Neuronal Degeneration in Spinocerebellar Ataxias (SCA)

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Neuronal Degeneration in Spinocerebellar Ataxias
(SCA)
Benjamin Downer Mentors Dr. Fratkin and Dr. Vig
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Objectives

• To develop technical skills such as fixing,
  embedding, cutting and staining tissue samples
• To become well-grounded in gross and fine
  neuroanatomy, particularly the cerebellum
• To be able to easily identify distinguishing
  features, especially neuronal changes in the
  brain stem and cerebellum, between normal brains
  and SCA-1 brains

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Methods

• Fixation / Embedding
• Microtome sectioning paraffin-embedded tissue
• Staining Hematoxylin / Eosin
• Staining Immunohistochemistry S100, calbindin,
  RAGE, GFAP, ubiquitin
• Staining Immunofluorescence

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What is SCA?

• SCA is the general term for autosomal dominant
  ataxias. The genetic causes vary from type to
  type, and principally include CAG repeat
  expansions, noncoding repeat expansions, and
  point mutations.
• There are currently 26 known types of SCA, all
  with separate phenotypes, and all displaying a
  large amount of variation from individual to
  individual.
• Though the specifics are different for each type,
  the areas affected include the cerebellum,
  midbrain, pons, medulla, spinal cord, cranial
  nerves, retina and basal ganglia.

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What about SCA-1?

• SCA-1 is a member of the SCA family. It is
  caused by a CAG trinucleotide repeat expansion in
  the ataxin-1 gene on chromosome 6.
• Some of the clinical features associated with
  SCA-1 are abnormalities of motor control such as

• stumbling gait
• slurred speech
• hypermetria
• nystagmus

• bulbar palsies
• dysphagia
• respiratory weakness

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A Brief Background
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Gross Pathology
In this SCA-1 patient, one can clearly see gross
signs of the disease. The pons and brain stem
are substantially smaller than normal, and the
cerebellum appears atrophied as well.
Comparatively, the cerebral cortex is not
shrunken or compressed, and the sulci are not
widened, indicating the localization of the
disease to the posterior fossa.
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The Purkinje Cell
A Purkinje cell is a pyramidal neuron with a
long, branching apical dendrite extending into
the molecular layer of the cortex. This highly
complex dendritic arbor allows for a very large
number of connections with granular neurons,
Bergmann glia and various afferent fibers.
Purkinje cells send efferent fibers throughout
the brain, including the pons, spinal cord and
thalamus, monitoring and helping control myriad
motor functions.
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Microscopic Pathology
Though the emphasis here is on Purkinje cells,
SCA-1 damages more than just these. Further
damage is seen in the pontine nuclei, the
inferior olivary nucleus, the spinal cord and
various cranial nerves. In the cerebellum we
also see loss of neurons in the dentate nucleus.
Pictured here is a myelin-stained (LFB) section
featuring one of the folia of the cerebellar
cortex. The extensive demyelination in the
folium indicates another feature of SCA-1, namely
the loss of Purkinje axons.
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Control
SCA-1 Patient
These HE-stained slides indicate the damage to
the surviving Purkinje neurons in SCA-1. In
contrast to the healthy neurons pictured in the
control, those in the SCA-1 are clearly
atrophied, with no clear nucleoli in most. The
healthy neurons present a more uniform structure
which the SCA-1 neurons lack.
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SCA-1 Patient
Control
These two calbindin-stained slides illustrate the
sometimes vast difference in Purkinje cell number
and health between SCA-1 and normal patients.
Though these two sections are from analogous
portions of the cerebellar cortex, there is a
large disparity in the development of the
dendritic arbor. Further, there is only one
Purkinje extant in the SCA-1 picture, and it is
atrophied.
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Neuronal Inclusions
In this section of tissue taken from a SCA-1
transgenic mouse, we see ubiquitin inclusions at
each of the arrows, indicating neuronal
degradation in the pons for the SCA-1 mice.
The existence of inclusions is confirmed in the
human model as well in this sample taken from a
SCA-3 patient. At each arrow ataxin-3 inclusions
can be seen, indicating the prevalence of the
mutated protein in the pons.
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Looking Ahead
From the work currently being done in the lab, we
can consider future research directions that have
been raised

• Studying trans-synaptic degeneration between
  Purkinje cells and neurons of the dentate nucleus
  in SCA-1
• Unifying research between our work on SCA-1 and
  other members of the SCA family of ataxias, e.g.
  SCA-3
• Looking further into the nature of the
  relationship between Purkinje cells and Bergmann
  glia in SCA-1
• Investigating the purpose of the cellular
  inclusions noticed in some SCA-affected neurons

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Acknowledgements

• Dr. Jonathan Fratkin
• Dr. Parminder Vig
• Marie Lopez
• Dr. David DSouza
• Dr. Ian Paul
• Detra Scott
• Scott Tran