The synapsins are a family of neuronal phosphoproteins evolutionarily conserved in invertebrate and vertebrate organisms with a molecular weight from 63 to 75 KDa. Their best-characterised function is to modulate neurotransmitter release at the pre-synaptic terminal, by reversibly tethering synaptic vesicles (SVs) to the actin cytoskeleton. However, many recent data have suggested novel functions for synapsins in other aspects of the pre-synaptic physiology, such as SV docking, fusion and recycling.
The synapsin (syn) family is composed of ten homologous proteins: syns Ia–b, IIa–b, and IIIa–f.
Synapsins are composed of a highly homologous N-terminal region that can be divided in three domains: A, B and C, and a more variable C-terminal portion composed of differently spliced domains (domains D–I).
In mammals, the different isoforms are generated by alternative splicing of three distinct genes, termed SYN1, SYN2 and SYN3, which have been mapped in human and mouse on chromosome X (Xp11), chromosome 3 (3p25) and chromosome 22 (22q12.3) respectively.
CHEMICAL STRUCTURE AND IMAGES
EVOLUTION OF SYNAPSIN
Synapsins or synapsin-like molecules have now been identified in a wide range of vertebrate and invertebrate organisms. The most accredited hypothesis to date postulates that the synapsin family originated from one ancestral gene, which was subjected to duplication when vertebrates diverged from invertebrates.
Domain A is a short N-terminal region, shared by all synapsin isoforms. It contains the phosphorylation site (site 1) for PKA, CaMKI and IV. Site 1 is the major phosphorylation site conserved between vertebrate and invertebrate synapsins.
Domain B, rich in small amino acids, displays only a weak similarity between different synapsins, and is considered as a
linker region connecting domain A to domain C. In syn I, domain B contains phosphorylation sites 4 and 5 for mitogen-activated protein kinase (MAPK)/Extracellular signal-regulated kinase (Erk).
Domain C is a large region of about 300 aa, containing both hydrophobic and highly charged sequences with the potential to assume a-helix and b-sheet conformation. It shows extensive homology amongst the various members of the family, and mediates the core functions of synapsins: the interaction with actin filaments and SV phospholipids and synapsin homo- and heterodimerisation. Domain C was found to modulate syn I binding to both SVs and actin through phosphorylation by the Tyr kinase c-Src.
After domain C, the amino acid sequence starts to diverge in the different synapsin gene products. However, all isoforms bear a proline-rich domain namely from D to I.
Domain D is a long, collagenase-sensitive region that contains phosphorylation site 2 for CaMKII, site 3 for CaMKII and p21-activated kinase (PAK), as well as sites 6 and 7 for MAPK/Erk, Cdk1 and Cdk5. Domain D binds to several SH3-containing proteins, such as c-Src, Grb2, the p85 subunit of PI3K, PLC-g and amphiphysin-I and II, as well as to other interactors such as CaMKII and Rab3.
PROTEIN AMINOACIDS PERCENTAGE
SYNTHESIS AND TURNOVER
To exert their functions in the pre-synaptic compartment, synapsins need to be anterogradely transported from the cell body to the axon terminals. Multiple pools of newly synthesised synapsin were identified: one pool moving at the rate of fast axonal transport, consistent with the transport of synapsin on pre-assembled SVs, and at least another slower pool possibly associated with cytoskeletal components. More recently, synapsin has been shown to be part of mobile packets that are anterogradely transported along the axon, and contain several pre-synaptic components.
Mass spectrometry analysis on hippocampal tissue has revealed at least 15 distinct synapsin forms, which contain, besides phosphorylation, also N-acetylation, methylation, pyroglutamate, tryptophan oxidation and deamidation sites.
All of these post-translational modifications can alter the interactions properties of synapsin.
Finally synapsins might be degraded in the pre-synaptic terminals via calpain.
Synapsin localised to pre-synaptic terminals to SVs far away from the plasmamembrane (distal pool), as well as to docked vesicles (proximal pool), albeit to a lesser extent. Within the nerve terminals, synapsin was found to be associated with the cytoplasmic surface of small SVs, containing non-peptidergic “classical” neurotransmitters, and virtually absent from large dense-core synaptic vesicles (LDCV) containing neuropeptides.
Synapsins were initially identified as major brain-specific phosphoproteins. They are now known to be substrates for several protein kinases, which all contribute to the modulation of synapsin function.
Sinapsin bind phospholipid membranes and SVs protein, both necessary for efficient vesicles fusion and neurotransmitter release and have been reported to bind virtually all the cytoskeletal components present in the pre-terminal and terminal compartments of the axon: F- and G-actin, spectrin, microtubules and neurofilaments.
SYNAPTOGENESIS AND SYNAPTIC PLASTICITY
Synapsin are also implicated in neurites outgrowth and synapse formation with different role for each isoform as shown below.
The influence of each synapsin isoform on the different biological processes is ranked as mild (0), intermediate (00), or strong (000).
In synaptic transmission synapsin phosphorylation allows an higher release of neurotransmitter, which is involved in several type of synaptic potentiation both in short-term plasticity and long-term plasticity.
Fine regulation of synapsin phosphorylation is a synaptic modification mechanism underlying learning and memory processes.
5′ region of synapsin gene is rich in GC, and contain a consensus motif for the cAMP responsive element (CRE), but no TATA or CAAT consensus elements. In addition, two motifs were identified, which were very similar to sequences found in the regulatory regions of the TrkA and 68 kDa neurofilament protein genes, opening the possibility that a subset of neuronal genes might be under the control of the same factors. Interestingly, although the CRE-binding (CREB) transcription factor was able to bind the CRE sequence in the promoter region of the SYN gene, synapsin transcription did not increase upon forskolin treatment; on the contrary, rising cAMP levels led to a slight decrease of synapsin mRNA. However, the CRE element was shown to bind several nuclear proteins, and to contribute to the basal activation of the syn I gene.
Moreover mutations in the silencer element of the synapsin gene were shown to promote expression in non-neuronal cells, by disrupting the binding of specific transcriptional repressor proteins. One of these factors was later identified in the neuron-restrictive silencer factor (NRSF), also known as repressor element-1 silencing transcription factor (REST), which regulates the transcription of several neuronal genes, including the BDNF gene.
To date there is no diagnostic use of synapsin, however genetic analyses in human populations have identified a nonsense mutation (Identification of a mutation in synapsin I, a synaptic vesicle protein, in a family with epilepsy 2004) in the gene coding for syn I, likely to cause mRNA degradation, as the cause of epilepsy in a family with history of epilepsy alone, or associated with aggressive behaviour, learning disabilities or autism. Several study show that anaesthetic agents decreased the levels of syn I phosphorylation whereas convulsant agents and intermittent amphetamine treatment increased it. Moreover lack of synapsin proteins can generate epileptic seizures involving distinct neuronal populations in different brain areas and is associated with behavioural and learning deficits.
Furthermore disfunction of synapsin proteins is also associated with schizophrenia (2) and other psychoses.