Combined deficiency of RAB32 and RAB38 in the mouse mimics Hermansky-Pudlak syndrome and critically impairs thrombosis

Abstract

The biogenesis of lysosome related organelles is defective in Hermansky-Pudlak syndrome (HPS), a disorder characterized by oculocutaneous albinism and platelet dense granule (DG) defects. The first animal model of HPS was the fawn-hooded rat, harboring a spontaneous mutation inactivating the small guanosine triphosphatase Rab38. This leads to coat color dilution associated with the absence of DGs and lung morphological defects. Another RAB38 mutant, the cht mouse, has normal DGs, which has raised controversy about the role of RAB38 in DG biogenesis. We show here that murine and human, but not rat, platelets also express the closely related RAB32. To elucidate the parts played by RAB32 and RAB38 in the biogenesis of DGs in vivo and their effects on platelet functions, we generated mice inactivated for Rab32, Rab38, and both genes. Single Rab38 inactivation mimicked cht mice, whereas single Rab32 inactivation had no effect in DGs, coat color, or lung morphology. By contrast, Rab32/38 double inactivation mimicked severe HPS, with strong coat and eye pigment dilution, some enlarged lung multilamellar bodies associated with a decrease in the number of DGs. These organelles were morphologically abnormal, decreased in number, and devoid of 5-hydroxytryptamine content. In line with the storage pool defect, platelet activation was affected, resulting in severely impaired thrombus growth and prolongation of the bleeding time. Overall, our study demonstrates the absence of impact of RAB38 or RAB32 single deficiency in platelet biogenesis and function resulting from full redundancy, and characterized a new mouse model mimicking HPS devoid of DG content.

Rab family of small GTPases: an updated view on their regulation and functions

“…HPS1 and HPS4 have subsequently been shown to heterodimerize to form the BLOC‐3 complex, which exhibits GEF activity toward Rab32 and its paralog Rab38 [21]. Furthermore, double KO of Rab32/38 in mice results in severe coat color dilution and defects in hemostasis and lung morphology [114], indicating involvement of these Rabs in LRO biogenesis. The number of dense granules in Rab32/38‐KO mouse platelets is reduced, and their morphology is abnormal.…”

LRRK2 binds to the Rab32 subfamily in a GTP-dependent manner via its armadillo domain

“…In this case, the positively-charged switch 1 residue is not critical for the interaction and may suggest a distinct mode of binding for Rab29. Rab32/38 functions are associated with biogenesis of lysosome-related organelles in epithelial cells [28,29], while expression of Rab38 has recently been linked to pancreatic cancer [30]. LRRK2 is widely expressed in human tissues, but the links between Rab32/38 and LRRK2 in vivo requires further investigation.…”

Rab GTPases: Key players in melanosome biogenesis, transport, and transfer

“…These mutants are considered models of human Hermansky–Pudlak syndrome (HPS), which is characterized by oculocutaneous albinism, a bleeding tendency, and, in many cases, pulmonary fibrosis (Table 1; reviewed in Huizing et al., 2020); however, no RAB38 mutation has been reported in HPS patients, probably because Rab32, the paralog of Rab38, compensates for the function of Rab38 in melanocytes (Wasmeier et al., 2006). Actually, Rab32 and Rab38 (Rab32/38) double‐knockout (KO) mice have recently been reported to exhibit phenotypes similar to those of HPS patients (Aguilar et al., 2019) (Table 1), and the heterodimeric BLOC‐3 complex, which is composed of HPS1 and HPS4 (Oh et al., 1996; Suzuki et al., 2002), is known to function as a GEF for Rab32/38 in melanocytes (Gerondopoulos et al., 2012; Ohishi et al., 2019). Intriguingly, dysfunctions of Rabggta, an α‐subunit of Rab geranylgeranyl transferase that is required for prenylation of Rabs, also cause similar HPS phenotypes (Detter et al., 2000).…”