Identification of altered dipeptidyl-peptidase activities as potential biomarkers for unipolar depression

Abstract

Background

Changes in circulatory aminopeptidases [dipeptidyl-peptidase-IV (DPP-IV), Prolyl-oligopeptidase (POP) and Leucine aminopeptidase (LAP)] activities have been found to be associated with psychiatric illnesses and inflammatory diseases.

Methods

The discriminatory indices of aminopeptidases activities were assessed by enzymatic assays in plasma samples from 240 unipolar depression (UD) patients and 264 matched controls. In addition the relationship between soluble and cellular DPP-IV activity was determined in plasma and blood cells from healthy subjects.

Results

Greater than 95% of the plasma DPP-IV activity could be blocked by inhibitors, demonstrating the specificity of the assay. Also, DPP-IV protein and activity levels were strongly correlated. In contrast, only 50% of the membrane-bound activity in blood cells was inhibited, which suggested that other similar peptidases may be present in these cells. UD patients had decreased plasma levels of DPP-IV and POP activities compared to healthy controls with a concomitant increase in LAP activity. Finally, testing of the LAP/DPP-IV ratio resulted in good discrimination of UD patients from controls with an area under the curve—receiver operating characteristic of 0.70.

Limitations

Further biological validation studies using different cohorts are warranted.

Conclusions

The finding that plasma DPP-IV activity was decreased and LAP activity was increased in UD patients suggests the potential value for testing the levels of these enzymes for improved classification of patients. In addition, the changes in these enzymes, suggests that the proteolytic maturation of their proneuropeptide and prohormone subtrates may also be affected in UD, resulting in altered production of the associated bioactive peptides.

Introduction

The aminopeptidase family of proteases is involved in the proteolytic processing of precursor proteins to produce biologically active neuropeptides and hormones. These include dipeptidyl-peptidase (DPP)-IV (Boonacker and Van Noorden, 2003), prolyl-oligopeptidase (POP) (Garcia-Horsman et al., 2007) and leucine aminopeptidase (LAP) (Matsui et al., 2006). DPP-IV selectively removes amino-terminal dipeptides from precursor proteins containing proline or alanine in the second position (Lambeir et al., 2003) and is widely distributed (Mentlein, 1999). It is present as both membrane-bound and circulating forms with indistinguishable protease activity (Drucker, 2003). LAP catalyses removal of leucine residues from the amino-terminus of proteins although its biological role is still not well understood. However, many studies have indicated that it plays a role in proteolytic maturation of proproteins involved in various central and peripheral processes (Lambeir et al., 2003). POP removes amino-terminal peptides from small proteins by cleavage on the carboxy-terminal side of proline resides and is highly expressed in the brain (Garcia-Horsman et al., 2007). It is also involved in the processing of bioactive peptides which are involved in regulation of mood and behaviour.

The aminopeptidases have been linked to the pathophysiology of various diseases including neuropsychiatric disorders and metabolic conditions such as type II diabetes mellitus (Drucker, 2003). Although UD is a severe mental disease affecting primarily the brain, it is becoming more apparent that the whole body is involved (Brandt et al., 2007, Hildebrandt et al., 2000). Studies over the last two decades have shown that many patients with UD have inflammatory, hormonal and metabolic abnormalities, similar to those seen in cardiovascular diseases and diabetes (Hildebrandt et al., 2000). DPP-IV has been proposed as a diagnostic or prognostic marker for various cancers and neuropsychiatric disorders. For example, previous studies have found that changes in aminopeptidase activity may be involved in neuropsychiatric conditions such as unipolar depression (UD) (Maes et al., 1997). Also, DPP-IV is used currently as a therapeutic target for diabetes as a means of increasing insulin release from pancreatic β-cells (Ahren and Schmitz, 2004). DPP-IV is also known to be involved in immune system regulation through effects on T cell activation and chemotaxis (Ohnuma et al., 2008), and it appears to be negatively associated with inflammation (Busso et al., 2005, Tanaka et al., 1994). Such studies also indicate a link between inflammation and depressive symptoms. Lower circulatory peptidase activity has also been proposed as a predictor for depressive symptoms following interferon-alpha (IFN-α) immunotherapy (Maes and Bonaccorso, 2004). Also, treatment of high-risk melanoma patients with IFN-α resulted in a decrease of POP serum activity and this was associated with increased symptoms of depression (Van Gool et al., 2004).

Neuropsychiatric conditions such as UD and anorexia nervosa have been associated with decreased serum DPP-IV levels (Hildebrandt et al., 1999). Conversely, serum POP activity has been found to be increased in bipolar disorder, mania and schizophrenia (Maes et al., 1994, Maes et al., 1995, Maes et al., 1996) and previous reports have also suggested its potential use as a prognostic (Tarrago et al., 2005) and/or diagnostic biomarker (Maes et al., 1996). This is interesting since POP catalyses the proteolytic production of hormones and neuropeptides (Polgar, 2002) that are known to be involved in regulation of mood disorders.

In this study, we have analysed plasma from a large cohort of UD patients and matched controls in attempt to characterise changes in the activities of DPP-IV, LAP and POP activity. It was of particular importance to determine whether any differences in these enzymatic activities could be used as a means of classifying UD patients compared to controls.