Extracellular matrix proteoglycans support aged hippocampus networks: a potential cellular-level mechanism of brain reserve

Abstract

One hallmark of normative brain aging is vast heterogeneity in whether older people succumb to or resist cognitive decline. Resilience describes a brain's capacity to maintain cognition in the face of aging and disease. One factor influencing resilience is brain reserve-the status of neurobiological resources available to support neuronal circuits as dysfunction accumulates. This study uses a cohort of behaviorally characterized adult, middle-aged, and aged rats to test whether neurobiological factors that protect inhibitory neurotransmission and synapse function represent key components of brain reserve. Histochemical analysis of extracellular matrix proteoglycans, which play critical roles in stabilizing synapses and modulating inhibitory neuron excitability, was conducted alongside analyses of lipofuscin-associated autofluorescence. The findings indicate that aging results in lower proteoglycan density and more lipofuscin in CA3. Aged rats with higher proteoglycan density exhibited better performance on the Morris watermaze, whereas lipofuscin abundance was not related to spatial memory. These data suggest that the local environment around neurons may protect against synapse dysfunction or hyperexcitability and could contribute to brain reserve mechanisms.

Keywords: CA3; Cognitive aging; Lipofuscin; Resilience; Spatial memory.

Figure 1.

Experimental design and age comparisons of spatial memory on the Morris watermaze. A) Image of the Morris watermaze testing environment. The watermaze apparatus consisted of a circular pool with an approximate diameter of 120cm and depth of 36 cm and an escape platform hidden beneath the surface of the water for the spatial version and just above the surface of the water for the cued version. Rats underwent 4 sequential days of testing with each rat performing 4 trials per day, and a Corrected Integrated Path Length (CIPL) to the escape platform was calculated (see Methods). Brains from all animals underwent histological and image-based analyses of extracellular matrix proteoglycan abundance (WFA) and lipofuscin-associated autofluorescence. B) Average CIPL scores across the 4 days of testing for the adult, middle-aged, and aged rats used in the histochemical analyses. Green represents adult rats, yellow represents middle-aged rats, and purple represents aged rats. Both aged and middle-aged rats exhibited higher CIPL scores than did young across all sessions, and aged rats exhibited higher CIPL scores on the 3^rd and 4^th day of testing compared to middle-aged rats. C) The difference measure between day 1 and day 4 of testing. Boxplots denote the middle 50% of the data, and horizontal lines indicate the median of each distribution. Aged rats exhibited a smaller difference, indicating poorer spatial learning. ** = p < 0.01.

Figure 2:

Aged rats with more extracellular matrix in CA3 exhibited better spatial memory. A) Representative photomicrograph of wisteria floribunda agglutinin (WFA) labelling in the CA3 region of the dorsal hippocampus from a young-adult and aged rat. B) WFA coverage of dorsal CA3 separated by age. Aged rats exhibited lower WFA coverage than young-adult rats. Boxplots denote the middle 50% of the data, and horizontal lines indicate the median of each distribution. C) Scatter plot of WFA coverage and Morris watermaze learning scores (Day 1 – Day 4 CIPL difference). There was no significant relationship observed between WFA coverage and CIPL difference scores for the adult and middle-aged rats. Amongst the aged rats, however, the animals with the most WFA coverage showed the best learning on the Morris watermaze. *** = p < 0.001.

Figure 3:

Lipofuscin increases with age but is not associated with spatial memory. A) Left: Representative photomicrograph of lipofuscin-associated autofluorescence in the CA3 region of the dorsal hippocampus. Right: A threshold image of the same micrograph to show the lipofuscin-associated autofluorescence. B) Lipofuscin-associated autofluorescent coverage of dorsal CA3 separated by age. Boxplots denote the middle 50% of the data, and horizontal lines indicate the median of each distribution. The older animals exhibited more lipofuscin than did middle-aged or young-adult animals. Middle-aged animals showed more lipofuscin than did the young adults. C) Scatter plot of lipofuscin coverage and Morris watermaze learning scores (Day 1 – Day 4 CIPL difference). There was no significant relationship observed between lipofuscin coverage and CIPL difference scores for any age group. * = p < 0.05; *** = p < 0.001.