Interactions
between a transcription factor and its target genes clearly require the
transcription factor to be expressed in the right place at the right time and
the presence of the corresponding recognition elements in the target gene. Just
as importantly, we need to know the molecular interactions that direct the
actions of these activators and repressors to specific promoters. We are
particularly interested in examining the role of REST/NRSF during neuronal
differentiation. The original proposed role for REST was that of a factor
responsible for restricting neuronal gene expression to the nervous system by
silencing expression of these genes in non-neuronal cells. Although it was
initially thought to repress neuronal genes in non-neuronal cells, the role of
REST is complex and tissue dependent. Moreover, it should be considered that
REST may function in different ways at different stages of cell differentiation
as a result of changing expression of associated proteins or other signals as
it may form complexes with other cellular factors, such as CoREST Sin3A and
histone deacetylase which contribute to modulate REST-mediated repression. An
N-terminal expression domain of REST interacts with Sin3A whilst the C-terminal
recruits a complex that contains CoREST and a component of the
chromatin-remodeling complex. We propose that REST is regulated by growth
factors, like IGF-I, and PKC activators in a time-dependent manner: it is
elevated during early steps of neural induction and could contribute to
down-regulate genes not yet required by the differentiation program while it
declines later for the acquisition of neural phenotypes, concomitantly with a
progressive neurite extension. This later decline is regulated by the proteasome
system activation and adds more evidences to the hypothesis that REST
down-regulation contributes to differentiation and arrest of proliferation of
neuroblastoma cells. The expression pattern of REST could be consistent with
the theory that, during early neuronal differentiation induced by IGF-I, it may
help to repress the expression of several genes not yet required by the
differentiation program and then it declines later. Interestingly, the
observation that REST expression is progressively reduced in parallel with cell
proliferation seems to indicate that the role of this transcription factor
could also be related to cell survival or to counteract apotosis events.
Finally, the glycosylation pattern of the REST protein
is analysed, moving from the observation that the molecular weight calculated
on REST sequence is about 116 kDa but using western blotting this transcription
factor appears to have distinct apparent molecular weight: this difference
could be explained by post-translational modifications of the proteins, like
glycosylation. In fact recently, several studies underlined the importance of
O-glycosylation in modulating transcriptional silencing, protein
phosphorylation, protein degradation by proteasome and protein–protein
interactions.
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