Increased Hippocampal ProBDNF Contributes to Memory Impairments in Aged Mice

Abstract

Memory decline during aging or accompanying neurodegenerative diseases, represents a major health problem. Neurotrophins have long been considered relevant to the mechanisms of aging-associated cognitive decline and neurodegeneration. Mature Brain-Derived Neurotrophic Factor (BDNF) and its precursor (proBDNF) can both be secreted in response to neuronal activity and exert opposing effects on neuronal physiology and plasticity. In this study, biochemical analyses revealed that increased levels of proBDNF are present in the aged mouse hippocampus relative to young and that the level of hippocampal proBDNF inversely correlates with the ability to perform in a spatial memory task, the water radial arm maze (WRAM). To ascertain the role of increased proBDNF levels on hippocampal function and memory we performed infusions of proBDNF into the CA1 region of the dorsal hippocampus in male mice trained in the WRAM paradigm: In well-performing aged mice, intra-hippocampal proBDNF infusions resulted in a progressive and significant impairment of memory performance. This impairment was associated with increased p-cofilin levels, an important regulator of dendritic spines and synapse physiology. On the other hand, in poor performers, intra-hippocampal infusions of TAT-Pep5, a peptide which blocks the interaction between the p75 Neurotrophin Receptor (p75NTR) and RhoGDI, significantly improved learning and memory, while saline infusions had no effect. Our results support a role for proBDNF and its receptor p75NTR in aging-related memory impairments.

Keywords: BDNF; TAT-Pep5; cofilin; memory; p75NTR; proBDNF; water radial arm maze.

Figure 1

Timeline of Experiments 1 and 2. In Experiment 2, mice were split into three groups based on their performance (unimpaired/impaired) and henceforth infused with different drugs (saline, proBDNF, TAT-Pep5). d, day of study; WRAM, behavioral training/testing in the water radial arm maze task; SAL, saline.

Figure 2

Memory impairment in aged mice positively correlates with increases in hippocampal proBDNF levels. (A) Average (±SEM) reference memory (RM) and working memory (WM) errors over four 4-session blocks of a WRAM task in 24-month old aged mice and 4-month old young mice. RM and WM errors positively correlate with proBDNF levels. (B) Increase in proBDNF, but not brain-derived neurotrophic factor (BDNF), in 24-month old aged mice relative to young 4-month old mice. (C) Relative to young 4-month old mice, aged 24-month old mice show increased p75 Neurotrophin Receptor (p75NTR) and decreased p-trk140 and trk140. (D) Relative to young 4-month old mice, aged 24-month old mice show decreased Tissue Plasminogen Activator (tPA), but not carboxypeptidase E (CPE). *p < 0.05; **p < 0.01.

Figure 3

Intra-hippocampal infusion of proBDNF impairs memory in well-performing (unimpaired) mice, while TAT-Pep5 infusion improves memory in poorly-performing (impaired) mice. (A) Average (±SEM) RM and WM errors in memory impaired 18-month old mice receiving intra-hippocampal saline infusions (impaired + SAL, n = 8, open triangles) and well-performing 18-month old aged mice receiving uncleavable proBDNF intra-hippocampal infusions (unimpaired + proBDNF, n = 16, open circles) over four daily sessions of a WRAM task. (B) Average (± SEM) RM and WM errors in memory impaired 18-month old mice infused with saline (impaired + SAL, n = 8, open triangles) and memory impaired 18-month old mice receiving intra-hippocampal infusions of TAT-Pep5 (impaired + TAT-Pep5, n = 9, closed circles) over four daily sessions of a WRAM task. (C) Representative images indicating the locations of drug infusions at two levels of the hippocampus. ns not significant; *p < 0.05; **p < 0.01.

Figure 4

Intra-hippocampal infusion of proBDNF increases p-cofilin levels in memory-unimpaired mice to levels seen in memory-impaired mice. (A) p-Cofilin to total cofilin ratio in memory-unimpaired mice, memory-impaired mice, and memory-unimpaired mice infused with proBDNF. (B) Representative p-cofilin and cofilin blots. **p < 0.01.

Figure 5

Theoretical model of the role of proBDNF in learning in memory in aged individuals. In young individuals, maturation of proBDNF to BDNF is controlled by plasmin and tPA, and is essential for learning and memory. Aged individuals show decreased levels of tPA and plasmin, and increased levels of uncleaved proBDNF, associated with increased spine remodeling and memory deficits. Blockade of p75NTR (e.g., by TAT-Pep5, as in the current study) leads to spine growth, and rescues learning and memory.