A novel GFPneo vector designed for the isolation and analysis of enhancer elements in transfected mammalian cells

Enhancing structural plasticity of PC12 neurons during differentiation and neurite regeneration with a catalytically inactive mutant version of the zRICH protein

BACKGROUND

Studies of the molecular mechanisms of nerve regeneration have led to the discovery of several proteins that are induced during successful nerve regeneration. RICH proteins were identified as proteins induced during the regeneration of the optic nerve of teleost fish. These proteins are 2',3'-cyclic nucleotide, 3'-phosphodiesterases that can bind to cellular membranes through a carboxy-terminal membrane localization domain. They interact with the tubulin cytoskeleton and are able to enhance neuronal structural plasticity by promoting the formation of neurite branches.

RESULTS

PC12 stable transfectant cells expressing a fusion protein combining a red fluorescent protein with a catalytically inactive mutant version of zebrafish RICH protein were generated. These cells were used as a model to analyze effects of the protein on neuritogenesis. Differentiation experiments showed a 2.9 fold increase in formation of secondary neurites and a 2.4 fold increase in branching points. A 2.2 fold increase in formation of secondary neurites was observed in neurite regeneration assays.

CONCLUSIONS

The use of a fluorescent fusion protein facilitated detection of expression levels. Two computer-assisted morphometric analysis methods indicated that the catalytically inactive RICH protein induced the formation of branching points and secondary neurites both during differentiation and neurite regeneration. A procedure based on analysis of random field images provided comparable results to classic neurite tracing methods.

Ubiquitin ligase Cbl-b is a negative regulator for insulin-like growth factor 1 signaling during muscle atrophy caused by unloading. Mol Cell Biol. 2009;29:4798-4811. [PMID: 19546233 DOI: 10.1128/mcb.01347-08]

Skeletal muscle atrophy caused by unloading is characterized by both decreased responsiveness to myogenic growth factors (e.g., insulin-like growth factor 1 [IGF-1] and insulin) and increased proteolysis. Here, we show that unloading stress resulted in skeletal muscle atrophy through the induction and activation of the ubiquitin ligase Cbl-b. Upon induction, Cbl-b interacted with and degraded the IGF-1 signaling intermediate IRS-1. In turn, the loss of IRS-1 activated the FOXO3-dependent induction of atrogin-1/MAFbx, a dominant mediator of proteolysis in atrophic muscle. Cbl-b-deficient mice were resistant to unloading-induced atrophy and the loss of muscle function. Furthermore, a pentapeptide mimetic of tyrosine(608)-phosphorylated IRS-1 inhibited Cbl-b-mediated IRS-1 ubiquitination and strongly decreased the Cbl-b-mediated induction of atrogin-1/MAFbx. Our results indicate that the Cbl-b-dependent destruction of IRS-1 is a critical dual mediator of both increased protein degradation and reduced protein synthesis observed in unloading-induced muscle atrophy. The inhibition of Cbl-b-mediated ubiquitination may be a new therapeutic strategy for unloading-mediated muscle atrophy.

An enhancer directs differential expression of the linked Mrf4 and Myf5 myogenic regulatory genes in the mouse

The myogenic regulatory factors, Mrf4 and Myf5, play a key role in skeletal muscle formation. An enhancer trap approach, devised to isolate positive-acting elements from a 200-kb YAC covering the mouse Mrf4-Myf5 locus in a C2 myoblast assay, yielded an enhancer, A17, which mapped at -8 kb 5' of Mrf4 and -17 kb 5' of Myf5. An E-box bound by complexes containing the USF transcription factor is critical for enhancer activity. In transgenic mice, A17 gave two distinct and mutually exclusive expression profiles before birth, which correspond to two phases of Mrf4 transcription. Linked to the Tk or Mrf4 minimal promoters, the nlacZ reporter was expressed either in embryonic myotomes, or later in fetal muscle, with the majority of Mrf4 lines showing embryonic expression. When linked to the Myf5 minimal promoter, only fetal muscle expression was detected. These observations identify A17 as a sequence that targets sites of myogenesis in vivo and raise questions about the mutually exclusive modes of expression and possible promoter/enhancer interactions at the Mrf4-Myf5 locus.

The transcriptional activator PAX3-FKHR rescues the defects of Pax3 mutant mice but induces a myogenic gain-of-function phenotype with ligand-independent activation of Met signaling in vivo

Pax3 is a key transcription factor implicated in development and human disease. To dissect the role of Pax3 in myogenesis and establish whether it is a repressor or activator, we generated loss- and gain-of-function alleles by targeting an nLacZ reporter and a sequence encoding the oncogenic fusion protein PAX3-FKHR into the Pax3 locus. Rescue of the Pax3 mutant phenotypes by PAX3-FKHR suggests that Pax3 acts as a transcriptional activator during embryogenesis. This is confirmed by a Pax reporter mouse. However, mice expressing PAX3-FKHR display developmental defects, including ectopic delamination and inappropriate migration of muscle precursor cells. These events result from overexpression of c-met, leading to constitutive activation of Met signaling, despite the absence of the ligand SF/HGF. Haploinsufficiency of c-met rescues this phenotype, confirming the direct genetic link with Pax3. The gain-of-function phenotype is also characterized by overactivation of MyoD. The consequences of PAX3-FKHR myogenic activity in the limbs and cervical and thoracic regions point to differential regulation of muscle growth and patterning. This gain-of-function allele provides a new approach to the molecular and cellular analysis of the role of Pax3 and of its target genes in vivo.

Green fluorescent protein as a second selectable marker for selection of high producing clones from transfected CHO cells

Mammalian cells are often used for the expression of recombinant proteins. The process of screening transfected cells randomly for high producing clones is tedious and time consuming. We evaluated using green fluorescent protein (GFP) for selection of high producing clones by fluorescence-activated cell sorter (FACS) to reduce screening effort. We expressed neurotrophin-3 (NT3), deoxyribonuclease (DNase), or vascular endothelial growth factor (VEGF) with GFP in Chinese hamster ovary cells. The vector expressed the desired secreted protein and the selectable marker, dihydrofolate reductase, in one expression unit and the intracellular GFP in a second expression unit. Transfected cells were grown in selection medium and sorted by FACS. High fluorescence clones were obtained and found to produce high amounts of the desired protein; VEGF productivity correlated well with GFP fluorescence in 48 clones. Further studies demonstrated that productivity correlated very well with RNA of the desired protein. For comparison, we randomly picked and screened 144 VEGF clones, and the highest producing VEGF clone obtained produced 0.7 pg/cell/day. In contrast, the highest producing VEGF clone obtained by FACS sorting produced 4.4 pg/cell/day. FACS sorting therefore selected high producing clones efficiently. Since an assay for the desired protein is not required, high producing clones for a protein of unknown function can be obtained by FACS sorting followed by measuring the RNA level of the desired protein in the highly fluorescent clones.