Table 2.

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

Target protein or Enzyme		Rationale
PROTEINS OF INTEREST
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.