The major neuronal post-translational modification of tubulin, polyglutamylation, can act as a molecular potentiometer to modulate microtubule-associated proteins (MAPs) binding as a function of the polyglutamyl chain length. The relative affinity of Tau, MAP2, and kinesin has been shown to be optimal for tubulin modified by ϳ3 glutamyl units. Using blot overlay assays, we have tested the ability of polyglutamylation to modulate the interaction of two other structural MAPs, MAP1A and MAP1B, with tubulin. MAP1A and MAP2 display distinct behavior in terms of tubulin binding; they do not compete with each other, even when the polyglutamyl chains of tubulin are removed, indicating that they have distinct binding sites on tubulin. Binding of MAP1A and MAP1B to tubulin is also controlled by polyglutamylation and, although the modulation of MAP1B binding resembles that of MAP2, we found that polyglutamylation can exert a different mode of regulation toward MAP1A. Interestingly, although the affinity of the other MAPs tested so far decreases sharply for tubulins carrying long polyglutamyl chains, the affinity of MAP1A for these tubulins is maintained at a significant level. This differential regulation exerted by polyglutamylation toward different MAPs might facilitate their selective recruitment into distinct microtubule populations, hence modulating their functional properties. Microtubules (MTs)1 are dynamic polymers, which are essential for a large variety of cellular functions such as cell morphology and polarity, cell motility, intracellular trafficking, and cell division. They are made up of ␣-and ␤-tubulin heterodimers, the two related subunits displaying a large isoform polymorphism due to the expression of multiple genes whose products are substrates for several post-translational modifications (for review, see Refs.

Using clonal cell lines isolated from murine neuroblastoma C1300, we investigated the mitochondrial changes related to neuronal differentiation and, more generally, the role played by the mitochondrion in this phenomenon. By different approaches (measurement of the mitochondrial mass, immunoquantification of specific mitochondrial proteins, or incorporation of Rhodamine 123), the differentiation of the inducible clone, N1E-115, was found associated with an important increase of the cellular content in mitochondria. This increase could be observed with differentiating N1E-115 cells maintained in suspension, i.e. under conditions where neurite outgrowth is prevented but other early stages of (biochemical) differentiation continue to occur. That these mitochondrial changes are likely to be correlated with these stages of neuronal differentiation, rather than with simple progression to the postmitotic stage, stems from comparative experiments with clone N1A-103, a neuroblastoma cell line variant that becomes postmitotic after induction but fails to differentiate and shows no modification in its cellular content in mitochondria. In accordance with these observations, chloramphenicol prevents differentiation when added together with the inducer. This effect is probably related to the inhibition of mitochondrial translation rather than to modification of the bioenergetic needs because oligomycine, a potent inhibitor of the mitochondrial ATP synthetase, shows no effect on neurogenesis. As a working hypothesis and in keeping with independently published models, we postulate that products resulting from mitochondrial translation could be involved in the organization of the cytoskeleton or of certain membrane components whose rearrangements should be the prerequisite or the correlates to early stages of neuronal differentiation.

No abstract