Figure 4. Insulin signaling affects age-associated morphological changes in adult touch neurons.

A–K. Wild type, reduction-of-function insulin receptor daf-2(e1370), transcription factor FOXO/daf-16(mgDf50) null mutant, and temperature-sensitive hsf-1(sy441) (all also harboring zdIs5[p_mec-4GFP]) were assayed at days of adult life indicated, having been reared at 20°C and shifted to 24°C at the L4 larval stage. Numbers of neurons scored for common phenotypes of ALM soma outgrowth (A, D, G), PLM wavy appearance (B, E, H) and PLM novel process branching (C, F, I) were: zdIs5 (data for wild type control assayed in daf-2 experiments): 172 day 2, 128 day 4, 106 day 8, 90 day 10, 30 day 15; zdIs5;daf-2: 198 day 2, 182 day 4, 156 day 8, 144 day 10, 118 day 15; zdIs5 (wild type control assayed in daf-16 experiments): 116 day 2, 88 day 4, 76 day 8, 64 day 10, 12 day 15; zdIs5;daf-16(mgDf50): 172 day 2, 130 day 4, 96 day 8, 62 day 10 (by day 15 all daf-16 animals were dead); zdIs5 (wild type control assayed in hsf-1 experiments): 54 day 2, 34 day 4, 19 day 8; zdIs5; hsf-1(sy441): 90 day 2, 59 day 4, 15 day 8 (no live hsf-1 mutants on adult day 10). Asterisk for p<0.05 in mutant vs. WT control on the indicated day, significance was determined using Fisher’s exact test. We observed similar outcomes for studies of aging touch neurons in daf-2(e1368); our daf-2 and daf-16 data are consistent with findings in Pan et al. (2011) and Tank et al., (2011) (daf-16(mu86)), although those studies did not feature a focus on specific abnormalities and individual touch neuron types. J. Cell autonomous expression of hsf-1 in touch neurons rescues early onset elevation of the ALM soma outgrowth phenotype. We expressed hsf-1 specifically in the touch neurons under the control of the mec-4 promoter, in the background of hsf-1(ts) 20°C (see Methods) and compared novel soma outgrowth to control animals that expressed only the p_mec-4mCherry construct used to visualize neuronal processes. Interestingly, expression of hsf-1 in the PLM neurons was associated with the presence of blunted dendrites in ~50% of neurons observed (data not shown), suggesting a dominant-negative effect on process maintenance (and possibly elevated breaking) for this subset of neurons; AVM and PVM are not significantly different from non-rescued lines.

K. Example of a 4 day old hsf-1 mutant touch neuron process (horizontal fluorescence) with an apparent break, ends indicated by arrows. Another touch neuron soma can be seen in the middle-right background. Similar beading at broken ends is seen immediately after laser axotomy. Scale bar represents 10 microns. L. Cell-specific quantitation of touch neuron “breaks” in the hsf-1(sy411) background on adult day 4. Strain measured is zdIs5; hsf-1(sy441): 90 day 2, 59 day 4, 15 day 8.