Immobile survival of motoneuron (SMN) protein stored in Cajal bodies can be mobilized by protein interactions

Abstract

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.

NUFIP and the HSP90/R2TP chaperone bind the SMN complex and facilitate assembly of U4-specific proteins

“…Interestingly, it was shown that the release of SMN from CBs by FGF-2 expression leads to an accumulation of Sm proteins and U4 snRNA in CBs ( 63 ). According to our model, this might result from a defect in the assembly of U4-specific proteins.…”

Signals controlling Cajal body assembly and function

“…SMN (survivor of motor neuron) is mutated in most cases of spinal muscular atrophy (SMA), and plays a crucial role in the cytoplasmic phase of snRNP biogenesis. The role of SMN in the CB is less clear, but may involve snRNA regeneration (Pellizzoni et al, 1998) and/or tri-snRNP assembly (Boulisfane et al, 2011, Forthmann et al, 2013). Coilin is known as the CB marker protein and is essential for canonical CB formation (Figure 1).…”

Regulation of Neuronal Differentiation by Proteins Associated with Nuclear Bodies

“…Neuroblastoma SK-N-BE(2) cells were differentiated for different periods with retinoic acid (5 µM), lysed by sonification in modified RIPA-buffer [7] and analyzed by Western blot for coilin, SMN and α-Tubulin (A). (B) Endogenous coilin protein levels were decreased significantly to 52% after 72h of differentiation, (C) while SMN protein levels were increased non-significantly after the same time of differentiation.…”

“…Immunocytochemistry and fluorescence microscopy were performed as described previously [7]. Antibodies used were anti-coilin (H-300, rabbit IgG, Santa Cruz), anti-SMN (mouse IgG, BD Bioscience), anti-α-tubulin (mouse IgG, Santa Cruz) and secondary antibodies Alexa Fluor® 488 goat anti-mouse IgG, Alexa Fluor® 555 goat anti-mouse IgG and Alexa Fluor® 555 goat anti-rabbit IgG (Invitrogen) or anti-mouse peroxidase-linked (sheep, Amersham) and anti-rabbit peroxidase-linked (goat IgG, Jackson ImmunoResearch).…”

“…Cellular decrease of SMN is responsible for pathogenesis of the neurodegenerative disease Spinal Muscular Atrophy [6]. The nuclear 23 kDa isoform of the Fibroblast Growth Factor – 2 (FGF-2 23 ) exhibits an antagonistic effect on the accumulation of SMN in CBs [7]. FGF-2 is not only an extracellular factor binding to receptor tyrosine kinase receptors but also expressed as different isoforms with intracellular functions [8].…”