![]() ![]() By using a very-low-level expression paradigm to express fluorescently tagged reporter proteins in neurons, a method to allow their triggered and 'synchronous' exit from the endoplasmic reticulum (RUSH), and live cell imaging, the authors describe a specific axonal trafficking pathway for the synaptic vesicle proteins Synaptotagmin-1 and Synaptobrevin-2. how these highly polarized cells achieve the specific and differential distribution of proteins and organelles into their axonal and dendritic compartments – the study is an important step forward in this context. The authors explored a key question in nerve cell biology, i.e. These experiments reveal a new-found membrane trafficking pathway, for SV proteins, in classically polarized mammalian neurons and provide a glimpse at the first steps of SV biogenesis. Moreover, we observed that SV constituents were first delivered to the presynaptic plasma membrane before incorporation into SVs. ![]() However, even moderate overexpression resulted in the spillover of SV proteins into dendrites, potentially explaining the origin of previous non-polarized transport models, revealing the limited, saturable nature of the direct axonal trafficking pathway. In sharp contrast to the selective retention model, both proteins selectively and specifically entered axons with minimal entry into dendrites. ![]() For these studies, the SV reporter constructs were expressed at carefully controlled, very low levels. Here, we used the RUSH (retention using selective hooks) system, in conjunction with HaloTag labeling approaches, to study the egress of two distinct transmembrane SV proteins, synaptotagmin 1 and synaptobrevin 2, from the soma of mature cultured rat and mouse neurons. The leading model posits that these proteins are randomly trafficked throughout neurons and are selectively retained in presynaptic boutons. Yet, how SV proteins are sorted to presynaptic nerve terminals remains the subject of debate. 2 nd ed.Neurotransmitter-filled synaptic vesicles (SVs) mediate synaptic transmission and are a hallmark specialization in neuronal axons. Nelson & Connaughton, Bipolar Cell Pathways in the Vertebrate Retina. There are situations where multiple axons arise, but that occurs only in neurons that have been tinkered around with genetically. Basically, the neuron has still just one axonal output, but collateral regulatory info is sent off back to the cell. The only situation where multiple axons arise from one cell is when the axon bifurcates along the way, sending one or more collaterals from the axon off back to the cell. bipolar cells), others as many as thousands of terminals (Brady et al., 2012). The axon can target neurons along the way ( en passant) and the axon can terminate in multiple terminals contacting various cells. Hence, dependent on the cell type, neurons can have one or as many as 200k dendritic connections.Īs far as I am aware, all neurons have just one axon. Hence they integrate massive amounts of (sensory) information and funnel it into one output signal (Purves et al., 2002). ![]() These cells have elaborate dendritic trees making 100,000 to 200,000 connections, but still there is just one axon. Multipolar neurons have multiple inputs (dendritic connections), and one output (the axon).There are also bipolar cells in the retina, these have one dendrite (input) and one axon (output) (Nelson & Connaughton, 2012).Ī striking example are the Purkinje cells in the cortex. ![]()
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