Caveolin-1 and Altered Neuregulin Signaling Contribute to the Pathophysiological Progression of Diabetic Peripheral Neuropathy

Abstract

OBJECTIVE Evaluate if Erb B2 activation and the loss of caveolin-1 (Cav1) contribute to the pathophysiological progression of diabetic peripheral neuropathy (DPN).

RESEARCH DESIGN AND METHODS Cav1 knockout and wild-type C57BL/6 mice were rendered diabetic with streptozotocin, and changes in motor nerve conduction velocity (MNCV), mechanical and thermal hypoalgesia, Erb B2 phosphorylation (pErb B2), and epidermal nerve fiber density were assessed. The contribution of Erb B2 to DPN was assessed using the Erb B2 inhibitors PKI 166 and erlotinib and a conditional bitransgenic mouse that expressed a constitutively active form of Erb B2 in myelinated Schwann cells (SCs).

RESULTS Diabetic mice exhibited decreased MNCV and mechanical and thermal sensitivity, but the extent of these deficits was more severe in diabetic Cav1 knockout mice. Diabetes increased pErb B2 levels in both genotypes, but the absence of Cav1 correlated with a greater increase in pErb B2. Erb B2 activation contributed to the mechanical hypoalgesia and MNCV deficits in both diabetic genotypes because treatment with erlotinib or PKI 166 improved these indexes of DPN. Similarly, induction of a constitutively active Erb B2 in myelinated SCs was sufficient to decrease MNCV and induce a mechanical hypoalgesia in the absence of diabetes.

CONCLUSIONS Increased Erb B2 activity contributes to specific indexes of DPN, and Cav1 may be an endogenous regulator of Erb B2 signaling. Altered Erb B2 signaling is a novel mechanism that contributes to SC dysfunction in diabetes, and inhibiting Erb B2 may ameliorate deficits of tactile sensitivity in DPN.

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes (1). Although hyperglycemia is the definitive cause of DPN (2), the vascular, glial, and neuronal damage that underlies the progressive axonopathy in DPN has a complex biochemical etiology involving oxidative stress (3,4), protein glycation (5), protein kinase C activation (6), polyol synthesis (7), and the hexosamine pathway (8). Altered neurotrophic support also contributes to sensory neuron dysfunction in DPN (9), but whether diabetes may alter growth factor signaling in Schwann cells (SCs), which also undergo substantial degeneration in diabetes, is poorly defined.

Neuregulins are growth factors that control SC growth, survival, and differentiation via their interaction with Erb B receptors (10). Although Erb B2 signaling promotes developmental myelination and is clearly trophic for SCs, pharmacological evidence supports that pathologic activation of Erb B2 after axotomy (11) or infection with leprosy bacilli (12) is sufficient to induce SC dedifferentiation and demyelination. Additionally, genetic evidence supports that Erb B2 can promote the development of sensory neuropathies independent of diabetes because expression of a dominant-negative Erb B4 in nonmyelinating (13) or myelinating (14) SCs induced a temperature or mechanical sensory neuropathy, respectively. Given the contribution of Erb B2 to the degeneration of SCs, endogenous proteins that regulate Erb B2 activity may influence the development of certain aspects of sensory neuropathies.

The interaction of Erb B2 with the protein caveolin-1 (Cav1) inhibits the intrinsic tyrosine kinase activity of the receptor (15). Cav1 is highly expressed in mature, myelinated SCs (16), and we have shown that prolonged hyperglycemia promoted the downregulation of Cav1 in SCs of sciatic nerve (17). Cav1 may regulate Erb B2 signaling in SCs because its forced downregulation was sufficient to enhance neuregulin-induced demyelination of SC–dorsal root ganglion (DRG) neuron cocultures (18). However, it is unknown whether an increase in Erb B2 activity may contribute to the pathophysiological development of DPN and if changes in Cav1 expression may alter Erb B2 activation in diabetic nerve.

In the current study, we demonstrate that diabetic Cav1 knockout mice showed an increased activation of Erb B2 and developed greater motor nerve conduction velocity (MNCV) deficits relative to their wild-type counterparts. Inhibition of Erb B2 with two structurally diverse inhibitors corrected the MNCV deficits and mechanical hypoalgesia evident after 6 or 15 weeks of diabetes. Also, induction of a constitutively active Erb B2 in myelinated SCs of adult mice was sufficient to recapitulate the MNCV and mechanical sensitivity deficits observed in the diabetic mice. These studies provide the first evidence that activation of Erb B2 contributes to deficits associated with myelinated fiber function in diabetic nerve and suggest that Cav1 may serve as an endogenous regulator of Erb B2.