Week 4 Topic 2 - From the dynamic synapse to synaptopathies

Question 1

Hello, and welcome to this lecture entitled, ‘From the dynamics synapse to synaptopathies’. My name is Deepak
Srivastava and I am the head of the neuronal circuitry and neurodevelopmental disorders research group here
at the Institute of Psychiatry, Psychology and Neuroscience, King’s College London.
In this lecture, we will focus on understanding the function of synapses in the healthy brain. In particular, we will
focus on tiny dendritic protrusions that decorate dendrites, which are known as ‘dendritic spines’. Dendritic
spines are the site for the majority of excitatory synapses in the mammalian brain. We’ll explore the basic
function of these structures, as well as the overall structure and what they contain.
We will go on to examine how dendritic spines make synaptic connections and how these synaptic connections
can be fine-tuned by a number of physiological stimuli. Finally, we’ll explore the evidence that indicates that
abnormal dendritic spine function is connected with mental illnesses and how studying genetic risk factors
associated with mental illnesses can tell us how dendritic spine dysfunction may contribute to the emergence of
disease.

Answer 1

PART 1 OF 4
In this part of the lecture, we’ll explore the basic function of synapses, the structure and content of dendritic
spines, and talk about two processes known as ‘spinogenesis’ and ‘synaptogenesis’.
Slide 5:

Question 2

PART 2 of 4

Answer 2

In this part of the lecture, we will now focus on the function of dendritic spines and discuss how the structure of
dendritic spines is thought to be linked with the functional properties of synapses.

Question 3

• Part 3 of 4

Answer 3

In this section, we will investigate the evidence indicating that abnormal dendritic spine function may be linked
with a range of mental illnesses.
Slide 4:
In the last two sections, we have focused on the role of dendritic spines in the healthy brain. Now, I would like to
explore the potential contribution of dendritic spine dysfunction in disease.

Question 4

• Part 4 of 4

Answer 4

In the final part of this lecture, I would like to explore synaptic deficits in schizophrenia and one way we can
study genetic mutations and link them with abnormal dendritic spine function.

I would like to now focus on how we believe dysfunction of dendritic spines may contribute to mental illnesses.
In order to explore this question, I want to focus on schizophrenia.

Question 5

What direction does synaptic communication occur?

Answer 5

Typically, the flow of information occurs only in one direction – from the pre-synaptic neuron to the postsynaptic
neuron.

Question 6

What happens if there is a disruption of synapse number or function?

Answer 6

There is increasing evidence that disruption of synapse number, and/or function is
strongly linked with brain dysfunction.

Question 7

What determines how many pre-synaptic cells a neuron can connect with?

Answer 7

In the diagram here, we can see a cartoon of a post-synaptic neuron in grey. You can see its cell body – or
‘soma’ – and its dendrites, that emerge out of it. The dendrites are where this neuron will receive information
and, thus, dictates the receptive field of the post-synaptic neuron. That is, the size of the dendritic arbor is
critical in determining how many pre-synaptic cells it can connect with.

Question 8

How does a neuron determine whether or not to send this information to the next neuron through the generation
of an action potential that is sent along its axon?

Answer 8

We can also see several bundles of blue axons, which are part of the pre-synaptic neuron – the arrow shows
the direction of information flow. Basically, information flows along the axons until they reach the synapse. The
information is then transferred across the synapse to the post-synaptic neuron. This neuron then collates the
information and then decides whether or not to send this information to the next neuron through the generation
of an action potential that is sent along its axon.

Question 9

What are the three ways synapses can be classified?

Answer 9

Within the mammalian brain, synapses can be classified in three different ways.

1. You have axodendritic or
   axospino synapses. This is where the axon of the pre-synaptic neuron synapses with the post-synaptic cell
   along its dendrite or on dendritic protrusions known as dendritic spines. These synapses account for the vast
   majority of synapses in the brain and can be excitatory, inhibitory or neuromodulatory.
2. You also have axosomatic synapses. These are synapses that occur on the cell body, or ‘soma’, of the postsynaptic
   cell. These are typically inhibitory or neuromodulatory.
3. Finally, you have axoaxonic synopses. This is
   where the pre-synaptic axon synapses directly on the axon of a post-synaptic cell and, thereby, controls the
   amount of information flow along the axon of the post-synaptic neuron. For the remainder of this lecture, we will
   focus on axodendritic or axospino synapses.

Question 10

Where on a neuron do most synapses occur?

Answer 10

As previously mentioned, a lot of synapses occur on highly specialised dendritic protrusions, known as
‘dendritic spines’. Here, on the right, we have an example of a pyramidal neuron located in layer 5 of the mouse
frontal cortex. You can see that it has a very typified morphology. There is a cell body, or soma, at the bottom
and, then, projecting to the top of the cortex – or the pia of the cortex – you can see a primary dendrite. It is
this typified structure that is quintessential of pyramidal neurons that are found within the cortex.
If we now zoom into the dendrite of one of these neurons, we can see that it is decorated by these funny little
protrusions that come off the dendrites. These protrusions are known as dendritic spines. And what we do
know is that dendritic spines form the post-synaptic compartment of synapses and that they are the site where
the majority of excitatory synapses occur within the mammalian forebrain.

Question 11

Where do synaptic vesicles

containing neurotransmitters reside? And where do the neurotransmitters go on the other side of the cleft?

Answer 11

In this cartoon of an excitatory synapse, we can see in the pre-synaptic terminal where the synaptic vesicles
containing neurotransmitters reside. Once an action potential arrives at the pre-synaptic terminal, the synaptic
vesicles move to the synaptic membrane, fuse with the membrane and release their neurotransmitter into the
synaptic cleft.
On the other side of the synaptic cleft, we have a dendritic spine, which is typified by its spine neck and spine
head.

Question 12

What is the PSD?

Answer 12

Within the spine head, you have the post-synaptic density – or PSD for short – which contains a large
number of proteins, including the neurotransmitter receptors. It is these receptors that receive the information
from the pre-synaptic neuron, in the form of neurotransmitters, and then translates these signals into a
response in the post-synaptic cell.

Question 13

Why have dendritic spines?

Answer 13

1. Firstly, as these structures are where the majority of excitatory synapses occur,

they increase the surface area and thus the potential number of synaptic
connections a post-synaptic neuron can make.

2. Secondly, it is emerging that dendritic spines can
   compartmentalise, both electrical and biochemical signals from the rest of the cell. What this means is that
   dendritic spines can filter – or even amplify – signals, both biochemical as well as electrical, before allowing
   them to pass into the rest of the cell and, thus, influence the output of the neuron.

Question 14

How do dendritic spines influence the output of a neuron?

Answer 14

1. dendritic spines have developed their specialised shapes,
2. also contain a vast
   number of proteins.

Question 15

What proteins, organelles, and fuel do dendritic spines have?

Answer 15

1. receptors – such as glutamate receptors;
2. adhesion proteins – that physically
   connect pre- and post-synapses together;
3. scaffold proteins – such as PSD95, that organises the PSD and
   proteins within dendritic spines.
4. A major component of dendritic spines is F-actin – it is the rearrangement of Factin
   that allows dendritic spines to change shape. We’ll explore this concept in more detail later.
5. It should also be noted that dendritic spines have a number of organelles within them, such as the endoplasmic
   reticulum and polyribosomes – these are required for the production of new proteins.
6. They also contain
   mitochondria, which provide the fuel needed for many processes.

Question 16

How do dendritic spines play a role in normal brain function?

Answer 16

Over recent years, we have really begun to develop an appreciation of the important role that dendritic spines
play in normal brain function.

For example, during early brain development, dendritic spines can be seen to
emerge out of dendrites and to search at the surrounding neuropil for an appropriate pre-synaptic partner.

Once it finds the appropriate pre-synaptic partner, it can make a synaptic connection.

It is thought to be one of
the ways that neural circuits or neural networks can be formed, and it is the basis by which wiring within the
brain occurs.

Question 17

How can dendritic spines change shape and size?

Answer 17

Interestingly, a number of signals, including synaptic activity as well as neuromodulating signals, can also cause dendritic spines to change shape and size as well as to increase or decrease in number. In this cartoon, we can
see that synaptic activity – such as long-term potentiation – seen here in red, causes the existing dendritic spine
to increase in its size, but also causes a new dendritic spine to emerge. This spine has the potential to form a
synapse. And, overall, this has led us to the emerging theme that synaptic connectivity within neural circuits – or
neural networks – can be remodelled and, thus, that wiring within the brain can be refined. Importantly, the
changes in synaptic connectivity can occur in a bi-directional manner.

A physiological
stimulus – such as synaptic activity – can cause either a change in the number or the shape of dendritic spines.
This can either lead to an increase or decrease in either the number or the strength of synaptic connections.
Moreover, it is these changes and synaptic connectivity – driven in part by changes in dendritic spine, shape or
number– that are thought to be essential for normal brain function.

Question 18

So, how do neurons make synapses?

Answer 18

There have been several different models by which synapses can be
formed.

1. The prevailing model that is used is dependent on the time of development as well as the region of the
   brain where this process is occurring. I would like to focus on one model of synapse formation that is thought to
   be the prevalent mechanism that occurs during development and within the adult forebrain. This model, known
   as the Filopodial model, can be easily broken down into two events: ‘spinogenesis’ and ‘synaptogenesis’.
   In this model, the axon and dendrites of the pre- and post-synaptic neurons have already been established.

Question 19

What is the Filopodial model?

Answer 19

One model of synapse formation that is thought to
be the prevalent mechanism that occurs during development and within the adult forebrain. This model, known
as the Filopodial model, can be easily broken down into two events: ‘spinogenesis’ and ‘synaptogenesis’.

In this model, the axon and dendrites of the pre- and post-synaptic neurons have already been established.

Question 20

What is a filopodia?

Answer 20

a dendritic
protrusion is known as a filopodia.

This is a very long protrusion that is very dynamic – that is, it moves around the
surrounding neuropil very quickly and can appear and disappear very quickly.

Filipodia do not have a
discernible head structure and do not contain the proteins necessary to create a synaptic connection.

For
example, these protrusions do not have a post-synaptic density – or ‘PSD’ – and they do not contain
neurotransmitter receptors.

The Filipodia then searches the surrounding neuropil looking for an appropriate
pre-synaptic partner – which is known as ‘target selection’.

Question 21

What is a neuropil?

Answer 21

a fibrous network of delicate unmyelinated nerve fibers interrupted by numerous synapses and found in concentrations of nervous tissue especially in parts of the brain where it is highly developed

Question 22

What is synaptogenesis?

Answer 22

Once a pre- and post-synaptic cell have identified each other as partners, the next stage is known as
‘synaptogenesis’.

The initial step of this is known as ‘synapse assembly’. Here, several key synaptic proteins are
recruited to the nascent dendritic protrusion.

These proteins include NMDA receptors, the scaffold protein,
‘PSD95’, and a number of adhesion proteins.

Question 23

What is the initial stage of creating a synapse and where does it occur? Spinogenosis is when the spine is created.

Answer 23

At this point, you can see a pre-synaptic terminal on the axon. In this model, the dendrite creates a dendritic
protrusion known as a filopodia. This is a very long protrusion that is very dynamic – that is, it moves around the
surrounding neuropil very quickly and can appear and disappear very quickly. Filipodia do not have a
discernible head structure and do not contain the proteins necessary to create a synaptic connection. For
example, these protrusions do not have a post-synaptic density – or ‘PSD’ – and they do not contain
neurotransmitter receptors. The Filipodia then searches the surrounding neuropil looking for an appropriate
pre-synaptic partner – which is known as ‘target selection’.

Question 24

What is synapse stabilisation?

Answer 24

The next step is known as synapse stabilisation. This is where synaptic activity
induces the recruitment of more adhesion molecules to further stabilise the dendritic spines, as well as NMDA
receptors and other synaptic proteins to establish these pre- and post-synaptic structures as fully functional
synaptic connections.

Question 25

What is the consequence of having dendritic spines with different morphologies?

Answer 25

Dendritic spines come in a myriad of shapes and sizes. The shape and size of a dendritic spine can tell you a lot
about its function. Here, we have a serial electron microscopy reconstruction of a small part of dendrite from a
hippocampal neuron. What you will hopefully be able to see is that dendritic spines can be large and small in
size, and even long and short.
So, what is the consequence of having dendritic spines with different morphologies? Well, in this image, we can
also see the size of the synaptic connection, which is shown in red. This nicely shows you that larger dendritic
spines typically have larger synaptic connections, whereas smaller or thinner spines have much smaller
synaptic connections.

Dendritic spine shape is also intimately linked with its function. For example, in this study, the authors have
labelled the dendrite of a neuron with green fluorescent protein, or GFP, to show the shape of the cell. They
have also immunostained the cell for GluA1-containing AMPA receptors, as a proxy for measuring synaptic
strength. More GluA1-containing AMPA receptors would indicate the stronger synapses.

LARGER SPINES

Hopefully, what you can see is that larger spines – shown here by these red arrows – contain a lot of GluA1-
containing AMPA receptors.

SMALLER SPINES

Whilst, on the other hand, much smaller or thinner spines – shown here by the
yellow arrows – have much smaller amounts of GluA1-containing AMPA receptors.

Question 26

What did Matsuzaki and colleagues in 2001 show in their study?

Answer 26

This was very nicely shown by a study carried out by Matsuzaki and colleagues in 2001. Here, the authors
performed a very elegant study to show that larger dendritic spines have much stronger responses to
glutamate whereas smaller dendritic spines have a smaller response to glutamate. In this study, the authors
have recorded excitatory post-synaptic currents in hippocampal neurons. The hippocampal neurons are
bathed in cage glutamate – that is, glutamate that is inactive unless you shine a particular wavelength of laser on
it, allowing it to become active. The authors were able to uncage glutamate at very specific sites, such as
directly over a dendritic spine – this means that you can activate only the AMPA receptors within a specific
dendritic spine. Thus, the authors uncaged glutamate over dendritic spines with different sizes and, at the same
time, recorded the excitatory postsynaptic current that it resulted.
In the left image, we can see four spines labelled ‘A’, ‘B’, ‘C’ and ‘D’ – all of which have different sizes. In the
middle and right panels, we can see that the measured, post-synaptic current induced by the uncaging – in
these images, yellow and red colours – indicate a larger response whereas darker and blue colours indicate a
smaller post-synaptic current. What the authors found was that if they uncaged glutamate over the spine
labelled ‘A’, that the current was much larger than if they uncaged glutamate over spines ‘C’ and ‘D’. Taken
together, these studies demonstrate that larger dendritic spines typically contain more AMPA receptors and
generate larger excitatory post-synaptic currents as compared to dendritic spines with smaller size. Thus,
indicating that dendritic spine structure is linked to synaptic function.
Slide 6:

Question 27

What’s the difference between larger and smaller dendritic spines?

Answer 27

What this tells us is that
larger dendritic spines not only contain more GluA1-containing AMPA receptors but that they are more likely to
have bigger responses to a glutamate or synaptic activity whereas thinner or smaller dendritic spines, that
have less AMPA receptors, are likely to have smaller responses to glutamate or synaptic activity.

Question 28

So, do dendritic spines change shape in response to different stimuli?

Answer 28

In short, yes, they can. As we discussed
earlier, physiological stimuli – such as changes in synaptic activity – can change the number and strength of
synaptic connections. This also leads to a change in dendritic spine size. For example, if we were to induce a
long-term potentiation- or LTP-like stimulus, we can see that dendritic spines can actually increase in size.
Conversely, if we were to induce a long-term depression- or LTD-like stimulus, we can see that dendritic spines
actually shrink in size.

Question 29

What is structural plasticity and what role does it play in encoding information?

Answer 29

The ability of dendritic spines to change size in response to stimulation is known as
‘structural plasticity’ and is thought that this process plays an essential role in the encoding of information.

If structural plasticity does play a central role in the encoding of information – based on our understanding that
larger dendritic spines have more AMPA receptors and, thus, make stronger synaptic connections – one would
expect that following a LTP-like stimulus that not only would dendritic spines change in size but also the amount
of AMPA receptors would also increase.

Question 30

In 2006, what idea did Kopec and colleagues test?

Answer 30

In 2006, Kopec and colleagues tested this idea. What they did was to monitor the amount of GluA1-containing AMPA receptors in dendritic spines, before and after the induction of LTP. Here, the authors induced LTP using
a chemical approach and, thus, have labelled this ‘chemically-induced long-term potentiation’ or ‘cLTP’. What the
authors did was to monitor both the size of the dendritic spines, by measuring spine volume, as well as the
amount of AMPA receptors within dendritic spines. This was done by making hippocampal neurons express a
red fluorescent protein, to outline the morphology of the cell, and to express GluA1-containing AMPA receptors
that would link to a special form of GFP, that only fluoresces when the receptor is expressed at the surface of
synapses.

What this means was that the authors could easily monitor the size of dendritic spines whilst
simultaneously measuring the amount of synaptic and, thus, active AMPA receptors within dendritic spines.

The authors then monitored both the size of dendritic spines as well as the amount of AMPA receptors in
dendritic spines 30 minutes before and up to 80 minutes after the induction of this chemical LTP. What the
authors found was that, as expected, the induction of chemical LTP caused dendritic spines to increase in size.

This can be seen in the left-hand image. In addition, the authors found that the amount of GluA1 in dendritic
spines also increased following the induction of chemical LTP. This can be seen in the middle panel where an
increase in the amount of GluA1 within dendritic spines is shown by an increase in the amount of yellow and red
colours. These data are summarised in the graph on the right and it shows that as spine size increases – shown
by the red line – the amount of AMPA receptors also increases.

Ultimately, what this tells us is that as dendritic
spines change size in response to stimulation, the amount of AMPA receptor also changes.

Thus, demonstrating
that structural and functional plasticity are linked.

Question 31

What does physiological stimuli – such as long-term potentiation (LTP)
or long-term depression (LTD) - do to dendritic spines?

Answer 31

So, to summarise, what we have shown here is that physiological stimuli – such as long-term potentiation (LTP)
or long-term depression (LTD) – can not only

1. change the number of dendritic spines but can also result in a
2. change in the size of dendritic spines.
3. This results in a concurrent change in the amount of AMPA receptors
   within these dendritic spines,
4. which underlies the changes in synaptic strength that is observed.
5. Thus, structural
   and functional plasticity are coordinated and can be changed, resulting in refinement of neuronal circuitry.
6. This
   process is, therefore, thought to be essential for normal brain function.

Question 32

How are dendrites and dendritic spine morphology affected in a range of
brain disorders?

Answer 32

ASD AND SCZ

In a range of different disorders, the overall dendritic arbour seems to be

1. simplified as compared to that seen in a normal neuron or healthy neuron.

Similarly, in studies where researchers have examined the post-mortem brains of individuals with different mental health issues, such as autism or schizophrenia, we can see that there is an

2. abnormal number of dendritic spines compared to healthy or controlled individuals.

ASD
increase in the number of dendritic spines, compared to healthy or controlled patients.

SCZ
Conversely, if we look at the number of dendritic spines of neurons found in the brains of patients with schizophrenia, there seems to be a reduction in the number of dendritic spines as compared to healthy individuals.

This has led to the overall idea that aberrant dendritic architecture or abnormal dendritic spine density could result in altered neuronal network or circuitry and, thus, wiring – which could, ultimately, result in cognitive
deficits that are seen in brain disorders, such autism spectrum disorders and schizophrenia.

Question 33

What are two major

issues with relying in post mortem studies, to help identify underlying causes of disease?

Answer 33

However, a major
issue with relying in post mortem studies, to help identify underlying causes of disease, is that

1. we do not know
   whether the observed deficits, such as impaired dendritic arbours or altered dendritic spine numbers, are a
   cause of the disease or have been caused by the disease progression.

Indeed, the post-mortem tissues have
been taken from individuals at the end of their life and after they have likely suffered from the disease for a long
time.

2. As such, a number of factors – such as chronic exposure to drugs – may have influenced the observed
   phenotypes.

Question 34

What evidence is there that dysfunction of dendritic spines may contribute to disease?

Answer 34

FIRST: TIMING

Well, if we look at typical neurodevelopment, we can see that dendritic growth and dendritic spine morphogenesis and, thus, synapse formation occurs early on in life.

If we examine when specific disease
symptoms occur, we can see that they coincide with critical periods of synapse formation.

ASD

For example,
symptoms associated with autism spectrum disorders emerge during early childhood, a period when there is
increased spine and synapse formation.

Current research indicates that an increase in the number of dendritic
spines, occurring early on in this disorder, may contribute to the symptoms.

SCZ

Symptoms associated with schizophrenia, on the other hand, typically emerge around adolescence or early
adulthood. This also coincides with a period when there is a refinement of synaptic connections. This is typified
by pruning of synaptic connections. One theory is that an increase in synapse elimination during this period may
contribute to the emergence of schizophrenic symptoms.

SECOND: STUDIES THAT INVESTIGATE GENETIC CAUSES FOR DISORDERS
However, perhaps the most compelling evidence that dendritic spine dysfunction may play an important role in
the emergence of disease, lies in recent large-scale studies investigating the underlying genetic causes for
neurodevelopmental and psychiatric disorders.

These studies have identified a large number of de novo protein coding mutations. That is, genetic variance that
would cause a change in the sequence of specific proteins that are associated with risk of developing diseases
– these include disorders such as intellectual disability, epilepsy and autism spectrum disorders, all of which
have an early onset, as well as disorders like schizophrenia and bipolar disorder, which have a late onset.
Interestingly, if we compare these de novo protein coding variants with the proteome of human post-synaptic
density – or the ‘PST’ – as a proxy for what proteins are present at synapses, we find that there is a large
overlap. What this strongly indicates is that many of the de novo protein coding mutations associated with
various neurodevelopmental and psychiatric disorders occur in proteins that are found at synapses. This
strongly supports the idea that dysfunction at synapses and, in turn, dendritic spines play an important role in
the emergence of disease.

Question 35

What do the genes that have been implicated with disease show us?

Answer 35

Indeed, if we examine which genes have been implicated with disease in more detail, we can start to see that
many of these genes, not only encode for proteins that localise to dendritic spines, but also have critical roles in
dendritic spine formation, maintenance and remodelling.

In this image, we can see several classes of synaptic proteins, all of which have been implicated with disease.
These include adhesion proteins, scaffold proteins and glutamate receptors, all of which we have discussed
earlier in this lecture as having critical roles in the basic function of dendritic spines.

In addition to this, we also
find a number of signalling molecules as well as voltage-gated calcium channels as being implicated.

These examples help to build a picture indicating that alterations in the function of some or many of these
proteins, could easily result in dysfunction of dendritic spines and, thus, impact synaptic communication and
connectivity.

Right now, there is a lot of work that is going on, trying to understand how these aberrant
structures occur and, moreover, if it is possible to reverse or stop these deficits from occurring.

And it is with
this that we hope we will be able to treat different mental illnesses.

Question 36

What are de novo protein coding mutations.

Answer 36

Genetic variance that
would cause a change in the sequence of specific proteins that are associated with risk of developing diseases. De novo means previously undetected.

Question 37

What does de novo mean?

Answer 37

De novo means previously undetected.

Question 38

What is SCZ, what % of the population does it affect?

Answer 38

This mental illness is a highly complex
disorder. Indeed, schizophrenia is a chronic disease that significantly impacts the psychological and the social
and cognitive functioning. It affects approximately 1% of the population.

Question 39

What are the three indications of SCA?

Answer 39

At the clinical level, it is described as having

1. positive symptoms – such as hallucinations and delusion,
2. negative
   symptoms – blunted affect, avolition, asociability – as well as
3. thought disorders. Working memory deficits or
   other cognitive deficits seem to be incorporated into these thought disorders.

Question 40

What does it mean that SCZ [ schizophrenia]

is a heterogeneous disorder?

Answer 40

That is, the symptoms that one patient displays may be very different to what
another patient experiences.

Question 41

What are current pharmacological treatments for SCZ and what are the 4 drawbacks?

Answer 41

Current treatments for schizophrenia rely on the use of antipsychotic drugs such as haloperidol, olanzapine
and clozapine. These drugs are particularly good at addressing the positive symptoms that are associated with
schizophrenia in the majority of patients.

1. However, about a fourth of patients are non-responsive to this type of
   drug treatment.
2. In addition to this, antipsychotics have little impact on the negative symptoms seen in
   schizophrenia as well as on the thought disorders for cognitive deficits associated with this disorder.
3. This has
   major implications, in terms of the functional recovery of the patient, as it seems that the severity of the
   negative and cognitive symptoms of schizophrenia that seem to be most associated with the functional
   recovery of the patient.
4. Furthermore, there are a number of severe side effects including sedation and weight
   gain as well as even motor deficits, which, again, seem to have a negative impact on patient functional recovery.

Question 42

What is another approach to treating schizophrenia and what are its drawbacks?

Answer 42

There are a number of other approaches to treating schizophrenia, such as

1. behavioural treatments, including
   cognitive behavioural therapy.

This is an approach that has been used as an adjunct to antipsychotic drug
treatment and can be effective in reducing relapse and resistant symptoms.

However, these behavioural
therapies have little impact on the negative and cognitive symptoms that are associated with schizophrenia and,
therefore, they have little impact on the patient’s functional recovery.

Question 43

So, how do we go about trying to understand what may be causing the negative and cognitive deficits in this SCZ
disorder? And how can we make more effective and safer therapies for this disorder?

Answer 43

1. Well, one thing we can do is to start looking at the neuropathology of the disorder.

Question 44

So, what do we know about

the neuropathology of the SCZ disorder?

Answer 44

Well, actually, we don’t know very much, mostly because there are many
inconsistencies between studies.

1. However, what is agreed upon by many is that there are reductions in the
   grey matter or patients compared to unaffected individuals.

Here we have structural MRI images of brains of healthy individuals or those who are suffering from
schizophrenia.

And we can see that there seems to be a difference in the overall volume of the brain of
schizophrenic patients as compared to the healthy individuals.

2. In addition to this, EEG and MEG studies have
   suggested that there is a dysfunction in neuronal network function in schizophrenic patients.
3. And, as we’ve seen
   already before, post mortem human studies suggest that there may be a reduction in the number of dendritic
   spines in patients with schizophrenia as those compared to healthy individuals.
4. The cause or causes of schizophrenia is likely to be multifaceted and likely involves a range of genetic as well as
   environmental factors. Each of these factors are unlikely to cause the disease by itself. But, a combination of
   both genetic and environmental factors would likely increase the chance of an individual developing the disease.
   While a number of environmentals have been linked with an increased risk of developing schizophrenia, there is
   also a very strong genetic component to this disease.

But for now, what I would like to highlight is that the genetic landscape of schizophrenia is highly
complex.

Question 45

What have Genetic studies shown about SCZ?

Answer 45

Genetic studies indicate that there are a large number of mutations that are associated with schizophrenia.

Some of these mutations are very rare, only occurring in fewer than 1% of patients with schizophrenia, but they
have a strong effect. That is, if you have this mutation, you are more likely to have the disease. Conversely,
there are a large number of genetic variants that have a weak effect. That is, they only slightly increase your
chance of developing the disease. Taken together, we believe that it is a combination of environmental and
genetic factors, both rare and common variants, that combine to underlie schizophrenia.

Question 46

So, how do we go about testing this theory that mutations in genes associated with schizophrenia can result in
altered synaptic structure or function and, therefore, impact brain wiring? And what are the drawbacks of each?

Answer 46

Well, the two most commonly used
approaches are to either

1. use animal models – where the gene of interest has been knocked out or mutated – or
   to
2. use primary neuronal cell cultures – where cells are grown in a dish and we, again, manipulate the
   expression of genes to try and understand the role that the protein may play in controlling synaptic structure or
   function.

Both of these experimental approaches have their benefits as well as their caveats.

For example, using an
animal model, we can not only look at the overall morphology of the cell, but we can also examine how altering
the expression of specific genes may impact the behaviour of an animal. But you may also argue, that how can
you model the behaviour of an animal with schizophrenia?

Whereas, on the other hand, looking at primary
neuronal cell cultures, it is a very easy way to manipulate gene expression and also allows you to examine
dendritic spines in quite a bit of detail.

Question 47

Example of growing neurons in a dish and what that means to understanding SCZ

Answer 47

Let’s take the approach of growing neurons in a dish and explore how we can use this experimental approach to
examine or model synaptic deficits in schizophrenia. What we can do in this approach is to grow neurons on a
glass coverslip. And, then, using some clever molecular biology, we manipulate the expression of our target
gene. After this, we can use a microscope to image the morphology of the neuron and then to perform detailed
analysis of the dendritic spines.

Let’s take a real-life example of this approach. In this experiment, we have chosen to target the DISC1 or
‘disrupted in schizophrenia one’ gene. This gene encodes for a protein that is found in dendritic spines and is
involved in a number of processes at the synapse. The mutation in this gene has been linked with a range of
psychiatric disorders, including schizophrenia, autism spectrum disorders, depression as well as a number of
other disorders. It was first identified in a Scottish family where a number of individuals were found to have
mutations in this gene and to also have schizophrenia or bipolar disorder. Mutations of the DISC1 gene often
seem to result in a reduction in the expression of the protein or in a dominant negative effect.
So, in order to test whether this one protein is important for regulating dendritic spine number – and,
therefore, wiring within the brain – we decided to try and reduce the expression of DISC1 in cultured neuronal
cells – or cells grown in a dish – and to compare them with a control cell. Hopefully, you can see here a control
cell and, if we zoom in on the dendrite, you can see the dendritic spines that are shown here with the red
arrows. However, in cells where there’s a reduction in the levels of DISC1, you can see that there are fewer
dendritic spines – as shown with the red arrows.
These data are consistent with a number of previous studies that have shown the same effect using a wide
range of different approaches. And, simply put, this experiment allows us to say that by reducing DISC1 levels,
we can negatively impact the number of dendritic spines. This allows us, therefore, to suggest that DISC1 plays
an important role in, at least, the maintenance of dendritic spines. And, therefore, alterations in the expression
of DISC1 protein, as seen in patients with various psychiatric disorders, may impact the synaptic connectivity
within their brain.

Question 48

Summary of Week 4 Topic 2

Answer 48

In this lecture, we have covered what the basic function of dendritic spines are, what their overall structure is
and what they contain. We have explored how dendritic spines are the site for where the majority of excitatory
synapses occur and discussed the model whereby dendritic spines can form new synaptic connections.
We then went on to investigate in more depth, the idea that dendritic spine structure is strongly linked with
synaptic function and that changing these two parameters are coordinated in response to different stimuli. We
then went on to examine the evidence that dysfunction in dendritic spine function – and, therefore, altered
synaptic connectivity – was linked with a number of neurodevelopmental and psychiatric disorders.

In particular, we focused on evidence coming from genetic studies that implicate these structures in the
pathogenesis of disease. Finally, we touched on some of the approaches whereby we can test the hypothesis
that dendritic spine dysfunction may contribute to a complex disorder, like schizophrenia. Moreover, I have
tried to show you that we could test the idea that altering the expression of proteins associated with disease,
allows us to see how these proteins may contribute to the pathophysiology of disorders, such as schizophrenia.