We have evaluated the effect of peripheral insulin deficiency on brain insulin pathway activity in a mouse model of type-1 diabetes, the parallels with Alzheimer’s disease (AD) and the effect of treatment with insulin. Nine weeks of insulin-deficient diabetes significantly impaired the learning capacity of mice, significantly reduced IDE protein expression and significantly reduced phosphorylation of the insulin-receptor and AKT. Phosphorylation of GSK3 was also significantly decreased, indicating increased GSK3 activity. This evidence of reduced insulin-signaling was associated with a concomitant increase in tau phosphorylation and amyloid β protein levels. Changes in phosphorylation levels of insulin receptor, GSK3 and tau were not observed in the brain of db/db mice, a model of type-2 diabetes, after a similar duration (8 weeks) of diabetes. Treatment with insulin from onset of diabetes partially restored the phosphorylation of insulin receptor and of GSK3, partially reduced the level of phosphorylated tau in the brain and partially improved learning ability in insulin-deficient diabetic mice. Our data indicate that mice with systemic insulin deficiency display evidence of reduced insulin-signaling pathway activity in the brain that is associated with biochemical and behavioral features of AD, and that it can be corrected by insulin treatment.

SummaryAssessment of cutaneous innervation in skin biopsies is emerging as a valuable means of both diagnosing and staging diabetic neuropathy. Immunolabeling, using antibodies to neuronal proteins such as protein gene product 9.5, allows for the visualization and quantification of intraepidermal nerve fibers. Multiple studies have shown reductions in intraepidermal nerve fiber density in skin biopsies from patients with both type 1 and type 2 diabetes. More recent studies have focused on correlating these changes with other measures of diabetic neuropathy. A loss of epidermal innervation similar to that observed in diabetic patients has been observed in rodent models of both type 1 and type 2 diabetes and several therapeutics have been reported to prevent reductions in intraepidermal nerve fiber density in these models. This review discusses the current literature describing diabetesinduced changes in cutaneous innervation in both human and animal models of diabetic neuropathy.

The quantification of epidermal innervation, which consists primarily of heat-sensitive C-fibers, is emerging as a tool for diagnosing and staging diabetic neuropathy. However, the relationship between changes in heat sensitivity and changes in epidermal innervation has not yet been adequately explored. Therefore, we assessed epidermal nerve fiber density and thermal withdrawal latency in the hind paw of Swiss Webster mice after two and four weeks of streptozotocin-induced diabetes. Thermal hypoalgesia developed after only two weeks of diabetes, but a measurable reduction in PGP9.5-immunoreactive epidermal nerve fiber density did not appear until four weeks. These data suggest that impaired epidermal nociceptor function contributes to early diabetes-induced thermal hypoalgesia prior to the loss of peripheral terminals.Keywords diabetic neuropathy; epidermal nerve fiber density; GAP-43; PGP9.5; substance P; thermal hypoalgesia Nearly half of all diabetic patients will develop some form of peripheral neuropathy, with distal symmetric polyneuropathy the most common form [24,33]. Depletion of intra-epidermal nerve fibers (IENFs), representing the peripheral terminals of nociceptive unmyelinated C-fibers, appears to be an early index of diabetic neuropathy [30]. Resulting sensory disorders in diabetic patients range from hyperalgesia and allodynia to progressive hypoalgesia that, in conjunction with microvascular disease and impaired wound healing, leaves limbs vulnerable to infections and amputation.Diabetes-induced reductions in epidermal innervation have been observed in multiple clinical studies, as well as in diabetic primates and rodents [for review see 1,17 and references therein]. IENF loss has been correlated with measures of nerve function to assess its predictive value for other aspects of diabetic neuropathy. In humans, reductions in IENF density have been correlated with changes in pressure and vibration perception, total neurological disability score and neuropathy status [25,31]. In rats, IENF loss correlates with changes in caudal sensory nerve conduction velocity [16].One would logically predict that loss of thermal sensation is a consequence of diminished epidermal innervation. Indeed, reductions in IENF density induced by topical application of Correspondence to: Andrew P. Mizisin, Ph.D., Department of Pathology 0612, School of Medicine, University of California, San Diego, 9500 Gilman Dr., La Jolla CA 92093-0612, Tel: (858) 822-3894, FAX: (858) 534-1886, email: amizisin@ucsd.edu. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. capsaicin to the skin ...

OBJECTIVETankyrase (TNKS) is a Golgi-associated poly-ADP-ribose polymerase that is implicated in the regulation of GLUT4 trafficking in 3T3-L1 adipocytes. Its chromosomal locus 8p23.1 is linked to monogenic forms of diabetes in certain kindred. We hypothesize that TNKS is involved in energy homeostasis in mammals.RESEARCH DESIGN AND METHODSGene-trap techniques were used to ablate TNKS expression in mice. Homozygous and wild-type littermates maintained on standard chow were compared.RESULTSWild-type mice express the TNKS protein abundantly in adipose tissue, the brain, and the endocrine pancreas but scarcely in the exocrine pancreas and skeletal muscle. TNKS-deficient mice consume increased amounts of food (by 34%) but have decreased plasma leptin levels and a >50% reduction in epididymal and perirenal fat pad size. Their energy expenditure is increased as assessed by metabolic cage studies and core body temperatures. These changes are not attributable to an increase in physical activity or uncoupled respiration (based on oxygraph analyses of mitochondria isolated from brown fat and skeletal muscle). The heightened thermogenesis of TNKS-deficient mice is apparently fueled by increases in both fatty acid oxidation (based on muscle and liver gene expression analyses and plasma ketone levels) and insulin-stimulated glucose utilization (determined by hyperinsulinemic-euglycemic clamps). Although TNKS deficiency does not compromise insulin-stimulated GLUT4 translocation in primary adipocytes, it leads to the post-transcriptional upregulation of GLUT4 and adiponectin in adipocytes and increases plasma adiponectin levels.CONCLUSIONSTNKS-deficient mice exhibit increases in energy expenditure, fatty acid oxidation, and insulin-stimulated glucose utilization. Despite excessive food intake, their adiposity is substantially decreased.

Charcot-Marie-Tooth disease type 4B (CMT4B) is a severe, demyelinating peripheral neuropathy characterized by slowed nerve conduction velocity, axon loss, and distinctive myelin outfolding and infolding. CMT4B is caused by recessive mutations in either myotubularin-related protein 2 (MTMR2; CMT4B1) or MTMR13 (CMT4B2). Myotubularins are phosphoinositide (PI) 3-phosphatases that dephosphorylate phosphatidylinositol 3-phosphate (PtdIns3P) and PtdIns(3,5)P2, two phosphoinositides that regulate endosomal-lysosomal membrane traffic. Interestingly, nearly half of the metazoan myotubularins are predicted to be catalytically inactive. Both active and inactive myotubularins have essential functions in mammals and in Caenorhabditis elegans. MTMR2 and MTMR13 are active and inactive PI 3-phosphatases, respectively, and the two proteins have been shown to directly associate, although the functional significance of this association is not well understood. To establish a mouse model of CMT4B2, we disrupted the Mtmr13 gene. Mtmr13-deficient mice develop a peripheral neuropathy characterized by reduced nerve conduction velocity and myelin outfoldings and infoldings. Dysmyelination is evident in Mtmr13-deficient nerves at 14 days and worsens throughout life. Thus, loss of Mtmr13 in mice leads to a peripheral neuropathy with many of the key features of CMT4B2. Although myelin outfoldings and infoldings occur most frequently at the paranode, our morphological analyses indicate that the ultrastructure of the node of Ranvier and paranode is intact in Mtmr13-deficient nerve fibers. We also found that Mtmr2 levels are decreased by Ϸ50% in Mtmr13-deficient sciatic nerves, suggesting a mode of Mtmr2 regulation. Mtmr13-deficient mice will be an essential tool for studying how the loss of MTMR13 leads to CMT4B2.MTMR2 ͉ myelin ͉ PtdIns3P ͉ PtdIns(3,5)P2 ͉ endosomal traffic C harcot-Marie-Tooth (CMT) disease (also called hereditary motor and sensory neuropathy) describes a group of inherited peripheral neuropathies that are both clinically and genetically heterogeneous (1). With a worldwide incidence of Ϸ1 in 2,500, CMT is one of the most common inherited neurological disorders (www.charcot-marie-tooth.org). CMT leads to progressive degeneration of the muscles of the extremities and loss of sensory function (2). Patients with demyelinating CMT (types 1, 3, and 4) show reduced nerve conduction velocity (NCV) (Ͻ38 m/s). In contrast, the axonal forms of CMT (type 2) are associated with normal or near normal NCVs and decreased compound muscle action potential amplitudes. Nerve biopsies from patients with demyelinating CMT show axonal loss and evidence of demyelination/remyelination, whereas nerves from axonal CMT patients show axonal loss without signs of demyelination and remyelination (2). Genetic studies have identified CMT-causing mutations in Ϸ30 distinct genes of diverse function (1). However, the cellular mechanisms by which these mutations lead to disease are generally poorly understood (1-4). In general, demyelinating and axonal forms of...