Prominent Axonopathy and Disruption of Axonal Transport in Transgenic Mice Expressing Human Apolipoprotein E4 in Neurons of Brain and Spinal Cord

The epsilon 4 allele of the human apolipoprotein E gene (ApoE4) constitutes an important genetic risk factor for Alzheimer's disease. Recent experimental evidence suggests that human ApoE is expressed in neurons, in addition to being synthesized in glial cells. Moreover, brain regions in which neurons express ApoE seem to be most vulnerable to neurofibrillary pathology. The hypothesis that the expression pattern of human ApoE might be important for the pathogenesis of Alzheimer's disease was tested by generating transgenic mice that express human ApoE4 in neurons or in astrocytes of the central nervous system. Transgenic mice expressing human ApoE4 in neurons developed axonal degeneration and gliosis in brain and in spinal cord, resulting in reduced sensorimotor capacities. In these mice, axonal dilatations with accumulation of synaptophysin, neurofilaments, mitochondria, and vesicles were documented, suggesting impairment of axonal transport. In contrast, transgenic mice expressing human ApoE4 in astrocytes remained normal throughout life. These results suggest that expression of human ApoE in neurons of the central nervous system could contribute to impaired axonal transport and axonal degeneration. The possible contribution of hyperphosphorylation of protein Tau to the resulting phenotype is discussed.