Semaphorin heterodimerization in cis regulates membrane targeting and neocortical wiring.

Disruption of neocortical circuitry and architecture in humans causes numerous neurodevelopmental disorders. Neocortical cytoarchitecture is orchestrated by various transcription factors such as Satb2 that control target genes during strict time windows. In humans, mutations of SATB2 cause SATB2 Associated Syndrome (SAS), a multisymptomatic syndrome involving epilepsy, intellectual disability, speech delay, and craniofacial defects. Here we show that Satb2 controls neuronal migration and callosal axonal outgrowth during murine neocortical development by inducing the expression of the GPI-anchored protein, Semaphorin 7A (Sema7A). We find that Sema7A exerts this biological activity by heterodimerizing in cis with the transmembrane semaphorin, Sema4D. We could also observe that heterodimerization with Sema7A promotes targeting of Sema4D to the plasma membrane in vitro. Finally, we report an epilepsy-associated de novo mutation in Sema4D (Q497P) that inhibits normal glycosylation and plasma membrane localization of Sema4D-associated complexes. These results suggest that neuronal use of semaphorins during neocortical development is heteromeric, and a greater signaling complexity exists than was previously thought.

SARS-CoV-2 Triggering Severe Acute Respiratory Distress Syndrome and Secondary Hemophagocytic Lymphohistiocytosis in a 3-Year-Old Child With Down Syndrome.

Down syndrome (DS) predisposes to severe immunologic reaction secondary to infectious triggers. Here, we report a pediatric DS patient with coronavirus disease 2019 (COVID-19) who developed a hyperinflammatory syndrome, severe acute respiratory distress syndrome, and secondary hemophagocytic lymphohistiocytosis requiring pediatric intensive care unit admission and treatment with steroids, intravenous immunoglobulin, and remdesivir. Investigations into genetic susceptibilities for COVID-19 and severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2)-associated complications warrant systematic clinical and scientific studies. We report a pediatric Down syndrome patient with coronavirus disease 2019 (COVID-19) who developed secondary hemophagocytic lymphohistiocytosis requiring treatment with steroids, intravenous immunoglobulin, and remdesivir. Investigations into genetic susceptibilities for COVID-19-associated complications warrant systematic clinical and scientific studies.

Defective Zn2+ homeostasis in mouse and human platelets with α- and δ-storage pool diseases.

Zinc (Zn2+) can modulate platelet and coagulation activation pathways, including fibrin formation. Here, we studied the (patho)physiological consequences of abnormal platelet Zn2+ storage and release. To visualize Zn2+ storage in human and mouse platelets, the Zn2+ specific fluorescent dye FluoZin3 was used. In resting platelets, the dye transiently accumulated into distinct cytosolic puncta, which were lost upon platelet activation. Platelets isolated from Unc13d-/- mice, characterized by combined defects of α/δ granular release, showed a markedly impaired Zn2+ release upon activation. Platelets from Nbeal2-/- mice mimicking Gray platelet syndrome (GPS), characterized by primarily loss of the α-granule content, had strongly reduced Zn2+ levels, which was also confirmed in primary megakaryocytes. In human platelets isolated from patients with GPS, Hermansky-Pudlak Syndrome (HPS) and Storage Pool Disease (SPD) altered Zn2+ homeostasis was detected. In turbidity and flow based assays, platelet-dependent fibrin formation was impaired in both Nbeal2-/- and Unc13d-/- mice, and the impairment could be partially restored by extracellular Zn2+. Altogether, we conclude that the release of ionic Zn2+ store from secretory granules upon platelet activation contributes to the procoagulant role of Zn2+ in platelet-dependent fibrin formation.

Functional Classification of Paediatric Patients with Non-syndromic Delta-Storage Pool Deficiency.

Storage pool disease (SPD) covers a group of platelet defects in which α- and/or delta-granules are reduced or cannot be secreted adequately in response to agonists. The detection of delta-granule release defects is hampered by a lack of fast and feasible tests. We aimed to implement a flow cytometry-based kinetic mepacrine assay to better identify and subgroup childhood patients with a mild to moderate bleeding diathesis and compare our method to established laboratory tests. We analysed 50 children with suspected SPD whose initial parameters were re-assessed in a second site visit. Mepacrine uptake and release patterns were correlated with CD63 exposure, platelet ADP/ATP release and content, and the bleeding score ascertained by the ISTH-BAT. Mepacrine release was overall significantly reduced in investigated patients compared with controls. Summarizing, our time-resolved approach proved to be a quick and inexpensive tool that was additionally able to distinguish between mepacrine uptake, mepacrine release, and combined defects. Classification of patients using such a kinetic assay makes it feasible to sensitively detect frequently missed SPD and to group these patients for further analyses and clinical correlations.