Figure 2

Spine SER forms an apparatus and boosts synapse enlargement following LTP. Serial section images (S#) and 3D reconstructions of spines (A) without SER (SER−) from LTP, (B) with single SER tubule (SER+ (Tub)) from control, and (C) with apparatus (SER+ (SA)) from LTP. The SA has dense material (S103, blue arrows) directed to the postsynaptic density (PSD; S103-104, black arrows), and a vesicle (S105, black arrow). Spine head diameters were measured at widest point (black line in 3Ds). (SER (green), spine (yellow), PSD (red). Scale bar in B is for all EMs and matches the edge of the cube, which is for all 3Ds). (D) Percentage of SER+ spines in control (12.7%, n = 267) and LTP (12.2%, n = 367) conditions versus SER− spines (χ² = 0.03, p = 0.86). (E) Percentages of SER+ spines with an apparatus (SA) versus tubule (T) in LTP (80%, n = 45) and control (47%, n = 34) conditions (χ² = 6.6, *p = 0.01). (F) Mean PSD areas differed between SER+ vs SER− in control (ANOVA, F_{(1, 265)} = 56, p < 0.0001) or LTP (ANOVA, F_(1,365) = 222, p < 0.0001) conditions, between control (0.081 ± 0.004 µm²) vs LTP (0.096 ± 0.005 µm²) for all spines (All Sp, hnANOVA, F_(1,616) = 4.40, *p = 0.036, η² = 0.0014), and between LTP vs Control for SER+ spines (hnANOVA, F_(1,61) = 5.7, **p = 0.020, η² = 0.070). SA+ spines had largest PSD areas in both control (0.24 ± 0.3) and LTP (0.31 ± 0.3µm²) conditions but did not reach significance between conditions (ANOVA, F_(1,50) = 3.1, p = 0.07). The PSD areas on SER− spines did not differ overall (hnANOVA, F_(1,537) = 2.4, p = 0.13) or for head diameters >0.15 µm (F_(1,533) = 3.1, p = 0.09) or >0.38 µm (hnANOVA, F_(1,207) = 2.0, p = 0.16). (G,H) The relationship between PSD area and head diameter was well-correlated in both conditions (p < 0.001) and had homogeneity of slopes across LTP and control conditions for both SER+ and SER− spines. (Gray arrow is smallest SER+ spine; vertical gray line is smallest SA+ spine). (G) The PSD areas on SER+ spines were greater in LTP than control conditions across head diameters (ANCOVA, F_(1,76) = 13.7, p = 0.00041; η² = 0.15, i.e. 15%). (H) For SER− spines, the statistically significant effect (ANCOVA, F_(1,552) = 8.7, p = 0.0034) was weak (η² = 0.009, 0.9%) and was accounted for by spine head diameters >0.38 µm (ANCOVA, F_(1,222) = 7.8, p = 0.006, η² = 0.022, 2.2%) compared to insignificant effect between LTP and control conditions for head diameters <0.38 µm (ANCOVA, F_(1,327) = 2.19, p = 0.14).