Nuclear movement in filamentous fungi

One of the most striking features of eukaryotic cells is the organization of specific functions into organelles such as nuclei, mitochondria, chloroplasts, the endoplasmic reticulum, vacuoles, peroxisomes or the Golgi apparatus. These membrane-surrounded compartments are not synthesized de novo but are bequeathed to daughter cells during cell division. The successful transmittance of organelles to daughter cells requires the growth, division and separation of these compartments and involves a complex machinery consisting of cytoskeletal components, mechanochemical motor proteins and regulatory factors. Organelles such as nuclei, which are present in most cells in a single copy, must be precisely positioned prior to cytokinesis. In many eukaryotic cells the cleavage plane for cell division is defined by the location of the nucleus prior to mitosis. Nuclear positioning is thus absolutely crucial in the unequal cell divisions that occur during development and embryogenesis. Yeast and filamentous fungi are excellent organisms for the molecular analysis of nuclear migration because of their amenability to a broad variety of powerful analytical methods unavailable in higher eukaryotes. Filamentous fungi are especially attractive models because the longitudinally elongated cells grow by apical tip extension and the organelles are often required to migrate long distances. This review describes nuclear migration in filamentous fungi, the approaches used for and the results of its molecular analysis and the projection of the results to other organisms.     

Nuclear migration advances in fungi

Nuclear migration encompasses three areas: separation of daughter nuclei during mitosis, congress of parental nuclei before they fuse during fertilization, and positioning of nuclei in interphase cells. This review deals primarily with interphase nuclear migration, which is crucial for events as disparate as vertebrate embryonic development and growth of fungal mycelia. Mutants of Aspergillus nidulans, Neurospora crassa and Saccharomyces cerevisiae have been particularly informative, and a detailed molecular analysis of this process is now well under way.     

Nuclear migration and positioning in filamentous fungi

Genetic analyses of nuclear distribution mutants have indicated that functions of the microtubule motor, cytoplasmic dynein, and its regulators are important for nuclear positioning in filamentous fungi. Here we review these studies and also present the need to further dissect how dynein and its associated microtubule cytoskeleton are involved mechanistically in nuclear positioning in the multinucleated hyphae.     

Cellular mechanisms of nuclear migration

Subcellular mobility, positioning, and directional movement of the nucleus in a certain site of the cell or cenocyte and, less frequently, intercellular translocation of the nucleus accompany the cell and tissue differentiation, change of their functions, and the organism growth and development and its response to stress, plant–microbial interactions, symbiosis, and many other processes in plants and animals. The nucleus movement is performed and directed through the interaction between dynamic cytoskeleton components and nucleus by means of signal-binding proteins, including motor and linker. The cell responds to the external signal by mobilization and polar reconstruction of the cytoskeleton components, as a result of which the nucleus displacement by means of actomyosin or microtubule mechanisms in cooperation with dynein and kinesin occurs. In plants, the actomyosin mechanism is involved in the nucleus migration; it allows the nucleus to move rapidly and over significant distances in response to environmental stimuli. An important role in the nucleus translocation belongs to the linker complexes of the proteins that are inserted in the nuclear envelope, that connect and transmit signals from the plasmalemma to the cytoplasm and nucleoplasm, and that provide the skeletal basis for many subcellular compartments. Changes in the protein composition, conformational modifications of the proteins, and displacement of linkers from the nuclear envelope result in the nucleus detachment from the cytoskeleton, and change in the form, mechanical rigidity, and positioning of the nucleus.     

Spore development and nuclear inheritance in arbuscular mycorrhizal fungi

Background  

A conventional tenet of classical genetics is that progeny inherit half their genome from each parent in sexual reproduction instead of the complete genome transferred to each daughter during asexual reproduction. The transmission of hereditary characteristics from parents to their offspring is therefore predictable, although several exceptions are known. Heredity in microorganisms, however, can be very complex, and even unknown as is the case for coenocytic organisms such as Arbuscular Mycorrhizal Fungi (AMF). This group of fungi are plant-root symbionts, ubiquitous in most ecosystems, which reproduce asexually via multinucleate spores for which sexuality has not yet been observed.     

A fungal actin-related protein involved in nuclear migration

The ro-4 mutant of the filamentous fungus Neurospora crassa forms distinctive colonies in which hyphae grow into rope-like aggregates. This unusual morphology coincides with a defect in hyphal nuclear migration. The ro-4 gene was cloned from a cosmid library by complementation of the closely linked pab-2 gene. The deduced 380 amino acid protein is most similar to the vertebrate actin-related protein/centractin. The R04 protein is not essential for cell viability, and new strains created by inducing point mutations at the ro-4 locus have a phenotype which is very similar to that of the original mutant. This study provides genetic evidence that an actin-related protein plays a role in nuclear motility. Since nuclear motility is believed to be a microtubule-dependent process, the ro-4 gene product may function as a component of the dynactin complex which activates force generation by cytoplasmic dynein.     

Approaches to resolving cephalopod movement and migration patterns

Cephalopod movement occurs during all phases of the life history, with the abundance and location of cephalopod populations strongly influenced by the prevalence and scale of their movements. Environmental parameters, such as sea temperature and oceanographic processes, have a large influence on movement at the various life cycle stages, particularly those of oceanic squid. Tag recapture studies are the most common way of directly examining cephalopod movement, particularly in species which are heavily fished. Electronic tags, however, are being more commonly used to track cephalopods, providing detailed small- and large-scale movement information. Chemical tagging of paralarvae through maternal transfer may prove to be a viable technique for tracking this little understood cephalopod life stage, as large numbers of individuals could be tagged at once. Numerous indirect methods can also be used to examine cephalopod movement, such as chemical analyses of the elemental and/or isotopic signatures of cephalopod hard parts, with growing interest in utilising these techniques for elucidating migration pathways, as is commonly done for fish. Geographic differences in parasite fauna have also been used to indirectly provide movement information, however, explicit movement studies require detailed information on parasite-host specificity and parasite geographic distribution, which is yet to be determined for cephalopods. Molecular genetics offers a powerful approach to estimating realised effective migration rates among populations, and continuing developments in markers and analytical techniques hold the promise of more detailed identification of migrants. To date genetic studies indicate that migration in squids is extensive but can be blocked by major oceanographic features, and in cuttlefish and octopus migration is more locally restricted than predictions from life history parameters would suggest. Satellite data showing the location of fishing lights have been increasingly used to examine the movement of squid fishing vessels, as a proxy for monitoring the movement of the squid populations themselves, allowing for the remote monitoring of oceanic species.     

Nuclear division,nuclear distribution and cytokinesis in filamentous fungi

Nuclear division, nuclear distribution and cytokinesis are fundamental processes of all eukaryotic organisms, and filamentous fungi, specificallyAspergillus nidulans andNeurospora crassa, have provided sophisticated genetic systems for identification of the genes required for these processes. Mutational analyses have led to identification of novel proteins that have subsequently been found to be conserved components required for nuclear-specific functions. Formation of the mitotic spindle inA. nidulans has been shown to be dependent onγ-tubulin, a central element of all microtubule organizing centres, and two kinesin-related proteins. Analysis ofA. nidulans mitotic mutants has led to identification of two important cell-cycle regulators, NIMA and BIME. The NIMA kinase is required for entry into mitosis, and BIME has recently been identified as a subunit of an anaphase-promoting complex that targets cyclins for proteolysis. The microtubule-associated motor protein cytoplasmic dynein has been discovered in bothA. nidulans andN. crassa, and it has been proposed that it provides motive force for the distribution of nuclei within hyphae. Future studies of nucleus-specific processes in filamentous fungi are likely not only to identify additional novel structural and regulatory proteins, but also lead to an understanding of how the processes of nuclear division, nuclear distribution and septation are altered to meet the developmental needs of the organism.     

Putting a notch in our understanding of nuclear migration

The nuclei of progenitor cells in developing neural epithelia change their position during the cell cycle through a process called interkinetic nuclear migration. Del Bene et al. (2008) report that defects in the machinery controlling this process lead to altered exposure to Notch signals and systemic effects on neurogenesis in the retina.     

Cell division and nuclear movement in the saccoderm desmidNetrium interruptus

Summary During anaphase in thisNetrium, the reforming daughter nuclei hardly pause at the poles before they elongate and rapidly and smoothly move along the daughter cells in one of the grooves in the chloroplast. Ahead of each nucleus is a pointed mass of cytoplasm that is distinctly striated; straight, mobile strands of cytoplasm emanate from this region ahead of the nucleus. When the nucleus reaches the large vacuole that divides the two chloroplasts, it steadily slides over to the chloroplast surface distal to the cleavage furrow. It then stops moving and slowly expands into the normal interphase morphology.Under the electron microscope, the chromosome-to-pole distance does not decrease much during anaphase (i.e., anaphase A is minimal) and so the half spindles remain about the same length by telophase. The poles of the open spindle are initially broad and contain typical spindle microtubules (MTs). These persist intact during anaphase and become focused upon a discrete Organizing Centre as the daughter nuclei reform. These MTs become a cone-shaped array that creates the pointed cytoplasmic mass ahead of the moving nucleus in live cells. Thus, this placoderm desmid behaves very likeClosterium during division and shows the lack of anaphase A, and the transformation of the telophase spindle into a MT-based motility system, now characteristic of many members of the Zygnematales.Abbreviations MT microtubule - MTOC microtubule organizing centre Dedicated to the memory of Professor Oswald Kiermayer     

Anastomosis formation and nuclear and protoplasmic exchange in arbuscular mycorrhizal fungi

We observed anastomosis between hyphae originating from the same spore and from different spores of the same isolate of the arbuscular mycorrhizal fungi Glomus mosseae, Glomus caledonium, and Glomus intraradices. The percentage of contacts leading to anastomosis ranged from 35 to 69% in hyphae from the same germling and from 34 to 90% in hyphae from different germlings. The number of anastomoses ranged from 0.6 to 1.3 per cm (length) of hyphae in mycelia originating from the same spore. No anastomoses were observed between hyphae from the same or different germlings of Gigaspora rosea and Scutellospora castanea; no interspecific or intergeneric hyphal fusions were observed. We monitored anastomosis formation with time-lapse and video-enhanced light microscopy. We observed complete fusion of hyphal walls and the migration of a mass of particles in both directions within the hyphal bridges. In hyphal bridges of G. caledonium, light-opaque particles moved at the speed of 1.8 +/- 0.06 microm/s. We observed nuclear migration between hyphae of the same germling and between hyphae belonging to different germlings of the same isolate of three Glomus species. Our work suggests that genetic exchange may occur through intermingling of nuclei during anastomosis formation and opens the way to studies of vegetative compatibility in natural populations of arbuscular mycorrhizal fungi.     

Acetylcholine receptor clustering and nuclear movement in muscle fibers in culture

We have studied the formation of acetylcholine receptor (AChR) clusters and the behavior of myonuclei in rat and chick skeletal muscle cells grown in cell culture. These cells were treated with a factor derived from Torpedo electric extracellular matrix, which causes a large increase in their number of AChR clusters. We found that these clusters were located preferentially in membrane regions above myonuclei. This cluster-nucleus colocalization is explained by our finding that most of the nuclei near clusters remain relatively stationary, while most of those away from clusters are able to translocate throughout the myotube. In some cases, clusters clearly formed first, then nuclei migrated underneath and became immobilized. If clustered AChRs later dispersed, their associated nuclei resumed moving. These results suggest that AChR clustering initiates an extensive cytoskeletal rearrangement that causes the subcluster localization of organelles, potentially providing a stable source of newly synthesized AChRs for insertion into the cluster.     

NudC is required for interkinetic nuclear migration and neuronal migration during neocortical development

NudC is a highly conserved protein necessary for cytoplasmic dynein-mediated nuclear migration in Aspergillus nidulans. NudC interacts genetically with Aspergillus NudF and physically with its mammalian orthologue Lis1, which is crucial for nuclear and neuronal migration during brain development. To test for related roles for NudC, we performed in utero electroporation into embryonic rat brain of cDNAs encoding shRNAs as well as wild-type and mutant forms of NudC. We show here that NudC, like Lis1, is required for neuronal migration during neocorticogenesis and we identify a specific role in apical nuclear migration in radial glial progenitor cells. These results identify a novel neuronal migration gene with a specific role in interkinetic nuclear migration, consistent with cytoplasmic dynein regulation.