Nucleolar PARP-1 Expression Is Decreased in Alzheimer's Disease: Consequences for Epigenetic Regulation of rDNA and Cognition

Abstract

Synaptic dysfunction is thought to play a major role in memory impairment in Alzheimer's disease (AD). PARP-1 has been identified as an epigenetic regulator of plasticity and memory. Thus, we hypothesize that PARP-1 may be altered in postmortem hippocampus of individuals with AD compared to age-matched controls without neurologic disease. We found a reduced level of PARP-1 nucleolar immunohistochemical staining in hippocampal pyramidal cells in AD. Nucleolar PARP-1 staining ranged from dispersed and less intense to entirely absent in AD compared to the distinct nucleolar localization in hippocampal pyramidal neurons in controls. In cases of AD, the percentage of hippocampal pyramidal cells with nucleoli that were positive for both PARP-1 and the nucleolar marker fibrillarin was significantly lower than in controls. PARP-1 nucleolar expression emerges as a sensitive marker of functional changes in AD and suggests a novel role for PARP-1 dysregulation in AD pathology.

Figure 1

Nucleolar PARP-1 immunoreactivity in AD ranged from absent to dispersed and less intense compared to that of controls. ((a) and (b)) Representative immunostaining with diaminobenzidine (DAB) of human hippocampal pyramidal neurons in CA1 region. (a) Prominent nucleolar staining of PARP-1 (arrows) was seen in most of pyramidal neurons of a control case. (b) The nucleolar staining of PARP-1 ranged from absent (arrowheads) to a more dispersed pattern with less intensity of label (arrows) in pyramidal neurons of an AD case. ((c) and (d)) Percentages of CA1 and CA4 hippocampal pyramidal neurons with PARP-1 positive nucleoli were significantly lower in AD cases compared to controls. (Control, n = 8; AD, n = 8; ^∗ p < 0.05.) Scale bar = 50 μm.

Figure 2

PARP-1 nucleolar immunoreactivity is altered in hippocampal pyramidal cells in AD brains. Representative confocal microscopy of PARP-1 immunostaining (red) with DAPI nuclear counterstaining (blue) of CA4 hippocampal pyramidal neurons. In controls brains (a–c) a high percentage of pyramidal cell nucleoli have intense and well- delineated PARP-1 staining (arrowheads). In contrast, in AD brains (d–f), the percentage of intensely stained and well-delineated nucleoli is less than in the controls and there is a more dispersed pattern with weak label intensity ((d) and (f), arrow). ((g) and (h)) The percentage of CA1 (g) and CA4 (h) hippocampal pyramidal neurons with PARP-1 positive nucleoli staining was less in AD cases compared to controls. (Control, n = 8 and n = 7 for CA1 and CA4, resp.; AD, n = 10 for both CA1 and CA4; ^∗ p < 0.05.) Scale bar = 25 μM.

Figure 3

Nucleolar proteins in hippocampal pyramidal cells are altered in AD. ((a)–(h)) Representative figures show colocalization ((d) and (h), yellow) of fibrillarin ((b) and (f), green) and PARP-1 ((c) and (g), red) in the nucleoli of pyramidal neurons. Control cases exhibit high intensity staining (a–d) compared to AD (e–h) (arrowheads). In AD compared to controls, there is a lower percentage of nucleoli that are both PARP-1(+) and fibrillarin(+) ((f)-(g), arrowhead) in CA1 (see Table 2) and CA4 (see Table 3) pyramidal cells and a higher percentage of nucleoli PARP-1(−)/fibrillarin(+) ((f) and (g), arrow) in CA1 (see Table 2) and CA4 (see Table 3), suggesting that different nucleolar proteins are affected in different ways in AD. Scale bar = 20 μm.

Figure 4

Model. Ribosome biogenesis in Alzheimer's disease. (a) Normal neuron. PARP-1 activity (PAR) prevents rDNA methylation. PAR = poly(ADPribosyl)ation. (b) AD neuron. Absence of PARP-1 in nucleoli allows DNA methyltransferase (Dnmt1) to methylate rDNA promoters silencing rRNA transcription resulting in nucleolar disruption.