Investigating the molecular mechanisms of circadian rhythms in bipolar disorder

Sally is a PhD student at Newcastle University working on a project that utilises iPSC*-derived retinal organoids to investigate molecular circadian rhythms in bipolar disorder. Previously, she studied a BSc in Medical Genetics and MSc in Translational Neuropathology at the University of Sheffield. She spent a few years working in a HTT diagnostics lab in Germany before returning to academia, where her passion for understanding the molecular basis of neuropsychiatric disorders was reignited.

Her research now integrates iPSC technology and circadian biology to uncover how circadian processes are altered in bipolar disorder. She hopes her work will contribute to a better understanding of the cellular mechanisms underlying mood disorders and support the development of novel therapeutic approaches. Sally is motivated by the potential of personalised medicine in mental health research, aiming to bridge the gap between molecular discoveries and tailored therapeutic strategies that can improve patient outcomes.

*iPSC - induced pluripotent stem cells, cells of a certain type (like blood or skin) that have been induced to become stem cells that can develop into almost any other kind of cell (like retinal cells)

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Cellular circadian rhythms are internal 24-hour clocks that maintain normal bodily functions. Simply, these clocks are multiple interacting transcription translation feedback loops. These are networks of genes that turn on and off at predictable times, influencing our body’s systems. These clocks are present in almost every cell of the body. These internal clock networks can be synchronised to the external environment by variables known as zeitgebers. The main zeitgeber input is light, detected by the eye’s retina. This information is sent to the Suprachiasmatic nucleus of the brain, which signals to the rest of the body via the pituitary gland. This gland releases the hormone cortisol that wakes you up in the mornings or melatonin that makes you sleepy at night, via regulation of further hormones and metabolism processes. This allows our bodies to prepare for the predictable changes that occur during the day or seasonally.

Illustration of how light influences our internal clocks

Bipolar disorder (BD) is a severe mood disorder with episodes of mania and episodes of depression. It affects 1% of the population and is one of the world’s leading causes of disability. Disruption of circadian rhythms is likely a key feature of BD, as patients have increased light sensitivity (particularly at night), display sleep disturbances and have abnormal melatonin levels. Furthermore, 25% of patients display a seasonal symptom pattern, with more hospitalisations with mania occurring in the summer and more hospitalisations with depression in the winter. Lithium is a common BD treatment, but there are many drawbacks, for example lithium is not effective for all patients and it has a narrow therapeutic index (meaning the helpful working dose is very close to the toxic dose). It is still unknown how the therapeutic effect of lithium works inside cells. It is likely that lithium functions by correcting disruptions in BD circadian rhythms.

Analysing mechanisms of circadian rhythm disruption is difficult in patients, therefore it is important to develop an alternative model. To do this, we took blood from BD patients and healthy controls to make stem cells (iPSCs) from this blood. These iPSCs can transform into any cell of the body depending on the nutrients and other conditions provided to them. As light is the main input to the circadian system the retina is a good starting point to study circadian rhythms in BD neurons. Therefore, we created model patient retinas, called retinal organoids.

These retinal organoids include most of the cell types present in the eye’s retina, with a comparable overall layout. They respond to light and behave similarly to the retina in the eye. As these retinal organoids are made from the patient cells, they have the same genetic make-up and are very similar to patient retinas. Experiments on this model tells us what is likely going on inside the actual patient retinas.

Illustration of the layers in the retina, and the layers in retinal organoids

To synchronise them, the organoids were exposed to light mimicking days and nights. Samples were then taken every 4-hours over a 36-hour period. The total RNA was extracted from these samples and measured using a technique called bulk RNA sequencing. This allowed us to track when different genes were turned on and off. We compared the patients to the healthy controls, identifying any differences in BD on a gene expression level. So far, we have identified 34 consistently different genes, along with abnormal oscillations of over 2000 genes. Currently we are working out which of these gene expression differences are likely to be having an influence on patients by carrying out functional studies, such as measuring the electrical responses of the retinal organoids upon illumination and looking at how the neurons form connections when they grow.

Overall, we aim to understand circadian dysfunction in BD and how lithium corrects this. Future work will involve repeating the experiments with various doses of lithium to observe any changes. This will give us an idea of what biological pathways are involved in the beneficial effects of lithium. Once this is identified we can look at alternatives for those patients where lithium is ineffective, as well as additional options for those taking lithium.

Blog by Sally Harwood