Different Doublecortin (DCX) Patient Alleles Show Distinct Phenotypes in Cultured Neurons: EVIDENCE FOR DIVERGENT LOSS-OF-FUNCTION AND "OFF-PATHWAY" CELLULAR MECHANISMS.

Doublecortin on the X-chromosome (DCX) is a neuronal microtubule-binding protein with a multitude of roles in neurodevelopment. In humans, DCX is a major genetic locus for X-linked lissencephaly. The best studied defects are in neuronal migration during corticogenesis and in the hippocampus, as well as axon and dendrite growth defects. Much effort has been directed at understanding the molecular and cellular bases of DCX-linked lissencephaly. The focus has been in particular on defects in microtubule assembly and bundling, using knock-out mice and expression of WT and mutant Dcx in non-neuronal cells. Dcx also binds other proteins besides microtubules, such as spinophilin (abbreviated spn; gene name Ppp1r9b protein phosphatase 1 regulatory subunit 9b) and the clathrin adaptors AP-1 and AP-2. Even though many non-sense and missense mutations of Dcx are known, their molecular and cellular defects are still only incompletely understood. It is also largely unknown how neurons are affected by expression of DCX patient alleles. We have now characterized several patient DCX alleles (DCX-R89G, DCX-R59H, DCX-246X, DCX-272X, and DCX-303X) using a gain-of-function dendrite growth assay in cultured rat neurons in combination with the determination of molecular binding activities and subcellular localization in non-neuronal and neuronal cells. First, we find that several mutants (Dcx-R89G and Dcx-272X) were loss-of-function alleles (as had been postulated) but surprisingly acted via different cellular mechanisms. Second, one allele (Dcx-R59H) formed cytoplasmic aggregates, which contained Hspa1B (heat shock protein 1B hsp70) and ubiquitinated proteins, trapped other cytoskeletal proteins, including spinophilin, and led to increased autophagy. This allele could thus be categorized as "off-pathway"/possibly neomorph. Our findings thus suggested that distinct DCX alleles caused dysfunction by different mechanisms.