Abnormal functional connectivity in the right dorsal anterior insula associated with cognitive dysfunction in patients with type 2 diabetes mellitus

Abstract Introduction Type 2 diabetes mellitus (T2DM) is a chronic disease with a high incidence worldwide. T2DM can cause cognitive impairment, but its neuropathological basis is unclear. A variety of neuropsychiatric studies have found that abnormal functional connectivity (FC) in the central executive network (CEN), default‐mode network (DMN), and salience network (SN) may be the neuropathological basis of cognitive dysfunction. The right dorsal anterior insula (dAI) is the core SN area. It plays an important role in regulating the CEN and the DMN. However, few studies have explored the relationship between cognitive impairment and FC among the right dAI, CEN, and DMN in patients with T2DM. Methods Resting‐state functional magnetic resonance imaging was used to investigate FC between the right dAI and the CEN and DMN in 44 patients with T2DM and 41 sex‐, age‐, and education‐matched healthy controls, as well as its relationship with clinical/cognitive variables. Results In patients with T2DM, FC between the right dAI and multiple brain regions of the CEN and DMN was generally decreased, and FC strength between the right dAI and the inferior frontal gyrus negatively correlated with trail making test A score (r = −0.421, p = 0.004). Conclusions Patients with T2DM exhibit abnormal FC between the right dAI and the CEN and DMN. This may be one of the neuromechanisms of cognitive impairment in patients with T2DM. In addition, reduced FC between the right dAI and the right inferior frontal gyrus may be related to abnormal attention regulation in patients with T2DM.


INTRODUCTION
Type 2 diabetes mellitus (T2DM) is a common chronic metabolic disease in middle-aged and older people. It is characterized by longterm hyperglycemia and insulin resistance. At present, the number of affected patients worldwide is as high as 451 million, and the incidence is increasing rapidly (Cho et al., 2018). Patients with T2DM often present with cognitive impairment, such as impairment in episodic memory, attention, and executive function(D. Zhang, Qi, et al., 2020;Qi et al., 2017;X. Zhang et al., 2021), which may increase the risk of Alzheimer's disease (AD) (Crane et al., 2013). Studies have shown that cognitive impairment in patients with T2DM is related to abnormal functional connectivity (FC) in different brain regions (D. Zhang, Qi, et al., 2020;L. Liu, Li et al., 2017;Sun et al., 2018;Tan et al., 2019).
However, the neuropathological mechanisms of cognitive dysfunction in T2DM are still not clear.
Neuroscientists consider that cognitive function relies on interactions between brain regions in multiple neural networks. The central executive network (CEN), default-mode network (DMN), and salience network (SN) play an important role in maintaining normal cognitive function (Menon, 2011). Aberrant organization and interconnectivity of the CEN, SN, and DMN are prominent features of many mental and neurological diseases, such as AD  and mild cognitive impairment (Chand et al., 2017).Triple network model of psychopathology proposes that deficits in engagement and disengagement in the three core neural cognitive networks play a significant role in many mental and neurological diseases, and emphasizes the core position of SN in three cores neural cognitive networks, for initiating network switching, leading to the engagement of CEN and the disengagement of DMN (Menon, 2011).
SN can identify the most salient stimuli among external inputs and internal events, so as to reasonably allocate cognitive resources, coordinating the activation status of the CEN and DMN (Menon & Uddin, 2010;Seeley et al., 2007;Sridharan et al., 2008). In salience processing, the SN is right dominant. Research by Sridaran and colleagues, as well as subsequent work, showed that the right dorsal anterior insula (dAI) plays a key causal role in switching/engaging or disengaging between the two major networks (the CEN and the DMN) and is a key node for initiating network switching/activation (Sridharan et al., 2008;Supekar & Menon, 2012;Uddin et al., 2011). Affected to the right dAI can lead to abnormal saliency processing, resulting in disorderly FC between the SN, CEN, and DMN, affecting cognitive function and behavioral activities Moran et al., 2013;Uddin et al., 2015).
Previous neuroimaging studies have confirmed a reduction in insula gray matter volume and abnormal spontaneous neuronal activity in patients with T2DM (J. Xia et al., 2017). These studies indicate that the insula is one of the most vulnerable brain regions in patients with T2DM. In addition, Liu et al. reported that FC between the right insula and the superior parietal lobule and precentral gyrus/postcentral gyrus of the CEN is decreased (L. Liu, Li et al., 2017). Several studies have demonstrated abnormal FC between the insula and the DMN brain regions (posterior cingulate and medial prefrontal cortex) in patients with T2DM (D. Tan et al., 2019). These studies demonstrated altered FC between the insula and the CEN and DMN regions. Yang et al. used seed points   to explore FC within and between five neural networks, including the   SN, CEN, and DMN, in patients with T2DM. They revealed extensively affected FC between these three networks, and the right insula was the most severely affected node in patients with T2DM and mild cognitive impairment (Yang et al., 2016). However, the region of interest (ROI) chosen in these studies was not the right dAI.
Considering the unique role of the right dAI in salience processing, it is necessary to explore FC between the right dAI and the CEN and DMN to understand the neuropathological mechanism of cognitive impairment in patients with T2DM. Seed-based FC analysis could reveal FC in specific brain regions according to prior anatomical knowledge or activation maps (Lowe et al., 2002), so it can reveal patterns of FC (aberrant or not) with the rest of the brain. Therefore, this study aimed to use the resting-state functional connectivity (rs-FC) method to observe FC between the right dAI and the CEN and DMN in patients with T2DM. We hypothesized that the right dAI in patients with T2DM has abnormal FC with the CEN and DMN, and speculate that there is a correlation between abnormal FC, cognitive scores, and clinical variables. (3) a fasting blood glucose level of <6.1 mmol/L (HCs only). Exclusion criteria for all subjects included (1) a history of acute metabolic complications, severe hyperglycemia episodes (blood glucose >33.3 mmol/L), or severe hypoglycemia episodes (blood glucose <3.9 mmol/L); (2) cerebrovascular accident, tumor, trauma, infection, or congenital abnormal brain development on conventional brain magnetic resonance imaging (MRI), including white matter score greater than grade 2; (3) major medical conditions, such as anemia, cancer, and thyroid dysfunction; (4) psychiatric or neurologic illness that could influence cognitive function, such as severe depression and Parkinson's disease; (5) a history of stroke or alcohol or other substance dependence; (6) minimental state examination (MMSE) score of <24 (HCs only); (7) contraindications to MRI.

Study subjects
The study was performed in accordance with the Declaration of Helsinki, was approved by the ethics committee of Shaanxi Provincial People's Hospital. All subjects were informed of the test content and methods in detail and provided written informed consent before MRI.
All subjects were scanned at 6:00 p.m. on the same day that clinical data were collected and neuropsychological tests were performed. The total scan time of per participant is about 50 min.

Neuropsychological tests
Before functional MRI (fMRI), all subjects completed a detailed standardized cognitive assessment covering multiple cognitive domains.

MRI image acquisition
All MRI data were acquired using a 3.0-T MR scanner (Ingenia, Philips Healthcare, the Netherlands) with a 16-channel phased-array head coil. Subjects were instructed to lie quietly, keep their eyes closed without falling asleep, and avoid thinking of anything during scanning. Soft cushions were placed on both sides of the coil to fix the head and minimize artefacts caused by head movement. Earplugs were given to reduce scanner noise. Conventional brain axial T1-weighted, T2-weighted, and fluid-attenuated inversion recovery imaging were acquired to exclude visible brain lesions. Images were obtained using resting-state fMRI and a gradient-echo planar sequence with the

Image preprocessing
Preprocessing of fMRI data was performed using Data Process-

Region of interest definition and FC analysis
According to prior literature (Deen et al., 2011), ROI with a radius of 6 mm were defined as centered in the right dAI (MNI = 35, 7, 3) in the MNI152 space. The mean time course of the ROI was calculated.
Then, Pearson's correlation coefficients between the mean time series of the ROI and the time series of other voxels in the whole brain for each subject were calculated. Correlation coefficients were converted to Z-values using the Fisher r-to-z transformation to improve normality. Z-values represent the strength of FC between the voxel and the ROI.
In the analysis between groups, the CEN and DMN in GIFT soft-

Clinical data and neuropsychological results
Clinical data and neuropsychological results of patients with T2DM and HCs are presented in Table 1

Intergroup differences in FC
Compared with HCs, FC between the right dAI and the right inferior frontal gyrus, right middle frontal gyrus, and right precuneus/posterior cingulate, bilateral medial prefrontal gyrus, bilateral angular gyrus was decreased in patients with T2DM (GRF correction, p < .05; Table 2 and Figure 1).

Correlation between FC and clinical/cognitive variables
In patients with T2DM, FC strength between the right dAI and the right inferior frontal gyrus was negatively correlated with TMT-A score (r = −0.421, p = .004) after Bonferroni correction only (Figure 2), no significant correlation was found in other cognitive scores and clinical variables. After having been corrected for age, sex, and education, FC strength between the right dAI and the right inferior frontal gyrus was still negatively correlated with TMT-A score (r = −0.536, p = .000).

DISCUSSION
In the current study, we used resting-state fMRI to explore FC between the right dAI and the CEN and DMN in patients with T2DM. The results show that FC between the right dAI and several brain regions of the CEN and DMN was decreased in patients with T2DM. In addition, reduced FC between the right dAI and right inferior frontal gyrus may be related to the patient's attentional deficits.

dAI reduced from CEN regions
The typical pathological basis of cognitive dysfunction in AD is the deposition of β-amyloid and tau proteins (Lemche, 2018). The pathological mechanism of cognitive impairment in T2DM is believed to be similar to that of AD (Bedse et al., 2015). Studies have found that the hub regions in AD patients are more likely to deposit β-amyloid (Crossley et al., 2014;Scheinin et al., 2009;Zhou et al., 2012), and insulin resistance can accelerate this process (Verdile et al., 2015). The anterior insula and the DLPFC (BA9/46) are the core brain regions of the SN and CEN, respectively (Habas et al., 2009), this may be the reason for the abnormal FC between anterior insula and DLPFC in T2DM patients.
The anterior insula and DLPFC constitute a cognitive control network (Cole & Schneider, 2007), which is the neural basis for individuals to effectively identify targets, manipulate and process related information, and guide behavior (Badre, 2011). Decreased FC between F I G U R E 1 Brain regions with decreased functional connectivity in patients with T2DM, corrected by GRF (threshold of p < .001, corrected threshold of p < .05). GRF, Gaussian random field correction; T2DM, type 2 diabetes mellitus F I G U R E 2 Correlation analysis of FC strength between the right dAI and the right inferior frontal gyrus with TMT-A score in patients with T2DM (r = −0.421, p = .004). dAI, dorsal anterior insula; FC, functional connectivity; T2DM, type 2 diabetes mellitus; Trail Making Test-A  (Hu et al., 2020). In this study, the MoCA score of patients with T2DM was significantly lower compared with HCs. Therefore, we speculated that dysfunction in the cognitive control network in patients with T2DM might be one of the reasons for cognitive decline in these patients.
The cognitive control network plays an important role in targetoriented behavior (Wu et al., 2020) and is involved in initiation and control of attention (Cole & Schneider, 2007;Seeley et al., 2007). The dAI is a key brain region for switching between internal and external attention (Sridharan et al., 2008) and the inferior frontal gyrus participates in the top-down attentional control process (Voegler et al., 2016). In the process of attention, the anterior insula is related to detection of salient stimuli and initiation of attention control signals, and the DLPFC is responsible for maintaining these processes (D'Esposito & Postle, 2015;Myers et al., 2017). Decreased FC between the right anterior insula and the inferior frontal gyrus may result in an individual's inability to effectively adjust external attention information (Voegler et al., 2016).
In our study, we found that FC strength between the right dAI and the inferior frontal gyrus negatively correlated with TMT-A score in patients with T2DM. TMT-A is often used to evaluate attentional selection (Baschi et al., 2019). This suggests that decreased FC between the right dAI and the inferior frontal gyrus may be related to abnormal attention adjustment in patients with T2DM.

dAI reduced from DMN regions
Episodic memory is mainly maintained by the DMN (McCormick et al., 2014), in which the posterior cingulate, precuneus, medial prefrontal cortex, and angular gyrus constitute a core network responsible for episodic memory retrieval (recall). The network plays a key role in successfully initiating retrieval and integrating recall content into a cohesive memory representation (Kim, 2010;King et al., 2015;Rugg & Vilberg, 2013). Research indicates that the insula is also involved in the retrieval phase of episodic memory (Daselaar et al., 2001;Kim, 2010;Spaniol et al., 2009). In addition, FC between the right posterior cingulate and the insula is positively correlated with episodic memory performance, both in immediate and delayed recall (Viard et al., 2019). In this study, we found that right dAI had reduced FC with multiple brain regions that make up the episodic memory retrieval network, which may indicate abnormal episodic memory retrieval in patients with T2DM. We compared MMSE and MoCA scores for immediate and delayed memory units between groups, and found that the delayed memory scores of T2DM patients were lower than those of HC group (See Table S2), which confirmed our speculation to a certain extent. Although previous studies have confirmed that abnormalities in episodic memory in patients with T2DM (Sadanand et al., 2016). Our study may provide new insight to explore the neuroimaging mechanisms of episodic memory impairment in patients with T2DM.

Limitations
This study has the following limitations. thus, we will improve the scale in future studies to further confirm our conclusions. Although there was no gender difference between groups in this study, male subjects were significantly more than female subjects in both T2DM and HC groups. Sala et al (2019) showed that gender may have some influence on brain metabolism and neural connectivity. Therefore, we will increase the sample size and balance the gender ratio of each group to in future studies to make the research results more rigorous.

CONCLUSION
In conclusion, decreased FC between the right inferior frontal gyrus in the CEN may underpin functional impairment in attention. Decreased FC between the right dAI and the DMN in multiple brain regions may help us to understand the underlying neuromechanisms of episodic memory impairment in patients with T2DM from another perspective.

CONFLICT OF INTEREST
The authors declare no conflict of interest.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available on requests from the corresponding author.

PEER REVIEW
The peer review history for this article is available at https://publons.