MALDI imaging mass spectrometry (IMS) is a powerful approach that facilitates the spatial analysis of molecular species in
biological tissue samples(2) (Fig.1). A 12 μm thin tissue section is covered with a MALDI matrix, which facilitates desorption and ionization of intact
peptides and
proteins that can be detected with a mass analyzer, typically using a MALDI TOF/TOF mass spectrometer. Generally hundreds of peaks can be assessed in a single rat brain tissue section. In contrast to commonly used imaging techniques, this approach does not require prior knowledge of the molecules of interest and allows for unsupervised and comprehensive analysis of multiple molecular species while maintaining high molecular specificity and sensitivity(2). Here we describe a MALDI IMS based approach for elucidating region-specific distribution profiles of
neuropeptides in the rat brain of an animal model
Parkinson's disease (PD). PD is a common
neurodegenerative disease with a prevalence of 1% for people over 65 of age(3,4). The most common symptomatic treatment is based on
dopamine replacement using L-DOPA(5). However this is accompanied by severe side effects including involuntary
abnormal movements, termed
L-DOPA-induced
dyskinesias (LID)(1,3,6). One of the most prominent molecular change in LID is an upregulation of the
opioid precursor
prodynorphin mRNA(7). The
dynorphin peptides modulate neurotransmission in brain areas that are essentially involved in movement control(7,8). However, to date the exact
opioid peptides that originate from processing of the
neuropeptide precursor have not been characterized. Therefore, we utilized MALDI IMS in an animal model of experimental
Parkinson's disease and
L-DOPA induced
dyskinesia. MALDI imaging mass spectrometry proved to be particularly advantageous with respect to
neuropeptide characterization, since commonly used antibody based approaches targets known
peptide sequences and previously observed post-translational modifications. By contrast MALDI IMS can unravel novel
peptide processing products and thus reveal new molecular mechanisms of
neuropeptide modulation of neuronal transmission. While the absolute amount of
neuropeptides cannot be determined by MALDI IMS, the relative abundance of
peptide ions can be delineated from the mass spectra, giving insights about changing levels in health and disease. In the examples presented here, the peak intensities of
dynorphin B,
alpha-neoendorphin and
substance P were found to be significantly increased in the dorsolateral, but not the dorsomedial, striatum of animals with severe
dyskinesia involving facial, trunk and orolingual muscles (Fig. 5). Furthermore, MALDI IMS revealed a correlation between
dyskinesia severity and levels of des-
tyrosine alpha-neoendorphin, representing a previously unknown mechanism of functional inactivation of
dynorphins in the striatum as the removal of N-terminal
tyrosine reduces the
dynorphin's
opioid-receptor binding capacity(9). This is the first study on
neuropeptide characterization in LID using MALDI IMS and the results highlight the potential of the technique for application in all fields of biomedical research.