novel medications against schizophrenia.

The PC12 clone, developed from a pheochromocytoma tumor of the rat adrenal medulla, has become a premiere model for the study of neuronal differentiation. When treated in culture with nanomolar concentrations of nerve growth factor, PC12 cells stop dividing, elaborate processes, become electrically excitable, and will make synapses with appropriate muscle cells in culture. The changes induced by nerve growth factor lead to cells that, by any number of criteria, resemble mature sympathetic neurons. These changes are accompanied by a series of biochemical alterations occurring in the membrane, the cytoplasm, and the nucleus of the cell. Some of these events are independent of changes in transcription, while others clearly involve changes in gene expression. A number of the alterations seen in the cells involve increases or decreases in the phosphorylation of key cellular proteins. The information available thus far allows the construction of a hypothesis regarding the biochemical basis of PC12 differentiation.

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PC12 cells were manipulated in such a way as to permit the study of differentiation‐specific responses independently from proliferative responses. Cells were starved for serum then exposed to nerve growth factor (NGF) or serum. Following addition of serum, cells incorporated thymidine in a synchronous manner. Subsequent to the wave of DNA synthesis, the cell number increased approximately two‐fold. Addition of NGF to serum‐starved cultures had no measurable effect on either parameter. Neurite outgrowth was more rapid and extensive and appearance of Na+ channels, measured as saxitoxin binding sites, more rapid than when NGF was added to exponentially‐growing cells. Epidermal growth factor receptors were heterologously down‐regulated by NGF with similar kinetics under both conditions. Induction of the proto‐oncogene c‐fos by NGF was also greater in the serum‐starved cells than in exponentially‐growing cultures. These results indicated that serum starvation resulted in synchronisation of the cultures and that NGF action may be cell cycle‐specific. Analysis of the cellular response to NGF at different times during the cell cycle showed that c‐fos was induced in the G1 phase but not in S or G2. Fluorescence‐activated cell sorter analysis demonstrated that addition of NGF to exponentially‐growing cells, resulted in their accumulation in a G1‐like state. With regard to the study of the mechanism of NGF action, these results illustrate that measurements of NGF effects on specific components in the signal transduction pathway may be confounded by the use of exponentially‐growing cultures.

In search for novel biomimetic scaffolds for application in vascular tissue engineering, we evaluated a series of fibrous scaffolds prepared by coelectrospinning tertiary blends of poly(lactide-co-glycolide) (PLGA), gelatin, and elastin (PGE). By systematically varying the ratios of PLGA and gelatin, we could fine-tune fiber size and swelling upon hydration as well as the mechanical properties of the scaffolds. Of all PGE blends tested, PGE321 (PLGA, gelatin, elastin v/v/v ratios of 3:2:1) produced the smallest fiber size (317 ± 46 nm, 446 ± 69 nm once hydrated) and exhibited the highest Young's modulus (770 ± 131 kPa) and tensile strength (130 ± 7 kPa). All PGE scaffolds supported the attachment and metabolization of human endothelial cells (ECs) and bovine aortic smooth muscle cells (SMCs) with some variances in EC morphology and cytoskeletal spreading observed at 48 h postseeding, whereas no morphologic differences were observed at confluence (day 8). The rate of metabolization of ECs, but not of SMCs, was lower than that on tissue culture plastic and depended on the specific PGE composition. Importantly, PGE scaffolds were capable of guiding the organotypic distribution of ECs and SMCs on and within the scaffolds, respectively. Moreover, the EC monolayer generated on the PGE scaffold surface was nonthrombogenic and functional, as assessed by the basal and cytokine-inducible levels of mRNA expression and amidolytic activity of tissue factor, a key player in the extrinsic clotting cascade. Taken together, our data indicate the potential application of PGE scaffolds in vascular tissue engineering.