Deciphering the developing brain

By Deepshika Arasu

Diseases can manifest at different stages of life in various parts of the body. But what about the disorders that compromise early development in your brain, the organ that greatly influences your personality, who your friends would be, your sense of self and the general ability to make sense and comprehend the world around you. Neurodevelopmental disorders (NDDs) occur early in life and alter brain function. To understand these disorders, scientists use a variety of tools to unravel how they affect the brain. So, let’s delve into the ways we model these disorders.

Clinically, these disorders can include motor, cognitive and communication deficits, repetitive stereotyped behaviours, learning disabilities, autism spectrum disorder, sensory dysfunction, and epilepsy. Mendelian NDDs are characterised by a single impaired gene and mostly have a clinical manifestation almost indistinguishable from non-syndromic NDDs. On the other hand, non-syndromic NDDs have heterogeneous, polygenic impairments and are impacted by environmental aspects. The latter makes up most of the NDDs, however, it is the former that is widely studied by scientists for its ease of genetic manipulation. 

From the humble fly to rodents: all’s a stage for studying NDDs

These disorders are modelled in a variety of ways, ranging from a cellular to an organismal level. Common cellular pathway disruptions such as cell proliferation or growth can be studied in non-neuronal cell types. At a higher organism level, genetically modified worms are utilized to understand NDDs. For example, the gene PQBP1 is involved in an X-Chromosome-linked intellectual disability and has a well-conserved worm homolog (gene equivalent) whose studies revealed the gene’s involvement in lipid metabolism1. Further, another small organism that has provided monumental insights into developmental mechanisms is Drosophila. The experimental tractability offered by these organisms has helped elucidate Fragile-X Syndrome, Angelman Syndrome, and many other NDDs 2,3.

Avian species have contributed to understanding neurodevelopmental disorders. The songbird model revealed that two language-related genes are heavily expressed during different time points of song learning. Fittingly, mutations in these two genes, FOXP2 (Forkhead box P2) and its transcriptional target in human, CNTNAP2, are implicated in severe speech deficits syndromes4. At the mammalian level, a variety of rodents, bats, and pigs are studied extensively to unravel the pathomechanisms of NDDs (Fig. 1). Besides offering a genetic and molecular basis for disease, most of the organismal studies uncover behavioural insights into the disease. They provide a platform to comprehend and correlate observed clinical behaviours. However, an important point to keep in mind is that results from animal studies do not always correlate to human brains since they can have different molecular mechanisms. 

Figure 1 : Worms, flies, birds and rodents – modeling neurodevelopment across species

Bridging the non-human and human gap 

The advent of the Yamanaka factors, a set of transcription factors that can direct cells to a pluripotent stage wherein they are capable of differentiation to most cell types, revolutionized the study of many neurological diseases. Skin cells from patients could be isolated, and reprogrammed to their primitive and immature state as induced pluripotent stem cells (iPSCs). These cells are differentiated into neurons and closely analyzed to understand their pathological state. A multitude of protocols now exist to achieve neurons of various types, subtypes, and developmental stages 5. Patient-derived neurons offer researchers the ability to study neurodevelopmental disease at much closer angles than ever before. The differentiation of human neurons can be monitored from inception and they mostly present with discernible disease-specific phenotypes that are analyzed and correlated with clinical phenotypes. Apart from 2D cellular systems, there has been a boom in creating 3D brain organoids which share remarkably similar organization and architecture as the human brain itself6. Advancements in research such as the development of iPSC-derived neurons and brain organoids have deepened our understanding of NDD pathology (Fig. 2). By employing these models, researchers are able to delve into the mechanistic dysfunction during neurodevelopment. 

Figure 2: A neuron derived from IPSCs grown on a bed of glial cells.

Edging towards therapeutics

Human cellular models are harnessed not only for their potential to study aberrant mechanisms, but also serve as valuable tools to test therapeutic options. Researchers have studied numerous neurodevelopmental disorders such as Autism, Tuberous Sclerosis Complex (TSC), Fragile X Syndrome (FXS) and STXBP1 Syndrome by utilizing human neuronal models to discover possible pharmaceutical options 7–9. However, induced neurons are a relatively new technology and vast amounts of characterizations are required before they can be employed for studies. Standardization of protocols, choosing a specific neuron type, and checking the suitability of various tools are essential for delving into the molecular, transcriptomic and electrophysiological phenotypes of any disease. Pinpointing the exact developmental stages during which impairment occurs can help researchers figure out the time frame appropriate for therapeutic intervention. The technological advancements in the past two decades have boosted neurodevelopmental studies and researchers are edging towards developing personalized medicine for treating neurodevelopmental dysfunction. By closely mimicking physiological environments, we can boost the efficiency of pre-clinical studies that look at therapeutic options. 

A combination that employs the strengths of animal studies and human cellular methods is the best way forward in understanding neurodevelopmental disorders. Animal models will further uncover behavioral aspects of impaired neurodevelopment such as sensory and social deficits, while also facilitating the study of cellular and molecular mechanisms in disease pathways. Complentarily, human neuronal modeling can extend the mechanistic studies by utilizing a human-specific context to scout for therapeutic options. This extensive repertoire of tools is vital to address the ever-challenging questions pertinent to neurodevelopmental dysfunction. 

About the writer

Deepshika completed her master’s thesis using iPSC-derived neurons to study STXBP1 syndrome at the FGA, CNCR. She is currently pursuing her PhD at Poietis, a start-up company in Bordeaux where she uses laser-assisted bioprinting to create brain organoids. When not at the lab, she usually spends her time wondering about the intersection of science, policy and tech culture.

Further reading

1. Takahashi, K. et al. Nematode Homologue of PQBP1, a Mental Retardation Causative Gene, Is Involved in Lipid Metabolism. PLoS One 4, e4104 (2009).

2. Burne, T. et al. Big ideas for small brains: What can psychiatry learn from worms, flies, bees and fish. Molecular Psychiatry 16, 7–16 (2011).

3. Wu, Y. et al. A Drosophila model for Angelman syndrome. Proc. Natl. Acad. Sci. U. S. A. 105, 12399–12404 (2008).

4. Panaitof, S. C. A songbird animal model for dissecting the genetic bases of autism spectrum  disorder. Dis. Markers 33, 241–249 (2012).

5. Salimi, A., Nadri, S., Ghollasi, M., Khajeh, K. & Soleimani, M. Comparison of different protocols for neural differentiation of human induced pluripotent stem cells. Mol. Biol. Rep. 41, 1713–1721 (2014).

6. Chiaradia, I. & Lancaster, M. A. Brain organoids for the study of human neurobiology at the interface of in vitro and in vivo. Nature Neuroscience 23, 1496–1508 (2020).

7. Costa, V. et al. MTORC1 Inhibition Corrects Neurodevelopmental and Synaptic Alterations in a Human Stem Cell Model of Tuberous Sclerosis. Cell Rep. 15, 86–95 (2016).

8. Park, C. Y. et al. Reversion of FMR1 Methylation and Silencing by Editing the Triplet Repeats in Fragile X iPSC-Derived Neurons. Cell Rep. 13, 234–241 (2015).

9. Zaslavsky, K. et al. SHANK2 mutations associated with autism spectrum disorder cause hyperconnectivity of human neurons. Nat. Neurosci. 22, 556–564 (2019).

Image credits: Cover photo/Image 2 taken by author during Internship project on STXBP1 project at CNCR. Image 1 was custom-made using word.