Autophagy, Mitophagy, Calpains and mTOR in Autism, but also in aging, cancer, diabetes, Alzheimer's, Parkinson's, and Huntington's etc.

 Source:  http://www.tanpaku.org/autophagy/

I am writing a science heavy post all about a protein called mTOR.  It is one of those "cancer proteins" that are now heavily researched, very complicated, but clearly very connected to autism.

In today’s lead-in post, that was not supposed to get complicated, I will introduce new terms, Autophagy, Mitophagy and Calpains. 

There are some very interesting implications from the research, not least that you can reduce mTOR levels just by eating (a lot) less.  Indeed, this “starvation” diet has now been shown by the University of Newcastle to be able to reverse the onset of type 2 diabetes.  It also may suggest another reason for those Somali Autism clusters in the US and Sweden, where refugees from Somalia have been settled.  Just as a starvation diet reduces mTOR, excessive eating increases mTOR.  Via several mechanisms we will see that autism associates with high levels of mTOR.  While the hygiene hypotheses can be used to explain these autism “hotspots” among Somali refugees, a completely different reason might be the switch from relative starvation to an overabundant diet; this would trigger an increase in mTOR and therefore the increase in autism (and later diabetes and cancer in the wider group).

In today’s post we will find out about Autophagy/Mitophagy and see how they are relevant to autism.

We will see how they are generally controlled by mTOR.  PINK1, which we encountered in a previous post will reappear, as will Verapamil, that L-type calcium channel blocker that seems to affect so many things.

Not only does verapamil appear protective towards developing type 2 diabetes, but also now Huntingdon’s Disease.

Autophagy

Autophagy is a very complex process.

Here is a relatively plain English explanation:

The word autophagy is derived from Greek words “auto” meaning self and “phagy” meaning eating. Autophagy is a normal physiological process in the body that deals with destruction of cells in the body.

It maintains homeostasis or normal functioning by protein degradation and turnover of the destroyed cell organelles for new cell formation.

During cellular stress the process of Autophagy is upscaled and increased. Cellular stress is caused when there is deprivation of nutrients and/or growth factors.

Thus Autophagy may provide an alternate source of intracellular building blocks and substrates that may generate energy to enable continuous cell survival.

Autophagy and cell death

Autophagy also kills the cells under certain conditions. These are form of programmed cell death (PCD) and are called autophagic cell death. Programmed cell death is commonly termed apoptosis.

Autophagy is termed a nonapoptotic programmed cell death with different pathways and mediators from apoptosis.

Autophagy mainly maintains a balance between manufacture of cellular components and break down of damaged or unnecessary organelles and other cellular constituents.

There are some major degradative pathways that include proteasome that involves breaking down of most short-lived proteins.

Autophagy and stress

Autophagy enables cells to survive stress from the external environment like nutrient deprivation and also allows them to withstand internal stresses like accumulation of damaged organelles and pathogen or infective organism invasion.

Autophagy is seen in all eukaryotic systems including fungi, plants, slime mold, nematodes, fruit flies and insects, rodents (laboratory mice and rats), humans.

Types of autophagy

There are several types of Autophagy. These are:-

·         microautophagy – in this process the cytosolic components are directly taken up by the lysosome itself through the lysosomal membrane.

·         macroautophagy – this involves delivery of cytoplasmic cargo to the lysosome through the intermediary of a double membrane-bound vesicle. This is called an autophagosome that fuses with the lysosome to form an autolysosome.

·         Chaperone-mediated autophagy – in this process the targeted proteins are translocated across the lysosomal membrane in a complex with chaperone proteins (such as Hsc-70).  

·         micro- and macropexophagy

·         piecemeal microautophagy of the nucleus

·         cytoplasm-to-vacuole targeting (Cvt) pathway

Autophagy & Autism

Loss of mTOR-dependent macroautophagy causes autistic-like synaptic pruning deficits

Developmental alterations of excitatory synapses are implicated in autism spectrum disorders (ASDs). Here, we report increased dendritic spine density with reduced developmental spine pruning in layer V pyramidal neurons in postmortem ASD temporal lobe. These spine deficits correlate with hyperactivated mTOR and impaired autophagy. In Tsc2 ± ASD mice where mTOR is constitutively overactive, we observed postnatal spine pruning defects, blockade of autophagy, and ASD-like social behaviors. The mTOR inhibitor rapamycin corrected ASD-like behaviors and spine pruning defects in Tsc2 ± mice, but not in Atg7(CKO) neuronal autophagy-deficient mice or Tsc2 ± :Atg7(CKO) double mutants. Neuronal autophagy furthermore enabled spine elimination with no effects on spine formation. Our findings suggest that mTOR-regulated autophagy is required for developmental spine pruning, and activation of neuronal autophagy corrects synaptic pathology and social behavior deficits in ASD models with hyperactivated mTOR.

Verapamil, Autophagy and Calpains

Here we need to introduce another new term, the calpain.

Hyper activation of calpains is a feature of Alzheimer’s and Huntingdon’s disease.  This does lead to altered calcium homeostasis.

Nobody has really studied calpains and autism.  There is research into calpains and TBI (traumatic brain injury).

Since we know there is aberrant calcium channel activity in autism and even excessive physical calcium present in autistic brains, it seems possible that hyper activation of calpains may be occurring in autism.

We also know that calpains play a role in degrading PTEN, which then affects BDNF, in turn affecting mTOR activation.  So everything is highly interrelated.

Calpain may be released in the brain for up to a month after a head injury, and may be responsible for a shrinkage of the brain sometimes found after such injuries.

However, calpain may also be involved in a "resculpting" process that helps repair damage after injury.

Moreover, the hyperactivation of calpains is implicated in a number of pathologies associated with altered calcium homeostasis such as Alzheimer's disease

  

Calpain-2-mediated PTEN degradation contributes to BDNF-induced stimulation of dendritic protein synthesis

From: Biomarkers of Brain Injury

 Source:-  Stanford Huntingdon Disease (HD) Project

So if it was the case that in autism, as in HD, that there is excessive calpain activity, then it would be possible to increase autophagy simply by reducing the flow of calcium into the cells. 

So this might be yet another reason why Verapamil may be a good therapeutic choice for some people with autism.

Mitophagy & PINK1

Mitophagy is a necessary ongoing “spring cleaning” of damaged bits of mitochondria.

It appears that in some autism, this process goes awry and damaged mitochondria accumulate.

We saw in early posts that in brain samples from younger people with autism, abnormal mitochondria are typically found.

Mitochondrial abnormalities in temporal lobe of autistic brain

Children with autism have mitochondrial dysfunction, study finds

I should point out that there are various types of mitochondrial disease and dysfunction.

It appears that some people’s autism is solely the result of mitochondrial disease, but a much broader group have some mitochondrial dysfunction.

Mitophagy is the selective degradation of mitochondria by autophagy. It often occurs to defective mitochondria following damage or stress. This process was first mentioned by J.J. Lemasters in 2005, although lysosomes in the liver that contained mitochondrial fragments had been seen as early as 1962, “As part of almost every lysosome in these glucagon-treated cells it is possible to recognize a mitochondrion or a remnant of one. It was also mentioned in 1977 by scientists studying metamorphosis in silkworms, “...mitochondria develop functional alterations which would activate autophagy."  Mitophagy is key in keeping the cell healthy. It promotes turnover of mitochondria and prevents accumulation of dysfunctional mitochondria which can lead to cellular degeneration. It is mediated by Atg32 (in yeast) and NIP3-like protein X (NIX). Mitophagy is regulated by PINK1 and parkin protein. The occurrence of mitophagy is not limited to the damaged mitochondria but also involves undamaged ones.

Autophagy and mitophagy in cellular damage control

Mitochondrial dysfunction due to aberrant mTOR-regulated mitophagy in autism

This Mentored Research Scientist Development Award (K01) is designed to characterize the molecular mechanism underlying mitochondrial dysfunction in autism, with the eventual goal of identifying therapeutic interventions for mitochondrial defects. The applicant (Dr. Guomei Tang) is an Associate Research Scientist at Columbia University Medical Center (CUMC), where internationally renowned basic neuroscience research in psychiatry has been ongoing for many years. CUMC provides a rich environment that supports and encourages Dr. Tang's development and this K01 award will be instrumental for her successful transition to an independent research investigator. Dr. Tang has recruited an outstanding team of mentors, co-mentors, consultants and collaborators with extensive experience in mitochondrial biology and diseases, neuropathology, psychiatry neuropathology, neuroscience, molecular and cell biology, and mTOR-autophagy signaling. These experts will provide her with critical guidance and advice, and enhance her technical and scientific skills for the proposed research. The career development activities include tutorials, directed readings, course work, workshops for mitochondrial biology, skills in collaborating with clinicians and senior scientists, grant writing and presentations, and responsible conduct of research. Dr. Tang's long term research goal is to elucidate the molecular and cellular mechanisms underlying synaptic pathology in autism, and to provide insights into the pathogenesis and potential treatment for autism. To accomplish this, Dr. Tang will use a multidisciplinary approach combining biochemical, histological and imaging techniques to examine mitochondrial autophagy in postmortem autistic brain and mouse models. Her preliminary evidence indicates an association between mitochondrial defects and a dysregulation of mTOR-autophagy signaling in autistic brain. In mouse embryonic fibroblasts (MEFs) and neuronal cultures, mTOR hyperactivation inhibits autophagy, decreases mitochondrial membrane potential and causes an accumulation of damaged mitochondria. These results suggest that mitochondrial dysfunction in autism may result from aberrant mTOR- mediated mitophagy signaling. To address this hypothesis, Dr. Tang proposes 3 specific aims: 1) To determine whether mTOR hyper regulation inhibits neuronal mitophagy and causes mitochondrial dysfunction in ASD mouse models;2) To examine whether enhancing mitophagy rescues mitochondrial dysfunction in ASD mouse models; and 3) To confirm mitophagy defects in ASD postmortem brain and lymphoblasts. These data will be important for understanding the mechanism by which mTOR kinase regulates mitophagy, elucidating the mitochondrial pathophysiology that underlies ASD pathogenesis, and ultimately to design interventions effective in treatment. The knowledge and experience gained from this proposal will lead directly to a study of the effects of mitophagy defects and mitochondria dysfunction on synaptic pathology in autism, which will be proposed in an R01 grant application in 3-4 years of the award

Obesity & Autism

Briefly to return to obesity, since I just saw something interesting…

Since we know that over eating with increase mTOR and that hyper-activated mTOR in associated with several dysfunctions in autism, being obese and autistic is not a good idea.

In the US, where potent “psychiatric” drugs are widely prescribed for autism, almost a third of all adolescents with autism are obese, not just over-weight.  Weight gain is a known side effect of some of these drugs.

CDC Study Flags High Rate of Obesity among Teens with Autism

Conclusion

It would appear that hyperactivated mTOR in autism causes dysfunctions in autophagy/mitophagy.  This causes at least two subsequent dysfunctions:-

 ·        Synaptic pruning dysfunction.  There is a post all about this subject.

 Dendritic Spines in Autism – Why, and potentially how, to modify them

 ·        Mitochondrial dysfunction
 

If hyper activation of calpains is occurring in autism, this would explain some of the odd behaviour of Ca²⁺.  It would also again suggest Verapamil for a broader group of autism.

The numerous other connections between mTOR and autism, will be covered in upcoming post on mTOR, which will even include food intolerance.