Faulty endoplasmic reticulum structure underlies human neurological disorder
A detailed scrutinisation of a neurodevelopmental disorder by researchers in the Department has revealed how the genetic defects interfere with a fundamental cell biological process.
Cartoon showing the folded membranes of the endoplasmic reticulum. Credit: Nicolle Rager, National Science Foundation
The work of Professor Francis Barr and colleagues, in collaboration with Drs Irene Aligianis and Mark Handley in Edinburgh, is published in the Journal of Cell Biology (1).
It provides insight into how defects in the construction of the endoplasmic reticulum, the convoluted membrane structure in all eukaryotic cells, may underlie a spectrum of human neurological disorders.
Warburg Micro syndrome (WMS) is a disorder in which children suffer from multiple specific developmental abnormalities in brain and eye development, profound global developmental delay and neurodegeneration. The genetic defect has been mapped to multiple genes – Rab18, a regulator of membrane traffic, and both subunits of a Rab regulatory complex known as Rab3GAP.
Because of the profound neurological defects in Warburg Micro syndrome, researchers have focused on a possible role for these genes in neurotransmission. But a link has remained elusive despite a number of studies.
A look at the conservation of Rab18 provides a clue as to why this might be the case. 'If Rab18 is part of a neurological pathway, then this is a problem because it is present in ancient eukaryotes without a nervous system and even plants,' comments Professor Barr. 'It appears to be a protein which has a fundamental role at the cellular level.'
Professor Barr, in collaboration with colleagues in Edinburgh who cloned the WMS genes, set out to determine whereabouts in the cell Rab18 acts and what its relationship with Rab3GAP is.
'We started looking at this as a pathway,' he explains. 'We knew that mutations in Rab3GAP subunits 1 and 2 and in Rab18 all cause the disease. So it looks like the components act in series, but in which order?'
Their experiments in cells showed that the Rab3GAP complex is required to promote guanine nucleotide exchange specifically by Rab18. Acting as a GEF (guanine nucleotide exchange factor), Rab3GAP triggers recruitment of Rab18 to the cellular membrane and its activation.
When the researchers tracked the two components by immunofluorescence, they found that the Rab3GAP complex directs Rab18 to the endoplasmic reticulum (ER). In the absence of functional Rab3GAP, Rab18 is lost from the ER tubular network and redistributed to a diffuse cytoplasmic pattern. The ER structure itself also changes, extending more into the cell periphery.
This suggested that Rab18 activation might play a role in controlling the structure of the ER, an important finding given that ER formation and maintenance has been studied little.
'The ER is in fine balance between extended tubular networks pushing out into the cell periphery and sheets of ER close to the nucleus,' explains Professor Barr. 'These look very different but they are all part of the same structure.'
'We found that if we take away Rab 18 or Rab3GAP, the balance of tubules to sheets is altered. The tubules are lost and the ER sheets take over. Cells from patients with WMS showed similar defects in their ER.
Fibroblasts fixed and stained with DAPI (DNA) and with antibodies to markers for the ER. Control (left) and WMS patient (right) cells
Professor Barr believes that the role of Rab18 and Rab3GAP in ER dynamics may explain why the WMS phenotype is predominantly neurological. Neurons have axons and dendrites that extend from the cell body, and this requires dynamic control of the ER.
'With these mutations, you lose the finger-like ER projections and get sheets of membrane. In a fibroblast, this would make little difference, but it matters in a neuron where the ability of the ER to colonise axons and dendrites may be reduced. Neuronal function is directly compromised, not due to a defect in neurotransmission but in the ability to make proper functional neurons.'
'It's amazing. You have a gene that is present in the last eukaryotic common ancestor, but not having it doesn't kill the cells. But obviously it has a big fitness cost.'
Looking across the eukaryotic world, the absence of Rab 18 in yeast stands out. The researchers speculate that this may be linked to the fact that yeast undergo 'closed' mitosis – mitosis in the absence of breakdown of the ER and nuclear envelope, followed by their reassembly.
'We would expect that Rab18's role in controlling ER dynamics would be important during breakdown and reassembly in open mitosis,' says Professor Barr. 'This is a shape change rather than disintegration, so maybe this pathway helps in that process.'
Other consequences of a faulty Rab18 pathway, such as changes in lipid droplet formation, have been observed in previous studies. From their own work, Professor Barr and colleagues believe that these are indirect effects of altered ER regulation – once the ER is disrupted, secondary effects propagate through the system.
The group's work on the molecular defects in WMS shines a spotlight on a wider spectrum of human neurological disorders. Mutations in ER-shaping factors such as alastins, reticulons and spastins are found in a group of disorders known as hereditary spastic paraplegias (HSPs). These are late-onset disorders in which ER function is defective.
'The genes in HSPs are present in multiple copies, so patients with a mutation in one of the copies of a gene get the disorder relatively late,' Professor Barr explains.
'We think that WMS could be considered an HSP. WMS is in a different category clinically because of its onset from birth, but this can be explained by the presence of only a single copy of each of Rab18/Rab3GAP. Maybe WMS cells can't establish a proper ER network as the neurons develop, while in HSPs, they establish it but can't maintain it. As they age, the ER starts to fragment.'
Professor Barr and his group would like to explore the role of Rab18 in more depth. An animal model would allow them to follow the impact of its disruption in neuronal cells at the earliest stages, whilst the identification of Rab18's target would open up many new avenues.