Cell–cell interaction

Cell–cell interaction refers to the direct interactions between cell surfaces that play a crucial role in the development and function of multicellular organisms.These interactions allow cells to communicate with each other in response to changes in their microenvironment. This ability to send and receive signals is essential for the survival of the cell. Interactions between cells can be stable such as those made through cell junctions. These junctions are involved in the communication and organization of cells within a particular tissue. Others are transient or temporary such as those between cells of the immune system or the interactions involved in tissue inflammation. These types of intercellular interactions are distinguished from other types such as those between cells and the extracellular matrix. The loss of communication between cells can result in uncontrollable cell growth and cancer. Cell–cell interaction refers to the direct interactions between cell surfaces that play a crucial role in the development and function of multicellular organisms.These interactions allow cells to communicate with each other in response to changes in their microenvironment. This ability to send and receive signals is essential for the survival of the cell. Interactions between cells can be stable such as those made through cell junctions. These junctions are involved in the communication and organization of cells within a particular tissue. Others are transient or temporary such as those between cells of the immune system or the interactions involved in tissue inflammation. These types of intercellular interactions are distinguished from other types such as those between cells and the extracellular matrix. The loss of communication between cells can result in uncontrollable cell growth and cancer. Stable cell-cell interactions are required for cell adhesion within a tissue and controlling the shape and function of cells. These stable interactions involve cell junctions which are multiprotein complexes that provide contact between neighboring cells. Cell junctions allow for the preservation and proper functioning of epithelial cell sheets. These junctions are also important in the organization of tissues where cells of one type can only adhere to cells of the same tissue rather than to a different tissue. Tight junctions are multi-protein complexes that hold cells of a same tissue together and prevent movement of water and water-soluble molecules between cells. In epithelial cells, they function also to separate the extracellular fluid surrounding their apical and basolateral membranes. These junctions exist as a continuous band located just below the apical surface between the membranes of neighboring epithelial cells. The tight junctions on adjacent cells line up so as to produce a seal between different tissues and body cavities. For example, the apical surface of gastrointestinal epithelial cells serve as a selective permeable barrier that separates the external environment from the body. The permeability of these junctions is dependent on a variety of factors including protein makeup of that junction, tissue type and signaling from the cells. Tight junctions are made up of many different proteins. The four main transmembrane proteins are occludin, claudin, junctional adhesion molecules (JAMs) and tricellulins. The extracellular domains of these proteins form the tight junction barrier by making homophilic (between proteins of the same kind) and heterophilic interactions (between different types of proteins) with the protein domains on adjacent cells. Their cytoplasmic domains interact with the cell cytoskeleton to anchor them. Of the three types of anchoring junctions, only two are involved in cell-cell interactions: adherens junctions and desmosomes. Both are found in many types of cells. Adjacent epithelial cells are connected by adherens junctions on their lateral membranes. They are located just below tight junctions. Their function is to give shape and tension to cells and tissues and they are also the site of cell-cell signaling. Adherens junctions are made of cell adhesion molecules from the cadherin family. There are over 100 types of cadherins, corresponding to the many different types of cells and tissues with varying anchoring needs. The most common are E-, N- and P-cadherins. In the adherens junctions of epithelial cells, E-cadherin is the most abundant. Desmosomes also provide strength and durability to cells and tissues and are located just below adherens junctions. They are sites of adhesion and do not encircle the cell. They are made of two specialized cadherins, desmoglein and desmocollin. These proteins have extracellular domains that interact with each other on adjacent cells. On the cytoplasmic side, plakins form plaques which anchor the desmosomes to intermediate filaments composed of keratin proteins. Desmosomes also play a role in cell-cell signaling. Gap junctions are the main site of cell-cell signaling or communication that allow small molecules to diffuse between adjacent cells. In vertebrates, gap junctions are composed of transmembrane proteins called connexins. They form hexagonal pores or channels through which ions, sugars, and other small molecules can pass. Each pore is made of 12 connexin molecules; 6 form a hemichannel on one cell membrane and interact with a hemichannel on an adjacent cell membrane. The permeability of these junctions is regulated by many factors including pH and Ca2+ concentration. Receptor proteins on the cell surface have the ability to bind specific signaling molecules secreted by other cells. Cell signaling allows cells to communicate with adjacent cells, nearby cells (paracrine) and even distant cells (endocrine). This binding induces a conformational change in the receptor which, in turn, elicits a response in the corresponding cell. These responses include changes in gene expression and alterations in cytoskeleton structure. The extracellular face of the plasma membrane has a variety of proteins, carbohydrates, and lipids which project outward and act as signals. Direct contact between cells allows the receptors on one cell to bind the small molecules attached to the plasma membrane of different cell. In eukaryotes, many of the cells during early development communicate through direct contact. Synaptic signaling, an integral part of nervous system activity, occurs between neurons and target cells. These target cells can also be neurons or other cell types (i.e. muscle or gland cells). Protocadherins, a member of the cadherin family, mediate the adhesion of neurons to their target cells at synapses otherwise known as synaptic junctions. In order to for communication to occur between a neuron and its target cell, a wave of depolarization travels the length of the neuron and causes neurotransmitters to be released into the synaptic junction. These neurotransmitters bind and activate receptors on the post-synaptic neuron thereby transmitting the signal to the target cell. Thus, a post-synaptic membrane belongs to the membrane receiving the signal, while a pre-synaptic membrane is the source of the neurotransmitter. In a neuromuscular junction, a synapse is formed between a motor neuron and muscle fibers. In vertebrates, acetylcholine released from the motor neuron acts as a neurotransmitter which depolarizes the muscle fiber and causes muscle contraction. A neuron’s ability to receive and integrate simultaneous signals from the environment and other neurons allows for complex animal behavior.