Table 2 Proteins and proteases of interest in development of β-cell specific biomarkers

Secretory Proteins

Insulin

· Insulin is highly specific to the β-cells and is produced in high amounts.

· Insulin degradation is a regulated process important for controlling insulin action by removing and inactivating the hormone.

· Abnormalities in degradation of insulin are present in various pathological conditions including T2DM, and may be associated with development of clinical symptoms[91].

Amylin (IAPP)

· Misfolding and deposit of IAPP is a major pathologic trait in a majority of T2DM patients[4].

· IAPP oligomers have been demonstrated to be toxic to β-cells by inducing apoptosis[6, 43–47].

· Depositions of IAPP become a pathological extracellular matrix surrounding the β-cells, and degradation of this matrix could potentially serve as marker of developing T2DM.

Β-cell Trans-membrane Proteins

Neuroligin-2, Neurexin 1α

· Β-cell exocytic machinery is very similar to that of neuronal synapses, and for this reason the β-cells and neurons have some common traits[92].

· It has been established that β-cells express specific proteins which are also found in the central nervous system (CNS), such as neuroligin-2 and neurexin-1 α[92].

· As these proteins are rather specific to β-cells and neurons within the CNS, they might be suitable biomarker candidates for evaluation of β-cell degradation.

GLP-1 receptor, GIP receptor

· The two incretin receptors GLP-1 receptor (GLP-1R) and GIP receptor (GIPR) are known to be expressed in pancreatic β-cells, but not exclusively by this cell type.

· Activation of both GLP-1R and GIPR is known to stimulate insulin synthesis and insulin release[93, 94], and both receptors have therefore been suggested as potential targets for the treatment of diabetes.

· GLP-1R and GIPR have been demonstrated to form heterodimers, which could be of importance for fine-tuning incretin response[95].

· Hyperglycemia has been found to lower the expression of both GLP-1R and GIPR, contributing to the diminished incretin action in hyperglycaemic states and diabetes[96, 97].

GLUT1, GLUT2

· Glucose transporters, GLUT1 and GLUT2, are important for the functionality of β-cells.

· GLUT1 and GLUT2 are expressed in several tissues. However, neo-epitopes, which are specific to the pathological events involved with loss of β-cells, could be potential β-cell markers.

T1DM Autoimmune Targets

GAD 65, IA-2, ZnT8

· GAD65, IA-2 and ZnT8 are all established autoantigens in T1DM[98–100].

· Autoantibodies directed against these autoantigens have also been identified in some T2DM patients[101].

· It has been found that GAD65 is released during β-cell injury, and circulating GAD65 would therefore be a suitable marker for β-cell ill-health[2, 102].

· It has been established that measurements of GAD65 are able to detect β-cell death at a time point preceding the onset of hyperglycemia[2, 102].

PROTEASES OF INTEREST

Caspase 3

· Caspase 3 is a key enzyme in the enzymatic cascade initiating cell apoptosis.

· Several pathological processes lead to β-cell apoptosis[4], rendering caspase 3 an interesting effector protease.

MMP-12

· MMP-12 is expressed primarily by macrophages and monocytes.

· Β-cell loss can occur as consequence of local inflammation, and therefore, MMP-12 could be a protease of interest.

MMP-9

· MMP-9 is expressed primarily by macrophages and T-cells

· Β-cell loss can occur as consequence of local inflammation, and therefore, MMP-9 could be a protease of interest.

Cathepsin B

· Cathepsin B is known to be present in pancreatic juice.

· Cathepsin B has been speculated to be involved in the pathology of pancreatitis[103, 104], and it could be hypothesized that similar mechanisms might, to some extent, be involved in development of T2DM.