A Structural Overview of TRPML1 and the TRPML Family

Intracellular calcium channels: Potential targets for type 2 diabetes mellitus?

Type 2 diabetes mellitus (T2DM) is a prevalent metabolic disorder. Despite the availability of numerous pharmacotherapies, a range of adverse reactions, including hypoglycemia, gastrointestinal discomfort, and lactic acidosis, limits their patient applicability and long-term application. Therefore, it is necessary to screen novel therapeutic drugs for T2DM treatment that have high efficacy but few adverse effects. AMP-activated protein kinase (AMPK) stands out as one of the most powerful targets for T2DM treatment. It can be activated through energy-sensing or calcium signaling. Medications that activate AMPK through the energy-sensing mechanism exhibit remarkable potency, but they are accompanied by lactic acidosis, carrying an alarmingly high mortality rate. Interestingly, medications that activate AMPK through calcium signaling, such as gliclazide, seldom induce lactic acidosis. However, the efficacy of gliclazide is much lower than metformin. Therefore, it is necessary to explore targets that activate AMPK via calcium signaling to avoid lactic acidosis while maintaining high potency. Ion channels are the main controller of intracellular calcium flow. Specific agonists and inhibitors targeting ion channels have been reported to activate AMPK. In this review, we will summarize the structure and function of calcium-permeable ion channels and discuss the potential of targeting these calcium channels for T2DM treatment.

TRPML-1 Dysfunction and Renal Tubulopathy in Mucolipidosis Type IV

Background

Loss-of-function mutations in the lysosomal channel transient receptor potential cation channel (TRPML-1) cause mucolipidosis type IV (MLIV), a rare lysosomal storage disease characterized by neurological defects, progressive vision loss, and achlorhydria. Recent reports have highlighted kidney disease and kidney failure in patients with MLIV during the second to third decade of life; however, the molecular mechanisms driving kidney dysfunction remain poorly understood.

Methods

A cross-sectional review of medical records from 21 patients with MLIV (ages 3–43 years) was conducted to assess kidney function impairment. In addition, we examined the kidney phenotype of MLIV mice at various ages, along with human kidney cells silenced for TRPML-1 and primary tubular cells from wild-type and MLIV mice. Immunohistology and cell biology approaches were used to phenotype nephron structure, the endolysosomal compartment, and inflammation. Kidney function was assessed through proteomic analysis of mouse urine and in vivo kidney filtration measurements.

Results

Of the 21 patients with MLIV, only adults were diagnosed with stage 2–3 CKD. Laboratory abnormalities included lower eGFR and higher levels BUN/creatine in blood and proteinuria. In MLIV mice, we observed significant alterations in endolysosomal morphology, function, and impaired autophagy in proximal and distal tubules. This led to the accumulation of megalin (LRP2) in the subapical region of proximal tubular cells, indicating a block in apical receptor–mediated endocytosis. In vivo and in vitro experiments confirmed reduced fluid-phase endocytosis and impaired uptake of ligands, including β-lactoglobulin, transferrin, and albumin in MLIV proximal tubular cells. Urine analysis revealed tubular proteinuria and enzymuria in mice with MLIV. In addition, early-stage disease was marked by increased inflammatory markers, fibrosis, and activation of the proinflammatory transcription factor NF-κB, coinciding with endolysosomal defects. Importantly, adeno-associated viral–mediated TRPML-1 gene delivery reversed key pathological phenotypes in MLIV mice, underscoring TRPML-1's critical role in kidney function.

Conclusions

Our findings link TRPML-1 dysfunction to the development of kidney disease in MLIV.

A Novel Homozygous Variant in the MCOLN1 Gene Associated With Severe Oromandibular Dystonia and Parkinsonism

BACKGROUND

Mucolipidosis type IV (MLIV) is a rare, progressive lysosomal storage disorder characterized by severe intellectual disability, delayed motor milestones and ophthalmologic abnormalities. MLIV is an autosomal recessive disease caused by mutations in the MCOLN1 gene, encoding mucolipin-1 which is responsible for maintaining lysosomal function.

OBJECTIVES AND METHODS

Here, we report a family of four Iranian siblings with cognitive decline, progressive visual and pyramidal disturbances, and abnormal movements manifested by severe oromandibular dystonia and parkinsonism. MRI scans of the brain demonstrated signal abnormalities in the white matter and thinning of the corpus callosum.

RESULTS AND CONCLUSIONS

Whole-exome sequencing identified a novel homozygous variant, c.362C > T:p. Thr121Met in the MCOLN1 gene consistent with a diagnosis of MLIV. The presentation of MLIV may overlap with a variety of other neurological diseases, and genetic analysis is an important strategy to clarify the diagnosis. This is an important point that clinicians should be familiar with. The novel variant c.362C > T:p. Thr121Met herein described may be related to a comparatively older age at onset. Our study also expands the clinical spectrum of MLIV associated with the MCOLN1 variants and introduces a novel likely pathogenic variant for testing in MLIV cases that remain unresolved.

Pathological Functions of Lysosomal Ion Channels in the Central Nervous System

Lysosomes are highly dynamic organelles that maintain cellular homeostasis and regulate fundamental cellular processes by integrating multiple metabolic pathways. Lysosomal ion channels such as TRPML1-3, TPC1/2, ClC6/7, CLN7, and TMEM175 mediate the flux of Ca2+, Cl-, Na+, H+, and K+ across lysosomal membranes in response to osmotic stimulus, nutrient-dependent signals, and cellular stresses. These ion channels serve as the crucial transducers of cell signals and are essential for the regulation of lysosomal biogenesis, motility, membrane contact site formation, and lysosomal homeostasis. In terms of pathophysiology, genetic variations in these channel genes have been associated with the development of lysosomal storage diseases, neurodegenerative diseases, inflammation, and cancer. This review aims to discuss the current understanding of the role of these ion channels in the central nervous system and to assess their potential as drug targets.

Phosphoinositide Regulation of TRP Channels: A Functional Overview in the Structural Era

Transient receptor potential (TRP) ion channels have diverse activation mechanisms including physical stimuli, such as high or low temperatures, and a variety of intracellular signaling molecules. Regulation by phosphoinositides and their derivatives is their only known common regulatory feature. For most TRP channels, phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] serves as a cofactor required for activity. Such dependence on PI(4,5)P2 has been demonstrated for members of the TRPM subfamily and for the epithelial TRPV5 and TRPV6 channels. Intracellular TRPML channels show specific activation by PI(3,5)P2. Structural studies uncovered the PI(4,5)P2 and PI(3,5)P2 binding sites for these channels and shed light on the mechanism of channel opening. PI(4,5)P2 regulation of TRPV1-4 as well as some TRPC channels is more complex, involving both positive and negative effects. This review discusses the functional roles of phosphoinositides in TRP channel regulation and molecular insights gained from recent cryo-electron microscopy structures.