number of genes have been implicated in neurodegenerative diseases such as
amyotrophic lateral sclerosis (ALS), also known as motor neurone disease, and
Frontotemporal lobar degeneration (FTLD). However, the core biological
processes involved in these disorders are extremely difficult to model and this
is hampering the effort to develop treatments. If we can resolve this and
increase our knowledge about these disease processes it may be possible to
develop new and improved treatments.
gene TDP-43 has been shown to be involved in the development of the
neurodegenerative diseases ALS and FTLD in humans, however the exact way(s) in
which it does this still remain unclear. This protein acts as a master
regulator of RNA, a precursor to proteins, and can therefore control the
expression of genes within cells. A recent study by a group led by Dr. Frank
Hirth from King’s College London has shed some light on this by using an
interesting choice of organism: drosophila melanogaster, the common fruit fly.
in a wide variety of fields are looking to drosophila to model complex
diseases. But why choose flies, they
couldn’t be any more different biologically to us could they? In fact,
fruit flies share many biological processes with us; they have a complex immune
system, an intricate brain and can even develop cancer! Genes that are similar
in structure and cellular function in flies and human are called homologues.
They can be retained from a common ancestor by a process called evolutionary
conservation. What makes the fruit fly a great model for studying disease is
that it has fewer genes than humans. Where humans often have multiple genes
that perform essentially the same function, a feature known as redundancy,
flies often only have one gene at each stage of a signalling pathway. This
means that if you alter that one gene the effect should be more obvious.
Imagine calling in sick for work but no one there is able to cover for you,
your work just won’t get done and others will see exactly what your role is!
fruit fly TBPH models neurodegenerative disease
fruit fly homologue of the TDP-43 gene is called TBPH, and it too has been
implicated in neurodegenerative disease. To investigate TBPH the researchers
from King’s made fruit flies which either lacked the gene, or had increased gene
expression. Interestingly, both types of fly have key symptoms of neurological
disorders such as a decreased life span and motor dysfunction – this was measured
by videoing the flies walking and then making recordings about the speed and
gait of their movements. TBPH may
therefore be considered a “goldilocks gene”, too much or too little can be
biologically bad and “just the right amount” is required for normal health.
discover how loss or gain of TBPH leads to altered motility of the flies the
researchers began to compare physical features of the normal fly to flies with
loss of TBPH. Firstly they noted that there were no visible differences between
muscle or synapse structure. This led them to dig deeper into the process of
motor function. Using a series of complex experiments measuring the electrical currents
generated at the gaps between neurons and muscles (neruo-muscular-junctions) of
TBPH expressing, non-expressing or over-expressing flies, the group were able
to suggest that the defects exist at the pre-synaptic stage of signal
transmission. The flies have trouble transmitting neurological signals from the
neurones to the muscles and this causes the physical difficulty in movement; as
seen in humans with disorders driven by dysfunctional motor neurones.
typically develop neurological diseases during old age, a fruit fly model where
TBPH loss, or over-expression, was targeted only to a specific area of neurones
allowed flies to age normally. It was not until flies had reached an “old age”
of 40 days that they started to lose pre-synaptic function. This led ultimately
to symptoms of neurodegeneration suggesting TBPH loss or over-expression has a
progressive effect on neurological degeneration. Therefore, optimal TBPH levels
in neurones are critical for maintaining proper motor neuron function during
aging and supports both the TDP-43 gene as a driver of motor neuron disorders.
This study elegantly highlights the usefulness of the TBPH fruit fly model as a
great way to research the underlying mechanisms behind these disorders. If more
is discovered about these mechanisms then new and exciting therapeutic agents
could be developed for treating neurodegenerative diseases. It is clear that
when it comes to neurodegenerative disease research the flies have it!