Most technical papers on SCA are more advanced than I can comprehend. Every sentence can be chock-full of references that I don’t fully understand. This article takes an example of what looks like a promising SCA3 study involving drug repurposing and dissects it. Here’s the study:
Breaking down the summary
Here’s a breakdown of the summary given at the front of the article:
Polyglutamine diseases are a class of dominantly inherited neurodegenerative disorders for which there is no effective treatment.
I give my own explanation of the term polyglutamine disease here; the term means that the disease is caused by CAG repeats in one’s DNA, as is the case for SCA3.
Here we provide evidence that activation of serotonergic signaling is beneﬁcial in animal models of SCA3.
Serotonin is a neurotransmitter in the nervous system, affecting mood, appetite, sleep, memory, and learning, though 90% of it is found in the gastrointestinal tract and regulates intestinal movements. There’s a whole other nervous system down there! See enteric nervous system (ENS).
Serotonin is also known as 5-HT, where HT stands for hydroxy·tryptamine.
We identiﬁed citalopram, a selective serotonin reuptake inhibitor [SSRI], in a small molecule screen of FDA-approved drugs that rescued neuronal dysfunction and reduced aggregation using a Caenorhabditis elegans model of mutant ataxin 3-induced neurotoxicity.
- Synapse: a junction between two nerve cells, consisting of a minute gap across which impulses pass by diffusion (spreading out) of a neurotransmitter.
- Presynaptic: relating to a nerve cell that releases a transmitter substance into a synapse during transmission of an impulse.
- Reuptake: the absorption by a presynaptic nerve ending of a neurotransmitter that it has secreted.
Small molecule drugs—most drugs are small molecule—are able to cross cell membranes, which allows them to regulate biological processes. What kinds of larger drug molecules are there? Protein is common.
Caenorhabditis elegans = C. elegans = roundworm, about 1 mm long, the first multicellular organism to have its whole genome sequenced, and used extensively as a model organism.
Protein aggregation is the toxic result of the SCA3 mutation. Presumably neuronal dysfunction is also related to the SCA3 polyglutamine problem.
MOD-5, the C. elegans orthologue of the serotonin transporter and cellular target of citalopram, and the serotonin receptors SER-1 and SER-4 were strong genetic modiﬁers of ataxin 3 neurotoxicity and necessary for therapeutic efﬁcacy.
SSRIs limit the reuptake of serotonin by blocking the serotonin reuptake transporter (SERT). An orthologue occurs when similar gene sequences occur in different species. In C. elegans, the serotonin transporter that citalopram blocks is called MOD-5.
When the reuptake of serotonin is blocked, more serotonin is available elsewhere. In C. elegans, the important serotonin receptors are called SER-1 and SER-4.
Moreover, chronic treatment of CMVMJD135 mice with citalopram signiﬁcantly reduced ataxin 3 neuronal inclusions and astrogliosis, rescued diminished bodyweight and strikingly ameliorated motor symptoms.
CMVMJD135 mice are transgenic mice with human SCA3. Transgenic means human DNA was introduced into the mice using genetic engineering. CMV stands for cytomegalovirus, and in genetic engineering, a CMV promoter helps force the desired results.
The genetic engineering takes place at the fertilization or embryonic stage (I don’t know which) so that the mice are born with the disease—rather than trying to give healthy, mature mice the disease, which would be impossible for a genetic disease.
It seems to me that a potential weakness in using mice is that toxic protein creation might occur more in glial cells than in neurons (“Unexpectedly, expression was more prominent in glial cells than in neurons”).
Ataxin 3 neuronal inclusions and astrogliosis seem to be yet more references to the negative effects of polyglutamine:
Inclusion: a body or particle recognizably distinct from the substance in which it is embedded (i.e., you don’t want toxic protein included in your brain cells).
Astrogliosis sounds to me like the neurons that are dying off are harming the good neurons left behind. This sounds like an autophagy failure, but I don’t know.
These results suggest that small molecule modulation of serotonergic signaling represents a promising therapeutic target for SCA3.
In my limited view, we have our very first readily-available treatment candidate for polyglutamine diseases such as SCA3. This isn’t merely symptom relief, this modifies the disease itself—on worms and mice. Sounds promising!
Breaking down the introduction
The density of information in the introduction is astounding. In the first paragraph, we are reminded (or learn) that with SCA3:
ATXN3 becomes highly aggregation prone, leading to an imbalance in cellular proteostasis, as aggregation-associated proteotoxicity dominates over folding and clearance.
Proteostasis: protein homeostasis. You want this. If you don’t have this, it’s a problem. Keeping a steady state seems to rely on garbage collection (autophagy) to prevent aggregation from gunking up necessary processes, such as protein folding. When protein folding fails, the protein can become toxic, hence proteotoxicity. I assume that clearance essentially refers to autophagy (i.e., clearing out dead cells).
A main point of the bulk of the introduction is this: existing experimental approaches use drugs that aren’t readily-available to the average person, drugs such as HDAC inhibitors. Being that citalopram is FDA-approved (i.e., readily available) and showed promising results makes it therapeutically viable.
Now, the closing of the introduction makes sense!
… [W]e demonstrate that modulation of serotonergic signalling by SSRIs suppressed mutant ATXN3 aggregation and neurotoxicity in both C. elegans and mice. This reveals the utility of the approach by which safe and highly effective bioactive small molecules can be repurposed … [S]erotonin recapture inhibition modulates proteotoxicity …
Breaking down the results
In the worms:
Treatment with citalopram caused complete rescue of mutant ATXN3-mediated neuronal dysfunction.
“Complete rescue” sounds good to me.
In the mice, there were some marginal improvements and some major improvements from a symptomatic standpoint.
Overall, these results demonstrate that citalopram can reduce the impairment in motor coordination of the [SCA3] mouse, with less beneﬁt on strength, suggesting effects at the level of the brainstem, midbrain, or cerebellum, rather than on spinocerebellar tracts and muscle innervation. Treatment also signiﬁcantly delayed disease progression in mice.
The deeper analysis also sounds promising:
The analysis of brain tissue from CMVMJD135 mice showed that citalopram treatment mitigated reactive astrogliosis and decreased ATXN3 intranuclear inclusions in the brainstem.
Breaking down the discussion
[M]odulators of the serotonergic pathway … strongly suppressed [SCA3] pathogenesis in vivo.
Pathogenesis: the biological mechanism that leads to a disease.
in vivo: on the whole, living organism (e.g, as opposed to part of it in a dish).
[A] faster desensitization of 5-HT1A autoreceptors by a direct agonist (e.g., buspirone) may also accelerate the therapeutic efﬁcacy of SSRIs.
Noted. Buspirone is a prescription drug.
[T]he release of serotonin from neurons signals to distal tissues the activation of protective mechanisms to prevent proteotoxicity.
In conclusion, the efﬁcacy of citalopram in suppression of ATXN3 pathogenesis in two disease models, as well as its safety record of being widely used in depression patients, prompts us to suggest this drug for clinical trials in patients [SCA3].
One concern I have with animal testing is that the drug dosage in testing could be very high, even to the point (or brink) of toxicity. The standard citalopram dosages are 10 mg, 20 mg, and 40 mg, yet the testing on mice was 8 mg/kg, which in a person like me scales to about 600 mg, which is 15 times the maximum.
Here’s a list of SSRIs. Strangely, they are all part of our popular culture, and I have heard of them all except one:
- Fluoxetine (Prozac), FDA 1987
- Sertraline (Zoloft), FDA 1991
- Paroxetine (Paxil), FDA 1993
- Fluvoxamine (Luvox), FDA 1994
- Citalopram (Celexa), FDA 1998
- Escitalopram (Lexapro), FDA 2002
I kind of wonder why the supplement known as 5-HTP (5-hydroxy·tryptophan) is not the solution to all our woes.