Identification of a truncated ß1-chimaerin variant that inactivates nuclear Rac1.

ß1-chimaerin belongs to the chimaerin family of GTPase-activating proteins (GAPs) and is encoded by the CHN2 gene, which also encodes the ß2- and ß3-chimaerin isoforms. All chimaerin isoforms have a C1 domain that binds diacylglycerol as well as tumor-promoting phorbol esters and a catalytic GAP domain that inactivates the small GTPase Rac. Nuclear Rac has emerged as a key regulator of various cell functions, including cell division, and has a pathological role by promoting tumorigenesis and metastasis. However, how nuclear Rac is regulated has not been fully addressed. Here, using several approaches, including siRNA-mediated gene silencing, confocal microscopy, and subcellular fractionation, we identified a nuclear variant of ß1-chimaerin, ß1-Δ7p-chimaerin, that participates in the regulation of nuclear Rac1. We show that ß1-Δ7p-chimaerin is a truncated variant generated by alternative splicing at a cryptic splice site in exon 7. We found that, unlike other chimaerin isoforms, ß1-Δ7p-chimaerin lacks a functional C1 domain and is not regulated by diacylglycerol. We found that ß1-Δ7p-chimaerin localizes to the nucleus via a nuclear localization signal in its N terminus. We also identified a key nuclear export signal in ß1-chimaerin that is absent in ß1-Δ7p-chimaerin, causing nuclear retention of this truncated variant. Functionally analyses revealed that ß1-Δ7p-chimaerin inactivates nuclear Rac and negatively regulates the cell cycle. Our results provide important insights into the diversity of chimaerin Rac-GAP regulation and function and highlight a potential mechanism of nuclear Rac inactivation that may play significant roles in pathologies such as cancer.

Endolysosomal dysfunction in radial glia progenitor cells leads to defective cerebral angiogenesis and compromised blood-brain barrier integrity.

The neurovascular unit (NVU) is a complex multicellular structure that helps maintain cerebral homeostasis and blood-brain barrier (BBB) integrity. While extensive evidence links NVU alterations to cerebrovascular diseases and neurodegeneration, the underlying molecular mechanisms remain unclear. Here, we use zebrafish embryos carrying a mutation in Scavenger Receptor B2, a highly conserved endolysosomal protein expressed predominantly in Radial Glia Cells (RGCs), to investigate the interplay among different NVU components. Through live imaging and genetic manipulations, we demonstrate that compromised acidification of the endolysosomal compartment in mutant RGCs leads to impaired Notch3 signaling, thereby inducing excessive neurogenesis and reduced glial differentiation. We further demonstrate that alterations to the neuron/glia balance result in impaired VEGF and Wnt signaling, leading to severe vascular defects, hemorrhages, and a leaky BBB. Altogether, our findings provide insights into NVU formation and function and offer avenues for investigating diseases involving white matter defects and vascular abnormalities.

Cellular Origins of the Lymphatic Endothelium: Implications for Cancer Lymphangiogenesis.

The lymphatic system plays important roles in physiological and pathological conditions. During cancer progression in particular, lymphangiogenesis can exert both positive and negative effects. While the formation of tumor associated lymphatic vessels correlates with metastatic dissemination, increased severity and poor patient prognosis, the presence of functional lymphatics is regarded as beneficial for anti-tumor immunity and cancer immunotherapy delivery. Therefore, a profound understanding of the cellular origins of tumor lymphatics and the molecular mechanisms controlling their formation is required in order to improve current strategies to control malignant spread. Data accumulated over the last decades have led to a controversy regarding the cellular sources of tumor-associated lymphatic vessels and the putative contribution of non-endothelial cells to this process. Although it is widely accepted that lymphatic endothelial cells (LECs) arise mainly from pre-existing lymphatic vessels, additional contribution from bone marrow-derived cells, myeloid precursors and terminally differentiated macrophages, has also been claimed. Here, we review recent findings describing new origins of LECs during embryonic development and discuss their relevance to cancer lymphangiogenesis.