Insulin-like growth factor 1 regulates developing brain glucose metabolism

Abstract

The brain has enormous anabolic needs during early postnatal development. This study presents multiple lines of evidence showing that endogenous brain insulin-like growth factor 1 (Igf1) serves an essential, insulin-like role in promoting neuronal glucose utilization and growth during this period. Brain 2-deoxy-d- [1-(14)C]glucose uptake parallels Igf1 expression in wild-type mice and is profoundly reduced in Igf1-/- mice, particularly in those structures where Igf1 is normally most highly expressed. 2-Deoxy-d- [1-(14)C]glucose is significantly reduced in synaptosomes prepared from Igf1-/- brains, and the deficit is corrected by inclusion of Igf1 in the incubation medium. The serine/threonine kinase Akt/PKB is a major target of insulin-signaling in the regulation of glucose transport via the facilitative glucose transporter (GLUT4) and glycogen synthesis in peripheral tissues. Phosphorylation of Akt and GLUT4 expression are reduced in Igf1-/- neurons. Phosphorylation of glycogen synthase kinase 3beta and glycogen accumulation also are reduced in Igf1-/- neurons. These data support the hypothesis that endogenous brain Igf1 serves an anabolic, insulin-like role in developing brain metabolism.

Figure 1

Glucose utilization is dramatically reduced in the Igf1−/− brain. (A) Anatomically matched coronal sections from forebrain (Top), midbrain (Middle), and hindbrain (Bottom) are shown. 2DGU is globally reduced in the Igf1−/− brains, most notably in the brain regions where Igf1 is most abundant in WT (left column). apt, anterior pretectal nuclei; mg, medial geniculate nuclei; vbl, ventrobasolateral complex; vnc, vestibular nuclear complex. (B) ¹⁴C-2DGU was quantified from film autoradiographs. The thalamus was measured at the level of the ventrobasolateral nuclear complex. Data are presented as mean ± SEM (arbitrary density units). n = 4–5 per group.

Figure 2

Akt phosphorylation and GLUT4 localization in WT and Igf1−/− cerebellar cortex. Total Akt immunoreactivity is homogeneously distributed in Purkinje perikarya and processes in WT (A) and Igf1−/− (B) brains. Phospho-specific Akt immunoreactivity (Thr³⁰⁸), in contrast, is granular-appearing and is detected predominantly in Purkinje dendritic processes in the WT (C, arrowhead) and is scarcely detected in the Igf1−/− brain (D). GLUT 4 immunoreactivity is concentrated in Purkinje dendrites and the cytoplasm just around the dendrite's origin (E). GLUT4 is present in Purkinje perikarya in the Igf1−/− cerebellum, but is not preferentially localized in dendrites (F). (Bar = 100 μm.)

Figure 3

(A–H) GSK3β-ser⁹ phosphorylation and glycogen synthesis in cerebellar cortex (A–D) and olfactory bulb (E–H). Representative micrographs show immunostaining specific for ser⁹-phosphorylated GSK3β on the left-hand side of the figure, and glycogen-PAS staining is shown on the right side. WT sections are shown in A, B, E, and F, and Igf1−/− sections are shown in C, D, G, and H. Intense ser⁹-phospho-GSK3β staining of projection neurons (arrows) is seen in the WT but is barely detectable in the Igf1−/− brain. (I) GSK3β total protein levels assessed by immunoblot are the same in WT and Igf1−/−. (J) This section was pretreated with α-amylase to remove tissue glycogen before PAS staining. gl, glomerular layer. (Bar = 100 μm.)

Figure 4

(A and B) Igf1 increases cerebrocortical glucose uptake. Igf1 (A) or fibroblast growth factor 2 (FGF2) (B) was injected into the cerebral cortex of anesthetized rats with vehicle injected simultaneously into the contralateral side. After the cerebral injections, 2DG was administered i.p. and the animals were killed 45 min later. Representative film autoradiographs from sections at the injection site show that only Igf1 produced increases in 2DGU, which is readily appreciated along the needle track. These findings were replicated in three animals. (C and D) Parallel patterns for Igf1 expression (C) and ¹⁴C-2DGU (D) in the adult rat brain 6 days after middle cerebral artery occlusion. Other growth factors such as Igf2 and platelet-derived growth factor did not show a similar correlation with 2DGU (data not shown).