Molecular motors and membrane traffic in Dictyostelium

Simple to Complex: The Role of Actin and Microtubules in Mitochondrial Dynamics in Amoeba, Yeast, and Mammalian Cells

Mitochondria are complex organelles that provide energy for the cell in the form of adenosine triphosphate (ATP) and have very specific structures. For most organisms, this is a reticular or tubular mitochondrial network, while others have singular oval-shaped organelles. Nonetheless, maintenance of this structure is dependent on the mitochondrial dynamics, fission, fusion, and motility. Recently, studies have shown that the cytoskeleton has a significant role in the regulation of mitochondrial dynamics. In this review, we focus on microtubules and actin filaments and look at what is currently known about the cytoskeleton’s role in mitochondrial dynamics in complex models like mammals and yeast, as well as what is known in the simple model system, Dictyostelium discoideum. Understanding how the cytoskeleton is involved in mitochondrial dynamics increases our understanding of mitochondrial disease, especially neurodegenerative diseases. Increases in fission, loss of fusion, and fragmented mitochondria are seen in several neurodegenerative diseases such as Parkinson’s, Alzheimer’s, and Huntington’s disease. There is no known cure for these diseases, but new therapeutic strategies using drugs to alter mitochondrial fusion and fission activity are being considered. The future of these therapeutic studies is dependent on an in-depth understanding of the mechanisms of mitochondrial dynamics. Understanding the cytoskeleton’s role in dynamics in multiple model organisms will further our understanding of these mechanisms and could potentially uncover new therapeutic targets for these neurodegenerative diseases.

Inositol hexakisphosphate kinase 1 (IP6K1) activity is required for cytoplasmic dynein-driven transport

Inositol pyrophosphates, such as diphosphoinositol pentakisphosphate (IP7), are conserved eukaryotic signaling molecules that possess pyrophosphate and monophosphate moieties. Generated predominantly by inositol hexakisphosphate kinases (IP6Ks), inositol pyrophosphates can modulate protein function by posttranslational serine pyrophosphorylation. Here, we report inositol pyrophosphates as novel regulators of cytoplasmic dynein-driven vesicle transport. Mammalian cells lacking IP6K1 display defects in dynein-dependent trafficking pathways, including endosomal sorting, vesicle movement, and Golgi maintenance. Expression of catalytically active but not inactive IP6K1 reverses these defects, suggesting a role for inositol pyrophosphates in these processes. Endosomes derived from slime mold lacking inositol pyrophosphates also display reduced dynein-directed microtubule transport. We demonstrate that Ser51 in the dynein intermediate chain (IC) is a target for pyrophosphorylation by IP7, and this modification promotes the interaction of the IC N-terminus with the p150(Glued) subunit of dynactin. IC-p150(Glued) interaction is decreased, and IC recruitment to membranes is reduced in cells lacking IP6K1. Our study provides the first evidence for the involvement of IP6Ks in dynein function and proposes that inositol pyrophosphate-mediated pyrophosphorylation may act as a regulatory signal to enhance dynein-driven transport.

Dynein dysfunction disrupts β-amyloid clearance in astrocytes through endocytic disturbances

We showed previously that aging attenuates the interaction between dynein-dynactin complexes in cynomolgus monkey brain and that dynein dysfunction reproduces age-dependent endocytic disturbances, resulting in intracellular β-amyloid (Aβ) accumulation, synaptic vesicle transport deficits, and neuritic swelling. It remains unclear whether such endocytic disturbances also occur in glial cells. Here, we show that endocytic pathology, such as intracellular accumulation of enlarged endosomes, occurs in astrocytes of aged monkey brains. Also, Aβ accumulates in these enlarged endosomes. RNA interference studies have shown that dynein dysfunction reproduces astroglial endocytic pathology and disrupts Aβ clearance in astrocytes through endocytic disturbances. These findings suggest that endocytic disturbances can alter astroglial functions and may also be involved in age-dependent Aβ pathology.

Tug-of-war between dissimilar teams of microtubule motors regulates transport and fission of endosomes

Intracellular transport is interspersed with frequent reversals in direction due to the presence of opposing kinesin and dynein motors on organelles that are carried as cargo. The cause and the mechanism of reversals are unknown, but are a key to understanding how cargos are delivered in a regulated manner to specific cellular locations. Unlike established single-motor biophysical assays, this problem requires understanding of the cooperative behavior of multiple interacting motors. Here we present measurements inside live Dictyostelium cells, in a cell extract and with purified motors to quantify such an ensemble function of motors. We show through precise motion analysis that reversals during endosome motion are caused by a tug-of-war between kinesin and dynein. Further, we use a combination of optical trap-based force measurements and Monte Carlo simulations to make the surprising discovery that endosome transport uses many (approximately four to eight) weak and detachment-prone dyneins in a tug-of-war against a single strong and tenacious kinesin. We elucidate how this clever choice of dissimilar motors and motor teams achieves net transport together with endosome fission, both of which are important in controlling the balance of endocytic sorting. To the best of our knowledge, this is a unique demonstration that dynein and kinesin function differently at the molecular level inside cells and of how this difference is used in a specific cellular process, namely endosome biogenesis. Our work may provide a platform to understand intracellular transport of a variety of organelles in terms of measurable quantities.

Dictyostelium discoideum: The Social Ameba

INTRODUCTIONDictyostelium discoideum is a unicellular eukaryote often referred to as a "social ameba" because it can form a multicellular structure when nutrient conditions are limiting. D. discoideum and related organisms, known as the Dictyostelia, have been studied for almost 150 years. The cellular and molecular aspects of their multicellular lifestyle have been studied in detail, and general principles for cell-to-cell communication, intracellular signaling, and cytoskeletal organization during cell motility have been derived from this work and have been found to be conserved across all eukaryotes. The bacteriovore nature of the unicellular stage provides an excellent model in which to study phagocytosis and the mechanisms of bacterial virulence. D. discoideum has also been used successfully to explore the molecular basis of various human diseases, as well as the mechanisms of drug action and the pathways that lead to resistance to certain therapeutic agents. The availability of a complete genome sequence has further widened the scope of studies using D. discoideum. A large potential for secondary metabolism has become apparent, which opens the door to discovering new compounds with potential medical applications. Numerous putative orthologs of genes responsible for diseases in humans, but whose molecular functions are still uncharacterized, are present in the D. discoideum genome. Finally, the availability of community resources, including the genome database dictyBase and the Dicty Stock Center, makes D. discoideum an easily accessible and powerful model organism to study.

Cytoskeletal elements and intracellular transport

Recent advances in the understanding of the functions of various components of the cytoskeleton indicate that, besides serving a structural role, the cytoskeletal elements may regulate the transport of several proteins in the cell. Studies reveal that there are co-operative interactions between the actin and microtubule cytoskeletons including functional overlap in the transport influenced by different motor families. Multiple motors are probably involved in the control of the dynamics of many proteins and intriguing hints about how these motors are co-ordinated are appearing. It has been shown that some of the intermediate elements also participate in selected intracellular transport mechanisms. In view of the author's preoccupation with the steroid receptor systems, special attention has been given to the role of the cytoskeletal elements, particularly actin, in the intracellular transport of steroid receptors and receptor-related proteins.

Binding of internalized receptors to the PDZ domain of GIPC/synectin recruits myosin VI to endocytic vesicles

Myosin VI (myo6) is the only actin-based molecular motor that translocates along actin filaments toward the minus end. Myo6 participates in two steps of endocytic trafficking; it is recruited to both clathrin-coated pits and to ensuing uncoated endocytic vesicles (UCV). Although there is evidence suggesting that the PDZ adaptor protein GIPC/synectin is involved in the association of myo6 with UCV, the recruitment mechanism is unknown. We show that GIPC/synectin is required for both internalization of cell surface receptors and for coupling of myo6 to UCV. This coupling occurs via a mechanism wherein engagement of the GIPC/synectin PDZ domain by C termini of internalized receptors facilitates in trans myo6 binding to the GIPC/synectin C terminus located outside of the PDZ domain. Analysis of megalin, a prototypical GIPC/synectin-binding receptor, revealed that deletion of its PDZ-binding motif drastically reduced GIPC/synectin and myo6 recruitment to UCV. Furthermore, interaction with GIPC/synectin was required for megalin's function, as megalin was mistargeted in the renal proximal tubules of GIPC/synectin-null mice and these mice exhibited proteinuria, a condition consistent with defective megalin trafficking.

Towards a molecular understanding of human diseases using Dictyostelium discoideum

The social amoeba Dictyostelium discoideum is increasingly being used as a simple model for the investigation of problems that are relevant to human health. This article focuses on several recent examples of Dictyostelium-based biomedical research, including the analysis of immune-cell disease and chemotaxis, centrosomal abnormalities and lissencephaly, bacterial intracellular pathogenesis, and mechanisms of neuroprotective and anti-cancer drug action. The combination of cellular, genetic and molecular biology techniques that are available in Dictyostelium often makes the analysis of these problems more amenable to study in this system than in mammalian cell culture. Findings that have been made in these areas using Dictyostelium have driven research in mammalian systems and have established Dictyostelium as a powerful model for human-disease analysis.

Dynamic microtubules in Dictyostelium

The term 'microtubule dynamics' is often used to describe assembly/disassembly characteristics of this important cytoskeletal polymer. The ability to image microtubules in live Dictyostelium cells has revealed additional dynamic components, acting on the individual assembled tubules. At least two separate forces are involved, in generation of pronounced bending motions during interphase and in creating tension with the cell cortex. This review attempts to summarize what is known about conventional microtubule dynamics in Dictyostelium as well as to describe these two additional motility components. We propose that these forces are important both in maintaining the overall structure of the microtubule array and in supporting intracellular traffic.

Unconventional myosins, actin dynamics and endocytosis: a ménage à trois?

Ever since the discovery of class I myosins, the first nonmuscle myosins, about 30 years ago, the history of unconventional myosins has been linked to the organization and working of actin filaments. It slowly emerged from studies of class I myosins in lower eukaryotes that they are involved in mechanisms of endocytosis. Most interestingly, a flurry of recent findings assign a more active role to class I myosins in regulating the spatial and temporal organization of actin filament nucleation and elongation. The results highlight the multiple links between class I myosins and the major actin nucleator, the Arp2/3 complex, and its newly described activators. Two additional types of unconventional myosins, myosinIX, and Dictyostelium discoideum MyoM, have recently been tied to the signaling pathways controlling actin cytoskeleton remodeling. The present review surveys the links between these three classes of molecular motors and the complex cellular processes of endocytosis and actin dynamics, and concentrates on a working model accounting for the function of class I myosins via recruitment of the machinery responsible for actin nucleation and elongation.

A transcriptional profile of multicellular development inDictyostelium discoideum

A distinct feature of development in the simple eukaryote Dictyostelium discoideum is an aggregative transition from a unicellular to a multicellular phase. Using genome-wide transcriptional analysis we show that this transition is accompanied by a dramatic change in the expression of more than 25% of the genes in the genome. We also show that the transcription patterns of these genes are not sensitive to the strain or the nutritional history, indicating that Dictyostelium development is a robust physiological process that is accompanied by stereotypical transcriptional events. Analysis of the two differentiated cell types, spores and stalk cells, and their precursors revealed a large number of differentially expressed genes as well as unexpected patterns of gene expression, which shed new light on the timing and possible mechanisms of cell-type divergence. Our findings provide new perspectives on the complexity of the developmental program and the fraction of the genome that is regulated during development.

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