Module 4: Signaling in the cell sheng. 1990 paper (ChangHui Pak)

Overview of the Mechanism of c-fos Transcription Activation

  • Background:

    • Understanding how signals from membrane depolarization and calcium influx activate gene expression is crucial in cellular function and memory retention.

    • c-fos is a proto-oncogene that serves as a model for studying immediate early gene (IEG) expression in neurons.

    • Cells can retain information via pre-existing protein modifications for short-term and gene expression for long-term memory.

Key Concepts

  • Calcium Response Element (CaRE):

    • The cis-acting element that mediates transcription induction by calcium influx, similar to the cAMP Response Element (CRE).

    • CREB (cAMP Response Element-Binding Protein) is a critical transcription factor that binds to both CaRE and CRE for regulating gene expression.

  • Phosphorylation of CREB:

    • CREB is phosphorylated by distinct signaling pathways activated by calcium and cAMP.

    • Phosphorylation occurs at a specific site that is important for transcriptional activation, which is rapidly responsive to both calcium influx and cAMP signaling.

  • Mechanism of Activation:

    • Upon membrane depolarization or during specific signals, calcium ions enter the cell through voltage-dependent channels.

    • This calcium influx triggers pathways that lead to the phosphorylation of CREB, enhancing its activity as a transcription factor.

Process Characteristics

  • Rapid Response:

    • The process of c-fos transcription activation is swift, with changes observable within minutes of calcium influx or membrane depolarization.

  • Distinct but Converging Pathways:

    • Calcium Pathway: Likely involves calcium/calmodulin-dependent kinases which phosphorylate CREB directly.

    • cAMP Pathway: The classical pathway involves activation of adenylate cyclase, increasing cAMP levels, which further activates PKA (cAMP-dependent protein kinase) to phosphorylate CREB.

Experimental Evidence

  • Studies on c-fos Expression:

    • Mutational analyses have shown that specific sequences in the c-fos promoter are essential for its calcium and cAMP responsiveness.

    • Experiments using various inhibitors indicate that activation does not require new protein synthesis, supporting the model of fast transcriptional responses in neuronal cells.

Cellular and Physiological Implications

  • Gene Expression Regulation:

    • Immediate early genes, including c-fos, mediate the cellular response to various stimuli and play roles in growth and differentiation.

  • Neuronal Plasticity:

    • Contributes to the understanding of how environmental signals translate into lasting changes in cellular function, impacting learning and memory.

Overall Significance

  • These findings elucidate the complex interplay between calcium and cAMP signaling in regulating immediate early gene expression, particularly in the context of neuronal activity and memory formation.

  • Insights from these mechanisms may reveal therapeutic targets for cognitive dysfunctions and hormonal regulation in neuronal settings.


  • Background: Understanding how signals from membrane depolarization and calcium influx activate gene expression is crucial for many processes in cellular function and memory retention. c-fos is a proto-oncogene that serves as a model for studying immediate early gene (IEG) expression in neurons, primarily due to its rapid response to stimuli. Cells can retain information via two principal mechanisms: short-term memory through pre-existing protein modifications and long-term memory through gene expression changes, making the study of c-fos essential to understanding the underlying biology of memory and learning.

Key Concepts

  • Calcium Response Element (CaRE): The cis-acting element that mediates transcription induction by calcium influx is crucial for the gene activation process. CaRE shares functional similarities with the cAMP Response Element (CRE), which further integrates signaling pathways. CREB (cAMP Response Element-Binding Protein) is a critical transcription factor that binds to both CaRE and CRE, prominently influencing the regulation of gene expression through transcriptional activations in response to cellular signaling events.

  • Phosphorylation of CREB: CREB activation occurs through phosphorylation by distinct signaling pathways activated by calcium and cAMP influx. The phosphorylation typically happens at serine 133 and is essential for CREB's ability to initiate transcription. This post-translational modification is rapidly responsive, ensuring that both calcium influx and cAMP signaling contribute swiftly and effectively to transcriptional activation, reflecting the dynamic nature of cellular responses to environmental stimuli.

  • Mechanism of Activation: Upon membrane depolarization or during specific signals, calcium ions enter the cell through voltage-dependent channels, such as L-type calcium channels and NMDA receptors. This influx of calcium ions serves as a second messenger, triggering pathways such as the calcium/calmodulin-dependent protein kinase pathway, which leads to the phosphorylation of CREB. The resultant modifications enhance CREB's activity as a transcription factor, thereby initiating transcription of c-fos and other immediate early genes.

Process Characteristics

  • Rapid Response: The process of c-fos transcription activation is both quick and efficient, with observable changes detectable within minutes of calcium influx or membrane depolarization. This rapid transcriptional response is key for processes such as synaptic plasticity and the formation of new neuronal connections.

  • Distinct but Converging Pathways:

    • Calcium Pathway: Likely involves calcium/calmodulin-dependent kinases (CaMKs), which directly phosphorylate CREB, linking calcium signaling to gene expression activation.

    • cAMP Pathway: The classical pathway involves the activation of adenylate cyclase following receptor stimulation, which increases intracellular cAMP levels. The elevation of cAMP further activates PKA (cAMP-dependent protein kinase), which subsequently phosphorylates CREB, amplifying its transcriptional activity. Together, these pathways highlight the complex interconnections between different signaling mechanisms in cellular responses.

Experimental Evidence

  • Studies on c-fos Expression: Mutational analyses have shown that specific sequences within the c-fos promoter are critical for its calcium and cAMP responsiveness. Through experiments employing various signaling inhibitors and modulators, it has been demonstrated that activation does not require new protein synthesis, supporting the notion of a fast transcriptional response in neuronal cells. Such findings underscore the potential for immediate early genes like c-fos to act as rapidly responding markers for cellular activity and adaptation.

Cellular and Physiological Implications

  • Gene Expression Regulation: The immediate early genes, inclusive of c-fos, mediate the cellular response to a variety of stimuli, playing pivotal roles in growth, differentiation, and adaptation to environmental changes. Their regulation is essential not only in neuronal contexts but also in other cell types where rapid transcriptional responses are necessary.

  • Neuronal Plasticity: Understanding the mechanisms by which c-fos and other immediate early genes function provides significant insights into how environmental signals can translate into lasting changes in neuronal circuits, impacting learning, memory, and behavioral responses. These insights offer essential clues for developing interventions in cognitive deficits and mental health disorders.

Overall Significance

  • These findings elucidate the complex interplay between calcium and cAMP signaling in regulating immediate early gene expression, particularly concerning neuronal activity and memory formation. A deeper understanding of these mechanisms may reveal therapeutic targets for cognitive dysfunctions, hormonal regulation, and other neurological disorders, enhancing our capability to design effective treatments tailored to individual needs in neuronal health.

Figure 1: Calcium Response Element (CaRE) and CREB Binding

  • Illustrates the interaction between CaRE and CRE in the gene activation process.

  • Highlights the role of CREB as a transcription factor influencing gene expression via these elements.

  • Emphasizes the importance of the calcium and cAMP signaling pathways in binding.

Figure 2: Phosphorylation of CREB

  • Shows the phosphorylation sites on CREB with emphasis on serine 133.

  • Details distinct signaling pathways activated by calcium influx versus cAMP.

  • Visual representation of how phosphorylation is crucial for initiating transcription.

Figure 3: Mechanism of Activation

  • Depicts the entry of calcium ions through voltage-dependent channels during membrane depolarization.

  • Illustrates downstream pathways triggered by calcium influx leading to CREB activation.

  • Highlights the role of calcium/calmodulin-dependent kinases in this process.

Figure 4: Rapid Response of c-fos Transcription Activation

  • Represents the time scale of changes in c-fos transcription following stimuli.

  • Visualizes the swift nature of transcriptional activation important for neuronal processes.

  • Highlights examples of synaptic plasticity responses during rapid activation.

Figure 5: Distinct but Converging Pathways

  • Compares the calcium pathway versus cAMP pathway regarding CREB phosphorylation.

  • Illustrates adenylate cyclase activation and the consequences for intracellular cAMP levels.

  • Highlights the interconnectedness of these signaling mechanisms contributing to gene expression.

Figure 6: Experimental Evidence on c-fos Expression

  • Presents findings from mutational analyses of the c-fos promoter focusing on responsiveness.

  • Shows data from signaling inhibitor experiments indicating the speed of transcriptional response.

  • Visualizes the importance of c-fos as a marker for cellular activity and adaptation.

Figure 7: Implications for Gene Expression Regulation

  • Visualizes the role of immediate early genes in cellular adaptation to stimuli.

  • Emphasizes the significance in both neuronal contexts and other cell types.

  • Highlights growth and differentiation processes mediated by these gene expressions.

Figure 8: Neuronal Plasticity Insights

  • Illustrates how c-fos contributes to translating environmental signals into cellular changes.

  • Shows connections between these mechanisms and their impact on learning and memory.

  • Highlights potential therapeutic applications for cognitive deficits and mental health.

Overall Significance Figure

  • Summarizes the complex interplay between calcium and cAMP signaling.

  • Visualizes importance for neuronal activity and memory formation.

  • Highlights future therapeutic targets for cognitive dysfunctions and neurological disorders.