Serum lipidomic determinants of human diabetic neuropathy in type 2 diabetes

Abstract

Objective: The serum lipidomic profile associated with neuropathy in type 2 diabetes is not well understood. Obesity and dyslipidemia are known neuropathy risk factors, suggesting lipid profiles early during type 2 diabetes may identify individuals who develop neuropathy later in the disease course. This retrospective cohort study examined lipidomic profiles 10 years prior to type 2 diabetic neuropathy assessment.

Methods: Participants comprised members of the Gila River Indian community with type 2 diabetes (n = 69) with available stored serum samples and neuropathy assessment 10 years later using the combined Michigan Neuropathy Screening Instrument (MNSI) examination and questionnaire scores. A combined MNSI index was calculated from examination and questionnaire scores. Serum lipids (435 species from 18 classes) were quantified by mass spectrometry.

Results: The cohort included 17 males and 52 females with a mean age of 45 years (SD = 9 years). Participants were stratified as with (high MNSI index score > 2.5407) versus without neuropathy (low MNSI index score ≤ 2.5407). Significantly decreased medium-chain acylcarnitines and increased total free fatty acids, independent of chain length and saturation, in serum at baseline associated with incident peripheral neuropathy at follow-up, that is, participants had high MNSI index scores, independent of covariates. Participants with neuropathy also had decreased phosphatidylcholines and increased lysophosphatidylcholines at baseline, independent of chain length and saturation. The abundance of other lipid classes did not differ significantly by neuropathy status.

Interpretation: Abundance differences in circulating acylcarnitines, free fatty acids, phosphatidylcholines, and lysophosphatidylcholines 10 years prior to neuropathy assessment are associated with neuropathy status in type 2 diabetes.

Figure 1

Flow diagram of the study strategy. Participants (n = 169) were originally recruited for the Renoprotection in Early Diabetic Nephropathy in Pima Indians trial from 1996 to 2001. Participants were followed for 6 years to assess the trial outcome. Of the original 169 clinical trial participants, 89 were subsequently enrolled in a long‐term observational study, which banked baseline serum and collected baseline clinical data (age, sex, height, weight, BMI, blood pressure, heart rate, diabetes duration, FPG, HbA_1c, total cholesterol, triglycerides, GFR, urine ACR, and medication use). At a mean 10‐year follow‐up, all 89 participants were reexamined, and peripheral neuropathy was assessed by MNSI examination (foot ulceration, vibration, and ankle reflexes) and questionnaire. Of these, 69 participants met the eligibility criteria for this study, which included the availability of a stored serum sample 10 years prior to the neuropathy evaluation. Banked serum from 10 years prior was analyzed by mass spectrometry; 435 lipids from the 18 classes were quantitated and their abundance by chain length and saturation were analyzed. ACR, albumin creatinine ratio; ESRD, end‐stage renal disease; FPG, fasting plasma glucose; GFR, glomerular filtration rate; HbA_1c, glycated hemoglobin; MNSI, Michigan Neuropathy Screening Instrument; PN, peripheral neuropathy. Figure created in BioRender.com. [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 2

Acylcarnitine abundance by neuropathy status. (A) Overall, mean C6–C14 acylcarnitine abundance was significantly decreased in participants with (dark gray, high MNSI index) versus without neuropathy (light gray, low MNSI index; p < 0.001). (B) Heatmap of acylcarnitine abundance by chain length (carbon number) and saturation (double bond number) for participants with (n = 27, high MNSI index) versus without neuropathy (n = 42, low MNSI index). The scale represents acylcarnitines species that are increased (red) or decreased (blue) in groups. A and B are based on generalized linear mixed models with carbon number, double bond number, and MNSI index groups as main effect variables and the MNSI index group by carbon number interaction, adjusted for other covariates. B, bars represent z‐score standardized mean values ± SEM. LC, L‐carnitine; MNSI, Michigan Neuropathy Screening Instrument. [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 3

Free fatty acid abundance by neuropathy status. (A) Overall, mean free fatty acid abundance was significantly increased in participants with (dark gray, high MNSI index) versus without neuropathy (light gray, low MNSI index; p = 0.042). (B) Heatmap of free fatty acid abundance by chain length (carbon number) and saturation (double bond number) for participants with (n = 27, high MNSI index) versus without neuropathy (n = 42, low MNSI index). The scale represents acylcarnitines species that are increased (red) or decreased (blue) in groups. (A and B) are based on generalized linear mixed models with a double bond number, carbon number, and MNSI index groups as main effect variables. (B) bars represent z‐score standardized mean values ± SEM. MNSI, Michigan Neuropathy Screening Instrument. [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 4

Phosphatidylcholine abundance by neuropathy status. (A) Overall, mean free fatty acid abundance was significantly decreased in participants with (dark gray, high MNSI index) versus without neuropathy (light gray; low MNSI index; p = 0.016). (B) Heatmap of phosphatidylcholine abundance by chain length (carbon number) and saturation (double bond number) for participants with (n = 27, high MNSI index) versus without neuropathy (n = 42, low MNSI index). The scale represents acylcarnitines species that are increased (red) or decreased (blue) in groups. A and B are based on generalized linear mixed models with a double bond number and MNSI index groups as main effect variables, adjusted for other covariates. B, bars represent z‐score standardized mean values ± SEM. MNSI, Michigan Neuropathy Screening Instrument. [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 5

Lysophosphatidylcholines abundance by neuropathy status. (A) Overall, mean free fatty acid abundance was significantly increased in participants with (dark gray, high MNSI index) versus without neuropathy (light gray, low MNSI index; p = 0.017). (B) Heatmap of lysophosphatidylcholine abundance by chain length (carbon number) and saturation (double bond number) for participants with (n = 27; high MNSI index) versus without neuropathy (n = 42, low MNSI index). The scale represents acylcarnitines species that are increased (red) or decreased (blue) in groups. A and B are based on generalized linear mixed models with a double bond number and MNSI index groups as main effect variables, adjusted for other covariates. B, bars represent z‐score standardized mean values ± SEM. MNSI, Michigan Neuropathy Screening Instrument. [Colour figure can be viewed at wileyonlinelibrary.com]