The present study showed that levels of BMP6 were increased approximately two to four-fold in the hippocampus of patients with AD and in APP tg mice compared to controls, however no significant differences were detected in the mRNA levels of two other BMPs, BMP2 and BMP7. A striking pattern of BMP6 distribution was also observed in plaque-containing regions of the hippocampus in both AD patients and APP tg mice, where Aβ-containing plaques were surrounded by a ring-like pattern of BMP6 immunoreactivity. Since BMP6 is a secreted protein, and its primary reported role in the brain is in regulating developmental neurogenesis, it is possible that abnormally elevated levels of this protein in AD might affect adult neurogenesis in the hippocampus.
It is important to note that neurogenesis persists in the aged brain, however its rate declines with increasing age, as revealed by previous studies in rodents (Kuhn et al., 1996
; Kempermann et al., 1998
), non-human primates (Gould et al., 1999
), and humans (Cameron and McKay, 1999
). Despite this natural decline with age, previous studies have shown that the adult brain remains responsive to therapeutic interventions that enhance neurogenesis (Jin et al., 2003
; Wise, 2003
). Understanding the molecular mechanisms involved in AD-related alterations in neurogenesis might help guide the development of new therapies in this direction. Interestingly, paralleling the decline in both the pool of NPCs and their proliferative potential in AD, the levels of various neurotrophic factors, including brain-derived neurotrophic factor (BDNF), stem cell factor (SCF), and neurosteroids among others, are dysregulated in AD and FAD-linked models (Weill-Engerer et al., 2002
; Laske et al., 2008
; for review see Schindowski et al., 2008
). These studies suggest that the neurogenic niche is dramatically altered in the pathogenesis of AD, and other growth factors may be aberrantly expressed as well.
In support of the possibility that alterations in BMP6 expression in AD may dysregulate adult neurogenesis, we showed that markers of neurogenesis were altered in the hippocampus of AD patients and APP tg mice compared to controls. Furthermore, in an in vitro model of adult hippocampal neurogenesis, we showed that exposure to Aβ protein upregulated BMP6 expression levels, and treatment with recombinant BMP6 reduced NPC proliferation.
The members of the BMP family of growth factors belong to various subclasses distinguished by amino acid sequence similarity, and approximately 20 unique BMP ligands have been identified to date (Wordinger and Clark, 2007
). Although the primary function of many of the BMPs is in osteogenesis, the class 1 (BMP2/4) and class 2 (BMP5/6/7/8) families have been previously implicated in CNS development and disease. For example, BMPs are important regulators of embryonic neurogenesis (Mehler et al., 1997
) and during embryonic brain development, the inhibition of these growth factors promotes neurogenesis (Nakashima and Taga, 2002
). The canonical role of BMPs in the developing brain is to promote glial differentiation via Smad signaling (Mehler et al., 1997
), and previous studies have shown that BMP2 induces astrocyte differentiation in mouse embryonic neuroepithelial cell cultures (Gross et al., 1996
; Fukuda and Taga, 2005
; Fukuda et al., 2007
). BMPs have been implicated in embryonic (Mehler et al., 1997
) and adult neurogenesis (Colak et al., 2008
), and BMP activity has been shown to regulate synaptic plasticity in the adult hippocampus (Sun et al., 2007
), however the role of BMPs in neurodegenerative disorders in less clear.
In support of a role for BMPs in AD, two recent publications have shown that BMP4 levels are increased in the brains of an animal model of AD (Li et al., 2008b
; Tang et al., 2009
). High levels of BMP4 were associated with reduced neurogenesis in the DG of mice expressing mutant forms of APP and presenilin-1 (PS1) (Li et al., 2008b
; Tang et al., 2009
). Increased BMP4 expression was accompanied by reduced expression of the BMP inhibitor Noggin (Tang et al., 2009
), however the precise mechanisms through which BMP4 levels are increased during the pathogenesis of AD remain unclear.
In contrast to the defective neurogenesis observed in association with elevated levels of BMPs in AD (present study and (Li et al., 2008b
; Tang et al., 2009
)), another related protein, BMP7 (osteogenic protein-1), has been proposed to have neuroregenerative capacity in acute CNS injury. BMP7 has been shown to be neuroprotective against nigrostriatal toxicity in a 6-hydroxydopamine rat model of Parkinson’s disease (Harvey et al., 2004
) and is under investigation as a potential therapy in acute CNS injuries such as stroke (Kawabata et al., 1998
; Ren et al., 2000
; Chou et al., 2006
). Interestingly, both BMP7 and its receptors are upregulated after acute CNS injury or stroke (Lewen et al., 1997
; Charytoniuk et al., 2000
; Chang et al., 2003
; Harvey et al., 2005
), suggesting that the increased expression of BMP7 post-injury might represent a protective response. Another recent study showed that BMP6, but not BMP7, is protective against apoptosis triggered by acute potassium withdrawal in cerebellar granule neurons (Barneda-Zahonero et al., 2009
). Taken together, it is possible that acute upregulation of BMP expression may be a compensatory response that is protective in the short term, while chronic upregulation of BMPs in the AD mouse models may result in defective neurogenesis possibly via modification of expression or function of receptors and downstream signaling pathways. Moreover, it may be important to exercise caution in long-term therapeutic studies utilizing BMPs in acute CNS injury, as it is possible that elevated levels of these factors may have negative side-effects on adult neurogenesis.
Since our results show that BMP6 is specifically increased in a mouse model overexpressing APP, it is possible that APP or APP products such as Aβ might play a role in producing the abnormal levels of BMP6. The precise transcriptional regulators involved remain to be determined, however the present data show that treatment with exogenous Aβ up-regulates expression of BMP6, and it is possible that Aβ may stimulate signaling pathways that activate transcription of BMP6. In the in vitro
Aβ preparations, multiple species including monomers, dimers and trimers were detected after 24 hours of exposure. This is consistent with the predominant Aβ species detected in human AD and APP tg mouse brains, where most recent studies suggest that relatively small oligomers are responsible for the synaptotoxic effects of Aβ (Lacor et al., 2004
). In this context, it is possible that these smaller oligomeric species may also be responsible for alterations in neurogenesis, however future studies will be necessary to elucidate the precise species that modulate BMP6 expression and neurogenesis. Previous studies have shown that BMP6 production is transcriptionally regulated by hormones such as estrogen (Zhang et al., 2005
) and glucocorticoids (Liu et al., 2004
). Although increased glucocorticoid production has been identified as an early feature in AD pathogenesis that specifically targets the hippocampus (Dhikav and Anand, 2007
), it is unknown whether Aβ or its oligomeric aggregates might interact with glucocorticoid receptors, and future studies will be necessary to elucidate the precise transcription factors involved in Aβ-mediated regulation of BMP6 expression.
In addition to uncovering a potential link between elevated levels of BMP6 and reduced neurogenesis in AD brains and APP tg mice, the present study suggests that the increased levels of BMP6 observed in the hippocampus of AD patients and APP tg mice might directly impair the proliferation of hippocampal NPCs. In this context, we showed that treatment with recombinant BMP6 in an in vitro model of adult neurogenesis reduced proliferation without an overt toxic effect, however understanding the precise downstream pathways that impair NPC proliferation will require further investigation. We propose that normalization of BMP expression in models of AD may present a novel therapeutic approach for protecting against the neurogenic alterations in AD.
Taken together, the data presented here suggest that increased levels of BMP6 in AD might directly interfere with the process of adult neurogenesis in the hippocampus. We also demonstrated that BMP6 protein levels were increased in the non-neurogenic cortex, suggesting that the observed changes in BMP6 expression may occur in multiple brain regions. This could be specific to regions that are more severely affected by AD-related pathological alterations, or it could be a global effect. For the present study, we focused on the effects of increased BMP6 expression on neurogenic cells because of the known role of BMP6 in embryonic neurogenesis, however future studies will be necessary to elucidate the impact of increased BMP6 expression on mature neuronal circuitries in non-neurogenic brain regions.
In both neurogenic and non-neurogenic regions of the brain, APP products such as Aβ may be responsible for increased BMP production, possibly via receptor-mediated transcriptional regulation. Moreover, BMPs might have a neuroprotective effect during the acute phase of CNS injury, but in chronic neurodegenerative processes such as AD they may contribute to pathophysiology. Further investigation will be necessary to elucidate the precise mechanisms involved in regulating BMP expression in AD and the effects on NPCs in the adult hippocampus, and in future studies BMPs may be important targets for therapeutic intervention to rescue the neurogenic deficits associated with the pathogenesis of AD.