Caveolae as potential macromolecule trafficking compartments within alveolar epithelium

https://doi.org/10.1016/S0169-409X(01)00142-9Get rights and content

Abstract

With inhalational delivery the alveolar epithelium appears to be the appropriate lung surface to target for the systemic delivery of macromolecules, such as therapeutic proteins. The existence of a high numerical density of smooth-coated or non-coated plasma membrane vesicles or invaginations within the alveolar epithelial type I cell has long been recognised. The putative function of these vesicles in macromolecule transport remains the focus of research in both pulmonary physiology and pharmaceutical science disciplines. These vesicles, or subpopulations thereof, have been shown to biochemically possess caveolin, a marker protein for caveolae. This review considers the morphometric and biochemical studies that have progressed the characterisation of the vesicle populations within alveolar type I epithelium. Parallel research findings from the endothelial literature have been considered to contrast the state of progress of caveolae research in alveolar epithelium. Speculation is made on a model of caveolae vesicle-mediated transport that may satisfy some of the pulmonary pharmacokinetic data that has been generated for macromolecule absorption. The putative transport function of caveolae within alveolar epithelium is reviewed with respect to in-situ tracer studies conducted within the alveolar airspace. Finally, the functional characterisation of in-vitro alveolar epithelial cell cultures is considered with respect to the role of caveolae in macromolecule transport. A potentially significant role for alveolar caveolae in mediating the alveolar airspace to blood transport of macromolecules cannot be dismissed. Considerable research is required, however, to address this issue in a quantitative manner. A better understanding of the membrane dynamics of caveolae in alveolar epithelium will help resolve the function of these vesicular compartments and may lead to the development of more specific drug targeting approaches for promoting pulmonary drug delivery.

Introduction

The permeability characteristics of the lung and recent advances in inhalational aerosol device technology have led to an increasing interest in exploiting the pulmonary route for the systemic delivery of macromolecule therapeutics, particularly recombinant proteins and polypeptides [1]. The lung deposition and absorption studies of Colthorpe et al. [2], [3] elegantly demonstrated that the extents of systemic absorption of insulin or growth hormone following lung administration positively correlate with the depth of deposition of these administered proteins within the lung. Anatomical determinants [4] would also support the view that the lung periphery, and the alveolar epithelium in particular, is the appropriate lung surface to target when aiming to systemically deliver macromolecules. Some of these anatomical determinants would include: location of the alveolar surface beyond the clearance mechanisms of the mucociliary escalator; the large alveolar surface area potentially available for absorption; the high blood flow to the alveolar region, and the thin cellular barrier from airspace to capillary blood presented by the alveolar epithelial and pulmonary capillary cells. Further, in the transport of macromolecules across the pulmonary alveolar epithelial–capillary endothelial barrier, evidence indicates that it is the alveolar epithelium that possesses a more restrictive paracellular pathway than that provided by the capillary endothelium [5]. As a corollary the mechanisms of transport of macromolecules within alveolar epithelium are the subject of genuine interest [6], and in particular the nature and extent of any vesicular trafficking mechanism(s) such as that potentially provided by caveolae.

Section snippets

Caveolae and the structural role of caveolins

At the electron-microscopic level caveolae are most frequently observed as “smooth coated” or “non-coated” omega-shaped invaginations (diameter of 50–100 nm at the widest point) connected to the plasmalemma or plasma membrane by a neck-like structure which affords spatial continuity with the extracellular environment (Fig. 1a). At least in endothelial cells caveolae-like vesicles may also be observed as fused lines or clusters of vesicles at the plasmalemma [7]. The term “smooth coated” or

Alveolar epithelial–pulmonary capillary barrier

The lower respiratory tract consists of the respiratory bronchioles, the alveolar ducts, and the alveoli themselves which represent the main location for gaseous exchange. Alveolar epithelium is predominantly comprised of two cell types, the terminally differentiated squamous alveolar epithelial type I (ATI) cell which constitutes approximately 93% of the alveolar epithelial surface area (33% of alveolar epithelial cells by number) and the surfactant producing cuboidal alveolar epithelial type

Transport role for caveolae in alveolar epithelium

While the morphometric data for the ATI cell vesicle populations is intriguing in terms of their putative function as endocytic or transcytotic compartments, direct evidence for their role in transport within alveolar epithelium is at present extremely limited. Certainly, the functional characterisation of ATI vesicle populations lags considerably behind the progress made in determining a transport role for caveolae in endothelium. However, until comparatively recently the role of endothelial

Caveolae and caveolin in cultured alveolar epithelium

Due to the complex nature of the lung architecture, the alveolar epithelium is not a readily accessible absorption surface to study. Therefore the use of cultures of alveolar epithelium cells as an in-vitro experimental model for the prediction of the extent, rate and mechanism of alveolar absorption of pharmaceuticals has gained acceptance amongst investigators [83].

Consistent with the in-vivo hypothesis of the ATII cell transdifferentiating into the in-vivo ATI cell [18], isolated ATII cells

Conclusion

There is a genuine clinical and commercial interest in exploiting the pulmonary route for the systemic delivery of macromolecules, particularly recombinant proteins and polypeptides. The alveolar epithelium appears to be the appropriate lung surface to target when aiming to systemically deliver macromolecules. The existence of a high numerical density of smooth-coated or non-coated plasma membrane vesicles or invaginations within the alveolar epithelial type I cell has long been recognised.

References (90)

  • H.S. Green et al.

    Calculations on the passage of small vesicles across endothelial cells by brownian motion

    J. Theor. Biol.

    (1972)
  • D. Simionescu et al.

    Differential distribution of the cell surface charge on the alveolar capillary unit. Characteristic paucity of anionic sites on the air–blood barrier

    Microvasc. Res.

    (1983)
  • T. Ohtani et al.

    Effect of absorption enhancers on pulmonary absorption of fluorescein isothiocyanate dextrans with various molecular weights

    Int. J. Pharm.

    (1991)
  • V. Rizzo et al.

    Rapid mechanotransduction in situ at the luminal cell surface of vascular endothelium and its caveolae

    J. Biol. Chem.

    (1998)
  • R.L. Roberts et al.

    Receptor-mediated endocytosis of insulin by cultured endothelial cells

    Tissue Cell

    (1992)
  • P.E. Lobie et al.

    Caveolar internalisation of growth hormone

    Exp. Cell Res.

    (1999)
  • J. Middleton et al.

    Transcytosis and surface presentation of IL-8 by venular endothelial cells

    Cell

    (1997)
  • A. Chaudhuri et al.

    Detection of duffy antigen in the plasma membranes and caveolae of vascular endothelial and epithelial cells of nonerythroid organs

    Blood

    (1997)
  • N.R. Mathias et al.

    Respiratory epithelial cell culture models for evaluation of ion and drug transport

    Adv. Drug Deliv. Rev.

    (1996)
  • L.G. Dobbs et al.

    Monoclonal antibodies specific to apical surfaces of rat alveolar type I cells bind to surfaces of cultured, but not freshly isolated, type II cells

    Biochim. Biophys. Acta

    (1988)
  • J.M. Cheek et al.

    Type 1 cell-like morphology in tight alveolar epithelial monolayers

    Exp. Cell Res.

    (1989)
  • J.S. Patton

    Breathing life into protein drugs

    Nat. Biotechnol.

    (1998)
  • P. Colthorpe et al.

    The pharmacokinetics of pulmonary delivered insulin: a comparison of intra-tracheal and aerosol administration to the rabbit

    Pharm. Res.

    (1992)
  • P. Colthorpe et al.

    The influence of regional deposition on the pharmacokinetics of pulmonary-delivered human growth hormone in rabbits

    Pharm. Res.

    (1995)
  • E.R. Weibel

    Morphological basis of alveolar–capillary gas exchange

    Physiol. Rev.

    (1973)
  • E.E. Schneeberger

    The integrity of the air–blood barrier

  • G. Clough et al.

    The role of vesicles in the transport of ferritin through frog endothelium

    J. Physiol.

    (1981)
  • A.M. Fra et al.

    De novo formation of caveolae in lymphocytes by expression of VIP21-caveolin

    Cell Biol.

    (1995)
  • U. Vogel et al.

    Expression of caveolin-1 and polarised formation of invaginated caveolae in Caco-2 and MDCK II cells

    J. Cell Sci.

    (1998)
  • S. Monier et al.

    VIP21-caveolin, a membrane protein constituent of caveolae coat, oligomerizes in-vivo and in-vitro

    Mol. Biol. Cell

    (1995)
  • M. Sargiacomo et al.

    Oligomeric structure of caveolin: implications for caveolae membrane organisation

    Proc. Natl. Acad. Sci. U.S.A.

    (1995)
  • J.D. Crapo et al.

    Cell number and cell characteristics of normal human lung

    Am. Rev. Respir. Dis.

    (1982)
  • B.D. Uhal

    Cell cycle kinetics in the alveolar epithelium

    Am. J. Physiol.

    (1997)
  • J. Gil et al.

    Distribution of vesicles in cells of the air–blood barrier in the rabbit

    J. Appl. Physiol.

    (1981)
  • J. Gil

    Number and distribution of plasmalemma vesicles in the lung

    Fed. Proc.

    (1983)
  • D.O. DeFouw, Ultrastructural features of alveolar epithelial transport, in: E.D. Crandall (Eds.) Fluid balance across...
  • D.O. DeFouw et al.

    Numerical denisties of cellular vesicles and cellular attentuation in the pulmonary alveolar septa of edematous dog lungs

    Microvasc. Res.

    (1982)
  • D.M. Haies et al.

    Morphometric study of rat lung cells: numerical and dimensional characteristics of parenchymal cell populations

    Am. Rev. Respir. Dis.

    (1981)
  • J. Gil et al.

    Morphometry of pinocytic vesicles in the capillary endothelium of rabbit lungs using automated equipment

    Circ. Res.

    (1980)
  • M. Simionescu et al.

    Morphological data on the endothelium of blood capillaries

    J. Cell Biol.

    (1974)
  • O.S. Atwal et al.

    An uptake of cationized ferritin by alveolar type I cells in airway-instilled goat lung: distribution of anionic sites on the epithelial surface

    J. Submicrosc. Cytol. Pathol.

    (1990)
  • R.E. Gordon et al.

    Endocytic vesicles of type I pneumocytes. Immunocytochemical colocalisation of calmodulin with clathrin molecules

    J. Histotechnol.

    (1989)
  • M.P. Lisanti et al.

    Characterisation of caveolin-rich domains isolated from an endothelial rich source: implications for human disease

    J. Cell Biol.

    (1994)
  • M. Kasper et al.

    Loss of caveolin-1 expression in type I pneumocytes as an indicator of subcellular alterations during lung fibrogenesis

    Histochem. Cell Biol.

    (1998)
  • G.R. Newman et al.

    Caveolin and its cellular and subcellular immunolocalisation in lung alveolar epithelium: implications for alveolar type I cell function

    Cell Tissue Res.

    (1999)
  • Cited by (101)

    • Type I Cells

      2021, Encyclopedia of Respiratory Medicine, Second Edition
    • Inhaled nanoparticles–An updated review

      2020, International Journal of Pharmaceutics
      Citation Excerpt :

      Immersion of the inhaled, slowly dissolving or insoluble nanomaterials in the fluid lining the lungs may enable them to be closely associated with epithelial cells and cells of the host-defense system for particle–cell interaction (Geiser et al., 2003). Subsequently, several post-defense mechanisms, including the mucociliary escalator transport, phagocytosis by macrophages and endocytosis, are involved in the removal of deposited nanoparticles and to maintain the lung mucosal surfaces (Gumbleton, 2001; Arredouani et al., 2004). The mucociliary escalator dominates clearance of nanoparticles from the upper airways.

    • Harnessing albumin as a carrier for cancer therapies

      2018, Advanced Drug Delivery Reviews
    View all citing articles on Scopus
    View full text