International Journal of Molecular Sciences Review Involvement of Bcl-xL in Neuronal Function and Development Julie Bas 1, Trang Nguyen 1 and Germain Gillet 1,2,* 1 Centre Léon Bérard, Centre de Recherche en Cancérologie de Lyon, Université de Lyon, Université Claude Bernard Lyon 1, INSERM 1052, CNRS 5286, 69008 Lyon, France; [email protected] (J.B.); [email protected] (T.N.) 2 Hospices Civils de Lyon, Laboratoire d’anatomie et Cytologie Pathologiques, Centre Hospitalier Lyon Sud, Chemin du Grand Revoyet, 69495 Pierre Bénite, France * Correspondence: [email protected] Abstract: The B-cell lymphoma (Bcl-2) family of proteins are mainly known for their role in the regulation of apoptosis by preventing pore formation at the mitochondrial outer membrane and subsequent caspase activation. However, Bcl-2 proteins also have non-canonical functions, inde- pendent of apoptosis. Indeed, the cell death machinery, including Bcl-2 homologs, was reported to be essential for the central nervous system (CNS), notably with respect to synaptic transmission and axon pruning. Here we focused on Bcl-xL, a close Bcl-2 homolog, which plays a major role in neuronal development, as bclx knock out mice prematurely die at embryonic day 13.5, showing massive apoptosis in the CNS. In addition, we present evidence that Bcl-xL fosters ATP generation by the mitochondria to fuel high energy needs by neurons, and its contribution to synaptic transmission. We discuss how Bcl-xL might control local and transient activation of caspases in neurons without causing cell death. Consistently, Bcl-xL may contribute to morphological changes, such as sprouting and retractation of axon branches, in the context of CNS plasticity. Regarding degenerative diseases and aging, a better understanding of the numerous roles of the cell death machinery in neurons may have future clinical implications. Citation: Bas, J.; Nguyen, T.; Gillet, Keywords: Bcl-xL; apoptosis; neurons; mitochondria; endoplasmic reticulum; caspases G. Involvement of Bcl-xL in Neuronal Function and Development. Int. J. Mol. Sci. 2021, 22, 3202. https:// doi.org/10.3390/ijms22063202 1. Introduction Academic Editor: Muriel Priault The Bcl-2 family of proteins plays a major role in the regulation of apoptosis, a highly conserved programmed cell death process that eliminates unwanted cells, including Received: 25 February 2021 damaged cells, which is essential during development [1]. Dysregulation of this process Accepted: 17 March 2021 can lead to cancer or degenerative diseases [2,3] and multiple clinical trials aim to inhibit Published: 21 March 2021 the Bcl-2 family proteins in cancer [4]. Bcl-2 is the founding member of this family that includes both apoptotic and non-apoptotic proteins. All of these actors are classified Publisher’s Note: MDPI stays neutral within three groups according to the presence of conserved Bcl-2 homology (BH) domains: with regard to jurisdictional claims in the anti-apoptotic members that contain the four BH domains (BH1-4, including Bcl-2, published maps and institutional affil- Bcl-xL, and Mcl-1), the pro-apoptotic members, that lack the BH4 domain (including iations. Bax and Bak), and the pro-apoptotic BH3-only proteins that contain the BH3 domain exclusively (including Bad, Bid, Bim) [5]. In addition, some proteins of the Bcl-2 family may contain a transmembrane domain in their C-terminus for their anchoring to intracellular membranes, including the mitochondrial outer membrane (MOM) and the endoplasmic Copyright: © 2021 by the authors. reticulum (ER) [5]. BH1, BH2, and BH3 domains form a hydrophobic groove allowing Licensee MDPI, Basel, Switzerland. anti-apoptotic Bcl-2 proteins to bind to the BH3 domain of apoptosis accelerators and thus This article is an open access article inhibit cell death [6]. The interactions between all of these proteins determine the formation distributed under the terms and of “pores” in the MOM as a consequence of the homodimerization of the multi-domain conditions of the Creative Commons apoptosis accelerators Bax and Bak, leading to the release of pro-apoptotic factors from Attribution (CC BY) license (https:// the mitochondria, including cytochrome c (Figure1). Cytochrome c release triggers the creativecommons.org/licenses/by/ formation of the apoptosome, the complex responsible for the activation of caspases, the 4.0/). Int. J. Mol. Sci. 2021, 22, 3202. https://doi.org/10.3390/ijms22063202 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, 3202 2 of 11 Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 2 of 12 executioners of apoptosis. Caspases are cysteine proteases that under normal conditions are present in a pro-caspase inactive state and are activated by proteolytic cleavage, in turn theleading executioners to the cleavageof apoptosis. of a Caspases large number are cysteine of downstream proteases that targets, under andnormal eventually condi- to cell tionsdeath are [6 present]. Beyond in a their pro-caspase highly inactive documented state and role are in activated apoptosis, by proteolytic Bcl-2 proteins cleavage, and caspases inhave turn been leading shown to the to cleavage be involved of a large in othernumber cellular of downstream functions targets, [5]. Notablyand eventually Bcl-xL, one of tothe cell most death studied [6]. Beyond anti-apoptotic their highly Bcl-2 documented homolog, role was in reported apoptosis, to Bcl-2 promote proteins tumor and stem cells caspasesproperties have [7 ],been promote shown migration to be involved in breast in other cancer cellular cells functions due to [5]. its effectNotably on Bcl-xL, mitochondrial onemetabolism of the most [8] andstudied to impact anti-apoptotic intracellular Bcl-2 homolog, calcium (Ca was2+ reported) trafficking to promote by interacting tumor with the steminositol cells triphosphate properties [7], receptor promote (IP3R)migration at thein breast ER [9 cancer]. Bcl-xL cells is due involved to its effect in many on mi- other non- 2+ tochondrialcanonical functions metabolism [5 ],[8] including and to impact neurite intracellular development calcium and (Ca growth) trafficking [10]. Thereby inter- is evidence acting with the inositol triphosphate receptor (IP3R) at the ER [9]. Bcl-xL is involved in that impaired Bcl-x activity is correlated with the onset of major diseases such as Parkinson’s many other non-canonical functions [5], including neurite development and growth [10]. disease (PD) [11,12], amyotrophic lateral sclerosis [13] spinal cord muscular atrophy [14], There is evidence that impaired Bcl-x activity is correlated with the onset of major diseases suchas well as Parkinson's as Friedrich disease ataxia (PD) [15], [11,12], however amyo thetrophic underlying lateral sclerosis molecular [13] mechanisms spinal cord remain muscularpoorly characterized. atrophy [14], as Actually,well as Friedrich Bcl-xL ataxia seems [15], to however play critical the underlying roles in amolecular large number of mechanismsnerve cells, remain including poorly astrocytes characterized. [13], Actually, neurons Bcl-xL of the seems cortex to [ 16play], thecritical hippocampus roles in [10], athe large midbrain number [of17 ],nerve the visualcells, including and the astr auditoryocytes [13], systems neurons [18 ,19of ],the motoneurons cortex [16], the (MN) [14] hippocampusand sensory [10], neurons the midbrain of the dorsal [17], the root visual ganglion and the [ 1auditory]. The bclx systemsgene [18,19], encodes moto- 5 isoforms, neuronsincluding (MN) Bcl-xL, [14] and the sensory major neurons isoform of in the the dorsal CNS. root The ganglion number [1]. of The neurons bclx gene in en- the CNS is codesdetermined 5 isoforms, by an including equilibrium Bcl-xL, between the major differentiation, isoform in the proliferation,CNS. The number and of apoptosis. neurons During inthe the development CNS is determined of the CNS, by an axonal equilibrium pruning between processes differentiation, take place, proliferation, leading to aand remodeling apoptosis.of neurons During in order the development to eliminate of “unnecessary” the CNS, axonal connections. pruning processes These take processes place, lead- involve the ingmitochondrial to a remodeling apoptosis of neurons pathway, in order in particularto eliminate the "unnecessary" activation of connections. caspases,leading These to very processes involve the mitochondrial apoptosis pathway, in particular the activation of localized degeneration at the subcellular level. However, during this process, the activation caspases, leading to very localized degeneration at the subcellular level. However, during thisof caspases process, the is independent activation of caspases of apoptosome is independent formation of apoptosome [20]. This formation reflects the [20]. importance This of reflectsnon-apoptotic the importance functions of non-apoptotic of the cell death functions machinery of the cell in thedeath development machinery in of the the de- CNS. This velopmentreview focuses of the CNS. on the This role review of Bcl-xL focuses and on the caspases role of Bcl-xL downstream and caspases in the downstream CNS with special inattention the CNSto with neurite special outgrowth. attention to neurite outgrowth. Figure 1. 1. TheThe interactions interactions between between the different the different member memberss of the B-cell of thelymphoma B-cell lymphoma(Bcl-2) family (Bcl-2) family control mitochondria outer membrane permeabilization (MOMP) and subsequent apoptosis. All control mitochondria outer membrane