Major depressive disorder (MDD) is a common mood disorder and a major cause of disability
worldwide. It is a debilitating illness which, if left untreated, carries high levels of morbidity
and mortality. Due to the heterogenic nature of this disease, the underlying pathophysiology is
still largely unresolved. It is well established that life stressors pose major environmental risk
factors for developing MDD. A complex interaction between environmental factors and the
genetic makeup is likely to underlie differences in stress-sensitivity, and may explain why
some people are more prone to develop MDD than others after severe adversity. MDD has a
neurobiological basis with complex molecular alterations, associated with functional and
structural brain abnormalities, with the prefrontal cortex (PFC) as one of the brain areas showing
profound alterations.
Extensive preclinical research has focused at unravelling the underlying pathophysiology of
MDD through genetic and more recently proteomic approaches. Mass spectrometry-based proteomics
provides a robust and sensitive identification of the global protein expression within a
given tissue or cell. This approach is highly advantageous for investigating a complex disease
as MDD, since proteins are likely the most ubiquitous molecules affected in disease. Many molecular
alterations associated with the pathophysiology of MDD reside within the synapse.
Thus, to reduce the complexity of the proteome analysed and to enrich for less abundant synaptic
proteins, nerve terminals (synaptosomes) can be purified by differential centrifugation
on a Percoll gradient and analysed by iTRAQ coupled to tandem mass spectrometry.
The aim of the present study was to investigate quantitative changes in protein abundance
in PFC synapses using a highly validated animal model, the Chronic Mild Stress (CMS) model of
depression. This model generates two stress-response phenotypes, reflecting depressive-like
behaviour (anhedonia) and stress-resilience. The large-scale, non-hypothesis driven proteomic
analysis of the two phenotypes was applied in order to investigate MDD-associated markers of
stress-susceptibility and stress-resilience. Furthermore, the behavioural task, the odour span
task, was set up in a pilot study with the aim of investigating the effect of CMS on working
memory.
Proteins involved in the biological pathway, synaptic transmission, were primarily regulated
between stress-resilient compared to anhedonic-like rats and control rats, whereas proteins
involved in synaptic transduction primarily were regulated between anhedonic-like rats
compared to control rats. Proteins involved in metabolism were significantly regulated in resilient
rats compared to anhedonic-like rats. Finally, proteins involved in cytoskeletal organisation
and oxidative stress were pathways shown to be affected in both phenotypes; however,
the proteins involved in these biological pathways, in each phenotype, were of different types.
In conclusion, stress-resilient and anhedonic-like rats showed a clear segregation in synaptic
proteome profiles reflecting the two hedonic responses to CMS. Stress-susceptibility and
stress-resiliency were associated with several proteomic aberrations, particularly those related
to metabolism, cytoskeletal organisation, synaptic transmission and signal transduction.
These proteins should be further investigated to confirm their relevance to depression.