Nuclear expression of S100B in oligodendrocyte progenitor cells correlates with differentiation toward the oligodendroglial lineage and modulates oligodendrocytes maturation.

The S100B protein belongs to the S100 family of EF-hand calcium binding proteins implicated in cell growth and differentiation. Here, we show that in the developing and the adult mouse brain, S100B is expressed in oligodendroglial progenitor cells (OPC) committed to differentiate into the oligodendrocyte (OL) lineage. Nuclear S100B accumulation in OPC correlates with the transition from the fast dividing multipotent stage to the morphological differentiated, slow proliferating, pro-OL differentiation stage. In the adult, S100B expression is down-regulated in mature OLs that have established contacts with their axonal targets, suggesting a nuclear S100B function during oligodendroglial cells maturation. In vitro, the morphological transformation and maturation of pro-OL cells are delayed in the absence of S100B. Moreover, mice lacking S100B show an apparent delay in OPC maturation in response to demyelinating insult. We propose that nuclear S100B participates in the regulation of oligodendroglial cell maturation.

Monitoring of S100 homodimerization and heterodimeric interactions by the yeast two-hybrid system.

The S100 family consists of 19 members, which function as transducers of calcium signals in a tissue-specific manner. Upon calcium binding, the conformation of many S100 proteins changes dramatically. Several hydrophobic residues are exposed, allowing the S100 proteins to interact with their target proteins, and thereby to transduce calcium signals into specific biological responses. To further elucidate the exact contribution of the S100 calciproteins in the calcium signalling pathways, several groups have applied the yeast two-hybrid technology to identify putative target proteins for the various S100 calciproteins. Two-hybrid large screens using S100 proteins as baits have confirmed the biochemical and structural feature of S100, which enable them to form homodimers and the ability of some members to form specific heterodimers in vivo. Yeast two-hybrid investigations have allowed the identification of conserved hydrophobic residues and domains that are crucial for the stabilization of S100 homo- and heterodimers. Furthermore, this method clearly underlines that the homo- and heterodimerization mechanisms differ among the members of the S100 family. However, several lines of evidence strongly suggest that two-hybrid methodology is limited to the analysis of interactions that are calcium-independent, since no target proteins other than S100 family members themselves have been detected with this methodology.

The zinc- and calcium-binding S100B interacts and co-localizes with IQGAP1 during dynamic rearrangement of cell membranes.

The Zn(2+)- and Ca(2+)-binding S100B protein is implicated in multiple intracellular and extracellular regulatory events. In glial cells, a relationship exists between cytoplasmic S100B accumulation and cell morphological changes. We have identified the IQGAP1 protein as the major cytoplasmic S100B target protein in different rat and human glial cell lines in the presence of Zn(2+) and Ca(2+). Zn(2+) binding to S100B is sufficient to promote interaction with IQGAP1. IQ motifs on IQGAP1 represent the minimal interaction sites for S100B. We also provide evidence that, in human astrocytoma cell lines, S100B co-localizes with IQGAP1 at the polarized leading edge and areas of membrane ruffling and that both proteins relocate in a Ca(2+)-dependent manner within newly formed vesicle-like structures. Our data identify IQGAP1 as a potential target protein of S100B during processes of dynamic rearrangement of cell membrane morphology. They also reveal an additional cellular function for IQGAP1 associated with Zn(2+)/Ca(2+)-dependent relocation of S100B.