Minibrain kinase and calcineurin coordinate activity-dependent bulk endocytosis through synaptojanin

Dysfunction of synaptic endocytic trafficking in Parkinson's disease

Parkinson's disease is characterized by the selective degeneration of dopamine neurons in the nigrostriatal pathway and dopamine deficiency in the striatum. The precise reasons behind the specific degeneration of these dopamine neurons remain largely elusive. Genetic investigations have identified over 20 causative PARK genes and 90 genomic risk loci associated with both familial and sporadic Parkinson's disease. Notably, several of these genes are linked to the synaptic vesicle recycling process, particularly the clathrin-mediated endocytosis pathway. This suggests that impaired synaptic vesicle recycling might represent an early feature of Parkinson's disease, followed by axonal degeneration and the eventual loss of dopamine cell bodies in the midbrain via a "dying back" mechanism. Recently, several new animal and cellular models with Parkinson's disease-linked mutations affecting the endocytic pathway have been created and extensively characterized. These models faithfully recapitulate certain Parkinson's disease-like features at the animal, circuit, and cellular levels, and exhibit defects in synaptic membrane trafficking, further supporting the findings from human genetics and clinical studies. In this review, we will first summarize the cellular and molecular findings from the models of two Parkinson's disease-linked clathrin uncoating proteins: auxilin (DNAJC6/PARK19) and synaptojanin 1 (SYNJ1/PARK20). The mouse models carrying these two PARK gene mutations phenocopy each other with specific dopamine terminal pathology and display a potent synergistic effect. Subsequently, we will delve into the involvement of several clathrin-mediated endocytosis-related proteins (GAK, endophilin A1, SAC2/INPP5F, synaptotagmin-11), identified as Parkinson's disease risk factors through genome-wide association studies, in Parkinson's disease pathogenesis. We will also explore the direct or indirect roles of some common Parkinson's disease-linked proteins (alpha-synuclein (PARK1/4), Parkin (PARK2), and LRRK2 (PARK8)) in synaptic endocytic trafficking. Additionally, we will discuss the emerging novel functions of these endocytic proteins in downstream membrane traffic pathways, particularly autophagy. Given that synaptic dysfunction is considered as an early event in Parkinson's disease, a deeper understanding of the cellular mechanisms underlying synaptic vesicle endocytic trafficking may unveil novel targets for early diagnosis and the development of interventional therapies for Parkinson's disease. Future research should aim to elucidate why generalized synaptic endocytic dysfunction leads to the selective degeneration of nigrostriatal dopamine neurons in Parkinson's disease.

DYRK1-mediated phosphorylation of endocytic components is required for extracellular lumen expansion in ascidian notochord

BACKGROUND

The biological tube is a basal biology structure distributed in all multicellular animals, from worms to humans, and has diverse biological functions. Formation of tubular system is crucial for embryogenesis and adult metabolism. Ascidian Ciona notochord lumen is an excellent in vivo model for tubulogenesis. Exocytosis has been known to be essential for tubular lumen formation and expansion. The roles of endocytosis in tubular lumen expansion remain largely unclear.

RESULTS

In this study, we first identified a dual specificity tyrosine-phosphorylation-regulated kinase 1 (DYRK1), the protein kinase, which was upregulated and required for ascidian notochord extracellular lumen expansion. We demonstrated that DYRK1 interacted with and phosphorylated one of the endocytic components endophilin at Ser263 that was essential for notochord lumen expansion. Moreover, through phosphoproteomic sequencing, we revealed that in addition to endophilin, the phosphorylation of other endocytic components was also regulated by DYRK1. The loss of function of DYRK1 disturbed endocytosis. Then, we demonstrated that clathrin-mediated endocytosis existed and was required for notochord lumen expansion. In the meantime, the results showed that the secretion of notochord cells is vigorous in the apical membrane.

CONCLUSIONS

We found the co-existence of endocytosis and exocytosis activities in apical membrane during lumen formation and expansion in Ciona notochord. A novel signaling pathway is revealed that DYRK1 regulates the endocytosis by phosphorylation that is required for lumen expansion. Our finding thus indicates a dynamic balance between endocytosis and exocytosis is crucial to maintain apical membrane homeostasis that is essential for lumen growth and expansion in tubular organogenesis.

A cellular atlas of calcineurin signaling

Intracellular Ca2+ signals are temporally controlled and spatially restricted. Signaling occurs adjacent to sites of Ca2+ entry and/or release, where Ca2+-dependent effectors and their substrates co-localize to form signaling microdomains. Here we review signaling by calcineurin, the Ca2+/calmodulin regulated protein phosphatase and target of immunosuppressant drugs, Cyclosporin A and FK506. Although well known for its activation of the adaptive immune response via NFAT dephosphorylation, systematic mapping of human calcineurin substrates and regulators reveals unexpected roles for this versatile phosphatase throughout the cell. We discuss calcineurin function, with an emphasis on where signaling occurs and mechanisms that target calcineurin and its substrates to signaling microdomains, especially binding of cognate short linear peptide motifs (SLiMs). Calcineurin is ubiquitously expressed and regulates events at the plasma membrane, other intracellular membranes, mitochondria, the nuclear pore complex and centrosomes/cilia. Based on our expanding knowledge of localized CN actions, we describe a cellular atlas of Ca2+/calcineurin signaling.

Triplication of Synaptojanin 1 in Alzheimer's Disease Pathology in Down Syndrome

Down Syndrome (DS), caused by triplication of human chromosome 21 (Hsa21) is the most common form of intellectual disability worldwide. Recent progress in healthcare has resulted in a dramatic increase in the lifespan of individuals with DS. Unfortunately, most will develop Alzheimer's disease like dementia (DS-AD) as they age. Understanding similarities and differences between DSAD and the other forms of the disease - i.e., late-onset AD (LOAD) and autosomal dominant AD (ADAD) - will provide important clues for the treatment of DS-AD. In addition to the APP gene that codes the precursor of the main component of amyloid plaques found in the brain of AD patients, other genes on Hsa21 are likely to contribute to disease initiation and progression. This review focuses on SYNJ1, coding the phosphoinositide phosphatase synaptojanin 1 (SYNJ1). First, we highlight the function of SYNJ1 in the brain. We then summarize the involvement of SYNJ1 in the different forms of AD at the genetic, transcriptomic, proteomic and neuropathology levels in humans. We further examine whether results in humans correlate with what has been described in murine and cellular models of the disease and report possible mechanistic links between SYNJ1 and the progression of the disease. Finally, we propose a set of questions that would further strengthen and clarify the role of SYNJ1 in the different forms of AD.

Fine-tuning activity-dependent bulk endocytosis via kinases and phosphatases

The regulation of activity-dependent bulk endocytosis, the dominant mode of membrane retrieval in response to intense neuronal activity, is poorly understood. In this JCB issue, Peng et al. (2021. J. Cell. Biol.https://doi.org/10.1083/jcb.202011028) propose a novel molecular mechanism for the coordination of activity-dependent bulk endocytosis that builds on Minibrain kinase and its presynaptic substrate synaptojanin-1.