The matrix protein Tiggrin regulates plasmatocyte maturation in Drosophila larva

The lymph gland (LG) is a major source of hematopoiesis during Drosophila development. In this tissue, prohemocytes differentiate into multiple lineages including macrophage-like plasmatocytes, which comprise the vast majority of mature hemocytes. Previous studies have uncovered genetic pathways that regulate prohemocyte maintenance and some cell fate choices between hemocyte lineages. However, less is known about how the plasmatocyte pool of the LG is established and matures. Here we report that Tiggrin, a matrix protein expressed in the LG, is a specific regulator of plasmatocyte maturation. Tiggrin mutants exhibit precocious maturation of plasmatocytes, while Tiggrin overexpression blocks this process, resulting in a buildup of intermediate progenitors (IPs) expressing prohemocyte and hemocyte markers. These IPs likely represent a transitory state in prohemocyte to plasmatocyte differentiation. We also found that overexpression of Wee1 kinase, which slows G2/M progression, results in a phenotype similar to Tiggrin overexpression while String/Cdc25 expression phenocopies Tiggrin mutants. Further analysis revealed that Wee1 inhibits plasmatocyte maturation through up-regulation of Tiggrin transcription. Our results elucidate connections between the extracellular matrix and cell cycle regulators in the regulation of hematopoiesis.