Synapsin
Encyclopedia
The synapsins are a family of protein
s that have long been implicated in the regulation of neurotransmitter
release at synapse
s. Specifically, they are thought to be involved in regulating the number of synaptic vesicles
available for release via exocytosis
at any one time.
Current studies suggest the following hypothesis for the role of synapsin: synapsins bind synaptic vesicles to components of the cytoskeleton
which prevents them from migrating to the presynaptic membrane and releasing transmitter. During an action potential
, synapsins are phosphorylated
by Ca2+/calmodulin-dependent protein kinase II
, releasing the synaptic vesicles and allowing them to move to the membrane and release their neurotransmitter
.
Gene knockout
studies in mice (where the mouse is unable to produce synapsin) have had some surprising results. Mice lacking all three synapsins are prone to seizures, and experience learning
defects. These results suggest that while synapsins are not essential for synaptic function, they do serve an important modulatory role. Conversely, studies using transgenic mice in which neuronal signaling is abolished in specific circuitries showed that synaptic
activity regulates, but is not essential to maintain, the expression of these proteins.
s possess three genes encoding three different synapsin proteins. Each gene in turn is alternatively spliced
to produce at least two different protein isoform
s for a total of six isoforms:
Different neuron terminals will express varying amounts of each of these synapsin proteins and collectively these synapsins will comprise 1% of the total expressed protein at any one time. Synapsin Ia has been implicated in bipolar disorder and schizophrenia.
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s that have long been implicated in the regulation of neurotransmitter
Neurotransmitter
Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...
release at synapse
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...
s. Specifically, they are thought to be involved in regulating the number of synaptic vesicles
Vesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
available for release via exocytosis
Exocytosis
Exocytosis , also known as 'The peni-cytosis', is the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane...
at any one time.
Current studies suggest the following hypothesis for the role of synapsin: synapsins bind synaptic vesicles to components of the cytoskeleton
Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
which prevents them from migrating to the presynaptic membrane and releasing transmitter. During an action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
, synapsins are phosphorylated
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
by Ca2+/calmodulin-dependent protein kinase II
Ca2+/calmodulin-dependent protein kinase
/calmodulin-dependent protein kinases II or CaM kinases II are serine/threonine-specific protein kinases that are regulated by the /calmodulin complex...
, releasing the synaptic vesicles and allowing them to move to the membrane and release their neurotransmitter
Neurotransmitter
Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...
.
Gene knockout
Gene knockout
A gene knockout is a genetic technique in which one of an organism's genes is made inoperative . Also known as knockout organisms or simply knockouts, they are used in learning about a gene that has been sequenced, but which has an unknown or incompletely known function...
studies in mice (where the mouse is unable to produce synapsin) have had some surprising results. Mice lacking all three synapsins are prone to seizures, and experience learning
Learning
Learning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...
defects. These results suggest that while synapsins are not essential for synaptic function, they do serve an important modulatory role. Conversely, studies using transgenic mice in which neuronal signaling is abolished in specific circuitries showed that synaptic
Synaptic
Synaptic may refer to:*Synapse, part of the nervous system*Synapsis, the pairing of two homologous chromosomes*Synaptic , a Linux graphical package management program for APT See also...
activity regulates, but is not essential to maintain, the expression of these proteins.
Family members
Humans and most other vertebrateVertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s possess three genes encoding three different synapsin proteins. Each gene in turn is alternatively spliced
Alternative splicing
Alternative splicing is a process by which the exons of the RNA produced by transcription of a gene are reconnected in multiple ways during RNA splicing...
to produce at least two different protein isoform
Protein isoform
A protein isoform is any of several different forms of the same protein. Different forms of a protein may be produced from related genes, or may arise from the same gene by alternative splicing. A large number of isoforms are caused by single-nucleotide polymorphisms or SNPs, small genetic...
s for a total of six isoforms:
| Gene | Protein | Isoforms |
|---|---|---|
| SYN1 | Synapsin I Synapsin I 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... |
Ia, Ib |
| SYN2 | Synapsin II | IIa, IIb |
| SYN3 | Synapsin III SYN3 Synapsin-3 is a protein that in humans is encoded by the SYN3 gene.-Further reading:... |
IIIa, IIIb |
Different neuron terminals will express varying amounts of each of these synapsin proteins and collectively these synapsins will comprise 1% of the total expressed protein at any one time. Synapsin Ia has been implicated in bipolar disorder and schizophrenia.