Beta cell functional changes can be attributed to variations at three levels: (1) cell-autonomous changes in beta cells, such as senescence and stimulus–secretion coupling; (2) changes in beta cell mass and proliferation; and (3) changes in insulin action, such as insulin resistance. In the following sections the potential contribution of each of these will be evaluated in the context of insulin secretion and ageing.
Cellular senescence
Ageing is defined as a time-dependent decline in cellular function. However, different tissues and cell types age through different mechanisms, described as the hallmarks of ageing [20]. Out of the nine hallmarks described, cellular senescence and the senescence-associated secretory phenotype (SASP) are one of the main mechanisms through which beta cells age [2]. By understanding senescent beta cells, we can further understand beta cell ageing. Cellular senescence is a stress response to an array of effects, such as DNA damage, ER stress and oncogene activation. The senescence state is characterised by a lack of cellular proliferation, increased β-galactosidase (βGal) activity and SASP secretion. SASP proteins include soluble and insoluble factors, such as chemokines, cytokines and extracellular matrix remodelling factors. These factors can induce dysfunction in surrounding cells and precipitate their entry into the senescence process [21], and by recruiting immune cells can favour a proinflammatory microenvironment. In addition, senescent cells upregulate anti-apoptotic pathways, which make them resistant to apoptosis. Since it is a stress response, senescence can occur at any time. But with age, cellular stressors increase and the immune response decreases, leading to an accumulation of senescent cells in tissues of aged animals (Fig. 1). We have shown that senescent beta cells accumulate in the islets of aged mice and humans and their proportion is further increased by insulin resistance, high BMI and type 2 diabetes [2]. To further understand the biology of the senescent beta cell subpopulation and its impact on insulin secretion, a beta cell senescence signature was generated through RNA sequencing (RNA-seq) comparing senescent (βGal+) and non-senescent (βGal−) beta cells from the same pool of islets [22]. It revealed a downregulation of beta cell identity genes (Ins1, Pdx1, Mafa, Neurod1), upregulation of genes that are usually suppressed (Cat, Ldha) and increased expression of markers of ageing (Igf1r, Bambi), senescence (p16Ink4a [also known as Cdkn2a] and p21Cip1 [also known as Cdkn1a]) and SASP (Ccl2, Il1a, Il6, Tnf) along with increased βGal activity. Conditions with a higher percentage of senescent beta cells, such as ageing and insulin resistance, were characterised by impaired beta cell function with higher basal insulin secretion.
Changes in stimulus–secretion coupling
The expression of genes involved in glucose stimulus–insulin secretion coupling was compared between senescent and non-senescent beta cells using the same RNA-seq data described above. Senescent beta cells were characterised by a significant downregulation of genes involved in glycolysis, cellular depolarisation (ATP-dependent K+ [KATP] channel, chloride channels, cation channels, voltage-gated calcium channels, sodium channels), incretin pathway receptors and components of insulin granules (Fig. 2a). These transcriptional changes mirror those observed during beta cell ageing, as seen in a microarray analysis of purified beta cells from 1- and 2-year-old MIP-GFP mice in which GFP expression is driven by the insulin gene promoter [16] (Fig. 2b). In both cases, there is a decrease in the transcription of genes involved in glucose metabolism, ionic channels, incretin signalling and insulin synthesis (Fig. 3), all of which predict a functional decline of beta cells with age and senescence. Some of these changes could also explain the functional characteristics of ageing beta cells such as increased basal insulin secretion. For example, decreased KATP channels would hypothetically lead to increased beta cell depolarisation at low glucose concentrations which would be reflected as higher insulin secretion in the basal state. Although this has not been proved experimentally, it represents a novel and interesting mechanism to be explored.
Other studies have provided supporting evidence for age-associated changes in important components of the stimulus–secretion coupling mechanism, such as a decline in the coordination of calcium dynamics, gap junction coupling and insulin secretion with age in humans [11]. Another key player of insulin secretion is mitochondrial activity, which is impaired in aged human islets as measured by NADPH fluorescence lifetime imaging [10, 23]. A study performed in single human beta cells suggested that the mechanistic deficit underlying changes in mitochondrial activity is located upstream of the citric acid cycle (CAC), thus altering the control of mitochondrial membrane potential [24]. Some of these changes in mitochondrial activity could be secondary to a decrease in mitochondrial number since a negative correlation between mtDNA copy number and islet donor age has been described [25]; accordingly, when this degree of mitochondrial depletion was simulated in beta cell-derived MIN6 cells, GSIS was impaired [26].
Another important player in age-related changes of insulin secretion could be insulinotropic hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1). Postprandial secretion of these hormones was shown to be increased in postmenopausal vs premenopausal women [27].
Age-associated changes in beta cell proliferation and mass
Although not a measure of function per se, changes in beta cell mass, due to the balance between proliferation and apoptosis, will have an impact on in vivo measurements of beta cell function.
In humans, beta cell mass in human does not change in response to ageing [28]. In addition, various studies indicate there is a functional reserve of beta cell mass that maintains overall insulin secretion and blood glucose levels unless a threshold is reached. This functional reserve has been estimated as 20–25% in rats [29], while in humans it ranges between 50% and 70% based on data on recent-onset type 1 [30, 31] and type 2 diabetes [32].
However, ageing does decrease the ability of beta cells to proliferate in response to higher metabolic demands in rodents [33] and humans [34], which is translated as a limited regenerative capacity of beta cells. In mice, beta cell regeneration is diminished by 12 months of age [35,36,37,38] and could be related to beta cell senescence [39,40,41,42,43,44]. This is suggested by studies knocking-out p16Ink4a (a marker and effector of senescence), which increased beta cell proliferation in older mice [40]. However, beta cell mass in rodents increases with age [45], which means the actual pool of replicating beta cells is greater in adults than in young animals due a greater number of cells [46, 47].
Taken together, we can conclude that age-related changes in beta cell mass and proliferation do not impact beta cell functional measures in nondiabetic settings.
Age-associated changes in insulin resistance
Peripheral insulin sensitivity plays a key role in determining beta cell function and development of type 2 diabetes. States of insulin resistance brought about by obesity or a sedentary lifestyle are initially accompanied by a compensatory increase in insulin secretion, which, in susceptible individuals, declines and develops into overt diabetes [48]. It is generally accepted that insulin resistance occurs with ageing [49], partly due to the accumulation of senescent cells in the adipose tissue and the subsequent local secretion of SASP, which promote sterile inflammation, a form of pathogen-free inflammation caused by mechanical trauma, ischaemia, stress or environmental conditions [50, 51]. However, some studies have found an improvement in insulin sensitivity with age [4], even in the context of a consistent worsening of insulin secretion. This highlights the heterogeneity of ageing in different individuals, a key concept of the pathophysiology of diabetes underscored by a recent publication that describes five subgroups of adult-onset diabetes, which vary according to the degree of beta cell dysfunction vs insulin resistance [52]. The clinical identification of the subgroup in which an individual lies should be used for personalised treatment.
Other mechanisms of cellular ageing
Changes in other cell types, such as blood vessels and exocrine cells, might also impact insulin secretion [53]. For example, it has been shown that revascularising islets from old mice with blood vessels from young ones restores their functional and proliferative capacity [54]. In addition, circulating factors provide another interesting and thought-provoking mechanism for regulation of insulin secretion. A study using parabiosis showed that the replicative capacity of islets in old mice was restored when parabiosed with young ones [55], suggesting the existence of a circulating factor(s) that confers rejuvenating properties to beta cells.