Monocytes Release Pro-Cathepsin D to Drive Blood-to-Brain Transcytosis in Diabetes.

BACKGROUND: Microvascular complications are the major outcome of type 2 diabetes progression, and the underlying mechanism remains to be determined. METHODS: High-throughput RNA sequencing was performed using human monocyte samples from controls and diabetes. The transgenic mice expressing human CTSD (cathepsin D) in the monocytes was constructed using CD68 promoter. In vivo 2-photon imaging, behavioral tests, immunofluorescence, transmission electron microscopy, Western blot analysis, vascular leakage assay, and single-cell RNA sequencing were performed to clarify the phenotype and elucidate the molecular mechanism. RESULTS: Monocytes expressed high-level CTSD in patients with type 2 diabetes. The transgenic mice expressing human CTSD in the monocytes showed increased brain microvascular permeability resembling the diabetic microvascular phenotype, accompanied by cognitive deficit. Mechanistically, the monocytes release nonenzymatic pro-CTSD to upregulate caveolin expression in brain endothelium triggering caveolae-mediated transcytosis, without affecting the paracellular route of brain microvasculature. The circulating pro-CTSD activated the caveolae-mediated transcytosis in brain endothelial cells via its binding with low-density LRP1 (lipoprotein receptor-related protein 1). Importantly, genetic ablation of CTSD in the monocytes exhibited a protective effect against the diabetes-enhanced brain microvascular transcytosis and the diabetes-induced cognitive impairment. CONCLUSIONS: These findings uncover the novel role of circulatory pro-CTSD from monocytes in the pathogenesis of cerebral microvascular lesions in diabetes. The circulatory pro-CTSD is a potential target for the intervention of microvascular complications in diabetes.

Calmodulin triggers activity-dependent rRNA biogenesis via interaction with DDX21.

Protein synthesis in response to neuronal activity, known as activity-dependent translation, is critical for synaptic plasticity and memory formation. However, the signaling cascades that couple neuronal activity to the translational events remains elusive. In this study, we identified the role of calmodulin (CaM), a conserved Ca2+-binding protein, in rRNA biogenesis in neurons. We found the CaM-regulated rRNA synthesis is Ca2+-dependent and necessary for nascent protein synthesis and axon growth in hippocampal neurons. Mechanistically, CaM interacts with nucleolar DDX21 in a Ca2+-dependent manner to regulate nascent rRNA transcription within nucleoli. We further found CaM alters the conformation of DDX21 to liberate the DDX21-sequestered RPA194, the catalytic subunit of RNA polymerase I, to facilitate transcription of rDNA. Using high-throughput screening, we identified the small molecules Batefenterol and Indacaterol that attenuate the CaM-DDX21 interaction and suppress nascent rRNA synthesis and axon growth in hippocampal neurons. These results unveiled the previously unrecognized role of CaM as a messenger to link the activity-induced Ca2+ influx to the nucleolar events essential for protein synthesis. We thus identified the ability of CaM to transmit information to the nucleoli of neurons in response to stimulation.Significance statement Protein synthesis in response to neuronal activity, known as activity-dependent translation, is critical for synaptic plasticity and long-term memory formation. In this study, we identify the novel role of calmodulin (CaM), a highly conserved Ca2+-binding protein, which is well-known by regulating myriad vital biological processes, in activity-dependent translation by regulating rRNA synthesis in neurons. We find that CaM can shuttle into the nucleolus upon depolarization and modulate the activity-induced de novo rRNA biogenesis, which is associated with ribosome assembly and protein synthesis in neurons. Mechanistically, CaM interacts with DDX21, an RNA helicase directly associated with Pol I subunit, to regulate the transcription of rDNA. Our study demonstrates CaM as a messenger linking neuronal activity to ribosome-dependent protein biosynthesis.