Plasma membrane ganglioside sialidase regulates axonal growth and regeneration in hippocampal neurons in culture.

It has been long recognized that the ganglioside GM1 plays a role in axonal growth and neuronal differentiation. However, the involvement of plasma membrane GM1 has been difficult to elucidate. This is possible now thanks to the recent cloning of plasma membrane ganglioside sialidase (PMGS), the enzyme responsible for the localized hydrolysis of oligosialogangliosides into GM1. In this work we show that PMGS mRNA and protein levels are high at early developmental stages of the hippocampus and low in adulthood both in vivo and in vitro. We also demonstrate that inhibition of PMGS activity blocks axonal elongation, whereas the increase in PMGS activity dramatically enhances axon growth and accelerates the polarization of cytoskeletal proteins. Finally, we show that axotomy close to the cell body in PMGS overexpressing neurons results in the regrowth of the original axon instead of randomly, as is the case in control neurons. In all, these results imply that PMGS activity through the modulation of GM1 surface levels is an important component of the machinery controlling axonal growth. We hypothesize that increasing PMGS activity in the adult nervous system may be useful to improve regeneration after nerve damage.

The Ras-like GTPase Gem is involved in cell shape remodelling and interacts with the novel kinesin-like protein KIF9.

Gem belongs to the Rad/Gem/Kir (RGK) subfamily of Ras-related GTPases, which also comprises Rem, Rem2 and Ges. The RGK family members Ges and Rem have been shown to produce endothelial cell sprouting and reorganization of the actin cytoskeleton upon overexpression. Here we show that high intracellular Gem levels promote profound changes in cell morphology and we investigate how this phenotype arises dynamically. We also show that this effect requires intact microtubules and microfilaments, and that Gem is associated with both cytoskeletal components. In order to investigate the mechanisms of Gem recruitment to the cytoskeleton, we performed a yeast two-hybrid screen and identified a novel kinesin-like protein, termed KIF9, as a new Gem interacting partner. We further show that Gem and KIF9 interact by co-immunoprecipitation. Furthermore, Gem and KIF9 display identical patterns of gene expression in different tissues and developmental stages. The Gem- KIF9 interaction reported here is the first molecular link between RGK family members and the microtubule cytoskeleton.

ENO1 gene product binds to the c-myc promoter and acts as a transcriptional repressor: relationship with Myc promoter-binding protein 1 (MBP-1).

The Myc promoter-binding protein-1 (MBP-1) is a 37-38 kDa protein that binds to the c-myc P2 promoter and negatively regulates transcription of the protooncogene. MBP-1 cDNA shares 97% similarity with the cDNA encoding the glycolytic enzyme alpha-enolase and both genes have been mapped to the same region of human chromosome 1, suggesting the hypothesis that the two proteins might be encoded by the same gene. We show here data indicating that a 37 kDa protein is alternatively translated from the full-length alpha-enolase mRNA. This shorter form of alpha-enolase is able to bind the MBP-1 consensus sequence and to downregulate expression of a luciferase reporter gene under the control of the c-myc P2 promoter. Furthermore, using alpha-enolase/green fluorescent protein chimeras in transfection experiments we show that, while the 48 kDa alpha-enolase mainly has a cytoplasmic localization, the 37 kDa alpha-enolase is preferentially localized in the cell nuclei. The finding that a transcriptional repressor of the c-myc oncogene is an alternatively translated product of the ENO1 gene, which maps to a region of human chromosome 1 frequently deleted in human cancers, makes ENO1 a potential candidate for tumor suppressor.

Chimeric amplicons containing the c-myc gene in HL60 cells.

The major amplicon present in HL60 cells is chimeric in nature being composed of 70 kb of DNA sequence derived from the MYC locus linked to 80 kb of novel DNA sequence derived from a non contiguous region located telomeric to the c-myc gene at 8q24 (Feo et al., 1996). Here we show by fluorescence in situ hybridization (FISH) that these coamplified sequences, MCR (Myc Coamplified Region), are derived from a locus located 3-4 Mb telomeric to the c-myc gene in the q24.2-24.3 region of chromosome 8. Genomic cloning and Southern blot analysis indicate the arrangement of chimeric amplicons are in tandem arrays. Analysis of the DNA sequences at the juncture of the MYC locus and the MCR suggest that these non syntenic regions were joined by nonhomologous recombination events. Visualization of the organization of the amplified DNA by fiber-FISH analysis illustrates we have cloned the complete amplicon. This is the first complete mammalian amplicon to be cloned and have its structure visualized. In addition to the major class of tandemly repeated amplicons, a second class of amplicons was detected by fiber-FISH in which the extent of the MCR component is about twice the size of the MCR component in the major amplicon. These longer amplicons most likely contain inverted repeats of MCR and MYC region sequences. Whether the amplicons contain mixtures of these two types of structures or separate amplicons only contain one type of structure has not yet been resolved. Properties of the MCR sequences responsible for retention in the chimeric HL60 amplicons upon long term passage are discussed.