In biological systems, liquid and solid-like biomolecular condensates may contain the same molecules but their behaviour, including movement, elasticity and viscosity, is different on account of distinct physicochemical properties. As such, it is known that phase transitions affect the function of biological condensates and that material properties can be tuned by several factors including temperature, concentration and valency. It is, however, unclear if some factors are more efficient than others at regulating their behaviour. Viral infections are good systems to address this question as they form condensates de novo as part of their replication programmes. Here, we used influenza A virus liquid cytosolic condensates, A.K.A viral inclusions, to provide a proof of concept that liquid condensate hardening via changes in the valency of its components is more efficient than altering their concentration or the temperature of the cell. Liquid IAV inclusions may be hardened by targeting vRNP interactions via the known NP oligomerizing molecule, nucleozin, both in vitro and in vivo without affecting host proteome abundance nor solubility. This study is a starting point for understanding how to pharmacologically modulate the material properties of IAV inclusions and may offer opportunities for alternative antiviral strategies.

Iron refractory iron deficiency anemia (IRIDA) is an autosomal recessive ferropenic anemia. Its hypochromic microcytic pattern is associated with low transferrin saturation, normal-high ferritin, and inappropriately high hepcidin level. This entity is caused by mutants of the TMPRSS6 gene that encodes the protein matriptase II, which influences hepcidin expression, an iron metabolism counterregulatory protein. We report two 29-year-old dizygotic female twins with ferropenic, hypochromic microcytic anemia with 20 years of evolution, refractory to oral iron therapy. After exclusion of gastrointestinal etiologies, IRIDA diagnosis was suspected and a novel mutation in the TMPRSS6 gene was identified. It was found in intron 11 (c.1396+4 A>T) and seems to affect the gene expression. In addition, 3 polymorphisms already associated with a higher risk of developing iron deficiency anemia were also found (D521D, V736A, and Y739Y). Our case reports an undescribed mutation causing IRIDA and supports the hypothesis that this clinical syndrome may be more common than previously thought and its genetics more heterogeneous than initially described.

Multiple viral infections form biomolecular condensates in the host cell to facilitate viral replication1. Accumulating evidence indicates that these viral condensates rely on specific material properties for function 2, but how these properties may be altered efficiently remains vastly unknown. Here, we use influenza A virus liquid cytosolic condensates 3, A.K.A viral inclusions, to provide a proof of concept that stabilizing transient interactions among the interactome in IAV inclusions more efficiently hardens these structures than varying temperature or concentration both in in situ and in in vivo models. This stabilization can be achieved by drug targeting, inducing changes in the solubility of viral proteome without affecting host cellular proteome. Our work supports the development of antivirals targeting the material properties of biomolecular condensates in viral infections. It also provides a framework for the efficient selection of pharmacological compounds with this activity and thus provides an advance in disease therapy.

Many viruses that threaten public health establish condensates via phase transitions to complete their lifecycles, and knowledge on such processes is key for the design of new antivirals. In the case of influenza A virus, liquid condensates known as viral inclusions are sites dedicated to the assembly of its 8-partite RNA genome. Liquid viral inclusions emerge near the endoplasmic reticulum (ER) exit sites, but we lack the molecular understanding on how the ER contributes to their biogenesis. We show here that viral inclusions develop at remodeled ER sites and display dynamic interactions using the ER, including fusion and fission events and sliding movements. We also uncover a novel role for the host factor, ATG9A, in mediating the exchange of viral inclusions between the ER and microtubules. Depletion of ATG9A arrests viral inclusions at microtubules and prevents their accumulation at the ER, leading to a significantly reduced production of viral genome complexes and infectious virions. In light of our recent findings, we propose that a remodeled ER supports the dynamics of liquid IAV inclusions, with ATG9A acting locally to facilitate their formation. This work advances our current knowledge regarding influenza genome assembly, but also reveals new roles for ATG9A beyond its classical involvement in autophagy.