Regular paperAllopregnanolone restores hippocampal-dependent learning and memory and neural progenitor survival in aging 3xTgAD and nonTg mice
Introduction
Generation of new neurons is well documented to occur throughout adulthood across various species, including rodents (Ma et al., 2009, Zhao et al., 2008) and humans (Eriksson et al., 1998). In most mammals, adult neurogenesis is restricted to the subgranular zone (SGZ) of the dentate gyrus in the hippocampus (Aimone et al., 2010) and subventricular zone (SVZ) of the lateral ventricle (Alvarez-Buylla et al., 2008). Both SGZ and subventricular zone neurogenesis have been shown to play a significant role in various forms of learning and memory (Bath et al., 2008, Kim et al., 2007, Lazarini et al., 2009, Saxe et al., 2006, Waddell and Shors, 2008, Wang et al., 2010). While the regenerative potential of the mammalian brain is sustained throughout the life span, the magnitude of the proliferative efficacy of neural progenitors declines with age and diseases, such as Alzheimer's disease (AD) (Hattiangady and Shetty, 2008, Hattiangady et al., 2007, Kuhn et al., 1996, Lazarov et al., 2010, Rao et al., 2005). Age- and AD-associated declines in hippocampal neurogenesis have been observed in multiple mouse models of AD along with a concomitant decline in cognitive function (Ermini et al., 2008, Niidome et al., 2008, Rodríguez et al., 2008, Rodríguez et al., 2009, Taniuchi et al., 2007, Verret et al., 2007, Zhang et al., 2007). Recent studies have shown that impaired neurogenesis is an early event in the etiology of mouse models of familial AD (Demars et al., 2010, Wang et al., 2010). Malfunctioning presenilin-1, misprocessing of amyloid precursor protein and toxic effects of hyperphosphorylated tau and amyloid beta (Aβ) could be contributing to impaired neurogenesis in AD models.
Neural stem cells have been proposed to be a therapeutic strategy to restore brain function in neurodegenerative disease. Several studies have shown restoration of cognitive function following neural stem cell transplants (Blurton-Jones et al., 2009, Ebert et al., 2008, Lu et al., 2003, Park et al., 2006). Ablated regions can be repopulated by neural stem cells under optimal conditions. Collectively, these data provide preclinical evidence for the potential of neural progenitor cells as a therapeutic avenue to regenerate neuronal circuits and to restore neurological function. While the transplant strategy shows efficacy, the clinical utility of this approach is limited. An alternative approach is to promote the intrinsic regenerative capacity of the brain. The challenge in this approach is the neurochemical and neuropathological milieu of the diseased brain. Changes in local biochemical milieu, including growth factors, cytokines, neurotransmitters, and neurosteroids can contribute to impaired neurogenesis (Brinton, 2009, Brinton et al., 2008, Demars et al., 2010, Wang et al., 2010). Reversing the decline in adult neurogenesis using regenerative factors could be a therapeutic strategy to reverse disease- and age-associated cognitive decline.
Neurosteroids are a class of therapeutic agents with blood-brain barrier-penetrating physicochemical properties (Brinton and Wang, 2006). One such neurosteroid is allopregnanolone (APα), a metabolite of progesterone, which is synthesized de novo in both embryonic and adult central nervous system (CNS) (Baulieu et al., 2001, Mellon and Griffin, 2002a, Mellon and Griffin, 2002b) as well as in pluripotent progenitor cells (Gago et al., 2004, Lauber and Lichtensteiger, 1996) and which exhibits an age- and AD-associated decline (Bernardi et al., 1998, Brinton et al., 2008, Marx et al., 2006, Weill-Engerer et al., 2002). We have recently shown that the triple-transgenic mouse model of Alzheimer's disease (3xTgAD), overexpressing APPSWE, PS1M146V, and tauP301L (Oddo et al., 2003b), has both neurogenic and cognitive deficits beginning at 3 months of age (Wang et al., 2010). Despite the lack of evident AD pathology, basal level of newborn cells in the SGZ of the hippocampus was significantly lower in these mice as compared with the age-matched nontransgenic (nonTg) mice. The concentration of APα was found to be significantly reduced within the brain and serum of 3xTgAD mice as compared with nonTg mice, similar to that observed in human AD Patients (Weill-Engerer et al., 2002). Furthermore, severe cognitive deficits were observed in 3-month-old 3xTgAD male mice as compared with nonTg mice in a hippocampal-dependent associative task. Treatment with APα reversed this cognitive deficit and restored learning and memory performance to the level of normal nonTg mice. APα also induced a significant increase in survival of neural progenitor cells, significantly correlated with APα-induced enhanced memory performance.
3xTgAD mice manifest age-dependent neuropathology of AD in the form of both Aβ plaques and neurofibrillary tangles along with age-dependent learning and memory deficits (Billings et al., 2005, Oddo et al., 2003a, Oddo et al., 2003b). The current study was directed at determining the efficacy of APα to restore associative learning and memory function and to promote neural progenitor generation and survival in the aged 3xTgAD mice at varying degrees of Aβ burden, from intraneuronal accumulation to extraneuronal plaque formation. Parallel analyses were conducted in the age-matched nonTg male mice to investigate the neurogenic and cognitive deficits that accompany aging in normal mice and probe the efficacy of APα to reverse those deficits. Results of our current analyses are further indicative of the potential of APα as a regenerative therapeutic to prevent or delay neurogenic and cognitive deficits associated with mild cognitive impairment, Alzheimer's disease, and late-stage normal aging.
Section snippets
Chemicals
Allopregnanolone (APα; 3α-hydroxy-5α-pregnan-20-1) (also known as AP, Allo, or THP) used for this study was purchased from Steraloids, Inc. (Newport, RI, USA). Chemicals were from MP Biomed (Irvine, CA, USA) unless otherwise noted.
Animals and treatment
Breeding pairs of the triple transgenic Alzheimer's disease mouse (3xTgAD, homozygous mutant of human APPSwe, PS1M146v, and tauP301L) and its background strain (129/Sv × C57BL/6) were obtained from Dr. Frank LaFerla (University of California, Irvine) and the colonies
Rationale for and design of APα treatment experiments
Our previous analyses demonstrated that APα reversed cognitive and neurogenic deficits of male 3xTgAD mice (3-month-old) prior to the onset of AD pathology (Wang et al., 2010). The current study was designed to determine the efficacy of APα to promote cognitive and neurogenic function in 3xTgAD male mice with mild (6-month-old), moderate (9-month-old), and severe (12-month-old) Aβ accumulation. Cognitive performance was assessed using the trace eyeblink conditioning paradigm (Fig. 1A) which
Discussion
Using a triple transgenic mouse model of Alzheimer's disease (3xTgAD) and its background strain control (nonTg), the data indicate that aging and Alzheimer's disease burden diminishes hippocampal neurogenesis and associated cognitive performance. These effects on hippocampal neurogenesis were differentially regulated by aging and AD pathology, with AD pathology affecting cell survival and hippocampal-dependent associative learning and memory more severely than normal aging. APα reversed the
Disclosure statement
Patents pending on allopregnanolone as a therapeutic for mild cognitive impairment and Alzheimer's disease.
All experiments strictly conformed to the Animal Welfare Act, Guide to Use and Care of Laboratory Animals, and the U.S. Government Principles of the Utilization and Care of Vertebrate Animals Used in Testing, Research, and Training guidelines on the ethical use of animals.
Acknowledgements
This research was supported by grants from by the National Institute on Aging U01AG031115, the Alzheimer Drug Discovery Foundation, and the Kenneth T. and Eileen L. Norris Foundation to RDB.
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