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Posts Tagged ‘insulin’

Neuroscientists show insulin receptor signaling regulates structure of brain circuits

A team of neuroscientists at Cold Spring Harbor Laboratory (CSHL) has demonstrated for the first time in living animals that insulin receptors in the brain can initiate signaling that regulates both the structure and function of neural circuits.

The finding suggests a significant role for this class of receptors and perhaps for insulin, not only in brain development, but also in cognition and in pathological processes in which cognition is impaired, as in Alzheimer’s disease, for example.

Insulin receptors on the surface of cells throughout the body have long been understood to play a central role in controlling metabolism through the regulation of glucose. When a molecule of insulin, a hormone, “docks” with the receptor, a complex signaling cascade is set in motion inside a cell, culminating in the cell’s uptake of insulin.

The Brain Is Not “Insulin-Insensitive” After All

Although insulin receptors are observed in certain parts of the mammalian brain, most scientists, until a few years ago, had assumed the organ was “insulin-insensitive,” knowing that glucose could be taken up by brain cells without the involvement of either insulin or insulin receptors.

In recent years, however, it has been shown that the brain is indeed an insulin target, and in cell-culture experiments that insulin receptor signaling in neurons can have an impact on the formation and development of neural circuits. This had never been demonstrated in living organisms until it was shown in experiments performed in the laboratory of CSHL Professor Hollis Cline, Ph.D., and reported this week in the journal Neuron.

These experiments, in Xenopus tadpoles, show that insulin receptor signaling in neurons regulates the maintenance of synapses, contributes to the processing of sensory information and is also involved in adjusting the plasticity of brain circuits in response to experience. The latter function is particularly interesting, notes Dr. Cline, since “it is required for the incorporation of neurons into brain circuits.”

Blocking the Receptor

To test the idea that insulin receptor signaling regulates the formation of brain circuits during development, the Cold Spring Harbor team used two different techniques to block the function of the receptor in neurons located in the visual pathway of tadpoles. One method “knocked down” expression of the receptors genetically, while the other left them in place but prevented them from initiating signaling cascades within the cell.

“Tadpoles are wonderful creatures for such experiments,” Dr. Cline explained, “in part because they have translucent bodies, which makes it easy for us to visualize and record what happens to individual neurons as we manipulate the insulin receptors on their surface.”

When insulin receptor function was blocked, neurons in the visual pathway connecting the tadpole’s retina to a brain region called the tectum responded very poorly to light stimuli. The tectum is the area in which brain cells process incoming visual signals. “We showed that the insulin receptor is critical for the proper operation of this circuit, and also that defects in receptor signaling cause a reduction in the animal’s visual responses,” Dr. Cline said.

Time-Lapse Images of Dendritic Branching

The team went on to perform other experiments that demonstrated two remarkable facts. One is that insulin receptor signaling correlates with the density of the synapses, or neuron-to-neuron connections, in brain circuits. In more technical terms, they found that insulin receptors maintain synaptic density and that synapse density decreases when insulin receptors are removed or dysfunctional.

The team also secured time-lapse images of dendritic formations, the ethereal, branch-like structures that receive chemical signals sent from one neuron to the next. Again, they found that when insulin receptors are engaged and sending signals inside the neuron, dendritic growth is enhanced, specifically in response to visual stimulation.

In this, as in the findings about synaptic density, the team found that insulin receptor signaling regulates the form and function of brain circuits in response to incoming visual information. Another way to put this is that the receptor regulates brain circuits in response to “experience.”

Possible Links to Disease

This suggests that insulin receptors in the brain may play a key role not only in the brain’s development early in life, but also in disease processes that usually occur late in life. People with advanced diabetes suffer memory loss and cognitive deficits, possibly because insulin receptor signaling in the brain is disrupted, synaptic connections are lost and brain circuits don’t work optimally.

In addition, other researchers have found a correlation between diminished insulin receptor signaling and Alzheimer’s disease. Results of the Cold Spring Harbor team’s research raise the question of whether deficits in learning and memory associated with Alzheimer’s might be linked causally to decreased synaptic density as a consequence of lowered insulin receptor signaling. “We are a long way from knowing this for sure, but it’s the direction in which our work now takes us,” Dr. Cline said.

Source: Cold Spring Harbor Laboratory

Shu-Ling Chiu, Chih-Ming Chen and Hollis T. Cline. “Insulin Receptor Signaling Regulates Synapse Number, Dendritic Plasticity, and Circuit Function In Vivo”. Neuron. June 11, 2008. doi:10.1016/j.neuron.2008.04.014.

Josh says:

If this turned out to be true, it could have huge implications for our eating habits and offer an even larger incentive to eat healthier other than the risk of type II diabetes.

Lifestyle can alter gene activity, lead to insulin resistance

A Finnish study of identical twins has found that physical inactivity and acquired obesity can impair expression of the genes which help the cells produce energy. The findings suggest that lifestyle, more than heredity, contributes to insulin resistance in people who are obese. Insulin resistance increases the chance of developing diabetes and heart disease.

The study, “Acquired obesity and poor physical fitness impair expression of genes of mitochondrial oxidative phosphorylation in monozygotic twins discordant for obesity,” appears in the online edition of the American Journal of Physiology-Endocrinology and Metabolism, published by The American Physiological Society (www.the-aps.org).

The study was carried out by Linda Mustelin and Kirsi Pietiläinen, of Helsinki University Central Hospital and the University of Helsinki; Aila Rissanen, Anssi Sovijärvi and Päivi Piirilä of Helsinki University Central Hospital; Jussi Naukkarinen, Leena Peltonen and Jaakko Kaprio, University of Helsinki and National Public Health Institute; and Hannele Yki-Järvinen of Helsinki University Central Hospital and Minerva Medical Research Institute.

Environment can influence genes

Recent studies have suggested that defects in expression of genes involved in the body’s conversion of food to energy, known as mitochondrial oxidative phosphorylation, can lead to insulin resistance. The researchers wanted to know if defects in the expression of these genes are primarily a result of heredity or lifestyle. Because the twins in the study were identical, any differences that were found could be attributed to environmental factors, the researchers reasoned.

Twenty four pairs of identical twins, born in Finland between 1975 and 1979, took part in the study. Fourteen pairs (eight male and six female) were discordant for obesity, that is, one twin was obese, while the other was not. The control group consisted of five male and five female twin pairs who were concordant for weight. Some of the concordant pairs were normal-weight while some pairs were overweight.

The researchers measured whole body insulin sensitivity, body composition and cardiorespiratory fitness. They also obtained a needle biopsy of abdominal subcutaneous fat tissue, although they were unable to obtain this measurement for one of the discordant pairs.

Among the discordant pairs, the study found the obese twin had significantly lower:

  • Insulin sensitivity, indicating the body has a harder time using glucose to produce energy.
  • Fitness levels, as measured by maximum oxygen uptake and maximum work capacity.
  • Transcription levels of genes that help cells convert food to energy (the genes of mitochondrial oxidative phosphorylation). Transcription is a multi-step process in which information in the genes is used to manufacture proteins. Proteins, in turn, direct cell activity. This suggests that the impaired expression of the genes may make it more difficult to lose excess weight, or may make additional weight gain more likely.

Heredity may still play role

“These data suggest that physical inactivity may have contributed to the defects in mitochondrial oxidative phosphorylation described in type 2 diabetic patients and prediabetic subjects,” the authors wrote. The authors also noted that, although environment plays a role in how these genes work, there still may be a hereditary component.

“Although we found that the reduced transcript levels of genes encoding mitochondrial oxidative phosphorylation in obesity is influenced by environmental and acquired factors, it does not exclude the possibility that genetic factors contribute to regulation of mitochondrial oxidative metabolism,” lead author Linda Mustelin noted.

The next step is to do a clinical study to see if exercise and other lifestyle changes can increase the expression of these genes.

Source: American Physiological Society

Josh says:

This is certainly possible, since so many things in our body are regulated by feedback mechanisms. Cells, and the body for that matter, are very adaptive. Take for example tolerance to drugs. Though, I’ve always assumed that insulin resistance was dietary more so than hereditary.

Unfortunately, the paper is not yet available.

Bypassing the insulin highway

An immune cell known as a neutrophil releases a protein that can suppress glucose production in the liver –without targeting insulin, researchers have found.

Neutrophils, a type of white blood cell, produce special immune proteins called defensins which seem to have a connection with glucose levels. During bacterial infection, defensin production can increase dramatically, a rise that frequently results in hypoglycemia. In addition, many patients with type II diabetes have decreased defensin levels. … Continue Reading »

Joslin researchers discover new effect for insulin

Researchers at the Joslin Diabetes Center have shown that insulin has a previously unknown effect that plays a role in aging and lifespan, a finding that could ultimately provide a mechanism for gene manipulations that could help people live longer and healthier lives.

The paper, published in the March 21st issue of Cell, reports that insulin inhibits a master gene regulator protein known as SKN-1, and that increased SKN-1 activity increases lifespan. SKN-1 controls what is called the Phase 2 detoxification pathway, a network of genes that defends cells and tissue against oxidative stress – damage caused by elevated levels of free radicals (byproducts of metabolism) – and various environmental toxins. The new finding was demonstrated in experiments on the digestive system of C. elegans, a microscopic worm often used as a model organism.

“We’ve found something new that insulin does and it has to be considered when we think about how insulin is affecting our cells and bodies,” said Dr. T. Keith Blackwell, senior investigator at Joslin and author of the paper. “This has implications for basic biology since under some circumstances insulin may reduce defense against the damaging effects of oxidative stress more than we realize.”

The idea down the line is that fine-tuning the activity of SKN-1 may lead to increased resistance to chronic diseases and influence longevity, he said. The work could be important as it relates to diabetes and the many problems associated with the disease, particularly vascular and renal complications.

But, today’s finding may be most important for what it teaches about aging in general, he said. … Continue Reading »