Identification of coelomocyte unconventional myosin and its association with in vivo particle/vesicle motility

Gata6+ large peritoneal macrophages: an evolutionarily conserved sentinel and effector system for infection and injury

There are striking similarities between the sea urchin cavity macrophage-like phagocytes (coelomocytes) and mammalian cavity macrophages in not only their location, but also their behaviors. These cells are crucial for maintaining homeostasis within the cavity following a breach, filling the gap and functioning as a barrier between vital organs and the environment. In this review, we summarize the evolving literature regarding these Gata6⁺ large peritoneal macrophages (GLPMs), focusing on ontogeny, their responses to perturbations, including their rapid aggregation via coagulation, as well as scavenger receptor cysteine-rich domains and their potential roles in diseases, such as cancer. We challenge the 50-year old phenomenon of the 'macrophage disappearance reaction' (MDR) and propose the new term 'macrophage disturbance of homeostasis reaction' (MDHR), which may better describe this complex phenomenon.

Aggregation of sea urchin phagocytes is augmented in vitro by lipopolysaccharide

Development of protocols and media for culturing immune cells from marine invertebrates has not kept pace with advancements in mammalian immune cell culture, the latter having been driven by the need to understand the causes of and develop therapies for human and animal diseases. However, expansion of the aquaculture industry and the diseases that threaten these systems creates the need to develop cell and tissue culture methods for marine invertebrates. Such methods will enable us to better understand the causes of disease outbreaks and to develop means to avoid and remedy epidemics. We report a method for the short-term culture of phagocytes from the purple sea urchin, Strongylocentrotus purpuratus, by modifying an approach previously used to culture cells from another sea urchin species. The viability of cultured phagocytes from the purple sea urchin decreases from 91.6% to 57% over six days and phagocyte morphology changes from single cells to aggregates leading to the formation of syncytia-like structures. This process is accelerated in the presence of lipopolysaccharide suggesting that phagocytes are capable of detecting this molecular pattern in culture conditions. Sea urchin immune response proteins, called Sp185/333, are expressed on the surface of a subset of phagocytes and have been associated with syncytia-like structures. We evaluated their expression in cultured phagocytes to determine their possible role in cell aggregation and in the formation of syncytia-like structures. Between 0 and 3 hr, syncytia-like structures were observed in cultures when only ∼10% of the cells were positive for Sp185/333 proteins. At 24 hr, ∼90% of the nuclei were Sp185/333-positive when all of the phagocytes had aggregated into syncytia-like structures. Consequently, we conclude that the Sp185/333 proteins do not have a major role in initiating the aggregation of cultured phagocytes, however the Sp185/333 proteins are associated with the clustered nuclei within the syncytia-like structures.

Formation of filopodia in earthworm (Lumbricus terrestris) coelomocytes in response to osmotic stress

Coelomocytes, the immunodefense cells of the earthworm Lumbricus terrestris, are exposed to changing osmotic pressures as the worm's coelomic fluid responds to fluctuating wet-dry conditions of the surrounding soil. Using light and fluorescence microscopy combined with actin and tubulin disrupting drugs, we determined the effects of changing osmotic pressure on coelomocyte morphology. The coelomocytes from L. terrestris respond to an increase in environmental osmotic pressure from isotonic conditions (170 mOsm) to hypertonic conditions (715 mOsm) by changing from a round/petalloid morphology to a filopodial morphology. Cytoskeletal fluorescent staining studies indicate that for filopodia to form, the actin cortical ring, present in most coelomocytes in isotonic conditions, must be disrupted. Breakdown of the actin ring by exposure to a hypertonic environment or actin disrupting drugs allows the formation of actin or tubulin-based filopodia. The filopodia, or podial-like extensions formed by earthworm coelomocytes, may enable the cells to better explore their environment.

Analysis of cytoskeletal and motility proteins in the sea urchin genome assembly

The sea urchin embryo is a classical model system for studying the role of the cytoskeleton in such events as fertilization, mitosis, cleavage, cell migration and gastrulation. We have conducted an analysis of gene models derived from the Strongylocentrotus purpuratus genome assembly and have gathered strong evidence for the existence of multiple gene families encoding cytoskeletal proteins and their regulators in sea urchin. While many cytoskeletal genes have been cloned from sea urchin with sequences already existing in public databases, genome analysis reveals a significantly higher degree of diversity within certain gene families. Furthermore, genes are described corresponding to homologs of cytoskeletal proteins not previously documented in sea urchins. To illustrate the varying degree of sequence diversity that exists within cytoskeletal gene families, we conducted an analysis of genes encoding actins, specific actin-binding proteins, myosins, tubulins, kinesins, dyneins, specific microtubule-associated proteins, and intermediate filaments. We conducted ontological analysis of select genes to better understand the relatedness of urchin cytoskeletal genes to those of other deuterostomes. We analyzed developmental expression (EST) data to confirm the existence of select gene models and to understand their differential expression during various stages of early development.

Monitoring chemical and physical stress using sea urchin immune cells

Coelomocytes are the cells freely circulating in the body fluid contained in echinoderm coelom and constitute the defence system, which, in response to injuries, host invasion, and adverse conditions, is capable of chemotaxis, phagocytosis, and production of cytotoxic metabolites. Red and colourless amoebocytes, petaloid and philopodial phagocytes, and vibratile cells are the cell types that, in different proportions, constitute the mixed coelomocyte cell population found in sea urchins. Advances in cellular and molecular biology have made it possible to identify a number of specific proteins expressed in coelomocytes under resting conditions or when activated by experimentally induced stress. Only recently, coelomocytes have been used for pollution studies with the aim of introducing a new biosensor for detection of stress at both cellular and molecular levels, as sentinel of sea health. In this chapter, we briefly review the important features of these valuable cells and describe studies on their use in the laboratory and in the field for the assessment of chemical and physical pollution of the sea.

Expression of phosphatidylinositol (4,5) bisphosphate-specific pleckstrin homology domains alters direction but not the level of axonal transport of mitochondria

Axonal transport of membranous organelles such as mitochondria is essential for neuron viability and function. How signaling mechanisms regulate or influence mitochondrial distribution and transport is still largely unknown. We observed an increase in the distal distribution of mitochondria in neurons upon the expression of pleckstrin homology (PH) domains of phospholipase Cdelta1 (PLCdelta-PH) and spectrin (spectrin-PH). Quantitative analysis of mitochondrial transport showed that specific binding of PH domains to phosphatidylinositol (4,5) bisphosphate (PtdIns(4,5)P2) but not 3' phosphorylated phosphatidylinositol species enhanced plus-end-directed transport of mitochondria two- to threefold and at the same time decreased minus-end-directed transport of mitochondria along axonal microtubules (MTs) without altering the overall level of motility. Further, the velocity and duration of mitochondrial transport plus the association of molecular motors with mitochondria remained unchanged by the expression of PH domains. Thus, PtdIns(4,5)P2-specific PH domains caused an increase in distal mitochondria by disturbing the balance of plus- and minus-end-directed transport rather than directly affecting the molecular machinery involved. Taken together our data reveal that level and directionality of transport are separable and that PtdIns(4,5)P2 has a novel role in regulation of the directionality of axonal transport of mitochondria.

Wound closure in the lamellipodia of single cells: mediation by actin polymerization in the absence of an actomyosin purse string

The actomyosin purse string is an evolutionarily conserved contractile structure that is involved in cytokinesis, morphogenesis, and wound healing. Recent studies suggested that an actomyosin purse string is crucial for the closure of wounds in single cells. In the present study, morphological and pharmacological methods were used to investigate the role of this structure in the closure of wounds in the peripheral cytoplasm of sea urchin coelomocytes. These discoidal shaped cells underwent a dramatic form of actin-based centripetal/retrograde flow and occasionally opened and closed spontaneous wounds in their lamellipodia. Fluorescent phalloidin staining indicated that a well defined fringe of actin filaments assembles from the margin of these holes, and drug studies with cytochalasin D and latrunculin A indicated that actin polymerization is required for wound closure. Additional evidence that actin polymerization is involved in wound closure was provided by the localization of components of the Arp2/3 complex to the wound margin. Significantly, myosin II immunolocalization demonstrated that it is not associated with wound margins despite being present in the perinuclear region. Pharmacological evidence for the lack of myosin II involvement in wound closure comes from experiments in which a microneedle was used to produce wounds in cells in which actomyosin contraction was inhibited by treatment with kinase inhibitors. Wounds produced in kinase inhibitor-treated cells closed in a manner similar to that seen with control cells. Taken together, our results suggest that an actomyosin purse string mechanism is not responsible for the closure of lamellar wounds in coelomocytes. We hypothesize that the wounds heal by means of a combination of the force produced by actin polymerization alone and centripetal flow. Interestingly, these cells did assemble an actomyosin structure around the margin of phagosome-like membrane invaginations, indicating that myosin is not simply excluded from the periphery by some general mechanism. The results indicate that the actomyosin purse string is not the only mechanism that can mediate wound closure in single cells.

Regulation and expression of metazoan unconventional myosins

Unconventional myosins are molecular motors that convert adenosine triphosphate (ATP) hydrolysis into movement along actin filaments. On the basis of primary structure analysis, these myosins are represented by at least 15 distinct classes (classes 1 and 3-16), each of which is presumed to play a specific cellular role. However, in contrast to the conventional myosins-2, which drive muscle contraction and cytokinesis and have been studied intensively for many years in both uni- and multicellular organisms, unconventional myosins have only been subject to analysis in metazoan systems for a short time. Here we critically review what is known about unconventional myosin regulation, function, and expression. Several points emerge from this analysis. First, in spite of the high relative conservation of motor domains among the myosin classes, significant differences are found in biochemical and enzymatic properties of these motor domains. Second, the idea that characteristic distributions of unconventional myosins are solely dependent on the myosin tail domain is almost certainly an oversimplification. Third, the notion that most unconventional myosins function as transport motors for membranous organelles is challenged by recent data. Finally, we present a scheme that clarifies relationships between various modes of myosin regulation.

Extremely low frequency electromagnetic fields and heat shock can increase microvesicle motility in astrocytes

The effect of extremely low frequency electromagnetic fields (EMF) on microvesicles was examined in rat astrocytes by video-enhanced microscopy in combination with a perfusable cell chamber. The EMF effect was compared with the effect of heat shock (HS) and with a combination of them both. The velocity of microvesicles was measured using image processing software (NIH Scion image 1.61). After exposure of astrocytes to EMF (50 Hz, 100microT, 1 h), the velocity of microvesicles in astrocytes increased from 0.32 +/- 0.03 microm/s (n = 120, 95% CI) in the untreated control group to 0.41 +/- 0.03 microm/s (n = 175, 95% CI). Fifteen minutes after HS (45 degrees C, 10 min) the microvesicles showed a velocity of 0.56 +/- 0.03 microm/s (n = 125, 95% CI). Combination of HS and EMF led to an increase in velocity up to 0.54 +/- 0.03 microm/s (n = 110, 95% CI). No significant difference between HS and HS+EMF was found. Compared to the untreated control group, the increased microvesicle velocity of the exposed cells might be a stress response of the cell. It is possibly a sign of intensified intracellular traffic required to adjust the metabolic needs.

Identification of a sea urchin Na(+)/K(+)/2Cl(−) cotransporter (NKCC): microfilament-dependent surface expression is mediated by hypotonic shock and cyclic AMP

We report the identification of an invertebrate Na(+)/K(+)/2Cl(−) cotransporter, NKCC. As a model system, we used the immune cells (coelomocytes) of the Mediterranean sea urchin Paracentrotus lividus. These cells are particularly interesting because they can be activated to undergo a rapid and dynamic change in cell shape. We demonstrate that forskolin, a cyclic AMP agonist known to regulate NKCC, induced coelomocyte transformation at doses of 10 micromol l(−)(1) and greater. Using two distinct monoclonal antibodies (T4 and T9) raised against the human intestinal epithelial NKCC, we have identified a high-molecular-mass (195 kDa) protein in coelomocyte extracts. We propose a novel method for the isolation of NKCC in one step by using bumetanide-Sepharose affinity chromatography under low-[Cl(−)] conditions. This method was successful in isolating coelomocyte 195 kDa NKCC. The T4 monoclonal antibody was used in immunocytochemical experiments to localize NKCC in resting and activated coelomocytes. In petalloid coelomocytes, a punctate, cytoplasmic distribution was observed in close proximity to actin filament bundles; in transformed coelomocytes, the immunofluorescence was distributed along the length of the filopodia and uniformly throughout the perinuclear region. The change in subcellular distribution of NKCC between the resting and the activated state was further investigated by using cell surface biotinylation followed by immunoprecipitation. These studies revealed an upregulation of NKCC at the plasma membrane upon activation, a process that was blocked by the F-actin-stabilizing drug phalloidin. These studies identify a novel model system in which to investigate a newly identified invertebrate Na(+)/K(+)/2Cl(−) cotransporter.

Characterization of sea urchin unconventional myosins and analysis of their patterns of expression during early embryogenesis

Early sea urchin development requires a dynamic reorganization of both the actin cytoskeleton and cytoskeletal interactions with cellular membranes. These events may involve the activities of multiple members of the superfamily of myosin motor proteins. Using RT-PCR with degenerate myosin primers, we identified 11 myosin mRNAs expressed in unfertilized eggs and coelomocytes of the sea urchin Strongylocentrotus purpuratus. Seven of these sea urchin myosins belonged to myosin classes Igamma, II, V, VI, VII, IX, and amoeboid-type I, and the remaining four may be from novel classes. Sea urchin myosins-V, -VI, -VII, and amoeboid-type-I were either completely or partially cloned and their molecular structures characterized. Sea urchin myosins-V, -VI, -VII, and amoeboid-type-I shared a high degree of sequence identity with their respective family members from vertebrates and they retained their class-specific structure and domain organization. Analysis of expression of myosin-V, -VI, -VII, and amoeboid-type-I mRNAs during development revealed that each myosin mRNA displayed a distinct temporal pattern of expression, suggesting that myosins might be involved in specific events of early embryogenesis. Interestingly, the onset of gastrulation appeared to be a pivotal point in modulation of myosin mRNA expression. The presence of multiple myosin mRNAs in eggs and embryos provides insight into the potential involvement of multiple specific motor proteins in the actin-dependent events of embryo development.

Actin-dependent regulation of neurotransmitter release at central synapses

Depolymerization of actin by latrunculin A transiently promotes neurotransmitter release. The mean rate of mEPSCs increases by a Ca2+-independent process, without a concomitant change in the mean amplitude. The readily releasable vesicle pool size and the rate of refilling of the readily releasable pool remain unaltered by latrunculin treatment. Evoked neurotransmitter release also increases in a manner consistent with an increase in vesicle release probability. The observed enhancement of neurotransmitter release is specific to actin depolymerization mediated by latrunculin A and is not caused by cytochalasin D. Our findings indicate that actin participates in a regulatory mechanism that restrains fusion of synaptic vesicles at the active zone.

Disassembly of actin filaments leads to increased rate and frequency of mitochondrial movement along microtubules

In activated sea urchin coelomocytes, cytoplasmic organelles move along distinct actin and microtubule dependent pathways, actin-based motility is driven by an unconventional myosin, and microtubule disassembly does not effect actin-dependent organelle motility [D'Andrea et al., 1994: J. Cell Sci. 107:2081-2094]. Given the growing evidence for potential interactions between components of the actin and microtubule cytoskeletons, we examined the effect of actin filament disassembly on the movement of mitochondria along microtubules in activated coelomocytes. Coelomocytes treated with cytochalasin B (CB), to disrupt actin filaments, exhibited a thinning of the cytoplasm, enhanced lateral undulation of microtubules, and ceased centripetal cortical flow of actin. Interestingly, the loss of actin filaments resulted in a approximately 1.5-fold increase in the average velocity of outward and inward moving mitochondria and increased the frequency of centripetal movement. To test if enhanced motility along microtubules was a consequence of decreased actin-myosin interaction, coelomocytes were treated with 2,3-butanedione monoxime (BDM), a potent inhibitor of myosin activity [Cramer and Mitchison, 1995: J. Cell Biol. 131:179-189]. BDM inhibited all types of actin-based motility observed in these cells including retrograde cortical flow, protrusion and retraction of the cell edge, and movement of intracellular organelles. Surprisingly, BDM treatment stopped the movement of mitochondria in CB-exposed cells, suggesting that BDM can also act as an inhibitor of organelle movement along microtubules. Collectively, these data demonstrated that microtubule-dependent mitochondrial motility and microtubule movement were sensitive to changes in the assembly state of the actin cytoskeleton.

Vesicle transport: the role of actin filaments and myosin motors

The transport of vesicles and the retention of organelles at specific locations are fundamental processes in cells. Actin filaments and myosin motors have been shown to be required for both of these tasks. Most of the organelles in cells associate with actin filaments and some of the myosin motors required for movement on actin filaments have been identified. Myosin V has been shown to transport endoplasmic reticulum (ER) vesicles in neurons, pigment granules in melanocytes, and the vacuole in yeast. Myosin I has been shown to be involved in the transport of Golgi-derived vesicles in epithelial cells. Myosin VI has been shown to be associated with Golgi-derived vesicles, and cytoplasmic vesicles in living Drosophila embryos. Myosin II may be a vesicle motor but its role in vesicle transport has not been resolved. Secretory vesicles, endosomes and mitochondria appear to be transported on actin filaments but the myosin motors on these organelles have not been identified. Mitochondria in yeast may be transported by the dynamic assembly of an actin "tail." The model that has unified all of these findings is the concept that long-range movement of vesicles occurs on microtubules and short-range movement on actin filaments. The details of how the microtubule-dependent and the actin-dependent motors are coordinated are important questions in the field. There is now strong evidence that two molecular motors, kinesin and myosin V, interact with each other and perhaps function as a complex on vesicles. An understanding of the interrelationship of microtubules and actin filaments and the motors that move cargo on them will ultimately establish how vesicles and organelles are transported to their specific locations in cells.

Vesicle-associated brain myosin-V can be activated to catalyze actin-based transport

Myosin-V has been linked to actin-based organelle transport by a variety of genetic, biochemical and localization studies. However, it has yet to be determined whether myosin-V functions as an organelle motor. To further investigate this possibility, we conducted a biochemical and functional analysis of organelle-associated brain myosin-V. Using the initial fractionation steps of an established protocol for the purification of brain myosin-V, we isolated a population of brain microsomes that is approx. fivefold enriched for myosin-V, and is similarly enriched for synaptic vesicle proteins. As demonstrated by immunoelectron microscopy, myosin-V associates with 30–40% of the vesicles in this population. Although a majority of myosin-V-associated vesicles also label with the synaptic vesicle marker protein, SV2, less than half of the total SV2-positive vesicles label with myosin-V. The average size of myosin-V/SV2 double-labeled vesicles (90+/−45 nm) is larger than vesicles that label only with SV2 antibodies (60+/−30 nm). To determine if these vesicles are capable of actin-based transport, we used an in vitro actin filament motility assay in which vesicles were adsorbed to motility assay substrates. As isolated, the myosin-V-associated vesicle fraction was nonmotile. However, vesicles pre-treated with ice-cold 0.1% Triton X-100 supported actin filament motility at rates comparable to those on purified myosin-V. This dilute detergent treatment did not disrupt vesicle integrity. Furthermore, while this treatment removed over 80% of the total vesicle proteins, myosin-V remained tightly vesicle-associated. Finally, function-blocking antibodies against the myosin-V motor domain completely inhibited motility on these substrates. These studies provide direct evidence that vesicle-associated myosin-V is capable of actin transport, and suggest that the activity of myosin-V may be regulated by proteins or lipids on the vesicle surface.

Novel characteristics of a myosin isolated from mammalian retinal pigment epithelial and endothelial cells

We have isolated a novel, high Mr protein from human retinal pigment epithelial cells and endothelial cells by affinity chromatography on Sepharose 4B. Two polypeptides are present on SDS-gels of the 8 M urea eluent with apparent molecular mass of approximately 210 and 47 kDa. In the absence of dithiothreitol, the two polypeptides migrate as one protein band with an apparent molecular mass of approximately 550 kDa. "Piglet," as this molecule is tentatively named, is present in retinal pigment epithelial and endothelial cells of several species, but could not be detected in the nonepithelial cells we examined. Immunofluorescent localization using an antibody to the 210-kDa polypeptide revealed a filamentous network in the cytoplasm of cultured cells. This antibody was used to identify a cDNA for piglet in a bovine aortic endothelial cell expression library. Sequence data indicate a high degree of identity with non-muscle myosin II heavy chain. We subsequently found that piglet had an actin-activated ATPase activity, colocalized with actin in cells, and reacted on Western blots with a pan-non-muscle myosin II heavy chain antiserum. The protein was also recognized by antibodies specific for myosin heavy chain isoform A, but did not react with anti-isoform B antibodies. Although piglet has several features in common with known forms of non-muscle myosin II, the distinctly unconventional features it displays suggest that it is a novel myosin.

Subcellular localization of myosin V in nerve growth cones and outgrowth from dilute-lethal neurons

Myosin V-null mice (dilute-lethal mutants) exhibit apparent neurological defects that worsen from birth until death in the third postnatal week. Although myosin V is enriched in brain, the neuronal function of myosin V is unclear and the underlying cause of the neurological defects in these mice is unknown. To aide in understanding myosin V function, we examined the distribution of myosin V in the rodent superior cervical ganglion (SCG) growth cone, a well characterized neuronal structure in which myosin V is concentrated. Using affinity purified, myosin V-specific antibodies in immunofluorescence and immunoelectron microscopy, we observed that myosin V is concentrated in organelle-rich regions of the growth cone. Myosin V is present on a distinct population of small (50–100 nm) organelles, and on actin filaments and the plasma membrane. Myosin V-associated organelles are present on both microtubules and actin filaments. These results indicate that myosin V may be carried as a passenger on organelles that are transported along microtubules, and that these organelles may also be capable of movement along actin filaments. In addition, we found no abnormalities in outgrowth, morphology, or cytoskeletal organization of SCG growth cones from dilute-lethal mice. These results indicate that myosin V is not necessary for the traction force needed for growth cone locomotion, for organization of the actin cytoskeleton, or for filopodial dynamics.

Examination of the endosomal and lysosomal pathways in Dictyostelium discoideum myosin I mutants

The role of myosin Is in endosomal trafficking and the lysosomal system was investigated in a Dictyostelium discoideum myosin I double mutant myoB-/C-, that has been previously shown to exhibit defects in fluid-phase endocytosis during growth in suspension culture (Novak et al., 1995). Various properties of the endosomal pathway in the myoB-/C- double mutant as well as in the myoB- and myoC- single mutants, including intravesicular pH, and intracellular retention time and exocytosis of a fluid phase marker, were found to be indistinguishable from wild-type parental cells. The intimate connection between the contractile vacuole complex and the endocytic pathway in Dictyostelium, and the localization of a myosin I to the contractile vacuole in Acanthamoeba, led us to also examine the structure and function of this organelle in the three myosin I mutants. No alteration in contractile vacuole structure or function was observed in the myoB-, myoC- or myoB-/C- cell lines. The transport, processing, and localization of a lysosomal enzyme, alpha-mannosidase, were also unaltered in all three mutants. However, the myoB- and myoB-/C- cell lines, but not the myoC- cell line, were found to oversecrete the lysosomal enzymes alpha-mannosidase and acid phosphatase, during growth and starvation. None of the mutants oversecreted proteins following the constitutive secretory pathway. Two additional myosin I mutants, myoA- and myoA-/B-, were also found to oversecrete the lysosomally localized enzymes alpha-mannosidase and acid phosphatase. Taken together, these results suggest that these myosins do not play a role in the intracellular movement of vesicles, but that they may participate in controlling events that occur at the actin-rich cortical region of the cell. While no direct evidence has been found for the association of myosin Is with lysosomes, we predict that the integrity of the lysosomal system is tied to the fidelity of the actin cortex, and changes in cortical organization could influence lysosomal-related membrane events such as internalization or transit of vesicles to the cell surface.

Molecular motors, membrane movements and physiology: emerging roles for myosins

Myosins are a large family of structurally diverse mechanoenzymes which, upon interaction with actin filaments, convert energy from ATP hydrolysis into mechanical force. Consistent with the ubiquitous association of actin with membranes, many of these novel myosins are membrane-associated. In the past two years, evidence has emerged that suggests roles for actin-based molecular motors in a wide range of membrane phenomena such as cell locomotion, phagocytosis, secretion, organelle transport, signal transduction and mechanoregulation of membrane protein function.

Membrane traffic motors

There is a wealth of data suggesting that microtubules and associated motor proteins play important roles in orchestrating membrane traffic within higher eukaryotes, with myosins and actin filaments fulfilling similar functions in organisms such as fungi, algae and plants. In addition, evidence is accumulating that both cytoskeletal systems can co-operate within one cell. Recent studies have highlighted how individual motor proteins can act at multiple steps in the membrane-traffic pathways, and in contrast, how more than one motor type may be involved in each transport step and in generating organelle morphology.

Regulation of calmodulin-binding myosins

Myosins constitute a diverse superfamily of actin-based mechanoenzymes that are involved in many essential cellular motilities. In addition to conventional muscle myosin II, ten other classes of unconventional myosins are known. Many unconventional myosins bind multiple calmodulin light chains and Ca2+, which can dramatically alter their mechanochemical and enzymatic activity. Calmodulin-binding myosins can also be regulated by phospholipid binding, phosphorylation of the heavy chain and actin-binding proteins. The molecular details linking unconventional-myosin regulation and function are just beginning to emerge.

Microtubule-associated movement of mitochondria and small particles in Acanthamoeba castellanii

Using video-enhanced differential interference microscopy and digital image processing, we have observed organelle motility in Acanthamoeba castellanii. In amoebae taken from cultures in rapid growth phase, mitochondria and small particles moved over distances of several microns and at an average velocity of approximately 2 microns/s. Mitochondrial motility was verified by intensified fluorescence microscopy of cells that were labeled in vivo with the DNA-binding dye DAPI or the mitochondria-specific dye MitoTracker. We further studied the role of microtubules (MTs) in the translocation of cell organelles. Double-labeling of fixed cells with mitochondrial markers (anti-F1 beta antibody, MitoTracker) and cytoskeletal markers (anti-tubulin antibody, rhodamine-phalloidin) demonstrate that the mitochondria colocalize with MTs in the subcortical cell area and are excluded from the F-actin-rich cell cortex. Colchicine treatment resulted in an almost complete depolymerization of MTs and an inhibition of organelle motility. Moreover, we have directly visualized MTs in vivo in flattened amoebae. Mitochondria and small particles moved along the MTs in a bidirectional mode at an average velocity of approximately 1 micron/s. We conclude that the observed movement of mitochondria and small particles in Acanthamoeba castellanii mainly occurs via microtubules and associated motor proteins.