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  • The idea that ASB can drive


    The idea that ASB11 can drive compartment expansion in the CNS is further supported by in vitro work in human neuronal precursors. Nerve growth factor (NGF)-induced differentiation of PC12 pheochromocytoma Andarine synthesis [11] and RA-induced human pluripotent teratocarcinoma carcinoma (NT2) cells [12] constitute well-established in vitro models for neuronal differentiation. The most important characteristics of these models are the inhibition of cell proliferation followed by the extension of neurites. Importantly, PC12 cells and NT2 cells overexpressing ASB11 continue to proliferate upon induction of neuronal differentiation. Conversely, when mean neurite length per soma was taken as a relative measure of neuronal differentiation and compared between cells overexpressing ASB11 to cells transfected with the control vector, the control cells showed significant neurite outgrowth, whereas ASB11 overexpression almost completely abolished any neurite outgrowth. The lack of NGF-induced neurite extension in ASB11-expressing cells may be caused by the incapacity of these cells to initiate neuronal differentiation or, alternatively, may reflect a block further down the neuronal differentiation program pathway. To distinguish between these possibilities, expression of GAP43, a marker of the neuronal precursor state, and of the neurofilament, a terminal neuronal marker, was used 13, 14, 15. Importantly, transfection of ASB11 into PC12 cells inhibited neurofilament expression, but strongly enhanced the NGF-dependent expression of GAP43. Thus, ASB11 does not mediate inhibition of the induction of neuronal differentiation per se, but blocks the transition from the proliferating neuronal precursor state to the postmitotic terminally differentiated phenotype, leading to an accumulation of neuronal precursors. We speculate that, in preneural cells, ASB11 maintains the neuroblast phenotype until its expression is downregulated by subsequent inductive differentiation signals. Later during development, upon disappearance of ASB11 expression, these cells would become available for secondary inductive signals or would pursue an alternative default fate. Indeed, in zebrafish, the timing of asb11 mRNA expression [16], the onset of which coincides with the appearance of primary neurons and the disappearance of which coincides with the birth of secondary neurons, suggests that asb11 functions to separate waves of primary and secondary neurogenesis by maintaining a pool of dividing precursors to respond to inductive signals operating later in development. The molecular mechanism by which Asb11 exerts these effects remains unclear, but its expression pattern is not inconsistent with an action in Delta-Notch signaling.
    Six-Ankyrin Repeat Domain-Containing ASB Proteins Asb family members contain a relatively divergent N-terminal domain, followed by a varying number of ankyrin repeats and a C-terminal SOCS box (Figure 3). Asb11 contains six ankyrin repeats, a feature that it shares with Asb5, Asb9, and Asb13, suggesting that these proteins constitute a separate subgroup within the Asb family of proteins. Analysis of the primary sequence shows high homology in the N-terminal domain (i.e., the amino acids in the N terminus of the ankyrin repeats, the most divergent region in the Asb family) of these four proteins, and analysis of the intron/exon boundaries also supports the notion that these six-ankyrin repeat domain-containing proteins are a separate subgroup within the Asb gene family. Homology is even higher in the 3′ untranslated region [17]. Furthermore, overexpression of ASB5, ASB9, and ASB11, but not of ASB1, ASB2, or ASB15, activates a Notch reporter construct in HeLa cells, demonstrating that members of the six-ankyrin repeat subfamily of Asb genes share functional characteristics that certain other Asb family members do not have. Functionally, these six-ankyrin repeat domain-containing Asb genes may all be involved in compartment size regulation. The expression pattern of d-asb9/11 in D. rerio revealed high expression during myogenesis. Forced expression of d-asb9/11 impaired terminal differentiation and caused hyperproliferation in the myogenic progenitor compartment, whereas either knockdown of d-asb9/11 or introduction of a germline mutation in the zebrafish d-asb9/11 gene (asb9/11cul) produced premature differentiation of muscle progenitors. Finally, d-asb9cul mutant fish were severely impaired in regenerative responses. Thus, d-asb9/11 is a principal regulator of both embryonic and regenerative myogenesis; in addition, six-ankyrin repeat domain-containing Asb proteins can act outside the ectodermal lineage and may also be active in derivatives from the mesodermal germline with respect to compartment expansion 18, 19.