BDNF transcripts, proBDNF and proNGF, in the cortex and hippocampus throughout the life span of the rat

Abstract

Neurotrophins are established molecular mediators of neuronal plasticity in the adult brain. We analyzed the impact of aging on brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) protein isoforms, their receptors, and on the expression patterns of multiple 5' exon-specific BDNF transcripts in the rat cortex and hippocampus throughout the life span of the rat (6, 12, 18, and 24 months of age). ProNGF was increased during aging in both structures. Mature NGF gradually decreased in the cortex, and, in 24-month-old animals, it was 30% lower than that in adult 6-month-old rats. The BDNF expression did not change during aging, while proBDNF accumulated in the hippocampus of aged rats. Hippocampal total BDNF mRNA was lower in 12-month-old animals, mostly as a result of a decrease of BDNF transcripts 1 and 2. In contrast to the region-specific regulation of specific exon-containing BDNF mRNAs in adult animals, the same BDNF RNA isoforms (containing exons III, IV, or VI) were present in both brain structures of aged animals. Deficits in neurotrophin signaling were supported by the observed decrease in Trk receptor expression which was accompanied by lower levels of the two main downstream effector kinases, pAkt and protein kinase C. The proteolytic processing of p75NTR observed in 12-month-old rats points to an additional regulatory mechanism in early aging. The changes described herein could contribute to reduced brain plasticity underlying the age-dependent decline in cognitive function.

Fig. 1

BDNF mRNA transcripts and proBDNF are differentially affected by aging in the rat cortex and hippocampus. a Western blot analysis of pro-/mature BDNF. Aging differentially affects proBDNF levels in the cortex and hippocampus. Protein levels are expressed as fold changes (mean ± SEM) relative to the control value (6-month-old animals). Each graph is accompanied by a representative immunoblot. Asterisk indicates p < 0.05 vs. 6-month-old rats. b RT-PCR expression analysis of exon I–IXA-containing BDNF mRNAs in 6- and 24-month-old animals. c Semiquantitative RT-PCR analysis of total and exon-specific BDNF transcript expression during aging. The increase in total BDNF mRNA in the cortex is mostly due to a twofold increases in levels of transcripts 4 and 6. In the hippocampus, age severely lowered the BDNF mRNA level already in 12-month-old animals. mRNA levels are expressed relative to the level of GAPDH mRNA ± SEM. Currency sign indicates p < 0.05 vs 6-month-old rats

Fig. 2

ProNGF increases during aging both in the cortex and in the hippocampus of rats. a The level of NGF protein is reduced in the cortex of aged animals, while b age did not significantly change the NGF level in the hippocampus. Protein levels are expressed as fold change (mean ± SEM) relative to the control value (6-month-old animals). Each graph is accompanied by a representative immunoblot. Asterisk indicates p < 0.05 vs 6-month-old rats. c Decrease in the level of cortical NGF is accompanied by increased transcription of the respective genes. mRNA levels were assessed by RT-PCR and are expressed relative to the level of GAPDH mRNA ± SEM. Asterisk indicates p < 0.05 vs 6-month-old rats

Fig. 3

Aging differentially affects the protein levels of total and phosphorylated Trk receptors in the hippocampus. Western blot analysis was used to determine the expression of Trk and pTrk (Tyr 496) receptors in the cortex (a) and hippocampus (b). Each graph is accompanied by a representative immunoblot. The data are expressed as the fold change (mean ± SEM) relative to the control value (6-month-old animals). Asterisk indicates p < 0.05 vs. 6-month-old rats

Fig. 4

p75NTR is subject to proteolytic processing both in the cortex and hippocampus of 12-month-old rats. Western blot analysis was used to determine the protein level of full-length p75NTR (a, b) as well as the levels of two specific cleavage products: p75ECD in the cortex (c) and p75ECDf in the hippocampus (d) of rats during aging. Graphs that show changes in protein levels are accompanied by representative immunoblots. The data are expressed as the fold change (mean ± SEM) relative to the control value (6-month-old animals). Asterisk indicates p < 0.05 vs 6-month-old rats

Fig. 5

Two effector kinases in the Trk receptors signaling pathway decrease in the hippocampus during aging. Western blot analysis was used to determine the expression of pAkt (Thr308) kinase and PKC in the cortex (a and c, respectively) and in the hippocampus (b and d, respectively). Each graph is accompanied by representative immunoblots. The data are expressed as the fold change (mean ± SEM) relative to the control value (6-month-old animals). Asterisk indicates p < 0.05 vs. 6-month-old rats