The molecular pathology of the neurodegenerative disease spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neurodegenerative disease, caused by reduced levels of the survival of motoneuron (SMN) protein. How SMN is involved in maintaining motoneuron integrity is unknown, so far. In the nucleus, SMN is associated to two compartments called nuclear gems and Cajal bodies (CBs). The binding of SMN to Cajal bodies occurs by a direct interaction with the CB marker protein coilin. Although the major amount of coilin is located diffuse in the nucleus outside of CBs, SMN and coilin are essential for maturation of small nuclear ribonucleoprotein particles (snRNPs) inside Cajal bodies. There, three single snRNPs are assembled to one tri-snRNP, a subunit of the spliceosom. Another direct binding partner of nuclear SMN is the 23 kDa isoform of fibroblast growth factor – 2 (FGF-223). The FGF-223/SMN interaction is already known to destabilize nuclear gems, the second nuclear compartment SMN is associated to. By using fluorescence recovery after photobleaching (FRAP) and photoconversion experiments in the present study of Forthmann et al., 2013 (Chapter I), we have discovered that FGF-223/SMN complex formation leads to a release of immobile SMN from CBs, followed by snRNP accumulation. Thus, tri-snRNP assembly at Cajal bodies seems to be associated with immobile SMN. In an unpublished study of Forthmann et al. (Chapter II), we show that co-expression of FGF-223 and SMN inhibits both FGF-223- dependent transcription and SMN promoted neurite outgrowth. Hence, we propose a model in which FGF-223 and SMN form an inactive complex, antagonizing both protein functions. Moreover, there is evidence to suggest that neuronal differentiation is likewise affected by diffuse nucleoplasmic coilin. Endogenous coilin levels decrease during differentiation of human neuroblastoma cells (NB), as it is already pointed out for murine PC12 cells. Expression of coilin reduces neurite outgrowth, indicating a new function of this protein in inhibition of neuronal differentiation. While SMN has coilin and FGF-223 as interaction partners, among others, FGF-223 has another nuclear binding partner beside SMN: the fibroblast growth factor receptor 1 (FGFR1). FGF-223/FGFR1 interaction heightens neuronal differentiation by inducing a pathway called integrative nuclear FGFR1 signaling (INFS). The INFS activates the tyrosine hydroxylase (TH) gene. During the development of TH-positive cells in the ventral midbrain (VM) of embryonic mice, the FGFR1 builds a nuclear complex with the orphan nuclear receptor Nurr1, as we could demonstrate in the study of Baron et al., 2012 (Chapter III). FRAP experiments reveal a dynamic change: FGFR1 is slowed down by interaction with Nurr1 into a chromatin-bound fraction. The TH gene promoter is activated by FGFR1/Nurr1 complex formation, similar to the FGFR1/FGF-223 binding. In conclusion, the regulation of the presented nuclear body and protein complexes influences neuronal differentiation and neurite outgrowth in vitro and in vivo. We assume the nuclear architecture to be relevant in differentiation processes of developing neuronal cells and therefore to be a possible target in neurodegenerative and neurodevelopmental disorders.