Rab38 targets to lamellar bodies and normalizes their sizes in lung alveolar type II epithelial cells

Abstract

Rab38 is a rat Hermansky-Pudlak syndrome gene that plays an important role in surfactant homeostasis in alveolar type II (ATII) pneumocytes. We examined Rab38 function in regulating lamellar body (LB) morphology in ATII cells. Quantitative electron microscopy revealed that LBs in ATII cells were ∼77% larger in Rab38-null fawn-hooded hypertension (FHH) than control Sprague-Dawley (SD) rats. Rab38 protein expression was restricted in lung epithelial cells but was not found in primary endothelial cells. In SD ATII cells, Rab38 protein level gradually declined during 5 days in culture. Importantly, endogenous Rab38 was present in LB fractions purified from SD rat lungs, and transiently expressed enhanced green fluorescent protein (EGFP)-tagged Rab38 labeled only the limiting membranes of a subpopulation (∼30%) of LBs in cultured ATII cells. This selective targeting was abolished by point mutations to EGFP-Rab38 and was not shared by Rab7 and Rab4b, which also function in the ATII cells. Using confocal microscopy, we established a method for quantitative evaluation of the enlarged LB phenotype temporally preserved in cultured FHH ATII cells. A direct causal relationship was established when the enlarged LB phenotype was reserved and then rescued by transiently reexpressed EGFP-Rab38 in cultured FHH ATII cells. This rescuing effect was associated with dynamic EGFP-Rab38 targeting to and on LB limiting membranes. We conclude that Rab38 plays an indispensible role in maintaining LB morphology and surfactant homeostasis in ATII pneumocytes.

Pulmonary Fibrosis in Hermansky-Pudlak Syndrome

“…In type II alveolar epithelial cells of the lung, Rab38 helps maintain lamellar body morphology and surfactant homeostasis (24). The role of this defect in the development of pulmonary fibrosis is not known, but it is intriguing to note that abnormalities in surfactant proteins have been identified in idiopathic pulmonary fibrosis and related interstitial lung disorders (24,26,27). Disease mechanisms remain uncertain for the occurrence of granulomatous colitis, which has also been reported in approximately 15% of patients with HPS, not necessarily tracking with the occurrence of subtype risk for pulmonary fibrosis.…”

“…proteins) are a type of LRO (27), making alveolar epithelial cells a good candidate for dysfunction in HPS.…”

Rab38 Modulates Proteinuria in Model of Hypertension-Associated Renal Disease

“…Fawn-hooded rats have dysfunctional lysosome-related organelles such as platelet dense granules, melanosomes, and alveolar cell lamellar bodies that can be normalized by restoration of Rab38 expression. 20,42,43 Abnormal lysosomal function in proximal tubular cells could conceivably diminish the breakdown of filtered proteins. Alternatively, aberrant trafficking could lead to misplacement or redistribution of receptors such as megalin and cubilin, causing decreased protein re-uptake.…”

Mechanism of platelet dense granule biogenesis: study of cargo transport and function of Rab32 and Rab38 in a model system

“…Rab7 also has been found in LROs such as melanosomes and lamellar bodies. 33,34 We reasoned that if MVBs/late endosomes are DG precursors, and Rab32 and Rab38 localize primarily to immature DGs, these Rabs should colocalize preferentially with Rab7 but only marginally with Rab5. Indeed, confocal fluorescence microscopy images and corresponding fluorescence intensity line scans showed significant colocalization between GFP-Rab7a and either Cherry-Rab32 or CherryRab38 (MOC ϭ 0.52 Ϯ 0.02 and 0.42 Ϯ 0.03, respectively; Figure 5B,D).…”

“…[15][16][17]34 Rab38 and its very close homolog Rab32 operate in a partially redundant manner in melanosome biogenesis. 17,18 We found that Rab32 and Rab38 partially colocalize with AP-3 and clathrin both in primary MKs and MEG-01 cells.…”