An Important Biological Paradox That May Hold The Key to Understanding Age-Related Diseases Such As Neurodegeneration

By Axol 2015/16 Scholarship Winner, Jonathan C. Grima.

This quote from Nobel laureate Francois Jacob in 1982 highlights an important biological paradox. Despite the complex and intricate developmental process, the human adult organism still goes on to age, senesce, and die. These “sophisticated” developmental programs that give rise to the mature organism inevitably fail to maintain their extraordinary work. What is the evolutionary rationale for senescence or biological aging? What are its fundamental mechanisms? These questions comprise one of the greatest unsolved mysteries in biology, that if unlocked by basic research would yield the greatest dividends for human health.

According to the World Health Organization, some of the top 10 leading causes of death worldwide are heart disease, cancer, and stroke. These conditions claimed the lives of 16.8 million people worldwide in 2012 and resulted in a total of $472.4 billion in US health care and economic costs in 2010 alone. This is $472.4 billion that instead could have been allocated to hospitals and biomedical research. One of the greatest risk factors underlying each of these diseases is biological aging, which is defined as the progressive molecular and cellular changes that lead to an organism’s death. In fact, of the 150,000 individuals worldwide who die each day, roughly 100,000 die due to age-related causes. This makes senescence, albeit indirectly, the leading cause of death worldwide. Unfortunately, why and how people age remain unanswered questions. Addressing this fundamental biological mystery, which underlies many of these diseases, can very well yield the greatest dividends for human health.

A classic example of an age-associated disease is neurodegeneration, which involves the progressive deterioration and death of neurons in the brain. Today, approximately 5 million Americans suffer from Alzheimer’s disease, 1 million from Parkinson’s disease, 400,000 from multiple sclerosis, 30,000 from ALS, and 30,000 from Huntington’s disease. If these are not addressed soon, more than 12 million Americans will suffer from a neurodegenerative disease by 2045. In the United States, the Alzheimer’s Association estimates the medical costs for Alzheimer’s patients to be $200 billion per year and may reach 1.1 trillion per year (in 2012 dollars) by 2050. Alzheimer’s Disease International (ADI) estimates the global cost of dementia to be $604 billion per year, which is roughly 1% of world gross domestic product. A study from the Johns Hopkins Bloomberg School of Public Health estimated that more than 26 million people worldwide were living with Alzheimer’s disease in 2006 and that the global prevalence of Alzheimer’s will grow to more than 106 million by 2050. By that time, 43% of these individuals will need high-level care as what is typically provided by nursing homes. Finally, according to a 2006 report from the NINDS, the cost of Parkinson’s disease in the United States is a little more than $6 billion per year and is estimated to be $23 billion when taking into consideration indirect costs such as lost productivity. Neurodegenerative diseases are without a doubt a tremendous emotional and economic burden worldwide.

As with cancer and stroke, the biology of aging also underlies all neurodegenerative diseases and is the greatest risk factor. The average age of onset for Parkinson’s disease is 60 years, 40 years for Huntington’s, 65 years for Alzheimer’s and 55 years for Amyotrophic Lateral Sclerosis (ALS). Understanding the fundamental aging process will hopefully provide insight into the pathophysiology of these age-related diseases of the brain and will hopefully translate to an extension of life and reduction of costs to society. For example, researchers from Dr. Martin Hetzer’s lab at the Salk Institute recently discovered that some of the longest-lived proteins in the brain are components of the Nuclear Pore Complex (NPC) and may represent the “weakest link” in the aging proteome. NPCs are the largest protein assemblies in eukaryotic cells. They span the nuclear envelope, consist of multiple copies of 30 different proteins called nucleoporins (NUPs), and serve as the only transport conduit between the nucleus and cytoplasm. These molecular machines not only regulate the flow of molecules into and out of the nucleus, but also have transport-independent functions such as regulating genome organization and gene expression. Dr. Hetzer’s team showed that NUPs persist for more than one year in rat brains, which suggests that NPCs can last an entire lifetime in human brains. Conversely, in order to combat functional deterioration, most proteins only last a day or two before being recycled with new functional copies. Declining neuron function may originate in extremely long-lived proteins like those of the NPC that deteriorate due to the accumulation of molecular damage over time. This allows toxins into the nucleus and causes alterations in DNA, loss of youthful gene expression, and cellular aging. As a result, NPC deterioration might be a general aging mechanism leading to age-related defects in nuclear function.

Excitingly, there is now increasing evidence implicating these proteins at the heart of neurodegenerative diseases such as dementia and ALS. For instance, familial and sporadic ALS and Frontotemporal Dementia (FTD) share a common genetic mutation in chromosome 9. An expanded hexanucleotide (GGGGCC) repeat within a non-coding region of the C9orf72 gene is the most common genetic cause of familial and sporadic ALS and FTD. It is also the most common cause of Huntington disease phenocopies. The repeat expansion leads to the loss of one alternatively spliced C9orf72 transcript, pathological inclusions of TDP-43 protein, the formation of nuclear RNA foci and accumulation of cytoplasmic dipeptide repeats. However, the underlying mechanisms by which this bi-directionally transcribed expanded repeat causes these diseases have not been fully elucidated. Nonetheless, it is becoming increasingly evident that NPC dysfunction may be a key pathogenic contributor. More specifically, work from our lab and others has shown that nucleocytoplasmic transport dysfunction is a fundamental pathway for C9orf72 ALS-FTD pathogenesis. The transport of transcription factors between the cytoplasm and the nucleus is a critical aspect of signal transduction and is especially arduous for neurons due to their highly polarized structure. Molecules smaller than 40 kDa are able to transit passively through the NPC. Molecules larger than 40 kDa are actively transported by nuclear transport receptors that recognize nuclear localization and export sequences. During nuclear import, cargo release occurs when the transport receptor interacts with Ran-GTP, a GTP-binding nuclear protein. During nuclear export, cargo is released into the cytoplasm upon GTP hydrolysis of Ran-GTP. This irreversible event is induced by RanGAP, which is a GTPase-activating protein located at the cytoplasmic filaments of the NPC. As a result, a Ran-GTP gradient between the nucleus and cytoplasm is required for this process. Work in our lab demonstrates that proteins in this pathway are potent genetic modifiers of GGGGCC repeat expansion-mediated cytotoxicity in a drosophila model and in iPS neurons derived from C9orf72 patients. Our ongoing studies suggest that products of the C9orf72 repeat expansion are likely to disrupt nucleocytoplasmic transport at the NPC. To this end, we have assessed the integrity of NUPs in C9orf72 human tissue, iPS neurons, and various transgenic animal models. Our preliminary data indicates that select NUPs are severely affected in the disease with aggregation at the nuclear membrane and altered nuclear to cytoplasmic distribution. This suggests NPC pathology and function are a fundamental defect in the pathway of C9orf72 ALS-FTD and provides an excellent demonstration of how studying basic mechanisms of aging, this case in the brain, can reveal a great deal regarding the pathophysiology of disease.

The biology of aging underlies all major human diseases. Unlocking a better understanding of the basic molecular mechanisms involved in senescence will more than likely reveal critical information regarding mechanisms involved in age-related disease just as knowledge of human development has advanced understanding of childhood disease. This will shed light on potential drug targets, such as the NPC, and ultimately lead to delayed onset of disease, improved quality of life, and reduction of associated costs to society.