Role of insulin in Alzheimer's disease:approaches emerging from basic animal research and neurocognitive studies in humans

Insulin signaling in the central nervous system, a possible pathophysiological mechanism of anesthesia-induced delayed neurocognitive recovery/postoperative neurocognitive disorder: a narrative review

INTRODUCTION

Impairment in neurocognitive functions ranges between delayed neurocognitive recovery (DNR) and postoperative neurocognitive disorders (pNCD). Incidence varies from 11% after noncardiac surgery to 60% after cardiac surgery.

AREAS COVERED

Insulin receptors (IRs) signaling pathway in the central nervous system (CNS) could be a possible pathophysiological mechanism of anesthesia-induced DNR/pNCD and perioperative intranasal insulin administration could be a preventive approach. This hypothesis is supported by the following evidence: effects of IRs-CNS signaling pathway on neuromodulation; higher incidence of DNR/pNCD in patients with insulin resistance; neurotoxicity of IRs signaling pathways after anesthetic exposure; improvement of neurocognitive impairment after insulin exposure. This narrative review was conducted after a literature search of PubMed, EMBASE and SCOPUS online medical data performed in May 2022.

EXPERT OPINION

Perioperative intranasal insulin is shown to be protective and future studies should address: the role of insulin as a neuromodulator; its integration into neuroprotection approaches; patient populations that might benefit from this approach; a well-defined protocol of intranasal insulin administration in a perioperative background and other disciplines; and possible collateral effects.

Reactivation of atrophic neurons in Alzheimer's disease

Decreased metabolic rate may precede cognitive impairment in Alzheimer's disease (AD) and is thus an early occurring hallmark. Several observations in post-mortem brain indicate that activated neurons are better able to withstand aging and AD, a phenomenon paraphrased by us as 'use it or lose it'. Moreover, a number of pharmacological and nonpharmacological studies support the concept that activation of the brain has beneficial effects and may to a certain degree restore several aspects of cognition and other central functions. For instance, the circadian system may be restimulated in Alzheimer patients by exposing them to more light or transcutaneous nerve stimulation. A procedure has been developed to culture human post-mortem brain tissue that allows testing of the efficacy of putative stimulatory compounds such as neurotrophins.

Branched-chain amino acids may improve recovery from a vegetative or minimally conscious state in patients with traumatic brain injury: a pilot study

OBJECTIVE

To investigate whether supplementation with branched-chain amino acids (BCAAs) may improve recovery of patients with a posttraumatic vegetative or minimally conscious state.

DESIGN

Patients were randomly assigned to 15 days of intravenous BCAA supplementation (n=22; 19.6g/d) or an isonitrogenous placebo (n=19).

SETTING

Tertiary care rehabilitation setting.

PARTICIPANTS

Patients (N=41; 29 men, 12 women; mean age, 49.5+/-21 y) with a posttraumatic vegetative or minimally conscious state, 47+/-24 days after the index traumatic event.

INTERVENTION

Supplementation with BCAAs.

MAIN OUTCOME MEASURE

Disability Rating Scale (DRS) as log(10)DRS.

RESULTS

Fifteen days after admission to the rehabilitation department, the log(10)DRS score improved significantly only in patients who had received BCAAs (log(10)DRS score, 1.365+/-0.08 to 1.294+/-0.05; P<.001), while the log(10)DRS score in the placebo recipients remained virtually unchanged (log(10)DRS score, 1.373+/-0.03 to 1.37+/-0.03; P not significant). The difference in improvement of log(10)DRS score between the 2 groups was highly significant (P<.000). Moreover, 68.2% (n=15) of treated patients achieved a log(10)DRS point score of .477 or higher (3 as geometric mean) that allowed them to exit the vegetative or minimally conscious state.

CONCLUSIONS

Supplemented BCAAs may improve the recovery from a vegetative or minimally conscious state in patients with posttraumatic vegetative or minimally conscious state.

Age- and sex-related reference values for serum insulin concentration and its biological determinants in a French healthy population. The STANISLAS cohort

Insulin is involved in coronary heart disease through diabetes and metabolic syndrome. A great deal is known about insulin and its correlates, as well as factors related to changes in insulin. However, few studies consider the broad variety of correlates simultaneously. Therefore, the aims of the present study were to characterize the main factors of biological variation affecting serum insulin concentration and to establish reference limits of insulinemia in a presumably healthy French population. Insulin was measured using a microparticular enzymatic immunoassay. A total of 646 subjects aged 11–58 years from the STANISLAS cohort and divided into four groups of 162 males, 157 females, 163 boys and 164 girls, were included in the statistical analyses. In the whole population, serum insulin concentration varied from 0.80 to 54.60 µU/ml. Significant factors affecting insulin were age, gender, body mass index and glucose, in addition to alanine aminotransferase and high-density lipoprotein cholesterol in men, triglycerides and oral contraceptive use in women, and alkaline phosphatase in girls.

In summary, we presented biological correlates of insulin in both healthy French male and female adults and children/adolescents and determined reference limits for insulin for each group. These results will contribute to a better interpretation of insulin data in further studies and laboratory investigations.

Therapeutic strategies for Alzheimer disease: focus on neuronal reactivation of metabolically impaired neurons

Based on several lines of evidence, it has been hypothesized that decreased neuronal metabolic rate may precede cognitive impairment, contributing to neuronal atrophy as well as reduced neuronal function in Alzheimer disease (AD). Additionally, studies have shown that stimulation of neurons through different mechanisms may protect those cells from the deleterious effects of aging and AD, a phenomenon we paraphrased as "use it or lose it." Therefore, it is attractive to direct the development of therapeutic strategies toward stimulation of metabolic rate/neuronal activity to improve cognition and other symptoms in AD. A number of pharmacological and nonpharmacological approaches discussed here support the concept that stimulation of the brain has beneficial effects and may, to a certain degree, restore several aspects of cognition and other central functions. For instance, the circadian system, which controls the sleep/wake cycle, may be stimulated in AD patients by exposing them to more light or transcutaneous nerve stimulation. We will also discuss a procedure that has been developed to culture human postmortem brain tissue, which allows testing of the efficacy of putative stimulatory compounds.

Potential roles of insulin and IGF-1 in Alzheimer's disease

Aging is characterized by a significant decline of metabolic and hormonal functions, which often facilitates the onset of severe age-associated pathologies. One outstanding example of this is the reported association of deranged signaling by insulin and insulin-like-growth-factor 1 (IGF-1) with Alzheimer's disease (AD). Recent compelling biological data reveal effects of insulin and IGF-1 on molecular and cellular mechanisms underlying the pathology of AD. This review discusses available biological data that highlight the therapeutic potential of the insulin-IGF-1 signaling pathway in AD.