Function of cytoplasmic dynein in mitochondrial maintenance – DyneMit

Cytoplasmic dynein (dynein) functions as the major molecular motor in moving cargoes towards the minus end of microtubules. It is required for various housekeeping cellular functions, including mitosis and axonal transport in neurons. Dynein has been involved in neurodegenerative diseases such as Alzheimer’s disease, Amyotrophic Lateral Sclerosis or Huntington’s disease and dynein is necessary for neuronal survival and morphology. Dynein is also responsible for mitochondrial transportation, and mitochondrial dysfunction has been repeatedly shown in neurodegenerative diseases but also in other pathologies such as diabetes.

The lack of appropriate genetic models has long precluded the study of the physiological functions of dynein in vivo. Recently, dynein mutations have been identified in mice with specific locomotor phenotypes. We recently observed that mice bearing the Cramping mutation in the dynein gene showed abnormalities in the striatum and features of diabetes. This was associated with a widespread and important defect in mitochondrial respiration. Thus, the phenotype of dynein mutant mice links together axonal transport defects, mitochondrial dysfunction and systemic abnormalities in energy homeostasis thereby bridging neurodegenerative diseases and diabetes.

The first aim of this project is to understand how a mutation in dynein leads to mitochondrial dysfunction in vivo. We reasoned that mitochondrial dysfunction in Cramping mutant mice is caused by dysfunctional degradation of senescent mitochondria, rather than impaired generation of new mitochondria. Preliminary work was indeed consistent with this hypothesis. Based on these results, the hypothesis underlying the current proposal is that dynein mutation leads to mitochondrial dysfunction through impaired targeting of dysfunctional mitochondria to degradation. Two different mechanisms might account for such an event. First, dynein mutation might lead to mitochondrial dysfunction through impaired clustering of abnormal mitochondria in a perinuclear structure called aggresome. Aggresome formation is required for autophagic degradation of a number of proteins and transport to the aggresome is known to be dependent upon dynein through its binding to HDAC6 and parkin. Second dynein has also been involved in the fusion between autophagosomes and lysosomes. A defect in such a fusion event is likely to lead to decreased autophagy of mitochondria. Our first aim is to determine which of these two hypothesis stands true. For this, we will first characterize the defect in autophagy and in mitochondrial function. In particular, we will determine whether dysfunctional autophagy is causative of mitochondrial dysfunction. We will further determine whether the aggresome pathway, and more specifically HDAC6 and parkin are involved in this phenotype.

The second aim of our application is to determine whether a loss of dynein in adult muscle or neurons leads to mitochondrial dysfunction. Indeed, while our observation of mitochondrial dysfunction in dynein mutant mice shows a genetic link between dynein and mitochondrial health, it does not provide definitive evidence that loss of dynein function leads to mitochondrial dysfunction in adult muscles and neurons. To this aim, we will generate and characterize new models with conditional loss of dynein expression.

In all, our application aims at determining the functional links between mitochondria, autophagy and cytoplasmic dynein. We think that the completion of this project will have several consequences : First, we will provide important insights into the fundamental mechanisms of mitochondrial autophagy. Second, the importance of dynein in neurodegeneration and in energy homeostasis will be relevant for neurodegenerative diseases and diabetes. Third, this project will lead to the creation of new tools of high interest such as conditional knock out mice for dynein.