The link between neurogenesis and pathology in AD mouse models has been somewhat controversial, as questions of cause and effect have cast a shadow over interpretation of the data. For example, it is not clear if altered neurogenesis stems from, or contributes to, amyloid-β (Aβ) pathology. Both papers avoid this complication by studying neurogenesis in very young animals, before the emergence of detectable amyloid deposits. Lazarov and colleagues examined double-transgenic mice carrying mutant human amyloid precursor protein (APP) and presenilin 1 (PS1) genes (APPSwe/PS1ΔE9), while Brinton’s group used the triple-transgenic (3xTG) animals developed in Frank Laferla’s lab at the University of California, Irvine (see Oddo et al., 2003). These animals carry a mutant human tau gene (TauP301L) in addition to APPswe and a point-mutated PS1 gene (PS1M146V).

Both groups assessed neurogenesis by measuring the incorporation of bromodeoxyuridine (BrdU), a marker of DNA synthesis, into proliferating cells in hotbeds of brain neurogenesis. Lazarov’s group looked at both the subventricular zone (SVZ) of the cortex, where new neurons destined for the olfactory lobe arise, and the subgranular zone (SGZ) of the dentate gyrus, which supplies new neurons to the hippocampus. Brinton’s group examined the SGZ only, but with the added bonus of correlating neurogenesis with hippocampal-dependent cognition.

First author Michael Demars and colleagues in Lazarov’s lab found that, compared to wild-type mice, the numbers of BrdU-positive proliferating cells and new neurons are drastically lower in the SVZ and SGZ of double-transgenic mice at two months of age, which is three months prior to the typical emergence of amyloid deposits in these animals. Similarly, Jun Ming Wang and colleagues in Brinton’s lab detected about half the normal number of proliferating cells in the SGZ of three-month-old 3xTg mice, despite the absence of Aβ immunoreactivity in the brains of these animals (3xTg mice typically show Aβ deposits starting at six months). Together, the findings indicate that neurogenesis could be an early casualty in mice expressing AD-associated genes.

Does this lost neurogenesis matter that much? Wang and colleagues collaborated with Richard Thompson, also at USC, to test their mice in a hippocampal-dependent associative learning task—the trace eye-blink conditioning paradigm—which is known to depend on the formation of new neurons (see Shors et al., 2001). Brinton explained that the animals have to associate temporally separated stimuli (a tone and a puff of air to the eye, much like that pesky pressure test at your ophthalmologist), and then retain this association over time (the animals are trained for five days, then tested nine days later). Brinton speculated that this test could be particularly relevant to AD. “People with Alzheimer’s have trouble remembering new information over time, and associating pieces of information received in one context with information from another context,” she said. The young AD mice flunked this test. Compared to wild-type controls, three-month-old 3xTg animals had trouble both learning the task and remembering the association. Again, the findings point to deficits that emerge prior to overt amyloid pathology.

What brings neurogenesis down in these animals? Brinton’s lab tested allopregnanolone, a progesterone derivative that stimulates proliferation of neuron precursors (see ARF related news story). Her associates found that its levels were lower in the 3xTg mice compared to wild-type animals. Intriguingly, when the researchers gave the steroid to three-month-old 3xTg mice, it dose-dependently boosted neurogenesis. On top of that, a single sub-cutaneous injection of allopregnanolone restored associative learning by the fifth day of training in the eye-blink test. Mice that received this single dose performed like wild-type animals when tested nine days after training. The data suggest that at least in young transgenic animals, allopregnanolone can restore learning and memory.

Brinton pointed out that allopregnanolone is unlikely to be the full story behind the neurogenesis deficit, and noted that growing evidence links PS1, which is mutated in both these animal models, to neurogenesis (see ARF related news story), possibly via Notch signaling.

Whatever causes the neurogenesis problem, the real question is whether it is causal or results from the degenerative process. “This is where Lazarov’s study could be really informative,” said Brinton. Demars and colleagues tested for tau pathology in four-month-old double-transgenic mice. They discovered increased hyperphosphorylated tau (AT8 immunoreactivity) in the SVZ, hippocampus, and cortex compared to single transgenic controls. In the SVZ, they also found increased immunoreactivity to PHF-1, an antibody that recognizes hyperphosphorylated forms of tau that are particularly prone to aggregation. The PHF-1 immunoreactivity in the SVZ colocalized with doublecortin, indicating the aberrant tau was in new neurons.

Neuroblasts that migrate from the SVZ to the olfactory bulb expressed hyperphosphorylated (PHF-1) tau as well. “The demonstration of hyperphosphorylated tau in this proliferative zone is disturbing, because that will impair migration and proliferation of cells,” said Brinton, noting that disturbed olfaction correlates with AD in humans (see ARF related news story).

It is currently impossible to study adult neurogenesis in humans directly; therefore, scientists do not know if the process is even required for cognition or changes in AD. But if animal work is anything to go by, then people could do worse than boosting their neurogenesis. Numerous strategies have shown that the process is needed for proper learning and memory in older rodents and that it declines as animals age. Just recently, researchers led by Gerd Kemperman, University of Freiburg, Germany, reported that neurogenic markers in aging humans parallel patterns seen in aging animals, suggesting human neurogenesis also tapers off with age (see also ARF related conference story on neurogenesis and aging). “If we can attenuate this decline, we are probably going to help Alzheimer’s patients significantly,” suggested Lazarov.

Brinton holds a patent on the use of allopregnanolone for treating dementia. With venture philanthropy funding from the Alzheimer’s Drug Discovery Foundation, she hopes to test the compound in preclinical and clinical trials. It is currently being tested for traumatic brain injury (see ClinicalTrials.gov). In the meantime, there may be a non-pharmacological way to egg on your neurogenesis. Exercise and/or environmental enrichment, where animals are removed from their boring lab cages and placed into a “fun” one with lots of toys to explore, enhance neurogenesis and even protect against Aβ pathology. Lazarov and colleagues recently confirmed as much (see Hu et al., 2010, ARF Sorrento story). “This is a very promising approach because it is not invasive; it just involves acquiring a specific lifestyle,” said Lazarov.—Tom Fagan.

References:
Wang JM, Singh C, Liu L, Irwin RW, Chen S, Chung EJ, Thompson RF, Brinton RD. Allopregnanolone reverses neurogenic and cognitive deficits in mouse model of Alzheimer’s disease. PNAS 2010, March 15 early online edition. Abstract

Demars M, Hu Y-S, Gadadhar A, Lazarov O. Impaired neurogenesis is an early event in the etiology of familial Alzheimer's disease in transgenic mice. Journal of Neuroscience Research 2010 March 5 early edition. Abstract