Dynamin I is dephosphorylated at Ser-774 and Ser-778 during synaptic vesicle endocytosis (SVE) in nerve terminals. Phosphorylation was proposed to regulate assembly of an endocytic protein complex with amphiphysin or endophilin. Instead, we found it recruits syndapin I for SVE and does not control amphiphysin or endophilin binding in rat synaptosomes. After depolarisation, syndapin exhibited a calcineurin-mediated interaction with dynamin. A phosphorylation sitemimicking peptide disrupted the dynamin-syndapin complex, not the dynamin-endophilin complex, arrested SVE and produced glutamate release fatigue after repetitive stimulation. Pseudo-phosphorylation of Ser-774 or Ser-778 inhibited syndapin binding without affecting amphiphysin recruitment. Site mutagenesis to alanine arrested SVE in cultured neurons. The effects of the sites were additive for syndapin I binding and SVE. Thus syndapin I is a central component of the endocytic protein complex for SVE via stimulus-dependent recruitment to dynamin I and plays a key role in synaptic transmission. KeywordsDynamin; syndapin; protein phosphorylation; endocytosis; synaptosomes; neurons Neurons communicate via the release of neurotransmitter by exocytosis from nerve terminals. After exocytosis, synaptic vesicles (SV) are retrieved by endocytosis to accommodate multiple cycles of synaptic transmission. Synaptic vesicle endocytosis (SVE) is triggered by a coordinated calcineurin-dependent dephosphorylation of a group of at least eight proteins called the dephosphins. They are dynamin I, amphiphysin I/II, synaptojanin, epsin, eps15, AP180 and PIP kinase Iγ 1. The dephosphins are constitutively phosphorylated in nerve terminals and their collective rephosphorylation after SVE is necessary for maintaining the continuity of SV recycling and thus maintenance of synaptic transmission. To date only one dephosphin kinase has been identified, cyclin-dependent kinase 5 (Cdk5) 2. It phosphorylates dynamin I, synaptojanin I and PIP kinase Iγ in vivo 2-4 and other dephosphins such as amphiphysin I in vitro 5. Cdk5 activity is required for SVE 2, yet it remains unknown whether each phosphorylation site in these substrates is functionally important for the basic mechanism of SVE and what functional role they serve in the process.Correspondence should be addressed to PJR: Tel, +61-2-9687-2800; Fax, +61-2-9687-2120; E-mail: phrobins@mail.usyd.edu.au.. Europe PMC Funders GroupAuthor Manuscript Nat Neurosci. Author manuscript; available in PMC 2007 November 20. Published Dynamin I is a large GTPase enzyme, the activity of which is required for vesicle fission in SVE 6. The proline-rich domain (PRD) at the C-terminus contains numerous binding motifs for src-3-homology (SH3) domains, through which it interacts with proteins such as amphiphysin I 7, endophilin I 8, and syndapin I 9. The SH3-mediated dynamin I interactions of amphiphysin and endophilin are involved in SVE 10, 11. An emerging idea is that different synaptic proteins like endophilin and amphiphysin are involved in ...

Synaptic vesicles (SVs) are retrieved by more than one mode in central nerve terminals. During mild stimulation, the dominant SV retrieval pathway is classical clathrin-mediated endocytosis (CME). During elevated neuronal activity, activity-dependent bulk endocytosis (ADBE) predominates, which requires activation of the calcium-dependent protein phosphatase calcineurin. We now report that calcineurin dephosphorylates dynamin I in nerve terminals only above the same activity threshold that triggers ADBE. ADBE was arrested when the two major phospho-sites on dynamin I were perturbed, suggesting that dynamin I dephosphorylation is a key step in its activation. Dynamin I dephosphorylation stimulates a specific dynamin I-syndapin I interaction. Inhibition of this interaction by competitive peptides or by site-directed mutagenesis exclusively inhibited ADBE but did not affect CME. The results reveal that the phospho-dependent dynamin-syndapin interaction recruits ADBE to massively increase SV endocytosis under conditions of elevated neuronal activity.

Glycogen synthase kinase-3 (GSK3) is a critical enzyme in neuronal physiology, however any specific role in presynaptic function is not yet known. We show that GSK3 phosphorylates a key residue on the large GTPase dynamin I (Ser-774) both in vitro and in primary rat neuronal cultures. This is dependent on prior phosphorylation of Ser-778 by cyclin-dependent kinase 5 (cdk5). We found a specific requirement for GSK3 in activity-dependent bulk endocytosis (ADBE), but not clathrin-mediated endocytosis (CME) using both acute inhibition with pharmacological antagonists and silencing of expression using shRNA. Moreover we showed that the specific phosphorylation of Ser-774 on dynamin I by GSK3 is both necessary and sufficient for ADBE. This is the first demonstration of a presynaptic role for GSK3 and reveals that a protein kinase signalling cascade prepares synaptic vesicles (SVs) for retrieval during elevated neuronal activity.

SummaryThe accurate formation of synaptic vesicles (SVs) and incorporation of their protein cargo during endocytosis is critical for the maintenance of neurotransmission. During intense neuronal activity, a transient and acute accumulation of SV cargo occurs at the plasma membrane. Activity-dependent bulk endocytosis (ADBE) is the dominant SV endocytosis mode under these conditions; however, it is currently unknown how ADBE mediates cargo retrieval. We examined the retrieval of different SV cargo molecules during intense stimulation using a series of genetically encoded pH-sensitive reporters in neuronal cultures. The retrieval of only one reporter, VAMP4-pHluorin, was perturbed by inhibiting ADBE. This selective recovery was confirmed by the enrichment of endogenous VAMP4 in purified bulk endosomes formed by ADBE. VAMP4 was also essential for ADBE, with a cytoplasmic di-leucine motif being critical for this role. Therefore, VAMP4 is the first identified ADBE cargo and is essential for this endocytosis mode to proceed.

Depolarization of nerve terminals stimulates rapid dephosphorylation of two isoforms of dynamin I (dynI), mediated by the calcium-dependent phosphatase calcineurin (CaN). Dephosphorylation at the major phosphorylation sites Ser-774/ 778 promotes a dynI-syndapin I interaction for a specific mode of synaptic vesicle endocytosis called activity-dependent bulk endocytosis (ADBE). DynI has two main splice variants at its extreme C terminus, long or short (dynIxa and dynIxb) varying only by 20 (xa) or 7 (xb) residues. Recombinant GST fusion proteins of dynIxa and dynIxb proline-rich domains (PRDs) were used to pull down interacting proteins from rat brain nerve terminals. Both bound equally to syndapin, but dynIxb PRD exclusively bound to the catalytic subunit of CaNA, which recruited CaNB. Binding of CaN was increased in the presence of calcium and was accompanied by further recruitment of calmodulin. Point mutations showed that the entire C terminus of dynIxb is a CaN docking site related to a conserved CaN docking motif (PXIXI(T/S)). This sequence is unique to dynIxb among all other dynamin variants or genes. Peptide mimetics of the dynIxb tail blocked CaN binding in vitro and selectively inhibited depolarization-evoked dynI dephosphorylation in nerve terminals but not of other dephosphins. Therefore, docking to dynIxb is required for the regulation of both dynI splice variants, yet it does not regulate the phosphorylation cycle of other dephosphins. The peptide blocked ADBE, but not clathrin-mediated endocytosis of synaptic vesicles. Our results indicate that Ca 2؉ influx regulates assembly of a fully active CaN-calmodulin complex selectively on the tail of dynIxb and that the complex is recruited to sites of ADBE in nerve terminals.