Axonal transport and neurodegenerative disease

Neurons have extensive processes and communication between those processes and the cell body is crucial to neuronal function and survival. Thus, neurons are uniquely dependent on microtubule based transport. Growing evidence supports the idea that deficits in axonal transport contribute to pathogenesis in multiple neurodegenerative diseases. We describe the motor, cytoskeletal, and adaptor proteins involved in axonal transport and their interactions. Data linking disruption of axonal transport to diseases such as ALS are discussed. Finally, we explore the pathways that may cause neuronal dysfunction and death.     

Cargo-carrying motor vehicles on the neuronal highway: transport pathways and neurodegenerative disease

Within axons vital cargoes must be transported over great distances along microtubule tracks to maintain neuronal viability. Essential to this system are the molecular motors, kinesin and dynein, which transport a variety of neuronal cargoes. Elucidating the transport pathways, the identity of the cargoes transported, and the regulation of motor-cargo complexes are areas of intense investigation. Evidence suggests that essential components, including signaling proteins, neuroprotective and repair molecules, and vesicular and cytoskeletal components are all transported. In addition newly emerging data indicate that defects in axonal transport pathways may contribute to the initiation or progression of chronic neuronal dysfunction. In this review we concentrate on microtubule-based motor proteins, their linkers, and cargoes and discuss how factors in the axonal transport pathway contribute to disease states. As additional cargo complexes and transport pathways are identified, an understanding of the role these pathways play in the development of human disease will hopefully lead to new diagnostic and treatment strategies.     

Small, monomeric G proteins in plants

The superfamily of small, monomeric GTP-binding proteins, in Arabidopsis thaliana comprising 93 members, is classified into four families: Arf/Sar, Rab, Rop/Rac, and Ran families. All monomeric G proteins function as molecular switches that are activated by GTP and inactivated by the hydrolysis of GTP to GDP. GTP/GDP cycling is controlled by three classes of regulatory protein: guanine-nucleotide-exchange factors (GEFs), GTPase-activating proteins (GAPs), and guanine-nucleotide-dissociation inhibitors (GDIs). Proteins of Arf family are primarily involved in regulation of membrane traffic and organization of the cytoskeleton. Arf1/Sar1 proteins regulate the formation of vesicle coat at different steps in the exocytic and endocytic pathways. Rab GTPases are regulators of vesicular transport. They are involved in vesicle formation, recruitment of cytoskeletal motor proteins, and in vesicle tethering and fusion. Rop proteins serve as key regulators of cytoskeletal reorganization in response to extracellular signals. Several data have also shown that Rop proteins play additional roles in membrane trafficking and regulation of enzymes activity. Ran proteins are involved in nucleocytoplasmic transport.     

The Emerging Role of Cytoskeletal Proteins as Reliable Biomarkers

Cytoskeletal proteins are essential building blocks of cells. More than 100 cytoskeletal and cytoskeleton‐associated proteins are known and for some, their function and regulation are understood in great detail. Apart from cell shape and support, they facilitate many processes such as intracellular signaling and transport, and cancer related processes such as proliferation, migration, and invasion. During the last decade, comparative proteomic studies have identified cytoskeletal proteins as in vitro markers for tumor progression and metastasis. Here, these results are summarized and a number of unrelated studies are highlighted, identifying the same cytoskeletal proteins as potential biomarkers. These findings might indicate that the abundance of these potential markers of tumor progression is associated with the biological outcome and are independent of the cancer origin. This correlates well with recently published results from the Cancer Genome Atlas, indicating that cancers show remarkable similarities in their analyzed molecular information, independent of their organ of origin. It is postulated that the quantification of cytoskeletal proteins in healthy tissues, tumors, in adjacent tissues, and in stroma, is a great source of molecular information, which might not only be used to classify tumors, but more importantly to predict patients’ outcome or even best treatment choices.     

Slow axonal transport: the subunit transport model

A central problem concerning slow transport of cytoskeletal proteins along nerve axons is where they are assembled and the form in which they are transported. The polymer and subunit transport models are the two major hypotheses. Recent developments using molecular and cellular biophysics, molecular cell biology and gene technology have enabled visualization of moving forms of cytoskeletal proteins during their transport. Here, we argue that these studies support the subunit transport theory.     

Constitutive protein secretion from the trans-Golgi network to the plasma membrane

Constitutive secretion is used to deliver newly synthesized proteins to the cell surface and to the extracellular milieu. The trans-Golgi network is a key station along this route that mediates sorting of proteins into distinct transport pathways, aided in part by clathrin and adaptor proteins. Subsequent movement of proteins to the plasma membrane can occur either directly or via the endocytic pathway. Moreover, multiple, parallel pathways from the trans-Golgi network to the plasma membrane appear to exist, not only in complex, polarized cells such as epithelial cells and neurons, but also in relatively simple cells such as fibroblasts. In addition to typical secretory vesicles, these pathways involve both small, pleiomorphic transport containers and relatively large tubular-saccular carriers that travel along cytoskeletal tracks. While production and movement of these membranous structures are typically described as constitutive, recent studies have revealed that these key steps in secretion are tightly regulated by Ras-superfamily GTPases, members of the protein kinase D family and tethering complexes such as the exocyst.     

Principles of cargo attachment to cytoplasmic motor proteins.

The attachment of cytoskeletal motor proteins to cargo-laden vesicles is crucial for efficient transport in intracellular membrane trafficking pathways. Recent studies have identified specific kinesin-binding and dynein-binding proteins that could serve as membrane-associated 'receptor' proteins for the respective motors. New insights have also emerged about the cargo-binding domains of the motor proteins, and the regulation of motor binding to cargoes.     

Protein Traffic in Gram-negative bacteria - how exported and secreted proteins find their way

Gram-negative bacteria assemble many proteins into the inner and outer membranes and export a large number of proteins to the periplasm or to the extracellular medium. During the billions of years bacteria have been around, they have evolved a number of different pathways with sophisticated machines to accurately and efficiently move proteins from one location to another. In this review, we first introduce specific proteins that are representative substrates of the protein transport pathways and describe their function. Then, their specific routes from synthesis to their destinations are described mentioning the signal peptide that may initiate their export and discuss what is known about the folding state of the substrates during transport. The membrane translocation device involved, the energy source required for transport, and whether a chaperone is needed will be discussed.     

Conserved interactions with cytoskeletal but not signaling elements are an essential aspect of Drosophila WASp function

Wiskott-Aldrich Syndrome proteins (WASp) serve as important regulators of cytoskeletal organization and function. These modular proteins, which are well-conserved among eukaryotic species, act to promote actin filament assembly in response to cues from various signal transduction pathways. Genetic analysis has revealed a requirement for the single Drosophila homolog, Wasp (Wsp), in cell-fate decisions governing specific neuronal lineages. We have used this unique developmental context to assess the contributions of established signaling and cytoskeletal partners of WASp. We present biochemical and genetic evidence that, as expected, Drosophila Wsp performs its developmental role via the Arp2/3 complex, indicating conservation of the cytoskeletal aspect of Wsp function in vivo. In contrast, we find that association with the key signaling molecules CDC42 and PIP2 is not an essential requirement, implying that activation of Wsp function in vivo depends on additional or alternative signaling pathways.     

The push and pull of the bacterial cytoskeleton

A crucial function for eukaryotic cytoskeletal filaments is to organize the intracellular space: facilitate communication across the cell and enable the active transport of cellular components. It was assumed for many years that the small size of the bacterial cell eliminates the need for a cytoskeleton, because simple diffusion of proteins is rapid over micron-scale distances. However, in the last decade, cytoskeletal proteins have indeed been found to exist in bacteria where they have an important role in organizing the bacterial cell. Here, we review the progress that has been made towards understanding the mechanisms by which bacterial cytoskeletal proteins influence cellular organization. These discoveries have advanced our understanding of bacterial physiology and provided insight into the evolution of the eukaryotic cytoskeleton.     

The 'ins' and 'outs' of intermediate filament organization

A major function shared by several types of cytoplasmic intermediate filaments (IFs) is to stabilize cellular architecture against the mechanical forces it is subjected to. As for other fibrous cytoskeletal arrays, a crucial determinant of this function is the spatial organization of IFs in the cytoplasm. However, very few crossbridging proteins are specific for IFs - most IF-associated proteins known to exert a structural role act to tether IFs to other major cytoskeletal elements, such as F-actin, microtubules or adhesion complexes. In addition, IFs are endowed with the ability to participate in their own organization. This intriguing property is probably connected to the unusual degree of sequence diversity and sequence-specific regulation that characterize IF genes and their proteins. This dependence upon a combination of extrinsic and intrinsic determinants contributes to distinguish IFs from other fibrous cytoskeletal polymers and is key to their function.     

Coordinated delivery and function of bacterial MARTX toxin effectors

Bacteria often coordinate virulence factors to fine‐tune the host response during infection. These coordinated events can include toxins counteracting or amplifying effects of another toxin or though regulating the stability of virulence factors to remove their function once it is no longer needed. Multifunctional autoprocessing repeats‐in toxin (MARTX) toxins are effector delivery toxins that form a pore into the plasma membrane of a eukaryotic cell to deliver multiple effector proteins into the cytosol of the target cell. The function of these proteins includes manipulating actin cytoskeletal dynamics, regulating signal transduction pathways and inhibiting host secretory pathways. Investigations into the molecular mechanisms of these effector domains are providing insight into how the function of some effectors overlap and regulate one another during infection. Coordinated crosstalk of effector function suggests that MARTX toxins are not simply a sum of all their parts. Instead, modulation of cell function by effector domains may depend on which other effector domain are co‐delivered. Future studies will elucidate how these effectors interact with each other to modulate the bacterial host interaction.     

中间纤维与细胞信号通路的研究进展

中间纤维家族由约70个中间纤维蛋白组成,在真核细胞内组成横跨核膜和胞质的网状骨架.中间纤维最初仅仅被当做是细胞骨架的一种,主要起机械支撑作用.这个观点正发生快速的改变,因为越来越多的研究发现中间纤维蛋白参与各种主要的细胞信号通路,如细胞应激、细胞凋亡和14-3-3信号通路等.     

Dynamics of nonmembranous cell components: Role of active transport along microtubules

Here we discuss some common mechanisms of microtubule-dependent active transport of nonmembranous components in animal cells. We summarize data about mRNA, cytoskeletal elements, structural proteins, and signaling complexes transport. We also characterize the series of molecular interactions that connect nonmembranous cargoes and microtubules and describe the regulatory pathways for these interactions.     

Mechanisms of receptor-mediated nuclear import and nuclear export

Nuclear transport of proteins and RNA occurs through the nuclear pore complex and is mediated by a superfamily of transport receptors known collectively as karyopherins. Karyopherins bind to their cargoes by recognition of specific nuclear localization signals or nuclear export signals. Transport through the nuclear pore complex is facilitated by transient interactions between the karyopherins and the nuclear pore complex. The interactions of karyopherins with their cargoes are regulated by the Ras-related GTPase Ran. Ran is assisted in this process by proteins that regulate its GTPase cycle and subcellular localization. In this review, we describe several of the major transport pathways that are conserved in higher and lower eukaryotes, with particular emphasis on the role of Ran. We highlight the latest advances in the structure and function of transport receptors and discuss recent examples of steroid hormone receptor import and regulation by signal transduction pathways. Understanding the molecular basis of nuclear transport may provide insight into human diseases by revealing how nucleocytoplasmic trafficking regulates protein activity.     

IQGAP1 as signal integrator: Ca2+, calmodulin,Cdc42 and the cytoskeleton

A family of proteins known as IQGAPs have been identified in yeast, amebas and mammals. IQGAPs are multidomain molecules that contain several protein-interacting motifs which mediate binding to target proteins. Mammalian IQGAP1 is a component of signaling networks that are integral to maintaining cytoskeletal architecture and cell-cell adhesion. Published data suggest that IQGAP1 is a scaffolding protein that modulates cross-talk among diverse pathways in complex regulatory circuits. These pathways include modulating the actin cytoskeleton, mediating signaling by Rho family GTPases and calmodulin, regulating E-cadherin and beta-catenin function and organizing microtubules.     

Proteomic analysis of the porcine platelet proteome and alterations induced by thrombin activation

Platelets are enucleated cells derived from bone marrow megakaryocytes and defects in platelet functions could be involved in many cardiovascular diseases. Proteomics can be used to provide a new insight in the study of these platelet functions and can help to identify the biochemical events underlying platelet activation. In this study, we have obtained a reference 2-DE map of porcine platelet proteins. A large number of cytoskeletal and metabolic proteins were found as well as some proteins related to cell mobility and immunological functions. Other proteins implicated in the cell signalling process, transport or apoptosis were also identified. Moreover, we have analysed, by 2D-DIGE methodology, quantitative modifications of platelet proteins following their activation by thrombin. 26 spots exhibited statistically significant differences, and a total of 16 spots corresponding to 13 different proteins were successfully identified. Using Ingenuity Pathway Analysis, the association of the deregulated proteins with canonical pathways highlighted two major pathways; coagulation system and integrin signalling. These results confirm that this proteomic approach (based on 2D-DIGE, mass spectrometry and bioinformatic and pathway databases) has proved to be a powerful tool when applied to studying signalling pathways that could play a relevant role in the activation of platelets.     

Function and regulation of Ena/VASP proteins

Regulation of cytoskeletal dynamics is required to coordinate cell movement, adhesion and shape change. The Ena/VASP protein family is thought to play an important role in linking signaling pathways to remodeling of the actin cytoskeleton. This review will examine the mechanisms by which Ena/VASP function might control actin dynamics and how these proteins are linked to various signaling pathways.     

Regulation of actin cytoskeleton dynamics by Arf-family GTPases

Small GTPases of the Arf family are best known for their role in vesicular transport, wherein they nucleate the assembly of coat proteins at sites of carrier vesicle formation. However, accumulating evidence indicates that the Arfs are also important regulators of actin cytoskeleton dynamics and are involved in a variety of actin-based processes, including cell adhesion, migration and neurite outgrowth. The mechanisms of this regulation are remarkably diverse, ranging from the integration of vesicular transport with cytoskeleton assembly to the direct regulation of Rho-family GTPase function. Here, we review recent progress in our understanding of how Arfs and their interacting proteins function to integrate membrane and cytoskeletal dynamics.