Valproic acid improves outcome after rodent spinal cord injury: Potential roles of histone deacetylase inhibition

Abstract

Histone deacetylases (HDAC) inhibitors including valproic acid (VPA) have emerged as a promising therapeutic intervention in neurological disorders. We investigated the levels of acetylated histone and the therapeutic potential of VPA in a rat model of spinal cord injury (SCI). At different time points (12 h, 1 day, 3 days, 1 week and 2 weeks) after SCI or sham surgery, the spinal cords were collected to evaluate the levels of acetylated histone H3 (Ac-H3) and H4 (Ac-H4). VPA or vehicle was injected for 1 week starting immediately after SCI and histone acetylation, apoptosis, as well as neurobehavior were observed to test the effect of VPA. The levels of Ac-H3 and Ac-H4 in the injured spinal cord started to significantly decrease as early as day 1, and remained below those in uninjured controls for at least 2 weeks after SCI. Injection of VPA markedly prevented the reductions of Ac-H3 and Ac-H4, upregulated the expressions of Hsp70 and Bcl-2, reduced apoptosis and finally promoted locomotion recovery. Our data demonstrated that SCI led to marked reduction in histone acetylation; VPA was neuroprotective in the SCI model, and the mechanism may involve HDAC inhibition and protective proteins induction.

Introduction

Histone modifications have the potential to influence many fundamental biological processes (Carey and La Thangue, 2006, Kouzarides, 2007). Histone deacetylases (HDACs) play a key role in the homeostasis of histone acetylation and in regulating fundamental cellular activities such as transcription. A wide range of brain disorders are linked to histone hypoacetylation and associated transcriptional dysfunction. HDAC inhibitors (HDACi), through enhancing acetylation of histone, restoring the balance of histone acetylation, and adjusting transcriptional dysfunction, have become a promising intervention for central nervous system (CNS) diseases (Abel and Zukin, 2008, Chuang et al., 2009, Dietz and Casaccia, 2010, Kazantsev and Thompson, 2008, Langley et al., 2005).

Valproic acid (VPA) is a drug widely prescribed to treat neurological and psychiatric disorders including seizures, neuropathic pain and bipolar mood disorder. It exerts great neuroprotective effects for a variety of neurological conditions, including spinal muscular atrophy (SMA), amyotrophic lateral sclerosis (ALS), transient or permanent middle cerebral artery occlusion (MCAO), intracerebral hemorrhage (ICH), traumatic brain injury (TBI), sciatic nerve axotomy and experimental autoimmune neuritis (EAN) (Brichta et al., 2006, Cui et al., 2003, Dash et al., 2010, Feng et al., 2008, Kim et al., 2007, Ren et al., 2004, Rouaux et al., 2007, Sinn et al., 2007, Sugai et al., 2004, Tsai et al., 2008, Zhang et al., 2008). VPA was shown to be an HDAC inhibitor in 2001 (Phiel et al., 2001). Through enhancing acetylation of histones H3 and H4 and adjusting gene transcription (Fukuchi et al., 2009), VPA reduced neuron death induced by lipopolysaccharide (LPS), excitotoxicity or aging (Chen et al., 2007, Jeong et al., 2003, Kanai et al., 2004). In animal models of ALS, ICH, MCAO and TBI, the protective effects of VPA are linking with HDAC inhibition (Dash et al., 2010, Kim et al., 2007, Ren et al., 2004, Rouaux et al., 2007, Sinn et al., 2007). VPA actions as a neuroprotectant and hence has new potential in the treatment of neurodegenerative diseases (Nalivaeva et al., 2009).

HDACi promote the transcription and expression of neuroprotective genes such as Hsp70 and Bcl-2 (Faraco et al., 2006, Ren et al., 2004, Sinn et al., 2007, Yildirim et al., 2008). For rats suffering from permanent focal cerebral ischemia, VPA treatment resulted in increased levels of acetylated histone H3 and Hsp70, as well as reduced brain damage and improved functional outcome (Kim et al., 2007); in rats with transient focal cerebral ischemia, VPA resulted in a time-dependent increase in acetylated H3 levels and time-dependently up-regulated Hsp70, suggesting the protective mechanism of VPA may involve HDAC inhibition and Hsp70 induction (Ren et al., 2004). VPA also increased levels of anti-apoptotic factors, Bcl-2 and Bcl-x (L), and ameliorated the degeneration of spinal motor neurons in spinal muscular atrophy (SMA) mice (Tsai et al., 2008).

It has been found that over expression of Hsp70 or Bcl-2 exert protective effects against spinal cord injury (SCI) in animal models (Khalatbary et al., 2010, Shin et al., 2007, Yune et al., 2004, Yune et al., 2008). Hsp70 is known to protect from both necrotic and apoptotic cell death. On severe spinal cord ischemic injury in rabbits, cyclosporin A elevated neurologic recovery with induced upregulation of Hsp70 (Shin et al., 2007). Bcl-2 overexpression prevents cells from undergoing apoptosis in response to a variety of stimuli. After SCI, estradiol treatment improved functional recovery in the injured rat, in part, by increasing expression of the anti-apoptotic factor Bcl-2 and reducing apoptotic cell death (Yune et al., 2004, Yune et al., 2008).

Hypoacetylation of histone occurs in a large number of CNS diseases and HDACi have emerged as a promising therapeutic intervention for these disorders. However, there are few investigations observing the levels of histone acetylation and the therapeutic potential of HDACi such as VPA after SCI. In the present research, in a rat model of spinal cord injury, we investigated the levels of acetylated histone, and the effect of VPA on functional recovery and apoptosis, as well as the possible mechanism.