Cortical Plasticity in Type II Diabetes Mellitus

Importance of the problem:

DM affects nearly 8.5% of the U.S. population, and DM2 accounts for 90% to 95%
of all diagnosed cases of DM in adults. Nearly three quarters (70%) of people
with DM2 have nervous system damage. Neurological complications of DM2 include
sensorimotor peripheral neuropathy leading to impaired sensation, weakness,
gait disturbance, balance problems, and pain in the feet or hands; and
autonomic neuropathy leading to slowed digestion of food and erectile
dysfunction. However, DM2 can also affect the central nervous system (CNS),
which can lead to cognitive decline. Cognitive dysfunction in DM2 results in a
significantly increased risk of dementia. There are currently about 250 million
people with DM2 worldwide, and this number is predicted to double by 2030. The
associated risk translates into a potential explosion in the appearance of
dementia with a burden to the patients, families and society.

Overcoming critical barriers to progress:

While peripheral and autonomic nervous system complications of DM2 can be
detected relatively early, characterized and longitudinally followed with
neurophysiologic techniques, we presently lack similarly reliable markers of
the pathological changes in cortical brain function associated with DM2. The
main goal of the present proposal is to evaluate the utility of transcranial
magnetic stimulation (TMS)-based measures of cortical reactivity and plasticity
to reveal early signs of CNS dysfunction in individuals with DM2. TMS is a
non-invasive brain stimulation technique that can be applied in humans to
transiently change neuronal activity. It is possible to apply TMS as
single-pulse TMS (spTMS, only one pulse in a certain time-window),
event-related TMS (erTMS, several pulses in a small time window), or repetitive
TMS (rTMS, several pulses with a certain frequency). RTMS can be divided into
online rTMS (rTMS during task execution, the effect does not outlast the
duration of the trial) and offline rTMS (rTMS before the start of task
execution, the effect outlasts the duration of the stimulation by up to 60
minutes). Receiving TMS is not painful. Depending on the parameters of magnetic
stimulation, TMS can lead to both longer lasting increases in cortical
excitability (facilitation) or decreases (inhibition), affecting underlying
brain functioning even beyond the magnetic stimulation itself. Usually,
classical rTMS protocols are used to induce such an effect. However, also the
now available patterned or thetaburst TMS (TBS) protocols, which are seen as a
subtype of offline rTMS, have shown to produce reliable and consistent after
effects that are mediated by fast synaptic plastic transmission efficiency
changes in the brain. Compared to classic rTMS, TBS has been found to produce
longer lasting effects despite being more time efficient in its application and
to vary significantly less among different individuals. Within the family of
TBS protocols, the so-called continuous TBS (cTBS) intervention has shown to
produce long lasting inhibition of the targeted brain region, while the
intermittent TBS (iTBS) protocol produces long term facilitation of the
stimulated brain region (Huang et al., 2005). One of the greatest advantages of
TMS is that it can be applied in vivo and that it allows for the investigation
of representational neuroplasticity mechanisms in humans (Siebner & Rothwell,
2003, Thickbroom, 2007). Moreover, TMS has the potential to non-invasively
create synaptic plasticity changes in focal regions of the cortex
(Pascual-Leone et al., 1998). The specifically designed theta burst stimulation
(TBS) protocols of TMS can induce both long term potentiation (LTP)- and long
term depression (LTD)-like mechanisms in the targeted stimulation area (Huang
et al., 2005). Such TBS induced changes in pre-synaptic activity result in the
dynamic expression of lasting changes in synaptic efficacy at the neural level
(Karabanov et al., 2015). These alterations of synaptic neuroplasticity
mechanisms are resembled by changes in TMS-induced corticospinal excitability
measures and can be assessed with single pulse TMS-induces motor evoked
potentials (MEPs) (Delvendahl et al., 2012). In combination with
electrodiagnostic techniques, such as electromyography, TBS-induced changes in
corticospinal excitability can be recorded and evaluated. Thus, differences in
TMS-based measures of corticospinal excitability change between healthy
individuals and patients with DM2 could provide a reliable and objective
assessment of DM2-associated CNS dysfunction. Furthermore, these measures of
neuroplasticity mechanisms could eventually serve as useful biomarkers for
cognitive dysfunction in DM2, inform about the development of effective
therapies and assess treatment response in future clinical trials. The proposed
TMS measures are safe and well-established in clinical neurophysiology and they
are testable across ages and levels of cognitive function.
Classical rTMS and TBS protocols have already been successfully applied by our
research group, as for example in our studies on simultaneous TMS and fMRI
measurements (SIMUL_07-03-071) as well as our experiments regarding neuronal
networks underlying attention (tbTMS_Att_08-3-085-7). It becomes apparent from
these two MEC approved studies that expertise on both, the experimental
paradigm as well as the TMS protocol, already resides within the Maastricht TMS
research group.


Increasing our understanding of the neurobiology of cognitive decline and
dementia in DM2:

Subjects with DM2 have a two-fold increased risk of dementia. The etiology of
cognitive decline in DM2 is probably multifactorial, including cerebral
microvascular disease mediated by chronic hyperglycemia, obesity, and altered
lipid metabolism. Other contributors include inflammatory mediators,
rheological factors and dysregulation of the hypothalamic-pituitary adrenal
axis. However, there appears to be a link between DM2 and Alzheimer*s Disease
(AD).
Studies in genetically engineered rodents reveal accelerated and more severe
cognitive deterioration when there is a concomitant insulin resistant brain
state (IRBS). Also, inhibition of neuronal insulin receptors has been proposed
as a model for sporadic AD. Exacerbation of beta-amyloid neurotoxicity by
advanced glycosylation end products may also contribute to the increased risk
of AD in individuals with DM2. Some have argued that AD may be *type 3
diabetes*. Some of the evidence for this argument includes: impaired insulin
secretion increases the risk of AD; insulin resistance is associated with
pathology in AD; there is an association between AD and specific polymorphisms
in genes involved in insulin signaling; there are complex interactions between
amyloid beta peptides and insulin degrading enzymes; DM2 accelerates AD
progression; and treatment with insulin in patients with DM2 and AD appears to
be associated with lower neuritic plaque density in their brains. Translational
application of such findings has led to consideration of cerebral glucose
metabolism and insulin resistance as therapeutic targets in AD, and to the
therapeutic administration of medications that improve insulin sensitivity
(e.g. rosiglitazone) and even insulin, which appears to improve selected
cognitive parameters in AD. Nonetheless, the mechanisms linking insulin, DM2
and AD remain insufficiently understood. We hypothesize that DM2 leads to
alterations in the mechanisms of synaptic plasticity, which increases the risk
of cognitive decline and AD.
A wealth of in vitro and in vivo evidence demonstrates alterations in
neuroplasticity and impaired synaptic function in AD. Alterations in synaptic
plasticity due to oligomeric amyloid toxicity are thought to emerge very early
in AD pathology. Here, we propose to use a TMS protocol to characterize and
measure synaptic plasticity in DM2 as a surrogate marker of the pathological
changes that could lead to dementia. If we can confirm that alterations of
cortical plasticity in DM2 matches those found in patients with early AD,
future investigations could examine whether the proposed measures might predict
the risk of development of dementia in patients with DM2.

Characterizing neurophysiologic CNS impact of DM2:

TMS has emerged as a method to measure focal cortical excitability and
plasticity in the human brain. Studies suggest that alterations in brain
plasticity might be the link between DM2, cognitive decline and dementia; TMS
offers an in vivo method to characterize CNS dysfunction in IFG and DM2, for
which we currently lack a reliable assay. We expect that iTBS effects on
corticospinal excitability, as a measure of neuroplasticity, will be reduced in
patients with IFG and, even more so, in patients with DM2, when compared to
healthy individuals. Such reduced neuroplasticity mechanisms would be
representative of accelerated neurodegenerative progression in these patients.
Therefore, measuring and characterizing altered neuroplasticity mechanisms in
IFG and DM2 would provide crucial information about the applicability of this
technique for the assessment of possible deviations from a regular course of
neurodegeneration in patient groups. Moreover, it would demonstrate that
neuroplasticity is already affected in pre- and early stages of DM2. Our vision
is that by showing differences in the characterization of neuroplasticity
mechanisms between DM2 and healthy individuals, TMS could eventually be applied
as a diagnostic tool for the detection of early signs of neurodegeneration.
TMS-based biomarkers could be developed, which could be assessed serially to
identify the degree and progression of cognitive decline in patients with IFG
and DM2, who might be at increased risk for expedited neurodegeneration. Such
biomarkers could also be used to identify the influence of possible
environmental risk factors that might interact with a genetic predisposition to
produce dementia in DM2. Furthermore, the advantages of such a non-invasive way
to accompany the clinical path of neurodegeneration would allow for a close
monitoring of affected patients, as well as for spontaneous adjustments in
their treatment and therapy.

Aims of the Study:

Our main aim is to characterize cortical brain plasticity in individuals with
impaired fasting glucose (IFG) and patients with type II diabetes mellitus
(DM2) as compared with age-, gender-, and IQ-matched healthy controls(HC).

• Objective 1
To compare TMS-based measures of cortical excitability and plasticity in
patients with IFG, DM2, and in healthy controls.
We predict that compared to controls, IFG and DM2 patients will show normal
cortical excitability, but reduced iTBS measures of plasticity. We expect the
plasticity measures of IFG patients to fall in between those of HC and DM2.
In order to control for external effects (other than iTBS) on the outcome
measures, all participants will undergo a second sham iTBS visit within three
months. We predict that sham iTBS will not induce plasticity and that it has no
effect on cortical excitability.

• Objective 2
To evaluate the association between fasting plasma glucose levels, insulin
resistance, and plasticity measures.
We predict that fasting plasma glucose levels and scores on the homeostatic
model assessment (HOMA) are correlated with plasticity impairments
• Exploratory Objectives
To analyze the impact of Apoliprotein E (APOE) and Brain Derived Neurotrophic
Factor (BDNF) polymorphisms on the measures of excitability and plasticity.
We predict that individuals with BDNF Val66Met polymorphism will show
significantly shorter-lasting TMS modulation, reflecting overall reduced
cortical plasticity.

To determine whether levels of tau influence the TMS measures of excitability
and plasticity.
We predict that individuals with elevated tau levels will show shorter-lasting
TMS modulation in M1, reflecting overall reduced cortical plasticity.

Overview:
We will study 15 individuals with DM2, 15 individuals with IFG and 15
age- and gender-matched (in aggregate) healthy controls. Each participant*s
participation in the study will consist of three visits that will last between
3 and 4 hours: At the beginning of the first visit informed consent will be
obtained and a thorough screening procedure will be applied.

TMS procedure:
The amplitude of single-pulse TMS (spTMS) induced motor evoked potentials
(MEPs) of the first dorsal interosseous (FDI) muscle contralateral to the
stimulation side over the left primary motor cortex (M1) will serve as a
measure for the determination of individual motor threshold (MT) and as a
measure of corticospinal excitability (Delvendahl et al., 2012). Individual MT
will be assessed at the very beginning of each session. Together with the
preparatory cortical mapping to find the motor hot spot this will require
approximately forty to fifty pulses. A batch of thirty spTMS pulses (Cuypers et
al., 2014) will be applied before (baseline) and at seven separate time points
within the first sixty minutes after iTBS modulation. There will be a minimum
of seven seconds between spTMS pulses to avoid carryover effects of previous
pulses. Each batch will last for approximately 3.5 to four minutes. There will
be a total of 240 pulses (8 batches x 30 pulses) for the MEP assessment as a
measure of corticospinal excitability before and after iTBS modulation. The
time between the onsets of the stimulation batches after iTBS will be five
minutes between the first two batches and ten minutes between the remaining
batches. Together with the ca. fifty pulses of cortical mapping and MT
determination participants will receive approximately 290 spTMS pulses during
each visit. The iTBS protocol will be applied once after the baseline spTMS
batch. The stimulation during this protocol lasts for three minutes and nine
seconds, whereas the actual stimulation time is reduced to blocks of two
seconds of TBS (repeated every ten seconds) followed by a no-stimulation period
of eight seconds. This protocol includes a total of 600 TMS pulses. Taken
together, participants will receive a total of approximately 890 TMS pulses
during one visit.


Study visits
Visit #1 - Informed consent and screening assessments
On visit 1, participants will come to the Cognitive Neuroscience
Department at the University of Maastricht for the following informed consent
and screening procedures:

• Informed consent
• Inclusion and exclusion criteria review
• Participant demographics and family history
• Medical history and medication review
• Questionnaires to help determine if it will be safe to receive TMS
• Weight and height assessment (to determine body mass index (BMI))
• Blood samples (for homeostasis model assessment (HOMA) & renal insufficiency
(estimated glomerular filtration rate, eGFR))
• Blood pressure assessment
• Two saliva samples will be taken:
o One sample for tau protein
o One sample for BDNF and APOE genotyping

Visit #2 - First TMS-EMG-EEG plasticity visit
On visit 2, participants will come to the Cognitive Neuroscience
Department at the University of Maastricht for the TMS-EMG-EEG measures and
will undergo the following procedures:

• Blood pressure assessment
• Two fasting glucose assessments
• TMS-EMG-EEG:
o Cortical (primary motor cortex (M1)) excitability and cortical plasticity
will be evaluated by TMS-EMG-EEG. Single pulse TMS and iTBS will be used in
conjunction with EMG and EEG (for more detailed information see section 7.3 on
study procedure).
o TMS side effects questionnaire will be done pre and post TMS at each visit.
o Participants will be given earplugs to be worn throughout TMS session.

In total, this visit will last approximately 3 to 4 hours.

Visit #3 -TMS-EMG-EEG plasticity visit - sham stimulation
The following measures from Visit #2 will be repeated, no sooner than
24 hours and no later than 3 months:

• Blood pressure assessment
• Two fasting glucose assessments
• TMS-EMG-EEG:
o iTBS modulation will be replaced with sham iTBS stimulation
o Cortical (M1) excitability will be evaluated by TMS-EMG-EEG. Single pulse TMS
and sham iTBS will be used in conjunction with EMG and EEG.
o TMS side effects questionnaire will be done pre and post TMS at each visit.
o Participants will be given earplugs to be worn throughout TMS session.

This visit will last approximately 3 to 4 hours.

The study consists of three separate sessions of approximately three to four
hours duration. Participants will undergo blood withdrawal and TMS in
combination with EMG and EEG.
TMS has been used in a growing number of laboratories worldwide since 1984. A
series of adverse events have been identified since then, and have been
thoroughly reviewed for the development of recommended safety guidelines and
precautions for the use of TMS, first at a consensus conference at the NIH in
June, 1996 and, more recently, in 2008 in Siena (Italy), in a meeting of an
international panel of TMS experts (The Safety of TMS Consensus Group). We will
carefully follow these updated safety guidelines in the present study. The most
severe and critical immediate side effect of TMS is its potential to induce
seizures. The seizures only last for the time of stimulation and no long-term
effects are known. This complication is calculated to occur in less than 0.1%
of patients and participants that have undergone TMS worldwide. Receiving TMS
is not painful but may eventually be uncomfortable depending on the used
stimulation intensity.