Potential effects of Rac1 GTPase signalling in the deregulation of Alzheimer's Disease relevant proteins

Alzheimer’s disease (AD) is a multifactorial pathology and the most common form of dementia in the elderly. Despite it was first described in 1906, the aetiology of AD is still poorly understood. Recently, different groups have highlighted the involvement of small Rho GTPases in AD (Bolognin et al., 2014), whose main function consists in the regulation of actin cytoskeleton dynamics (Hall and Lalli, 2010). Moreover, this family of protein plays an important role in dendritic spine morphology (Etienne‐Manneville and Hall, 2002), whose alterations are responsible for synaptic deficits. Since the progression of AD is characterized by a wide synaptic loss, alterations of cytoskeleton dynamics might be a key pathogenic event contributing to AD neuropathology, and Rho GTPases could be directly connected to the disease. Rac1, one of the three best characterized Rho GTPases, is known to act as neuronal survival promoter (Le et al., 2005; Loucks et al., 2006). Interestingly, it has been found that Rac1 is dysregulated in AD brain, suggesting a possible involvement in the processing of beta amyloid (Aβ) from its precursor APP (Stankiewicz and Linseman, 2014). Moving from these assumptions, the aim of this thesis was to investigate the molecular pathways connecting Rac1 and AD relevant proteins, Aβ and tau. All the experiments were performed in vitro on primary cortical neurons, taking into account the modulation of the single proteins. The project has started with Aβ administration and tau induced hyperphosphorylation, and both approaches did not result in any modification of Rac1 cellular distribution or activation. We proceeded then with the mis‐regulation of Rac1 activity, through the administration of mutant proteins fused with a TAT domain, which allows their internalization into the cells. This approach showed a perturbation of Aβ metabolism, and elicited the translocation of SET, a protein directly connected to tau hyperphosphorylation. More in details, only Rac1 activation was able to enhance the levels of Aβ or its precursor, while the over‐expression of the protein was sufficient to promote SET translocation from the nucleus to the plasma membrane. Taken together, the data describe a putative pathway in which Rac1 is up‐stream and it is able to affect the regulation of AD relevant proteins. The involvement of Rac1 in the processing of Aβ is not completely new (Boo et al., 2008; Wang et al., 2009), but, to our knowledge, nobody has already investigated which fragments can be produced after Rac1 activation. On the other hand, the data on SET translocation after Rac1 over‐expression constitute a new insight and future experiments could be better clarify this direct connection between SET and Rac1 in AD context. Indeed, only two groups found the same pathway, but not in a neuronal population (ten Klooster et al., 2007; Switzer et al., 2011).