International Journal of Molecular Sciences Review Ostm1 from Mouse to Human: Insights into Osteoclast Maturation Jean Vacher 1,2,3,*, Michael Bruccoleri 1,2 and Monica Pata 1 1 Institut de Recherches Cliniques de Montreal (IRCM), Montreal, QC H2W 1R7, Canada; [email protected] (M.B.); [email protected] (M.P.) 2 Departement de Medecine, Universite de Montreal, Montreal, QC H2W 1R7, Canada 3 Department of Medicine, Division of Experimental Medicine, McGill University, Montreal, QC H3A 1A3, Canada * Correspondence: [email protected] Received: 16 July 2020; Accepted: 4 August 2020; Published: 5 August 2020 Abstract: The maintenance of bone mass is a dynamic process that requires a strict balance between bone formation and resorption. Bone formation is controlled by osteoblasts, while osteoclasts are responsible for resorption of the bone matrix. The opposite functions of these cell types have to be tightly regulated not only during normal bone development, but also during adult life, to maintain serum calcium homeostasis and sustain bone integrity to prevent bone fractures. Disruption of the control of bone synthesis or resorption can lead to an over accumulation of bone tissue in osteopetrosis or conversely to a net depletion of the bone mass in osteoporosis. Moreover, high levels of bone resorption with focal bone formation can cause Paget’s disease. Here, we summarize the steps toward isolation and characterization of the osteopetrosis associated trans-membrane protein 1 (Ostm1) gene and protein, essential for proper osteoclast maturation, and responsible when mutated for the most severe form of osteopetrosis in mice and humans. Keywords: osteoclast; osteopetrosis; grey-lethal; Ostm1; bone resorption; trafficking 1. Introduction Osteoclasts derive from hematopoietic stem cells that are shared with early myeloid lineage precursors. Differentiation of osteoclast precursors is dependent on mature osteoblasts that produce macrophage colony-stimulating factor (M-CSF), receptor activator of NF-κB Ligand (RANKL), and osteoprotegerin (OPG) a soluble decoy receptor of RANKL [1–5]. Upon recruitment and attachment to bone, mononuclear pre-osteoclasts undergo a process of fusion and these newly-formed multinucleated cells are structurally and functionally induced to generate active osteoclasts [6]. Mature osteoclasts are large multinucleated cells with numerous mitochondria, vacuoles, and lysosomes, which resorb mineralized cartilage and bone [7]. The biochemical characterization of osteoclasts have been hampered by the fact that these giant cells are tightly attached to the bone matrix and are therefore difficult to isolate. Moreover, as these cells are terminally differentiated and non-proliferative, a large number of cells have to be isolated at once. However, despite these impediments, osteoclast specific markers have been defined and novel efficient tools have been developed to analyze osteoclast biology ex vivo and in vivo [8]. When osteoclasts are activated, a resorption cycle is induced causing several proteins to be relocalized along with cytoskeletal rearrangement. Active osteoclasts are polarized and show two cellular histo-morphologic characteristics: an actin ring and a ruffled border. The actin ring, devoided of organelles, is enriched in dynamic and adhesive projections of the cell membrane called podosomes and in αVβ3 integrins that allow spreading and tight attachment to the bone surface [9–11]. The plasma Int. J. Mol. Sci. 2020, 21, 5600; doi:10.3390/ijms21165600 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2020, 21, 5600 2 of 14 membraneInt. J. Mol. in contactSci. 2020, with21, x FOR the PEER bone REVIEW surface enlarges into the ruffled border that induces polarization 2 of 14 of the osteoclast. Following this attachment, the osteoclast secretory lysosomes, also found in immune cells andinduces melanocytes polarization [12 ,of13 ],th wille osteoclast. associate Following and move this along attachment the microtubules,, the osteoclast fuse secretory to the plasma lysosomes, membrane,also found and thenin immune participate cells inand ru fflmelanocytesed border formation [12,13], will [14 associate,15]. The and ruffl moveed border along is the an infoldedmicrotubules, finger-likefuse to distortion the plasma of themembrane, plasma membraneand then participate adjacent toin theruffled bone border surface formation that participates [14,15]. The with ruffled lysosomalborder proton is an pumpinfolded H+ finger-like/V-ATPase anddistortion chloride of exchangerthe plasma ClC-7 membrane in acidification adjacent ofto thethe extracellularbone surface that resorbingparticipates lacunae towith ensure lysosomal bone matrix proton demineralization pump H+/V-ATPase [16–18 and]. In chloride the lacunae, exchanger release ClC-7 from in secretory acidification lysosomesof the ofextracellular tartrate resistant resorbing acid lacunae phosphatase to ensure (Trap), bone matrix matrix metallopeptidases demineralization [16–18]. (Mmp 9,14)In the [19 lacunae,], and cathepsinrelease Kfrom (Ctsk) secretory result in osteoidlysosomes degradation of tartrate which resistant is principally acid type phosphatase I collagen [ 20(Trap),] whereas matrix high aciditymetallopeptidases potentiates (Mmp dissolution 9,14) of[19], hydroxyapatite, and cathepsin theK (Ctsk) bone mineralresult in component. osteoid degradation The protein which is and mineralprincipally degradation type I collagen products [20] are whereas phagocytosed high acidity at the potentiates ruffled border dissolution into of of the hydroxyapatite, osteoclast as the digestivebone vacuole. mineral Thus, component. bone resorption The protein involves and exocytosismineral degradation and endocytosis products at the are ru fflphagocytoseded border and at the exocytosisruffled on border the contralateral into of the sideosteoclast of osteoclasts as digestive [10,21 vacuole.]. Importantly, Thus, bone osteoclast resorption bone resorptioninvolves exocytosis has been demonstratedand endocytosis to beat the critical ruffled for normalborder hematopoieticand exocytosis progenitorson the contralateral recruitment side and of osteoclasts proliferation [10,21]. that linkImportantly, bone remodeling osteoclast to hematopoiesisbone resorption regulation has been [22]. demonstrated Furthermore, numerousto be critical fundamental for normal bone–immunehematopoietic interactions progenitors through recruitment shared factors and prolifer have beenation discovered that link andbone are remodeling the subject to of hematopoiesis the field of osteoimmunologyregulation [22]. [23Furthermore,–26]. numerous fundamental bone–immune interactions through shared Defectivefactors have osteoclast been discovered differentiation and are or the generation subject of of the ine fieldfficient of os osteoclaststeoimmunology leads to[23–26] the severe bone pathologyDefective called osteoclast osteopetrosis, differentiation a heterogenous or generation inherited of inefficient disease of osteoclasts bone metabolism leads to [27 the,28 ].severe This diseasebone pathology was first describedcalled osteopetrosis, by Albers-Schönberg a heterogenous [29] and inherited results indisease accumulation of bone ofmetabolism mineralized [27,28]. osteoidThis and disease cartilage was due first to described loss of bone by Albers-Schönberg resorption [4,30]. Di[29]fferent and results forms ofin osteopetrosisaccumulation have of mineralized been characterizedosteoid and in variouscartilage vertebrate due to loss species of bone [ 31resorption–33] and [4,30]. mouse Different models forms were essentialof osteopetrosis toward have our been understandingcharacterized of mammalian in various vertebrate osteoclast formationspecies [31– and33] functionand mouse [12, 34models]. In humans, were essential three clinicaltoward our groupsunderstanding have been defined: of mammalian osteoclast formation and function [12,34]. In humans, three clinical groups have been defined: infantile-malignant autosomal recessive (ARO) which is fatal within the first few years of life; • intermediate• infantile-malignant recessive (IRO) autosomal which appears recessive during (ARO) the which first decade is fatal ofwithin life but the does first notfew mediate years of a life; • malignant• intermediate response; recessive (IRO) which appears during the first decade of life but does not mediate autosomala malignant dominant response; (ADO), with full-life expectancy but with major bone malformations. • • autosomal dominant (ADO), with full-life expectancy but with major bone malformations. Each form of the disease is characterized by a reduced bone marrow compartment leading to hematopoieticEach defects form of including the disease anemia is characterized and high susceptibility by a reduced to infections bone marrow [35,36]. compartment Characterization leading of to autosomalhematopoietic recessive defects osteopetrotic including mutations anemia and in high mouse susc modelseptibility and to infections in human [35,36]. patients Characterization defined ‘osteoclast-poor’of autosomal (impaired recessive osteoclastosteopetrotic diff mutationserentiation) in andmouse ‘osteoclast-rich’ models and in (inactive human osteoclasts)patients defined osteopetrosis‘osteoclast-poor’ leading to (impaired more targeted osteoclast therapies differentiation) [37–39]. and ‘osteoclast-rich’ (inactive osteoclasts) osteopetrosis leading to more targeted therapies [37–39]. 2. Osteopetrotic Grey-Lethal Mouse Model 2. Osteopetrotic Grey-Lethal Mouse Model The spontaneous osteopetrotic grey-lethal