Stability of dentate gyrus granule cell mossy fiber BDNF protein expression with age and resistance of granule cells to Alzheimers disease neuropathology in a mouse model

Abstract

The neurotrophin brain-derived neurotrophic factor (BDNF) is important in development and maintenance of neurons and their plasticity. Hippocampal BDNF has been implicated Alzheimers disease (AD) because hippocampal levels in AD patients and AD animal models are consistently downregulated, suggesting that reduced BDNF contributes to AD. However, the location where hippocampal BDNF protein is most highly expressed, the mossy fiber (MF) axons of dentate gyrus (DG) granule cells (GCs), has been understudied, and never in controlled in vivo conditions. We examined MF BDNF protein in the Tg2576 mouse model of AD. Tg2576 and wild type (WT) mice of both sexes were examined at 2-3 months of age, when amyloid-{beta} (A{beta}) is present in neurons but plaques are absent, and 11-20 months of age, after plaque accumulation. As shown previously, WT mice exhibited high levels of MF BDNF protein. Interestingly, there was no significant decline with age in either genotype or sex. Notably, we found a correlation between MF BDNF protein and GC {Delta}FosB, a transcription factor that increases after 1-2 weeks of elevated neuronal activity. Remarkably, there was relatively little evidence of A{beta} in GCs or the GC layer even at old ages. Results indicate MF BDNF is stable in the Tg2576 mouse, and MF BDNF may remain unchanged due to increased GC neuronal activity, since BDNF expression is well known to be activity-dependent. The resistance of GCs to long-term A{beta} accumulation provides an opportunity to understand how to protect other vulnerable neurons from increased A{beta} levels and therefore has translational implications. SIGNIFICANCEDeclining hippocampal brain-derived neurotrophic factor (BDNF) has been implicated in the pathogenesis of Alzheimers disease (AD). However, few studies have examined where hippocampal BDNF protein has its highest concentration, and plays a critical role in memory, the dentate gyrus granule cell (GC) axons (mossy fibers; MFs). Using a well-established mouse model of cerebral amyloid overexpression, the Tg2576 mouse model of AD, we found that MF BDNF did not decline with age, suggesting a notable exception to the idea that reduced hippocampal BDNF contributes to AD pathobiology. We also identified that Tg2576 GC activity correlates with MF BDNF protein based on GC expression of the transcription factor {Delta}FosB. These data are consistent with the activity-dependence of MF BDNF. In addition, we found that Tg2576 GCs were relatively resistant to accumulation of amyloid-b, providing insight into AD resilience, which has strong therapeutic implications.