Hi!! Can I please have some help understanding the (short) linked article in sim

Question

Hi!! Can I please have some help understanding the (short) linked article in simpler terms, specifically focusing on the 4 figures within it? I'm having a lot of trouble trying to understand it. The article is about the reactivation of neuroblasts in Drosophila via TOR and glial insulin relays. Please be as detailed as possible when referring to the figures. :) The figures are also located within the linked article.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3146047/

Fig. 1 TOR/PI3K signaling in fat body and neuroblasts regulates reactivation a, Diagram depicting larval fat body (FB) and CNS with central brain (CB), thoracic (Th) and abdominal (Ab) neuromeres, mNSCs, mushroom body (MB NBs) and other neuroblasts (circles) indicated. b, Brain lobe (inset in Eig.la), showing EdU incorporation in postembryonic neuroblasts (large cells; e.g dotted circle) and their progeny (smaller cells), labelled with nab-GAL4 driving membrane GFP (NeuroblastsmGFP). c, EdU incorporation timecourse from first-instar (LI) to third-instar (L3) larval stages in the wild-type (WT) CNS (OL, optic lobes). d,fg, EdU-labelled CNSs from larvae expressing TOR/PI3K components driven by Cg-GAL4 Fat bo or nab-GAL4 Near blasts > e Histograms of Ed vox els from thoracic NSs of fed la ae normalized to controls. In this and all subsequent figures, error bars are se n ·peo.os. See tect Methods and Supplementary Fig.2 for details of molecules expressed.

Explanation / Answer

This paper essentially talks about the signalling conditions and reqquirements for Drosophila neuroblast reactivation after quiescence. Quiescence is described as a state off cellular dormancy which many stem or cancer cells go through- a sort of lying-in-wait period before they emerge active and ready for battle. This emergence is characterised by extracellular signals that stimulate these cells into waking up.

This paper talks about the nutrient based reactivation of neuroblasts through fat bodies (which are a source of amino acids) which activate glia, which in turn stimulate neuroblast wake-up.

Figure 1:

Growing cells incorporate the fluorescent molecule EdU which enables us to visualise cell growth and replication patterns. Drosophila larvae in the first late instar stage awaken their neuroblasts through cell-enlargement, in a seuential pattern: central brain neuromeres, then thoracic then abdominal neuromeres. However some neuroblasts (Mushroom bodies) do not undergo quiescence in FED larvae. This leads us to believe that amino acids (derived from fat) are not just required as a nutrient component

using a dominant-negative Shibire Dynamin (ShiDN), investigating what happens when fat body derived signal is BLOCKED in a larva shows REDUCED EdU incorporation (implies the development of neuroblasts is much lower). This shows that a signal based on dietary requirements is needed to move neuroblasts out of quiescence.

Furthermore with reference to 1d and 1e it is seen that an overexpression of Tor, or fat body specific of TSC 1/2 (which are TOR inhibitors), reduce neuroblast exit from quiescence. This, in conjugation with the earlier experiment points towards some sort of TOR dependent FDS (fat body derived signal) which is reuired to wake up neuroblasts from quiescence.

Figure 1e-g : investigating the signalling pathways essential within neuroblasts for their reactivation

TOR signalling is essential for exit from quiesence. In fed larvae, there is a clear inhibition of TOR signalling (througgh TSC 1/2 overexpression), but forced overexpression of TOR Triggers rapid exit from quiescence (Last three green bars in fig 1e)

In nutrient restricted (NR) larvae, where lesser cells are activated from quiescence, the pathway may be regularised simply by overexpressing Rheb or activated p110 protein (which bypasses the NR restriction to reactivating neuroblasts), with or without fat body supplementation (1f and g)

Figure 2:

To test the signalling pathways which may bridge the FDS to TOR/PI3k pathways, insulin-like receptor (InR) pathways were tested, since according to fig 1f, InR activation was enought ot bypass nutrient restrcition and activate neuroblasts. The seven insulin-like peptides (Ilps) are tested for this function.

From fig 2a- reactivation is moderately delayed in Ilp6,2,3 deficient larvae, severely delayed in  Ilp2, Ilp3 and Ilp5 deficient cases or if all througgh Ilp 1-5 is deficient.

However neuroblast reactivation eventually starts, but at a later time point, hence indicating that it is merely a spatio-temporal delay.

Fig3: To trace the source of Ilps which would help neuroblast reactivation.

figures 3b and c indicate that  Pan-glial or pan-neuronal overexpression of Ilp4, Ilp5 or Ilp6 led to precocious reactivation under fed conditions, as well as NR conditions . Figure 3d further shows that Ilp3 overexpression in neurons also bypasses NR block to reactivation. Again, only temporal pattern of reactivation is affected in all these cases, spatially, the order of reactivation remains the same (Central brain first)

In combination with figure 3 data, figure 4 a and graph b (latter part of the graph under glia) indicate that the glial expression is more important to bypass these blocks and reactivate neuroblasts, as opposed neuronal expression.

FOCUSSING ON Ilp6:

figure 4b and c furthermore go to show that glial overexpression of Ilp6 makes FDS unnecessary to exit the neuroblast from quiescence. The larval fat body and glia act as substitutes for each other- since in case off NR condition, the Il6 mRNA in the fat body is upregulated and works on bringing the cells out of quiescence, while in normal conditions, Il6 is produced in the glia.

Hope this helps :)

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