Reduced levels of survival of motoneuron (SMN) protein lead to spinal muscular 
atrophy, but it is still unknown how SMN protects motoneurons in the spinal cord 
against degeneration. In the nucleus, SMN is associated with two types of 
nuclear bodies denoted as gems and Cajal bodies (CBs). The 23 kDa isoform of 
fibroblast growth factor-2 (FGF-2(23)) is a nuclear protein that binds to SMN 
and destabilizes the SMN-Gemin2 complex. In the present study, we show that 
FGF-2(23) depletes SMN from CBs without affecting their general structure. FRAP 
analysis of SMN-EGFP in CBs demonstrated that the majority of SMN in CBs 
remained mobile and allowed quantification of fast, slow and immobile nuclear 
SMN populations. The potential for SMN release was confirmed by in vivo 
photoconversion of SMN-Dendra2, indicating that CBs concentrate immobile SMN 
that could have a specialized function in CBs. FGF-2(23) accelerated SMN release 
from CBs, accompanied by a conversion of immobile SMN into a mobile population. 
Furthermore, FGF-2(23) caused snRNP accumulation in CBs. We propose a model in 
which Cajal bodies store immobile SMN that can be mobilized by its nuclear 
interaction partner FGF-2(23), leading to U4 snRNP accumulation in CBs, 
indicating a role for immobile SMN in tri-snRNP assembly.