Rab5 is necessary for the biogenesis of the endolysosomal system in vivo

An outstanding question is how cells control the number and size of membrane organelles. The small GTPase Rab5 has been proposed to be a master regulator of endosome biogenesis. Here, to test this hypothesis, we developed a mathematical model of endosome dependency on Rab5 and validated it by titrating down all three Rab5 isoforms in adult mouse liver using state-of-the-art RNA interference technology. Unexpectedly, the endocytic system was resilient to depletion of Rab5 and collapsed only when Rab5 decreased to a critical level. Loss of Rab5 below this threshold caused a marked reduction in the number of early endosomes, late endosomes and lysosomes, associated with a block of low-density lipoprotein endocytosis. Loss of endosomes caused failure to deliver apical proteins to the bile canaliculi, suggesting a requirement for polarized cargo sorting. Our results demonstrate for the first time, to our knowledge, the role of Rab5 as an endosome organizer in vivo and reveal the resilience mechanisms of the endocytic system. The small GTPase Rab5 has been proposed to be a master regulator of endosome biogenesis; using in vivo RNA interference and mathematical modelling it is shown here that the endolysosomal system is resilient to loss of Rab5 until its concentration drops below a critical level, at which point endosomes are lost, leading to increased serum low-density lipoprotein levels, alterations in metabolism and hepatocellular polarity. Rab5 is endosome organizer in vivo Regulation of organelle size and number is a fundamental question in biology. The small GTPase Rab5 has been proposed as a master regulator of the biogenesis of endosomes: membrane-associated vacuoles involved in endocytosis. In this study, Marino Zerial and colleagues use a combination of mathematical modelling and in vivo RNA interference analysis to reduce the levels of Rab5 in mouse liver. They demonstrate that Rab5 is a principal component of endosome biogenesis in vivo . Consistent with the loss of endosomes following reduction in Rab5 levels below a key point, animals had elevated levels of serum low-density lipoprotein as a result of decreased endocytosis.