Sil1 is an endoplasmic reticulum (ER)-resident protein and a nucleotide exchange factor for the molecular chaperone protein Bip [1,4,6,7]. It plays a crucial role in the ER protein-folding process. Bip, an ER HSP70 chaperone, interacts with unfolded client proteins in a nucleotide-dependent manner, and Sil1 tightly regulates this process by facilitating the conversion from the ADP-bound to the ATP-bound state of Bip, closing the chaperone folding cycle [1,4,7]. This function is essential for ensuring proper protein maturation in the ER; otherwise, misfolded proteins may accumulate, triggering the unfolded protein response (UPR) [3]. Genetic models, such as mouse models, have been valuable in studying Sil1's functions.

Loss-of-function mutations in Sil1 are the leading cause of Marinesco-Sjögren syndrome (MSS), an autosomal recessive multisystem disorder [1]. In Sil1-deficient fibroblasts from MSS patients, transcriptomic analysis revealed 664 differentially expressed transcripts, with issues in membrane trafficking, and an impact on the extracellular space/extracellular matrix (ECM) and cell adhesion machinery. Functional assays showed reduced ECM remodelling capacity, motility, and slower spreading during adhesion in these fibroblasts, and TEM analysis of mouse tendons indicated a disorganization of collagen fibres, suggesting that aberrant ECM is a key pathological feature of MSS [3]. In SIL1-deficient zebrafish embryos and mice, morphological changes were detected in the peripheral nervous system (PNS) and neuromuscular junctions (NMJs), suggesting that impaired neuromuscular transmission might be part of MSS pathophysiology [5]. In mice with Sil1 deficiency, the expression of Reelin receptors was diminished, impairing the Reelin signalling pathway, inhibiting the developmental expression of GluN2A, and impairing spatial learning, indicating a role for Sil1 in central nervous system development [2]. In a cortical neuron model with SIL1 protein deficiency, proteomic analysis identified that loss of SIL1 affects actin dynamics, leading to abnormal neural migration [6].

In conclusion, Sil1 is essential for maintaining ER homeostasis and normal physiology through its role in the protein-folding process in the ER. Studies using gene-knockout (KO) mouse models and other loss-of-function experiments have revealed its significance in diseases like MSS, as well as its role in processes such as neural development, ECM regulation, and neuromuscular function. Understanding Sil1's functions provides potential avenues for developing treatments for MSS and other related conditions [1,2,3,5,6].

References:

1. Ichhaporia, Viraj P, Hendershot, Linda M. 2021. Role of the HSP70 Co-Chaperone SIL1 in Health and Disease. In International journal of molecular sciences, 22, . doi:10.3390/ijms22041564. https://pubmed.ncbi.nlm.nih.gov/33557244/

2. Xu, Shilian, Zhu, Jia, Mi, Kai, Shen, Yan, Zhang, Xiaomin. 2019. Functional Role of SIL1 in Neurodevelopment and Learning. In Neural plasticity, 2019, 9653024. doi:10.1155/2019/9653024. https://pubmed.ncbi.nlm.nih.gov/31531014/

3. Amodei, Laura, Ruggieri, Anna Giulia, Potenza, Francesca, De Laurenzi, Vincenzo, Sallese, Michele. 2024. Sil1-deficient fibroblasts generate an aberrant extracellular matrix leading to tendon disorganisation in Marinesco-Sjögren syndrome. In Journal of translational medicine, 22, 787. doi:10.1186/s12967-024-05582-0. https://pubmed.ncbi.nlm.nih.gov/39180052/

4. Bracher, Andreas, Verghese, Jacob. . Nucleotide Exchange Factors for Hsp70 Molecular Chaperones: GrpE, Hsp110/Grp170, HspBP1/Sil1, and BAG Domain Proteins. In Sub-cellular biochemistry, 101, 1-39. doi:10.1007/978-3-031-14740-1_1. https://pubmed.ncbi.nlm.nih.gov/36520302/

5. Phan, Vietxuan, Cox, Dan, Cipriani, Silvia, Weis, Joachim, Roos, Andreas. 2018. SIL1 deficiency causes degenerative changes of peripheral nerves and neuromuscular junctions in fish, mice and human. In Neurobiology of disease, 124, 218-229. doi:10.1016/j.nbd.2018.11.019. https://pubmed.ncbi.nlm.nih.gov/30468864/

6. Xu, Yuanyuan, Sun, Hongji, Chen, Junyang, Zhong, Zhaoming, Zhang, Xiaomin. 2024. Loss of SIL1 Affects Actin Dynamics and Leads to Abnormal Neural Migration. In Molecular neurobiology, 62, 335-350. doi:10.1007/s12035-024-04272-8. https://pubmed.ncbi.nlm.nih.gov/38850350/

7. Bracher, Andreas, Verghese, Jacob. . GrpE, Hsp110/Grp170, HspBP1/Sil1 and BAG domain proteins: nucleotide exchange factors for Hsp70 molecular chaperones. In Sub-cellular biochemistry, 78, 1-33. doi:10.1007/978-3-319-11731-7_1. https://pubmed.ncbi.nlm.nih.gov/25487014/