Functions
Based on their brain/endocrine distribution and biochemical properties, in particular C2 domains of certain synaptotagmins bound to calcium, synaptotagmins were proposed to function as calcium sensors in the regulation of neurotransmitter release and hormone secretion. Although synaptotagmins share a similar domain structure and a high degree of homology in the C2 domains, not all synaptotagmins bind to calcium. In fact, only eight out of the fifteen synaptotagmins are capable of calcium binding. The calcium binding synaptotagmins include synaptotagmins 1, 2, 3, 5, 6, 7, 9, and 10. The remaining seven synaptotagmins do not bind to calcium due to the lack of calcium coordinating residues or spatial orientation of the acidic residues (see the section on C2 domains for details).
Calcium-binding synaptotagmins act as Ca2+ sensors and are involved in both:
- early synaptic vesicle docking to the presynaptic membrane via interaction with β-neurexin [4] or SNAP-25 [5]
- late steps of Ca2+-evoked synaptic vesicle fusion with the presynaptic membrane. [6] [7] [8] It was also shown that synaptotagmin 1 can displace complexin from the SNARE complex in the presence of calcium. This is thought to be one of the last steps in exocytosis. [9] Calcium-bound synaptotagmin binding to the SNARE complex, causes the fusion clamp effect of complexin to be released, allowing vesicle fusion to occur and exocytosis to proceed. [10]
Synaptotagmins directly affect the synchronicity of calcium-dependent neurotransmission. While the suppression of Syt1 blocks fast, synchronous neurotransmission, it also enhances slow, asynchronous neurotransmission. [11] On the other hand, suppression of Syt7 hinders the slower, asynchronous release of neurotransmitters. This suggests that synaptotagmin-7 is responsible for mediating a slower form of Ca(2+)-triggered release while the faster release is induced by synaptotagmin-1. These discrepancies illustrate important distinctions between synaptotagmin isoforms and how they underlie the kinetics of neurotransmission and long-term potentiation.
C-terminal C2-domains
The C2 domain is a widely occurring conserved sequence motif of 130-140 amino acid residues, which was first defined as the second constant sequence in PKC isoforms. [12] The C2 domain was first shown to bind to calcium in synaptotagmin-1. Subsequent atomic structure analysis of synaptotagmin-1 at 1.9 Å resolution indicated that its C2 domains are composed of a stable eight-stranded β-sandwich with flexible loops emerging from the top and bottom. Nuclear magnetic resonance (NMR) studies of synaptotagmin-1 revealed that calcium binds exclusively to the top loops, and the binding pockets are coordinated by five conserved aspartate residues: three calcium ions bind to C2A via D172, D178, D230, D232, S235 and D238, and two calcium ions bind to C2B via D303, D309, D363, 365 and D371. Not all synaptotagmin C2 domains bind to calcium. In fact, based on sequence similarities and subsequent confirmation by biochemical analyses, only eight synaptotagmins bind to calcium, namely, synaptotagmin-1, -2, -3, -5, -6, -7, -9 and -10. The lack of critical residues involved in calcium binding accounts for the majority of failure in calcium-binding by the other synaptotagmins. This includes both C2 domains of synaptotagmin-11, -12, -13, -14 and -15, and C2A domain of synaptotagmin-4 and -8. Synaptotagmin-4 and -11 C2B domains, which possess all five acidic residues in the top loops, however, do not bind to calcium due to spatial orientation of the calcium ligands that fail to form proper calcium binding sites. For calcium-binding synaptotagmins, although amino acid residues in the top loops other than those mentioned above are not directly involved in coordinating calcium binding, they affect calcium binding affinity, such as R233 in synaptotagmin-1. The diversity of sequences and structures flanking the calcium-coordinating amino acid residues renders the eight synaptotagmins bind to calcium at various affinities, covering the full range of calcium requirements for regulated exocytosis.
The C2A domain regulates the fusion step of synaptic vesicle exocytosis. [13] [14] Consistent with this, the kinetics of Ca2+-dependent phospholipid binding activity of the C2A domain in vitro are compatible with the very fast nature of neurotransmitter release (within 200 μs). [15] The C2A domain was shown to bind negatively charged phospholipids in a Ca2+-dependent fashion. Ca2+-binding alters the protein-protein interactions of synaptotagmin such as increasing the affinity of synaptotagmin for syntaxin.
The C2B domain binds to phosphatidyl-inositol-3,4,5-triphosphate (PIP3) in the absence of calcium ions, and to phosphatidylinositol bisphosphate (PIP2) in their presence, [16] suggesting that a lipid-interaction switch occurs during depolarization. Ca2+-binding to the C2B domain confers synaptotagmin dimerization involved in the fusion step of synaptic vesicles by Ca2+-dependent self-clustering via the C2B domain. [17] Ca2+-independent is the interaction between the C2B domain and SNAP-25, and between the C2B domain and the "synprint" (synaptic protein interaction) motif of the pore-forming subunit of voltage-gated calcium channels. The C2B domain regulates also the recycling step of synaptic vesicles by binding to the clathrin assembly protein, AP-2.
Chemical synapses are biological junctions through which neurons' signals can be sent to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous system. They are crucial to the biological computations that underlie perception and thought. They allow the nervous system to connect to and control other systems of the body.
Exocytosis is a form of active transport and bulk transport in which a cell transports molecules out of the cell. As an active transport mechanism, exocytosis requires the use of energy to transport material. Exocytosis and its counterpart, endocytosis, are used by all cells because most chemical substances important to them are large polar molecules that cannot pass through the hydrophobic portion of the cell membrane by passive means. Exocytosis is the process by which a large amount of molecules are released; thus it is a form of bulk transport. Exocytosis occurs via secretory portals at the cell plasma membrane called porosomes. Porosomes are permanent cup-shaped lipoprotein structure at the cell plasma membrane, where secretory vesicles transiently dock and fuse to release intra-vesicular contents from the cell.
In a neuron, synaptic vesicles store various neurotransmitters that are released at the synapse. The release is regulated by a voltage-dependent calcium channel. Vesicles are essential for propagating nerve impulses between neurons and are constantly recreated by the cell. The area in the axon that holds groups of vesicles is an axon terminal or "terminal bouton". Up to 130 vesicles can be released per bouton over a ten-minute period of stimulation at 0.2 Hz. In the visual cortex of the human brain, synaptic vesicles have an average diameter of 39.5 nanometers (nm) with a standard deviation of 5.1 nm.
SNARE proteins – "SNAPREceptors" – are a large protein family consisting of at least 24 members in yeasts, more than 60 members in mammalian cells, and some numbers in plants. The primary role of SNARE proteins is to mediate the fusion of vesicles with the target membrane; this notably mediates exocytosis, but can also mediate the fusion of vesicles with membrane-bound compartments. The best studied SNAREs are those that mediate the release of synaptic vesicles containing neurotransmitters in neurons. These neuronal SNAREs are the targets of the neurotoxins responsible for botulism and tetanus produced by certain bacteria.
Synaptosomal-Associated Protein, 25kDa (SNAP-25) is a Target Soluble NSF (N-ethylmaleimide-sensitive factor) Attachment Protein Receptor (t-SNARE) protein encoded by the SNAP25 gene found on chromosome 20p12.2 in humans. SNAP-25 is a component of the trans-SNARE complex, which accounts for membrane fusion specificity and directly executes fusion by forming a tight complex that brings the synaptic vesicle and plasma membranes together.
A C2 domain is a protein structural domain involved in targeting proteins to cell membranes. The typical version (PKC-C2) has a beta-sandwich composed of 8 β-strands that co-ordinates two or three calcium ions, which bind in a cavity formed by the first and final loops of the domain, on the membrane binding face. Many other C2 domain families don't have calcium binding activity.
Complexin (also known as synaphin) refers to a one of a small set of eukaryotic cytoplasmic neuronal proteins which binds to the SNARE protein complex (SNAREpin) with a high affinity. These are called synaphin 1 and 2. In the presence of Ca2+, the transport vesicle protein synaptotagmin displaces complexin, allowing the SNARE protein complex to bind the transport vesicle to the presynaptic membrane.
Syntaxin-1A is a protein that in humans is encoded by the STX1A gene.
Synaptotagmin-1 is a protein that in humans is encoded by the SYT1 gene.
Regulating synaptic membrane exocytosis protein 1 is a protein that in humans is encoded by the RIMS1 gene.
DnaJ homolog subfamily C member 5, also known as cysteine string protein or CSP is a protein, that in humans encoded by the DNAJC5 gene. It was first described in 1990.
Double C2-like domain-containing protein beta is a protein that in humans is encoded by the DOC2B gene.
Axon terminals are distal terminations of the branches of an axon. An axon, also called a nerve fiber, is a long, slender projection of a nerve cell that conducts electrical impulses called action potentials away from the neuron's cell body in order to transmit those impulses to other neurons, muscle cells or glands. In the central nervous system, most presynaptic terminals are actually formed along the axons, not at their ends.
The ribbon synapse is a type of neuronal synapse characterized by the presence of an electron-dense structure, the synaptic ribbon, that holds vesicles close to the active zone. It is characterized by a tight vesicle-calcium channel coupling that promotes rapid neurotransmitter release and sustained signal transmission. Ribbon synapses undergo a cycle of exocytosis and endocytosis in response to graded changes of membrane potential. It has been proposed that most ribbon synapses undergo a special type of exocytosis based on coordinated multivesicular release. This interpretation has recently been questioned at the inner hair cell ribbon synapse, where it has been instead proposed that exocytosis is described by uniquantal release shaped by a flickering vesicle fusion pore.
Syntaxins are a family of membrane integrated Q-SNARE proteins participating in exocytosis.
Vesicle fusion is the merging of a vesicle with other vesicles or a part of a cell membrane. In the latter case, it is the end stage of secretion from secretory vesicles, where their contents are expelled from the cell through exocytosis. Vesicles can also fuse with other target cell compartments, such as a lysosome. Exocytosis occurs when secretory vesicles transiently dock and fuse at the base of cup-shaped structures at the cell plasma membrane called porosome, the universal secretory machinery in cells. Vesicle fusion may depend on SNARE proteins in the presence of increased intracellular calcium (Ca2+) concentration.
Synapsin I, is the collective name for Synapsin Ia and Synapsin Ib, two nearly identical phosphoproteins that in humans are encoded by the SYN1 gene. In its phosphorylated form, Synapsin I may also be referred to as phosphosynaspin I. Synapsin I is the first of the proteins in the synapsin family of phosphoproteins in the synaptic vesicles present in the central and peripheral nervous systems. Synapsin Ia and Ib are close in length and almost the same in make up, however, Synapsin Ib stops short of the last segment of the C-terminal in the amino acid sequence found in Synapsin Ia.
The active zone or synaptic active zone is a term first used by Couteaux and Pecot-Dechavassinein in 1970 to define the site of neurotransmitter release. Two neurons make near contact through structures called synapses allowing them to communicate with each other. As shown in the adjacent diagram, a synapse consists of the presynaptic bouton of one neuron which stores vesicles containing neurotransmitter, and a second, postsynaptic neuron which bears receptors for the neurotransmitter, together with a gap between the two called the synaptic cleft. When an action potential reaches the presynaptic bouton, the contents of the vesicles are released into the synaptic cleft and the released neurotransmitter travels across the cleft to the postsynaptic neuron and activates the receptors on the postsynaptic membrane.
Synaptic stabilization is crucial in the developing and adult nervous systems and is considered a result of the late phase of long-term potentiation (LTP). The mechanism involves strengthening and maintaining active synapses through increased expression of cytoskeletal and extracellular matrix elements and postsynaptic scaffold proteins, while pruning less active ones. For example, cell adhesion molecules (CAMs) play a large role in synaptic maintenance and stabilization. Gerald Edelman discovered CAMs and studied their function during development, which showed CAMs are required for cell migration and the formation of the entire nervous system. In the adult nervous system, CAMs play an integral role in synaptic plasticity relating to learning and memory.
Ferlins are an ancient protein family involved in vesicle fusion and membrane trafficking. Ferlins are distinguished by their multiple tandem C2 domains, and sometimes a FerA and a DysF domain. Mutations in ferlins can cause human diseases such as muscular dystrophy and deafness. Abnormalities in expression of myoferlin, a human ferlin protein, is also directly associated with higher mortality rate and tumor recurrence in several types of cancer, including pancreatic, colorectal, breast, cervical, stomach, ovarian, cervical, thyroid, endometrial, and oropharyngeal squamous cell carcinoma. In other animals, ferlin mutations can cause infertility.
