Cell-Cell Interactions

Overview of Cell-Cell Interactions and the Cell Surface

Biological systems rely on the ability of cells to interact with their surroundings and each other. Unicellular organisms must contend with constant shifts in environmental conditions, requiring them to be highly adaptive to external changes. In contrast, cells in multicellular organisms function as an interdependent community where they must communicate and cooperate to ensure the survival and function of the organism as a whole. This interaction begins at the cell surface. The plasma membrane consists of a phospholipid bilayer studded with proteins, which may be integral or peripheral. These membrane proteins serve critical roles by regulating transport to create an internal environment distinct from the exterior, attaching to cytoskeletal elements on the interior surface, and anchoring the cell to a complex array of extracellular structures.

Structure and Function of the Extracellular Matrix

Most cells possess a protective layer or wall that forms just beyond the plasma membrane. This is known as the extracellular matrix (ECM). The ECM is fundamental to the cell as it helps define cell shape, provides a mechanism for attachment to other cells, and acts as a first line of defense against the external environment. In plants, new cells secrete a fiber composite called the primary cell wall. This structure consists of long strands of the polysaccharide cellulose bundled into cable-like microfibrils that form a crisscrossed network. Between these microfibrils are gelatinous polysaccharides, such as pectin, which function to keep the cell wall moist. Some mature plant cells eventually secrete a secondary cell wall located between the plasma membrane and the primary cell wall. The composition of this secondary wall correlates with the specific function of the cell; for example, wood-forming cells contain lignin for structural rigidity, while leaf cells contain waxes for protection.

Animal cells also secrete a fiber composite ECM that provides structural support. The fibrous component of the animal ECM is primarily composed of collagen. Collagen proteins form triple helixes that coalesce to create collagen fibrils. These fibers are embedded in a ground substance made of proteoglycans, which are proteins attached to many polysaccharides. Proteoglycans are responsible for the rubber-like consistency of tissues such as cartilage. The composition and amount of ECM vary significantly between different types of tissues, which are groups of similar cells functioning as a unit. For instance, the protein elastin provides lung tissue with the ability to stretch. To maintain structural integrity, membrane proteins called integrins bind to cross-linking proteins in the ECM, such as laminins. These attachments link the ECM to the plasma membrane and anchor the internal cytoskeleton to the external matrix.

Cell-Cell Attachments and Adhesion in Animal and Plant Cells

Cells must be physically connected to form tissues. In plants, cells are "glued" together by the middle lamella, a layer comprised of gelatinous pectins that is continuous with the primary cell walls of adjacent cells. In animals, several specialized structures facilitate adhesion. Tight junctions form a watertight seal between adjacent cells, preventing the passage of molecules between them. Desmosomes provide strong cell-cell attachment, particularly in epithelial and muscle cells. Within desmosomes, intermediate filaments inside the cell link to cadherin proteins. These cadherins extend into the extracellular space and bind to cadherins of the same type on the adjacent cell. This specificity allows cells of the same type to recognize and attach to one another.

Communication Portals: Gap Junctions and Plasmodesmata

Direct communication between adjacent cells is facilitated by specialized channels. In animal tissues, gap junctions connect adjacent cells by forming protein channels that allow for the flow of small molecules and regulatory ions. These portals help cells coordinate their activities through the rapid passage of signals. In plant cells, similar functions are performed by plasmodesmata. These are gaps in the cell walls where the plasma membranes, cytoplasm, and smooth endoplasmic reticulum (ER) of two cells connect, forming a continuous internal pathway for communication.

Cell-Cell Signaling in Multicellular Organisms

Communication between distant cells often involves signaling molecules. For example, neurotransmitters may open or close channels in distant target cells. Hormones are information-carrying molecules secreted from a cell that circulate throughout the body to act on target cells far from the signaling source. The presence of a receptor determines if a cell responds to a specific signal. Receptors for lipid-soluble signaling molecules are located in the target cell's cytoplasm because these molecules can diffuse across the plasma membrane. Conversely, lipid-insoluble signaling molecules cannot cross the plasma membrane and must be recognized by receptors located on the cell surface. Once a signal is received, the cell must initiate a response through signal transduction, converting the extracellular signal into an intracellular message.

Signal Transduction via GPCRs and Enzyme-Linked Receptors

G-protein-coupled receptors (GPCRs) represent a major class of surface receptors. When a signaling molecule binds to a GPCR, it triggers the activation of a G protein. This often involves a process where the G protein activates an enzyme like adenylyl cyclase, which converts $ATP$ into cyclic adenosine monophosphate ($cAMP$). The $cAMP$ acts as a second messenger, activating molecules such as Protein Kinase A. Both the alpha subunit and the beta/gamma complex of the G protein can activate other molecules to cause signal transduction.

Enzyme-linked receptors, such as Receptor Tyrosine Kinases (RTKs), are transmembrane proteins that directly catalyze reactions inside the cell. The RTK signaling process involves five distinct steps. First, a hormone binds to two RTK subunits, causing them to form a dimer. Second, the RTK is phosphorylated by $ATP$. Third, bridge proteins connect the RTK to a Ras protein, activating it. Fourth, the activated Ras phosphorylates a protein kinase. Finally, this triggers a phosphorylation cascade of protein kinases. These cascades are often initiated by mitogens, such as mitogen-activated protein kinases (MAPKs), which are signaling molecules that activate cell division.

Signal Deactivation and Crosstalk

Cells possess built-in systems to turn off intracellular signals rapidly. Phosphatases are enzymes that remove phosphate groups from proteins in a phosphorylation cascade, effectively deactivating the signal. Furthermore, signal transduction pathways do not operate in isolation; they intersect and connect through a process called crosstalk. This forms complex signaling networks where sequences in Pathway A may inhibit or stimulate components of Pathway B or C. This integration allows a cell to synthesize input from many different signals and produce a coordinated response.

Signaling and Quorum Sensing in Unicellular Organisms

Unicellular organisms also communicate, often to respond to environmental changes. Quorum sensing is a signaling mechanism where bacteria release species-specific signaling molecules to monitor population density. When the concentration of these molecules reaches a specific threshold, the bacteria coordinate their activities. This can lead to the formation of biofilms, such as dental plaque, which glue a community of microbes to a surface. In some cases, quorum sensing occurs via G-protein-coupled receptors. In organisms like slime molds, this signaling can cause previously free-living cells to aggregate into a multicellular structure.