The main hallmark of type 2 diabetes mellitus (T2DM) is insulin desensitisation. The discovery that the incretin hormone glucagon-like peptide 1 (GLP-1) facilitates insulin release during episodes of hyperglycaemia and has several additional properties to overcome insulin desensitisation made GLP-1 an ideal candidate as a treatment for diabetes (see Bayliss & Starling (1902) and reviews in this special issue). Several drugs have been developed and brought onto the market as treatments, and therefore, GLP-1 is primarily known in the context of diabetes. However, it has become apparent that GLP-1 has additional properties as well, which have not been researched to such a high degree as its properties in maintaining normoglycaemia.

One possibility for the development of neurodegenerative disorders is an impairment in signalling by growth factors such as insulin and IGF1. The desensitisation would reduce vital gene expression for cell repair and growth and could put neurons at an increased risk over time if additional stressors occur ( Holscher 2011 ). Neurons do not divide and cannot be replaced, and most of them live for the duration of a person's lifetime. The amount of actual neurogenesis is far too insufficient to compensate for the loss. This means neurons are exposed to stressors over a long time frame, and damage may accumulate over decades and will finally result in synaptic loss and neuronal dysfunction and ultimately, neuronal death ( Holscher 2011 ).

Initial studies of patients with PD found similar biochemical changes in insulin signalling in brain regions that are relevant to this disease. It was found that the levels of insulin receptor phosphorylation were increased in the basal ganglia and the substantia nigra ( Moroo et al. 1994 ). Furthermore, increased IRS2 phosphorylation, a marker of IGF1 resistance, was found in the basal ganglia of the 6-hydroxydopamine lesion rat model of PD ( Morris et al. 2008 ). Animal studies show similar changes. In a high-fat mouse model of T2DM, learning and memory and synaptic plasticity in the brain were impaired ( Porter et al. 2010b ). In a high-fat-diet rat model of early-stage T2DM, insulin resistance was observed while dopamine release was attenuated and dopamine clearance was diminished in the basal ganglia, indicating that dopaminergic signalling is compromised in T2DM ( Morris et al. 2011 ).

Importantly, biochemical studies of brain tissue demonstrate that insulin signalling in the brain is desensitised in AD patients, and the brain tissue shows a very similar profile to that in diabetic people with respect to insulin signalling biomarkers of desensitisation in the periphery ( Steen et al. 2005 , Lester-Coll et al. 2006 , Talbot et al. 2012 ). In an initial study, insulin receptor levels were found to be phosphorylated and expression levels down-regulated in the brains of patients with AD ( Steen et al. 2005 ). In a histological study of AD brain tissue, IGF1 and insulin receptors were found to be internalised in neurons, and the second messengers insulin receptor substrate 1 (IRS1) and IRS2 were reduced in total levels but had increased levels of phospho- Ser 312 ( Moloney et al. 2010 ). Furthermore, in a recent study, it was found that in brain tissue of AD patients, IGF1 and insulin signalling was strongly desensitised. Phosphorylation of the insulin receptor β chain was reduced at positions IRβ pY 1150/1151 and IRβ pY 960 , while the IRS1 was hyperphosphorylated at positions IRS1 pS 616 and IRS1 pS 636 , which deactivates IRS1 signalling, and IRS1 binding to phosphatidylinositol 3-kinase (PI3K) p85α was also much reduced. In addition, it was found in a AD brain tissue incubation study that treating brain tissue with insulin induced a reduced downstream second messenger activation ( Talbot et al. 2012 ). The observed biochemical changes were very pronounced and also occurred in AD patients that were not diabetic. This type of CNS insulin signalling desensitisation is not dependent on glucose levels.

GLP-1 mimetics have neuroprotective properties

AS T2DM had been identified as a risk factor for AD, the concept developed that drugs that can treat T2DM successfully may also have neuroprotective properties. In diabetes, a range of drugs is on the market or under development, which could be tested for potential neuroprotective properties. As described in this special issue, use of mimetics of the incretin GLP-1 is a successful strategy to treat T2DM (Holst 2004, Drucker & Nauck 2006, Campbell & Drucker 2013). Not only have a range of effective and long-lasting mimetics been developed and tested, three of these have received approval as treatments for T2DM (Madsbad et al. 2011, Elkinson & Keating 2013). In the brain, GLP-1 receptors are expressed by neurons, in particular on pyramidal neurons in the hippocampus and neocortex, and Purkinje cells in the cerebellum (During et al. 2003, Perry et al. 2007, Hamilton & Holscher 2009). Glia cells were not found to express this receptor but induced expression when activated in an inflammatory response (Iwai et al. 2006). In initial studies of synaptic plasticity in the hippocampus, we found that novel GLP-1 analogues such as Val8GLP-1, liraglutide or exendin-4, which are dipeptidyl peptidase 4 protease-resistant and have a much enhanced biological half-life in the body (Holst 2004), have profound effects on memory formation and on synaptic plasticity in the brain (Holscher 2010). In addition, GLP-1 mimetics can protect synapses from the detrimental effects that β-amyloid has on synaptic plasticity in the hippocampus (Gault & Holscher 2008). Most of these novel mimetics can cross the blood–brain barrier (Kastin et al. 2002, Kastin & Akerstrom 2003, McClean et al. 2011, Hunter & Holscher 2012, McGovern et al. 2012), a property that is of vital importance if they are to be used to treat neurodegenerative disorders of the CNS.

GLP-1 is a growth factor, and the neuroprotective effects are most probably due to classic growth factor effects such as increased expression of genes that are linked to cell growth and repair and replacement, increase of cell metabolism, inhibition of apoptosis and reduction of inflammatory responses (see Fig. 2 for details on the underlying molecular mechanisms). Other growth factors have shown similar neuroprotective properties (Bradbury 2005, Kuipers & Bramham 2006). However, most neuroprotective growth factors such as nerve growth factor and brain-derived neurotrophic factor do not cross the blood–brain barrier and therefore have no protective effect in the CNS when injected peripherally (Holscher 2011).

Figure 2 Download Figure

Download figure as PowerPoint slide Overview of the main pathways induced by GLP-1 in neurons. The GLP-1 receptor is a member of a different classes of receptors compared with the IR. Activation of the GLP-1R activates an adenylyl cyclase and increases cAMP levels. This activates PKA and other downstream kinases that are related to growth factor signalling. This may be the reason why GLP-1 mimetics can compensate for insulin desensitisation in diabetics and in AD. For more details, see Holscher (2010) and Holscher & Li (2010). Citation: Journal of Endocrinology 221, 1; 10.1530/JOE-13-0221

Parkinson's disease There are several preclinical studies that have demonstrated neuroprotective effects of exendin-4 in animal models of PD. The protective effects of exendin-4 on neural stem/progenitor cells in the subventricular zone in the rat brain and the beneficial effects in an animal model of PD as well as in cell culture had been tested (Bertilsson et al. 2008). Exendin-4 increased the number of neural stem/progenitor cells in cell culture experiments. Furthermore, in an in vivo experiment, i.p. injection of exendin-4 enhanced the numbers of BrdU-positive progenitor cells in the subventricular zone. Neuronal precursor cell counts were also increased, suggesting that new neurons form that may compensate for the loss of dopaminergic neurons in the substantia nigra (Bertilsson et al. 2008). Exendin-4 was injected i.p. to test its effect in the 6-hydroxy-dopamine (6-OHDA) PD animal model which demonstrates neuronal loss in the substantia nigra. Five weeks after unilateral 6-OHDA lesion, the rats received i.p. injections of extendin-4 for 3 weeks. In a functional test of the dopaminergic system, amphetamine was injected, which that enhances dopamine release in the basal ganglia. A reduction of rotations in the movement of the exendin-4 group demonstrated a reduced functional impairment in this group. The expression of dopamine-synthesis-related enzymes was also elevated in the drug group. This result demonstrates that exendin-4 has cellular and functional beneficial properties in protecting rodents from the loss of dopaminergic neurons and transmission induced by 6-OHDA (Bertilsson et al. 2008). This was confirmed in a second study that employed the 6-OHDA and the LPS-induced substantia nigra injection lesion model of PD, which were used to test the effects of exendin-4. Seven days after inducing the pharmacological lesions, exendin-4 was injected i.p. After 7 days of treatment, amphetamine-induced circling behaviour was reduced in the exendin-4 groups. The levels of dopamine measured in the basal ganglia were also increased. Histological markers also confirmed that dopamine production was increased compared with the lesion-only groups (Harkavyi et al. 2008). An additional study tested exendin-4 in cultured dopaminergic rat neurons. These cells are vulnerable to 6-OHDA exposure. Exendin-4 protected the neurons taken from wild-type mice, but not those taken from GLP-1-receptor-knockout mice. In an in vivo study, exendin-4 protected dopaminergic neurons and rescued motor function in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine lesion mouse model of PD (Li et al. 2009). Patient data analyses also confirmed that a higher percentage of PD patients were diabetic or had elevated levels of glucose compared with age-matched control subjects. It was found that 8–30% of PD patients are diabetic, a significantly higher percentage compared with age-matched controls (Pressley et al. 2003, Aviles-Olmos et al. 2012). Based on the encouraging preclinical studies, a clinical trial testing exendin-4 in PD patients has been conducted (see below).

Amyotrophic lateral sclerosis A different progressive neurodegenerative disorder that may be treated with GLP-1 mimetics is amyotrophic lateral sclerosis (ALS). The dominant symptom is increased motor neuron degeneration in the cortex, the brain stem and spinal cord. As a consequence, patients with ALS show fast developing paralysis and muscle wasting and die within 3–5 years after diagnosis, mainly due to respiratory failure (Kunst 2004). In order to test the effects of exendin-4 in this disease, the drug was tested in NSC-19 neuroblastoma cells and a mouse model of ALS (SOD1 G93A mutant mice). It was found that exendin-4 protected NSC-19 cells and elevated the biomarker for acetylcholine neurotransmission, choline acetyltransferase (ChAT) activity and also protected cells from hydrogen peroxide-induced stress. The SOD1 mutant mice were treated with exendin-4 from 6 weeks of age onwards until the final stage of disease progression. The drug-treated SOD1 mice had near normal motor activity. In histological analyses, extending 4-treated mice had a reduced rate of degeneration of motor neurons in the spinal cord. In immunohistochemical analyses, motor neuron markers such as ChAT were normalised (Li et al. 2012). In a different approach, injection of cells that release GLP-1 into the brains of SOD1 mice, their survival was significantly extended and motor impairment and weight loss were much delayed. In motor activity analysis, an improvement of function was also observed. The chronic inflammatory response in the CNS was also reduced (Knippenberg et al. 2012). These neuroprotective effects of the GLP-1 mimetic exendin-4 support the concept that GLP-1 signalling is neuroprotective and may be a treatment strategy for ALS.

Peripheral neuropathy Peripheral neuropathy is a degeneration of the neurons of the peripheral nervous system and can be induced by a range of causes. In chronic T2DM, peripheral neuropathy is often observed. To test the potential neuroprotective effect of exendin-4, the drug was tested in the diabetic polyneuropathy that is found in the streptozotocin (STZ) animal model of diabetes. STZ is toxic to β cells in the pancreas and reduces insulin production. The effects of GLP-1 (7–37) or exendin-4 were tested in cultured dorsal ganglion neurons from the peripheral nervous system. Both drugs accelerated the neurite outgrowth of cultured ganglion neurons. In the STZ-induced diabetes mouse model, exendin-4 was injected i.p. for 4 weeks. When testing the motor and sensory nerve conduction velocity of peripheral nerves, both GLP-1 (7–37) and exendin-4 protected the conduction of neurons. In behavioural tests, pain perceptions and motor and sensory neuronal conduction were improved by exendin-4. In histological studies, the skin nerve fibre densities were also normalised by exendin-4 (Himeno et al. 2011). Exendin-4 has been tested in a different model of peripheral neuropathy that was induced by vitamin B6 overdose. In anatomical studies, axon sizes were normalised by GLP-1. In motor tasks, the rats were partially protected from the effects of high B6 doses (Perry et al. 2007). Again, GLP-1 signalling has neuroprotective effects on peripheral neuropathy and may be of use in treating patients with such conditions.

Ischaemia and stroke The anti-inflammatory properties and the neuroprotective effects of GLP-1 mimetics indicate that these drugs may be useful in treating stroke victims. Exendin-4 showed good neuroprotection in a transient middle cerebral artery occlusion stroke model in rats. It was found that exendin-4 reduced the brain area that degenerated after the stroke had been induced. In a functional score of motor activity, the drug-treated group performed better (Li et al. 2009). In a transient cerebral ischaemia model in gerbils, the effect of exendin-4 treatment was measured for the hippocampal CA1 region. It was found that GLP-1 receptor expression was increased after 1 day, and GLP-1R immunoreactivity was found not only in pyramidal neurons but also in astrocytes and GABAergic interneurons. Exendin-4 reversed the ischaemia-induced hyperactivity, reduced neuronal loss and also reduced microglial inflammatory activation in a dose-dependent manner (Lee et al. 2011). A further study tested the neuroprotective effect of exendin-4 injected i.v. after a 60-min focal cerebral ischaemia induction. The drug reduced infarct volume and protected the mice from motor impairment. It also reduced oxidative stress, induction of an inflammatory response and neuronal death after reperfusion (Teramoto et al. 2011). In a neuronal cell culture study, exendin-4 showed good neuroprotection under hypoxic conditions. This process was dependent on PKA the kinase that is activated by the GLP-1 receptor via adenylyl cyclase activation (Wang et al. 2012). Taken together, these preclinical studies demonstrate good efficiency of GLP-1 signalling in protecting neurons from stressors and in reducing the inflammatory response in stroke and ischaemia. GLP-1 mimetics therefore show promise in preventing some of the secondary damage that occurs after a stroke or an ischaemic insult.