Ablation of cyclase-associated protein 2 (CAP2) leads to cardiomyopathy

Cyclase-associated proteins are highly conserved proteins that have a role in the regulation of actin dynamics. Higher eukaryotes have two isoforms, CAP1 and CAP2. To study the in vivo function of CAP2, we generated mice in which the CAP2 gene was inactivated by a gene-trap approach. Mutant mice showed a decrease in body weight and had a decreased survival rate. Further, they developed a severe cardiac defect marked by dilated cardiomyopathy (DCM) associated with drastic reduction in basal heart rate and prolongations in atrial and ventricular conduction times. Moreover, CAP2-deficient myofibrils exhibited reduced cooperativity of calcium-regulated force development. At the microscopic level, we observed disarrayed sarcomeres with development of fibrosis. We analyzed CAP2’s role in actin assembly and found that it sequesters G-actin and efficiently fragments filaments. This activity resides completely in its WASP homology domain. Thus CAP2 is an essential component of the myocardial sarcomere and is essential for physiological functioning of the cardiac system, and a deficiency leads to DCM and various cardiac defects.     

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

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-2²³) is a nuclear protein that binds to SMN and destabilizes the SMN-Gemin2 complex. In the present study, we show that FGF-2²³ 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-2²³ accelerated SMN release from CBs, accompanied by a conversion of immobile SMN into a mobile population. Furthermore, FGF-2²³ 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-2²³, leading to U4 snRNP accumulation in CBs, indicating a role for immobile SMN in tri-snRNP assembly.