Is sporadic Alzheimer’s disease a form of diabetes?

Observed relationships between Alzheimer’s disease (AD) and diabetes/insulin resistance have long been speculated about and could provide important clues about AD etiology and pathophysiology. The fact that the insulin pathway activation state appears reduced in AD has given rise to speculation that amyloid-beta perturbs insulin signaling, causing a “diabetes” of the brain, and to speculation that a central nervous system manifestation of a general insulin resistance state promotes the production or blocks the elimination of amyloid-beta. Another possibility to consider is that this could all reflect a common upstream feature that gives rise to insulin resistance in the brain, peripheral insulin resistance, and amyloid-beta production. For example, mitochondrial dysfunction is known to cause insulin resistance and to increase processing of amyloid precursor protein to amyloid-beta. Dr. Moreira nicely explains the rationale for trying to treat AD like we treat diabetes, and of summarizing past and recent clinical trials that featured such interventions. These trials have thus far not proved transformative although results from particular trials of particular agents are pending. One caution is that if insulin resistance is contributing to neurodegeneration and neurodysfunction in AD, interventions that address the actual cause of that insulin resistance would seem to make the most sense. If perturbed mitochondrial function is the root cause, the most rationale approach would arguably be to repair or reverse the perturbed mitochondrial function.

As posted by Paula I. Moreira, type 2 diabetes (T2D) constitutes such a public health concern that, recently, the World Health Organization considered it an epidemic of the 21st century [1,2]. This can be further aggravated by the ever-increasing population aging, modern lifestyle-related risk factors (especially the “obesity boom” in the so-called “under development” nations), and morbidity and mortality associated with its severe long-term complications [1,3,4]. Among such chronic complications, one can find severe brain dysfunction/degeneration, cognitive decline and, ultimately, neurodegenerative disorders (including Alzheimer disease, AD) [5-7].

Though it is not surprising that T2D constitutes a risk factor for AD, the opposite (i.e., the higher risk of AD patients to T2D) [8,9] seems to be more striking. Another point focused was that, despite the knowledge on the subcellular commonalities between both pathologies, their precise (inter)actions during T2D progression that culminate in AD remain debatable. On one hand, it seems highly probable that chronic hyperglycemia- and/or insulin resistance-related injury (e.g., on brain neurovascular function) may be further aggravated over patient’s aging, culminating in cognitive decline and AD [6,8,10-12]. As such, insulin could not only constitute a (or the) missing link between both pathologies, but the restoration of its brain levels/action could provide a successful therapeutic strategy [13,14]. But, on the other hand, things have shown not to be so straightforward and the opinions divide between the hypotheses that AD could be a brain-specific, type 3 diabetes, metabolic-cognitive syndrome or insulin-resistant state [15]. Apart from these controversies, it seems more consensual that, given the lack of chronically-efficient or safe anti-AD drugs, and the similarities between T2D and AD, a beneficial anti-T2D (or, at least, anti-hyperglycemic) treatment can be also beneficial against AD [9,16]. This may occur, e.g., by allowing the management/prevention (especially before midlife) or delay of T2D (or, at least, its peripheral features) and the subsequent chronic brain injury [6,17].

Among the anti-T2D drugs (or classes of drugs) with a promising potential against AD, most of them (including insulin itself) have shown contradictory results in patients. Although these can be explained by the different doses used, duration of treatment and parameters analyzed, one can also ask if the recurrent hypoglycemic episodes affecting patients brains under these treatments cannot underlie (at least partially) some of the negative effects reported. If this is true, then the very promising glucagon-like peptide-1 (GLP-1) analogues could be also the most beneficial against AD in clinical trials. This because GLP-1 mimetics were shown not only to readily cross the blood-brain barrier and exert neuroprotective effects in models of T2D and AD [5,8,14], but their hypoglycemic potential was found to be very low [6].

In sum, there is a hope for anti-T2D drugs as one (or the) “hidden secret” for the treatment of AD. But, if the clinical trials fail, a possible future “exit plan” may involve more “sophisticated” medicines, with less recurrent hypoglycemia episodes (or other adverse effects). Alternatively, one can expect that, at least by attenuating peripheral T2D hallmarks, the drugs currently available may blunt its chronic brain damage and, ultimately, the complications affecting the central nervous system.

References
[1] World Health Organization (2016) Diabetes fact sheet 312, revised June 2016 (available online: http://www.who.int/mediacentre/factsheets/fs312/en/). Accessed June 26, 2016.
[2] NCD Risk Factor Collaboration (NCD-RisC) (2016) Worldwide trends in diabetes since 1980: a pooled analysis of 751 population-based studies with 4.4 million participants. Lancet 387, 1513-1530.
[3] de Visser A, Hemming A, Yang C, Zaver S, Dhaliwal R, Jawed Z, Toth C (2014) The adjuvant effect of hypertension upon diabetic peripheral neuropathy in experimental type 2 diabetes. Neurobiol Dis 62, 18-30.
[4] Shaw JE Sicree RA, Zimmet PZ (2010) Global estimates of the prevalence of diabetes for 2010 and 2030. Diabetes Res Clin Pract 87, 4–14.
[5] Candeias EM, Sebastião IC, Cardoso SM, Correia SC, Carvalho CI, Plácido AI, Santos MS, Oliveira CR, Moreira PI, Duarte AI (2015) Gut-brain connection: The neuroprotective effects of the anti-diabetic drug liraglutide. World J Diabetes 6, 807-827.
[6] Duarte AI, Candeias E, Correia SC, Santos RX, Carvalho C, Cardoso S, Plácido A, Santos MS, Oliveira CR, Moreira PI (2013) Crosstalk between diabetes and brain: glucagon-like peptide-1 mimetics as a promising therapy against neurodegeneration.Biochim Biophys Acta 1832, 527-541.
[7] Sima AA (2010) Encephalopathies: the emerging diabetic complications. Acta Diabetol 47, 279-293.
[8] Sebastião I, Candeias E, Santos MS, de Oliveira CR, Moreira PI, Duarte AI (2014) Insulin as a bridge between type 2 diabetes and Alzheimer disease - how anti-diabetics could be a solution for dementia. Front Endocrinol (Lausanne) 5, 110.
[9] Cai H, Cong WN, Ji S, Rothman S, Maudsley S, Martin B (2012) Metabolic dysfunction in Alzheimer's disease and related neurodegenerative disorders. Curr Alzheimer Res 9, 5-17.
[10] Carvalho C, Katz PS, Dutta S, Katakam PV, Moreira PI, Busija DW (2014) Increased susceptibility to amyloid-β toxicity in rat brain microvascular endothelial cells under hyperglycemic conditions. J Alzheimers Dis 38, 75-83.
[11] Duarte AI, Santos MS, Seiça R, Oliveira CR (2004) Oxidative stress affects synaptosomal gamma-aminobutyric acid and glutamate transport in diabetic rats: the role of insulin. Diabetes 53, 2110-2116.
[12] Moreira PI, Santos MS, Moreno AM, Seiça R, Oliveira CR (2003) Increased vulnerability of brain mitochondria in diabetic (Goto-Kakizaki) rats with aging and amyloid-beta exposure. Diabetes 52, 1449-1456.
[13] Duarte AI, Moreira PI, Oliveira CR (2012) Insulin in central nervous system: more than just a peripheral hormone. J Aging Res 2012, 384017.
[14] Hunter K, Hölscher C (2012) Drugs developed to treat diabetes, liraglutide and lixisenatide, cross the blood brain barrier and enhance neurogenesis. BMC Neurosci 13, 33.
[15] Frisardi V, Solfrizzi V, Seripa D, Capurso C, Santamato A, Sancarlo D, Vendemiale G, Pilotto A, Panza F (2010) Metabolic-cognitive syndrome: a cross-talk between metabolic syndrome and Alzheimer's disease. Ageing Res Ver 9, 399-417.
[16] Corbett A, Ballard C (2013) Is a potential Alzheimer's therapy already in use for other conditions? Can medications for hypertension, diabetes and acne help with the symptoms? Expert Opin Investig Drugs 22, 941-943.
[17] Lin B, Koibuchi N, Hasegawa Y, Sueta D, Toyama K, Uekawa K, Ma M, Nakagawa T, Kusaka H, Kim-Mitsuyama S (2014) Glycemic control with empagliflozin, a novel selective SGLT2 inhibitor, ameliorates cardiovascular injury and cognitive dysfunction in obese and type 2 diabetic mice. Cardiovasc Diabetol 13, 148.

Neither is necessarily a form of the other per se. Just as Alzheimer himself once said that an infectious process in the CNS can have the same cause and yet land on two different places in the brain creating two different brain disorders [1], it makes more sense if both Alzheimer’s disease and diabetes are the result of the same underlying ongoing amyloid-producing systemic pathology involving two different organ-systems in the body.

As far back as 2006, it became obvious that epidemiological studies were finding a link between diabetes, Alzheimer disease, and dementia [2]. Shock waves from the Madrid Alzheimer's meeting sent scientists scurrying for an explanation for how a Seattle VA study showed that diabetes could lead to a nearly 70% increase in Alzheimer's disease. Why this posed a particularly challenging problem was that it left the possibility of the explosion of diabetes with approximately 20 million in the US already having the disease and 40 million prediabetics, closing ranks with the rapidly growing Alzheimer’s victims. If this were to occur, such a pool would vastly escalate the number of the 4.5 million American Alzheimer's sufferers at the time.

A Puget Sound Seattle VA study showed subtle improvement in Alzheimer's patients placed on insulin mist. As a result researchers began to reemphasize that diabetes not only attacks the body but the mind. Conclusion: These results supported longer trials of intranasal insulin therapy for patients with amnestic mild cognitive impairment and patients with AD [3].Yet another study concluded that among well-functioning older adults, DM and poor glucose control among those with DM are associated with worse cognitive function and greater decline. This suggested that the severity of DM might contribute to accelerated cognitive aging [4].   

As studies continued to appear, the one thing that most agreed upon was that as blood sugar control in diabetes worsens, Alzheimer's risk climbed.

While some speculated that diabetic's insulin deficient or resistant cells led to a cut-off of vital blood sugar to brain neurons, disabling their ability to remove clumps of incapacitating Alzheimer's amyloid –others thought it was a one-two punch: Alzheimer's tendency to attack cell's mitochondria or energy factories, leading to a neuronal death facilitated by diabetes's own attack on the brain's neurons. Or perhaps, although it was a not yet proven – there was a link between diabetes and Alzheimer disease due to malfunctioning of a gene that is involved in the function of the insulin-degrading enzyme (IDE). There were even those who, as a result of such studies, now consider diabetes as a precursor for Alzheimer's [2].

But to pathologist Phillip Schwartz, lead research pathologist for the State Facility in Warren Pennsylvania it was none of these. After a 50-year-autospsy-driven study of cases ranging from age 16 through 87, Schwartz published a report of 331 autopsied cases of diabetes, finding amyloidosis of the pancreas in 224 out of the 331 autopsies with the same chronic mycobacterial infectious focus in the body, either active or quiescent, that he had documented for a similar amyloid-generating process going on in autopsy after autopsy on Alzheimer’s brains [5].

Schwartz, who was the originator of the Thioflavin-S dye used to detect amyloid, reported that in the case of diabetes, most of those diagnosed as diabetic prior to death showed intense islet cell amyloidosis.  And Schwartz hypothesized that once amyloidosis of the pancreatic islet cells hit a critical mass, the result was diabetes mellitus. Thus most cases of pancreatic amyloidosis, as well as the inflammatory infiltrate of the islet cells characteristic of Juvenile diabetes, and Alzheimer's, ought to be considered one and the same immunopathy – from, in all but a few cases, a foci of chronic mycobacterial infection in the body, which he found in just about every case in which he did a thorough and exhaustive examination.

Schwartz once said this regarding infectious amyloidosis: “Our investigations have disclosed amyloidosis to be one of the most frequent diseases of the human species, and cerebral amyloidosis, because of its enormous incidence in the aged, to be the most important condition in neuropathology.” Yet the fact that it could hit the pancreas with its islet cells was to him also a given. One immunologic process of infectious origin, two diseases.

REFERENCES

1.      Alzheimer A (1911) On certain peculiar diseases of old age. Zeit fur die Ges Neur und Psych 4: 356-485.

2.      Susman E. News From the International Conference on Alzheimer’s Disease and Related Disorders Neurology Today. 19 September 2006  6:18: 25-26.

3.      Craft S, Baker LD, Montine TJ, Minoshima S, Watson GS, Claxton A, Arbuckle M, Callaghan M, Tsai E, Plymate SR, Green PS, Leverenz J, Cross D, Gerton B. Intranasal insulin therapy for Alzheimer disease and amnestic mild cognitive impairment: a pilot clinical trial. Arch Neurol. 2012 Jan; 69(1):29-38.

4.      Yaffe K, Falvey C, Hamilton N, Schwartz AV, Simonsick EM, Satterfield S, Cauley JA, Rosano C, Launer LJ, Strotmeyer ES, Harris TB. Diabetes, glucose control, and 9-year cognitive decline among older adults without dementia. Arch Neurol. 2012 Sep;69(9):1170-5

5.      P. Schwartz, Amyloidosis—Cause and Manifestation of Senile Deterioration (Springfield, Illinois: Charles C Thomas, 1970), 363pp