Neurodegenerative diseases studied in human brain and rodent models

Abstract

Parkinson’s and Alzheimer’s disease are the two most common neurodegenerative diseases world-wide, but are still little understood. Papers in this thesis examine some of the possible pathogenic mechanisms with the help of mouse models and analysis of human post-mortem tissue.

Gene-based animal models have been developed to study pathological pathways during disease progression. Using mouse models with overexpression or ablation of disease-related genes we analyzed effects of pathogenic mutations on the function of the proteins. A prominent feature of patients carrying the G2019S mutation in leucine rich repeat kinase 2 (LRRK2) is a high load of Lewy Bodies, an intracellular protein accumulation highly enriched in α-synuclein. We investigated the interaction of LRRK2 and α-synuclein in transgenic mice and did not find an influence of LRRK2 expression on fibrillization of α-synuclein or dopamine neurons pathology, indicating separate pathways for the two genes causing PD.

Using another genetic model with a conditional knockout of the mitochondrial transcription factor A in dopamine neurons, the MitoPark mouse, we detected reduced dopamine release and pacemaker activity in the substantia nigra several weeks before these mice develop motor dysfunction. The mRNA levels of the hyperpolarization-activated cyclic nucleotide gated channels 1-4 (HCN1-4), responsible for the pacemaking activity, were not altered in MitoPark mice, indicating that post-translational modifications occur early in the presymptomatic stages of Parkinson’s disease.

To evaluate behavioral and cellular changes related to L-DOPA therapy we used older MitoPark mice, which model late stages of Parkinson’s disease. Chronic L-DOPA treatment normalized gait parameters but induced also progressing dyskinetic behavior in MitoPark mice. The treatment also caused a robust increase of TH mRNA expression in the striatum, as evidenced by RNA-Sequencing. The induction of TH in striatal neurons with an interneuronal phenotype was dependent on the degree of dopamine depletion and the L-DOPA dose.

In disease-affected brain areas of patients with Parkinson’s or Alzheimer’s disease we found a significant increase in small-sized cells expressing the lysosomal enzyme myeloperoxidase. This finding supports involvement of a neuroinflammatory component in these diseases and encourages the research for anti-inflammatory treatments. Another protein implicated in the pathogenesis of neurodegeneration is the serine peptidase HTRA2. We detected altered enzyme activity and expression of HTRA2 in frontal cortex samples from Alzheimer’s disease patients. The association of a mutation in a HTRA2 allele with Alzheimer’s disease in our case-control material further supported a role of mitochondrial dysfunction in the pathology.

Taken together, the studies presented in this thesis uncover changes in gene and protein expression in mouse and human samples, as well as behavioral changes in animal models of disease and will aid the development of better treatment options by increasing our knowledge of underlying pathological mechanisms.