Figure 7.

Neuronal and astrocytic uptake of exogenous sterols. Neurons and astrocytes were cultured in defined cholesterol-free medium supplemented with free isotopically labeled cholesterol (D₇-Chol), D₇-Chol precomplexed with ApoE, and D₇-Chol precomplexed with BSA as illustrated in Panel A. The labeled moiety is highlighted in red. Final concentration of D₇-Chol in all conditions was 500 nM; protein final concentration was 0.7 μg/mL (either ApoE or BSA). Panels B and C show the uptake of D₇-Chol from the medium by neurons and astrocytes, respectively. D₇-Chol levels were normalized to the total cell count at the end point of the experiment (DIC3). Sterol values correspond to the mean ± SEM of 8–12 replicates. Note that ApoE facilitates the uptake of D₇-Chol by neurons but not by astrocytes. Panel D denotes the experimental design to test the incorporation of ¹³C₃−Lanosterol by neurons and astrocytes. Cells were cultured in defined cholesterol-free medium and with free isotopically labeled lanosterol (¹³C₃−Lan), ¹³C₃−Lan precomplexed with ApoE, and ¹³C₃−Lan precomplexed with BSA. The labeled moiety is highlighted in red. Panels E and F show ¹³C₃−Chol levels derived from the conversion of ¹³C₃−Lan by neurons and astrocytes, respectively. ¹³C₃−Chol levels were normalized to the total cell count at the end point of each experiment. Sterol values correspond to the mean ± SEM of 8–12 replicates. Note that the uptake and conversion of lanosterol into cholesterol is 40-fold higher in neurons when lanosterol is precomplexed with ApoE, while in astrocytes the same treatment increases only by 15%. In all panels, statistical significance is denoted by asterisks (**p < 0.01, ***p < 0.001; ****p < 0.0001).