Control of protein trafficking by reversible masking of transport signals

Local Secretory Trafficking Pathways in Neurons and the Role of Dendritic Golgi Outposts in Different Cell Models

A fundamental characteristic of neurons is the relationship between the architecture of the polarized neuron and synaptic transmission between neurons. Intracellular membrane trafficking is paramount to establish and maintain neuronal structure; perturbation in trafficking results in defects in neurodevelopment and neurological disorders. Given the physical distance from the cell body to the distal sites of the axon and dendrites, transport of newly synthesized membrane proteins from the central cell body to their functional destination at remote, distal sites represents a conundrum. With the identification of secretory organelles in dendrites, including endoplasmic reticulum (ER) and Golgi outposts (GOs), recent studies have proposed local protein synthesis and trafficking distinct from the conventional anterograde transport pathways of the cell body. A variety of different model organisms, including Drosophila, zebrafish, and rodents, have been used to probe the organization and function of the local neuronal secretory network. Here, we review the evidence for local secretory trafficking pathways in dendrites in a variety of cell-based neuronal systems and discuss both the similarities and differences in the organization and role of the local secretory organelles, especially the GOs. In addition, we identify the gaps in the current knowledge and the potential advances using human induced pluripotent stem cells (iPSCs) in defining local membrane protein trafficking in human neurons and in understanding the molecular basis of neurological diseases.

Reversible association with motor proteins (RAMP): A streptavidin-based method to manipulate organelle positioning

We report the development and characterization of a method, named reversible association with motor proteins (RAMP), for manipulation of organelle positioning within the cytoplasm. RAMP consists of coexpressing in cultured cells (i) an organellar protein fused to the streptavidin-binding peptide (SBP) and (ii) motor, neck, and coiled-coil domains from a plus-end-directed or minus-end-directed kinesin fused to streptavidin. The SBP-streptavidin interaction drives accumulation of organelles at the plus or minus end of microtubules, respectively. Importantly, competition of the streptavidin-SBP interaction by the addition of biotin to the culture medium rapidly dissociates the motor construct from the organelle, allowing restoration of normal patterns of organelle transport and distribution. A distinctive feature of this method is that organelles initially accumulate at either end of the microtubule network in the initial state and are subsequently released from this accumulation, allowing analyses of the movement of a synchronized population of organelles by endogenous motors.

Commonly used trafficking blocks disrupt ARF1 activation and the localization and function of specific Golgi proteins

Cold temperature blocks used to synchronize protein trafficking inhibit GBF1 function, leading to a decrease in ARF1-GTP levels and mislocalization of the ARF1 effector golgin-160. Several other, but not all, Golgi proteins including ARL1 also mislocalize. ARF1 activity and golgin-160 localization require more than 30 min to recover from these blocks.

ADP-ribosylation factor (ARF) proteins are key regulators of the secretory pathway. ARF1, through interacting with its effectors, regulates protein trafficking by facilitating numerous events at the Golgi. One unique ARF1 effector is golgin-160, which promotes the trafficking of only a specific subset of cargo proteins through the Golgi. While studying this role of golgin-160, we discovered that commonly used cold temperature blocks utilized to synchronize cargo trafficking (20 and 16°C) caused golgin-160 dispersal from Golgi membranes. Here, we show that the loss of golgin-160 localization correlates with a decrease in the levels of activated ARF1, and that golgin-160 dispersal can be prevented by expression of a GTP-locked ARF1 mutant. Overexpression of the ARF1 activator Golgi brefeldin A–resistant guanine nucleotide exchange factor 1 (GBF1) did not prevent golgin-160 dispersal, suggesting that GBF1 may be nonfunctional at lower temperatures. We further discovered that several other Golgi resident proteins had altered localization at lower temperatures, including proteins recruited by ARF-like GTPase 1 (ARL1), a small GTPase that also became dispersed in the cold. Although cold temperature blocks are useful for synchronizing cargo trafficking through the Golgi, our data indicate that caution must be taken when interpreting results from these assays.

Improving the Solubility of Artificial Ligands of Streptavidin to Enable More Practical Reversible Switching of Protein Localization in Cells

Chemical inducers that can control target-protein localization in living cells are powerful tools to investigate dynamic biological systems. We recently reported the retention using selective hook or "RUSH" system for reversible localization change of proteins of interest by addition/washout of small-molecule artificial ligands of streptavidin (ALiS). However, the utility of previously developed ALiS was restricted by limited solubility in water. Here, we overcame this problem by X-ray crystal structure-guided design of a more soluble ALiS derivative (ALiS-3), which retains sufficient streptavidin-binding affinity for use in the RUSH system. The ALiS-3-streptavidin interaction was characterized in detail. ALiS-3 is a convenient and effective tool for dynamic control of α-mannosidase II localization between ER and Golgi in living cells.

Drive the Car(go)s-New Modalities to Control Cargo Trafficking in Live Cells

Synaptic transmission is a fundamental molecular process underlying learning and memory. Successful synaptic transmission involves coupled interaction between electrical signals (action potentials) and chemical signals (neurotransmitters). Defective synaptic transmission has been reported in a variety of neurological disorders such as Autism and Alzheimer’s disease. A large variety of macromolecules and organelles are enriched near functional synapses. Although a portion of macromolecules can be produced locally at the synapse, a large number of synaptic components especially the membrane-bound receptors and peptide neurotransmitters require active transport machinery to reach their sites of action. This spatial relocation is mediated by energy-consuming, motor protein-driven cargo trafficking. Properly regulated cargo trafficking is of fundamental importance to neuronal functions, including synaptic transmission. In this review, we discuss the molecular machinery of cargo trafficking with emphasis on new experimental strategies that enable direct modulation of cargo trafficking in live cells. These strategies promise to provide insights into a quantitative understanding of cargo trafficking, which could lead to new intervention strategies for the treatment of neurological diseases.