Impact of gut-peripheral nervous system axis on the development of diabetic neuropathy

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Impact of gut-peripheral nervous system axis on the development of diabetic neuropathy

Thalita Mázala-de-Oliveira

, Yago Amigo Pinho Jannini de Sá

, Vinicius de Frias Carvalho

/

1Fundação Oswaldo Cruz-Fiocruz, Instituto Oswaldo cruz, Rio de Janeiro, RJ, Brasil

2Cedars-Sinai Medical Center, Medicine Department, Los Angeles, CA, United States of America

3Instituto Nacional de Ciência e Tecnologia em Neuroimunomodulação, Rio de Janeiro, RJ, Brasil

Diabetes is a chronic metabolic disease caused by a reduction in the production and/or action of insulin, with consequent development of hyperglycemia. Diabetic patients, especially those who develop neuropathy, presented dysbiosis, with an increase in the proportion of pathogenic bacteria and a decrease in the butyrate-producing bacteria. Due to this dysbiosis, diabetic patients presented a weakness of the intestinal permeability barrier and high bacterial product translocation to the bloodstream, in parallel to a high circulating levels of pro-inflammatory cytokines such as TNF-α. In this context, we propose here that dysbiosis-induced increased systemic levels of bacterial products, like lipopolysaccharide (LPS), leads to an increase in the production of pro-inflammatory cytokines, including TNF-α, by Schwann cells and spinal cord of diabetics, being crucial for the development of neuropathy.

Key words: diabetes - dysbiosis - gut microbiota - inflammation - neuropathy

Diabetes is a chronic metabolic disease characterized by hyperglycemia due to a reduction in the production and/or action of insulin.

Currently, diabetes is one of the most serious and frequent chronic diseases in world- wide. Uncontrolled diabetes is accompanied by the de- velopment of several disabling and costly complications, which reduce patients’ life expectancy and can be fatal.

(2)

In 2021, the global prevalence of diabetes reached pan- demic proportions with 537 million people living with diabetes in the world, accompanied by an expense of 699 billion USD in global healthcare. In addition, future projections suggest that up to 2045 the number of people with diabetes will increase by 46%, with an estimate of health expenditures for the care of this disease that will exceed one trillion USD.

Neuropathy is the most prevalent complication of di- abetes, occurring in up to half of all people living with this disease.

In addition, neuropathy is responsible for frequent hospitalization compared to other diabetes morbidities,

and it is the most common reason for non- traumatic amputation.

Neuropathic pain is manifested as spontaneous or induced pain, such as hyperalgesia and allodynia.

Although neuropathy is the strongest predic- tor of mortality in diabetes, it remains without specific treatment.

This scenario leads to high individual costs for patients, including pain, inability to work, poor quality of life, multiple hospitalizations for ulcers and eventual amputations. Therefore, we performed a narrative review with the aim of increasing knowledge about the role of gut dysbiosis in the development and/or progression of neuropathy in diabetic patients, besides how essential this microenvironment is for better control of the disease.

doi: 10.1590/0074-02760220197

+ Corresponding author: vfrias@ioc.fiocruz.br

 https://orcid.org/0000-0003-2136-8958 Received 26 August 2022

Accepted 14 February 2023

Pathogenesis of diabetic neuropathy

In diabetic patients, the development of neuropathy is multifactorial and involves uncontrolled glycemia, diabetes duration, and age-related neuronal attrition.

Although the precise order of cellular injury in diabe- tes is unknown, the alterations in the peripheral nervous system that culminate in diabetic neuropathy are well established. These changes include (i) progressive loss of neurofilament polymer, which are essential structur- al scaffolds of the axon;

(ii) modification in the key plasticity molecules in the dorsal root ganglia (DRG), including decrease in the synthesis of growth-associated protein 43 (GAP43) and β-tubulin and increase in the expression of heat shock proteins (HSP) and poly(ADP- ribose) polymerase (PARP);

(iii) axonal degen- eration;

(iv) reduction in the Schwann cells to support axons, through decrease in the provision of cytoskeletal support, trophic factors or ribosome transfer that allows intra-axonal mRNA translation within distal axons;

(v) demyelination, which occurs in more severe cases of diabetic neuropathy;

(vi) reduction in the blood flow in the DRG

(Fig. 1).

The exact mechanism that promotes alterations in the peripheral nervous system and, consequently, neu- ropathic pain in diabetic patients is not fully elucidated.

Nonetheless, several evidences showed that hypergly- cemia and dyslipidemia induce pathological changes in neurons, glia, and vascular cells that culminate in nerve dysfunction and neuropathy.

Hyperglycemia increases glycolysis, polyol, advanced glycation end products, protein kinase C, and hexosamine pathways in Schwann cells, DRG neurons and axons.

These metabolic changes result in increased reactive

oxygen species (ROS) formation and release of pro-

inflammatory mediators. In parallel, the dyslipidemia

observed in diabetic patients also induced a rise in ROS

production and systemic and local inflammation.

In diabetes, the overproduction of ROS induces ac- tivation of PARP-1 with a concomitant decrease in ATP formation.

Altogether, high levels of ROS and loss of ATP production cause mitochondrial failure and meta- bolic and oxidative damage in Schwann cells and DRG neurons.

The increase in ROS formation, dysfunc- tional mitochondria, and microvascular insufficiency result in axonal degeneration in the peripheral nervous system.

In addition, the oxidative damage in- duces hyperexcitability in the axons and DRG neurons, causing neuropathic pain.

Several pre-clinical and clinical studies showed up- regulation of pathways involved in inflammation in peripheral nerves.

Furthermore, numerous ex- perimental models of diabetic neuropathy showed an inflammatory response, characterized by infiltration of macrophages and T cells and increased levels of pro- inflammatory cytokines, in the sciatic nerve and DRG.

(35,36,37,38)

The inflammatory response was accompanied by

loss of myelinated and unmyelinated nerve fibers and ax- onal damage in diabetic animals with neuropathy.

The presence of inflammation biomarkers was also associ- ated with onset and progression of neuropathy in diabetic patients.

Moreover, diabetes-induced inflammation altered mitochondrial bioenergetics in DRG neurons

and HSP content in sensory neurons.

Besides, the systemic low-grade inflammation has been implicated in neuropathic pain observed in diabetic patients, since some circulating inflammatory mediators was positive related to the severity of neuropathic pain in a subgroup of pa- tients with distal symmetrical polyneuropathy.

Diabetes and gut microbiota

The human gut microbiota contains up to 100 tril- lion of microbes, including commensal, symbiotic, and pathogenic bacteria, as well as archaea, fungi, and vi-

ruses.

Gut microbiota has various physiological functions in the host, including strengthening epithelial- intestinal barrier integrity,

maintenance of energy ho- meostasis,

protection against pathogens,

and regu- lation of host immunity.

Diet, early-life microbiota exposure, antibiotic ther- apy, changing hygiene status, pollution, socioeconomic status, and other environmental factors can directly in- fluence the composition of gut microbiota and its meta- bolic products, making it unique to each individua.

Furthermore, patients with some pathologic conditions, for instance inflammatory diseases, infections, neu- rodegenerative diseases, and metabolic diseases, pre- sented an imbalance in the microbes’ composition into the gut with the predominance of pathogenic bacteria, known as dysbiosis.

In early stages of life, the extensive exposure to an- tibiotics may lead to dysbiosis, resulting in underweight or overweight.

Moreover, the large consumption of antibiotics is usually been related to the development of metabolic disorders in later stages of life.

In addition, the intake of sweeteners, as sugar substitute, also has considerable impact over microbiota population, induc- ing glucose intolerance.

These observations suggest that an imbalance in the composition of the gut micro- biota may be related to the development of metabolic diseases. In fact, the composition of gut microbiota can alter the immune response of non-obese diabetic (NOD) mice, a classical model of autoimmune diabetes. Germ- free (GF) NOD mice showed an acceleration in the in- sulitis in parallel to a rise in the Th1 and Th17 cells in the mesenteric and pancreatic lymph nodes,

however, the incidence of diabetes was not modified.

Fur- thermore, the administration of probiotics in NOD mice decreased the incidence of diabetes, through reduction of insulitis and gut permeability and modulation of cy-

Fig. 1: alterations observed in the peripheral nervous system in diabetic neuropathy. In the dorsal root ganglion (DRG), localization of the cell bod- ies of sensory neurons, diabetes induced alterations in the key plasticity molecules, including reduction in growth-associated protein 43 (GAP43) and β-tubulin in parallel with an increase in the heat shock proteins (HSP) and poly(ADP-ribose) polymerase (PARP) expression. Furthermore, diabetics showed loss of neurofilament polymer, decrease in the Schwann cells-axon support, axonal degeneration, and ischemia in the sensory nerves, in addition to a demyelination in more severe cases of diabetic neuropathy. Altogether, these alterations evoked pain in diabetic patients.

tokine profile, Treg cells and T helper cell polarization.

(65,66,67)

These data indicate that gut microbiota is impor-

tant to the development of diabetes.

It is well known that diabetic patients showed dys- biosis.

Although the composition of gut microbiota is diverse among diabetic patients, the relationship be- tween Firmicutes and Bacteroidetes is unbalanced in these patients.

In addition, patients with type 1 diabetes presented a clear depletion of species like Pre- votella copri and Bifidobacterium longum, probiotic bacteria, and enrichment of families like Ruminococ- caceae, Clostridiaceae, Clostridiales, and Oscillibacter, bacteria associated with infection and inflammation.

The maintenance of homeostasis in gut environment is important not only to slow-down diabetes develop- ment but it is also central in the control of its compli- cations.

Dysbiosis in diabetic patients and animals lead to an increase in the intestinal permeability

in parallel to a rise in the bacterial content to the blood- stream, as lipopolysaccharide (LPS).

Interestingly, the supplementation of diabetic mice with a microbial anti-inflammatory molecule, which is a metabolite of a commensal bacteria Faecalibacterium prausnitzii, im- proved the intestinal barrier permeability and reduced circulating levels of LPS.

In diabetic patients, the gut leakiness is accompanied by a low-grade inflammation, with increased levels of IL- 1β, IL-6, and TNF-α in the blood.

Furthermore, type 1 and type 2 diabetic patients as well as NOD mice showed a reduction in the abundance of butyrate-producing bac- teria.

Butyrate is a short-chain fatty acid (SCFA) that induce mucin production, regulating the permeability of the intestinal barrier, and reduce translocation of bacte- ria and their products, oxidative stress, and inflamma- tion.

The reduction of butyrate-forming bacteria, with consequent inadequate butyrate secretion, aggravates the pathogenesis of diabetes,

through increase of inflam- mation and oxidative stress. Treatment with butyrate decreased ROS production and the homeostatic levels of inflammatory markers in diabetic mice.

Remarkably, treatment with probiotics is one of the most used strategies to modulate intestinal microbiota and their used can prevent diabetes establishment and is effec- tive as adjuvant in insulin resistance therapies.

According to Food and Agriculture Organization (FAO) and World Health Organization (WHO), probiotics are de- fined as live micro-organisms which when administered in adequate amounts confer a health benefit on the host.

(88)

Furthermore, treatment of diabetic rats with probiotics slow down the progression of diabetes in clear association with a decrease in the plasma levels of LPS.

Besides, the treatment of diabetic animals with probiotics contain- ing the Lactobacillus rhamnosus NCDC17 improved the insulin resistance, in parallel to a reduction in IL-6 and TNF in the epididymal fat.

Although the use of adju- vant therapy with probiotics seems to be interesting to treat some comorbidities of diabetes related to dysbio- sis, some effects of them are controversy in diabetic pa- tients. In general, the treatment with probiotics is useful to control insulin resistance and improved the intestinal barrier permeability in type 2 diabetic patients, however,

it is ineffective in reducing systemic inflammation. Fur- thermore, the use of probiotics to reduce the glycemia and serum insulin levels in these patients is very controversy.

Although the studies with type 1 diabetic patients are still scarce, it was observed that treatment with probiotics re- duced glycemia and systemic inflammatory markers as compared to placebo (Table).

Gut microbiota and neuropathic pain

The gut microbiota plays a role in the maintenance of nervous system function, through immunological, hormonal, and neuronal signals.

Several studies showed the participation of gut microbiota in the devel- opment of pain, since GF mice exhibited visceral hyper- sensitivity that was controlled by postnatal colonization with conventional microbiota.

In addition, the transference of fecal microbiota from irritable bowel syndrome patients to GF mice increased visceral hyper- sensitivity.

Likewise, probiotic treatment improves stress- and inflammation-induced visceral hypersensi- tivity.

In addition, gut dysbiosis has been de- scribed in patients with irritable bowel syndrome who present abdominal pain.

Furthermore, the gut microbiota is also involved in the pathophysiology of neuropathic pain. For instance, some chemotherapy drugs, including paclitaxel and ox- aliplatin, cause chemotherapy-induced peripheral pain (CIPN) during anti-cancer treatment,

and affects up to 48% of patients undergoing chemotherapy.

Sev- eral chemotherapy drugs that induced neuropathic pain change the composition of gut microbiota, inducing dys- biosis.

Likewise, oxaliplatin-induced mechanical hy- peralgesia is decreased in GF mice or in animals treated with antibiotics,

suggesting that CIPN depends on dysbiosis. Another case of gut microbiota participating in neuropathic pain is observed in a murine model of chronic constriction injury (CCI).

In this model, the oral treatment with antibiotics resulted in gut microbiota changes and decreased the development of CCI-induced neuropathic pain, through a skewing from a pro-inflam- matory to an anti-inflammatory immune profile,

indicating that dysbiosis is an important factor in the development of CCI-induced neuropathic pain. In ad- dition, the transplantation of fecal microbiota from rats with spared nerve injury to pseudo-GF mice increased mechanical stimulus-induced pain,

suggesting that gut microbiota possess a significant role in the spared nerve injury-induced neuropathic pain.

Since the development of neuropathic pain can be related to changes in the gut microbiota and diabetic patients presented dysbiosis in association with neu- ropathy, a central question arises: can dysbiosis and the consequent pro-inflammatory status be critical for the development of neuropathic pain in diabetes?

Dysbiosis is important to the establishment and aggravation of diabetic neuropathy

In diabetes, dysbiosis occurs in parallel to a break in

the epithelial-intestinal barrier and translocation of bacte-

rial contents to the bloodstream.

In addition, the hyper-

glycemia observed in diabetic patients is followed by an

TABLE Clinical trials in diabetic patients after probiotic administration ProbStudy design/subjectsSample sizeStudy periodAdvantageDisadvantageRef Lactobacillusacidophilus NCFM® (1010CFU)Double-blinded, randomized, placebo-controlled, T2D males. Ages: 48-65 years-oldProb:n = 21 Cont: n = 244 weeks↑ insulin sensitivityNo alteration in the inflammatory markers (TNF, IL-6, IL-1ra and CRP)

(91) Lactobacillus acidophilus (2 ×109CFU); L. casei (7×109 CFU); L. rhamnosus (1.5×109 CFU); L. bulgaricus (2×108 CFU); Bifidobacterium breve (2×1010CFU); B. longum (7×109 CFU), Streptococcus thermophilus (1.5×109CFU)

Randomized, double-blinded, placebo-controlled, T2D males and females. Ages: 35-70 years-oldProb: n = 27 Cont: n = 278 weeks↓ FPG and CRPNo alteration in the serum insulin levels and HOMA-IR(92) Lactobacillus sporogenes (1×107 CFU) + 0.04 g inulin (as prebiotic)Randomized, placebo-controlled, T2D. Ages: 35-70 years-oldProb: n = 35 Cont: n = 356 weeks↓ Insulin and CRPNo alteration in the HOMA-IR and FPG(93) Lactobacillus acidophilus (4x108 CFU/100mL); Bifidobacterium bifidum (4x108 CFU/100mL)Randomized, double-blind, placebo-controlled, T2D females. Ages: 50-60 years-oldProb: n = 10 Cont: n = 105.5 weeks↓ FPG and Glycemia---(94) Lactobacillus acidophilus La-5(109 CFU); Bifidobacterium animalis subsp lactis BB-12 (109 CFU)Randomized, double-blind, placebo-controlled, T2D patients. Ages: 35-60 years-oldProb: n = 23 Cont: n = 226 weeks↓ HbA1c and TNF- α (both groups)No alteration in the IL-10 and IL-6 levels(95) Lactobacillus casei (4×1010 CFU)Randomized, placebo-controlled, T2D patients. Ages: 30-79 years-oldProb: n = 34 Cont: n = 3416 weeks

↑ Counts of the total Lactobacillus and L. casei subgroup ↑ Counts of the total Clostridium coccoides group and the C. leptum subgroup ↓ L. gasseri and L. reuteri subgroups ↓ Translocation of gut bacteria to the blood No alteration in the HbA1c, FPG, and inflammatory markers (LBP, IL-6, TNF-α, CRP)

(96) Lactobacillus acidophilus La5 (7.23x106 CFU/g); Bifidobacterium lactis Bb12 (6.04x106 CFU/g)Randomized, double-blind, controlled clinical, T2DM. Ages: 30-60 years-oldProb: n = 30 Cont: n = 306 weeks↓ FPG and HbA1cNo alteration in the insulin levels(97) Lacticaseibacillus paracasei(3x108 CFU)strain Shirota (previously Lactobacillus caseistrain Shirota); Bifidobacterium breve (3x108 CFU) strain Yakult, and galactooligosaccharides)

Randomized, double-blind, controlled clinical, T2DM and obese patients. Ages: 30-80 years-oldProb: n = 44 Cont: n = 4224 weeks↑ B. adolescentis, B. pseudocatenulatum, Lactobacillus ↓ B. vulgatus

↑ levels of FPG and HbA1c (12 weeks) No alteration in the glycemia and inflammatory markers (IL-6, LBP, CRP)

(98) Lactobacillus acidophilus (2×109 CFU); Lactobacillus casei (7×109 CFU); Lactobacillus rhamnosus (1.5×109 CFU); Lactobacillus bulgaricus (2×108 CFU); Bifidobacterium breve (3×1010 CFU); Bifidobacterium longum (7×109 CFU); Streptococcus thermophilus (1.5×109 CFU)

Randomized, double-blind, controlled clinical, T2DM. Ages: 30-75 years-oldProb: n = 30 Cont: n = 306 weeks↓ FPGNo alteration in the insulin levels(99) Lactobacillus casei (108 CFU/L)Block randomized, controlled clinical, T2DM. Ages: 30-50 years-oldProb: n = 20 Cont: n = 208 weeks↓ FPG, serum insulin level, and HOMA-IRNo alteration in the HbA1c(100) Lactobacillus + Lactococcus(6×1010 CFU/g); Bifidobacterium (1×1010CFU /g); Propionibacterium (3×1010 CFU/g); Acetobacter (1×106 CFU /g)Randomized, double-blind, controlled clinical, T2D. Ages: 18-75 years-oldProb: n = 31 Cont: n = 228 weeks↓ HbA1c and HOMA-IR↑ TNF-α, IL-1β, IL-6, INF- γ levels No alteration in the FPG and serum insulin and IL-8 levels

(101) Lactobacillus salivarius (subsp.salicinius AP-32); Lactobacillus johnsonii (MH-68); Bifidobacterium animalis (subsp.lactis CP-9) contain 1×1010CFU/day)

Randomized, double-blind, placebo-controlled trial. T1DM. Ages: 6-18 years-oldProb: n = 27 Cont: n = 2924 weeks↑ B. animalis, Akkermansia muciniphila and Lactobacillus salivariusin the gut ↓ FPG, HbA1c, IL-8, IL-17, MIP-1β, TNF-α ↑ TGF-β---(102) CFU: colony-forming units; Cont: control; CRP: C-reactive protein; FPG: fasting plasma glucose; HbA1c: hemoglobin A1c; HOMA-IR: homeostasis model of assessment-insulin resistance; IL: interleukin; IL-1ra: interleukin-1 receptor antagonist; INF: interferon; LBP: lipopolysaccharide binding protein; MIP: macrophage inflammatory proteins; Prob: probiotic; TGF: trans- forming growth factor; TNF: tumor necrosis factor; T1D: type 1 diabetes; T2D: type 2 diabetes.

increase in the pro-inflammatory cytokines IL-1β, IL-6, and TNF-α, marking low-grade inflammation.

Further- more, the reduction in the butyrate-producing bacteria in the gut microbiota of diabetic patients is related to the high permeability of the epithelial-intestinal barrier.

Interestingly, the incubation of Schwann cells in a high glucose environment induced an increase in the apoptosis of those cells in association with overexpres- sion of TLR4, the receptor activated by LPS, and a rise in the TNF-α production.

In addition, the expression of TLR4 mRNA and the protein levels TNF-α were in- creased in the spinal cord of streptozotocin-induced diabetes. These raises in TLR4 and TNF-α were posi- tively correlated with mechanical/thermal hypersen- sitivity in diabetic rats.

Reinforcing the hypothesis that the increase in TLR4 expression in the Schwann cells and spinal cord is important to diabetic neuropa- thy, the inhibition of TLR4 signaling in the spinal cord attenuated mechanical hyperalgesia in diabetic rats with neuropathy and downregulated the local levels of TNF-α.

Furthermore, the continuous delivery of IL- 10 in the nerve fibers of DRG blocked the nociceptive response in diabetic animals, in parallel to a decrease in the expression of TLR4.

TNF-α is produced primarily by endoneurial mac- rophages and Schwann cells,

and is increased at the injury site after CCI of the sciatic nerve in rats.

The administration of TNF-α inhibitor or antibodies to TNF-α reduced nerve injury- and CCI-induced hyper- sensitivity, respectively.

Furthermore, the IL1R1/

TNFR1 double knock-out mice showed a decrease in the nociceptive sensitivity after nerve injury compared to wild-type littermates.

In addition, direct injection of TNF-α into the sciatic nerve induced painful neurop- athy.

It is well known that patients with diabetic peripheral neuropathy showed elevated levels of both TNF-α and soluble TNF-α receptors in their serum.

(137,138,139)

The inhibition of TNF-α, using a recombinant

human TNF-α receptor-antibody fusion protein, recov- ery lower nerve conduction velocity, demyelination of nerve fibers, disorganization of lamellar and axonal structures, and decreased expression of myelin basic protein in the nerve tissue of diabetic rats that developed peripheral neuropathy.

Furthermore, the blockage of TNF-α signaling using either TNF-α knockout mice or anti-TNF-α monoclonal antibody improved neuropathy in diabetic mice.

Sodium butyrate has anti-inflammatory and neu- roprotective effects in spinal cord injury, including the improvement of motor function and the reduction of neutrophils accumulation and pro-inflammatory cyto- kine expression.

Furthermore, sodium butyrate and sodium propionate improved nitroglycerin-induced pain attacks, reducing the damage in the trigeminal nerve nu- cleus and the expression of pro-inflammatory mediators.

(143)

Likewise, the pain and discomfort in healthy human

were reduced after intraluminal administration of butyr- ate into the distal colon.

Altogether, these evidences suggested that the decrease in the butyrate-producing bacteria in diabetic patients can be related to the devel- opment of neuropathic pain.

The composition of the gut microbiota can change during the development of diabetes. Interestingly, the di- versity of gut microbiota from patients with type 2 dia- betes with gastrointestinal autonomic neuropathy was modified compared to type 2 diabetic patients without this condition. Diabetic patients with neuropathy pre- sented an increase in the relative abundance of patho- genic bacteria of phyla Proteobacteria,

a LPS-produc- ing bacteria phylum.

Jinmaitong, a natural compound rich in flavonoid and its glycosides, triterpenoids, and phenolic acids,

improves nerve conduction velocity, pain, and temperature sensation in diabetic rats with neuropathy,

as well as markedly ameliorated clini- cal symptoms of pain in the extremities of diabetic pa- tients with peripheral neuropathy.

In parallel, Jinmai- tong enriched nine species of gut microbiota of diabetic rats with neuropathy, avoiding dysbiosis.

Furthermore, the flavonoid quercetin reversed me- chanical pain and intraepidermal nerve fiber density in streptozotocin-induced diabetic rats, in clear associa- tion with the reduction of pathogenic bacteria species and the enrichment of two prebiotic species.

These data suggest that both Jinmaitong and quercetin improve neuropathy in diabetic subjects by modulating pheno- type-associated gut microbiota. In agreement with the proposition that dysbiosis can be important to the devel- opment of neuropathy in diabetes, a case report showed that fecal microbiota transplantation decrease limb pain and paresthesia in a diabetic patient with neuropathy that did not use any painkillers or drugs for alleviating the pain. In addition, this patient showed an improvement of motor conduction velocity in tibial nerve, attested by electromyogram, and a reduction in the visual analogue scale pain score from severe pain to mild pain after the treatment with fecal microbiota transplantation.

In conclusion

In conclusion, we postulate that the increase in the richness of pathogenic bacteria and a reduction in the abundance of butyrate-producing bacteria in the gut microbiota of diabetic patients may be responsible for the onset of peripheral neuropathy. The dysbiosis in diabetes triggers a break in the intestinal barrier with consequent increase in the bacterial products, such as LPS, into the bloodstream. Possibly, the activation of TLR4 in the Schwann cells and spinal cord of diabet- ics induces an overproduction of TNF-α, resulting in the increase of pain (Fig. 2). In this respect, new thera- peutic strategies founded on probiotics or bacterial me- tabolites, as butyrate, seem to be potentially practical approaches for adjuvant treatment of neuropathy in dia- betic patients.

AUTHORS’ CONTRIBUTION

VFC conceived of the presented idea; TMO, YAPJS and VFC wrote the manuscript and designed the figures. All au- thors discussed the research and contributed to the final manu- script. The authors declare that the research was conducted without any financial relationships that could be construed as a potential conflict of interest.

REFERENCES

1. Insuela DBR, Ferrero MR, Gonçalves-de-Albuquerque CF, Chaves ADS, da Silva AYO, Castro-Faria-Neto HC, et al. Gluca- gon reduces neutrophil migration and increases susceptibility to sepsis in diabetic mice. Front Immunol. 2021; 12: 633540.

2. Heald AH, Stedman M, Davies M, Livingston M, Alshames R, Lunt M, et al. Estimating life years lost to diabetes: outcomes from analysis of National Diabetes Audit and Office of National Statis- tics data. Cardiovasc Endocrinol Metab. 2020; 9(4): 183-5.

3. Sun H, Saeedi P, Karuranga S, Pinkepank M, Ogurtsova K, Dun- can BB, et al. IDF diabetes atlas: global, regional and country- level diabetes prevalence estimates for 2021 and projections for 2045. Diabetes Res Clin Pract. 2022; 183: 109119.

4. Feldman EL, Callaghan BC, Pop-Busui R, Zochodne DW, Wright DE, Bennett DL, et al. Diabetic neuropathy. Nat Rev Dis Primers.

2019; 5(1): 41.

5. Patel S, Farkash C, Simmons D. Type 1 diabetes management and hospitalisation in the over 25’s at an Australian outer urban diabe- tes clinic. BMC Endocr Disord. 2022; 22(1): 143.

6. Hicks CW, Selvin E. Epidemiology of peripheral neuropathy and lower extremity disease in diabetes. Curr Diab Rep. 2019; 19(10): 86.

7. Kamerman PR, Wadley AL, Davis KD, Hietaharju A, Jain P, Kopf A, et al. World Health Organization essential medicines lists: where are the drugs to treat neuropathic pain? Pain. 2015; 156(5): 793-7.

8. Finnerup NB, Attal N, Haroutounian S, McNicol E, Baron R, Dworkin RH, et al. Pharmacotherapy for neuropathic pain in adults: a systematic review and meta-analysis. Lancet Neurol.

2015; 14(2): 162-73.

9. Scott JN, Clark AW, Zochodne DW. Neurofilament and tubu- lin gene expression in progressive experimental diabetes: fail- ure of synthesis and export by sensory neurons. Brain. 1999;

122(11): 2109-18.

10. Ma J, Pan P, Anyika M, Blagg BS, Dobrowsky RT. Modulating molecular chaperones improves mitochondrial bioenergetics and decreases the inflammatory transcriptome in diabetic sensory neurons. ACS Chem Neurosci. 2015; 6(9): 1637-48.

11. Urban MJ, Pan P, Farmer KL, Zhao H, Blagg BS, Dobrowsky RT.

Modulating molecular chaperones improves sensory fiber recov- ery and mitochondrial function in diabetic peripheral neuropathy.

Exp Neurol. 2012; 235(1): 388-96.

12. Ilnytska O, Lyzogubov VV, Stevens MJ, Drel VR, Mashtalir N, Pacher P, et al. Poly(ADP-ribose) polymerase inhibition allevi- ates experimental diabetic sensory neuropathy. Diabetes. 2006;

55(6): 1686-94.

13. Lupachyk S, Shevalye H, Maksimchyk Y, Drel VR, Obrosova IG.

PARP inhibition alleviates diabetes-induced systemic oxidative stress and neural tissue 4-hydroxynonenal adduct accumulation:

correlation with peripheral nerve function. Free Radic Biol Med.

2011; 50(10): 1400-9.

14. Cashman CR, Höke A. Mechanisms of distal axonal degeneration in peripheral neuropathies. Neurosci Lett. 2015; 596: 33-50.

15. Feldman EL, Nave KA, Jensen TS, Bennett DLH. New horizons in diabetic neuropathy: mechanisms, bioenergetics, and pain. Neu- ron. 2017; 93(6): 1296-1313.

16. Dunnigan SK, Ebadi H, Breiner A, Katzberg HD, Lovblom LE, Perkins BA, et al. Conduction slowing in diabetic sensorimotor polyneuropathy. Diabetes Care. 2013; 36(11): 3684-90.

17. Gumy LF, Bampton ET, Tolkovsky AM. Hyperglycaemia inhibits Schwann cell proliferation and migration and restricts regenera- tion of axons and Schwann cells from adult murine DRG. Mol Cell Neurosci. 2008; 37(2): 298-311.

18. Mizisin AP, Shelton GD, Wagner S, Rusbridge C, Powell HC.

Myelin splitting, Schwann cell injury and demyelination in feline diabetic neuropathy. Acta Neuropathol. 1998; 95(2): 171-4.

19. Zochodne DW, Ho LT. The influence of sulindac on experimen- tal streptozotocin-induced diabetic neuropathy. Can J Neurol Sci.

1994; 21(3): 194-202.

20. Kang Q, Yang C. Oxidative stress and diabetic retinopathy: mo- lecular mechanisms, pathogenetic role and therapeutic implica- tions. Redox Biol. 2020; 37: 101799.

21. Jang ER, Lee CS. 7-ketocholesterol induces apoptosis in differen- tiated PC12 cells via reactive oxygen species-dependent activation of NF-κB and Akt pathways. Neurochem Int. 2011; 58(1): 52-9.

22. Legrand-Poels S, Esser N, L’homme L, Scheen A, Paquot N, Piette J. Free fatty acids as modulators of the NLRP3 inflammasome in obesity/type 2 diabetes. Biochem Pharmacol. 2014; 92(1): 131-41.

23. Burkart V, Wang ZQ, Radons J, Heller B, Herceg Z, Stingl L, et al. Mice lacking the poly(ADP-ribose) polymerase gene are resis- tant to pancreatic beta-cell destruction and diabetes development induced by streptozocin. Nat Med. 1999; 5(3): 314-9.

Fig. 2: mechanisms associated with diabetic neuropathy induced by gut microbiota dysbiosis and the therapeutic effects of prebiotics/probiotics.

Dysbiosis in diabetic patients results in a leak of epithelial-intestinal barrier, increasing its permeability and the absorption of molecules from lumen to the blood such as lipopolysaccharide (LPS). LPS directly activates Schwann cells to release tumor necrosis factor-α (TNF-α), favoring the nociception in diabetics. Changes in diet, prebiotics or probiotics may induces changes in microbiota, restoring butyrate producing micro- organisms, leading to anti-inflammatory effects, reducing the TNF-α production and, consequently, the pain in diabetics tumor necrosis factor.

24. Fernyhough P, McGavock J. Mechanisms of disease: mitochon- drial dysfunction in sensory neuropathy and other complications in diabetes. Handb Clin Neurol. 2014; 126: 353-77.

25. Chowdhury SK, Smith DR, Fernyhough P. The role of aberrant mitochondrial bioenergetics in diabetic neuropathy. Neurobiol Dis. 2013; 51: 56-65.

26. Huvers FC, De Leeuw PW, Houben AJ, De Haan CH, Hamulyak K, Schouten H, et al. Endothelium-dependent vasodilatation, plasma markers of endothelial function, and adrenergic vasocon- strictor responses in type 1 diabetes under near-normoglycemic conditions. Diabetes. 1999; 48(6): 1300-7.

27. Rumora AE, Lentz SI, Hinder LM, Jackson SW, Valesano A, Levin- son GE, et al. Dyslipidemia impairs mitochondrial trafficking and function in sensory neurons. FASEB J. 2018; 32(1): 195-207.

28. Hink U, Li H, Mollnau H, Oelze M, Matheis E, Hartmann M, et al.

Mechanisms underlying endothelial dysfunction in diabetes mel- litus. Circ Res. 2001; 88(2): e14-e22.

29. Vinik A, Ullal J, Parson HK, Casellini CM. Diabetic neuropa- thies: clinical manifestations and current treatment options. Nat Clin Pract Endocrinol Metab. 2006; 2(5): 269-81.

30. Kasznicki J, Kosmalski M, Sliwinska A, Mrowicka M, Stanczyk M, Majsterek I, et al. Evaluation of oxidative stress markers in patho- genesis of diabetic neuropathy. Mol Biol Rep. 2012; 39(9): 8669-78.

31. Hur J, O’Brien PD, Nair V, Hinder LM, McGregor BA, Jagadish HV, et al. Transcriptional networks of murine diabetic peripheral neuropathy and nephropathy: common and distinct gene expres- sion patterns. Diabetologia. 2016; 59(6): 1297-306.

32. McGregor BA, Eid S, Rumora AE, Murdock B, Guo K, de Anda- Jáuregui G, et al. Conserved transcriptional signatures in human and murine diabetic peripheral neuropathy. Sci Rep. 2018; 8(1): 17678.

33. Hur J, Sullivan KA, Pande M, Hong Y, Sima AA, Jagadish HV, et al. The identification of gene expression profiles associated with progression of human diabetic neuropathy. Brain. 2011; 134(Pt 11): 3222-35.

34. Hur J, Sullivan KA, Callaghan BC, Pop-Busui R, Feldman EL.

Identification of factors associated with sural nerve regeneration and degeneration in diabetic neuropathy. Diabetes Care. 2013;

36(12): 4043-9.

35. Baum P, Kosacka J, Estrela-Lopis I, Woidt K, Serke H, Paeschke S, et al. The role of nerve inflammation and exogenous iron load in experimental peripheral diabetic neuropathy (PDN). Metabolism.

2016; 65(4): 391-405.

36. Kosacka J, Woidt K, Toyka KV, Paeschke S, Klöting N, Bechmann I, et al. The role of dietary non-heme iron load and peripheral nerve inflammation in the development of peripheral neuropathy (PN) in obese non-diabetic leptin-deficient ob/ob mice. Neurol Res. 2019; 41(4): 341-53.

37. Paeschke S, Baum P, Toyka KV, Blüher M, Koj S, Klöting N, et al.

The role of iron and nerve inflammation in diabetes mellitus type 2-induced peripheral neuropathy. Neuroscience. 2019; 406: 496-509.

38. Cheng HT, Dauch JR, Oh SS, Hayes JM, Hong Y, Feldman EL.

p38 mediates mechanical allodynia in a mouse model of type 2 diabetes. Mol Pain. 2010; 6: 28.

39. Kosacka J, Nowicki M, Klöting N, Kern M, Stumvoll M, Bechmann I, et al. COMP-angiopoietin-1 recovers molecular biomarkers of neuropathy and improves vascularisation in sciatic nerve of ob/ob mice. PLoS One. 2012; 7(3): e32881.

40. Herder C, Kannenberg JM, Huth C, Carstensen-Kirberg M, Rath- mann W, Koenig W, et al. Proinflammatory cytokines predict the incidence and progression of distal sensorimotor polyneuropathy:

KORA F4/FF4 study. Diabetes Care. 2017; 40(4): 569-76.

41. Hall BE, Macdonald E, Cassidy M, Yun S, Sapio MR, Ray P, et al. Transcriptomic analysis of human sensory neurons in painful diabetic neuropathy reveals inflammation and neuronal loss. Sci Rep. 2022; 12(1): 4729.

42. Chowdhury SK, Zherebitskaya E, Smith DR, Akude E, Chat- topadhyay S, Jolivalt CG, et al. Mitochondrial respiratory chain dysfunction in dorsal root ganglia of streptozotocin-induced dia- betic rats and its correction by insulin treatment. Diabetes. 2010;

59(4): 1082-91.

43. Fernyhough P, Roy Chowdhury SK, Schmidt RE. Mitochondrial stress and the pathogenesis of diabetic neuropathy. Expert Rev En- docrinol Metab. 2010; 5(1): 39-49.

44. Ma J, Farmer KL, Pan P, Urban MJ, Zhao H, Blagg BS, et al. Heat shock protein 70 is necessary to improve mitochondrial bioener- getics and reverse diabetic sensory neuropathy following KU-32 therapy. J Pharmacol Exp Ther. 2014; 348(2): 281-92.

45. Ma J, Pan P, Anyika M, Blagg BS, Dobrowsky RT. Modulating molecular chaperones improves mitochondrial bioenergetics and decreases the inflammatory transcriptome in diabetic sensory neurons. ACS Chem Neurosci. 2015; 6(9): 1637-48.

46. Bäckryd E, Themistocleous A, Larsson A, Gordh T, Rice AS, Tesfaye S, et al. Hepatocyte growth factor, colony-stimulating factor 1, CD40, and 11 other inflammation-related proteins are associated with pain in diabetic neuropathy: exploration and rep- lication serum data from the Pain in Neuropathy Study. Pain.

2022; 163(5): 897-909.

47. Thursby E, Juge N. Introduction to the human gut microbiota. Bio- chem J. 2017; 474(11): 1823-36.

48. Vemuri R, Shankar EM, Chieppa M, Eri R, Kavanagh K. Beyond just bacteria: functional biomes in the gut ecosystem including virome, mycobiome, archaeome and helminths. Microorganisms.

2020; 8(4): 483.

49. Tsang DK, Wang RJ, De Sa O, Ayyaz A, Foerster EG, Bayer G, et al. A single cell survey of the microbial impacts on the mouse small intestinal epithelium. Gut Microbes. 2022; 14(1): 2108281.

50. Gao Z, Yin J, Zhang J, Ward RE, Martin RJ, Lefevre M, et al.

Butyrate improves insulin sensitivity and increases energy expen- diture in mice. Diabetes. 2009; 58(7): 1509-17.

51. Edwinson A, Yang L, Chen J, Grover, M. Colonic expression of Ace2, the SARS-CoV-2 entry receptor, is suppressed by commen- sal human microbiota. Gut Microbes. 2021; 13(1): 1984105.

52. Romero R, Zarzycka A, Preussner M, Fischer F, Hain T, Her- rmann JP, et al. Selected commensals educate the intestinal vas- cular and immune system for immunocompetence. Microbiome.

2022; 10(1): 1-18.

53. Kau AL, Ahern PP, Griffin NW, Goodman AL, Gordon JI. Hu- man nutrition, the gut microbiome and the immune system. Na- ture. 2011; 474(7351): 327-36.

54. Pérez-Cobas AE, Gosalbes MJ, Friedrichs A, Knecht H, Artacho A, Eismann K, et al. Gut microbiota disturbance during antibiotic therapy: a multi-omic approach. Gut. 2013; 62(11): 1591-1601.

55. Levy M, Kolodziejczyk AA, Thaiss CA, Elinav E. Dysbiosis and the immune system. Nat Rev Immunol. 2017; 17(4): 219-232.

56. Weiss GA, Hennet T. Mechanisms and consequences of intestinal dysbiosis. Cell Mol Life Sci. 2017; 74(16): 2959-77.

57. Weintraub AS, Ferrara L, Deluca L, Moshier E, Green RS, Oak- man E, et al. Antenatal antibiotic exposure in preterm infants with necrotizing enterocolitis. J Perinatol. 2012; 32(9): 705-9.

58. Cotten CM, Taylor S, Stoll B, Goldberg RN, Hansen NI, Sánchez PJ, et al. Prolonged duration of initial empirical antibiotic treat- ment is associated with increased rates of necrotizing enterocolitis and death for extremely low birth weight infants. Pediatrics. 2009;

123(1): 58-66.