Overlapping Role of Dynamin Isoforms in Synaptic Vesicle Endocytosis

Abstract

The existence of neuron specific endocytic protein isoforms raises questions about their importance for specialized neuronal functions. Dynamin, a GTPase implicated in the fission reaction of endocytosis, is encoded by three genes, two of which, dynamin 1 and 3, are highly expressed in neurons. We show that dynamin 3, thought to play a predominantly postsynaptic role, has a major presynaptic function. While lack of dynamin 3 does not produce an overt phenotype in mice, it worsens the dynamin 1 KO phenotype, leading to perinatal lethality and a more severe defect in activity-dependent synaptic vesicle endocytosis. Thus, dynamin 1 and 3, which together account for the overwhelming majority of brain dynamin, cooperate in supporting optimal rates of synaptic vesicle endocytosis. Persistence of synaptic transmission in their absence indicates that if dynamin plays essential functions in neurons, such functions can be achieved by the very low levels of dynamin 2.

Dynamin 2 regulates biphasic insulin secretion and plasma glucose homeostasis

“…For example, low levels of dynamin 3 (and possibly extremely low levels of dynamin 1) may, at least partially, support endocytosis in Dnm2 KO β cells. A similar scenario has been reported at rodent nerve terminals lacking dynamin 1 (36,75), in which dynamin 3 partially supports vesicle recycling (38,39). It is unclear whether the dynamin-independent endocytosis at some nonsecretory cells (45,71) and nerve terminals (i.e., bulk endocytosis) (40,72,76) also occurs in dynamin 2 KO β cells.…”

“…Given the proposed dynamin function in endocytosis and vesicle biogenesis from the TGN (48-51), it is interesting that we found no significant decrease in granule density in KO β cells ( Figure 5, B and C, and Supplemental Figure 5, B and C), even under 20 mM glucose stimulation for 20 minutes (Supplemental Figure 5D). Moreover, we observed no obvious cell surface expansion in KO cells, which is different from the severe vesicle depletion at nerve terminals after dynamin perturbations (31,38). Mature and immature types of insulin granules are readily distinguishable in our preparations based on their characteristic ultrastructures.…”

“…The meandering tubules in KO β cells were approximately 20 to 80 nm in diameter and up to hundreds of nanometers in length. These structures are reminiscent of endocytic defects in central synapses lacking dynamin 1 (and dynamin 3) (36,38,40) but more complicated than those in fibroblasts lacking dynamin (45). A unique feature of these endocytic intermediates is that each tubule was capped by a CCP, suggesting a severe defect in the membrane fission reactions of CME.…”

“…Dynamin 1 is highly expressed in the nervous system, dynamin 2 is ubiquitous in all tissues, and dynamin 3 is found primarily in neuronal tissues and at low levels in testis (35,36). At nerve terminals, dynamin 1 and 3 contribute to synaptic vesicle reformation and neurotransmission (36)(37)(38)(39)(40); synapses lacking dynamin 1 (and dynamin 3) have fewer vesicles and abundant clathrin-coated profiles (CCPs) accumulated on the PM (36,38,40). In nonneuronal cells, dynamin isoforms appear cell-type specific and may play distinct roles in multiple endocytosis pathways (32,35,41).…”

Three-dimensional architecture of extended synaptotagmin-mediated endoplasmic reticulum–plasma membrane contact sites

“…COS-7 cells were cultured in Dulbecco's modified essential Eagle medium (DMEM) (Life Technologies) supplemented with 10% (vol/vol) FBS at 37°C and 5% CO 2 . Neuronal culture was prepared as previously described (37). Transfection of plasmids was carried out with Lipofectamine 2000 (Life Technologies), according to the manufacturer's instructions.…”