9.3 Response to the Signal

Cell Signaling and Cellular Processes

Cell signaling pathways are how cells receive information from their environment and translate it into action. Whether a cell grows, divides, changes its metabolism, or dies depends on the signals it receives and how it responds. This section focuses on that response phase: what happens inside the cell after a signal has been transduced.

Signaling Pathways in Cellular Processes

Signal transduction is the process of relaying an extracellular stimulus to intracellular effectors. Here's the general flow:

1. A ligand (like a hormone or neurotransmitter) binds to a cell surface receptor.
2. That binding triggers a cascade of intracellular events, often involving second messengers like cAMP or calcium ions.
3. The cascade ultimately activates transcription factors (such as CREB or NF-κB), which enter the nucleus and change gene expression.

Regulating protein expression. Transcription factors activated by signaling cascades bind to specific DNA sequences called promoters and enhancers. Depending on the factor, this either promotes or represses transcription of target genes, which directly alters which proteins the cell makes.

Modulating metabolism. Signaling cascades can activate or inhibit metabolic enzymes. A classic example is the insulin signaling pathway:

1. Insulin binds to its receptor, triggering a phosphorylation cascade inside the cell.
2. This causes the glucose transporter GLUT4 to move to the cell membrane, allowing glucose to enter the cell.

This is how your body regulates blood sugar after a meal.

Controlling cell growth and proliferation. Growth factors like EGF and PDGF act as mitogenic signals, meaning they stimulate cell division. A key pathway here is the MAP kinase (MAPK) pathway:

1. Growth factors activate receptor tyrosine kinases (RTKs) on the cell surface.
2. RTKs initiate the MAPK cascade, a chain of phosphorylation events.
3. Downstream targets like cyclin D and CDK4/6 are activated, pushing the cell through the cell cycle toward division.

Function of Protein Kinase C

Protein kinase C (PKC) is a family of serine/threonine kinases that plays a central role in signal transduction. It's activated by two second messengers: diacylglycerol (DAG) and calcium ions ($Ca^{2+}$).

Here's how PKC gets activated:

1. A ligand binds to a G protein-coupled receptor (GPCR), which activates the enzyme phospholipase C (PLC).
2. PLC cleaves the membrane lipid $PIP_2$ into two products: DAG (which stays in the membrane) and $IP_3$ (which diffuses into the cytoplasm).
3. $IP_3$ triggers the release of $Ca^{2+}$ from the endoplasmic reticulum.
4. DAG and $Ca^{2+}$ together activate PKC.

Once active, PKC phosphorylates a variety of target proteins, altering their activity, localization, or stability. Its targets include enzymes, ion channels, and transcription factors.

PKC is involved in a wide range of cellular processes:

• Cell growth and differentiation (e.g., in keratinocytes and neurons)
• Apoptosis (through phosphorylation of Bcl-2 family proteins)
• Immune cell activation (T cells and B cells)
• Neurotransmitter release and synaptic plasticity (including long-term potentiation, a process tied to learning and memory)

Importance of Programmed Cell Death

Apoptosis is a tightly regulated form of programmed cell death. Unlike necrosis (which is messy and inflammatory), apoptosis is orderly. The cell shrinks, its chromatin condenses, and it breaks into small apoptotic bodies that are quickly engulfed by neighboring cells or macrophages, with minimal damage to surrounding tissue.

Why apoptosis matters for development. During embryonic development, apoptosis sculpts tissues by removing excess or unwanted cells. For example, the webbing between your fingers is removed by apoptosis during limb development. Neurons that fail to form proper connections are also eliminated this way.

Why apoptosis matters in adults. In mature tissues, apoptosis balances cell proliferation to maintain constant cell numbers. It also clears out senescent, damaged, or infected cells before they can cause problems.

What happens when apoptosis goes wrong:

• Too little apoptosis can lead to cancer (cells that should die instead keep dividing), autoimmune disorders, and persistent viral infections.
• Too much apoptosis is linked to neurodegenerative diseases. In Alzheimer's and Parkinson's, premature neuron death contributes to disease progression.

The two apoptotic pathways both converge on the same endpoint but start differently:

• Intrinsic pathway: Triggered by intracellular stress like DNA damage or oxidative stress.

  1. Cytochrome c is released from the mitochondria into the cytoplasm.
  2. This activates caspase-9, an initiator caspase.

• Extrinsic pathway: Triggered by external death signals, such as FasL binding to the Fas receptor on the cell surface.

  1. This activates caspase-8, another initiator caspase.

Both pathways converge on the activation of executioner caspases (caspase-3, -6, and -7), which cleave key cellular substrates and carry out the actual dismantling of the cell.

Cellular Response Regulation

Several mechanisms work together to regulate how cells respond to signals.

Cell cycle control. Cyclins and cyclin-dependent kinases (CDKs) regulate progression through each phase of the cell cycle. Checkpoints at key transitions ensure that each phase is completed correctly before the cell moves on. If something is wrong (like damaged DNA), the checkpoint can halt the cycle.

Gene expression changes. Signaling cascades activate transcription factors that bind specific DNA sequences to turn genes on or off. Epigenetic modifications (like histone acetylation or DNA methylation) can also change how accessible genes are in response to signals, adding another layer of regulation.

Signal amplification through second messengers. Second messengers like cAMP, cGMP, and $Ca^{2+}$ amplify and spread signals within the cell. A single receptor activation can generate many second messenger molecules, which in turn activate many downstream effectors. This amplification is why even a tiny amount of hormone can produce a large cellular response.

Phosphorylation and dephosphorylation. Protein kinases add phosphate groups to target proteins, changing their activity or function. This is one of the most common ways signals are transmitted through a cascade. Equally important, phosphatases remove those phosphate groups. Without phosphatases, signals would never turn off. This balance between kinases and phosphatases is what allows cells to respond to signals and then return to baseline.