ADNP/ADNP2 expression in oligodendrocytes: implication for myelin-related neurodevelopment

Oligodendrocytes, the myelin-forming cells of the central nervous system, play important roles in brain development and maintenance. Activity-dependent neuroprotective protein (ADNP), an early marker essential for brain formation, interacts with microtubule end-binding proteins (EB1, EB2, and EB3). EB1 and EB3 are highly expressed in neurons (axons and dendritic spines, respectively) and EB1 enhancement of neurite outgrowth is attenuated by EB2. ADNP/EB presence in oligodendrocytes has not been studied so far. Here, we measured messenger RNA (mRNA) levels of ADNP and EB1–EB3 in rat brain oligodendrocytes during culture maturation and in rat brains during development (1, 35, and 75 days) in comparison with rat astrocytes, dorsal root ganglion (DRG) neurons, and the oligodendroglia cell lines (OLN-93 cell line, not expressing the microtubule-associated protein (MAP) tau, and OLN-93 cells stably transfected to express various forms of tau). Results showed that all transcripts studied were expressed in oligodendrocytes. ADNP and EB2 mRNA transcript content peaked at the time of oligodendrocyte maturation (5 days in vitro) and was highest in newborn rat brains compared with mature brains. ADNP2 (the only family member of ADNP), and EB1, although expressed in lower quantities, essentially paralleled ADNP and EB2 expression patterns, respectively. EB3 mRNA, peaking upon oligodendrocyte maturation, showed an apparent second peak of expression (10 days in vitro) and increased in the mature rat brain compared with the newborn brain. DRG cells expressed the highest levels of EB3, when compared with oligodendrocyte precursors and with astrocytes but not when compared with mature oligodendrocytes. Mature oligodendrocytes and oligodendrocyte precursors expressed ~30–40-fold more EB2 vs. EB3, and ~4–7-fold vs. ADNP. DRGs expressed ~5-fold more EB2 vs. EB3 and astrocytes showed an in-between (~20-fold) ratio. Only DRGs expressed similar EB1 and EB3 transcript levels, contrasting with oligodendrocyte and astrocytes (~10–30-fold more EB1). Astrocytes expressed more ADNP than DRGs and oligodendrocyte precursor cells (~2-fold) but not compared with mature oligodendrocytes. EB1 and EB3 were previously found to be associated with tau. Immortalized oligodendrocytes showed an intermediate phenotype of mRNA expression compared with oligodendrocyte precursor cells and mature oligodendrocytes with tau transfection reducing overall ADNP and EB expression. In summary, ADNPs and EBs are highly expressed in oligodendrocytes suggesting an impact on myelin formation in health and disease.