Authors: David Howett
Neena Singh and Ajai Tripathi’s recent research from Case Western Reserve University School of Medicine (OH, USA) may offer exciting new insights into prion protein’s (PrP) binding motif and its interactions with hemin, a toxic byproduct of hemoglobin. The implications of this research, published in The Journal of Alzheimer’s Disease recently, identify hemin as a potential therapeutic target for sporadic Creutzfeldt-Jakob disease (CJD) and cerebral hemorrhage.
Cellular prion protein (PrPc) is a membranous protein implicated in a variety of physiological functions, from long-term potentiation to stem cell renewal. However, PrPc has gained notoriety as the pathological substrate PrP-scrapie (PrPSc) in prion diseases such as CJD.
Prior research has emphasized the therapeutic potential of hemoproteins as potential drug targets for CJD, which led Singh and Tripathi to explore the interaction between hemin and PrPc in their most recent study.
In addition, the clinical significance of this interaction resonates in determining the outcome of cerebral hemorrhage. Following hemorrhage, surrounding penumbric neurons upregulate PrPC for its neuroprotective effect. Concurrently however, heme, released by hemolyzed red blood cells, is rapidly oxidized by PrPC to produce hemin. This self-propagating cycle results in an abundance of hemin inhibiting the beneficial function of PrPc through endocytosis, causing further neurotoxicity.
These observations led to this eloquently executed series of experiments exploring the PrPc–hemin interaction.
Utilizing fluorescent labelled PrPC, the interaction between PrPC and hemin was investigated in neuroblastoma (SH-SY5Y) and hematopoietic (K562) cell cultures.
Results indicated that exposure to a pathologically-representative concentration (10-30 μM) of hemin induced rapid endocytosis of PrPC in neuroblastoma cells. In contrast, hematopoietic cells exhibited a decrease in PrPC expression independent of internalization.
In both cell lines, deletion of the hemin-binding octapeptide repeat domain inhibited PrPC-mediated increases in hemoglobin synthesis and PrPC internalization. Pharmaceutical inhibition of PrPC-hemin elicited increases in hemoglobin synthesis and also limited the propagation of pathological PrPsc, for example in CJD.
Conversely, outcomes in cerebral hemorrhage may be improved by inhibiting hemin-induced endocytosis of PrPC. The downstream effects of such inhibition would preclude the upregulation of hemoglobin and improve neuronal viability by clearing extracellular hemin and increasing respiratory potential.
Tripathi and Singh’s in vitro findings, that under physiological conditions PrPC regulates hemoglobin synthesis, were further validated using ex vivo samples from rodent and pathological human brain tissue.
In rodents, the concentration of hemoglobin was observably reduced in erythrocytes and brain tissue of PrP–/– mice compared to PrP+/+ controls. In addition, reduced concentrations of hemoglobin were observed in human sporadic CJD brains.
Collectively, these results implicate PrPC in promoting neuronal survival in physiological conditions and in cerebral hemorrhage by clearing hemin from the neuronal microenvironment and upregulating the synthesis of hemoglobin. This upregulation observed in sporadic CJD brains may indicate a dysregulation of hemoglobin synthesis or a compensatory mechanism by surviving neurons.
The implications of these findings may offer insights into two separately untreatable diseases, which are both in desperate need of therapeutic interventions. Singh commented: “We think that manipulation of neuronal hemoglobin may provide an effective method of improving neuronal survival. Further studies are necessary to explore viable options that take advantage of PrPC and hemin in this process.”
Sources: Case Western Reserve press release; Tripathi AK, Singh N. Prion Protein-Hemin Interaction Upregulates Hemoglobin Synthesis: Implications for Cerebral Hemorrhage and Sporadic Creutzfeldt-Jakob Disease. J Alzheimer’s Dis. 51, 107–121 (2016).