3.3: Changes in Protein Shape Can Cause Disease

While the conformation of a protein determines its biological function, an allosteric (shape) change can moderate or disrupt its function. Under normal circumstances, cells use changes in protein shape to regulate metabolism. Such allosteric regulation is well documented in familiar biochemical pathways such as glycolysis and is discussed in more detail elsewhere. Less well understood is how (or why) conformational change in some protein’s cells can have devastating effects.

3.3.1. Sickle Cell Anemia

Mutations of globin genes can cause hemoglobin disorders characterized by inefficient oxygen delivery by blood. In the 1940s, the British biochemist J.B.S. Haldane studied southern African regions and made a correlation between high incidences of hemoglobin disorders and low incidences of malaria, suggesting that heterozygous individuals (i.e., those that had only one copy of a mutant hemoglobin gene) were somehow protected from malaria. Sickle cell anemia is a well-known example of a hemoglobin disorder and is caused by a single base change in the gene for human \(\beta\)−hemoglobin (one of the polypeptides in hemoglobin). Since red blood cells are rich in hemoglobin, the abnormal shape of the \(\beta\)−hemoglobin can cause the cells themselves to become sickle shaped. Sickle cells disrupt capillary flow and oxygen delivery, causing the symptoms of anemia.

While sickle cell anemia originated in Africa, it probably spread to the United States because of the slave trade. It may even have abetted the slave trade. Europeans exploiting their African colonies’ natural resources were dying of malaria, but African natives seemed unaffected. Europeans, having brought stowaway malarial mosquitos to the new world in the first place, figured that enslaved Africans would survive the illness in the Americas.

We know now that individuals heterozygous for the mutant \(\beta\)−hemoglobin suffer sickle-cell trait and are generally unaffected, because at least some of their hemoglobin is normal. Homozygous individuals that make only the sickle-cell \(\beta\)−hemoglobin variant suffer more frequent and more severe episodes of the disease. Stressors that can trigger sickling include infection or dehydration.

Compare normal red blood cells to a sickle cell below in Figure 3.7.

Figure 3.7: Light micrograph of a sickled erythrocyte (the red blood cell at the top)

The sickle-cell gene mutation affects perhaps more than one hundred million people worldwide, including 8–10% of African Americans. For more demographic information, see Sickle Cell Trait Demographics and Sickle Cell Data. In Africa, heterozygotes with sickle-cell trait are protected from malaria, confirming Haldane’s hypothesis. But patients homozygous for the \(\beta\)−hemoglobin mutation derive little benefit from its antimalarial effect.

In the meantime, despite a 33% reduction in cases of malaria in recent years, this disease, caused by a mosquito-borne parasite, still threatens half of the people on the planet, causing over a half-million deaths per year. There are treatments (other than mosquito nets and killing mosquitos), but at this time there is still no preventive vaccine.

3.3.2. The Role of Misshapen and Misfolded Proteins in Alzheimer's Disease

Prion proteins, when first discovered, seemed to behave as infectious agents that could reproduce without DNA or other nucleic acid. As you can imagine, this highly unorthodox and novel hereditary mechanism generated its share of controversy. Read about research on the cellular prion (\(\rm PrP^c\))protein at Wikipedia Take on Prions. Of course, prions turned out not to be reproductive agents of infection after all. Recent studies of prions have revealed several normal prion protein functions, including roles in memory formation in mice and sporulation in yeast(check out Prion Proteins and the Formation of Memory).

A mutant version of the prion protein (\(\rm PrP^{\rm Sc}\)) can mis-fold and to take on an abnormal shape. The deformed \(\rm PrP^{\rm Sc}\) can then induce abnormal folding in other, normal \(\rm PrP^c\) molecules. These events, illustrated in Figure 3.8, result in the formation of so-called amyloid plaques.

Figure 3.8: Mutations in the gene for the normal prion \(\rm PrP^c\) protein (left) may produce some abnormally folded prion \(\rm PrP^{Sc}\) proteins(at the right). The misfolded \(\rm PrP^{Sc}\) molecules interact with other (even normal) prions, causing them to mis-fold, precipitating \(\rm PrP^{Sc}\) proteins into aggregated amyloid plaques.

Abnormally folded prions (\(\rm PrP^{Sc}\)) have been associated with Alzheimer’s disease, which affects about 5.5 million Americans. \(\rm PrP^{Sc}\) is also associated with mad cow disease and Creutzfeldt-Jakob disease (mad cow disease in humans), as well as scrapie in sheep, among others. We are beginning to understand that the role of prion proteins in Alzheimer’s disease is less causal than indirect.

3.3.2.a The Amyloid Beta (\(A_{\beta}\)) peptide

Postmortem brains from patients Alzheimer’s disease patients show the characteristic extracellular amyloid plaques to be composed largely of the beta-amyloid (\(A_{\beta}\)) peptide. In affected cells, an enzymatic digest of the APP (amyloid precursor protein) generates thirty-nine to forty-three extracellular amino acid (\(A_{\beta}\)) peptides.

Normally, excess (\(A_{\beta}\)) peptides are themselves digested. However, an unregulated (\(A_{\beta}\)) peptide accumulation leads to the formation of the beta-amyloid plaques seen in Alzheimer’s disease (Figure 3.9).

Figure 3.9: APP (amyloid precursor protein) on cell membranes (left) are enzymatically cleaved to release beta amyloid peptide fragments (middle) that aggregate to form beta amyloid plaques that characterize Alzheimer's disease

The scissors in the illustration represents two enzymes that digest the APP. Prion proteins are not a proximal cause of Alzheimer’s disease but may have a role in initiating events that lead to it. Normal prion protein (\(textbf{\rm PrP^c}\)) is itself a membrane receptor and is thought to bind(\(A_{\beta}\))peptides, effectively preventing their aggregation into plaques. An experimental reduction of \(textbf{\rm PrP^c}\) was shown to increase the extracellular (\(A_{\beta}\)) peptides. Presumably, prion protein aggregation induced by the mutant PrP protein(\(textbf{\rm PrP^{Sc}}\)) prevents prion proteins from binding to (\(A_{\beta}\)) peptides, leading to (\(A_{\beta}\)) peptide accumulation and ultimately to amyloid plaque formation and neurodegeneration.

3.3.2.b The Tau Protein

The tau protein called is also associated with Alzheimer’s disease. MAP-T (Microtubule-Associated Protein-Tau) is as its name indicates, is one of several microtubule-associated protens (MAPS). In normal neurons, MAP-T is phosphorylated, and the phosphorylated MAP-T binds to, and stabilizes microtubules. But the misshapen tau that accumulates in neurofibrillary tangles in hippocampus brain neurons may be a more immediate cause of the neuronal dysfunction associated with Alzheimer’s disease than (\(\textbf{A_{\beta}}\)) peptides.

When neuronal tau becomes hyperphosphorylated, its conformation changes, making it unable to associate with microtubules. No longer stabilized, the microtubules disassemble and the deformed tau proteins form neurofibrillary tangles. Immunostaining of hippocampal neurons with antibodies against tau protein localizes the neurofibrillary tau protein tangles, as seen in the light micrograph below (Figure 3.10).

Figure 3.10: Immunofluoresce micrograph of tau proteins in brain cells of an Alzheimer’s brain; pointers indicate tau tangles.

The formation of neurofibrillary tau protein tangles in a diseased neuron is compared to normal neurons in Figure 3.11. The tangled clumps of tau proteins in this illustration are what appear as deep purple in Figure 3.10.

Figure 3.11: The formation of tau tangles (lower left) occurs in diseased neurons that cannot maintain normal microtubule structure (upper middle). In the absence of stabilizing tau molecules, microtubules degenerate and tau proteins clump (lower right).

There is no cure yet for Alzheimer’s disease, although treatments with cholinesterase inhibitors seem to slow its advancement. For example, the drug Aricept inhibits acetylcholine breakdown by acetylcholinesterase, thereby enhancing cholinergic neurotransmission, which may in turn prolong the brain’s neural function.

Sadly too, there is as yet no treatment to restore lost memories or the significant cognitive decline associated with Alzheimer’s disease. Perhaps more promising in this respect is a recent development. Proteins or peptides associated with Alzheimer’s disease have been detected in the blood and serum. Amyloid beta-(\(A_{\beta}\)) and/or tau-protein fragments that escape into the blood stream can be detected six to eight (or more) years before Alzheimer’s symptoms appear. A neurofilament light chain (NfL) that is seen in a familial form of Alzheimer’s disease (among other neuropathies) is detectable sixteen years before the symptoms!

The ability to detect these marker proteins and Alzheimer’s-associated peptides so far in advance of symptoms raises hopes for early monitoring of at-risk individuals and for new therapies for Alzheimer’s disease. For brief reviews, see Early Detection of Circulating Ab Peptides and Early Detection of Circulating Tau Peptides. For a recent report on tracking the NfL protein, see Early Detection of Circulating Neurofilament Light-Chain Protein.

3.3.2.c Some Relatives of Alzheimer’s Disease

Some of the same protein abnormalities seen in Alzheimer’s disease are also seen in other neurodegenerative diseases as well as traumatic brain damage. An abnormal accumulation of tau protein is diagnostic of Chronic Traumatic Encephalopathy (CTE).

First described in the early twentieth century, disoriented boxers staggering about after a fight were called “punch drunk,” suffering from dementia pugilistica. We now know they suffered from CTE, as do other athletes exposed to repetitive mild-to-severe brain trauma, such as football players. Immunostaining of whole brains and brain tissue from autopsied CTE patients using antibodies to tau protein, show accumulations of abnormal tau proteins and tau neurofibrillary tangles very much like those found in Alzheimer’s patients.

Many National Football League and other football players have been diagnosed postmortem with CTE, and many still living show signs of degenerative cognition and behavior consistent with CTE. (See a 2020 List of NFL players with CTE to see how many!)

Parkinson’s disease is yet another example of a neurodegenerative disease that results when a single protein changes shape in brain cells. Though they are not characterized as plaques, aggregates can form in brain cells when the protein alpha-synuclein undergoes anomalous conformational change. The change results in MSA (Multiple System Atrophy) or Parkinson’s disease. To read details of this recent research and how it may lead to treatments, see Targeting Alpha Synuclein for Parkinson’s Therapy.

Much of the high-resolution electron microscopy that can reveal details of the structure and conformational changes in proteins came from the work of Jacques Dubochet, Joachim Frank, and Richard Henderson, who shared the 2017 Nobel Prize for developing and refining cryoelectron microscopy for biomolecular imaging (For details, see Nobel Prize-Cryoelectron Microscopy)