Treating autism spectrum disorders by targeting connections in the brain

12 April 2019

By David A. Menassa, @DavidMenassa1, University of Southampton, Neuroscience Theme Lead of The Physiological Society

The United Kingdom has seen a rise in the number of people diagnosed with autism spectrum disorders (ASDs). More recently, some trusts in Northern Ireland have reported a three-fold increase in diagnoses since 2011. Some cases of ASD are linked to a genetic mutation but most of the time, we do not actually know why they occur.

ASD is an umbrella term for disorders characterised by impairments in social interaction and language acquisition, sensory and motor problems and stereotypical behaviours of variable severity according to the Diagnostic and Statistical Manual of Mental Disorders.

Multiple studies report that the physiology and structure of synapses (the gaps between our neurons) are affected in ASDs (1, 2) and that addressing these changes could offer clues for therapy.

The genetic mutations giving rise to different ASDs are predominantly involved in synaptic function. A large-scale analysis of 2000 human brains from individuals with ASDs, schizophrenia and bipolar disorder showed that genes involved in controlling the release of neurotransmitters into the synapse are least active in ASDs (3, 4).

An attempt using the CRISPR/Cas9 gene editing method in a genetic form of ASD known as Fragile-X syndrome (FXS) showed some promise (5). When the researchers turned on a gene that is turned off in this condition, this changed the cells derived from affected individuals from diseased to normal.

Because the number of synapses in ASD post-mortem brains is altered (1), microglia (the brain’s resident immune cells) are thought to be directly involved (as these cells can determine whether synapses stick around) (6-8). Minocycline, which inhibits these microglia, has been used in trials on FXS individuals with promising outcomes including improvements in language, attention and focus as well as an alleviation of core symptoms (9, 10). Furthermore, novel approaches involving inducing microglia to self-destruct (11) or shielding synapses from microglia (12) are being explored. At least in terms of symptom alleviation, addressing synaptic dysfunction and microglia seem to be promising avenues for treatment.

Environmental factors such as changes during pregnancy could contribute to ASD such as the transfer of maternal antibodies against fetal synapses (13) or maternal immune activation (14) or lack of oxygen to the mum or the baby (15). Ways to reduce the risk of injury to the brain with low oxygen in the mum for example have involved delivering antioxidants to the placenta (16). Furthermore, in the early life of the newborn when low oxygen is detected, inhalation of xenon gas combined with cooling also seem to provide promising results that improve neurological outcome (17).

The impact of altered brain development extends beyond the individual influencing their family, carers and the healthcare system. More funding is needed to support this research in order to elucidate the underlying physiology and identify effective treatments.

The figure is artwork showing neurons in various colours (black, green and blue) making connections with each other and some of these connections are interrupted too. This is to show that ASD is a ‘connectivity disorder’. Microglia are the pink cells that are near the synapses which we can see in the highlighted squares – a) A dendrite with very few boutons/dots on it meaning less synapses than in b) More black dots on the dendrite and that means more synapses. You see these two profiles in post-mortem tissue stained with Golgi – either less dots (Rett’s syndrome for example) or more dots (idiopathic autism) compared to controls. Microglia are thought to be involved here either by over-pruning or under-pruning.

 

References

  1. Penzes P, Cahill ME, Jones KA, Van Leeuwen JE, Woolfrey KM. Dendritic spine pathology in neuropsychiatric disorders. Nat Neurosci. 2011;14(3):285-93.
  2. Lima Caldeira G, Peça J, Carvalho AL. New insights on synaptic dysfunction in neuropsychiatric disorders. Curr Opin Neurobiol. 2019;57:62-70.
  3. Zhu Y, Sousa AMM, Gao T, Skarica M, Li M, Santpere G, et al. Spatiotemporal transcriptomic divergence across human and macaque brain development. Science. 2018;362(6420).
  4. Gandal MJ, Zhang P, Hadjimichael E, Walker RL, Chen C, Liu S, et al. Transcriptome-wide isoform-level dysregulation in ASD, schizophrenia, and bipolar disorder. Science. 2018;362(6420).
  5. Liu XS, Wu H, Krzisch M, Wu X, Graef J, Muffat J, et al. Rescue of Fragile X Syndrome Neurons by DNA Methylation Editing of the FMR1 Gene. Cell. 2018;172(5):979-92.e6.
  6. Schafer DP, Lehrman EK, Kautzman AG, Koyama R, Mardinly AR, Yamasaki R, et al. Microglia sculpt postnatal neural circuits in an activity and complement-dependent manner. Neuron. 2012;74(4):691-705.
  7. Schafer DP, Heller CT, Gunner G, Heller M, Gordon C, Hammond T, et al. Microglia contribute to circuit defects in Mecp2 null mice independent of microglia-specific loss of Mecp2 expression. Elife. 2016;5.
  8. Paolicelli RC, Bolasco G, Pagani F, Maggi L, Scianni M, Panzanelli P, et al. Synaptic pruning by microglia is necessary for normal brain development. Science. 2011;333(6048):1456-8.
  9. Utari A, Chonchaiya W, Rivera SM, Schneider A, Hagerman RJ, Faradz SM, et al. Side effects of minocycline treatment in patients with fragile X syndrome and exploration of outcome measures. Am J Intellect Dev Disabil. 2010;115(5):433-43.
  10. Paribello C, Tao L, Folino A, Berry-Kravis E, Tranfaglia M, Ethell IM, et al. Open-label add-on treatment trial of minocycline in fragile X syndrome. BMC Neurol. 2010;10:91.
  11. Kim HJ, Cho MH, Shim WH, Kim JK, Jeon EY, Kim DH, et al. Deficient autophagy in microglia impairs synaptic pruning and causes social behavioral defects. Mol Psychiatry. 2017;22(11):1576-84.
  12. Lehrman EK, Wilton DK, Litvina EY, Welsh CA, Chang ST, Frouin A, et al. CD47 Protects Synapses from Excess Microglia-Mediated Pruning during Development. Neuron. 2018;100(1):120-34.e6.
  13. Coutinho E, Menassa DA, Jacobson L, West SJ, Domingos J, Moloney TC, et al. Persistent microglial activation and synaptic loss with behavioral abnormalities in mouse offspring exposed to CASPR2-antibodies in utero. Acta Neuropathol. 2017;134(4):567-83.
  14. Careaga M, Murai T, Bauman MD. Maternal Immune Activation and Autism Spectrum Disorder: From Rodents to Nonhuman and Human Primates. Biol Psychiatry. 2017;81(5):391-401.
  15. Kolevzon A, Gross R, Reichenberg A. Prenatal and perinatal risk factors for autism: a review and integration of findings. Arch Pediatr Adolesc Med. 2007;161(4):326-33.
  16. Phillips TJ, Scott H, Menassa DA, Bignell AL, Sood A, Morton JS, et al. Treating the placenta to prevent adverse effects of gestational hypoxia on fetal brain development. Sci Rep. 2017;7(1):9079.
  17. Mayor S. Xenon shows promise to prevent brain injury from lack of oxygen in newborns. BMJ. 2010;340:c2005.
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