The Principal Inferior Olivary Nucleus in Aging and Alzheimer's Disease

Neuronal degeneration is a commanding event in Alzheimer’s disease (AD). Neuronal loss is one of the major hallmarks and is frequently reported in regions with presence of neuritic plaques (NPs) and neurofibrillary tangles (NFTs). The causative role of β-amyloid (Aβ) and tau protein in neurofibrillary pathology is controversial, and the main trigger for neurodegeneration in AD is still unknown. Thus we investigated neuronal and glial changes in the principal inferior olivary nucleus (PO) in normal aging and AD. This region is known not to develop NPs or NFTs in sporadic AD, although a few diffuse Aβ accumulations can be found. To obtain a precise number of cells an unbiased quantitative methodology should be applied. In the first study using a stereological method, optical fractionator, we estimated the total number of neurons in PO, where we reported a significant neuronal loss of up to 34% in AD group compared to age-matched controls. It is well established that neurons are in very close contact and cohabitation with glial cells. Whether neurons in PO die solely or other cells are involved was not known. In the second study we expanded a quantitative analysis and estimated a total number of oligodendrocytes and astrocytes in PO in AD and control brains. The total number of astrocytes remained unchanged, but oligodendroglial cell number was diminished by 46% indicating interdependent degeneration among neurons and oligodendrocytes. We could conclude that neurodegeneration takes place in AD brains, even in absence of neurofibrillary pathology. The patomechanisms behind the neuronal loss are still unknown. In the third study we applied immunohistochemistry with unbiased quantification and ELISA method to analyse astroglial markers (S100B, GFAP and vimentin) known to be involved in neurodegeneration. A 2-fold significant increase of S100B was found by immunohistochemistry and by ELISA in AD vs. age-matched controls. Due to the fact that S100B can have both beneficial and toxic effects, depending on its concentration, we performed additional immunostaining for iNOS and NT-3, as downstream markers for oxidative stress and neurotoxicity. In AD brains remarkably stronger and widespread staining pattern was noted both in neurons and glial cells compared to controls indicating predominantly neurotoxic events. These results strongly pointed toward oxidative damage to be involved in neuronal and glial degeneration. In the last study we investigated the presence of [Ca]i related neuronal markers, calcium binding proteins (calbindin-CB, calretinin-CR, parvalbumin-PV) as pivotal molecules in understanding Ca-dependent neurodegeneration. We observed a significant reduction of CB and CR up to 65% in AD brains. Thus loss of the calcium binding proteins can play an additional role in neuronal and glial degeneration in PO. In summary, using a quantitative approach based on stereology we were able to demonstrate that neurodegeneration with glial changes occurs also in the brain region that is not affected by neurofibrillary pathology. Furthermore, the changes occurring in PO can be used as a model to investigate diverse pathomechanisms involved in neuronal and glial pathology.