The newly identified striatal marker Doublecortin-like kinase 3 (Dclk3) may be a molecular determinant of striatal vulnerability in Huntington’s disease
Résumé
Huntington's disease (HD) is an inherited neurodegenerative disorder caused by an abnormal polyglutamine expansion in the protein huntingtin (htt). Mutant htt, despite its ubiquitous expression in the brain, leads to preferential neurodegeneration of the striatum through unknown mechanisms. One hypothesis is that gene products selectively expressed in the striatum may be involved in the high vulnerability of striatal neurons to mutant htt. In the present study, we first show that (i) expression of mRNA coding for a newly identified "striatal" gene product, Doublecortin-like kinase 3 (Dclk3) is reduced in the striatum of BAC-HD mice and knock-in Hdh140 CAG mice, and (ii) overexpression of Dclk3 using lentiviral vectors is neuroprotective against mutant htt in primary culture of striatal neurons and in the mouse striatum in vivo. We next aimed at identifying the mechanisms of Dclk3 neuroprotective effects. Dclk3 contains two putative domains: a doublecortin-like domain in the N-terminal part of the protein and a kinase domain in its C-terminus. Thus, we generated mutants and truncated fragments of Dclk3 to investigate the role of the kinase domain. We found that a mutation inactivating the kinase was devoid of protective effects in striatal neurons. Overexpression of the kinase domain alone was sufficient to produce neuroprotection against mutant htt in vivo. Autophosphorylation experiments demonstrated that Dclk3 is actually a functional kinase, further suggesting that Dclk3 likely acts against mutant htt primarily through its kinase activity. We also found that recombinant Dclk3 is co-localized with microtubules with a higher density in the cell perinuclear region, according to a network-like architecture reminiscent of the cytoskeleton. The kinase domain shows a more diffuse localization throughout the cell. Interestingly, biochemical analyses indicated that Dclk3 is cleaved into several breakdown products, separating its doublecortin-like domain from its kinase domain. An abnormal cleavage of recombinant Dclk3 was found in the striatum of transgenic BAC-HD mice as compared to WT mice. Thus, reduced expression of Dclk3 and possibly alteration of its cleavage and cellular localization may lead to a loss of its “pro-survival” activity in HD models, rendering the striatum highly vulnerable to mutant htt. These novel results suggest that Dclk3 may be an important determinant of striatal vulnerability in HD. The identification of the mechanisms underlying the neuroprotective effects of Dclk3 may lead to novel potential therapeutic strategies for HD.