Post-Injury Treatment with 7,8-Dihydroxyflavone, a TrkB Receptor Agonist, Protects against Experimental Traumatic Brain Injury via PI3K/Akt Signaling
November
Post-Injury Treatment with 7,8-Dihydroxyflavone, a TrkB Receptor Agonist, Protects against Experimental Traumatic Brain Injury via PI3K/Akt Signaling
Chun-Hu Wu 0 1 4
Tai-Ho Hung 2 4
Chien-Cheng Chen 3 4
Chia-Hua Ke 0 3 4
Chun-Yen Lee 3 4
Pei-Yi Wang 3 4
Szu-Fu Chen 0 3 4
0 Departments of Physiology and Biophysics, National Defense Medical Center , Taipei, Taiwan , Republic of China,
1 Graduate Institute of Life Sciences, National Defense Medical Center , Taipei, Taiwan , Republic of China,
2 Department of Obstetrics and Gynecology, Chang Gung Memorial Hospital at Taipei and College of Medicine, Chang Gung University , Taipei, Taiwan , Republic of China,
3 Department of Physical Medicine and Rehabilitation, Cheng Hsin General Hospital , Taipei, Taiwan , Republic of China
4 Editor: Masuo Ohno, Nathan Kline Institute and New York University Langone Medical Center , United States of America
Tropomyosin-related kinase B (TrkB) signaling is critical for promoting neuronal survival following brain damage. The present study investigated the effects and underlying mechanisms of TrkB activation by the TrkB agonist 7,8-dihydroxyflavone (7,8-DHF) on traumatic brain injury (TBI). Mice subjected to controlled cortical impact received intraperitoneal 7,8-DHF or vehicle injection 10 min post-injury and subsequently daily for 3 days. Behavioral studies, histology analysis and brain water content assessment were performed. Levels of TrkB signaling-related molecules and apoptosis-related proteins were analyzed. The protective effect of 7,8-DHF was also investigated in primary neurons subjected to stretch injury. Treatment with 20 mg/kg 7,8-DHF attenuated functional deficits and brain damage up to post-injury day 28. 7,8-DHF also reduced brain edema, neuronal death, and apoptosis at day 4. These changes were accompanied by a significant decrease in cleaved caspase-3 and increase in Bcl-2/Bax ratio. 7,8-DHF enhanced phosphorylation of TrkB, Akt (Ser473/Thr308), and Bad at day 4, but had no effect on Erk 1/2 phosphorylation. Moreover, 7,8-DHF increased brain-derived neurotrophic factor levels and promoted cAMP response element-binding protein (CREB) activation. This beneficial effect was attenuated by inhibition of TrkB or PI3K/Akt. 7,8-DHF also promoted survival and reduced apoptosis in cortical neurons subjected to stretch injury. Remarkably, delayed administration of 7,8-DHF
Signaling
at 3 h post-injury reduced brain tissue damage. Our study demonstrates that
activation of TrkB signaling by 7,8-DHF protects against TBI via the PI3K/Akt but
not Erk pathway, and this protective effect may be amplified via the PI3K/Akt-CREB
cascades.
Traumatic brain injury (TBI) triggers a complex cascade of apoptotic events
which can contribute to delayed secondary injury processes [1]. Clinically,
activation of caspases [2], profiles of Bcl-2 family proteins, and the release of
proapoptotic proteins from mitochondria have been linked to TBI outcomes [3].
Experimental studies have also demonstrated that caspase inhibitors [2] or
overexpression of Bcl-2, an anti-apoptotic molecule, were protective against TBI [4].
These studies indicate that apoptosis could serve as a therapeutic target following
TBI.
On the other hand, TBI also activates endogenous protective mechanisms to
counteract secondary injury [5]. Tropomyosin-related kinase B (TrkB) signaling
has been considered to be an important protective mechanism induced by brain
damage and a key regulator of neuronal survival [5, 6]. The TrkB is activated by
binding to brain-derived neurotrophic factor (BDNF), which results in activation
of downstream phosphatidylinositol 3-kinase (PI3K)/Akt, mitogen-activated
protein kinase (MAPK)/Erk, or PLC-c signaling via receptor autophosphorylation
and dimerization [6, 7]. PI3K/Akt and Erk signaling pathways are the major
TrkBmediated survival pathways that promote neuronal survival and protect against
apoptosis [6, 7]. In addition, BDNF/TrkB signaling can promote further BDNF
production through cAMP-response element binding protein (CREB), a key
transcription factor for BDNF induction via activation of PI3K/Akt or Erk
signaling [810], which is emerging as a positive-feedback loop. TBI induces an
acute but transient increase in BDNF and TrkB mRNA, presumably indicating a
transient but failed endogenous protective response [11, 12]. These data suggest
that modulation of TrkB signaling have a therapeutic role in brain damage.
However, recombinant BDNF has so far yielded disappointing results in clinical
trials [13], possibly because of its short plasma half-life and poor blood-brain
barrier (BBB) penetration [14].
7,8-Dihydroxyflavone (7,8-DHF), a flavone derivative, has recently been
identified as a specific TrkB agonist which passes the BBB after peripheral
administration [15]. 7,8-DHF binds to the extracellular domain of TrkB, inducing
its dimerization and autophosphorylation, and activates the downstream PI3K/
Akt and Erk pathways in pri (...truncated)