Introduction

The mechanisms of apoptosis may help elucidate the pathologies of uncontrolled cell growth or death. Apoptosis is under the mediation of two major pathways: the extrinsic or death receptor pathway and the intrinsic or mitochondrial pathway.[1][2] The intrinsic pathway is controlled and regulated by the Bcl-2 (B-cell lymphoma 2) family of proteins.  The proteins in this family can be classified by their anti-apoptotic (Bcl-2, Bcl-x, Bcl-w, Mcl-1, and A1/Bfl-1) or pro-apoptotic (Bax, Bak, and Bok/Mtd) actions.[3] Both pathways result in the activation of caspases that lead to the termination of cell life.[4] 

The BAX gene (Bcl-2 Associated X-protein) is a pro-apoptotic member of the Bcl-2 gene family; it encodes a 21-kDa protein named BAX-alpha, whose association with Bcl-2 researchers believe plays a critical role in regulating intrinsic apoptosis.[4] 

Development

Research has shown that the Bax gene is located on chromosome 19 by utilizing fluorescent in-situ hybridization and human-hamster somatic cell hybrid DNA.[4] Using isolated mitochondria treated to block permeability transition (PT) mitochondrial pore opening, scientists determined that Bax protein allows membrane depolarization and release of mitochondrial contents, including cytochrome c, via a Ca+ dependent opening of PT pores.[5] However, PTP-/- knockout mice still exhibit Bax-dependent cell death; therefore, the suggestion has been that Bax is capable of forming pores in the mitochondrial outer membrane (MOM) on its own by inserting its amphipathic alpha-helical structure directly into the MOM.[6]

Cellular

The intrinsic pathway of apoptosis becomes activated via decreased survival signals. The Bax gene encodes BCL2L4 protein that, upon activation, heterodimerizes with Bcl2 family proteins and alters cellular mitochondria to induce cell death.[7]  In healthy cells, Bax protein is cytosolic with minimal binding and insertion into the outer mitochondrial membrane. Pro-survival signals, such as Bcl-xl and Bcl-2, ensure Bax does not accumulate on the outer mitochondrial membrane of healthy cells.[8] Although the complete mechanism by which Bax permeabilizes the MOM is unclear, it is known that Bax, upon activation, causes membrane depolarization and mitochondrial leaking of contents such as cytochrome c.[3] Morphological changes in the apoptotic cell include pyknosis (nuclear condensation and shrinkage), cell shrinkage, and membrane blebbing, leading to the release of vesicles containing cellular contents that will be phagocytosed by surviving cells.[9]

Biochemical

Due to alternative splicing, the Bax gene produces multiple isoforms, with Bax-alpha being the most predominant.[4] The hydrophobic pocket of the BH3 domain is the active binding site for regulators such as BH3 and Bcl-xl pro- and anti-apoptotic members of the Bcl-2 family.[3] Transcriptional regulation of the Bax gene occurs in large part by tumor suppressor P53.[1]  Although the conformational changes that occur upon activation are not entirely understood, it is known that the carboxy-terminal end of the protein is responsible for mitochondrial membrane location and binding.[6] Once bound, the BH1 and BH2 domain is suspected to be pore-forming, resulting in increased membrane permeability and leakage of interspace proteins, which induce downstream cellular effects resulting in cell death.[6][3]

Molecular Level

Bax and other pro-apoptotic members of the Bcl-2 family contain 4 Bcl-2 homologous (BH) domains. The BH3 domain contains the active binding site, which consists of alpha-helices embedded within a hydrophobic groove.[3] The BH3 domain allows for heterodimerization with Bcl-2 and Bcl-xl, while its hydrophobic c-terminal allows for outer mitochondrial membrane binding. BH1 and BH2 domains are suspected to be the pore-forming domains of the protein.[6] The full extent of the conformational changes that occur during protein binding to both regulators and the MOM has yet to be seen but is an area of research that may elucidate possible pharmacological intervention of pathological processes that involve apoptotic dysregulation. 

Function

The Bax gene encodes BCL2L4 protein that, upon activation, heterodimerizes with Bcl2 family proteins and alters cellular mitochondria to induce cell death.[7] When Bax protein is activated, its function is to bind and induce MOM permeabilization. Such permeabilization results in mitochondrial swelling and rupture with subsequent leakage of intermembrane space proteins, specifically cytochrome c and endonuclease G.[10]  Every mechanism in which Bax increases mitochondrial membrane permeability is not well understood, but scientists know that binding to permeability transition pore (PTP) causes conformational changes that increase permeability.[11] However, PTP- knockout mice still exhibit Bax-dependent cell death; therefore, there have been suggestions that Bax is capable of forming pores in the MOM on its own by inserting its amphipathic alpha-helical structure directly into the MOM.[6]

Mechanism

Bax protein exists constitutively in the cytoplasm until activated. It binds to multiple anti-apoptotic proteins such as Bcl-xl that inhibit Bax translocation to the MOM; therefore, Bax mediated apoptosis is dependant on the concentration of both pro- and anti-apoptotic proteins.[6] Bax becomes activated by binding upregulated death signals such as BH3 only proteins or t-Bid at its BH3 hydrophobic pocket domain. Activation causes conformational changes that lead to translocation to the MOM, where Bax induces mitochondrial outer membrane permeabilization (MOMP) by two possible mechanisms described above.[6][2] MOMP leads to mitochondrial swelling and rupture, causing the release of periplasmic proteins such as cytochrome C. [2] Cytochrome C binds and activated cytosolic caspases, which are known to be the effector proteases of cell death.[2][10]

Testing

Researchers originally discovered Bax using immunoprecipitation studies with Bcl-2.[6] They discovered its genetic location using FISH, and they parsed out its function using various methods.[5] Most recent studies aim to identify Bax's role in pathological diseases using genetic knockout models or virally infected over-expression of Bax.[2][12]

Pathophysiology

Because of the Bax gene's role in cell apoptosis, dysfunction carries links to various pathologies related to cell accumulation (i.e., cancer) or cell loss (i.e., heart failure, Alzheimer disease).[2] Tumorigenesis is largely dependant on the cell's ability to overcome cell cycle regulators and cell apoptosis. Research has shown that both loss of function and gain of function mutations in Bcl-2 family proteins, including Bax, increases the likelihood of human tumorigenesis.[13] Research demonstrates that Bax deletion increases the oncogenic gene activation and the progression of certain cancers such as Philadelphia positive leukemia.[12]

Clinical Significance

Bax dysregulation can lead to apoptotic dysfunction that increases the likelihood of multiple pathologies.  As a tumor suppressor gene, Bax has a critical role in preventing tumorigenesis, and loss of function often occurs in human tumors. Loss of function mutations can also lead to incomplete negative selection and peripheral tolerance dysfunction, thus causing autoimmune complications. Chronic neurodegenerative conditions, such as Huntington disease, Parkinson disease, or Alzheimer disease, often depend on aberrant apoptotic cell death. Lastly, although necrosis is the primary mechanism of injury during myocardial infarction, it has been seen that reperfusion injury is an apoptosis driven event.[13]