Characterization of age-dependent and progressive cortical neuronal degeneration in presenilin conditional mutant mice.

Mark R Cookson,Mary Wines-Samuelson,Eva C Schulte,Chiye Aoki,Miriam J Smith,Raymond J Kelleher,Xinran Liu,Jie Shen

doi:10.1371/journal.pone.0010195

Abstract

Presenilins are the major causative genes of familial Alzheimer's disease (AD). Our previous study has demonstrated essential roles of presenilins in memory and neuronal survival. Here, we explore further how loss of presenilins results in age-related, progressive neurodegeneration in the adult cerebral cortex, where the pathogenesis of AD occurs. To circumvent the requirement of presenilins for embryonic development, we used presenilin conditional double knockout (Psen cDKO) mice, in which presenilin inactivation is restricted temporally and spatially to excitatory neurons of the postnatal forebrain beginning at 4 weeks of age. Increases in the number of degenerating (Fluoro-Jade B+, 7.6-fold) and apoptotic (TUNEL+, 7.4-fold) neurons, which represent ∼0.1% of all cortical neurons, were first detected at 2 months of age when there is still no significant loss of cortical neurons and volume in Psen cDKO mice. By 4 months of age, significant loss of cortical neurons (∼9%) and gliosis was found in Psen cDKO mice. The apoptotic cell death is associated with caspase activation, as shown by increased numbers of cells immunoreactive for active caspases 9 and 3 in the Psen cDKO cortex. The vulnerability of cortical neurons to loss of presenilins is region-specific with cortical neurons in the lateral cortex most susceptible. Compared to the neocortex, the increase in apoptotic cell death and the extent of neurodegeneration are less dramatic in the Psen cDKO hippocampus, possibly in part due to increased neurogenesis in the aging dentate gyrus. Neurodegeneration is also accompanied with mitochondrial defects, as indicated by reduced mitochondrial density and altered mitochondrial size distribution in aging Psen cortical neurons. Together, our findings show that loss of presenilins in cortical neurons causes apoptotic cell death occurring in a very small percentage of neurons, which accumulates over time and leads to substantial loss of cortical neurons in the aging brain. The low occurrence and significant delay of apoptosis among cortical neurons lacking presenilins suggest that loss of presenilins may induce apoptotic neuronal death through disruption of cellular homeostasis rather than direct activation of apoptosis pathways.

Highlights

Presenilins (Psen 1 and 2) are the major causative genes of earlyonset familial Alzheimer’s disease (FAD) and harbor,90% of the identified FAD-linked mutations
Loss of presenilins causes a delayed onset of apoptotic cell death in the cerebral cortex In Psen cDKO mice, Presenilin inactivation is dependent on expression of Cre recombinase under the control of the aCaMKII
Our in situ hybridization and western analyses have shown that expression of Cre mRNAs and loss of Psen1 mRNAs and proteins begins at 3 weeks of age in the cerebral cortex ([14]; MWS and JS, unpublished data)

Summary

Introduction

Presenilins (Psen 1 and 2) are the major causative genes of earlyonset familial Alzheimer’s disease (FAD) and harbor ,90% of the identified FAD-linked mutations. To circumvent the requirement of presenilins in development, we previously generated a presenilin conditional double knockout (Psen cDKO) mouse, in which presenilin inactivation is restricted spatially and temporally to excitatory neurons of the postnatal forebrain using the Cre/loxP technology [9,10]. Psen cDKO mice permit assessment of direct consequences of presenilin inactivation in excitatory pyramidal neurons of the adult cerebral cortex, where presenilins are normally expressed highly and AD pathogenesis occurs. Analysis of these mutant mice demonstrated that loss of presenilins in mature neurons of the cerebral cortex results in progressive impairment in synaptic plasticity and learning and memory, followed by age-dependent neurodegeneration [10,11]

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Conclusion