Tag Archives: neurexin1

Understanding Schizophrenia 2: What causes schizophrenia?

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The Royal Bethlem psychiatric hospitalThe previous post in this series described the symptoms of schizophrenia. Here we turn to the causes of schizophrenia. There has been major progress in this area over the last twenty years. A number of factors have been identified which carry a risk for schizophrenia. Some of these factors are genetic, others impact during the course of life.

Usually schizophrenia emerges in late adolescence or early adulthood, as the intellect, personality and neural networks are being sculpted. The population risk is slightly less than 1%, with a slight excess of male sufferers (1.4:1). Males also tend to show a more severe pattern of illness, with more impoverishment of the personality and psychological decline.

Risk Factor Categories

The risk factors for schizophrenia can be grouped into several categories (Figure 1). The perinatal category includes hypoxic and nutritional insults to the developing brain in-utero. The second category includes being brought up in city environment, particularly for immigrants. The third category includes drugs of abuse, specifically strong cannabinoid CB1 receptor agonists. Finally, there is the genetic category, which can be subdivided into single nucleotide polymorphisms (SNPs) and copy number variants (CNVs).

Figure 1. Risk factors for schizophrenia.

Figure 1. Risk factors for schizophrenia.

Genetic risk factors

It has long been recognized that schizophrenia runs in families (Figure 2), but until the last decade attempts to identify specific genes floundered. Technological advances have revolutionized the field. It is now feasible to screen an individual’s DNA at every base pair (A, T, C, G) in every chromosome. Variants (say the substitution of an A for a T) are called single nucleotide polymorphisms (SNPs) when they occur in at least 5% of the overall population.

In the technique known as GWAS (genome wide association study) tens of thousands of patients are compared against tens of thousands of controls. So far, 145 SNPs have been shown to confer risk for schizophrenia (Figure 3). Each SNP on its own carries a very small risk, but they are common in the population, and their effects are additive. Two SNPs of considerable interest are the gene for a calcium channel (CaV1.2) and the gene for a protein called complement C4a.

Figure 3. Single nucleotide polymorphisms which confer a risk for schizophrenia.

Figure 3. Single nucleotide polymorphisms which confer a risk for schizophrenia.

The second major breakthrough in schizophrenia genetics are copy number variants (CNVs). Copy number variants are deletions or duplications of a long stretch of DNA, typically incorporating half a dozen genes or so. So far eight CNVs which confer a risk for schizophrenia have been identified. Each of these CNVs carry a very high risk. A CNV of considerable interest is NRXN1 (Figure 4).The NRXN1 protein forms a physical bridge which stabilses synaptic connections in the brain. The NRXN1 story provides strong evidence for a long-held theory that the pathology of schizophrenia stems from abnormal connectivity within neural networks.

Figure 4. Copy number variants associated with schizophrenia.

Figure 4. Copy number variants which carry a risk for schizophrenia.

We can recap. A number of factors confer risk for the development of schizophrenia. These can be categorized into several categories – perinatal, environmental, cannabinoid CB1 drugs, and genes. The gene category includes SNPs (such as, complement C4a, the calcium channel CaV1.2) and CNVs (such as NRXN1). In the next post in this series we will look at the neurobiology of these components and cannabinoid CB1 drugs.

Further posts in this series:

What exactly is schizophrenia?

 Future posts in this series:

What happens to the nervous system in schizophrenia?

The prognosis of schizophrenia.

How is schizophrenia treated?

Psychosis Research. Where have we been & where are we going?

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phenotype and genotype

The Institute of Psychiatry at The Maudsley is the largest centre for psychiatric research in Europe. Recently a group of leading researchers were tasked with summarising an area of research as it pertains to psychosis and psychopharmacology.

The outcome was a series of short lectures, delivered to a lively audience of psychiatrists, mental health workers and psychologists at The Maudsley. The lecture slides and audio are now available below and constitute a unique training resource for those who treat patients.

1. Sir Robin Murray,
“Psychosis research: Deconstructing the dogma“
2. David Taylor,
“Current Psychopharmacology: Facts & Fiction“
3. Oliver Howes,
“How can we Treat psychosis better?“
4. Marta DiForti,
“An idiot's guide to psychiatric genetics“
5. Sameer Jauhar,
“Ten psychosis papers to read before you die!”
6. Paul Morrison,
“Future antipsychotics“

 

Cognitive disorders: the role of dendritic spines.

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Neuronal plasticity:

A major contribution of neuroscience to the humanities is the knowledge that the structure of the brain is moulded by the experiences the mind goes through – the phenomenon known as plasticity. It means that the circuits of the brain are sculpted by habitat, schooling, language, relationships, and culture, as well as by the unfolding genetic programme. The action occurs below the micrometre scale – at synapses (the points of connection between neurons) – and involves the exquisite choreography of a number of molecular machines. These molecular processes are so fundamental for cognition that their failure (whether driven by gene mutation or by harsh environments) results in neuropsychological disability. A major locus of plasticity (and hence, cognitive disability) is the dendritic spine.

pyramidal neuron

The dendrites of pyramidal neurons express thousands of dendritic spines. P=pyramidal neuron.

Principal neurons in the brain, such as cortical pyramidal neurons, express tens of thousands of small protruberances on their dendritic trees. These structures (dendritic spines) receive excitatory information from other neurons, and are highly dynamic. They can adjust their responsiveness to glutamate (the major excitatory neurotransmitter), becoming stronger (potentiation) or weaker (depression), as local circumstances dictate. This strengthening (LTP) or weakening (LTD) can be transient, or persist over long periods and as such, serves as an ideal substrate for learning and memory at synapses and in circuits. Potentiated spines increase in size, and express more AMPA glutamate receptors, whilst the opposite pattern occurs in synaptic depression to the extent that spines can be 'absorbed' back into the dendritic tree.

Over the course of childhood, dendritic spines (excitatory synapses) increase in number, but their numbers are 'pruned' back during adolescence to reach a plateau. Enriched environments have been shown to increase spine density, impoverished environments the opposite. In common psychiatric disorders, spine density is altered. For example, the most robust histological finding in schizophrenia is a reduction of spine density in the frontal cortex, auditory cortex and the hippocampus. In major depression, spines (and dendrites) are lost in the hippocampus. In autism, spine density actually increases. Finally, in Alzheimer's and other dementias there is a catastrophic, and progressive loss of cortical and sub-cortical spines.

Regulation of the spine:

The molecular biology of dendritic spines involves hundreds of proteins, but the outlines are now reasonably well understood. Scaffolding proteins [such as PSD95, shank(s), AKAP, stargazin and homer(s)] provide structural support and provide orientatation for membrane bound receptors, ion-channels and their downstream signalling pathways. The scaffold (post-synaptic density), facilitates effective signalling by ensuring that the correct protein partners are in close apposition. The scaffold is also tethered to proteins which bridge the synaptic cleft (cell adhesion molecules) and to bundles of actin filaments which provide the structure and force for spine enlargement (and retraction).

dendritic spine

Spine plasticity is fundamental for learning and memory. The shuttling of AMPA receptors underlies early phase plasticity. Modification of the actin cytoskeleton and local protein synthesis underlie long term plastic changes.

There is a constant remodelling of the actin cytoskeleton within the spine in response to synaptic and network signalling. Remodelling is via small, cytoplasmic G-proteins from the RHO family. Some family members promote the growth and stabilisation of actin filaments, whereas others promote actin disassembly. Mutations in the proteins which regulate actin dynamics are a cause of learning disability. Finally local protein synthesis (and degradation) occurs within dendritic spines, is tightly controlled and is essential for plasticity. Abnormalities in local protein synthesis within the spine underlie learning disability syndromes such as fragile X, neurofibromatosis and tuberous sclerosis.

Spine pathology:

Recent years have seen glutamate synapses move to centre stage in neuropsychiatry. This is not surprising given the role of pyramidal neurons (glutamate containing neurons) in information processing, and the role of glutamate transmission in learning and memory [see link]. But it is remarkable that so many psychological and cognitive disorders appear to 'coalesce' at dendritic spines.

The enclosed vector-graphic image [click here] highlights a selection of some of the proteins which are now known to be involved in autism, learning disability and schizoprenia.

Research will continue to decipher the complexity (and beauty) of the dendritic spine, but potential treatments are starting to emerge for disorders like fragile X, (which until recently were thought to be not amenable for drug treatment, as was the case for schizophrenia until the 1950s). Molecular neuroscientists will, almost certainly, continue to uncover more treatment targets. The task for psychiatry, as ever, is to keep abreast of neuroscience in all it's complexity (and beauty).