The critical roles for MT1-MMP in hydrocephalus and isthmin1 in kidney branching morphogenesis

Abstract

Hydrocephalus is characterized by abnormal accumulation of cerebrospinal fluid (CSF) in the ventricular cavity. The circulation of CSF in the brain ventricles is controlled by the coordinated beating of motile cilia at the surface of ependymal cells (ECs). ECs are essential for regulating the neurogenic niche and maintaining CSF flow. Thus, defects in motile cilia and ECs are closely related to hydrocephalus. Mice deficient for Membrane type-1 matrix metalloproteinase (MT1-MMP) exhibit a dome-shaped skull and enlarged lateral ventricles after birth, suggesting a potential critical role for MT1-MMP in the development of hydrocephalus. The present work shows that in the central nervous system (CNS) MT1-MMP is highly expressed in glial cell lineage including radial glial cells, ECs and astrocytes. Loss of MT1-MMP in mice leads to reduced and disorganized motile cilia and the compromised maturation of ECs. Consistently, the pro-multiciliogenic transcription factors were significantly downregulated and the activation of Notch signaling was observed in Mmp14-/- mice. Inhibition of Notch signaling reversed the defective ciliogenesis in Mmp14-/- mice, indicating that MT-MMP knockout triggers defects in ciliogenesis of ECs through Notch hyperactivation. Together, this study reveals MT1-MMP is essential for ependymal cells maturation and ciliogenesis, and loss of MT1-MMP promotes hydrocephalus pathogenesis. In addition, impairment of ECs is often accompanied by abnormal development of other lineage cells. Loss of MT1-MMP in ECs disturbs the neurogenic activity in subventricular zone (SVZ). Moreover, more activated astrocytes were observed during the differentiation of neural stem cells isolated from MT1-MMP mutant SVZ, suggesting that MT1-MMP may also regulate the activation of astrocytes and maintenance of SVZ stem cells niche during early brain development in mouse. Isthmin1 (ISM1) is a secreted protein originally identified in the midbrain of Xenopus laevis. It contains two functional domains: thrombospondin (TSR) and adhesion-associated AMOP domain. ISM1 has been shown to have anti-angiogenic, anti-tumorigenic, and proapoptotic properties. However, the biological function of ISM1 in mammals remains largely unknown. This work shows that Ism1 deficient mice exhibit a spectrum of renal abnormalities including renal agenesis and dysgenesis. Ism1 mutant kidneys fail to generate T-shaped ureteric bud (UB) and mesenchymal condensation at E11.5, which subsequently result in majority of apoptosis in metanephric mesenchyme (MM). ISM1 is expressed mainly in condensed MM and weakly in UB at E11.5. The exogenous ISM1 promotes branching in cultured kidney explants ex vivo, suggesting that ISM1 plays a critical role in renal branching morphogenesis. At the initiation of the first branching event, lack of Gdnf was observed in mutant MM; the exogenous GDNF rescued the defects in UB branching in cultured kidney explants, implicated that the defective branching morphogenesis in Ism1-/- kidneys may be attribute to the absence of GDNF and ISM1 may be involved in the regulation of GDNF production. Moreover, genes in the regulation of actin cytoskeleton organization, cell motility and integrin pathway, essential for mesenchymal condensation, were significantly downregulated in mutant kidney MM identified by scRNA- Seq analysis, which gives more evidence that ISM1 may be associated with the regulation of MM condensation. My current data indicate that ISM1 is required for renal branching morphogenesis and may regulate GDNF/Ret signaling by controlling the condensation of mesenchymal cells in the developing mouse kidney. This work will help to understand the biological function of ISM1 and kidney development, but the molecular mechanisms need to be further investigated.