# The Effects of C-peptide on Type 1 Diabetic Polyneuropathies and Encephalopathy

Authors: Anders A. F. Sima; Weixian Zhang; Zhen-guo Li; Hideki Kamiya
Journal: Experimental Diabetes Research
 (2008)

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## Abstract

Diabetic polyneuropathy (DPN) occurs more frequently in type 1 diabetes resulting in a more severe DPN.  The differences in DPN between the two types of diabetes are due to differences in the availability of insulin and C-peptide.  Insulin and C-peptide provide gene regulatory effects on neurotrophic factors with effects on axonal cytoskeletal proteins and nerve fiber integrity.  A significant abnormality in type 1 DPN is nodal degeneration.  In the type 1 BB/Wor-rat, C-peptide replacement corrects metabolic abnormalities ameliorating the acute nerve conduction defect.  It corrects abnormalities of neurotrophic factors and the expression of neuroskeletal proteins with improvements of axonal size and function. C-peptide corrects the expression of nodal adhesive molecules with prevention and repair of the functionally significant nodal degeneration.
Cognitive dysfunction is a recognized complication of type 1 diabetes, and is associated with impaired neurotrophic support and apoptotic neuronal loss.  C-peptide prevents hippocampal apoptosis and cognitive deficits.  It is therefore clear that substitution of C-peptide in type 1 diabetes has a multitude of effects on DPN and cognitive dysfunction.
Here the effects of C-peptide replenishment will be extensively described as they pertain to DPN and diabetic encephalopathy, underpinning its beneficial effects on neurological complications in type 1 diabetes.


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## Body

## 1. Introduction

Diabetes is an increasingly common metabolic disorder that affects the nervous system in
a variety of ways. It impacts on the
peripheral nervous system (PNS) in a progressive fashion resulting in diabetic
polyneuropathies (DPNs), which as a group is the most common chronic diabetic
complication [1]. It also affects the central nervous system (CNS) resulting in
progressive cognitive impairment and is associated with an increased risk for
the development of Alzheimer’s disease [2, 3].
The mechanisms underlying these complications are several and are not
necessarily the same in type 1 and type 2 diabetes [2, 4–6]. Historically, hyperglycemia, which is a
common clinical attribute of both types of diabetes, has been regarded as the
major underlying factor initiating the complications. However, this does not explain differences in
the neurological complications in the two types of diabetes, nor does it
explain the only partial benefits in curbing the progression or preventing the
complications in trials aimed at optimal hyperglycemic control, such as the
DCCT and UKPDS trials [7, 8]. Downstream
effects of hyperglycemia on the polyol pathway and oxidative stress have been
the targets for numerous clinical trials with marginal effects at best [9–11]. These data strongly suggest that
factors other than hyperglycemia are involved in the initiation and progression
of DPN. Such factors may differ in the
two types of diabetes as suggested by epidemiological studies. The prevalence of DPN in type 2 diabetes is
about 50% after 25 years of diabetes, whereas in type 1 diabetes it is close to
100% after 15-years disease duration [12–14], suggesting a more rapid
progression of DPN in type 1 diabetic subjects.DPN involves both somatic and autonomic peripheral nerves and is characterized as a
progressive dying back axonopathy. The
structural pathology, however, differs in the two types of diabetes in that the
axonopathy is more severe in type 1 DPN and is in type 2 DPN associated with a
greater frequency of primary segmental demyelination. Type 1 DPN is also characterized by
progressive nodal and paranodal degeneration with significant impact on nerve
function, abnormalities which do not occur in type 2 diabetes [4, 5, 15].One factor that differs between type 1 and type 2 diabetes and is likely the
explanation for some of the differences in DPN is the degree of perturbed
insulin signaling due to insulin deficiency in type 1 diabetes and insulin
resistance associated with hyperinsulinemia in type 2 diabetes. Insulin signaling exerts besides its
hypoglycemic effect a multitude of metabolic and molecular effects, which are
not commonly recognized. Pertaining to
DPN, insulin signaling has prominent effects on Na+/K+-ATPase
and NO activities important for the metabolically induced acute nerve
dysfunction. It transduces strong
neurotrophic effects on its own and possesses generegulatory functions on
other neurotrophic factors such as IGF-1, NGF, and NT-3 as well as their
receptors. Furthermore, it is an
important regulator of postranslational modifications of neuroskeletal and cell
adhesive proteins, and besides that it possesses a strong antiapoptotic
effect. Considering these effects, it is
not totally surprising that strict hyperglycemic control alone will not provide
total protection against DPN [7, 8], or CNS for that matter, in diabetic
subjects and that mechanistic, functional, and structural differences exist
between the neurological complications occurring in the two types of diabetes [16].Insulin is secreted from pancreatic beta cells in response to glucose. Simultaneously, proinsulin C-peptide is
secreted in equimolar quantities.
Insulin’s half-life in the circulation is short whereas that of
C-peptide is substantially longer [17, 18].
C-peptide was initially believed to be a waste product of insulin
synthesis until the molecular bases for its intriguing insulin-like effects
were delineated [19, 20].In this review, we will briefly describe recent data pertaining to the interaction
between insulin and C-peptide, outline the pathogenetic mechanisms underlying
type 1 DPN, and contrast these to those of type 2 DPN. We will describe the effects of C-peptide on
somatic and small fiber neuropathy and briefly summarize the effects on primary
diabetic encephalopathy.


## 2. Insulin and C-peptide Interactions

After its discovery in the 1960s by Steiner [21–23], it was believed that
C-peptide, which plays an intricate part in the biosynthesis and folding of
insulin, would have an insulin-like glucose lowering effect. Since this turned out not to be the case,
C-peptide was abandoned and dismissed as a nonfunctional peptide. However, in the 1990s, the Karolinska group
and others demonstrated effects on blood flow, incipient diabetic nephropathy,
and neuropathy in type 1 diabetic subjects [24–27]. This led to renewed interests in the action
of C-peptide. The Karolinska group
demonstrated specific binding of C-peptide to cell surfaces and suggested that
it acted via a G-protein-related receptor mechanism [28]. Detailed studies by Grunberger et al. [19, 20, 29] demonstrated that C-peptide autophosphorylates the insulin receptor in
the presence of insulin and stimulates p38 MAP-kinase and PI-3 kinase activity
and reduces the activation of JNK phosphorylation, with subsequent dose-related
effects on Na+/K+-ATPase activity and NO [30–32]. These experiments seemed to suggest an
insulinomimetic effect, although despite years of effort by us and the
Karolinska group, we failed to identify a specific C-peptide receptor. Further studies revealed an interesting
stoichiometric relationship between insulin and C-peptide pertaining to insulin
signaling activities. It was shown that
in the presence of high concentrations of insulin, C-peptide has an inhibitory
effect on the combined insulin-signaling activity, whereas in the presence of
low insulin concentrations C-peptide enhances insulin signaling [19, 20, 33]. Recent data have suggested that the enhanced
insulinomimetic effect displayed by C-peptide is due to its ability to
dehexamerize insulin and thereby enhance the intrinsic actions of insulin
itself [34]. As of yet unpublished data
have demonstrated that the effects exerted by C-peptide on insulin action can
be prolonged by its binding of metal-ions such as chromium and iron. It therefore appears that C-peptide interacts
in a complex way with insulin to produce its supporting insulinomimetic
effects.


## 3. Mechanisms Underlying Type 1 and Type 2 Dpn

The progressive evolution of pathogenetic factors responsible for DPN can be
divided into an early and reversible metabolic phase and a partly overlapping
progressively irreversible structural phase [1, 35] (Figure 1).Figure 1
Scheme of pathogenetic events in type 1
(hyperglycemic and insulin and C-peptide deficient) and type 2 (hyperglycemic)
BBZDR/Wor-rats. Insulin and C-peptide
deficiencies add significantly to early metabolic abnormalities such as Na+/K+-ATPase
and NO activities underlying the acute and reversible nerve conduction defect
(dark gray). Subsequent changes with
respect to gene regulatory mechanisms and suppression of major neurotrophic
factors and their receptors lead to severe axonal degeneration, atrophy, and
loss; nodal and paranodal degenerative changes; and impaired nerve fiber
regeneration (dark gray). Such changes
are responsible for the chronic and increasingly irreversible nerve
dysfunction, which are more severely expressed in type 1 diabetes. Mechanisms on which C-peptide has preventive
or corrective effects are indicated with (*).An early metabolic perturbation is activation of the polyol pathway by excessive
glucose, resulting in accumulation of sorbitol and fructose and depletion of
other osmolytes such as taurin and myoinositol [36–38]. Myoinositol depletion results in insufficient
diacylglycerol for Na+/K+-ATPase activation [36]. The more severe Na+/K+-ATPase
defect in type 1 DPN is accounted for the additional defects in protein kinase
C activity caused by insulin and C-peptide deficiencies [39] (Figure 1). Impaired endoneurial blood flow underlies
endoneurial hypoxemia caused by impaired eNOS expression and NO activity,
abnormalities, which are magnified by insulin and C-peptide deficiencies [32, 40, 41] (Figure 1). These aberrations
have also been associated with hyperglycemia-induced mitochondrial dysfunction,
overproduction of superoxide, oxidative, and nitrosative stress [41, 42]. Such early reversible metabolic abnormalities
are associated with nerve conduction slowing, which is significantly more
severe in type 1 BB/Wor-rats than in their type 2 counterparts the BBZDR-rats [39, 43] (Figure 2). These differences appear
to be mainly due to differences in the Na+/K+-ATPase
defect [37, 39, 43, 44]. Since the
excitation of the nodal membrane underlying the propagation of nerve conduction
depends on the inward flux of Na+, decreased Na+/K+-ATPase
activity results in improper inactivation of intra-axonal Na+ with
decreased permeability and intra-axonal Na+ accumulation, potentially
resulting in conduction block [45, 46].Figure 2
Longitudinal measurements of motor nerve
conduction velocities (MNCVs) in the sciatic-tibial conducting system. Note a progressive decline in MNCV in type 1
BB/Wor-rats with duration of diabetes. This decline is significantly milder in type 2 BBZDR/Wor-rats and only
become significant after 4 months of diabetes.C-peptide replacement from onset of diabetes had significant effects on
MNCVs although these are not completely prevented [17, 39, 47].Functional abnormalities of small nerve fibers, particularly unmyelinated fibers and small
myelinated Aδ fibers, occur
early and underlie hyperalgesia and allodynia or neuropathic pain [48–50] (Figure 3). This is associated with increased
formation of Na+-channels and α-adrenergic receptors resulting in
hyperexcitability and ectopic discharges in C-fibers, which appears to be the
initiating event [51–54]. Other
mechanisms which contribute to and sustain pain are related to remodeling of large Aβ fibers which form collaterals with excitotoxic
effects on nociceptive spinal cord neurons which amplify pain [54, 55]. Additional central nervous system mechanisms
involving noradrenalin and serotonin reuptake as well as gabaergic effects are
involved leading to different levels of sensitization of pain. These functional defects occur earlier and to
a greater extent in type 1 DPN as compared to that of type 2 DPN [48] (Figure 3). On the other hand, hyperalgesia appears to
persist for a longer period of time in type 2 diabetic rats (Figure 3), which
may explain the fact that nociceptive neuropathy is more common in type 2 than
in type 1 patients [56]. The early
damage to small peripheral nerve fibers appears to result from decreased
neurotrophic support by insulin and nerve growth factor (NGF) both of which are
particularly neurotrophic to small nociceptive ganglion cells [57, 58].Figure 3
Longitudinal measurements of thermal
hyperalgesia in type 1 BB/Wor-rats without and with C-peptide replacement from
onset of diabetes compared to duration- and hyperglycemia-matched type 2
BBZDR/Wor-rats and age-matched control rats.
Note more severe hyperalgesia in type 1 as compared to type 2 rats and
with partial but significant prevention in C-peptide treated rats [48, 49].Impaired
insulin and/or NGF support may also explain the occurrence of painful diabetic
neuropathy in prediabetic patients with impaired insulin function [59, 60], and
nociceptive neuropathy in prediabetic rats with impaired glucose tolerance but
without overt hyperglycemic diabetes [61].
It therefore appears that although hyperglycemia remains an important
factor in the pathogenesis of DPN, differences in metabolic influences due to
the presence or absence of insulin action modulate the severity of DPN and is
likely to be the main explanation for the differences in DPN between the two
types of diabetes.The structural and progressively irreversible DPN is characterized by axonal
atrophy and loss, which is more severely expressed in type 1 as compared to
type 2 DPN in experimental diabetes [39, 43, 48] (Figures 1 and 4). Additional changes that characterize
experimental and human type 1 DPN is a progressive degenerative process
affecting the paranodal and nodal apparati [4, 5, 16, 62] (Figure 5). On the other hand, primary segmental
degeneration is a more common feature of type 2 human and experimental
diabetes, which may relate to abnormalities in caveolin-1 signaling, which in
turn is modulated by cholesterol levels [4, 5, 16, 39, 63].Magnitudes of myelinated axon degeneration as
assessed by teased fiber analysis (a) and ultrastructural quantification of
axoglial dysjunction (b), a measure of paranodal degeneration. Note a significantly more severe axonal
degeneration in type 1 as compared to type 2 rats. C-peptide treatments from onset of diabetes
and as an intervention between 5 and 8 months had significant preventive and
corrective effects on axonal degeneration.
In (b) type 2 diabetes was not affected by paranodal degeneration in contrast to
type 1 diabetes showing profound degeneration.
C-peptide treatments had significant preventive and therapeutic effects
on paranodal degeneration [17, 39, 47].
(a)(b)Figure 5
Schematic illustration of the nodal and paranodal molecular architecture in the normal situation (top left) and in the
type 1 DPN (top right). The intricate relationships between several paranodal adhesive molecules emanating from the
terminal myelin loops and the paranodal axolemma are depicted. Note the colocalization of the insulin
receptor (IR) (bottom left). At the node
the gated Na-α-channels are “anchored” to the axolemma via interaction with β-Na+-channels, RPTPβ, contactin, and their interaction with ankyrinG,
(bottom right). For further explanation of the molecular perturbations in type 1 diabetes and the effect of C-peptide,
see text [18].Cytoskeletal
neurofilaments (NFs) and tubulins are major constituents of the axon and their
expression levels and phosphorylation status determine axonal function and size
[64, 65]. Reduced expression of NFs and
tubulins occurs in
experimental models of diabetes [66–69] and is associated with
decreased axonal transport of NFs [70, 71] due to aberrant phosphorylation by
phosphorylating protein kinases [71–73].
NFs are unique to neurons and interact with microtubules thereby forming
the basis for axonal transport. NFs
consist of three intermediate filaments, NFL, NFM, and NFH, forming coiled-coil
dimers which align in a staggered fashion.
Several neurotrophic factors like NGF, NT-3, IGF-1, insulin, and
C-peptide stabilize NF transcripts [14, 74, 75]. Aberrant phosphorylation of NFs perturbs
their function and interaction with other cytoskeletal components resulting in
malalignment of the cytoskeleton, impaired axonal function, atrophy, and
eventually loss [14, 76–78]. Several
kinases are involved in NF phosphorylation such as cyclin-dependent kinases
including Cdk5 and the MAP kinases Erk 1/2, SAPK [72], and
GSKβ [79–81].Tubulins assemble
into microtubules and provide for axonal transport and polarity [1]. Microtubule-associated proteins like MAP1B
and tau regulate their assembly [82–84].
Inhibition of GSK-3β abolishes
MAP1B phosphorylation which impacts on microtubule stability [85]. Reduced expression of NFs and tubulins occur
already in 2-mo diabetic type 1 BB/Wor-rats and tend to progress with duration
of diabetes, whereas similar changes occur later and are significantly milder
in type 2 BBZDR/Wor-rats [43, 66]. Simultaneously,
neurofilaments become hyperphosphorylated in type 1 diabetic rats via
upregulation of phosphorylating stress kinases like SAPK and GSK-3β which emanate from impaired insulin, IGF-1,
and C-peptide signaling [86]. The
structural consequences as would be expected, therefore affect type 1 DPN more
severely than type 2 DPN with unmyelinated fibers being particularly vulnerable
[16, 48] (Figures 2, 3, 4, and 6). A
further difference between type 1 and type 2 DPN occurs in sympathetic
autonomic nerves. STZ- and BB/Wor-rats
develop dystrophic axonal changes consisting of accumulations of NFs,
tubolovesicular conglomerates, and degenerated organelles. These changes have been related to insulin
and IGF-1 deficits and do not occur to a significant degree in type 2
BBZDR/Wor-rats [87].The effect of type 1 and type 2 diabetes on unmyelinated axonal size (a) and numbers (b) in the sural nerve in 7-8 month
diabetic rats. Note in type 1 rats, significant atrophy (a) and loss (b) of unmyelinated fiber, whereas no
significant deficits were detectable in type 2 BBZDR/Wor-rats. Replenishment with C-peptide resulted in
significant prevention of C-fiber atrophy (a) and loss (b) [17, 47].
(a)(b)The differences in insulin-deficiency-mediated effects on neurotrophic factors and
downstream deficits in the expression and phosphorylation status of neuroskeletal
proteins also affect
the regenerative capacity of injured nerves.
Hence, the immediate gene responses following nerve injury and
upregulation of the expression of neuroskeletal, mRNAs, and proteins are more
severely perturbed in type 1 BB/Wor-rats as compared to their type 2
counterpart, the BBZDR/Wor-rats [43, 66, 69].In recent years, it has been suggested by several investigators [88, 89] that DPN
is in part caused by mitochondrial dysfunction-related apoptosis of dorsal root
ganglion cells. However, it is difficult
to reconcile this loss of DRG neurons in the absence of peripheral sensory
nerve fiber loss in the streptozotocin diabetic rat. Although apoptotic stresses do occur, more so
in type 1 diabetic DRG cells than in those of type 2 diabetes, these appear to
be counteracted by antiapoptotic mechanism [90, 91]. Instead the degeneration and eventually loss,
particularly of small nociceptive neurons, of DRGs in type 1 BB/Wor-rats appear
to be due to degeneration and vacuolation of the Golgi apparatus [92].Probably the most intriguing difference encountered in DPN in the two types of diabetes
is the progressive degeneration of the paranodal ion-channel barrier in type 1
DPN, which is unaffected in DPN accompanying type 2 diabetes [4, 5, 39, 62] (Figures
1 and 5). This abnormality when first
described [4, 62] caused some controversy, since it could not be identified in
mostly type 2 diabetic nerve [93–95].
The tight junctions which make up the paranodal barrier are composed of
cell adhesive molecules localized to the axolemma such as casper, Na β-channels and contactin and receptor protein
tyrosin phosphatase β (RPTP-β) on the terminal loops of the myelin sheath
(Figure 5). The interaction of these
adhesive molecules depends on their posttranslational modifications, which
become progressively compromised in type 1 DPN, resulting in a breakup of tight
junctional structures and the barrier itself [62, 96–99]. Simultaneous defects in Na β-channels and ankyrinG of the nodal
axolemma dislodge the Na-α-channels which
become lateralized [97–99] (Figure 5).
These abnormalities result in decreased density of nodal Na-α-channels with profound consequences as to the
propagation of conduction impulses [45, 62, 96–98]. Interestingly, the insulin receptor, which is
markedly downregulated in type 1 diabetes, colocalizes with paranodal tight
junctions and decorates the nodal axolemma [100].


## 4. The Effect of C-Peptide Replacement on Type 1 DPN

Initial in vitro studies on the effect of C-peptide, demonstrated
insulin-like effects [19, 20, 29, 101–105]. With regard to DPN, we and several other
groups demonstrated a dose-related beneficial effect on neural Na+/K+-ATPase
activity [17, 31, 47], which constitutes the most important early metabolic
abnormality with consequences pertaining to nerve conduction velocity as
outlined above. Neurovascular
dysfunction associated with oxidative stress has emerged as a contributing
factor in the acute development of DPN [42, 106–109].C-peptide promotes the release of NO
in endothelial cells in a concentration-dependent manner [110]. In addition, it increases the expression of
eNOS protein and mRNA which appears to be mediated via a MAP-kinase-dependent mechanism [102, 110–112]. These observations are
consistent with in vivo findings in humans and animal models [24, 25, 27, 32, 33, 113, 114].The effect of C-peptide replacement
in type 1 BB/Wor-rats, resulted in correction of endoneurial perfusion, the
nerve conduction defect, and attenuated thermal hyperalgesia [32]. It did not demonstrate an effect on oxidative
stress. Inhibition of eNOS, but not of
cyclooxygenase, reversed the positive effects of C-peptide [32]. Interestingly, in hyperglycemia-matched type
2 BBZDR/Wor-rats, neurovascular deficits and increased oxidative stress were
not accompanied by nerve conduction slowing or hyperalgesia [32]. These findings indicate that sensory nerve
conduction deficits and small fiber function are not inevitably consequences of increased oxidative
stress or decreased endoneurial blood flow in this type 2 rodent model [32].Insulin and C-peptide exert on their
own neurotrophic and antiapoptotic effects [115–117]. In addition, C-peptide has corrective effects
on the expression of several neurotrophic factors such as NGF, IGF-1, and NT-3
and their respective receptors [49, 50, 118] (Figure 7). These regulatory effects appear to be
mediated by early gene regulatory effects of c-fos particularly on NGF as well
as by transcriptional factor NFκB with wider implications [66, 115]. The insulin receptor itself is in peripheral
nerve located primarily to the paranodal and nodal regions of myelinated fibers
and to small nociceptive neurons in the DRGs [58, 100].Expression of the receptors of neurotrophic
factors in dorsal root ganglia in 8-month type 1 diabetic (D1) and type 2
diabetic (D2) rats, as well as type 1 rats replaced with C-peptide. (c) compared to age-matched control rats
. Note marked decreases in the expression of insulin receptor, IGF-1 receptor, and NGF-TrkA receptor in type 1
rats. These defects were significantly
milder in type 2 diabetic rats and were significantly prevented by C-peptide
replacement in type 1 diabetic rats [50].
(a)
Insulin receptor(b)
IGF-1 receptor(c)
NGFR- TrKAIn the sciatic nerve, the expression
of the insulin receptor is upregulated in the BB/Wor-rat, whereas its
expression in type 2 BBZDR/Wor-rats is downregulated by more than 50% [66], in
contrast the insulin receptor expression becomes progressively downregulated in
DRGs of the type 1 model and remains unchanged in type 2 rats [48]. Systemic IGF-1 is decreased in both models [3],
whereas NGF and NF-3 are impaired in sciatic nerves of the BB/Wor-rat but not
in the type 2 BBZDR/Wor-rat [48] and their respective receptors are
significantly more severely affected in the type 1 model [48]. These aberrations in the expression of
neurotrophic factors and their receptors in the BB/Wor-rat are fully prevented
by full continuous substitution of C-peptide [49] and are significantly
improved following intervention with C-peptide [50]. Such beneficial effects on the neurotrophic
supporting network transcend into effects on major neuroskeletal proteins such
as NFs and neurotubulins [86, 118], their postranslational modifications, and
ultimately axonal size, a major determinant of axonal function, hence resulting
in prevention and even reversal of nerve dysfunction [6, 17, 31, 49, 50] (Figures
2 and 3). As mentioned earlier,
nociceptive DRG neurons are specifically responsive to insulin and NGF. It is therefore not totally surprising that
nociceptive nerve fibers are particularly vulnerable to the diabetic
insult. In the type 1 model, they are
more severely affected than in the type 2 rat [48] showing a progressive axonal
atrophy coupled with nociceptive neuronal atrophy with ultimate C-fiber loss
and loss of substance P and calcitonin-gene-related neurons [49, 50]. The progressive distal fiber loss and
subsequent neuronal atrophy and loss are not likely to reflect apoptotic cell death. Instead, apoptotic stresses which indeed do
occur are likely to be counteracted by antiapoptotic elements such as heat
shock proteins [119]. In a recent study,
we demonstrated profound changes of the Golgi apparatus particularly in small
sensory DRG neurons in the type 1 BB/Wor-rat and suggested that this may
reflect neurotrophic withdrawal with degeneration of cytoskeletal binding
proteins and microtubules [92].The impact of insulin-signaling on
regulation of neurotrophic support is also reflected by the effect of C-peptide
on normalizing nerve fiber regeneration in the BB/Wor-rat [118].As mentioned above, one of the most characteristic abnormalities occurring in type 1 human
and experimental diabetes is the progressive nodal and paranodal degeneration [4, 16, 62]. Axoglial dysjunction is a
progressive degeneration of the paranodal ion-channel barrier which eventually
results in paranodal degeneration and reparative intercalated internodes [4, 62]. This abnormality is not specific for type 1 DPN,
but occurs in a series of clinical and experimental neuropathies [120]. At the node, the voltage-gated Na+α-channels are
held in place by auxiliary subunits β1 and β2 Na-channels which act as adhesive
molecules. Interaction between
contactin, ankyrinG, and β-subunits are critical for the enrichment and
localization of Na+α-channels to the nodal axolemma. AnkyrinG1 interacts with other
nodal cell adhesion molecules and its postranslational modifications are
important for these interactions. It interacts
with the Na-channel β-subunits which
in turn interact
with RPTP-β.At the paranode, the myelin loops
adhere to the axolemma via tight junctions.
Caspr is part of these and interacts with contactin and RPTP-β.
Caspr's cytoplasmic tail mediates protein-protein interaction through binding with p85 at
SH3 domains [99]. In
myelinated nerve fibers, insulin receptors are particularly concentrated to the
node and the paranode [100]. In type 1
DPN, caspr and contactin become significantly downregulated together with RPTP-β associated with a defect in caspr's p85
binding. p85, the regulatory subunit of
phosphatidyl-inositol 3-kinase, is possibly mediated by insulin signaling (Figure 5). This sequence of events leads to
disruption of the tight junctions [99]. At
the node of Ranvier, the expression of Na+-channel α-subunits is not altered, although the β1-subunit is downregulated together
with contactin and ankyrinG.
In addition, the latter undergoes O-linked N-acetylglucosylation, which
inhibits its phosphorylation and interaction with the Na-channel β-units and contactin. This leads to dislodgement of Na-channel α-subunits, which now migrate laterally through
the breached paranodal ion-channel barrier [97, 99].C-peptide substitution in type 1
BB/Wor-rats prevents the degenerative processes of the paranode and the node of
Ranvier [99] and intervention with C-peptide repairs the paranodal apparatus as
evidenced by an increased number of intercalated internodes [17]. It therefore appears as if these functionally
significant lesions in type 1 DPN relate to abnormalities in insulin-signaling.


## 5. Primary Diabetic Encephalopathy in Type 1 Diabetes and the Effect of C-peptide

Cognitive deficits occur more commonly in diabetic patients than in the nondiabetic
population [121–125].
This is probably in part due to ischemic pathologies due to cerebral
micro- and macrovascular disease, which may be confounded by hypertensive
cerebral angiopathy or to repeated episodes of severe hypoglycemia. Such conditions have been referred to as
secondary diabetic encephalopathy.
However, there is now growing evidence to suggest that cognitive
impairments may be consequent to perturbed metabolism in diabetes or so-called
primary diabetic encephalopathy [126]. Impaired
memory, problem solving ability, and intellectual development have been
documented in patients with type 1 diabetes.
Such signs and symptoms have been accompanied by electrophysiological
and structural abnormalities [127–130]. These appear to be more common in patients
with early onset of diabetes and may in part relate to interference with normal
brain development [124, 131, 132].Cognitive decline in patients with type 2 diabetes may be associated with an increased
risk for the development of Alzheimer’s disease due to CNS insulin resistance
and other confounding factors, such as overweight and hypercholesterolemia [2, 122, 123].Deficits in cognitive function have also been documented in experimental models of
diabetes. In the streptozotocin-induced
diabetic rat, impaired cognitive performances have been associated with
abnormalities in hippocampal long-term potentiation indicative of abnormal
synaptic plasticity, changes that are reversed by insulin treatment [133, 134]. We have demonstrated that impaired spatial
memory in diabetic BB/Wor-rats is preceded by significant reductions in the
expression of IGF-1, IGF-II, IGF-1 receptor and insulin receptor in hippocampus
in 2 months diabetic rats [135]. These
early findings were followed by increasingly impaired deficits in Morris water
maze-testing, laddering of genomic DNA in hippocampus and frontal cortex
associated with elevated Bax/Bcl-XL ratios, increased caspase 3
activity, and neuronal loss in hippocampus [117, 135]. In these studies, full replacement with
proinsulin C-peptide attenuated the functional cognitive deficits, normalized
hippocampal expression of insulin and IGF-1 receptors, Bax expression, and that
of cleaved PARP, active caspase 3, and caspase 12. These effects were associated with
significant reductions in hippocampal neuronal loss [117, 136].On the other hand, in a recent study [3] of the type 2 BBZDR/Wor-rat, we
demonstrated in the frontal cortex perturbed amyloid precursor protein (APP)
metabolism with increased accumulation of β-amyloid, soluble APP, and a 3-fold increase of
Aβ C-terminal fragments. These changes were associated with insulin resistance and decreased expression of insulin and IGF receptors
and increased deposition of phospho-tau.
The consequence of these abnormalities was decreased synapse density,
neuritic degeneration, and neuronal loss [2, 3]. Parallel studies in the type 1 counterpart,
the BB/Wor-rat, showed similar changes although they were significantly milder
as compared to type 2 rats [3]. Interestingly
though, amyloid deposition and increased phospho-tau were not affected by
C-peptide replacement (unpublished data, Li and Sima).It is therefore clear that cognitive deficits occur in rodent models of diabetes,
which have not been genetically manipulated.
The underlying molecular abnormalities appear to differ in type 1 and
type 2 diabetes. In the former, it
appears to be mainly caused by a deficit in insulin signaling and availability
of neurotrophic support, which can be modified by C-peptide replacement. In contrast, the rather profound
Alzheimer-like changes in type 2 diabetes appear to relate to
insulin-resistance and possibly elevated cholesterol levels, abnormalities
which do not appear to be responsive to C-peptide treatment.


## 6. Concluding Thoughts and Appeals

It is becoming increasingly evident that DPN differs in the two types of
diabetes. This is not totally surprising
when considering the underlying pathophysiologic differences between type 1 and
type 2 diabetes. The only commonality of
the two disorders is hyperglycemia.
Although hyperglycemia remains a prominent factor in the pathogenesis of
the chronic complications, probably equally important is the role of insulin or
lack thereof together with its prime assistant C-peptide. Recognizing such differences will open up
areas of untapped therapeutic possibilities.
One of these concerns C-peptide.
As outlined in this review, unlike earlier examined therapeutic
approaches which have met with disappointing results, C-peptide corrects a
number of key pathogenetic mechanisms involved in DPN and has experimentally
and in limited clinical trials proven to be highly efficacious in preventing
and even reversing DPN in type 1 diabetes.
In view of this, it is surprising that major insulin manufacturing
companies as well as main granting agencies have approached this new evolving
area with such skepticism. The
overriding concept is almost embarrassingly simple: since the discovery of
insulin and the lack thereof in type 1 diabetes, we have for more than 80 years
replaced it in type 1 patients and thereby saved millions of lives, who however
still develop the late complications with significant disabilities. Would not it now be about time to also
replace insulin’s companion and thereby prevent millions of type 1 patients
from developing the devastating late complications? This concept takes on an even greater
dimension and urgency, when considering the preliminary data eluded to in this
review, indicating the potential effect of C-peptide substitution in preventing
cognitive impairments and even dementia in type 1 diabetic patients. Therefore, we appeal to the pharmaceutical
industry and federal and private agencies to get involved. A great leap in the treatment of type 1
diabetes may be just around the corner.Table 1
Summary of the corrective effects of
C-peptide on metabolic, molecular, functional, and structural parameters in DPN
and primary diabetic encephalopathy. Arrows indicate a decrease (↓) or
increase (↑) in the parameter in the non-C-peptide-treated
situation. The original findings are referenced.
Diabetic NeuropathyAbnormalityReferences↓Na+/K+-ATPase[29, 88]↓eNOS, NO[31, 95]↓c-fos[63]↓NFκB[84]↓NGF, TrkA[49, 50, 98]↓IGF-1, IGF-1R[49, 50, 98]↓NT-3, TrkC[49, 50]↓Insulin Receptor[49, 50]↓Neurofilaments[74, 98]↓Tubulins[74, 98]↓Substance P[49, 50]↓CGRP[49, 50]↓Cell adhesive molecules[82]↑DRG neuronal atrophy[49, 50]and loss↓NCV[16, 29, 31, 49, 50, 74, 82, 88, 95]↑Axonal atrophy and loss[16, 49, 50, 74, 88, 98]↓Nerve fiber regeneration[98]↑Axoglial dysjuction[16, 82, 88]Primary Diabetic Encephalopathy↓Insulin Receptor[96, 97, 104, 114, 115]↓IGF-1, IGF-1R[97, 104, 114, 115]↑Caspase 3[114, 115]↑Caspase 12[114, 115]↑TauUnpublished data↑Apoptosis[113, 115]Cognitive function[114, 115]



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*Source: 230458-2008-04-16.xml*