Regulation of the actin cytoskeleton in the neuronal growth cone

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Biology, 2005.Includes bibliographical references (leaves 94-111).During the development of the nervous system, axons and dendrites are guided to their targets throughout the brain and body through the detection of diffusible and surface-bound guidance cues. The growth cone, a specialized structure found at the tips of axons and dendrites, detects and interprets these cues. Activation of downstream intracellular singling pathways leads to changes in the cytoskeleton of the growth cone that affect its motility. Although a number of the key players in this system have been identified, the means by which the growth cone is able to directly rearrange its cytoskeleton in response to guidance cues is not known. In the following experiments, I have examined the roles of several known actin- binding proteins in the remodeling of the growth cone cytoskeleton. The Arp2/3 protein complex binds to an existing actin filament and nucleates formation of a daughter branch. Arp2/3 activity generates a dense actin network in fibroblast cells that drives membrane protrusions. However, the role of this activity in neuronal growth cones has not been established. We have used an inhibitory strategy to explore the requirement for Arp2/3 in growth cone morphology and function. Arp2/3 is not required for growth cone morphology or the generation of protrusive structures; however, it may be required for proper motility in response to axon guidance cues. Activity the EnaNASP family of proteins promotes the formation of long, unbranched filaments in fibroblast systems. Ena/VASP has also been genetically linked to axon guidance pathways in Drosophila, C. elegans, and mice.(cont.) To examine the role of Ena/VASP proteins in growth cones, we used a strategy to simultaneously inhibit all three mammalian family members in neurons. Ena/VASP proteins were found to have a key role in filopodia formation in growth cones. During the course of my graduate work I was involved in several collaborations that examined the roles of several other proteins involved in the regulation of the actin cytoskeleton. Relevant excerpts from these experiments are presented.by Geraldine A. Strasser.Ph.D

Arp2/3 Is a Negative Regulator of Growth Cone Translocation

Arp2/3 is an actin binding complex that is enriched in the peripheral lamellipodia of fibroblasts, where it forms a network of short, branched actin filaments, generating the protrusive force that extends lamellipodia and drives fibroblast motility. Although it has been assumed that Arp2/3 would play a similar role in growth cones, our studies indicate that Arp2/3 is enriched in the central, not the peripheral, region of growth cones and that the growth cone periphery contains few branched actin filaments. Arp2/3 inhibition in fibroblasts severely disrupts actin organization and membrane protrusion. In contrast, Arp2/3 inhibition in growth cones minimally affects actin organization and does not inhibit lamellipodia protrusion or de novo filopodia formation. Surprisingly, Arp2/3 inhibition significantly enhances axon elongation and causes defects in growth cone guidance. These results indicate that Arp2/3 is a negative regulator of growth cone translocation.National Institutes of Health (U.S.) (Grant 6895154)W. M. Keck Foundation (Distinguished Young Scholar Award

The Murine Nck SH2/SH3 Adaptors Are Important for the Development of Mesoderm-Derived Embryonic Structures and for Regulating the Cellular Actin Network

Mammalian Nck1 and Nck2 are closely related adaptor proteins that possess three SH3 domains, followed by an SH2 domain, and are implicated in coupling phosphotyrosine signals to polypeptides that regulate the actin cytoskeleton. However, the in vivo functions of Nck1 and Nck2 have not been defined. We have mutated the murine Nck1 and Nck2 genes and incorporated β-galactosidase reporters into the mutant loci. In mouse embryos, the two Nck genes have broad and overlapping expression patterns. They are functionally redundant in the sense that mice deficient for either Nck1 or Nck2 are viable, whereas inactivation of both Nck1 and Nck2 results in profound defects in mesoderm-derived notochord and embryonic lethality at embryonic day 9.5. Fibroblast cell lines derived from Nck1(−/−) Nck2(−/−) embryos have defects in cell motility and in the organization of the lamellipodial actin network. These data suggest that the Nck SH2/SH3 adaptors have important functions in the development of mesodermal structures during embryogenesis, potentially linked to a role in cell movement and cytoskeletal organization

Combined absence of TRP53 target genes ZMAT3, PUMA and p21 cause a high incidence of cancer in mice

Transcriptional activation of target genes is essential for TP53-mediated tumour suppression, though the roles of the diverse TP53-activated target genes in tumour suppression remains poorly understood. Knockdown of ZMAT3, an RNA-binding zinc-finger protein involved in regulating alternative splicing, in haematopoietic cells by shRNA caused leukaemia only with the concomitant absence of the PUMA and p21, the critical effectors of TRP53-mediated apoptosis and cell cycle arrest respectively. We were interested to further investigate the role of ZMAT3 in tumour suppression beyond the haematopoietic system. Therefore, we generated Zmat3 knockout and compound gene knockout mice, lacking Zmat3 and p21, Zmat3 and Puma or all three genes. Puma-/-p21-/-Zmat3-/- triple knockout mice developed tumours at a significantly higher frequency compared to wild-type, Puma-/-Zmat3-/- or p21-/-Zmat3-/-deficient mice. Interestingly, we observed that the triple knockout and Puma-/-Zmat3-/- double deficient animals succumbed to lymphoma, while p21-/-Zmat3-/- animals developed mainly solid cancers. This analysis suggests that in addition to ZMAT3 loss, additional TRP53-regulated processes must be disabled simultaneously for TRP53-mediated tumour suppression to fail. Our findings reveal that the absence of different TRP53 regulated tumour suppressive processes changes the tumour spectrum, indicating that different TRP53 tumour suppressive pathways are more critical in different tissues.This work was supported by grants and fellowships from the Australian Phenomics Network (APN), the Australian National Health and Medical Research Council (NHMRC) to MJH and AS (1143105), Programme Grant to AS (1016701), Investigator grant to AS (2007887), Investigator Grant to MJH (2017971); the Leukaemia and Lymphoma Society of America to AS and MJH (LLS SCOR 7015-18); the Cancer Council of Victoria Project grant to AS (1052309) and Venture Grant to MJH and AS; support to AS from the estate of Anthony (Toni) Redstone OAM; support to AJ from Spanish Ministry of Economy and Development Grant (PID2021-127710OB-I00) and Programa Captació de Talent Investigador ‘La Caixa’ foundation (51110009). AJ is supported by Ramon y Cajal Research Fellowship (RYC2018-025244-I), AS and MJH are supported by NHMRC Fellowships (1020363 and 1156095), MSB is supported by Cancer Council Victoria Postdoctoral Fellowship and Swedish Cancer Society (21 0355 PT). This work was made possible through the Victorian Government Operational Infrastructure Support and Australian Government, the ‘Unidad de Excelencia María de Maeztu’ funded by the Spanish Government

A dysfunctional desmin mutation in a patient with severe generalized myopathy

Mice lacking desmin produce muscle fibers with Z disks and normal sarcomeric organization. However, the muscles are mechanically fragile and degenerate upon repeated contractions. We report here a human patient with severe generalized myopathy and aberrant intrasarcoplasmic accumulation of desmin intermediate filaments. Muscle tissue from this patient lacks the wild-type desmin allele and has a desmin gene mutation encoding a 7-aa deletion within the coiled-coil segment of the protein. We show that recombinant desmin harboring this deletion cannot form proper desmin intermediate filament networks in cultured cells, nor is it able to assemble into 10-nm filaments in vitro. These findings provide direct evidence that a mutation in desmin can cause human myopathies

Memory CD8+ T cells vary in differentiation phenotype in different persistent virus infections.

The viruses HIV-1, Epstein-Barr virus (EBV), cytomegalovirus (CMV) and hepatitis C virus (HCV) are characterized by the establishment of lifelong infection in the human host, where their replication is thought to be tightly controlled by virus-specific CD8+ T cells. Here we present detailed studies of the differentiation phenotype of these cells, which can be separated into three distinct subsets based on expression of the costimulatory receptors CD28 and CD27. Whereas CD8+ T cells specific for HIV, EBV and HCV exhibit similar characteristics during primary infection, there are significant enrichments at different stages of cellular differentiation in the chronic phase of persistent infection according to the viral specificity, which suggests that distinct memory T-cell populations are established in different virus infections. These findings challenge the current definitions of memory and effector subsets in humans, and suggest that ascribing effector and memory functions to subsets with different differentiation phenotypes is no longer appropriate

Physiological and morphological responses of the temperate seagrass Zostera muelleri to multiple stressors: investigating the interactive effects of light and temperature

Understanding how multiple environmental stressors interact to affect seagrass health (measured as morphological and physiological responses) is important for responding to global declines in seagrass populations. We investigated the interactive effects of temperature stress (24, 27, 30 and 32°C) and shading stress (75, 50, 25 and 0% shade treatments) on the seagrass Zostera muelleri over a 3-month period in laboratory mesocosms. Z. muelleri is widely distributed throughout the temperate and tropical waters of south and east coasts of Australia, and is regarded as a regionally significant species. Optimal growth was observed at 27°C, whereas rapid loss of living shoots and leaf mass occurred at 32°C. We found no difference in the concentration of photosynthetic pigments among temperature treatments by the end of the experiment; however, up-regulation of photoprotective pigments was observed at 30°C. Greater levels of shade resulting in high photochemical efficiencies, while elevated irradiance suppressed effective quantum yield (ΔF/FM'). Chlorophyll fluorescence fast induction curves (FIC) revealed that the J step amplitude was significantly higher in the 0% shade treatment after 8 weeks, indicating a closure of PSII reaction centres, which likely contributed to the decline in ΔF/FM' and photoinhibition under higher irradiance. Effective quantum yield of PSII (ΔF/FM') declined steadily in 32°C treatments, indicating thermal damage. Higher temperatures (30°C) resulted in reduced above-ground biomass ratio and smaller leaves, while reduced light led to a reduction in leaf and shoot density, above-ground biomass ratio, shoot biomass and an increase in leaf senescence. Surprisingly, light and temperature had few interactive effects on seagrass health, even though these two stressors had strong effects on seagrass health when tested in isolation. In summary, these results demonstrate that populations of Z. muelleri in south-eastern Australia are sensitive to small chronic temperature increases and light decreases that are predicted under future climate change scenarios