The PICALM Protein Plays a Key Role in Iron Homeostasis and Cell Proliferation

Abstract

The ubiquitously expressed phosphatidylinositol binding clathrin assembly (PICALM) protein associates with the plasma membrane, binds clathrin, and plays a role in clathrin-mediated endocytosis. Alterations of the human PICALM gene are present in aggressive hematopoietic malignancies, and genome-wide association studies have recently linked the PICALM locus to late-onset Alzheimer's disease. Inactivating and hypomorphic Picalm mutations in mice cause different degrees of severity of anemia, abnormal iron metabolism, growth retardation and shortened lifespan. To understand PICALM’s function, we studied the consequences of PICALM overexpression and characterized PICALM-deficient cells derived from mutant fit1 mice. Our results identify a role for PICALM in transferrin receptor (TfR) internalization and demonstrate that the C-terminal PICALM residues are critical for its association with clathrin and for the inhibitory effect of PICALM overexpression on TfR internalization. Murine embryonic fibroblasts (MEFs) that are deficient in PICALM display several characteristics of iron deficiency (increased surface TfR expression, decreased intracellular iron levels, and reduced cellular proliferation), all of which are rescued by retroviral PICALM expression. The proliferation defect of cells that lack PICALM results, at least in part, from insufficient iron uptake, since it can be corrected by iron supplementation. Moreover, PICALM-deficient cells are particularly sensitive to iron chelation. Taken together, these data reveal that PICALM plays a critical role in iron homeostasis, and offer new perspectives into the pathogenesis of PICALM-associated diseases.

A CALM-derived nuclear export signal is essential for CALM-AF10–mediated leukemogenesis

“…5 We have recently determined that the CALM clathrinbinding domain spans aa 583-652 at the extreme C terminus, which is distinct from the NES. 19 Similarly, Stoddart et al 21 have shown that endocytosis is not perturbed in CALM-AF10 cells. Instead, these authors suggested that homo-oligomerization of CALM-AF10 may mediate its transformation potential, based on the ability of the last 55 aa of CALM within CALM-AF10 (aa 593-648) to facilitate dimerization.…”

“…19,22 Calm NULL CALM-AF10 leukemias were generated by transplanting CALM-AF10-transduced Calm NULL E14 fetal liver cells into lethally irradiated B6(Cg)-Tyr C-2J /J mice. Upon development of symptoms, mice were sacrificed, and bone marrow cells were cultured with interleukin (IL)-3.…”

“…17 Structurally, CALM contains domains that are involved in endocytosis, including an epsin N-terminal homology domain 18 and a clathrin-binding domain in the carboxy-terminus. 17, 19 CALM has also been shown to shuttle between the cytoplasm and the nucleus, where it may activate transcription, although this remains poorly understood. 20 Although perturbation of endocytosis as a result of CALM gene translocations has been hypothesized to play a role in leukemogenesis, 5,21 the specific contributions of CALM to CALM-AF10-dependent leukemogenesis remain unclear.…”

Cargo Recognition in Clathrin-Mediated Endocytosis

“…By binding to multiple adaptors or CLASPs, cargoes participate directly in the network, and thus modulate the coat assembly process (Mettlen et al 2010). A corollary of this densely wired redundant network is that gene silencing or RNAi of numerous individual CLASPs does not have a dramatic penetrant effect on endocytosis (Garcia et al 2001;Kang-Decker et al 2001;Morris et al 2002;Kamikura and Cooper 2003;Holmes et al 2007;Koh et al 2007;Wang et al 2008;Chen et al 2009;Mullen et al 2012;Pozzi et al 2012;Scotland et al 2012;Suzuki et al 2012;Umasankar et al 2012;Kononenko et al 2013;Tsushima et al 2013). Rather, subtle or tissue-specific defects are manifest linked to im- proper signal relay by the cargo affected.…”

Regulation of heme biosynthesis and transport in metazoa

“…Three studies unanimously reported that the phosphatidylinositol binding clathrin assembly protein (PICALM) plays an essential role in TFR endocytosis [19][20][21]. Picalm-deficient mice developed severe anemia, which is due to impaired uptake of iron in erythroid precursors.…”