Protein phosphorylation and the control of exocytosis in neurons

Molecular Machines Determining the Fate of Endocytosed Synaptic Vesicles in Nerve Terminals

The cycle of a synaptic vesicle (SV) within the nerve terminal is a step-by-step journey with the final goal of ensuring the proper synaptic strength under changing environmental conditions. The SV cycle is a precisely regulated membrane traffic event in cells and, because of this, a plethora of membrane-bound and cytosolic proteins are devoted to assist SVs in each step of the journey. The cycling fate of endocytosed SVs determines both the availability for subsequent rounds of release and the lifetime of SVs in the terminal and is therefore crucial for synaptic function and plasticity. Molecular players that determine the destiny of SVs in nerve terminals after a round of exo-endocytosis are largely unknown. Here we review the functional role in SV fate of phosphorylation/dephosphorylation of SV proteins and of small GTPases acting on membrane trafficking at the synapse, as they are emerging as key molecules in determining the recycling route of SVs within the nerve terminal. In particular, we focus on: (i) the cyclin-dependent kinase-5 (cdk5) and calcineurin (CN) control of the recycling pool of SVs; (ii) the role of small GTPases of the Rab and ADP-ribosylation factor (Arf) families in defining the route followed by SV in their nerve terminal cycle. These regulatory proteins together with their synaptic regulators and effectors, are molecular nanomachines mediating homeostatic responses in synaptic plasticity and potential targets of drugs modulating the efficiency of synaptic transmission.

Synaptic proteins as multi-sensor devices of neurotransmission

Neuronal communication is tightly regulated in time and space. Following neuronal activation, an electrical signal triggers neurotransmitter (NT) release at the active zone. The process starts by the signal reaching the synapse followed by a fusion of the synaptic vesicle (SV) and diffusion of the released NT in the synaptic cleft. The NT then binds to the appropriate receptor and induces a membrane potential change at the target cell membrane. The entire process is controlled by a fairly small set of synaptic proteins, collectively called SYCONs. The biochemical features of SYCONs underlie the properties of NT release. SYCONs are characterized by their ability to detect and respond to changes in environmental signals. For example, consider synaptotagmin I (Syt1), a prototype of a protein family with over 20 gene and variants in mammals. Syt1 is a specific example of a multi-sensor device with a large repertoire of discrete states. Several of these states are stimulated by a local concentration of signaling molecules such as Ca2+. The ability of this protein to sense signaling molecules and to adopt multiple biochemical states is shared by other SYCONs such as the synapsins (Syns). Specific biochemical states of Syns determine the accessibility of SV for NT release. Each of these states is defined by a specific alternative spliced variant with a unique profile of phosphorylation modified sites. The plasticity of the synapse is a direct reflection of SYCON's multiple biochemical states. State transitions occurs in a wide range of time scales, and therefore these molecules need to cope with events that last milliseconds (i.e., exocytosis in fast responding synapses) and with events that can carry on for many minutes (i.e., organization of SV pools). We suggest that SYCONs are optimized throughout evolution as multi-sensor devices. A full repertoire of the switches leading to alternation of protein states and a detailed characterization of protein-protein network within the synapse is critical for the development of a dynamic model of synaptic transmission.

Concentration-dependent stimulation and inhibition of growth cone behavior and neurite elongation by protein kinase inhibitors KT5926 and K-252a

We examined the concentration- and time-dependent effects of two related protein kinase inhibitors, KT5926 and K-252a, on neurite formation and nerve growth cone migration of chick embryo sensory neurons. The effects of these drugs on neurite formation over an 18-h period were dissimilar. KT5926 stimulated neurite formation at concentrations between 100 and 500 nM and inhibited neurite formation at 5 microM. K-252a had no stimulatory effects on neurite formation, and it inhibited neurite formation at concentrations above 50 nM. This difference may occur because K-252a inhibits activation of the nerve growth factor receptor trk A, while KT5926 does not inhibit trk A. Both drugs, however, had similar immediate effects on growth cone migration. Growth cone migration and lamellipodial spreading were rapidly stimulated by 500 nM concentrations of KT5926 and K-252a. At 2 microM levels of either drug, growth cone spreading was still stimulated, but growth cone migration was inhibited by both drugs. These results show that changes in protein phosphorylation/dephosphorylation can rapidly regulate the cellular machinery that is responsible for driving growth cone migration and neurite elongation. The different effects of 2 microM concentrations of either KT5926 or K-252a on growth cone spreading versus migration suggests that the actin-dependent protrusive motility of the growth cone leading margin is regulated differently by changes in protein phosphorylation and dephosphorylation than the cytoskeletal mechanism that drives neurite elongation.