ER-to-Golgi protein delivery through an interwoven, tubular network extending from ER.

ER-phagy (reticulo-phagy) defines the degradation of portions of the endoplasmic reticulum (ER) within lysosomes or vacuoles. It is part of the self-digestion (i.e., auto-phagic) programs recycling cytoplasmic material and organelles, which rapidly mobilize metabolites in cells confronted with nutrient shortage. Moreover, selective clearance of ER subdomains participates to the control of ER size and activity during ER stress, the re-establishment of ER homeostasis after ER stress resolution and the removal of ER parts, in which aberrant and potentially cytotoxic material has been segregated. ER-phagy relies on the individual and/or concerted activation of the ER-phagy receptors, ER peripheral or integral membrane proteins that share the presence of LC3/Atg8-binding motifs in their cytosolic domains. ER-phagy involves the physical separation of portions of the ER from the bulk ER network, and their delivery to the endolysosomal/vacuolar catabolic district. This last step is accomplished by a variety of mechanisms including macro-ER-phagy (in which ER fragments are sequestered by double-membrane autophagosomes that eventually fuse with lysosomes/vacuoles), micro-ER-phagy (in which ER fragments are directly engulfed by endosomes/lysosomes/vacuoles), or direct fusion of ER-derived vesicles with lysosomes/vacuoles. ER-phagy is dysfunctional in specific human diseases and its regulators are subverted by pathogens, highlighting its crucial role for cell and organism life.

ER-phagy: mechanisms, regulation, and diseases connected to the lysosomal clearance of the endoplasmic reticulum

Osteogenesis Imperfecta (OI) comprises a heterogeneous group of patients who share bone fragility and deformities as the main characteristics, albeit with different degrees of severity. Phenotypic variation also exists in other connective tissue aspects of the disease, complicating disease classification and disease course prediction. Although collagen type I defects are long established as the primary cause of the bone pathology, we are still far from comprehending the complete mechanism. In the last years, the advent of next generation sequencing has triggered the discovery of many new genetic causes for OI, helping to draw its molecular landscape. It has become clear that, in addition to collagen type I genes, OI can be caused by multiple proteins connected to different parts of collagen biosynthesis. The production of collagen entails a complex process, starting from the production of the collagen Iα1 and collagen Iα2 chains in the endoplasmic reticulum, during and after which procollagen is subjected to a plethora of posttranslational modifications by chaperones. After reaching the Golgi organelle, procollagen is destined to the extracellular matrix where it forms collagen fibrils. Recently discovered mutations in components of the retrograde transport of chaperones highlight its emerging role as critical contributor of OI development. This review offers an overview of collagen regulation in the context of recent gene discoveries, emphasizing the significance of transport disruptions in the OI mechanism. We aim to motivate exploration of skeletal fragility in OI from the perspective of these pathways to identify regulatory points which can hint to therapeutic targets.

/pdf/collagen-transport-and-related-pathways-in-osteogenesis-563x0rihpr.pdf

Collagen transport and related pathways in Osteogenesis Imperfecta.

ER exit sites in Drosophila display abundant ER-Golgi vesicles and pearled tubes but no megacarriers.

Targeting cellular stress in vitro improves osteoblast homeostasis, matrix collagen content and mineralization in two murine models of osteogenesis imperfecta.

Mechanisms of procollagen and HSP47 sorting during ER-to-Golgi trafficking

Cellular response to protein misfolding underlies multiple diseases. Collagens are the most abundant vertebrate proteins, yet little is known about cellular response to misfolding of their procollagen precursors. Osteoblasts (OBs)—the cells that make bone—produce so much procollagen that it accounts for up to 40% of mRNAs in the cell, which is why bone bears the brunt of mutations causing procollagen misfolding in osteogenesis imperfecta (OI). The present study of a G610C mouse model of OI by multiple transcriptomic techniques provides first solid clues to how OBs respond to misfolded procollagen accumulation in the endoplasmic reticulum (ER) and how this response affects OB function. Surprisingly, misfolded procollagen escapes the quality control in the ER lumen and indirectly triggers the integrated stress response (ISR) through other cell compartments. In G610C OBs, the ISR is regulated by mitochondrial HSP70 (mt‐HSP70) and ATF5 instead of their BIP and ATF4 paralogues, which normally activate and regulate ISR to secretory protein misfolding in the ER. The involvement of mt‐HSP70 and ATF5 together with other transcriptomic findings suggest that mitochondria might initiate the ISR upon disruption of ER‐mitochondria connections or might respond to the ISR activated by a yet unknown sensor.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/advs.202201273

ER, Mitochondria, and ISR Regulation by mt‐HSP70 and ATF5 upon Procollagen Misfolding in Osteoblasts

Mechanisms underlying chronic pain are insufficiently understood. Preclinical evidence suggests a potential contribution of excitatory glutamatergic and inhibitory GABAergic imbalances in pain-relevant brain areas, such as a lower excitatory/inhibitory tone in the brainstem periaqueductal grey (PAG). This cross-sectional magnetic resonance spectroscopy (MRS) study investigated whether a lower excitatory/inhibitory tone is also observed in the PAG of patients with non-specific chronic low back pain (CLBP) and whether this would relate to altered psychophysical measures of descending pain modulation and experimental pressure pain sensitivity. Specifically, the ratio between pooled glutamate and glutamine and GABA levels (Glx/GABA), Glx and GABA in the PAG were compared between CLBP patients and pain-free controls. Further, associations of Glx/GABA with conditioned pain modulation (CPM) effects and pressure pain thresholds (PPTs) were assessed. MRS was acquired on a 3T Philipps MR system using a point-resolved spectroscopy sequence optimized with very selective saturation pulses (OVERPRESS) and voxel-based flip angle calibration in a 1.1 mL volume of interest. Data from 41 CLBP patients (median [interquartile range]: 54 years [41 - 65], 22 females) and 29 age- and sex-matched controls (47 years [34 - 67], 17 females) fulfilled MRS quality criteria. CPM and PPTs were assessed at the lower back as most painful area and the non-dominant hand as pain-free control area. The CPM paradigm consisted of PPTs applied before, during (parallel CPM effect) and after a cold water bath and an ambient temperature water bath as control paradigm to identify 'true' CPM effects. In the PAG of CLBP patients, a lower Glx/GABA ratio, i.e. a lower excitatory/inhibitory tone, was observed (P = 0.002, partial {eta}2 = 0.14) driven by decreased Glx (P = 0.012, partial {eta}2 = 0.11) and increased GABA (P = 0.038, d = 0.46). CLBP patients showed disrupted associations between Glx/GABA and PPTs compared to controls in both areas (lower back: P = 0.004, partial {eta}2 = 0.12; hand: P = 0.002, partial {eta}2 = 0.16). In controls, lower Glx/GABA was associated with lower PPTs (lower back: r = 0.48, P = 0.009, hand: r = 0.53, P = 0.003), but this link was missing in CLBP patients (r's > -0.23, P's > 0.150). Additionally, CLBP patients with more severe clinical pain showed smaller CPM effects at the hand (rho = 0.54, P = 0.003). These findings suggest a dysfunction of the PAG in patients with CLBP and might indicate altered descending inhibition of deep tissue afferents.

/pdf/the-periaqueductal-grey-in-chronic-low-back-pain-1eiyfxip.pdf

Laura Gorrell

Papers

Mechanisms of procollagen and HSP47 sorting during ER-to-Golgi trafficking

ER, Mitochondria, and ISR Regulation by mt‐HSP70 and ATF5 upon Procollagen Misfolding in Osteoblasts

The periaqueductal grey in chronic low back pain: dysregulated metabolites and function