RAB39B is a novel PD associated gene with a putative role in intracellular trafficking. Currently, little is known about the distribution of RAB39B in brain tissue, and how dysregulation of its function(s) can lead to the development of disease. To address this knowledge gap, we generated mouse models with knockout of Rab39b, and investigated the distribution of RAB39B (RNA and protein) in mouse brain tissue. In this study, we showed that RAB39B is present throughout postnatal life, with the protein being abundant in the neuronal soma of cortical and hippocampus, which have implications for its proposed roles in synaptic maintenance and cognition. These results are consistent with previous reports on the localization of Rab39b expression and RAB39B protein in mouse brain tissue (3, 9). Notably, we demonstrated that RAB39B is an abundant protein in dopaminergic neurons in the SNpc, the neuronal subtype selectively lost in PD.
The distribution of RAB39B (RNA and protein) in cortical and midbrain regions could potentially explain the wide spectrum of clinical features that have been observed in individuals with mutations in RAB39B. Specifically, the function(s) of RAB39B in cortical and hippocampal regions are likely to be associated with the development of cognitive impairment, whilst its function(s) in the SNpc could be linked to the development of PD. Consistent with the mouse data reported herein, we have recently demonstrated substantial steady-state levels of RAB39B in human cortex, hippocampus and substantia nigra (14). These observations suggest the mouse represents an appropriate model for further studies to investigate the pathological mechanisms underlying parkinsonism and cognitive dysfunction in RAB39B-mediated disease.
The abundance of RAB39B protein in neuronal somas is suggestive of a role for RAB39B in trafficking events in the cell body as opposed to axonal projections and neuronal synapses. This hypothesis is supported by the lack of RAB39B localized to dopaminergic synapses in the striatum by ISH and IHC. To date, the intracellular localization of endogenous RAB39B has not been well defined in vitro or in vivo. However, one study investigated the intracellular localization of RAB39B using neural differentiated PC12 and SK-N-BE(2)C cells overexpressing RAB39B. Immunofluorescent microscopy showed that wildtype RAB39B could be observed throughout the cytoplasm of cell bodies, at the plasma membranes and at neuritic processes (15). In order to definitively define the specific intracellular localization of endogenous RAB39B, further studies are required, which will be facilitated by the mouse models generated in this study, in addition to previously generated stem cell models (13).
In conclusion, we have expanded on current knowledge of the distribution of RAB39B in mouse brain, and demonstrated that neuronal localization of the endogenous protein is not limited to cortical regions as has been previously reported. Further, we generated and validated monoclonal antibodies for RAB39B and mouse models with knockout (KO) of Rab39b. Ongoing studies are now investigating the subcellular localization of RAB39B and testing the behavioral and biochemical phenotypes observed in aged knockout mice. These novel resources represent valuable tools for future investigations into the physiological function(s) of RAB39B and its role in human disease states.