Neuronal degeneration, synaptic defects, and behavioral abnormalities in tau45-230 transgenic mice
Introduction
The microtubule-associated protein (MAP) tau plays an important role during neuronal development by stabilizing the microtubule network in growing axons (Drubin and Kirschner, 1986, Ferreira et al., 1989, Dreschel et al., 1992, Bramblett et al., 1993). Therefore, conditions that altered the levels of expression of this MAP during development led to abnormal axonal elongation (Caceres and Kosik, 1990, Knops et al., 1991, Dawson et al., 2001). Tau has also been implicated in axonal degeneration and cell death in the context of Alzheimer’s disease (AD) and related disorders known as tauopathies (Kosik et al., 1986, Wood et al., 1986, Kondo et al., 1988, Rapoport et al., 2002, Yancopoulou and Spillantini, 2003, Parihar and Hemnani, 2004, Roberson et al., 2007). The mechanisms by which tau mediates neuronal degeneration are not completely understood; however, a growing body of evidence indicates that abnormal posttranslational modifications of this MAP underlie tau pathology. Numerous studies have focused on the role of tau phosphorylation in AD because neurofibrillary tangles, pathological hallmarks of this disease, are formed mainly by hyperphosphorylated tau isoforms (Kosik et al., 1986, Wood et al., 1986, Kondo et al., 1988, Takashima et al., 1993, Ferreira et al., 1997, Alvarez et al., 1999, Ekinci et al., 1999, Parihar and Hemnani, 2004). More recently, it has been suggested that cleavage could also underlie tau toxicity. Thus, we have shown that calpain-mediated tau cleavage is a conserved mechanism in multiple tauopathies (Ferreira and Bigio, 2011). This cleavage induces the generation of the 17 kDa tau45-230 fragment (Park and Ferreira, 2005; Park et al., 2007, Reinecke et al., 2011). The toxic effects of this tau fragment were first detected in hippocampal neurons transfected with a tau45-230-green fluorescent protein (GFP) construct (Park and Ferreira, 2005). Similar effects were demonstrated in a Drosophila model of tauopathy (Reinecke et al., 2011).
Conflicting results regarding the identity and toxicity of this tau fragment have been recently published (Garg et al., 2011). Although these authors described the formation of a tau fragment of similar apparent molecular weight, it contained a different N-terminus as a result of the cleavage by a different calpain isoform. This fragment failed to induce neurodegeneration in cultured neurons (Garg et al., 2011).
To address this discrepancy and obtain insights into the toxic effects of the 17-kDa tau45-230 fragment in mammalian central neurons that develop in situ, we generated and characterized transgenic mice that express this fragment in hippocampal neurons. Our results showed enhanced cell death of pyramidal neurons and synaptic loss in the hippocampus of transgenic tau45-230 mice. In addition, these changes were accompanied by behavioral abnormalities. Collectively, these data indicate that indeed tau45-230 has toxic effects that could contribute to the progressive degeneration of central neurons in AD and related disorders.
Section snippets
Generation of tau45-230-GFP transgenic mice
Transgenic mice were generated by injecting the pronucleus of a single-cell fertilized C57BL/6J mouse embryo with the tau45-230-GFP transgene under the control of the Thy 1.2 promoter. This transgene was derived from a peGFP-N1 plasmid (Invitrogen, Grand Island, NY, USA) containing the human cDNA coding sequence for the tau45-230 fragment cloned into the multiple cloning site as previously described (Park and Ferreira, 2005). The 7.8-kb transgene containing all regulatory elements and the
Generation of transgenic tau45-230 mice
We have previously shown that the expression of the 17-kDa tau45-230 fragment in otherwise healthy cultured hippocampal neurons led to degeneration and cell death (Park and Ferreira, 2005). Similar results were obtained in a tauopathy model system in Drosophila (Reinecke et al., 2011). To test whether the expression of this fragment could also induce neurodegeneration in vertebrate hippocampal neurons that develop in situ, we generated strains of transgenic mice carrying tau45-230-GFP cDNA
Discussion
This report provides the first direct evidence of a tau fragment (tau45-230) generated in the context of AD and related disorders capable of inducing neuronal death and synaptic loss when expressed in mouse hippocampal neurons that develop in situ. In addition, the data presented herein indicate that these hippocampal changes are accompanied by functional abnormalities that could have some bearings on the cognitive and/or behavioral deficits associated with these diseases.
Previous studies have
Acknowledgments
This work was supported by NIH grant NS39080 and Northwestern University start-up funds to AF. The genetically engineered mice were generated with the assistance of Northwestern University Transgenic and Targeted Mutagenesis Laboratory. The Northwestern University Behavioral Phenotyping Core conducted the behavioral tests included in this study. The authors are grateful to Lindsey Wold for her participation in the initial stages of the establishment of the mouse colonies. The authors declare no
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