Rat tyrosine hydroxylase promoter directs tetracycline-inducible foreign gene expression in dopaminergic cell types

An Efficient and Versatile System for Visualization and Genetic Modification of Dopaminergic Neurons in Transgenic Mice

Background & Aims

The brain dopaminergic (DA) system is involved in fine tuning many behaviors and several human diseases are associated with pathological alterations of the DA system such as Parkinson’s disease (PD) and drug addiction. Because of its complex network integration, detailed analyses of physiological and pathophysiological conditions are only possible in a whole organism with a sophisticated tool box for visualization and functional modification.

Methods & Results

Here, we have generated transgenic mice expressing the tetracycline-regulated transactivator (tTA) or the reverse tetracycline-regulated transactivator (rtTA) under control of the tyrosine hydroxylase (TH) promoter, TH-tTA (tet-OFF) and TH-rtTA (tet-ON) mice, to visualize and genetically modify DA neurons. We show their tight regulation and efficient use to overexpress proteins under the control of tet-responsive elements or to delete genes of interest with tet-responsive Cre. In combination with mice encoding tet-responsive luciferase, we visualized the DA system in living mice progressively over time.

Conclusion

These experiments establish TH-tTA and TH-rtTA mice as a powerful tool to generate and monitor mouse models for DA system diseases.

Synergy between glutathione peroxidase-1 and astrocytic growth factors suppresses free radical generation and protects dopaminergic neurons against 6-hydroxydopamine

The degeneration of dopaminergic neurons in the course of Parkinson disease is largely blamed on oxidative damage in the brain. This study examined the potency of glutathione peroxidase-1 (GPX-1) to protect dopaminergic neurons against toxicity induced by the parkinsonian neurotoxin 6-hydroxydopamine (6-OHDA). We generated pLV-GPX1, a recombinant lentivirus vector carrying the coding sequence for human GPX-1, into the SK-N-MC neuroblastoma cell line. The pLV-GPX1-infected neurons showed an over 3-fold increase in enzyme expression and a 2.6-fold increase in enzyme activity compared to the pLV-EGFP-infected control cells. In the pLV-GPX1-infected cells, we also detected significantly increased neuronal survival and resistance to 6-OHDA-mediated toxicity compared to our controls (75 ± 4% versus 51 ± 7%, p < 0.001). To maximize this protection, the neurons were treated with conditioned medium taken from growing primary astrocytes (astro-CM). We found the treated pLV-GPX1-infected neurons even more significantly resistant to 6-OHDA toxicity compared to their untreated counterparts (86 ± 5% versus 75 ± 4%, p < 0.001). Concomitant with increased neuroprotection, co-presence of overexpressed GPX-1 and astro-CM significantly increased glutathione (GSH) levels compared to when either of the two was present (p < 0.001). Further analysis showed nearly 2.7-fold reduction, in the presence of astro-CM, of hydrogen peroxide (H(2)O(2)) levels released from the pLV-GPX1-infected neurons compared to control groups (p < 0.001). Finally, regression analysis between H(2)O(2) levels and cell viability showed that co-presence of GPX-1 and astro-CM reduced 33% of cell death rate (p < 0.05). These data highlight the antioxidant properties of GPX-1 in protecting dopaminergic neurons and further emphasize the capacity of astrocytes in pumping growth-inducing factors that may synergize with GPX-1 to accelerate neuroprotection.

Neuron-specific conditional expression of a mitochondrially targeted fluorescent protein in mice

Mitochondrial dysfunction contributes to the pathophysiology of both acute and chronic neurodegenerative disorders. Quantification of mitochondrial bioenergetic properties generally requires the use of isolated brain mitochondria. However, the involvement of neuronal mitochondrial dysfunction in these disorders is limited by the lack of markers, and therefore isolation procedures, that distinguish neuronal compared with astrocyte mitochondria. To address this and other issues concerning neuronal mitochondria in the CNS, transgenic mice were generated that express a fluorescent protein targeted specifically to neurons. A neuron-specific promoter, CaMKIIalpha (calcium/calmodulin-dependent kinase IIalpha) driven tTA (tetracycline transactivator) mice were crossed with TRE (tetracycline responsive element) driven mitochondrial targeted enhanced yellow fluorescent protein (eYFP) mice. Expression of eYFP in the bigenic mouse brain was observed only in neuronal mitochondria of striatum, forebrain, and hippocampus and was enhanced by the removal of the tetracycline analog doxycycline (Dox) in the diet. The respiratory control ratio of synaptic and nonsynaptic mitochondria isolated from eYFP-expressing mice was the same as control mice, suggesting that neuronal mitochondria expressing eYFP maintain normal bioenergetic functions. More importantly, the development of Dox-inducible, neuron targeted mito/eYFP transgenic mice offer a unique in vivo model for delineating the participation of neuronal mitochondria in neuronal survival and death.

Efficient control of gene expression in the hematopoietic system using a single Tet-on inducible lentiviral vector

This work addresses the problem of efficient control of gene expression in the context of viral vectors, which still represents a difficult challenge. A number of lentiviral vectors incorporating the different elements of regulatable transcriptional systems have been described, but they fail to perform satisfactorily either because of a poor dynamic range of transcription levels or because they display high background activities in the uninduced state and mediocre inducer response. We report here on the systematic comparison of vector designs containing the elements of the doxycycline-inducible Tet-on system in their most advanced versions (rtTA2S-M2 transactivator and tTS(Kid) repressor). We show that a simple "all-in-one" vector can be obtained and used for efficient control of transgene expression in long-term tissue culture and in the hematopoietic system of mice following bone marrow transplantation. Using this vector, the uninduced state can be kept at background levels and induction factors of 100-fold are repeatedly obtained over months both in tissue culture and in vivo. Interestingly, the low background activity of the all-in-one vector renders the use of the tTS repressor dispensable, avoiding the problem of progressive loss of inducibility over time associated with irreversible modifications of the chromatin surrounding proviral sequences.