Puthalakath - Regulation of apoptosis by Bcl-2 family proteins
Apoptosis or cell suicide is a normal process during embryonic development as well as throughout ones life. For example, apoptosis shapes our body during development. As our brains develop, the body creates millions more cells than it needs; the ones that don't form synaptic connections undergo apoptosis so that the remaining cells function well. Similarly, during an infection, our immune cells get activated and multiply. When the infection is cleared, these cells have to be removed by apoptosis.
Apoptosis is not a perfect process. Sometimes, the wrong cells kill themselves off, and sometimes, the ones that are damaged/mutated stick around. Deregulated apoptosis can cause various diseases such as cancer, heart failure, immune system failure during sepsis, autoimmune diseases etc. Understanding the apoptosis process is important in finding a cure for these diseases. Our lab is interested understanding the molecular basis of apoptosis regulation during heart failure, sepsis and in chemo resistance.
Bim and heart failure
Apoptosis of cardiomyocytes (heart muscle cells) the underlying cause Heart Failure (HF). Excessive stimulation of beta-adrenergic receptors (βAR), either by excessive catecholamine during chronic stress or by autoantibodies, leads to the apoptosis of cardiomyocytes. Work conducted in our lab using transgenic mouse models revealed the molecular mechanism regulating the expression of the apoptotic protein Bim and hence targeting Bim expression offers an avenue for developing new drugs for treating HF.
Despite the common use of βAR antagonists (or β-blockers), heart failure remains poorly controlled with a 5-year survival rate of 50%. Moreover, achieving right dosage without compromising the heart function can be difficult. Patients with unstable hemodynamics usually cannot tolerate β-blockers apparently due to blockade of sympatho-βAR-PKA mediated functional compensation. Therefore there is a need to develop a novel drug that improves the biological properties of the failing heart i.e. one that maintains the β-AR-mediated functional compensation and at the same time is capable of blocking the apoptotic arm of the β-AR pathway.
Our understanding of the molecular regulation of Bim expression during βAR stimulation has helped us to screen for drugs that target only the apoptotic arm of this pathway maintaining the functional compensation arm. These drug hits have the ability to block Bim induction during βAR signalling without having any effect on PKA activity (PKA activity is necessary for the contractile function of the heart). We are currently attempting to improve their in vivo efficacy and conducting pharmacokinetics to understand the drug metabolism.
Role of Bim in sepsis-induced lymphopenia
Sepsis leads to an annual financial burden in excess of $16 billion in the US alone. In Australia, there has been a four-fold increase in sepsis incidents between 1997 and 2005, owing to an ageing population. Sepsis is defined as the host inflammatory response to severe, life-threatening infection. This response can be divided into two stages, a hyper-inflammatory phase and a hypo-inflammatory phase. During the hyper-inflammatory phase, activated immune cells (mostly the innate immune system) produce copious amounts of inflammatory cytokines, which can lead to multiple organ failure. However, improved treatment protocols have resulted in most patients surviving this stage and entering a protracted immune suppressive phase. This phase is characterized by extensive apoptosis of B cells, and T cells leading to prolonged lymphopenia where the patients are susceptible to nosocomial infections.
Experimental drug therapies for sepsis are at cross roads with more than 30 drug trials failing in the last 25 years. This is largely attributed to the fact that these drugs were aimed at the hyper inflammatory phase of sepsis. Current thinking in the field is to block immune cell apoptosis to prevent lymphopenia. Bim is a critical mediator of immune cell homeostasis. We have recently identified the factor that is released by macrophages during sepsis that kills B and T cells. Efforts are underway to identify the receptor(s) for this factor. Understanding the molecular basis of the interaction between this factor and the receptor will help in developing new therapeutics for sepsis.
Role of Bok in chemo-resistance
Bok (Bcl-2 related ovarian killer) is a protein that is believed to be a member of multidomain Bax/Bak sub-family member. It is widely reported to be a pro-apoptotic protein. However, genomic knock-out of this protein by our collaborator (Dr Thomas Kaufmann, Bern, Switzerland) revealed that these mice did not have any apparent phenotype. To better understand its function, we conducted a yeast two-hybrid screen and identified a protein that regulates pyramidine metabolism. Work is underway to further elucidate its role in pyramidine biosynthesis through structure determination by X-ray crystallography and to understand its role in drug resistance during cancer chemotherapy.