Mol. Neurobiol. 2014; Nov. 19

miR-126 regulates growth factor activities and vulnerability to toxic insult in neurons

Kim W, Noh H, Lee Y, Jeon J, Shanmugavadivu A, McPhie DL, Kim KS, Cohen BM, Seo H, Sonntag KC

 

Department of Psychiatry, McLean Hospital, Harvard Medical School, 115 Mill Street, Belmont, MA, 02478, USA.

ksonntag@mclean.harvard.edu

 

Abstract

Dysfunction of growth factor (GF) activities contributes to the decline and death of neurons during aging and in neurodegenerative diseases. In addition, neurons become more resistant to GF signaling with age. Micro (mi)RNAs are posttranscriptional regulators of gene expression that may be crucial to age- and disease-related changes in GF functions. MiR-126 is involved in regulating insulin/IGF-1/phosphatidylinositol-3-kinase (PI3K)/AKT and extracellular signal-regulated kinase (ERK) signaling, and we recently demonstrated a functional role of miR-126 in dopamine neuronal cell survival in models of Parkinson’s disease (PD)-associated toxicity. Here, we show that elevated levels of miR-126 increase neuronal vulnerability to ubiquitous toxicity mediated by staurosporine (STS) or Alzheimer’s disease (AD)-associated amyloid beta 1-42 peptides (Aβ1-42). The neuroprotective factors IGF-1, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and soluble amyloid precursor protein α (sAPPα) could diminish but not abrogate the toxic effects of miR-126. In miR-126 overexpressing neurons derived from Tg6799 familial AD model mice, we observed an increase in Aβ1-42 toxicity, but surprisingly, both Aβ1-42 and miR-126 promoted neurite sprouting. Pathway analysis revealed that miR-126 overexpression downregulated elements in the GF/PI3K/AKT and ERK signaling cascades, including AKT, GSK-3β, ERK, their phosphorylation, and the miR-126 targets IRS-1 and PIK3R2. Finally, inhibition of miR-126 was neuroprotective against both STS and Aβ1-42 toxicity. Our data provide evidence for a novel mechanism of regulating GF/PI3K signaling in neurons by miR-126 and suggest that miR-126 may be an important mechanistic link between metabolic dysfunction and neurotoxicity in general, during aging, and in the pathogenesis of specific neurological disorders, including PD and AD.

PMID: 25407931

 

Supplement

One of the main hurdles in developing novel therapeutics for age-related disorders is the still limited understanding of both the biology of normal aging and the mechanisms of disease pathology. Aging and age-related disorders are characterized by slow progressive deterioration or death of neurons, and are influenced by age- and disease-specific factors and molecules that are important in cell survival, including growth factors (GFs). If the cellular defence mechanisms are compromised, the penetrance of disease-specific mechanisms becomes higher and cell survival less likely. In fact, there is evidence that neurons in aging and neurodegeneration become resistant to GF activities, such as, e.g., Insulin/IGF-1, nerve growth factor (NGF), glial cell-derived neurotrophic factor (GDNF), and others. Among the regulatory factors that govern gene and protein networks and, consequently, influence neuronal health and function are small molecules, including miRNAs, which are non-coding RNAs that mainly regulate gene expression at the posttranscriptional level. There is an increasing body of evidence that miRNAs contribute to aging and the pathology of neurological disorders (1, 2).

 

Fig.1

Figure 1. Schematic of miR-126 as a master regulator in GF signaling cascades. miR-126 targets key factors in PI3K/AKT/GSK-3b and ERK signaling, including IRS-1, p85β, and SPRED1, which are downregulated when miR-126 levels are increased. Consequently, expression and activation levels of factors in PI3K and ERK signaling cascades are modulated leading to an increase of pan-neuronal (STS) or disease-associated (6-OHDA, Ab1-42) toxicity, and an impairment of neuroprotective GFs. Ab1-42 competes with IGF-1 and NGF on insulin (IR), IGF-1R or TrkA/p75NTR receptor binding and appears to directly interfere with PI3K activation of AKT. sAPPa acts synergistically with NGF or IGF-1 and reverses Ab1-42 toxicity.

To gain insight in the molecular disturbances of neurons in neurodegeneration, we previously analyzed the transcriptional profiles of substantia nigra (SN) dopamine (DA) neurons isolated by laser-assisted microdissection from postmortem Parkinson’s disease (PD) patients’ brains and age-matched controls, demonstrating that PD DA neurons exhibit dysregulated gene expression networks (3, 4). To further understand the regulatory mechanisms involved in these expression networks, we also determined their miRNA profiles and identified a network of miRNA/target-gene associations that is consistent with dysfunctional gene and signaling pathways in PD pathology (5). One of the upregulated miRNAs in PD DA neurons was miR-126, which has recently been described as a master regulator of GF-activated PI3K/AKT and MAPK/ERK signaling pathways in non-neuronal cells. In a series of experiments in DA neuronal cell systems, we could show that elevated levels of miR-126 impaired the trophic effects of IGF-1 and increased cellular vulnerability to 6-OHDA, an inducer of oxidative stress to mimicking aspects of PD pathology (6). Mechanistically, increased expression of miR-126 downregulated factors within the Insulin/IGF-1/PI3K/AKT and ERK signaling cascades, including its targets IRS-1, PIK3R2 (p85β), and SPRED1 (Fig. 1). In turn, inhibition of miR-126 increased IGF-1 trophism and conferred neuroprotection to 6-OHDA toxicity. Together, these data indicated that miR-126 may play a role in (DA) neuronal cell survival and in the pathology of neurodegenerative disorders, such as PD, at least in part by regulating GF/PI3K/AKT and MAPK/ERK signaling.

Based on these results we hypothesized that miR-126 may play a general role in regulating GF activities in neurons. We, therefore, extended our studies to test its function in context of other non-specific and disease-specific toxic factors, such as Staurosporine (STS) or Alzheimer’s disease (AD)-associated amyloid beta 1-42 oligomers (Aβ1-42), and neuroprotective GFs other than IGF-1, including NGF, brain-derived neurotrophic factor (BDNF), and soluble amyloid precursor protein α (sAPPα), one of the cleaved products of amyloid precursor protein (APP) that has neurotrophic and neuroprotective properties by synergistically acting with NGF and IGF-1 and reversing the toxic effects of Aβ (7). We found that small increases of miR-126 are toxic in cortical and hippocampal neurons and impair the neuroprotective effects of all GFs tested in context of STS and Aβ1-42 toxicity by profoundly downregulating factors in the PI3K/AKT/GSK-3β and MAPK/ERK signaling pathways (Fig. 1). In turn, inhibition of miR-126 enhances the actions of GFs and is neuroprotective without disturbing normal neuronal cell function.

 

Fig.2

Figure 2. Working model of miR-126 as a key factor in neurotoxocity and -protection. The DNA sequence of miR-126 is located in Intron 7 of the EGFL7 gene. In neurons, miR-126 is expressed at low levels without affecting normal homeostasis (A). Elevated miR-126 levels, e.g., in response to stress, increase neurotoxicity by impairing PI3K and ERK signaling and diminishing the neuroprotective effects of GFs (B). Inhibition of miR-126 is neuroprotective, because its interference in GF/PI3K and ERK signaling cascades is abolished (C).

Altogether, our studies identified a novel mechanism in neurons, mediated by miR-126, which modulates the effects of numerous neurotrophic and neuroprotective GFs by regulating target factors in the PI3K/AKT/GSK-3β and MAPK/ERK signaling pathways (Fig. 2). Our finding that miR-126 is expressed at low levels in neurons and that even small increases (4-5 fold) can have profound effects on cell survival and vulnerability to toxic insult is consistent with the view that the slow progressive deterioration or death of neurons in aging and age-related disorders is a consequence of small disturbances in the balance between neuroprotection and neurotoxic stress. The observation that miR-126 is a master regulator of PI3K and MAPK/ERK activities in neurons could contribute to further understanding the molecular mechanisms involved in aging and neurodegeneration, and the influence of age- and disease-specific factors, such as genetic predisposition, dysfunctional proteins, compensatory mechanisms in cell survival, inflammation, and also neuronal resistance to GF activities that have been implicated in both aging and diseases, such as AD and PD. In fact, our data suggest that miR-126 may be an important mechanistic link between metabolic dysfunction and neurotoxicity. The observation that inhibition of miR-126 is neuroprotective, while seemingly not impairing normal neuronal cell function, makes it a potential therapeutic target to treat or prevent age-associated dysfunction and degeneration.

 

 

 

References

  1. Sonntag KC. MicroRNAs and deregulated gene expression networks in neurodegeneration. Brain Res. 2010;1338:48-57.
  2. Sonntag KC, and Wahlestedt C. RNA mechanisms in CNS systems and disorders. Brain Res. 2010;1338:1-2.
  3. Simunovic F, Yi M, Wang Y, Macey L, Brown LT, Krichevsky AM, Andersen SL, Stephens RM, Benes FM, and Sonntag KC. Gene expression profiling of substantia nigra dopamine neurons: further insights into Parkinson’s disease pathology. Brain. 2009;132(7):1795-809.
  4. Simunovic F, Yi M, Wang Y, Stephens R, and Sonntag KC. Evidence for gender-specific transcriptional profiles of nigral dopamine neurons in Parkinson disease. PLoS One. 2010;5(1):e8856.
  5. Briggs C, Kong B, Wang Y, Iyer L, and Sonntag K-C. Midbrain dopamine neurons in Parkinson’s disease exhibit a dysregulated miRNA and target-gene network. Brain Res. 2015; In Press.
  6. Kim W, Lee Y, McKenna ND, Yi M, Simunovic F, Wang Y, Kong B, Rooney RJ, Seo H, Stephens RM, et al. miR-126 contributes to Parkinson’s disease by dysregulating the insulin-like growth factor/phosphoinositide 3-kinase signaling. Neurobiol of Aging. 2014;35(7):1712-21.
  7. Kim W, Noh H, Lee Y, Jeon J, Shanmugavadivu A, McPhie DL, Kim KS, Cohen BM, Seo H, and Sonntag KC. MiR-126 Regulates Growth Factor Activities and Vulnerability to Toxic Insult in Neurons. Mol Neurobiol. 2014. Nov. 19 [Epub ahead of print].

 

Acknowledgements

This work was supported by a grant from the Massachusetts’ Alzheimer’s Disease Research Center and the Harvard NeuroDiscovery Center, and National Institute of Neurological Disorders and Stroke R21NS067335 to Kai C Sonntag. Additional support was from Drs. Bruce Cohen and Kwang-Soo Kim, Mclean Hospital, Harvard Medical School.

 

Contact

Kai-Christian Sonntag, M.D., Ph.D.

Assistant Professor in Psychiatry (Neuroscience)

McLean Hospital, Harvard Medical School

MRC 223, Mailstop 326,

115 Mill Street, Belmont, MA 02478

ksonntag@mclean.harvard.edu

 

 

 

Multiselect Ultimate Query Plugin by InoPlugs Web Design Vienna | Webdesign Wien and Juwelier SchönmannMultiselect Ultimate Query Plugin by InoPlugs Web Design Vienna | Webdesign Wien and Juwelier Schönmann