AGGF1 is an angiogenic factor with G‐Patch and FHA domains 1 described by our group. Gain‐of‐function mutations in AGGF1 cause Klippel–Trenaunay syndrome, whereas somatic loss‐of‐function mutations cause cancer. Paraspeckles are small membraneless subnuclear structures with a diameter of 0.5–1 μm, and composed of lncRNA NEAT1 as the scaffold and three core RNA‐binding proteins NONO, PSPC1, and PSF. Here, we show that AGGF1 is a key regulatory and structural component of paraspeckles that induces paraspeckle formation, forms an outside rim of paraspeckles, wraps around the NONO/PSF/PSPC1/NEAT1 core, and regulates the size and number of paraspeckles. AGGF1‐paraspeckles are larger (>1 μm) than conventional paraspeckles. RNA‐FISH in combination with immunostaining shows that AGGF1, NONO, and NEAT1_2 co‐localize in 20.58% of NEAT1_2‐positive paraspeckles. Mechanistically, AGGF1 interacts with NONO, PSF, and HNRNPK, and upregulates NEAT1_2, a longer, 23 kb NEAT1 transcript with a key role in regulation of paraspeckle size and number. RNA‐immunoprecipitation shows that AGGF1 interacts with NEAT1, which may be another possible mechanism underlying the formation of AGGF1‐paraspeckles. NEAT1_2 knockdown reduces the number and size of AGGF1‐paraspeckles. Functionally, AGGF1 regulates alternative RNA splicing as it decreases the exon skipping/inclusion ratio in a CD44 model. AGGF1 is also localized in some nuclear foci without NEAT1 or NONO, suggesting that AGGF1 is an important liquid–liquid phase separation (LLPS) driver for other types of AGGF1‐positive nuclear condensates (referred to as AGGF1‐bodies). Our results identify a special type of AGGF1‐coated paraspeckles and provide important insights into the formation, structure, and function of paraspeckles.

Background Skeletal muscle atrophy is a common condition without a pharmacologic therapy. AGGF1 encodes an angiogenic factor that regulates cell differentiation, proliferation, migration, apoptosis, autophagy and endoplasmic reticulum stress, promotes vasculogenesis and angiogenesis and successfully treats cardiovascular diseases. Here, we report the important role of AGGF1 in the pathogenesis of skeletal muscle atrophy and attenuation of muscle atrophy by AGGF1. Methods In vivo studies were carried out in impaired leg muscles from patients with lumbar disc herniation, two mouse models for skeletal muscle atrophy (denervation and cancer cachexia) and heterozygous Aggf1+/− mice. Mouse muscle atrophy phenotypes were characterized by body weight and myotube cross‐sectional areas (CSA) using H&E staining and immunostaining for dystrophin. Molecular mechanistic studies include co‐immunoprecipitation (Co‐IP), western blotting, quantitative real‐time PCR analysis and immunostaining analysis. Results Heterozygous Aggf1+/− mice showed exacerbated phenotypes of reduced muscle mass, myotube CSA, MyHC (myosin heavy chain) and α‐actin, increased inflammation (macrophage infiltration), apoptosis and fibrosis after denervation and cachexia. Intramuscular and intraperitoneal injection of recombinant AGGF1 protein attenuates atrophy phenotypes in mice with denervation (gastrocnemius weight 81.3 ± 5.7 mg vs. 67.3 ± 5.1 mg for AGGF1 vs. buffer; P < 0.05) and cachexia (133.7 ± 4.7 vs. 124.3 ± 3.2; P < 0.05). AGGF1 expression undergoes remodelling and is up‐regulated in gastrocnemius and soleus muscles from atrophy mice and impaired leg muscles from patients with lumbar disc herniation by 50–60% (P < 0.01). Mechanistically, AGGF1 interacts with TWEAK (tumour necrosis factor‐like weak inducer of apoptosis), which reduces interaction between TWEAK and its receptor Fn14 (fibroblast growth factor‐inducing protein 14). This leads to inhibition of Fn14‐induced NF‐kappa B (NF‐κB) p65 phosphorylation, which reduces expression of muscle‐specific E3 ubiquitin ligase MuRF1 (muscle RING finger 1), resulting in increased MyHC and α‐actin and partial reversal of atrophy phenotypes. Autophagy is reduced in Aggf1+/− mice due to inhibition of JNK (c‐Jun N‐terminal kinase) activation in denervated and cachectic muscles, and AGGF1 treatment enhances autophagy in two atrophy models by activating JNK. In impaired leg muscles of patients with lumbar disc herniation, MuRF1 is up‐regulated and MyHC and α‐actin are down‐regulated; these effects are reversed by AGGF1 by 50% (P < 0.01). Conclusions These results indicate that AGGF1 is a novel regulator for the pathogenesis of skeletal muscle atrophy and attenuates skeletal muscle atrophy by promoting autophagy and inhibiting MuRF1 expression through a molecular signalling pathway of AGGF1‐TWEAK/Fn14‐NF‐κB. More importantly, the results indicate that AGGF1 protein therapy may be a novel approach to treat patients with skeletal muscle atrophy.