Impact of HLD6-associated TUBB4A mutant proteins on cell morphogenesis

Oligodendroglial cells are a type of glial cell in the central nervous system (CNS) that surround neuronal axons with a specialized cell membrane called the myelin sheath. Their physiological functions and their functional defects in pathological conditions have been well studied so far. However, the molecular mechanisms that control oligodendroglial cell differentiation and its destruction remain unclear. In oligodendroglial diseases such as congenital hypomyelinating leukodystrophies, differentiation often appears to be arrested at an immature stage. Here, for the first time, we report that hypomyelinating leukodystrophy type 6 (HLD6)-associated tubulin beta 4 A (TUBB4A) mutated proteins (6 types of missense mutations), but not the wild type one, disrupt the cell morphological changes with myelin sheath-like widespread membranes in FBD-102b cells, a mouse oligodendroglial model undergoing differentiation. Consistently, expression levels of oligodendroglial cell differentiation marker proteins and Akt kinase phosphorylation, a key signaling pathway promoting oligodendroglial differentiation and myelination, were reduced. Notably, mutated TUBB4A proteins failed to undergo the proper post-translational processing necessary for microtubule formation. Instead, these proteins localized aberrantly to intracellular organelle-like structures. These findings suggest that HLD6-asociated TUBB4A mutations lead to defective cell morphogenesis, providing novel insights into the pathological molecular mechanisms underlying differentiation arrest before myelination, at least at the molecular and cellular levels.