Influence of the constitutive, conditional and inducible astroglia-derived tenascin-C ablation on synaptic function
Final Report Abstract
In order to exclude developmental mechanisms underlying tenascin-C deficiency from those present only in the adult and in particular in GFAP-positive cells, we had proposed to generate conditionally tenascin-C deficient mice, in which the gene was ablated specifically in astrocytes. For this, we bred the mice carrying the flox sites in the tenascin-C gene with animals that express the cre recombinase under the control of the mouse GFAP promoter. First attempts failed, because in these mice the cre recombinase was also active in neurons, a phenomenon that could have compromised our results, because tenascin-C is expressed in a subclass of inhibitory interneurons in the spinal cord and cerebellum during early stages of development. Also, the mice generated by crossing the floxed tenascin-C mice with animals that allow the progesterone (tamoxifen)-specific inducible expression of cre recombinase under the universal prion protein promoter, showed that in these mice injection of tamoxifen intraperitoneally, intravenously or directly into the brain itself, did not completely ablate tenascin-C expression neither at the in situ hybridization nor at the immunoblotting levels. The mice were thus not usable for inducible deletion of tenascin-C expression in the adult. We thus started to breed the floxed tenascin-C mice with mice, kindly given to us on a collaborative basis by Frank Kirchhoff, then at the Max-Planck-Institute for Experimental Medicine, Göttingen, which express, under the human GFAP promoter, the tamoxifen inducible cre recombinase (GFAP-CreERT2 mouse). According to the experience of Frank Kirchhoff, there is specific expression of the cre recombinase only in astrocytes of adult mice. Unfortunately, breeding attempts to generate three lines of conditionally deficient in tenascin-C failed, first because we lost the transgene in the cre recombinase mice - and this took us several months to find out - and then because of problems with the genotyping that suddenly occurred and we could not correct. We therefore sent the tenascin-C flox mice to Frank Kirchhoff, but he then had no capacity to generate the desired mice for the planned experiments. In order to be productive, we decided to extend our investigations on the tenascin-C constitutively deficient mice and were successful in finding that the olfactory system of these mice was defective. Delayed onset of odor detection in neonatal mice lacking tenascin-C. This finding was not unexpected, since these mice are deficient in neural stem cell generation. Another study was initiated to find out - in complementation to a conditionally acute ablation approach - whether tenascin-C was involved in learning and memory by injecting antibodies into the chicken brain. The results showed that tenascin-C is important in a species other than the mouse for learning and memory. Regional and cellular distribution of the extracellular matrix protein tenascin-C in the chick forebrain and its role in neonatal learning. We also found that the tenascin-C constitutively deficient mice were impaired in several parameters important for the mouse organism. Enhanced novelty-induced activity, reduced anxiety, delayed resynchronization to daylight reversal and weaker muscle strength in tenascin-C-deficient mice. Genetic ablation of tenascin-C expression leads to abnormal hippocampal CA1 structure and electrical activity in vivo. Our previous studies could be extended to show that tenascin-C is important for hippocampal function. As we had hoped for a function of tenascin-C, namely the demarcation of functional domains in boundaries, tenascin-C was found to be instrumental in containment of functional units in the olfactory bulb, a structure known for its exemplary compartmentalization. Tenascin-C is an inhibitory boundary molecule in the developing olfactory bulb. In search for the functions of binding partners for tenascin-C and on the basis of the knowledge that tenascin-C is important in the physiological competence of neurons, we found in collaboration with our colleagues in the United States that hyaluronic acid, for which binding sites in tenascin-C had been found, affects a particular type of calcium channel The extracellular matrix molecule hyaluronic acid regulates hippocampal synaptic plasticity by modulating postsynaptic L-type Ca(2+) channels. Compartmentalization from the outside: the extracellular matrix and functional microdomains in the brain. We also found that tenascin-C is important for spinal cord regeneration, emphasizing the point that a molecule is not only repellent in a boundary function as in the olfactory bulb or the whisker representation in the barrel field of the cortex, but can be also conducive in an acute neurological problem, namely trauma in the spinal cord of a mammal. The extracellular matrix glycoprotein tenascin-C is beneficial for spinal cord regeneration. These results were complemented by the finding that in fish, which are able to spontaneously regenerate in the adult, tenascin-C is needed for spinal cord regeneration. The extracellular matrix glycoprotein tenascin-C promotes locomotor recovery after spinal cord injury in adult zebrafish. The regenerative capacity of tenascin-C was hypothesized to be due to its ability to close the blood-brain barrier after spinal cord injury, a view which was supported by the finding that in tenascin-C deficient mice the blood-spinal cord barrier was not closed as efficiently as in wild type mice. Adhesion molecules close homolog of L1 and tenascin-C affect blood-spinal cord barrier repair. Interestingly, tenascin-C affects the immune system of the brain in a mouse model of Alzheimer's disease, which, similar to many neurodegenerative diseases, displays an immune system component. This immune system component of tenascin-C was summarized in a review. Finally, in collaboration with the Department of Anatomy at the University Hospital Hamburg-Eppendorf we found that also in a model of spinal cord regeneration involving hemisection (and not complete transection as described above) tenascin-C was found to be adverse to spinal cord regeneration. Extracellular matrix alterations, accelerated leukocyte infiltration and enhanced axonal sprouting after spinal cord hemisection in tenascin-C-deficient mice. This finding underscored the view that tenascin-C as a boundary molecule is inhibitory for neurons and subcompartments of neurons, such as axons, but is conducive for regeneration in the situation, where boundaries are not present.
