Of note, the broader transgene expression pattern of SLICK-H transgenic mice compared to other SLICK lines facilitates widespread inducible neuron-specific conditional genetic manipulation, but renders SLICK-H transgenic mice unsuitable for single neuron imaging purposes.
The "single-neuron labeling inducible Cre-mediated knockout" (SLICK) transgene is designed with two separate copies of the modified mouse Thy1 promoter region (each containing the sequences required for neuronal expression but lacking the sequences required for expression in non-neural cells) driving expression of a CreERT2 fusion protein and an enhanced yellow fluorescent protein (EYFP). Mice harboring the SLICK transgene are viable, fertile, normal in size and do not display any gross physical or behavioral abnormalities. mRNA coexpression of both CreERT2 and YFP is highly correlated. YFP expression is directed primarily to projection neurons with variations in the extent and brightness of labeling observed in different founder lines (see below). CreERT2 fusion gene activity is inducible; observed at high levels following tamoxifen administration (see below). When SLICK transgenic mice are bred with mice containing loxP-flanked sequences, tamoxifen-inducible Cre-mediated recombination will result in deletion of the floxed sequences in the Thy1-expressing cells of the double mutant offspring.
SLICK transgenic mice derived from founder line H (SLICK-H) exhibit both widespread and strong expression of YFP in projection neurons throughout the peripheral and central nervous system; and are thus not suitable for single neuron imaging purposes. Transgene expression is restricted to neurons; no YFP fluorescence or tamoxifen-induced Cre recombinase activity is observed in astrocytes, oligodendrocytes, or non-neural populations. As designed, transgene expression is limited during early developmental stages due to the late onset/activity of the Thy1 promoter used in these mice. Transgene expression increases postnatally, and robust YFP expression/highly-efficient induction of recombination in most projection neuron populations is possible in adult SLICK-H mice from six weeks of age and older. Specifically, SLICK-H transgenic mice have extensive/strong YFP labeling of cell somas in the cortex (visual, parietal, somatosensory, motor, insular, piriform and entorhinal areas), the major hippocampal fields (except CA2), the amygdala (basal nucleus), the inferior colliculus, the superior colliculus, the thalamic nuclei, the brainstem nuclei, the anterior olfactory nucleus, the cerebellum (granule and Purkinje cells), and olfactory bulb (mitral cells). Outside the brain, extensive/strong YFP labeling is found in the spinal cord (sensory neurons and motor neurons), the dorsal root ganglion neurons, and the retina (photoreceptors and retinal ganglion cells). YFP expression can be detected by direct YFP fluorescence as well as immunohistochemical methods. In addition, SLICK-H transgenic mice have high tamoxifen-induced Cre recombinase efficiency/activity in projection neurons of the cortex, hippocampus, amygdala, brainstem, thalamus, inferior colliculus, superior colliculus, spinal cord, dorsal root ganglia, and retina following tamoxifen administration. CreERT2 activation in the absence of tamoxifen is minimal (less than 1% of the levels observed after tamoxifen induction). Therefore, these SLICK-H transgenic mice allow fluorescent labeling in the majority of projection neurons populations in the central and peripheral nervous system, with inducible Cre-lox genetic manipulation in the same widespread population of labeled neurons.
Other SLICK transgenic lines have sparse labeling of single neurons. The SLICK-A mice (Stock No. 007606) express the transgene in small subsets of motor neurons, dorsal root ganglion neurons, some hippocampal regions and cortical layer V; but fluorescence is less uniform with leaky and less-efficient Cre recombinase activity. The SLICK-V mice (Stock No. 007610) express the transgene sparsely in neurons of the central nervous system. Therefore, the broader transgene expression pattern of SLICK-H transgenic mice facilitates widespread inducible neuron-specific conditional genetic manipulation, but renders SLICK-H transgenic mice unsuitable for single neuron imaging purposes.
The Cre-ERT2 fusion protein consists of Cre recombinase fused to a triple mutant form of the human estrogen receptor which does not bind its natural ligand (17β-estradiol) at physiological concentrations but will bind the synthetic estrogen receptor ligands 4-hydroxytamoxifen (OHT or tamoxifen) and, with lesser sensitivity, ICI 182780. Restricted to the cytoplasm, Cre-ERT2 can only gain access to the nuclear compartment after exposure to tamoxifen. To counteract the mixed estrogen agonist effects of tamoxifen injections, which can result in late fetal abortions in pregnant mice, progesterone may be coadministered.
This strain is one of many from the same transgene creator/donating investigator with fluorescent protein expression/neurobiological research applications, including:
SLICK line A (Stock No. 007606),
SLICK line V (Stock No. 007610),
SLICK line H (Stock No. 012708),
GAD67-GFP line 3 (Stock No. 007673),
Thy1-GFP line O (Stock No. 007919),
Thy1-YFP line G (line 17) (Stock No. 014130),
Thy1-CFP line I (Stock No. 014131),
Thy1-ChR2-YFP line 18 (Stock No. 007612),
Thy1-ChR2-YFP line 9 (Stock No. 007615),
Thy1-eNpHR-YFP line 2 (Stock No. 012332),
Thy1-eNpHR-YFP line 4 (Stock No. 012334),
Thy1-vChR1-YFP line 1 (Stock No. 012341),
Thy1-vChR1-YFP line 4 (Stock No. 012344),
Thy1-vChR1-YFP line 8 (Stock No. 012348),
Thy1-mhChR2-YFP line 20 (Stock No. 012350),
Prv-mhChR2-YFP line 15 (Stock No. 012355),
ChAT-ChR2-YFP line 5 (Stock No. 014545),
ChAT-ChR2-YFP line 6 (Stock No. 014546),
VGAT-ChR2-YFP line 8 (Stock No. 014548),
TpH2-ChR2-YFP line 5 (Stock No. 014555),
and R26-CAG-LSL-2XChETA-tdTomato reporter mice (Stock No. 017455).
