The beneficial effects of C-Peptide on diabetic polyneuropathy

Abstract

Diabetic polyneuropathy (DPN) is a common complication in diabetes. At present, there is no adequate treatment, and DPN is often debilitating for patients. It is a heterogeneous disorder and differs in type 1 and type 2 diabetes. An important underlying factor in type 1 DPN is insulin deficiency. Proinsulin C-peptide is a critical element in the cascade of events. In this review, we describe the physiological role of C-peptide and how it provides an insulin-like signaling function. Such effects translate into beneficial outcomes in early metabolic perturbations of neural Na+/K+-ATPase and nitric oxide (NO) with subsequent preventive effects on early nerve dysfunction. Further corrective consequences resulting from this signaling cascade have beneficial effects on gene regulation of early gene responses, neurotrophic factors, their receptors, and the insulin receptor itself. This may lead to preventive and corrective results to nerve fiber degeneration and loss, as well as, promotion of nerve fiber regeneration with respect to sensory somatic fibers and small nociceptive nerve fibers. A characteristic abnormality of type 1 DPN is nodal and paranodal degeneration with severe consequences for myelinated fiber function. This review deals in detail with the underlying insulin-deficiency-related molecular changes and their correction by C-peptide. Based on these observations, it is evident that continuous maintenance of insulin-like actions by C-peptide is needed in peripheral nerve to minimize the sequences of metabolic and molecular abnormalities, thereby ameliorating neuropathic complications. There is now ample evidence demonstrating that C-peptide replacement in type 1 diabetes promotes insulin action and signaling activities in a more enhanced, prolonged, and continuous fashion than does insulin alone. It is therefore necessary to replace C-peptide to physiological levels in diabetic patients. This will have substantial beneficial effects on type 1 DPN.

Figure 1

A: Na⁺/K⁺-ATPase activities in sciatic nerve of acutely diabetic type 1 BB/Wor-rats, C-peptide replaced BB/Wor-rats and age-matched non-diabetic control rats. For comparison, Na⁺/K⁺-ATPase activity in age- and duration-matched type 2 diabetic BBZDR/Wor-rats is included. Note markedly reduced Na⁺/K⁺-ATPase activities in BB/Wor-rats, which is fully restored in C-petide treated rats. Type 2 BBZDER/Wor rats show a modest decrease in Na⁺/K⁺-ATPase activity. ^* p < 0.001, ^# p < 0.01 vs. control rats. B: Sciatic motor nerve conduction velocity (NCV) deficits developing in acutely diabetic BB/Wor-rats, C-peptide-treated BB/Wor-rats (from 1 week duration of diabetes) and type 2 BBZDR/Wor-rats compared to age-matched non-diabetic control rats. Note the sharp and rapid decline in NCVs in type 1 BB/Wor-rats (B) corresponds to severely diminished activity of Na⁺/K⁺-ATPase (A). C-peptide-treated rats show an immediate partial but significant correction of NCV corresponding to a normalization of Na⁺/K⁺-ATPase activity. Both NCV and Na⁺/K⁺-ATPase activity defects are substantially milder in type 2 BBZDR/Wor-rats compared with type 1 BB/Wor-rats. These differences have led us to suggest a hyperglycemic component, and an insulin/C-peptide deficiency component, of the nerve conduction slowing.

Figure 2

ELISA assessment of NGF (A) and NT-3 (B) in sciatic nerves of control, diabetic BB/Wor-rats and those with full substitution of C-peptide. The amounts of NGF and NT-3 are substantially decreased in diabetic rats and corrected by C-peptide to concentrations not significantly different from control rats. ^* p < 0.005 vs. control rats, ^† p < 0.05 vs. untreated BB/Wor-rats B: Protein expression of neurotrophic receptors in four mo diabetic DRG neurons. Note marked suppression of both the insulin (C), IGF-1 (D), NGF-TrkA (E) and the TrC (F) receptors in diabetic rats, and significant restorations of the expression of these receptors in diabetic rats treated with full C-peptide replacement from onset of diabetes. ^*** p < 0.001; ^** p < 0.005 vs. control rats; ^†† p < 0.005; ^† p < 0.05 vs. untreated BB/Wor-rats. Data are compiled from references [58] and [74].

Figure 3

Impaired neurotrophic support leads to suppressed expression of tubulins and NFs and their aberrant phosphorylation [42, 73, 75]. These abnormalities lead eventually to axonal degeneration of myelinated fibers. Here depicted ultrastructurally, showing axonal atrophy and sequestration by the inner Schwann cell lip (A), which can be morphometrically assessed (B), and decreased numbers of myelinated fibers (C). ml in (A) indicate myelin lamellae. Full C-peptide substitution for eight months results in significant prevention of axonal degeneration (B), prevention of myelinated fiber loss (C) and enhances nerve fiber regeneration (D). For comparison, type 2 BBZDR/Wor-rats with the same duration of diabetes, and with the same levels of hyperglycemia, show significantly milder degrees of axonal degeneration, fiber loss (C), but the same degree of regenerative capacity (D) as non-treated BB/Wor-rats. Data are rearranged from references [28] and [85].

Figure 4

Diabetic BB/Wor-rats show progressive increases of hyperalgesia, as assessed from withdrawal latencies following thermal stimulation (A). Hyperalgesia is partly, but significantly, prevented in iso-hyperglycemic C-peptide replaced diabetic rats (A). These beneficial effects were underpinned by prevention of the decline in substance P (B) and CGRP (C) in small nociceptive DRG neurons, and by prevention of unmyelinated c-fiber atrophy (D), and partial but significant prevention of c-fiber loss in sural nerve (E). B and C: ^** p < 0.001; ^* p < 0.005 vs. control rats; † p < 0.05 vs. untreated BB/Wor-rats. D and E: ^* p < 0.05, ^** p < 0.005 and ^*** p < 0.001 vs. control rats; ⁺ p < 0.05 and ⁺⁺ p < 0.001 vs. untreated BB/Wor-rats. Data compiled from references [58] and [74].

Figure 5

Schematic depiction of paranodal degeneration of the paranodal apparatus in diabetic nerve (top panel, right side) as compared to a normal paranode (top panel, left side). The molecular architecture of the paranode is shown in the lower panel, left side. Caspr interacts with RPTP-β via binding of p85 to SH3 domains. RPTP-β is a ligand of contactin. At the nodal gap (lower panel, right side) ankyrinG interacts with RPTP-β and β-Na⁺-channel subunits, which in turn anchor the α-Na⁺-channels to the nodal axolemma.

Figure 6

Disruption of the paranodal ion-channel barrier (axo-glial dysjunction) is shown ultrastructurally in A. Note the loss of electron dense tight junctions (small arrowheads), and structurally intact tight junctions (large arrowheads). Morphometric assessment of axoglial dysjunction (B) shows increased frequency in diabetic BB/Wor-rats, an increase that is fully prevented in C-peptide-replaced BB/Wor-rats. Type 2 BBZDR-rats show a normal frequency of axoglial dysjunction. Reproduced with permission from reference [57].

Figure 7

Immunocytochemical localization of caspr (A). Caspr is strictly localized to the paranodal areas in control, C-peptide-replaced BB/Wor-rats, and for comparison in type 2 diabetic BBZ-rats. In untreated type 1 BB/Wor-rats caspr is dispersed along the axolemma beyond the confines of the paranodal apparatus. Protein expression of caspr is decreased in diabetic BB/Wor-rats, and completely prevented in C-peptide replaced diabetic rats (B). In type 2 diabetic rats caspr is unaltered (B). The expression of contactin (C) and RPTP-β (D) was significantly decreased in BB/Wor-rats and significantly prevented by C-peptide substitution. Type 2 BBZ-rats showed no change. ^* p < 0.01 vs. controls; ^† p < 0.05 vs. untreated BB/Wor-rats. Data modified and reproduced with permission from reference [57].