Centriolar Satellite Markers

The term “centriolar satellites” was coined in the second half of the 20th century to name the 70–100 nm electron-dense particles in diameter surrounding the centrosome. Centriolar satellites contain several centrosomal proteins and are the core structural elements of centrosomes and cilia.

Proteomics and systematic genome-wide analysis suggest that the human centrosome comprises hundreds of different components. According to the Subcellular section of the Human Protein Atlas (HPA) project, three percent of all protein-coding human genes (564 genes) encode proteins that localize to the centrosome or the centriolar satellites (proteinatlas.org).

Centriolar satellites, ubiquitous in vertebrate cells, are small, cytoplasmic membrane-less spherical granules that localize and move around the centrosomes and cilia. Centriolar satellites can also move along microtubules with the help of motor proteins. Thus, they function as vehicles for protein transport towards and away from the centrosome. However, they are mostly only detectable in interphase cells and undergo dissolution during mitosis (Kubo 2003).

Satellites have been implicated in multiple critical cellular functions, including centriole duplication, centrosome maturation, and ciliogenesis; however, their precise composition and assembly properties still need to be explored (Kubo 1999; Hori 2017).

Because of the substantial overlap of functions and proteomes between the centriolar satellites with centrosomes and cilia, research on centriolar satellites has so far been undertaken with a biased view from a centrosome/cilium-centered perspective. 

As a result, centriolar satellites are often referred to as the “third component” of the vertebrate centrosome/cilium complex and as a member of the emerging class of membrane-less organelles. 

Only about 50% of the centrosome proteome overlaps with the satellite proteome, challenging the current classification of proteins into “centrosome” or “satellite” categories.

Supporting this view, data show that satellite proteins composition is mainly unaffected by centriole depletion; most proteins of the centriolar satellites are present in cells that lack centrioles, meaning that satellite assembly is centrosome-independent.

Centriolar satellites are detectable in almost all mammalian cell types. Their cellular distribution ranges from clustering at the centrosomes, the nucleus or basal bodies, to scattered throughout the cytoplasm, depending on the cell types and tissues.

However, their size, molecular composition, abundance, and localization can vary considerably so, type and tissue-specific functions remain primarily unexplored.

A complete picture regarding the full set of satellite components and the spatiotemporal constitution of centriolar satellites has not yet been established because the number of proteins identified as satellite components has continued to increase over the last several years. They were reported to be 11 in 2011 (Bärenz 2011) and over 100 according to the most recent studies (Gupta 2015).

The analysis of the satellite interactome, combined with subdiffraction imaging, reveals the existence of multiple unique microscopically resolvable satellite populations that display distinct protein interaction profiles, paving the way for future studies aimed at better understanding the biogenesis and functions of these enigmatic structures (Gheiratmand 2019).

Atlas Antibodies offers numerous markers targeting centriolar satellites proteins, some of which are highlighted in this white paper.

Figures 1-7 show representative immunofluorescent (ICC-IF) stainings of centriolar satellite proteins in various human cell lines.

PCM1: the bona fide centriolar satellite marker

PCM1 (pericentriolar material 1) is a large protein (~230 kDa) and the first identified molecular component of centriolar satellites: it is now considered the bona fide centriolar satellite marker in cells (Balczon 1994; Baron 1988; Kubo 1999). Additional PCM1 location includes the cytosol.

PCM1 serves as a scaffold protein and recruits centrosomal proteins to centriolar satellites (Hames 2005). It is essential to correctly position proteins such as CEP250, CETN3, PCNT, and NEK2 and anchoring microtubules to the centrosome.

PCM1 represents a structural platform for centriolar satellites: when PCM1 becomes dysfunctional, either by depletion, deletion or mutation, satellite particles disassemble (Dammermann 2002).

Therefore, the evaluation of new proteins as components of centriolar satellites is formally made according to two criteria: the first is colocalisation and physical interaction with PCM1, and the second is delocalisation from pericentrosomal locations upon PCM1 depletion (Lopes 2011).

PCM1 accumulates at the nuclear surface of differentiated, non-cycling myocytes. Research studies have demonstrated that antibodies against PCM1 specifically label cardiomyocyte nuclei, and as such, PCM1 has been used by several independent groups to identify cardiomyocytes both in human and rodents (Bergmann 2009, 2012; Hirai 2016; Preissl 2015).

Mutation of genes encoding PCM1 and other centriolar satellite components or regulatory proteins involved in centriolar satellite integrity can cause ciliopathy-related human diseases such as Bardet–Biedl syndrome, Joubert syndrome, Meckel Gruber syndrome, primary microcephaly (MCPH), and oral-facial-digital syndrome (Kodani 2015; Stephen 2015; Sang 2011).

Centriolar satellites disease involvement

Centriolar satellites are crucial regulators of a wide range of cellular processes. Research on centriolar satellites focuses on how important they are in centrosome activities such as centrosome maintenance and centriole duplication.

Scientists examine mice deficient in centriolar satellite formation and the relationship with the organelle. In addition, they are looking at cell lines from patients with protein aggregation disorders to see if there are effects on the composition and function of the centriolar satellite.

Centriolar satellites contain numerous proteins involved in a variety of mechanisms that can go wrong, thus causing ciliopathic diseases, carcinogenesis, neurogenesis, and other diseases such as dwarfisms and microcephaly (Firat-Karalar 2014; Ge 2010; Gupta 2015; Mahmood 2011, Rauch 2008, Silva 2016; Stowe 2012; Ye 2014).

There are dozens of publications reporting the involvement of centriolar satellite proteins in diverse kinds of diseases. Table 1. summarizes some examples.

Centriolar satellites are required for efficient ciliogenesis and ciliary content regulation.

Centriolar satellites play pivotal roles in centrosome assembly and primary cilium formation through the delivery of centriolar/centrosomal components from the cytoplasm to the centrosome.

Odabasi et al. (2019) generated a kidney epithelial cell line (IMCD3) lacking satellites using CRISPR/Cas9-mediated PCM1 deletion and investigated the cellular and molecular consequences of satellite loss. 

They found that cells lacking satellites could still form full-length cilia, although at significantly lower numbers, with changes in the centrosomal and cellular levels of key ciliogenesis factors. Surprisingly, other functions of satellites, namely proliferation, cell cycle progression, and centriole duplication, were unaffected in these cells.

Recent studies implicate primary cilium proteins in the etiologies of various polycystic kidney diseases. The mammalian serine/threonine kinases, Nek1 are involved in primary cilium formation. Mice with NEK1 mutation suffer from polycystic kidney diseases, suggesting that NEK1 may be engaged in cilium control. Moreover, NEK1 overexpression inhibits ciliogenesis (Shalom 2008).

Centriolar proteins as tumor suppressor

DMBT1 (the gene deleted in malignant brain tumors 1) located in the centriolar satellite is an intracellular, secreted protein considered a candidate tumor suppressor gene for brain, lung, esophageal, gastric, and colorectal cancers.

For example, in gastric cancer, DMBT1 may mediate mucosal protection, reducing the risk of developing gastric precancerous lesions. However, the increased expression in human gastric precancerous lesions points to a more complex role of DMBT1 in gastric carcinogenesis (Garay 2017).

Dysregulation of LATS2 (large tumor suppressor 2) gene functions has been found in different tumors. As a core kinase in the Hippo pathway, LATS2 plays a pivotal role in organ size control and tumor suppression by restricting proliferation and promoting apoptosis. LATS2 is a putative tumor suppressor gene with potential roles in regulating cell proliferation and apoptosis in lung cancer (Luo, 2014).

Moreover, LATS2 is downregulated in gliomas and exhibits a negative correlation with the prognosis of glioma: over-expression of LATS2 inhibited glioma cell proliferation and migration/invasion, while LATS2 downregulation promoted them (Shi 2019). Furthermore, data from a genome-wide array-based comparative genomic hybridization analysis show that LATS2 can act as a tumor suppressor gene for malignant mesothelioma (an aggressive neoplasm associated with asbestos exposure) (Murakami 2011).

Chromosomal aberrations involving the PCM1 gene are associated with papillary thyroid carcinomas and a variety of hematological malignancies, including atypical chronic myeloid leukemia and T-cell lymphoma. For example, PCM1 had a significantly lower expression in primary ovarian carcinoma than controls promoting PCM1 as a potential tumor suppressor (Pils 2005).

Table 1. Centriolar satellites disease involvement

Genome-wide association studies have identified several risk factors for neurodegenerative diseases and dementia involving centriolar satellite proteins. 

Due to its cerebrovascular role, the centriolar satellite CD2-associated protein (CD2AP) is a leading genetic risk factor for Alzheimer’s disease (Liao 2015; Furusawa 2019). Furthermore, the role of CD2AP in maintaining the blood-brain barrier integrity was reported by Cochran et al. (2015), using the anti-CD2AP polyclonal antibody (HPA003326).

Lan et al. (2016) used CRISPR/Cas9 genome editing to generate primary glioblastoma (GBM) cell lines depleted from PCM1. The results suggest that PCM1 plays multiple roles in GBM pathogenesis and that PCM1-associated pathways could be targeted to augment current or future anti-GBM therapies.

A more recent study emphasizes the role of PCM1 in the postnatal brain. Using the anti-PCM1 polyclonal antibody HPA023370, Monroe et al. (2020) support a contributory role for PCM1 in some individuals diagnosed with schizophrenia.

PLA2G6 is involved in parkinsonism-induced dystonia and neurodegeneration (Karkheiran 2015). In addition, PLA2G6 has also been certified as a causative gene in patients with autosomal recessive early-onset Parkinson’s disease (Paisán-Ruiz 2010, Shen 2019).

A significantly reduced head circumference characterizes primary microcephaly. Mutations in the centromere proteins genes, such as WDR62 and CENPU, cause primary microcephaly (Mahmood 2011). 

Truncating mutation in intracellular phospholipase A1 gene (DDHD2) is involved in hereditary spastic paraplegia with intellectual disabilities (Alrayes 2015).

Table 2. Centriolar satellite antibody markers

References

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