As demonstrated by this study, taurine ameliorated diabetes-induced axonal injury of spinal nerve, and enhanced axonal regeneration by activating NGF dependent Akt/mTOR pathway. Our study has the characteristics as follows. (1) Taurine ameliorated SC damage in STZ-induced DM rats. (2) Taurine promoted axonal growth of SC, and improved neurological function of DN rats. (3) Taurine enhanced NGF expression level that was lowered in SC of DN. (4) Taurine activated the NGF-dependent PI3K/Akt/mTOR pathway.
DN patients have exhibited various motor dysfunctions such as falling risk increase, gait and balance change, and body sway increase (Muramatsu et al., 2021). According to present studies, motor system dysfunctions such as cerebral cortex and SC may also induce motor dysfunctions of diabetes patients (Ferris et al., 2020; Muramatsu, 2020). DN study give priority to PNS, with CNS ignored relatively. A pilot study first reported that the pathogenesis of diabetic distal symmetrical polyneuropathy lies in the substantial involvement of SC (Eaton et al., 2001). It was also confirmed that SC involvement occurred in DN patients (Selvarajah et al., 2006). With high concentration of glucose, local microenvironment in SC can be changed. Necropsy findings of microvascular disease in SC are similar to those in peripheral nerve, indicating the same pathogenetic mechanisms in both areas and the concomitant processes (Reske-Nielsen and Lundbaek, 1968). As for peripheral motor nerves of diabetes, main distal decrease shows in conduction velocity. According to previous reports, conduction velocity and morphological abnormalities were found at axons of spinal level in diabetic animal models (Muramatsu et al., 2018; Muramatsu, 2020). As revealed by morphological analysis in this study, the axons of SC in diabetic rats were characterized by morphological deformation, irregular arrangement, pits and vacuoles. According to immunofluorescence staining results, there was weaker immunofluorescence intensity and lower SMI312 expression in axons of SC of DN rats. Moreover, by comparing with the rats in control group, diabetic rats exhibited obviously decreased TWL, MWT, and latency to fall. As indicated by the results, there were SCAI and sensory and motor dysfunctions in DN rats.
With taurine supplementation, insulin sensitivity and normalized glycemia, insulinemia, hypertension, and dyslipidemia were improved in T2DM experimental models (Tsuboyama-Kasaoka et al., 2006; El Mesallamy et al., 2010). As discovered in our previous study, by administrating taurine, blood glucose level and nerve conduction function were ameliorated, axonal damage was improved in sciatic nerve of diabetic peripheral neuropathy rats, and DRG neurons were cultured with HG exposure. According to substantiating experimental data, taurine’s protection against several injury mechanisms was demonstrated in CNS (Menzie et al., 2014). In animal models, taurine concentration is altered in SC after injury (McAdoo et al., 1999), and increased when treating SCI (Benton et al., 2001). As clarified by Sobrido-Cameán et al., axonal regeneration was promoted by taurine after a complete SCI in lampreys (Sobrido-Cameán et al., 2020). By administrating taurine, axonal damage was attenuated in SC induced by HG in rats. Furthermore, taurine supplementation enhanced GAP43 level, that was lowered by HG exposure. Referred to as neurodulin, GAP43 is an axon membrane protein, involving in nerve extracellular growth, synaptic development and regeneration. GAP-43 is up-regulated in SC neurons during regeneration (Petruska and Mendell, 2004). As further exhibited by our results, taurine promoted the regeneration and repair of damaged spinal axons and improved the neurological function of DN rats. As suggested by beneficial effects of taurine in DN models, taurine may be a promising candidate for DN treatment.
According to our results, taurine corrected the deregulated NGF level in SC caused by HG exposure. The function of neurotrophins in DN is supported by numerous findings in experimental animal studies and human studies. NGF plays an important role in regulating axon growth and guidance, with the neuroprotective and regenerative effects exhibited. As illustrated by Obrosova et al., taurine counteracts NGF deficiency in early experimental diabetic peripheral neuropathy (Obrosova et al., 2001). According to our previous studies, taurine protected sciatic nerve in diabetic peripheral neuropathy rats against myelin damage by inhibiting apoptosis via NGF (Li et al., 2019a; Wu et al., 2020), with axon outgrowth and GAP-43 expression level enhanced (Zhang et al., 2021).
In addition, further investigation was given to the potential molecular mechanisms responsible for NGF regulation of axon regeneration during DN. According to the previous studies on action mechanism of NGF, the binding of NGF to its receptor TrkA and the neuron survival and neurite outgrowth driving via PI3K/Akt signaling were observed (Yuan et al., 2003; Wang et al., 2016). As known, Akt activation can promote differentiation. Also, mTOR (mammalian target of rapamycin) pathway is activated by the survival of neurons, thereby controlling cell growth and size by protein translation initiation regulation, and further causing extensive axon regeneration in CNS neurons (Park et al., 2010; Li et al., 2022). As demonstrated, Akt/mTOR signaling pathway could be modulated by taurine (Li et al., 2012; Li et al., 2019b). According to our findings both in vitro and in vivo, after taurine administration, phosphorylation of TrkA, Akt, and mTOR were decreased and tended to normal. Besides, as discovered in HG-treated cortical neurons with taurine treatment, the protein expression of GAP43 was attenuated by suppressing NGF, Akt, or mTOR activation, further causing a delay of axon regeneration. This also indicated that taurine up-regulated GAP43 and promoted axon regeneration by activating NGF dependent Akt/mTOR signaling pathway. The mechanism, that taurine up-regulates NGF expression in nervous tissue of DN rats, remains unclear. According to a report, oxidative stress was involved in NGF concentration decrease in diabetic nerve (Obrosova et al., 2001). Moreover, antioxidant therapy inhibited nerve NGF decrease of experimental DN (Garrett et al., 1997). As taurine is a well-established antioxidant, the upregulation of reduced NGF levels in diabetic neural tissue by taurine may be partly attributed to the antioxidant effect.
Neurotrophins are a family of proteins, playing a vital role in regulating growth, survival, and differentiation of neurons in CNS and PNS. As exhibited, the neurotrophic factors such as brain-derived neurotrophic factor (BDNF) and neurotrophin 3 (NT-3) can promote nerve regeneration in damaged nervous tissue (Fang et al., 2017). According to the report of Wu et al., taurine up-regulated BDNF expression in hippocampus of depressive rats (Wu et al., 2017). It is implied that there may also be other nutritional factors involved in taurine, thereby promoting regeneration of spinal axon injury of diabetic rats. Hence, for more accurate explanation of the action mechanism of taurine, it is necessary to further examine effects of taurine on other neurotrophic factors in SC in diabetic rats in the later studies. Besides, in the present study, the absence of controls designed for taurine itself or the intervention reagents themselves may hamper the precise elucidation of the mechanism of beneficial action on taurine. Hence, it is necessary to add these control groups in future studies.
To sum up, our study offers evidence for protective role of taurine administration in attenuating the damage to axons in SC of DN model and in improving the nerve function. As indicated by our results, the mechanism of NGF dependent Akt/mTOR pathway may have relation with the beneficial effect of taurine. In view of its safety record in human clinical trials of diabetes or many other neurodegenerative diseases, taurine may be a promising candidate for central neuropathy improvement of patients with diabetes in the future.