Introduction:
The cell signaling pathways triggered by the steroid hormone maintain specific genes in the DNA (deoxy-ribo nucleic acid) of the cell. The hormones and receptors play as transcription regulators by increasing or decreasing the production of mRNA (a type of RNA which produces protein) of specific genes. Signaling pathways are influenced by the level of signals, signaling components, the location of these components, and their modification.
What Are the Types of Signaling Pathways?
Chemical Signaling:
Usually, there are two types of signaling between living organisms:
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Interaction between cells is called intercellular signaling.
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Interactions within a cell are called intracellular signaling.
Chemical signaling is released by ligands, which are signaling cells in the form of small, volatile, or soluble molecules. A ligand is a molecule attached to another specific molecule, sometimes delivering the signal during the process, where ligands are thought of as signaling molecules. Ligands interact with target cell proteins; these cells are affected by chemical signals, whereas these proteins are also known as receptors. Ligands and receptors are of different forms, where a specific receptor will have a specific ligand that typically binds with only that ligand.
What Are the Forms of Signaling?
There are four types of chemical signaling in multicellular organisms. The main difference between the different types of signaling is the distance the signals travel through the organism to reach the target cell. The same signal does not influence all cells.
The four types of signaling are:
Paracrine Signaling:
Paracrine signals act between close cells, where these cells transfer through the extracellular matrix. These signals bring out a quick response that lasts for a short period. To maintain the response localized, paracrine ligand molecules are degraded quickly by the enzymes or eliminated by the neighboring cells. Eliminating the signals will cause a concentration gradient for the signals, allowing them to diffuse quickly through the intracellular space if released again.
Endocrine Signaling:
These signals are produced by the endocrine cells, and these signals are produced by distant cells. In the body, many endocrine cells exist in the endocrine gland, like the thyroid gland, hypothalamus, and pituitary gland. These signals produce a slower Response but have a long-lasting effect. Hormones are the ligands released from endocrine signaling; hormones are signaling molecules formed in one part of the body and affect other body regions. Hormones travel longer distances between the endocrine cells and the target cells through the bloodstream, which is the slow way to move through the body. Due to their form of transport, hormones are diluted and are present in low concentration when they act on the target cells. It differs from paracrine signaling, where the ligand concentration is high.
Autocrine Signaling:
Autocrine signals are released by signaling cells that bind to the released ligand; that is, the signaling cells and the target cells can be the same or similar cells. These signalings are produced during the early development of an organism to ensure the development of cells in the correct tissue and take up the proper function. It also regulates pain sensation and inflammatory response. If a cell is affected by a virus, the cell can signal itself to cell death. Sometimes the neighboring cells of the same type are also affected by the released ligand.
Direct Signaling Across Cell Junction:
The connection between the plasma membrane of neighboring cells is the gap junction in animals and plasmodesmata in plants. Water-filled pathways allow these small signal molecules called intracellular mediators to diffuse between the cells. Smaller molecules like calcium ions can diffuse through the channel, whereas larger molecules like the DNA and protein can not. The transfer of signaling molecules communicates the current state of the cell next to the target cells; this allows a group of cells to coordinate a response to a signal received by a single cell.
What Are the Types of Receptors?
The ligand binds to the receptors and protein molecules in the target cell or its surface. There are two types of receptors:
Internal Receptor:
It is also called an intracellular or cytoplasmic receptor, is present in the cytoplasm, and responds to hydrophobic ligand molecules that travel across the plasma membrane. Once they enter the cell, these molecules bind to protein regulators of mRNA synthesis to regulate gene expression (it is a cellular process of changing the information in the DNA of a cell into amino acid, which forms a protein).
When a ligand binds to the internal receptor, a change is stimulated, which exposes a DNA-binding site on the protein. This ligand-receptor complex moves to the nucleus and binds to the chromosomal DNA-specific regulatory regions. It initiates transcription (copying information in a DNA cell into a special messenger RNA). Internal receptors can directly affect gene expression without passing the signals to other receptors or messengers.
Cell-Surface Receptors:
These are transmembrane receptors; cell-surface, membrane-anchored proteins bind to external ligand molecules. This type of receptor converts an extracellular signal to an intercellular signal. Cell-surface receptors are known as specific proteins or makers as they are specific to individual cell types. A malfunction in any of these proteins results in severe consequences.
Three components of cell-surface receptors are mentioned below
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External ligand binding domain or extracellular domain.
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Hydrophobic membrane-spanning region.
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Intracellular domain inside the cell.
The size and extent of each domain vary depending on the type of receptors. Cell-surface receptors play a major role in signaling in multicellular organisms. Three types of cell-surface receptors are
Ion Channel-Linked Receptors:
These receptors bind to a ligand and open a channel through a membrane that allows the passage of specific ions.
G-Protein Membrane Receptor:
It binds to a ligand and activates a G-protein membrane, which interacts either with an ion channel or an enzyme in the membrane.
Enzyme-Linked Receptors:
These are cell-surface receptors with an intracellular domain connected with an enzyme.
What Are Signaling Molecules?
- Small Hydrophobic Ligands:
These diffuse directly through the plasma membrane and communicate with internal receptors. Steroid hormones are the important members of these ligands; these include female sex hormones, estradiol (a type of estrogen), male sex hormone, and cholesterol. Hydrophobic ligands must bind to carrier proteins to dissolve in blood as they are transported through the bloodstream.
2. Water-Soluble Ligands:
Water-soluble ligands are polar and can not pass through the plasma membrane. Most of these ligands bind to the extracellular domain of the cell-surface receptor. These ligands include small molecules, peptides, and proteins.
3. Other Ligands:
Nitric oxide is a gas that is a form of ligand, which diffuses directly through the plasma membrane and interacts with receptors in smooth muscles, and induces relaxation of the tissues. This ligand functions for only short distances as it has a very short half-life.
4. Hormones:
Hormones bind to specific hormone receptors and regulate the chances in the targeted cell. Even though these hormones come in contact with different cells as they move through the bloodstream, they affect only those cells which possess the necessary receptor. Cells may possess many receptors for the same hormone; the number of receptors that respond to a hormone indicates the sensitivity of the cell to that hormone which may increase or decrease over time.
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Up-Regulation:
The number of receptors increases in response to increased hormone levels, which makes the cell more sensitive to the hormone and increases cellular activity.
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Down-Regulation:
Down-regulation is when the number of receptors reduces to raise the hormone level resulting in reduced cellular activity.
Conclusion:
Hormones are signals or chemical messengers released from the endocrine gland, which is very important for the functioning of various systems. These hormone signals help the hormone to bind with the specific receptor to perform its function in a specific cell.
