What we can learn from animal models about cerebral multi-morbidity.

Late-onset diseases such as Alzheimer's disease, Parkinson's disease, or frontotemporal lobar degeneration are considered to be protein-folding disorders, with the accumulation of protein deposits causing a gain-of-toxic function. Alzheimer's disease is characterized by two histological hallmark lesions: amyloid-β-containing plaques and tau-containing neurofibrillary tangles. However, signature proteins, including α-synuclein, which are found in an aggregated fibrillar form in the Lewy bodies of Parkinson's disease brains, are also frequently found in Alzheimer's disease. This highlights the fact that, although specific aggregates form the basis for diagnosis, there is a high prevalence of clinical overlap between neuropathological lesions linked to different diseases, a finding known as cerebral co- or multi-morbidity. Furthermore, the proteins forming these lesions interact, and this interaction accelerates an ongoing degenerative process. Here, we review the contribution that transgenic animal models have made to a better mechanistic understanding of the causes and consequences of co- or multi-morbidity. We discuss selected vertebrate and invertebrate models as well as the insight gained from non-transgenic senescence-accelerated mouse-prone mice. This article is part of a series on 'Cerebral multi-morbidity of the aging brain'.