Cholinergic basal forebrain degeneration due to sleep-disordered breathing exacerbates pathology in a mouse model of Alzheimer's disease.

Although epidemiological studies indicate that sleep-disordered breathing (SDB) such as obstructive sleep apnea is a strong risk factor for the development of Alzheimer's disease (AD), the mechanisms of the risk remain unclear. Here we developed a method of modeling SDB in mice that replicates key features of the human condition: altered breathing during sleep, sleep disruption, moderate hypoxemia, and cognitive impairment. When we induced SDB in a familial AD model, the mice displayed exacerbation of cognitive impairment and the pathological features of AD, including increased levels of amyloid-beta and inflammatory markers, as well as selective degeneration of cholinergic basal forebrain neurons. These pathological features were not induced by chronic hypoxia or sleep disruption alone. Our results also revealed that the cholinergic neurodegeneration was mediated by the accumulation of nuclear hypoxia inducible factor 1 alpha. Furthermore, restoring blood oxygen levels during sleep to prevent hypoxia prevented the pathological changes induced by the SDB. These findings suggest a signaling mechanism whereby SDB induces cholinergic basal forebrain degeneration.

The basal forebrain volume reduction detected by MRI does not necessarily link with the cholinergic neuronal loss in the Alzheimer's disease mouse model.

Degeneration of cholinergic neurons in the basal forebrain (BF) contributes to cognitive impairment in Alzheimer's disease (AD) and other disorders. Atrophy of BF volume measured by structural MRI is thought to represent the loss of cholinergic neurons in this structure. As there are multiple types of neurons in the BF as well as glia and axons, whether this MRI measure actually reflects the change of cholinergic neurons has not been verified. In this study, we assessed BF cholinergic neuron number by histological counts and compared with the volume measurements by in vivo MRI in 3xTg mice, a model of familial AD. Both manual and template-based segmentation revealed atrophy of the medial septum (MS), consistent with a significant reduction in cholinergic neuron number. However, MRI-measured volume reduction did not correlate with the reduced cholinergic neuron number. To directly test whether specific loss of cholinergic neurons results in BF atrophy, we selectively ablated the cholinergic neurons in the MS. However, no detectable change in MRI volume was observed between lesioned and unlesioned mice. The results indicate that although loss of cholinergic neurons within the BF likely contributes to volume loss, this volume change cannot be taken as a direct biomarker of cholinergic neuron number.

Is there a role for the p75 neurotrophin receptor in mediating degeneration during oxidative stress and after hypoxia?

Cholinergic basal forebrain (cBF) neurons are particularly vulnerable to degeneration following trauma and in neurodegenerative conditions. One reason for this is their characteristic expression of the p75 neurotrophin receptor (p75NTR ), which is up-regulated and mediates neuronal death in a range of neurological and neurodegenerative conditions, including dementia, stroke and ischaemia. The signalling pathway by which p75NTR signals cell death is incompletely characterised, but typically involves activation by neurotrophic ligands and signalling through c-Jun kinase, resulting in caspase activation via mitochondrial apoptotic signalling pathways. Less well appreciated is the link between conditions of oxidative stress and p75NTR death signalling. Here, we review the literature describing what is currently known regarding p75NTR death signalling in environments of oxidative stress and hypoxia to highlight the overlap in signalling pathways and the implications for p75NTR signalling in cBF neurons. We propose that there is a causal relationship and define key questions to test this assertion.