β-propeller protein-associated neurodegeneration (BPAN), also known as static encephalopathy of childhood with neurodegeneration in adulthood (SENDA), was firstly described in 2009 [1]. BPAN is a X-linked dominant disorder, and the patients are characterized by global developmental delay in early childhood with essentially static, slow motor and cognitive gains until adolescence or early adulthood. In young adulthood, affected individuals develop progressive dystonia, parkinsonism, extrapyramidal signs, and dementia resulting in severe disability [2]. In 2012, Haack et al. discovered that mutations in WDR45 were responsible for BPAN [3]. WDR45 is a homologue of yeast Atg18 which is important for autophagosome formation [4,5]. So far, it is well established that the impairment of macroautophagy resulted by mutations in WDR45 is the cause of BPAN [[6], [7], [8]]. As a subtype of neurodegeneration with brain iron accumulation (NBIA), BPAN also shares the common hallmark feature of NBIA that iron accumulation in the globus pallidus and substantia nigra [3]. Furthermore, BPAN patients or WDR45 deficient cells also showed malfunctioning mitochondria and endoplasmic reticulum (ER) stress without aberrant iron homeostasis, suggesting that WDR45 is a multifunction protein [9]. Recently, iron overload was supposed to be the primary cause for those disorders [9,10]. However, the mechanism of how WDR45 regulates those processes especially the iron homeostasis remains unclear.

Iron is an essential element for cell function so its intracellular delivery has to be tightly regulated. Cells import irons through transferrin-bound iron and nontransferrin-bound iron uptake pathways [11]. Studies have demonstrated that these two pathways contribute for the iron accumulation in BPAN as the transferrin receptor (TfRC) and the divalent metal transporter 1 (DMT1) are increased in WDR45 deficient cells [[12], [13], [14]]. Imported irons were utilized as cofactors of various enzymes in cells. Excess irons will be either stored by ferritin, or exported out of the cell via ferroportin 1 (FPN1) to maintain the iron homeostasis [15]. Ferritin-bound iron is considered harmless because the iron is not able to react with surrounding molecules, while the free irons (Fe2+, also known as labile iron pool, LIP) would increase the generation of reactive oxygen species (ROS) which consequently causes oxidative damage to cells through Haber-Weiss and Fenton reactions [16,17]. Thereby the LIP was tightly regulated [18,19]. Iron accumulating at toxic levels within neurons may lead to cell death via apoptosis, autophagy, necrosis and ferroptosis [20]. Recently, we have demonstrated that overexpressing a BPAN-related mutation of WDR45 in HeLa cells led to iron overload and consequently cell ferroptosis [12], suggesting that ferroptosis is also involved in BPAN.

Consistent with the in vivo observation, iron accumulation was also observed in the skin fibroblasts and induced pluripotent stem cells (iPSCs)-derived neurons from BPAN patients [10,21]. Besides increased iron incorporation into WDR45 deficient cells [12,13,22], accelerated ferritinophagy also increased cellular Fe2+ [23]. Indeed, the reduced ferritin was detected in patient derived fibroblasts and mutant WDR45 overexpressing HeLa cells [12,21,24]. However, BPAN patients with WDR45 mutations presented impaired macroautophagy [[6], [7], [8],14,21], thereby ferritinophagy seems not to be the reason for decreased ferritin. Therefore, dysregulation of iron homeostasis in BPAN still need to be elucidated. In previous, we reported a novel de novo mutation (c.1040_1041del, p.Glu347GlyfsTer7) in WDR45 in a Chinese BPAN patient, and generated a HeLa cell line that stably overexpressing this mutant WDR45 [7]. Our previous results showed that macroautophagy was impaired in this cell line suggested that this mutation in WDR45 is pathogenic [7]. Furthermore, overexpressing this mutant WDR45 in HeLa cells resulted in a reduction of GPX4 and FTH [12]. Considering that chaperone-mediated autophagy (CMA) could be compensatory upregulated when macroautophagy is compromised [25,26], and CMA plays an important role in the degradation of GPX4 [27,28], we hypothesize that CMA may be involved in BPAN. Thus, the goal of this study is to define the contribution of CMA to iron homeostasis. We demonstrated that FTH could be degraded via CMA. Overexpressing mutant WDR45 activates CMA in an ER stress/p38 pathway-dependent manner. Inhibition of this pathway blocked the activation of CMA thereby alleviated the decrease of FTH and reduced the level of Fe2+. Thus, our study revealed that a dysfunction in the macroautophagy machinery promotes CMA which regulates the iron homeostasis by degradation of FTH.