Crossing the blood-brain barrier is a crucial, rate-limiting step of brain metastasis. Understanding of the mechanisms of cancer cell extravasation from brain microcapillaries is limited as the underlying cellular and molecular processes cannot be adequately investigated using in vitro models and end-point in vivo experiments. Using ultrastructural and functional imaging, we demonstrate that dynamic changes of activated brain microcapillaries promote the mandatory first steps of brain colonization. Successful extravasation of arrested cancer cells occurred when adjacent capillary endothelial cells (ECs) entered into a distinct remodeling process. After extravasation, capillary loops were formed, which was characteristic of aggressive metastatic growth. Upon cancer cell arrest in brain microcapillaries, matrix-metalloprotease 9 (MMP9) was expressed. Inhibition of MMP2/9 and genetic perturbation of MMP9 in cancer cells, but not the host, reduced EC projections, extravasation, and brain metastasis outgrowth. These findings establish an active role of ECs in the process of cancer cell extravasation, facilitated by crosstalk between the two cell types. This extends our understanding of how host cells can contribute to brain metastasis formation and how to prevent it.

Short stature is a common phenotype in children with Schaaf–Yang syndrome (SYS). Prader–Willi syndrome (PWS) and SYS share several phenotypic features including short stature, muscular hypotonia and developmental delay/intellectual disability. Evidence exists that similar to PWS, growth hormone (GH) deficiency may also be a feature of SYS. Recombinant human GH (rhGH) therapy has been approved for PWS, but the effects of rhGH therapy in individuals with SYS have not yet been documented. This retrospective, questionnaire‐based study analyzes the prevalence of rhGH therapy in children with SYS, the effects of rhGH therapy on anthropometric measures, and parental perception of the treatment. Twenty‐six individuals with SYS were sent a clinical questionnaire and a request for growth charts. We found a significant increase in height z‐score (p* = 0.04) as well as a significant decrease in body mass index 6 months after rhGH therapy initiation (p* = 0.04). Furthermore, height z‐scores of the treated group (mean z‐score = −1.00) were significantly higher than those of the untreated group (mean z‐score = −3.36, p = 0.01) at time of enrollment. All parents reported an increase in muscle strength and endurance, and several families noted beneficial effects such as improved cognition and motor development.