Current models evoke the plasma membrane (PM) as the exclusive platform from which Ras regulates signalling. We developed a fluorescent probe that reports where and when Ras is activated in living cells. We show that oncogenic H-Ras and N-Ras engage Raf-1 on the Golgi and that endogenous Ras and unpalmitoylated H-Ras are activated in response to mitogens on the Golgi and endoplasmic reticulum (ER), respectively. We also demonstrate that H-Ras that is restricted to the ER can activate the Erk pathway and transform fibroblasts, and that Ras localized on different membrane compartments differentially engages various signalling pathways. Thus, Ras signalling is not limited to the PM, but also proceeds on the endomembrane.

Ras proteins regulate cellular growth and differentiation, and are mutated in 30% of cancers. We have shown recently that Ras is activated on and transmits signals from the Golgi apparatus as well as the plasma membrane but the mechanism of compartmentalized signalling was not determined. Here we show that, in response to Src-dependent activation of phospholipase Cgamma1, the Ras guanine nucleotide exchange factor RasGRP1 translocated to the Golgi where it activated Ras. Whereas Ca(2+) positively regulated Ras on the Golgi apparatus through RasGRP1, the same second messenger negatively regulated Ras on the plasma membrane by means of the Ras GTPase-activating protein CAPRI. Ras activation after T-cell receptor stimulation in Jurkat cells, rich in RasGRP1, was limited to the Golgi apparatus through the action of CAPRI, demonstrating unambiguously a physiological role for Ras on Golgi. Activation of Ras on Golgi also induced differentiation of PC12 cells, transformed fibroblasts and mediated radioresistance. Thus, activation of Ras on Golgi has important biological consequences and proceeds through a pathway distinct from the one that activates Ras on the plasma membrane.

Endothelial cells establish an instructive vascular niche that reconstitutes haematopoietic stem and progenitor cells (HSPCs) through release of specific paracrine growth factors, known as angiocrine factors. However, the mechanism by which endothelial cells balance the rate of proliferation and lineage-specific differentiation of HSPCs is unknown. Here, we demonstrate that Akt activation in endothelial cells, through recruitment of mTOR, but not the FoxO pathway, upregulates specific angiocrine factors that support expansion of CD34 − Flt3 − KLS HSPCs with long-term haematopoietic stem cell (LT-HSC) repopulation capacity. Conversely, co-activation of Akt-stimulated endothelial cells with p42/44 MAPK shifts the balance towards maintenance and differentiation of the HSPCs. Selective activation of Akt1 in the endothelial cells of adult mice increased the number of colony forming units in the spleen and CD34 − Flt3 − KLS HSPCs with LT-HSC activity in the bone marrow, accelerating haematopoietic recovery. Therefore, the activation state of endothelial cells modulates reconstitution of HSPCs through the upregulation of angiocrine factors, with Akt-mTOR-activated endothelial cells supporting the self-renewal of LT-HSCs and expansion of HSPCs, whereas MAPK co-activation favours maintenance and lineage-specific differentiation of HSPCs.Acute injury to the bone marrow microenvironment, after treatment with chemotherapy and irradiation, or myelotoxin, suppresses haematopoiesis, which results in the depletion of HSPCs and the development of life-threatening pancytopenias. The interaction of the surviving HSPCs with the bone marrow niche cells rapidly reconstitutes haematopoiesis, rescuing the host from complications associated with long-term bone marrow suppression. Bone marrow niches orchestrate maintenance, expansion and trafficking of HSPCs [1][2][3][4][5] . The osteogenic niche modulates the quiescence of the HSPCs 1-2 , whereas the vascular niche, demarcated by the bone marrow sinusoidal endothelial cells (SECs), regenerates and replenishes the HSPC Results Endothelial cells support both self-renewal and lineage-specific differentiation of HSPCsStudying the role of primary human endothelial cells (PECs) in the regulation of haematopoiesis has been hampered by the need for growthfactor deprivation during culture, which leads to apoptosis of PECs. Supplementation with serum and angiogenic factors, such as VEGF-A and basic-fibroblast growth factor (FGF2), are therefore necessary to maintain PECs for co-culture with HSPCs. However, serum inhibits the self-renewal of HSPCs, whereas FGF2 promotes self-renewal of HSPCs 16 , rendering it difficult to assess the cell-autonomous capacity of PECs to support HSPC homeostasis. To circumvent this problem, PECs can be transduced with an adenovirus gene, early region 4 encoded open reading frame-1 (E4ORF1), which leads to constitutive activation of Akt and enables co-culturing of PECs with HSPCs in serum-and growth factor-free medium for weeks, while maintaining their angio...

We show that Nras is transiently localized in the Golgi prior to the plasma membrane (PM). Moreover, green fluorescent protein (GFP)-tagged Nras illuminated motile, peri-Golgi vesicles, and prolonged BFA treatment blocked PM expression. GFP-Hras colocalized with GFP-Nras, but GFP-Kras4B revealed less Golgi and no vesicular fluorescence. Whereas a secondary membrane targeting signal was required for PM expression, the CAAX motif alone was necessary and sufficient to target proteins to the endomembrane where they were methylated, a modification required for efficient membrane association. Thus, prenylated CAAX proteins do not associate directly with the PM but instead associate with the endomembrane and are subsequently transported to the PM, a process that requires a secondary targeting motif.