Research interests:

Semaphorin Function in the Mammalian Brain

Semaphorins are a major family of conserved axon guidance molecules and are known to also regulate other aspects of neural development. Several subfamilies of semaphorins exist and we have been studying the functions of Class 6 semaphorins. These are transmembrane proteins, with a long cytoplasmic tail, and are thought to be capable of acting both as signals to other cells and as receptors on the surface of migrating neurons or axons.

We have focused on in vivo analyses of the functions of Semaphorin-6A (Sema6A). Mutation of Sema6A in mice leads to widespread defects across the brain, including abnormal thalamocortical and hippocampal axon guidance and altered cell migration in the cerebellum and spinal cord. We have been investigating the functions of Sema6A in these processes and the involvement of its known binding partners, PlexinA2 and PlexinA4. These studies have revealed a complex set of context-dependent interactions between these proteins that contributes to guidance decisions in many parts of the developing brain. To better understand these, we are also investigating the signaling pathways activated by Semaphorin-Plexin interactions, in collaboration with Dr. Isabella Graef at Stanford University and Prof. Yvonne Jones at Oxford University.

Finally, we are studying the functional consequences of cellular disorganisation and neuronal dysconnectivity arising from mutations in these genes, through integrative physiological, behavioural and pharmacological analyses. In combination with results from parallel human genetic analyses, these have suggested a possible link with the etiology of psychiatric disorders, particularly schizophrenia.

Links to papers:

Kerjan, G., Dolan, J., Haumaitre, C., Schneider-Maunoury, S., Fujisawa, H., Mitchell, K.J., Chedotal A. (2005) The transmembrane semaphorin Sema6A controls cerebellar granule cell migration. Nature Neuroscience 8, 1516-24.

Bron R, Vermeren M, Kokot N, Andrews W, Little GE, Mitchell KJ, Cohen J. (2007) Boundary cap cells constrain spinal motor neuron somal migration at motor exit points by a semaphorin-plexin mechanism. Neural Develop. Oct 30;2:21.

Renaud J, Kerjan G, Sumita I, Zagar Y, Georget V, Kim D, Fouquet C, Suda K, Sanbo M, Suto F, Ackerman SL, Mitchell KJ, Fujisawa H, Chédotal A. (2008) Plexin-A2 and its ligand, Sema6A, control nucleus-centrosome coupling in migrating granule cells. Nature Neurosci. Apr;11(4):440-9.

Suto, F., Tsuboi, M., Kamiya, H., Mizuno, H., Kiyama, Y., Komai, S.,, Shimizu, M., Sanbo, M., Yagi, T., Hiromi, Y., Chedotal, A., Mitchell, K.J., Manabe, T., and Fujisawa, H. (2007) Interactions between Plexin-A2, Plexin-A4 and Semaphorin 6A Control Lamina-restricted Projection of Hippocampal Mossy Fibers. Neuron, 53(4):535-47.

Little GE, Lopez-Bendito G, Runker AE, Garcia N, Pinon MC, Chedotal A, Molnar Z, Mitchell KJ. (2009) Specificity and plasticity of thalamocortical connections in Sema6A mutant mice. PLoS Biol. Apr 28;7(4):e98.

 

Searching for Novel Neurodevelopmental Genes

While many genes involved in controlling the morphogenetic processes underlying brain wiring, including cell migration, axon guidance, synaptic target selection and synapse formation have now been identified, these known genes are insufficient to explain the full trajectory and connectivity choices of even the best understood neurons in simple model systems such as the fly ventral nerve cord. The staggering complexity of the wiring diagram in the mammalian brain and its stereotyped pattern of formation suggest that many more genes are involved in carrying out this developmental programme.

We have used a bioinformatics approach to search for genes that are good candidates to encode proteins involved in neural development, specifically in cell-cell interactions. In collaboration with Prof. Guy Tear at King’s College London, we have compiled a database documenting the evolutionary relationships of all predicted proteins in the genomes of nematode, fruitfly, mouse and human. Using a clustering algorithm, Tribe-MCL, we can group orthologous and paralogous proteins based on BLAST scores. All proteins are also annotated with the results of transmembrane domain and other protein motif predictions. We are particularly interested in secreted, transmembrane or GPI-linked proteins that contain any of a large number of motifs found in known axon guidance genes. Using this approach we have identified a number of subfamilies of such proteins, many of which are conserved from mammals to invertebrates. We are currently investigating the functions of Plxdc2, as well as Neto and Odz family genes.

We have also focused in particular on the Leucine-Rich Repeat superfamily of proteins. LRR proteins are involved in many aspects of neural development including especially synaptic target recognition and synapse formation, both in flies and mammals. The LRR superfamily has expanded and diverged rapidly in mammals and may thus help specify the complexity of synaptic interactions observed in the mammalian brain. We are currently investigating the functional roles in neural development of the novel Elfn and Lrtm subfamilies.

Links to Papers:

Dolan J, Walshe K, Alsbury S, Hokamp K, O'Keeffe S, Okafuji T, Miller SF, Tear G, Mitchell KJ. (2007) The extracellular leucine-rich repeat superfamily; a comparative survey and analysis of evolutionary relationships and expression patterns. BMC Genomics. 14;8:320.

Miller, S.F.C., Summerhurst, K., Runker, A.E., Kerjan, G., Friedel, R.H., Chedotal, A., Murphy, P. and Mitchell, K.J. (2007) Expression of Plxdc2/TEM7R in the developing nervous system of the mouse. Gene Expression Patterns, 7(5):635-44.

 

The Genetics of Neurodevelopmental Psychiatric Disorders

 

Many psychiatric disorders have their origins in disturbances in neural development.  This is true even for conditions such as schizophrenia, where the most florid symptoms that lead to a diagnosis tend not to emerge until late adolescence.  In fact, schizophrenia is associated with much more stable, though subtle, deficits in cognitive, motor and sensory domains that are evident from very early childhood.  The existence of prenatal risk factors, such as maternal infection or low vitamin D levels, especially during the second trimester of gestation reinforce the notion of an early insult. 

This view has now been bolstered by genetic studies which have found numerous single-gene mutations or copy number variants (deletions or duplications) that confer a very high risk of developing schizophrenia.  Importantly, many such mutations also confer a high risk of developing other psychiatric or neurological disorders such as autism spectrum disorders, bipolar disorder, epilepsy and intellectual disability.  Many of the genes thus identified have important roles in neurodevelopment.  Disturbances in neurodevelopment may thus predispose to a wide range of disorders, with the possibility of incomplete penetrance and variable expressivity. 

 

These findings, along with theoretical considerations and other evidence, argue strongly against the prevailing model in the field, which proposes that schizophrenia (and other so-called complex disorders) is caused by the co-inheritance of a large number of risk alleles that are common in the population.  Any one of these alone is not thought to confer a large increase in risk but particular combinations (or exceeding a certain number of them) can.  A more realistic model is that the disorder is not polygenic, but rather heterogeneous.  That is, it is caused by single mutations in each individual, but these can be in many different genes across the population. 

Thankfully, such single mutations can readily be modeled in mice.  These studies promise to reveal the pathogenic mechanisms whereby mutations in neurodevelopmental genes lead to altered connectivity, physiological dysfunction and ultimately psychopathology.  We are currently investigating these links in mutants of Semaphorin-6A, along with parallel genetic studies in humans, and have obtained evidence that these mutants may represent a useful model for the neurodevelopmental etiology of psychiatric disorders.  This bottom-up approach to psychiatric disorders is a collaboration with the groups of Prof. John Waddington and Dr. David Henshall at the Royal College of Surgeons in Ireland, and of Prof. Michael Gill at St. James’s Hospital in Dublin.

Links to papers:

Waddington JL, Corvin AP, Donohoe G, O'Tuathaigh CM, Mitchell KJ, Gill M. (2007) Functional genomics and schizophrenia: endophenotypes and mutant models. Psychiatr Clin North Am. Sep;30(3):365-99.

Mitchell, K.J. (2007) The genetics of brain wiring; from molecule to mind. PLoS Biology Apr 17;5(4):e113.

Mitchell KJ, Porteous DJ. (2009) GWAS for psychiatric disease: is the framework built on a solid foundation? Mol Psychiatry. Aug;14(8):740-1.

 

Synaesthesia (meaning “mixing of the senses”) is a heritable condition of involuntary sensory cross-activation whereby particular stimuli elicit secondary sensory-perceptual experiences.  These may include, for example, experiencing colours in response to sounds, words, music, letters, smells and many other stimuli, tasting words, personification of numbers, spatial locations for numbers, dates, letters, calendar units, feeling sounds, perceiving shapes in response to taste and many other forms.  (See http://home.comcast.net/~sean.day/html/types.htm for a current list). 

 

It is thought to be caused by aberrant cross-activation of one cortical area or processing stream by another, but whether this reflects altered structural connectivity or differences in function is a matter of intense debate.  We have been investigating synaesthesia as a model to understand how cortical areas become specialized during development and how their patterns of connectivity are established.  These interlinked processes depend on both genetic patterning and circuit specification mechanisms as well as experience-dependent plasticity.  Very little is currently known about any of these processes, however, or about how they interact.  Due to its relatively benign presentation, comparatively simple phenotype and apparently Mendelian inheritance pattern, synaesthesia provides a very powerful model in which to study these mechanisms that is likely to be of wide relevance. 

Through a collaboration with Dr. Fiona Newell in the Trinity College Institute of Neuroscience and Dr. Aiden Corvin at St. James’s Hopsital in Dublin, we have carried out a familial survey of synaesthesia in Ireland, with several important findings.  Notably, different types of synaesthesia can co-occur within the same family.  This has led us to suggest a model of initially widespread cross-wiring, or mis-wiring more generally, that can be resolved differently in different individuals, even yielding different phenotypes in monozygotic twins.  In order to better define the phenotype of synaesthesia we have conducted behavioural, electroencephalography and magnetic resonance imaging experiments. 

Using EEG, we found, surprisingly, that synaesthetes differ in the earliest components of the visual evoked potential, even to very simple visual stimuli that do not induce a synaesthetic experience.  Together with another study, which found similar differences in early auditory processing, these data suggest that the phenotype of synaesthesia may be much broader than the apparently discrete manifestation would suggest.  Using voxel-based morphometry and diffusion tensor imaging we have also found structural differences in the brains of synaesthetes.  Again, these are more widespread in the brain than might have been expected from the overt phenotype, confirming and extending recent results of a number of other groups. 

Our current aims are to explore the phenotypic correlates of synaesthesia in more detail using high-resolution tractography and connectivity analyses and to use these phenotypic criteria in a genetic screen for the variants that predispose to this condition. 

Links to papers:

Barnett KJ, Finucane C, Asher JE, Bargary G, Corvin AP, Newell FN, Mitchell KJ. (2008) Familial patterns and the origins of individual differences in synaesthesia. Cognition. 106(2): 871-93.

Bargary G, Barnett, K.J., Mitchell, K.J.* and Newell, F.N.* (2009) Coloured-speech synaesthesia is triggered by multisensory, not unisensory, perception. Psychological Science May;20(5):529-33. (*joint senior authors).

Kylie J. Barnett, John J. Foxe, Sophie Molholm, Simon P. Kelly, Shani Shalgi, Kevin J. Mitchell*, Fiona N. Newell* (2008) Differences in early sensory-perceptual processing in synesthesia: A visual evoked potential study. NeuroImage 43 (2008) 605–613. (*joint senior authors).

Bargary G, Mitchell KJ (2008). Response to Cohen Kadosh and Walsh: Synaesthesia: evaluating competing theories. Trends Neurosci. Sep 17. [Epub ahead of print]

 

 

jadolan@tcd.ie

littleg@tcd.ie

millers@tcd.ie

okafujit@tcd.ie

perezbrf@tcd.ie

tawarayh@tcd.ie

Dr. Kevin Mitchell

Dr. Annette Runker, Dr. Suzanne Miller & Dr. Graham Little

Jackie Dolan

The Mitchell Lab, 2006

Dr. Gary Bargary, Dr. Graham Little & Dr. Maria Yusta Boyo

 

 

Protocols:

Dye tracing

Slice culture

Generation of template DNA for ISH probes

DIG labelling of RNA probes

In Situ Hybridisation

Contact Us:

Dr. Kevin Mitchell,

Developmental Neurogenetics,

Smurfit Institute of Genetics,

University of Dublin,

Trinity College,

Dublin 2.

Email: Kevin.Mitchell@tcd.ie

Phone: +353-1-8963067

Useful Links:

Wiring the Brain

International Conference

Adare Manor, Limerick

21st-24th April 2009

http://wiringthebrain.com/

Wiring the brain blog available at:

http://wiringthebrain.blogspot.com/

 

http://www.neuroscience.tcd.ie/

 

http://www.tcd.ie/Genetics/

 

MRC Centre for Developmental Neurobiology

King's College London:

http://www.kcl.ac.uk/depsta/biomedical/mrc/

 

Funded by: