The role of Ubiquitin ligases Nedd4-1 and Nedd4-2 in the brain and their crosstalk with alpha-synuclein and other dementia-related proteins

Abstract

Parkinson’s disease (PD) and Dementia with Lewy Bodies (DLB) are a progressive and currently incurable neurodegenerative disease and the most common movement disorder. α-synuclein protein is thought to be the main aetiological agent of PD and DLB, particularly when levels are increased in substantia nigra (SN) dopaminergic neurons, causing cell death. Current PD and DLB treatments are symptomatic and do not target the disease at its source or prevent its progression. In order to develop protective treatments for PD and DLB, it is important to first understand its pathophysiology. Using mice which are genetically altered to exhibit PD-and DLB-like traits at a molecular level, allows the study of which proteins may be involved in the mechanisms of PD and DLB, and may alter α-synuclein accumulation and aggregation. Neural precursor cell-expressed developmentally derived 4 (Nedd4) is a family of proteins of which some members have been shown to target α-synuclein for breakdown through ubiquitination, both in cellular models, and in fly and rat PD models. It is therefore of interest to research the effect of deleting Nedd4 family members in dopaminergic neurons. In order to investigate the function of some Nedd4 family members role in the dopaminergic system, we have generated mice in which two members of this family, Nedd4-1 and Nedd4-2, are deleted specifically in dopaminergic neurons. Mice deficient for Nedd4-2 in their dopaminergic neurons were found to have a progressive loss of striatal dopaminergic innervation and dopaminergic neurons in the SN. This was accompanied by striatal gliosis and both gliosis and inflammation in the SN. Mice deficient for Nedd4-1 in their dopaminergic neurons were found to have an increased α-synuclein burden in these cells but did not exhibit the dopaminergic cell loss in the SN as Nedd4-2-deficient mice did. In the ventral midbrain the Mammalian Target of Rapamycin (mTOR) inhibitor RTP801 was found to be increased in Nedd4-1 and Nedd4-2 deficient mice, AKT phosphorylation only in Nedd4-1 deficient mice and the autophagy related p62 protein only in double deficient mice. Mice deficient for Nedd4-1 and Nedd4-2 and also overexpressing human mutant A53T α- synuclein exhibit an enhanced, age-dependent striatal dopaminergic axon loss and cell loss in the SN compared to mice just overexpressing human mutant A53T α-synuclein. Nedd4-1 and Nedd4-2 deletion did not elevate levels of A53T α-synuclein in dopaminergic neurons, but it did increase its propensity to aggregate. This was accompanied by reduced inflammation in the striatum and SN compared to mice just overexpressing human mutant A53T α-synuclein. This study therefore allows us to conclude that that Nedd4-1 and Nedd4-2 have distinct roles in the dopaminergic system, with Nedd4-1 controlling α-synuclein levels and Nedd4-2 being responsible for dopaminergic neuron survival. With this information, molecular therapies may be developed for use in human PD patients to increase Nedd4-1 and Nedd4-2 activity for clearance of α-synuclein and protection of dopaminergic neurons. Additional data are provided from mice deficient for Nedd4-1 and Nedd4-2 in cortical neurons showing for example reduced mTOR signalling and increased ABCB1 protein levels, which are alterations associated with dementia. These data support the idea that enhancing Nedd4- 1 and Nedd4-2 signalling might be also used to treat dementia.