From Cannabinoids and Neurosteroids to Statins and the Ketogenic Diet: New Therapeutic Avenues in Rett Syndrome?

Abstract

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder caused mainly by mutations in the MECP2 gene, being one of the leading causes of mental disability in females. Mutations in the MECP2 gene are responsible for 95% of the diagnosed RTT cases and the mechanisms through which these mutations relate with symptomatology are still elusive. Children with RTT present a period of apparent normal development followed by a rapid regression in speech and behavior and a progressive deterioration of motor abilities. Epilepsy is one of the most common symptoms in RTT, occurring in 60 to 80% of RTT cases, being associated with worsening of other symptoms. At this point, no cure for RTT is available and there is a pressing need for the discovery of new drug candidates to treat its severe symptoms. However, despite being a rare disease, in the last decade research in RTT has grown exponentially. New and exciting evidence has been gathered and the etiopathogenesis of this complex, severe and untreatable disease is slowly being unfolded. Advances in gene editing techniques have prompted cure-oriented research in RTT. Nonetheless, at this point, finding a cure is a distant reality, highlighting the importance of further investigating the basic pathological mechanisms of this disease. In this review, we focus our attention in some of the newest evidence on RTT clinical and preclinical research, evaluating their impact in RTT symptomatology control, and pinpointing possible directions for future research.

Keywords: GABAAR; Rett syndrome; cannabinoids; cholesterol; epilepsy; ketogenic diet; neurosteroids.

FIGURE 1

The potential of GABA_AR modulators on RTT. (1) Exclusive deletion of Mecp2 from GABAergic neurons results in almost the full range of neuropsychiatric symptoms of RTT (Chao et al., 2010). (2) Blocking GABA reuptake using tiagabine (El-Khoury et al., 2014) or enhancing GABA_AR activity with the benzodiazepine Midazolam increases the life-spam and reduces symptoms in Mecp2 mutant mice (Voituron and Hilaire, 2011).

FIGURE 2

Research topics to be addressed in this review. In red: the use of direct and indirect modulators of GABAergic signaling, respectively, benzodiazepines (BZD) and GABA enhancers. In orange: Abnormal cholesterol metabolism occurring in RTT, the use of statins (STA) to improve RTT symptoms; the impact of deficits on cholesterol uptake on neurosteroidogenesis; the possible therapeutic actions of the neurosteroid allopregnanolone (ALLO). In blue: the use of derivatives of the cannabis plant to ameliorate RTT symptoms; cannabidiol (CBD) has shown promising anti-epileptic effects via GABA-dependent mechanisms; the effects of CBD on CB₁R and TRPV₁ and its impact on RTT symptoms are still undisclosed; Cannabidivarin (CBDV) has been identified as a promising therapeutic drug in an RTT-mice model. In green: Ketogenic Diet (KD) has potent anticonvulsive actions via GABAergic and glutamatergic (GLU) signaling systems; the KD can also improve RTT symptoms via BDNF-mediated effects or through increases in adenosine production. BNDF signaling is known to be impaired in RTT (Li and Pozzo-Miller, 2014), while unpublished data obtained in our lab points to deregulations in adenosinergic signaling. Direct arrows denote direct interaction with GABAergic signaling system, while doted arrows denote known or undisclosed involvement on RTT pathophysiology.

FIGURE 3

De novo synthesis of some important neurosteroids. (1) After being transported from the outer to the inner membrane, cholesterol is transported from the outer to the inner membrane of the mitochondria by steroidogenic acute regulatory protein (StAR). (2) Once inside the mitochondria, cholesterol is transformed in pregnenolone, the mother of all steroid hormones, by cytochrome P450 cholesterol side-chain cleavage (P450scc) enzyme (CYP11A1). (3) Pregnenolone can be, subsequently, transformed into progesterone by 3β-hydroxysteroid dehydrogenase (3β-HSD), which in turn, can originate allopregnanolone through the action of 3α-hydroxysteroid dehydrogenase (3α-HSD). (4) Allopregnanolone is viewed as the most potent endogenous modulator of the GABAergic system via interaction with the GABA_A receptor (for a detailed review see Melcangi et al., 2014).

FIGURE 4

Binding sites for neurosteroids on the postsynaptic and extrasynaptic GABA_AR (Reddy, 2011; Cai et al., 2018). Contrary to benzodiazepines, which require the mandatory γ subunit, neurosteroids can act on GABA_AR whenever they are composed by α subunits (Bianchi and Macdonald, 2003). The extrasynaptic GABA_AR usually contains the highly neurosteroid sensitive δ subunit, which opens an exploitable therapeutic window, especially in cases of benzodiazepine tolerance (Brown et al., 2002; Uusi-Oukari and Korpi, 2010; Meera et al., 2011; Rogawski et al., 2013; Carver and Reddy, 2016).

FIGURE 5

Time-dependent modulation by allopregnanolone on GABA_AR signaling. (1) Allopregnanolone actions are known to contribute to neuroprotection of the fetal brain (Liu and Wong-Riley, 2010). In rats, allopregnanolone levels change immediately after birth and during the first weeks of life (Grobin and Morrow, 2001). (2) By exclusively stimulating the GABA_A-ergic neurons in the locus coeruleus it was found that allopregnanolone increases the amplitude, frequency and decay time of GABA_AR inhibitory post-synaptic currents (IPSCs) both in wild-type and Mecp2 mutant animals. (3) However, after the first 2 weeks, allopregnanolone effects are lost in Mecp2-null animals, coinciding with the onset of RTT symptoms in such animals (Jin et al., 2013b). The drop in allopregnanolone levels seems to coincide with drastic reductions in progesterone levels immediately after birth (Paoletti et al., 2006). Modifications in the composition of the GABA_AR can also be responsible for the effect (Jin et al., 2013b).