Cardiac preservation using ex vivo organ perfusion: new therapies for the treatment of heart failure by harnessing the power of growth factors using BMP mimetics like THR-184

Abstract

As heart transplantation continues to be the gold standard therapy for end-stage heart failure, the imbalance between the supply of hearts, and the demand for them, continues to get worse. In the US alone, with less than 4,000 hearts suitable for transplant and over 100,000 potential recipients, this therapy is only available to a very few. The use of hearts Donated after Circulatory Death (DCD) and Donation after Brain Death (DBD) using ex vivo machine perfusion (EVMP) is a promising approach that has already increased the availability of suitable organs for heart transplantation. EVMP offers the promise of enabling the expansion of the overall number of heart transplants and lower rates of early graft dysfunction. These are realized through (1) safe extension of the time between procurement and transplantation and (2) ex vivo assessment of preserved hearts. Notably, ex vivo perfusion has facilitated the donation of DCD hearts and improved the success of transplantation. Nevertheless, DCD hearts suffer from serious preharvest ischemia/reperfusion injury (IRI). Despite these developments, only 40% of hearts offered for transplantation can be utilized. These devices do offer an opportunity to evaluate donor hearts for transplantation, resuscitate organs previously deemed unsuitable for transplantation, and provide a platform for the development of novel therapeutics to limit cardiac injury. Bone Morphogenetic Protein (BMP) signaling is a new target which holds the potential for ameliorating myocardial IRI. Recent studies have demonstrated that BMP signaling has a significant role in blocking the deleterious effects of injury to the heart. We have designed novel small peptide BMP mimetics that act via activin receptor-like kinase (ALK3), a type I BMP receptor. They are capable of (1) inhibiting inflammation and apoptosis, (2) blocking/reversing the epithelial-mesenchymal transition (EMT) and fibrosis, and (3) promoting tissue regeneration. In this review, we explore the promise that novel therapeutics, including these BMP mimetics, offer for the protection of hearts against myocardial injury during ex vivo transportation for cardiac transplantation. This protection represents a significant advance and a promising ex vivo therapeutic approach to expanding the donor pool by increasing the number of transplantable hearts.

Keywords: BMP; Bone morphogenetic protein; Ex-vivo heart machine perfusion; NMP; TGF; THR-123; THR-184; mimetics.

Figure 1

Revolutionizing heart transplantation: exploring donations after brain death (DBD) and donations after circulatory death (DCD).

Figure 2

The paragonix sherpapak® cardiac transport system. The heart is submerged in cold cardioplegic solution. The transport system uses proprietary phase change cold packs to maintain temperatures 4–8°C.

Figure 3

This figure illustrates an organ care system (OCS®) heart machine with the schematic diagram, breaking down the processes involved.

Figure 4

The death receptor and mitochondrial pathways of apoptosis: DNA damage triggers the death receptor and mitochondrial apoptotic pathways. Death ligands that bind to their respective receptors trigger the recruitment of adaptor molecules (FADD), and subsequent recruitment and activation of caspases to mediate apoptosis. Activated caspases can also cleave BID to promote the mitochondrial pathway of apoptosis. This pathway is defined by mitochondrial outer membrane permeabilization (MOMP) and is regulated by the BCL-2 proteins. Pro-apoptotic proteins such as BID or BIM promote BAX or BAK homo-oligomerization in the mitochondrial membrane, whereas anti-apoptotic proteins such as BCL-2 and BCL-xL inhibit this process. De-repressor proteins BAD, PUMA or NOXA bind the anti-apoptotic proteins and reduce the threshold for BAX/BAK activation. MOMP results in cytochrome c release into the cytosol, which promotes APAF-1 oligomerization, caspase activation, and apoptosis.

Figure 5

The TGF beta/BMP/activin pathways are mediated by their specific type I and type II receptors. BMP binds any of three type I receptors: BMPR-IA (ALK3), BMPR-IB (ALK6) and a Type IA activin receptor ActR-IA (ALK2) (–140).

Figure 6

BMP signaling pathways. BMP mimetic/BMP-7 transduces signals in target cells by binding to a specific membrane bound receptor BMPR-II and phosphorylates BMPR-I, which activates both the canonical and the non-canonical pathways. In the canonical pathway, activated BMPR-II leads to phosphorylation of Smad-1/5/8 which complexes with Smad-4 and translocate the signal. In the non-canonical pathway, p38, Mitogen-activated protein kinase (MAPK), c-Jun-N terminal Kinase (JNK), ERK and NFKB were activated via the activation of X-linked inhibitor of apoptosis protein (XIAP), TGF-beta activated kinase 1 (TAK1) and TAK1 binding protein (TAB1) whereas PI3K/Akt were activated by both BMPR-II and Smad-1/5/8. Altogether, this influences the different transcription factors and regulates the gene expression.

Figure 7

Structure diagrams of the BMP-7 monomer and the region covered by the mimetic. (A) Ribbon diagram showing the secondary structure of the BMP monomer, which contains three structural regions: antiparallel beta sheets of “Finger 1” (with the large terminal loop), “Finger 2” (with the tight beta-turn), and the “Heel” alpha helix. Initial targets for mimetic development were the terminal loops of fingers 1 and 2, loops at the C-terminal, and N-terminal loops at the ends of the heel helix. (B) The region around the beta turn of finger 2 proved to have activity similar to BMP-7 and became the lead for further mimetic development. (C) The “beta-turn” region covered by the mimetic is immediately C-terminal to the “knuckle” region covered by other BMP mimetics. Residue position numbers are based on BMP-2 residue numbers. Secondary structure: beta sheet (>>>>), segments of which are labelled e.g., “b6”; beta turn (bt, tttt). Peptide disulfide bond: C C.

Figure 8

THR-123 and THR-184 binding like BMP-7, BMP mimetics THR-123 and THR-184 bind the BMP type II receptor BMPR-II and selective BMP type I receptors actR-IA (ALK2) and BMPR-IA (ALK3). Unlike BMP-7, they do not bind BMPR-IB (ALK6).