The role of TGF-ß signaling in muscle atrophy, sarcopenia and cancer cachexia.

Skeletal muscle, comprising a significant proportion (40 to 50 percent) of total body weight in humans, plays a critical role in maintaining normal physiological conditions. Muscle atrophy occurs when the rate of protein degradation exceeds protein synthesis. Sarcopenia refers to age-related muscle atrophy, while cachexia represents a more complex form of muscle wasting associated with various diseases such as cancer, heart failure, and AIDS. Recent research has highlighted the involvement of signaling pathways, including IGF1-Akt-mTOR, MuRF1-MAFbx, and FOXO, in regulating the delicate balance between muscle protein synthesis and breakdown. Myostatin, a member of the TGF-ß superfamily, negatively regulates muscle growth and promotes muscle atrophy by activating Smad2 and Smad3. It also interacts with other signaling pathways in cachexia and sarcopenia. Inhibition of myostatin has emerged as a promising therapeutic approach for sarcopenia and cachexia. Additionally, other TGF-ß family members, such as TGF-ß1, activin A, and GDF11, have been implicated in the regulation of skeletal muscle mass. Furthermore, myostatin cooperates with these family members to impair muscle differentiation and contribute to muscle loss. This review provides an overview of the significance of myostatin and other TGF-ß signaling pathway members in muscular dystrophy, sarcopenia, and cachexia. It also discusses potential novel therapeutic strategies targeting myostatin and TGF-ß signaling for the treatment of muscle atrophy.

An Aerolysin-like Pore-Forming Protein Complex Targets Viral Envelope to Inactivate Herpes Simplex Virus Type 1.

Because most of animal viruses are enveloped, cytoplasmic entry of these viruses via fusion with cellular membrane initiates their invasion. However, the strategies in which host cells counteract cytoplasmic entry of such viruses are incompletely understood. Pore-forming toxin aerolysin-like proteins (ALPs) exist throughout the animal kingdom, but their functions are mostly unknown. In this study, we report that ßγ-crystallin fused aerolysin-like protein and trefoil factor complex (ßγ-CAT), an ALP and trefoil factor complex from the frog Bombina maxima, directly blocks enveloped virus invasion by interfering with cytoplasmic entry. ßγ-CAT targeted acidic glycosphingolipids on the HSV type 1 (HSV-1) envelope to induce pore formation, as indicated by the oligomer formation of protein and potassium and calcium ion efflux. Meanwhile, ßγ-CAT formed ring-like oligomers of ∼10 nm in diameter on the liposomes and induced dye release from liposomes that mimic viral envelope. Unexpectedly, transmission electron microscopy analysis showed that the ßγ-CAT-treated HSV-1 was visibly as intact as the vehicle-treated HSV-1, indicating that ßγ-CAT did not lyse the viral envelope. However, the cytoplasmic entry of the ßγ-CAT-treated HSV-1 into HeLa cells was totally hindered. In vivo, topical application of ßγ-CAT attenuated the HSV-1 corneal infection in mice. Collectively, these results uncovered that ßγ-CAT possesses the capacity to counteract enveloped virus invasion with its featured antiviral-acting manner. Our findings will also largely help to illustrate the putative antiviral activity of animal ALPs.

A secreted pore-forming protein modulates cellular endolysosomes to augment antigen presentation.

Bacterial pore-forming toxin aerolysin-like proteins are widely distributed in animals and plants. Emerging evidence supports their roles in host innate immunity, but their direct actions in adaptive immunity remain elusive. In this study, we found that ßγ-CAT, an aerolysin-like protein and trefoil factor complex identified in the frog Bombina maxima, modulated several steps of endocytic pathways during dendritic cell antigen presentation. The protein augmented the antigen uptake of dendritic cells and actively neutralized the acidification of cellular endocytic organelles to favor antigen presentation. In addition, the release of functional exosome-like extracellular vesicles was largely enhanced in the presence of ßγ-CAT. The cellular action of ßγ-CAT increased the number of major histocompatibility complex (MHC) I-ovalbumin and MHC II molecules on dendritic cell surfaces and the released exosome-like extracellular vesicles. An enhanced antigen presentation capacity of dendritic cell for priming of naive T cells was detected in the presence of ßγ-CAT. Collectively, these effects led to strong cytotoxic T lymphocyte responses and antigen-specific antibody responses. Our findings provide evidence that a vertebrate-secreted pore-forming protein can augment antigen presentation by directly modulating cellular endocytic and exocytic pathways, leading to robust activation of adaptive immunity.

Pore-forming toxin-like protein complex expressed by frog promotes tissue repair.

Tissue repair is a highly dynamic process, and the immediate onset of acute inflammation has been considered necessary for repair. Pore-forming proteins are important, both in pathogen invasion and host immunity. However, their roles in wound healing and tissue repair are unclear. ßγ-crystallin fused aerolysin-like protein (α-subunit) and trefoil factor (ß-subunit) complex (ßγ-CAT) is a complex of a bacterial pore-forming toxin aerolysin-like protein and trefoil factor identified in the frog Bombina maxima. In this study, we established mouse cutaneous wound models to explore the effects of ßγ-CAT on skin wound healing. ßγ-CAT accelerated the healing of full-thickness wounds by improving re-epithelialization. This complex relieved dermal edema and promoted scarless healing. ßγ-CAT treatment resulted in a rapid release of IL-1ß, which initiated an acute inflammation response in the early stage of healing. Meanwhile, the expression levels of TGF-ß1, VEGF, and bFGF and the recruitment of M2 macrophages around the wound significantly increased after ßγ-CAT treatment. ßγ-CAT protected skin wounds against methicillin-resistant Staphylococcus aureus by improving neutrophil recruitment at the site of the wound. Overall, our results suggest that ßγ-CAT can promote tissue repair and protect skin wounds against antibiotic-resistant bacterial infection by triggering the acute inflammatory response. This is the first example that aerolysin-like pore-forming proteins widely existing in plants and animals may act in wound healing and tissue repair.-Gao, Z.-H., Deng, C.-J., Xie, Y.-Y., Guo, X.-L., Wang, Q.-Q., Liu, L.-Z., Lee, W.-H., Li, S.-A., Zhang, Y. Pore-forming toxin-like protein complex expressed by frog promotes tissue repair.

Animal secretory endolysosome channel discovery.

Secretory pore-forming proteins (PFPs) have been identified in organisms from all kingdoms of life. Our studies with the toad species Bombina maxima found an interaction network among aerolysin family PFPs (af-PFPs) and trefoil factors (TFFs). As a toad af-PFP, BmALP1 can be reversibly regulated between active and inactive forms, with its paralog BmALP3 acting as a negative regulator. BmALP1 interacts with BmTFF3 to form a cellular active complex called ßγ-CAT. This PFP complex is characterized by acting on endocytic pathways and forming pores on endolysosomes, including stimulating cell macropinocytosis. In addition, cell exocytosis can be induced and/or modulated in the presence of ßγ-CAT. Depending on cell contexts and surroundings, these effects can facilitate the toad in material uptake and vesicular transport, while maintaining mucosal barrier function as well as immune defense. Based on experimental evidence, we hereby propose a secretory endolysosome channel (SELC) pathway conducted by a secreted PFP in cell endocytic and exocytic systems, with ßγ-CAT being the first example of a SELC protein. With essential roles in cell interactions and environmental adaptations, the proposed SELC protein pathway should be conserved in other living organisms.