Synaptotagmin‐1 promotes the formation of axonal filopodia and branches along the developing axons of forebrain neurons

Abstract

Synaptotagmin-1 (syt1) is a Ca(2+)-binding protein that functions in regulation of synaptic vesicle exocytosis at the synapse. Syt1 is expressed in many types of neurons well before synaptogenesis begins both in vivo and in vitro. To determine if expression of syt1 has a functional role in neuronal development before synapse formation, we examined the effects of syt1 overexpression and knockdown on the growth and branching of the axons of cultured primary embryonic day 8 chicken forebrain neurons. In vivo these neurons express syt1, and most have not yet extended axons. We present evidence that syt1 plays a role in regulating axon branching, while not regulating overall axon length. To study the effects of overexpression of syt1, we used adenovirus-mediated infection to introduce a syt1-YFP construct, or control GFP construct, into neurons. Syt1 levels were reduced using RNA interference. Overexpression of syt1 increased the formation of axonal filopodia and branches. Conversely, knockdown of syt1 decreased the number of axonal filopodia and branches. Time-lapse analysis of filopodial dynamics in syt1-overexpressing cells demonstrated that elevation of syt1 levels increased both the frequency of filopodial initiation and their lifespan. Taken together these data indicate that syt1 regulates the formation of axonal filopodia and branches before engaging in its conventional functions at the synapse.

Optic nerve crush induces spatial and temporal gene expression patterns in retina and optic nerve of BALB/cJ mice

“…Syts have a conserved mechanism of action and are crucial for neuronal Ca 2+ -triggered vesicle fusion [115]. Previous studies have shown Syt1 to participate in axonal regeneration, including synaptic projection and proper axonal targeting [80,116]. In our study, Syt1 was identified in ON neuron projection and synaptic transmission clusters .…”

“…Expression of Synpr has been shown in neurons while Syt1 has been shown in both neurons and is critical in fusion events of astrocytes [77-81]. In addition to synapse formation, Syt1 has also been shown to regulate the formation of axonal filopodia and branching [80]. The induction of both Synpr and Syt1 expression may be related to synaptic vesicle fusion and release, and the roles of both genes in ONC need to be further explored.…”

Scale Invariant Disordered Nanotopography Promotes Hippocampal Neuron Development and Maturation with Involvement of Mechanotransductive Pathways

“…The functionality of neuronal cells depends on a complex synaptic protein machinery which regulates e.g., vesicle trafficking. In developing neurons this machinery appears before the synapses are even operative and electrically active (Greif et al, 2013). Therefore, as a read-out we counted synapses present in the different culturing conditions.…”

“…Presynaptic varicosities were immunolabeled with an antibody against synaptotagmin-I/p65 and juxtaposed to dendrites (MAP2) (Figures 3A–H). Synaptotagmin-I/p65 is presynaptic marker and an integral synaptic vesicle protein (Matthew et al, 1981; Greif et al, 2013) involved in determining neuronal polarity and axon formation/specification (Greif et al, 2013; Inoue et al, 2015). …”

Involvement of Rho-family GTPases in axon branching

“…29,89,90 Given that Rho-GTPases have been ascribed a variety of cellular functions, it will be necessary to determine their specific contributions to cytoskeletal dynamics, subcellular organization, organelle function, and membrane traffic, all components of axon branching. 3,29,91 Through regulation of the cytoskeleton, Rho-GTPases may also control aspects of somatic protein synthesis. 71 It will thus be of interest to determine if they may have such roles in axons in the context of axonal protein synthesis dependent axon branching.…”