Jin Xu

Axonal regeneration in the central nervous system is imperative for functional restoration following spinal cord injury (SCI). Myeloid cells are key regulators of axonal regeneration, yet their roles are not fully revealed. SCI perturbs glucose metabolism; however, its precise impact on axonal regeneration remains undefined. Moreover, whether myeloid cells orchestrate glucose metabolic responses to facilitate regeneration is unclear. Here, using the zebrafish Mauthner cell axon transection model, we demonstrate that following SCI, myeloid cell deficiency leads to a late-stage glucose surge, which leads to impaired axonal regeneration. We further identify glucagon signalling as a critical molecular determinant of this metabolic dysregulation and show that targeted mutations in gcga or its receptors (gcgra, gcgrb) rescue the axonal regeneration defects caused by myeloid cell deficiency. Finally, cell-depletion experiments demonstrated that macrophages are responsible for the late-stage hyperglycemia and defective axon regeneration of Mauthner cells. These findings suggest that glucose metabolism plays a critical role in macrophage-warranted axon regeneration in the spinal cord, positioning glucose homeostasis as a potential therapeutic target for enhancing axon regeneration and recovery.

Approximately 30% of patients with myelodysplastic syndrome (MDS) progress to secondary acute myeloid leukemia (sAML) via accumulating gene mutations. Genomic analyses reveal a complex interplay among mutant genes, with co-occurring and mutually exclusive patterns. Hyperactivation of c-MYB and deficiency of PU.1 have been linked to myeloid disorders. We report a case of AML with concurrent PU.1 and c-MYB mutations, exhibiting early onset, high blast count, chemo-resistance, indicating high-risk features, along with elevated Pelger–Huët anomaly (PHA). However, the synergistic mechanism of c-MYB and PU.1 in sAML remains unclear. Using c-Myb-hyperactivation and Pu.1-deficient double-strain ( c-myb ^hyper ;pu.1 ^G242D/G242D ) zebrafish, we investigated MDS/sAML progression. Surprisingly, the double mutant exhibited a distinct type of neutrophil resembling clinical PHA cells and demonstrated a higher rate of MDS/sAML transformation. Further expression analysis revealed reduced lmnb1 expression in double-mutant zebrafish. Knockdown of lmnb1 resulted in PHA and increased blast cells, while overexpression of lmnb1 in c-myb ^hyper ;pu.1 ^G242D/G242D reduced PHA cell level. This suggests that c-Myb hyperactivation and Pu.1 deficiency synergistically reduce lmnb1 expression, inducing the development of PHA-like neutrophils and promoting MDS/sAML progression in zebrafish. Moreover, coadministration of cell cycle inhibitor cytarabine (Ara-C) and the differential inducer all-trans retinoic acid (ATRA) could effectively relieve the neutrophil expansion and PHA symptoms in c-myb ^hyper ;pu.1 ^G242D/G242D zebrafish. Our findings revealed that c-Myb hyperactivation and Pu.1 deficiency played a synergistic role in sAML development and suggests a phenotypic association between the emergence of PH-like cells and the transformation to sAML. Furthermore, c-myb ^hyper ;pu.1 ^G242D/G242D zebrafish might serve as a suitable sAML model for drug screening.

Hemolysis induces tissue damage by releasing cellular contents into the plasma. It is widely accepted that hemolysis-derived toxic molecules are cleared by macrophages or metabolized in hepatocytes. In zebrafish, we found that scavenger endothelial cells (SECs), a specialized endothelium with remarkable endocytosis capability, engulf both macromolecular hemoglobin (Hb) and small molecular unconjugated bilirubin (UCB), two primary toxic byproducts of hemolysis. These engulfment processes are mediated by the scavenger receptor Stab2. To demonstrate the protective function of SECs during hemolysis, we employed a zebrafish model of erythropoietic porphyria, characterized by excessive protoporphyrin IX (PPIX) accumulation due to ferrochelatase mutation, leading to light-sensitive hemolysis and larva death. We found that SECs facilitate the clearance of excess PPIX via Stab2, thereby mitigating PPIX-induced larval mortality. In addition, mouse SECs possess a conserved capability of scavenging Hb/UCB/PPIX. In conclusion, our study identifies SECs as a detoxification system during physiological and pathological hemolysis, shedding light on their protective role against hemolysis-induced damage.

Microglia, the crucial immune cells inhabiting the central nervous system (CNS), perform a range of vital functions, encompassing immune defense and neuronal regulation. Microglia subsets with diverse functions and distinct developmental regulations have been identified recently. It is generally accepted that all microglia originate from hematopoiesis and depend on the myeloid transcription factor PU.1. However, a recent study reported the existence of mrc1 ⁺ microglia in zebrafish embryos, which are seemingly independent of Pu.1 and reliant on lymphatic vessels, sparking great interest in the possibility of lymphatic-originated microglia. To address this, we took advantage of a pu.1 knock-in zebrafish allele for a detailed investigation. Our results conclusively showed that almost all zebrafish embryonic microglia (~95% on average) express pu.1 . Further, lineage tracing and mutant analysis revealed that these microglia neither emerged from nor depended on lymphatic vessels. In essence, our study refutes the presence of pu.1 -independent but lymphatic-dependent microglia.

Styxl2, a poorly characterized pseudophosphatase, was identified as a transcriptional target of the Jak1-Stat1 pathway during myoblast differentiation in culture. Styxl2 is specifically expressed in vertebrate striated muscles. By gene knockdown in zebrafish or genetic knockout in mice, we found that Styxl2 plays an essential role in maintaining sarcomere integrity in developing muscles. To further reveal the functions of Styxl2 in adult muscles, we generated two inducible knockout mouse models: one with Styxl2 being deleted in mature myofibers to assess its role in sarcomere maintenance, and the other in adult muscle satellite cells (MuSCs) to assess its role in de novo sarcomere assembly. We find that Styxl2 is not required for sarcomere maintenance but functions in de novo sarcomere assembly during injury-induced muscle regeneration. Mechanistically, Styxl2 interacts with non-muscle myosin IIs, enhances their ubiquitination, and targets them for autophagy-dependent degradation. Without Styxl2, the degradation of non-muscle myosin IIs is delayed, which leads to defective sarcomere assembly and force generation. Thus, Styxl2 promotes de novo sarcomere assembly by interacting with non-muscle myosin IIs and facilitating their autophagic degradation.

During the development of the vertebrate nervous system, 50% of the nerve cells undergo apoptosis shortly after formation. This process is important for sculpting tissue during morphogenesis and removing transiently functional cells that are no longer needed, ensuring the appropriate number of neurons in each region. Dysregulation of neuronal apoptosis can lead to neurodegenerative diseases. However, the molecular events involved in activating and regulating the neuronal apoptosis program are not fully understood. In this study, we identified several RB1 mutations in patients with neurodegenerative diseases. Then, we used a zebrafish model to investigate the role of Rb1 in neuronal apoptosis. We showed that Rb1-deficient mutants exhibit a significant hindbrain neuronal apoptosis, resulting in increased microglia infiltration. We further revealed that the apoptotic neurons in Rb1-deficient zebrafish were post-mitotic neurons, and Rb1 inhibits the apoptosis of these neurons by regulating bcl2/caspase through binding to Kmt5b. Moreover, using this zebrafish mutant, we verified the pathogenicity of the R621S and L819V mutations of human RB1 in neuronal apoptosis. Collectively, our data indicate that the Rb1-Kmt5b-caspase/bcl2 axis is crucial for protecting post-mitotic neurons from apoptosis and provides an explanation for the pathogenesis of clinically relevant mutations.

Animals evolve diverse pigment patterns to adapt to the natural environment. Countershading, characterized by a dark-colored dorsum and a light-colored ventrum, is one of the most prevalent pigment patterns observed in vertebrates. In this study, we reveal a mechanism regulating xanthophore countershading in zebrafish embryos. We found that csf1a and csf1b mutants altered xanthophore countershading differently: csf1a mutants lack ventral xanthophores, while csf1b mutants have reduced dorsal xanthophores. Further study revealed that csf1a is expressed throughout the trunk, whereas csf1b is expressed dorsally. Ectopic expression of csf1a or csf1b in neurons attracted xanthophores into the spinal cord. Blocking csf1 signaling by csf1ra mutants disrupts spinal cord distribution and normal xanthophores countershading. Single-cell RNA sequencing identified two col1a2 ⁺ populations: csf1a ^high csf1b ^high muscle progenitors and csf1a ^high csf1b ^low fibroblast progenitors. Ablation of col1a2 ⁺ fibroblast and muscle progenitors abolished xanthophore patterns. Our study suggests that fibroblast and muscle progenitors differentially express csf1a and csf1b to modulate xanthophore patterning, providing insights into the mechanism of countershading.