What Does the Endocannabinoid System Have To Do With Autism Spectrum Disorder?
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Many of the body’s critical functions, including learning and memory, temperature control and inflammatory responses, are regulated by the endocannabinoid system (ECS), an intricate network of receptors and signals found throughout the brain and body. The ECS is activated when cannabinoid receptors are stimulated – a process that can occur when the body produces naturally occurring molecules called endocannabinoids or from the administration of cannabis.
In recent years, interest has grown in deciphering the role that the ECS could play in the development of neurodevelopmental disorders such as autism spectrum disorder (ASD), and whether any conditions that co-occur with ASD could be targeted therapeutically.
To learn more about the link between endocannabinoids and ASD and how innovative models are helping to study the ECS and its effects on the developing brain, Technology Networks spoke to Drs. Karen Litwa and Ken Soderstrom from East Carolina University.
Anna MacDonald (AM): What do scientists know about the role of the ECS in neurodevelopment and inflammatory responses?
Karen Litwa (KL): We know the ECS is active during the earliest stages of embryonic growth and therefore plays a critical role in the developing fetal brain. There are two primary cannabinoid receptors: CB1 and CB2. CB1 is mainly expressed in the central nervous system (CNS), while CB2 is present in cells of the immune system. In the brain CB2 is almost exclusively expressed by microglia. Thus, the endocannabinoid system uniquely serves as a link between the immune system and the CNS.
Given its abundance in the brain, the ECS plays a significant role in neurodevelopmental processes such as neurogenesis, neural specification and maturation, axonal tract formation, glia formation and neuronal migration. In other words, the ECS is essential for regulating proper neuron communication and plasticity to establish the complex circuitry that mediates many functions such as memory, appetite, anxiety, pain and learning.
AM: How does the ECS link to the development of neurodevelopmental disorders such as ASD?
KL: Over the years, observations in both animals and humans have implicated the ECS in the pathogenesis of ASD. In humans, post-mortem brains of ASD patients show reduced CB1 expression, while children with ASD have lower circulating levels of endocannabinoids. There is also a strong association between ASD and variants in endocannabinoid synthesizing enzymes. Additionally, the use of cannabis during pregnancy is linked to an increased risk of ASD in the child, suggesting that disruption of endogenous ECS signaling may be involved in the pathogenesis. The evidence of ECS involvement continues to mount.
While not the focus of our research, there is evidence to suggest a role of neuroinflammation in ASD. Elevation in pro-inflammatory markers is a consistent finding in ASD and may even predict severity or disease phenotype. Researchers are particularly interested in studying whether microglia have sustained activity that results in neurotoxicity seen in ASD.
Ruairi J Mackenzie (RM): Many autistic people and groups do not view their condition as something that needs to be treated. However, much of the literature still broadly characterizes behavioral features of ASD as symptoms. In targeting the endocannabinoid system, which features of ASD would be addressed and which autistic people would such an approach be aimed at?
Ken Soderstrom (KS): Currently available treatments for ASD are typically supplemental to behavioral strategies and individualized based on the intensity of the presentation. These treatments do not address the condition itself, but rather symptoms of co-occurring conditions (anxiety, attention deficit hyperactivity disorder, seizures, sleep disorders, etc.) and address behaviors that could impede a child’s development.
Researchers expect that treatments targeting the ECS will offer similar benefits and may provide an opportunity to develop safer, effective therapies and improve quality of life for people living with ASD—not to change who they are as a person. A growing interest in cannabinoids, which act on the ECS, is also opening doors for conditions other than ASD. One example is epilepsy, for which a cannabidiol preparation called Epidiolex received FDA approval in 2018.
Interestingly, epilepsy is also seen in a subset of ASD patients, a connection that may further support the development of treatments targeting the ECS in ASD. An important feature of cannabinoid-based treatments is that some appear to address multiple symptoms with a single preparation. For example, in addition to anti-seizure activity, anecdotal evidence suggests cannabidiol may also enhance social behavior. There is also clear evidence that cannabidiol is anti-neuroinflammatory which is a distinct problem in ASD. This anti-inflammatory effect of cannabidiol is at least partly due to potent inhibition of microglial activation, which researchers are working to better understand.
While ECS-based therapies are promising, more research is needed to establish their efficacy and safety. To be clear, we are not endorsing any particular treatment.
AM: What are some of the main challenges of studying the endocannabinoid system in the developing brain and the effects of its mediation?
KL: Studying embryonic brain development involves great complexity and presents numerous ethical barriers. Often with animal models of disease, only a few of the defining characteristics are replicated, which means we are only looking at part of the larger picture—rarely the complete disease. Neurodevelopmental disorders have added challenges given their variable presentation and features like verbal or nonverbal communication. These features do not translate well to animal models and are therefore difficult to study.
AM: How are new models such as cortical spheroids helping to overcome these hurdles?
KL: Cortical spheroids, which are 3D in vitro models generated from induced pluripotent stem cells (iPSCs), replicate fetal brain development—something we don’t have easy access to otherwise. These “mini-brain” models offer several advantages. The ECS is quite dynamic as fetal development progresses, so it is important to consider the “age” of the brain we are modeling. With spheroids, cells grow over time, allowing researchers to estimate the window of development that the spheroid model represents. While the spheroids we used in our research modeled neurotypical control patients, they can also be grown on the background of a person with ASD using iPSCs derived from patients with the condition. The cells self-organize to form functional neural circuits with spontaneous activity and supporting cells, such as astrocytes. Thus, they develop much like the human brain does in utero, which ensures that the relevant signaling pathways are intact.
The ECS is established in our spheroid model. We confirmed the expression of CB1 and enzymes required to synthesize the endogenous ligands. Another useful feature of this model is that we can easily study the neural circuits that form using microelectrode array (MEA) technology. This tool enables us to measure and monitor electrical activity of neurons over time or in response to a drug—a variable that is key to understanding neurodevelopmental diseases, possible interventions and their effects.
RM: What has your own research contributed to this growing field?
KL: Our research specifically focuses on the role of CB1 in controlling the synaptic strength and balance of inhibitory and excitatory signaling, which is altered in ASD. When we disrupted CB1 activity in our cortical spheroid model, we observed asynchronous activity of the neural networks, similar to findings in ASD.
We used a compound that prevented signaling through the CB1 receptor. Using an MEA assay, we were able to measure localized activity (spikes) as well as rapid communication between cell populations (bursts). We observed a high degree of variability in spikes and bursts, which suggested a lack of synchronous activity and less mature neural circuit. This finding helps us understand the importance of intact CB1 signaling and how loss of signaling through this receptor produces a phenotype seen in ASD.
AM: Can you tell us about current research exploring the use of cannabis and cannabidiol for ASD?
KS: Only two drugs, both of which focus on managing irritability, are FDA-approved for ASD. But these medications carry an increased risk of obesity and metabolic syndrome, so finding new therapies is an important area of research. To date, however, only a couple of clinical studies have assessed the use of cannabinoids as treatment for ASD. It is important to note that the cannabinoids used in these studies were extracted and purified from botanical cannabis. These kinds of extracts contain at least traces of other bioactive molecules that potentially contribute to complex effects.
A recently published placebo-controlled study showed that cannabidiol preparations were well tolerated in ASD and have potential to improve core symptoms like irritability. The side effect profile was favorable, with drowsiness and decreased appetite being the most common.
Cannabidivarin (CBDV) is another cannabidiol variant under study to assess effects on irritability in children with ASD. The Children’s Hospital of Philadelphia (CHOP) initiated an observational registry study to gather information about the use of medical cannabis in the pediatric ASD population.
This is an emerging area with great potential. A challenge for targeting the ECS is identifying treatments that modulate the right balance of widely distributed cellular processes to achieve the desired effect. I am certainly excited to see where the research takes us, especially given the central role the ECS plays in ASD pathogenesis.
Drs. Karen Litwa and Ken Soderstrom were speaking to Anna MacDonald, Science Writer, and Ruairi J Mackenzie, Senior Science Writer for Technology Networks.