Summary Ral GTPases are RAS effector molecules and by implication a potential therapeutic target for RAS mutant cancer. However, very little is known about their roles in stem cells and tissue homeostasis. Using Drosophila , we identified expression of RalA in intestinal stem cells (ISCs) and progenitor cells of the fly midgut. RalA was required within ISCs for efficient regeneration downstream of Wnt signaling. Within the murine intestine, genetic deletion of either mammalian ortholog, Rala or Ralb , reduced ISC function and Lgr5 positivity, drove hypersensitivity to Wnt inhibition, and impaired tissue regeneration following damage. Ablation of both genes resulted in rapid crypt death. Mechanistically, RALA and RALB were required for efficient internalization of the Wnt receptor Frizzled-7. Together, we identify a conserved role for RAL GTPases in the promotion of optimal Wnt signaling, which defines ISC number and regenerative potential.

Adult tissue homeostasis requires a tight balance between the removal of old or damaged cells and the production of new ones. Such processes are usually driven by dedicated stem cells that reside within specific tissue locations or niches (Nystul and Spradling, 2006).The intestinal epithelium has a remarkable regenerative capacity, which has made it a prime paradigm for the study of stem cell-driven tissue self-renewal. The discovery of the presence of stem cells in the adult midgut of the fruit fly Drosophila melanogaster has significantly impacted our understanding of the role of stem cells in intestinal homeostasis. Here we will review the current knowledge of the main mechanisms involved in the regulation of tissue homeostasis in the adult Drosophila midgut, with a focus on the role of stem cells in this process. We will also discuss processes involving acute or chronic disruption of normal intestinal homeostasis such as damage-induced regeneration and ageing.3

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RAS-like (RAL) GTPases function in Wnt signalling-dependent intestinal stem cell proliferation and regeneration. Whether RAL proteins work as canonical RAS effectors in the intestine, and the mechanisms of how they contribute to tumourigenesis remain unclear. Here, we show that RAL GTPases are necessary and sufficient to activate EGFR/MAPK signalling in the intestine, via induction of EGFR internalisation. Knocking down Drosophila RalA from intestinal stem and progenitor cells leads to increased levels of plasma membrane-associated EGFR and decreased MAPK pathway activation. Importantly, in addition to impacting stem cell proliferation during damage-induced intestinal regeneration, this role of RAL GTPases impacts on EGFR-dependent tumorigenic growth in the intestine and in human mammary epithelium. However, the effect of oncogenic RAS in the intestine is independent from RAL function. Altogether, our results reveal previously unrecognised cellular and molecular contexts where RAL GTPases become essential mediators of adult tissue homeostasis and malignant transformation.

SummaryRAS-like (RAL) GTPases function in Wnt signalling-dependent intestinal stem cell proliferation and regeneration. Whether RAL proteins work as canonical RAS effectors in the intestine, and the mechanisms of how they contribute to tumorigenesis remain unclear. Here, we show that RAL GTPases are necessary and sufficient to activate EGFR/MAPK signalling in the intestine. We identify non-canonical roles of RAL GTPases, not as RAS effectors, but rather by acting upstream of RAS activation via induction of EGFR internalization. Knocking down Drosophila RalA from intestinal stem and progenitor cells leads to increased levels of plasma membrane-associated EGFR and decreased MAPK pathway activation. Importantly, in addition to impacting stem cell proliferation and damage-induced intestinal regeneration, this function of RAL GTPases drives EGFR-dependent tumorigenic growth in the intestine and in human mammary epithelium. Altogether, our results reveal previously unrecognised cellular and molecular contexts where RAL GTPases become essential mediators of EGFR-driven tissue homeostasis and malignant growth.

Calcium ions are well-known intracellular signalling molecules. A new study identifies local cytoplasmic calcium as a central integrator of metabolic and proliferative signals in Drosophila intestinal stem cells.

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Cholesteatoma is a rare condition affecting 9-12.6 adults and 3-15 children per 100,000 per annum [1–4], with a more aggressive presentation in the paediatric population [5]. Intermittent otorrhea (ear discharge) is the presenting complaint in over half of cholesteatoma patients [6, 7]. The peak incidence of cholesteatoma is 5-15 years of age [8] which overlaps significantly with a period of high incidence in otitis media [9] and externa [10], diseases that often present the same way as cholesteatoma. This results in diagnosis that may take several years. Left untreated, cholesteatoma can cause significant lasting damage in the form of deafness, vertigo, facial paralysis, meningitis, and brain abscesses which may prove fatal [11]. Current treatment options are limited to surgical excision with the aim to establish a safe and manageable ear, while maintaining hearing is secondary. Improving surgical instrumentation has allowed a better success rate, however, revision surgeries remain a mainstay of practice. In practical terms, this means that those affected by bilateral disease often undergo surgery 4 or more times [12]. This represents a significant burden for patients. The decision about the exact surgical approach (canal wall up vs canal wall down) is a careful balancing act of safety versus functionality, and the pros and cons must be weighed in light of available evidence and the skill of the surgeon [13].