SUMMARY We address the mechanism by which adult intestinal stem cells (ISCs) become localized to base of each villus during embryonic development. We find that, early in gut development, proliferating progenitors expressing ISC markers are evenly distributed throughout the epithelium, in both the chick and mouse. However as the villi form, the putative stem cells become restricted to the base of the villi. This shift in the localization is driven by mechanically influenced reciprocal signaling between the epithelium and underlying mesenchyme. Buckling forces physically distort the shape of the morphogenic field causing local maxima of epithelial signals, in particular Shh, at the tip of each villus. This induces a suite of high threshold response genes in the underlying mesenchyme to form a signaling center called the “villus cluster.” Villus cluster signals, notably Bmp4, feed back on the overlying epithelium, to ultimately restrict the stem cells to the base of each villus.

The six-transmembrane protein glycerophosphodiester phosphodiesterase 2 (GDE2) induces spinal motor neuron differentiation by inhibiting Notch signaling in adjacent motor neuron progenitors. GDE2 function requires activity of its extracellular domain that shares homology with glycerophosphodiester phosphodiesterases (GDPD). GDPDs metabolize glycerophosphodiesters into glycerol-3-phosphate and corresponding alcohols but whether GDE2 inhibits Notch signaling by this mechanism is unclear. Here, we show that GDE2, unlike classical GDPDs, cleaves glycosylphosphatidylinositol (GPI)-anchors. GDE2 GDPD activity inactivates the Notch activator RECK by releasing it from the membrane by GPI-anchor cleavage. RECK release disinhibits ADAM protease-dependent shedding of the Notch ligand Delta-like 1 (Dll1) leading to Notch inactivation. This study identifies a previously unrecognized mechanism to initiate neurogenesis that involves GDE2 mediated surface cleavage of GPI-anchored targets to inhibit Dll1-Notch signaling.

Significance We report a quantitative Raman microscopy method that measures the concentration of protein and lipid in cells at high spatial resolution in living and in fixed samples of tissues, allowing quantitative studies of cell size and organelle regulation both in cell culture and in tissue slices; it can be applied to problems of cell size control, intracellular crowding, and lipid metabolism in the context of cell growth, cell differentiation, cell senescence, and pathology.

The specification of spinal interneuron and motor neuron identities initiates within progenitor cells, while motor neuron subtype diversification is regulated by hierarchical transcriptional programs implemented postmitotically. Here, we find that mice lacking GDE2, a six-transmembrane protein that triggers motor neuron generation, exhibit selective losses of distinct motor neuron subtypes, specifically in defined subsets of limb-innervating motor pools that correlate with the loss of force-generating alpha motor neurons. Mechanistically, GDE2 is expressed by postmitotic motor neurons but utilizes extracellular glycerophosphodiester phosphodiesterase activity to induce motor neuron generation by inhibiting Notch signaling in neighboring motor neuron progenitors. Thus, neuronal GDE2 controls motor neuron subtype diversity through a non cell-autonomous feedback mechanism that directly regulates progenitor cell differentiation, implying that subtype specification initiates within motor neuron progenitor populations prior to their differentiation into postmitotic motor neurons.