We have identified a novel protein that functions as a transcriptional coactivator interacting with Smad to modulate BMP signaling. Our data support a model where Sizn1 associates with Smads in the nucleus upon BMP signaling activation. Sizn1 also recruits, or stabilizes, CBP to the transcription complex to positively regulate signaling. We have also demonstrated that BMP signaling can directly regulate ChAT gene transcription, a necessary factor for the cholinergic phenotype. Furthermore, we have found that Sizn1 knockdown reduces the BMP-dependent induction of cholinergic-neuron-specific genes (ChAT and VaChT). Finally, we have shown that the coactivator function of Sizn1 is dependent on R-Smads entering the nucleus.
TGF-β family members, including BMPs and activin, play multiple roles in development, differentiation, and tumorigenesis through signaling (13
). Given that TGF-β family members and their receptors show broad temporal and spatial expression patterns during development, the expression patterns of ligands and receptors alone are not sufficient to explain the distinct roles played by TGF-β family members in restricted cell populations. Thus, additional modulators are required to control TGF-β signaling levels. The regulation of pathway activation is found at many levels, including ligand availability, receptor binding and activation, the translocation of the activated Smad complex to the nucleus, and the recruitment of transcription factors and transcriptional coregulators (coactivators or corepressors). Sizn1 can now be added to the list of known transcriptional coregulators. Unlike most ubiquitously expressed transcriptional coregulators that have been reported so far, Sizn1 exhibits restricted spatial and temporal expression patterns in the brain (Msg1 also shows some restricted expression) (24
), which can permit further refinement of BMP signaling. Cholinergic-neuron-specific expression of Sizn1 in the septal nucleus is a good example of this postulate. Once its own expression is induced by BMP signaling, Sizn1 can participate in the activation of other genes necessary for inducing and/or maintaining the cholinergic-neuron phenotype.
Although our studies have focused on Sizn1 expression in septal cholinergic neurons, our data also show that Sizn1 is expressed in the developing thalamus, amygdala, putamen (Fig. ), and brain stem (data not shown) as well as in the septum. Preliminary data indicate that its expression is not restricted to cholinergic neurons in other brain regions. Definition of these areas of expression, the cell types, and the function of Sizn1 in these cells is under active investigation. Outside the nervous system, we know that Sizn1 is also expressed in the testis (Fig. ). TGF-β signaling, including through BMP-7, BMP-4, BMP-8a, and BMP-8b, plays multiple roles in spermatogenesis and maintaining epididymal integrity (12
). It is reasonable to consider that Sizn1 may also have some role in these processes.
Given the fact that Sizn1 is expressed in the forebrain and hindbrain regions, where cholinergic neurons do not reside, we can postulate that Sizn1 likely has roles in the development/maintenance of other types of neurons. It is also possible that Sizn1 may be involved in other signaling pathways, not only in the BMP pathway. We have some evidence supporting the idea that Sizn1 can modulate Wnt signaling but not Shh signaling (unpublished data). In this manner, Sizn1, along with other transcription factors and transcriptional coregulators, contributes to generating the complexity of neuronal subtypes in the mammalian nervous system. Theoretically, this may also account for the lack of a Sizn1 ortholog, as well as many other transcriptional coactivators/corepressors, in invertebrates, which have simpler nervous systems and fewer neuronal subtypes. It should be noted that Sizn1 is also expressed in the adult brain (Fig. ). We speculate that it may have a role in maintaining cholinergic-neuron-specific gene expression. Further studies are obviously required to define the expression of Sizn1 in the adult nervous system and to characterize any functions that it has in the mature brain.
BFCNs project to the cortex and limbic system, where they function in processes such as attention, learning, and memory (7
). Previous studies have shown that BMP-9 contributes to the maturation of BFCNs by regulating multiple genes involved in the induction and maintenance of the cholinergic phenotype (17
). Our data support these findings and provide the mechanism of how BMP signaling can directly modulate the cholinergic phenotype. In summary, Sizn1 is a novel transcriptional coactivator in the BMP-signaling pathway, functioning by interacting with Smad1 and CBP. It contributes to the BMP-induced enhancement of cholinergic-neuron-specific gene expression, including that of ChAT
. Given the requirement of ventral-forebrain cholinergic neurons in cognition, future studies with animal models will provide further insight into the potential role of Sizn1 in memory and other related cognitive functions.