Geon GSK-3, Valproic Acid and Epilepsy Topics

 

Glycogen synthase kinase-3 (GSK-3) is a kinase that catalyzes phosphorylation of a number of proteins. A growing body of evidence suggests that the activation of GSK-3 enhances neuronal excitability (Hsu et al., 2016; Paul et al., 2016; Crofton et al., 2017; Paul et al., 2017). On the other hand, the antiepileptic drug, valproic acid (VPA), may also act on GSK-3. This article will explain how the activation of GSK-3 increases neuronal excitability and how VPA has antiepileptic effect.

The Role of GSK-3 in Excitability

GSK-3 has two subtypes: GSK-3α and GSK-3β. They are constitutively active, but can be inactivated through the phosphorylation of a single residue: serine 21 for GSK-3α or serine 9 for GSK-3β. Akt (also called PKB) is a major kinase that regulates the activity of GSK-3 via phosphorylation. Alternatively, calpain may remove the N-terminal regulatory domain, rendering GSK-3 persistently active (Goñi-Oliver et al., 2007).

Akt plays a central role in cell signaling (Hemmings and Restuccia, 2012). Many signaling pathways converge to the activation of Akt, including Trk and Wnt pathways (Figure 1).

Image

Figure 1. Signaling pathways toward Akt and GSK-3. Trk stands for tyrosine (or tropomyosin) receptor kinase. One of its subtypes, TrkB, is the receptor for brain-derived neurotrophic factor (BDNF). [Source: Wildburger and Laezza, 2012]

GSK-3 may act on a variety of targets to modulate neuronal excitability.

  1. Potassium channels. Under normal physiological conditions, opening of the potassium channel allows K+ ions to flow outward, resulting in membrane hyperpolarization. This has inhibitory effect on neuronal firing. GSK-3 may phosphorylate and inactivate the potassium channel Kv7.2 (encoded by KCNQ2) (Wildburger and Laezza, 2012) or Kv4.2 (Scala et al., 2015). Thus, through its action on potassium channels, GSK-3 may enhance excitability.
  2. Sodium channels. In contrast to potassium channels, the Na+ influx through sodium channels makes the membrane more depolarized, which has excitatory effect on neuronal firing. The sodium channels, Nav1.6 and Nav1.2, are modulated by their association with the protein FGF14 (Laezza et al., 2009). GSK-3 may promote the association between the sodium channel and FGF14, thereby enhancing excitability (Shavkunov et al., 2013).
  3. AMPA receptors. They conduct mainly Na+ ions. Elevated GSK-3 activity enhances the insertion of AMPA receptors into the postsynaptic membrane, resulting in higher excitability (Wei et al., 2010; Wildburger and Laezza, 2012).
  4. Tau phosphorylation. The Tau protein is a major target of GSK-3. Hyperphosphorylated Tau at the axon initial segment has been shown to reduce excitability (see The Role of Microtubules and Tau Proteins in Neuronal Excitability).

We see that activation of GSK-3 leads to higher excitability except #4. Therefore, the net effect of GSK-3 activation on excitability is most likely positive.

Differential Effects of Akt on Excitability

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Figure 2. The effects of Akt on neuronal excitability. GSK-3 may modulate excitability via a variety of targets as described in the last section. This figure shows only Nav1.6 and Kv7.2. [Modified from Bozzi and Borrelli, 2013]

Akt has opposite effects on excitability. Its activation enhances mTOR activity, leading to increased excitability (see Epilepsy and mTOR), but reduces GSK-3 activity, resulting in decreased excitability. Since GSK-3 acts directly on ion channels to modulate excitability, the impact of GSK-3 on excitability is more direct and faster than mTOR. However, the mTOR-modulated excitability can last longer, because the lifetime of Tau protein is usually longer than the phosphorylation state of GSK-3.

The Antiepileptic Mechanism of VPA

VPA is an effective broad-spectrum antiepileptic drug (Tomson et al., 2015). Its underlying mechanism could involve both GSK-3 and mTOR. As discussed above, the activities of GSK-3 and mTOR cannot be simultaneously repressed by Akt alone. However, VPA may stimulate two independent pathways that lead to the suppression of short-term seizures by GSK-3 as well as the prevention of chronic epilepsy by mTOR.

Suppressing GSK-3 Activity Through Wnt Signaling

In the animal model for epilepsy, status epilepticus (prolonged seizures) may be induced by kainate, pentylenetetrazole (PTZ), or pilocarpine (Zeng et al., 2009; San et al., 2015; Hong et al., 2013). During status epilepticus, phosphatidylinositol (3-5) trisphosphate (PIP3) is reduced (Chang et al., 2014), which could arise from the cleavage of full-length TrkB by calpain (Xie et al., 2014), thereby disrupting the BDNF/TrkB pathway that contributes to the production of PIP3 (Figure 1). Calpain is a Ca2+-dependent protease responsible for the cleavage of proteins. It has been implicated in epilepsy (Feng et al., 2011).

Seizure activity may cause Ca2+ overload to activate calpain, resulting in the reduction of PIP3, which in turn enhances excitability and seizures by increasing GSK-3 activity. This explains the positive feedback between seizures and PIP3 (Chang et al., 2014). VPA can increase PIP3 and inhibit GSK-3 via the Wnt pathway (Figure 1 and Bug et al., 2005). Consistent with this notion, VPA also preserves M-current during seizures (Kay et al., 2015). The Kv7.2 channel regulated by GSK-3 is a major contributor to the M-current (Brown and Passmore, 2009).

Suppressing mTOR Expression Via PPAR-γ Activation

After status epilepticus, mTOR is activated (Zeng et al., 2009; San et al., 2015), together with the increase of Tau proteins (Pollard et al., 1994), which may set the stage for later spontaneous recurrent seizures. The peroxisome proliferator activated receptor-γ (PPAR-γ) is a transcription factor regulating gene expression. It has been shown that VPA can directly bind and activate PPAR-γ (Zuckermann et al., 2015). The anti-epileptogenic property of VPA could be due to the activation of PPAR-γ, thereby suppressing the mTOR expression (San et al., 2015; Dell'Accio and Sherwood, 2015).

Valproic acid is effective not only for epilepsy, but also for bipolar disorder. The link between mental disorders and epilepsy is further supported by the finding that pioglitazone, a PPAR-γ agonist, improves autism (Boris et al., 2007; Ghaleiha et al., 2015), major depressive disorder (Sepanjnia et al., 2012) and bipolar disorder (Kemp et al., 2014).

Prenatal Exposure to Valproic Acid

Although VPA is well tolerated and safe in adults, prenatal exposure to VPA can induce developmental abnormalities such as autism (Ranger and Ellenbroek, 2015). This agrees with the finding that VPA can stimulate both Wnt and PPAR-γ pathways. The Wnt pathway enhances mTOR signaling which is suppressed by the PPAR-γ pathway in adults. However, in the fetal brain, the expression level of PPAR-γ is very small (Michalik et al., 2002). Hence, VPA may cause hyperactive mTOR in the fetus, resulting in autism (see Autism Spectrum Disorders). Consistent with this notion, resveratrol - an inhibitor of mTOR signaling (Liu et al., 2010) - prevents social deficits in animal model of autism induced by valproic acid (Bambini-Junior et al., 2014).

Acknowledgement

The author wishes to thank Prof. Robin Williams for stimulating discussions on the antiepileptic mechanism of valproic acid.

 

Author: Frank Lee
First published: December 16, 2015
Last updated: July 3, 2017