In recent years it has become evident that impaired protein folding plays a key role in a wide variety of diseases.
There are considerable overlaps between these categories and in many cases, it is not clear which of the mechanisms that are the dominating cause of pathology. It is therefore important to have a holistic view of the folding and misfolding areas to understand the molecular basis underlying these diseases.
Common denominators for these proteins are: Aberrant folding, partially folded intermediates, conformational plasticity and protein aggregation.
Amyloid disease
The amyloid diseases is a part of a larger group of conformational diseases, many of which are termed misfolding diseases. This group includes for example devastating diseases such as Alzheimer’s disease, Parkinson’s disease and Huntington’s disease.
Neurodegeneration is associated with amyloid diseases of the central nervous system, and most often the direct cause of symptoms. It is not known how neurodegeneration commences following misfolding of these proteins and many are the suggestions as of which of the multitude of misfolded self-assembled protein aggregates are the most toxic species.
It is likely that neurons are particularly sensitive to this type of proteinaceous toxins. Part of our research is focused on addressing this question. This is done by combining conformation sensitive, fluorescent, amyloid targeting probes that can be used both in the test tube and on tissue sections for microscopy. We address the test tube samples with biophysical analysis and compare disease properties of different amyloid conformations in animal models or in real patient samples displaying the same amyloid conformations, determined by fluorescence microscopy.
Conformation sensitive fluorescent probes can be used to detect amyloid and discriminate between different types of amyloid within one amyloid deposit. Small changes in the chemical structure of the probes modulate their binding. Image adapted from Zhang et al Journal of Medicinal Chemistry 2019 https://pubs.acs.org/doi/10.1021/acs.jmedchem.8b01681
Using conformation sensitive fluorescence probe and a range of fluorescence imaging techniques we can delineate differences in amyloid plaques with high resolution and specificity. Image adapted from Nyström et al Journal of visualized experiments 2017 https://www.jove.com/video/56279/imaging-amyloid-tissues-stained-with-luminescent-conjugated
Systemic amyloidosis
Amyloids can be found in virtually any organ of the body and over 35 different proteins are known to form amyloid in humans.
Familial Amyloidotic Polyneuropathy (FAP), also known as Skellefteåsjukan, is caused by amyloid formation of Transthyretin (TTR). FAP is hereditary and carriers of the disease associated mutant TTR will invariably come down with the disease. Successful basic research has paved the way for the treatment of this disease by stabilizing the culprit protein, the mutated TTR, and thereby preventing it from misfolding into amyloid.
This and other successful treatment strategies has proven that targeting protein misfolding can prevent or cure amyloid diseases.
Many different protein cause amyloid formation and disease in the human body. However, amyloid steming from the same protein does not always adopt the same amyloid structure. This conformational polymorphism is one of our research topics. Image from Fändrich et al, Journal of Internal Medicine 2018 https://onlinelibrary.wiley.com/doi/full/10.1111/joim.12732
Infectious protein misfolding
The prion diseases Creutzfeldt-Jakob’s disease and Bovine Spongiform Encephalopathy (Mad cow disease) are often associated with amyloid deposition. The term Prion is an abbreviation for PRotein Infectious ONly. Prions, in the classic sense, are protein infectious agents composed of the prion protein PrP. However, the debate is ongoing regarding prion properties also exhibited by amyloid assemblies comprised of other proteins.
The prion protein particle was the first example of an infectious agent that lacks nucleic acid, as opposed to conventional infectious agents such as parasites, bacteria or viruses. Prions cause a variety of diseases in humans and animals including bovine spongiform encephalopathy (BSE) or mad cow disease, Creutzfeldt-Jakob disease (in humans), Chronic wasting disease (in deer and elk) and scrapie (in sheep). Collectively prion diseases are in medical terms named transmissible spongiform encephalopathies or TSE´s due to the transmissibility of the diseases and the pathological feature of microvacuole formation in affected areas of the brain.
A dramatic feature of the transmissible spongiform encephalopathies, TSEs, is the rapid and efficient neuronal degeneration process. Misfolding of the prion protein (PrP) is a crucial step in these diseases. Here, the PrP misfolding process entails the conversion of a helical protein to a largely insoluble β–sheet rich state.
But, how can a misfolding process be infectious? The novel and controversial concept of the prion hypothesis is the propagation of protein conformations through structure based templated folding presented by Stanley Prusiner and is replicated in vitro (see Figure below).
PrP - The full-length soluble domain of mature human PrP protein is composed of 209 amino acids (res. 23-231) and is, in the natively folded PrPC form, folded into two domains. The N-terminal unstructured domain comprising the initial 90 residues has very high flexibility and affinity for Cu2+. The C-terminal globular domain folds into four helices and two short β–strands. The structure of the disease causing and infectious misfolded PrPSC particle is not known in detail due to the insolubility of the protein. PrPSC adopts a stable oligomeric structure with a large amount of β–sheet structure. Prion strains appear to be encoded in the PrPSC structure.
The prion protein can convert into its disease causing amyloid conformation both through a sporadic route and by templated seeding by adding a small fraction already formed amyloids. The spontaneous, unseeded reaction is significantly slower than the seeded reaction. Image adapted from Nyström et al, Journal of Biological Chemistry 2012 http://www.jbc.org/content/287/31/25975.long
Prion disease in mammals
Prion diseases, also termed TSEs (transmissible spongiform encephalopathy) are known to afflict a large number of mammals. The disease is mediated either by sporadic events of misfolding of the endogenously expressed PrP, or by transmission of preformed, misfolded prion particles from a source outside of the host.
BSE in cattle, Scrapie in sheep, MSE in mink and Chronic wasting disease in wild cervids has been known for several decades. More recently, prion disease has been discovered in dromedary camels in North Africa, demonstrating again the wide spread of prion disease among mammalian species.
The three-dimensional fold of native PrP from a wide range of mammals is well conserved. However, the amino acid sequence can vary. There are a number of single nucleotide polymorphisms (SNPs) that are hypothetically protective against infection by prion disease. Dogs and pigs appear resistant to prion disease, while human, cat and cow all are susceptible to prion infection.
We have shown that, although being resistant to prion disease, both dog PrP and pig PrP readily form amyloid fibrils in the test tube and the resulting fibrils are capable of enhancing amyloid fibril formation of any of the PrP sequences we tested, regardless of species origin, prion susceptible or resistant.
We study prion protein sequences from several different mammals and how their similarities and differences dictate their ability to form amyloid and interfere with each other. Image adapted from Nyström and Hammarström Scientific Report 2015 https://www.nature.com/articles/srep10101
One intriguing feature about prion biology is the strain phenomenon and how it is coupled to a species barrier that moderates which species can infect and be infected by which.
Although very similar in native three-dimensional structure, PrP can misfold into numerous misfolded states and thereby cause disease with different clinical attributes as well as histologic lesion profiles. Often, the strain rather than the amino acid sequence dictates the course of disease, as well as intra- and inter species transmission. When a prion strain is transmitted from one species to another, it can reside in, and transmit between individuals of the new host species without causing clinical disease, for several generations before it eventually has adapted to its new host. The novel, adapted strain can then cause disease with different clinical symptoms than were seen in the original donor species.
Prions in wild animals has been on our agenda for some time. Prions strains rather than amino acid sequence dictate their infectivity. Prions are known to be able to adapt to novel species, generating new infectious strains.
Since 2016 it has been known that Chronic Wasting disease is present in Europe, after the discovery of the disease in a reindeer in Norway. As of March 2019 the disease is also a fact in Sweden. Already at this early stage of investigation (May 2019) there are strong indications that the CWD prion strains found in Scandinavia are different from those found in North America.
We run a BSL3 lab dedicated to the studies of prions, specializing in experiments in the intersection between recombinant proteins and tissue samples.
