Recombinant Mouse Transmembrane protein 216 (Tmem216)

What is Transmembrane protein 216 (Tmem216) and what is its cellular localization?

Transmembrane protein 216 (Tmem216) is a ciliary transition zone tetraspan transmembrane protein that localizes to the base of primary cilia or adjacent basal body in ciliated cells. Immunostaining studies in ciliated cell lines (inner medullary collecting duct [IMCD3] and retinal pigment epithelium [hRPE]) have demonstrated this localization pattern, as marked by acetylated or glutamylated tubulin staining . This specific positioning at the ciliary transition zone is consistent with Tmem216's critical functions in ciliogenesis and ciliary maintenance.

What genetic disorders are associated with TMEM216 mutations?

Mutations in TMEM216 are linked to two related ciliopathies: Joubert syndrome (JBTS) and Meckel syndrome (MKS). These conditions represent a spectrum of developmental disorders with overlapping features. Research has confirmed that the JBTS2 and MKS2 genetic loci are allelic and result from mutations in TMEM216 .

The phenotypic spectrum associated with TMEM216 mutations includes:

A notable genetic finding is the p.R73L founder mutation identified in all patients of Ashkenazi Jewish descent tested (n=10) .

What is the expression pattern of Tmem216 during development?

In zebrafish, in situ hybridization with tmem216 antisense probe revealed wide distribution at 3 days post-fertilization (dpf) across multiple organs including the eye, pronephros, brain, liver, intestine, and muscle . Within the retina specifically, expression was observed in all neural retina layers (outer nuclear layer, inner nuclear layer, and ganglion cell layer), with this expression pattern maintained through 7-dpf .

RT-PCR analysis has demonstrated that TMEM216 mRNA is detectable in:

• Freshly laid eggs

• 7-dpf whole larvae

• Adult eye tissue

• Adult brain tissue

• Adult skeletal muscle

In human embryonic tissues, in situ hybridization confirmed expression in the central nervous system, limb bud, kidney, and cartilage . This broad and relatively low-level expression pattern mirrors that of other Joubert syndrome and Meckel syndrome-associated genes.

What protein complexes does Tmem216 participate in?

Tmem216 functions as a component of the transition zone tectonic complex, a protein assembly critical for ciliary formation and function . This complex consists of multiple proteins associated with Meckel and Joubert syndromes, including:

• Secreted protein: TCTN1

• Transmembrane proteins: TCTN2, TCTN3, meckelin (TMEM67), and TMEM216

• Intracellular proteins: B9 domain–containing protein 1 (B9D1), CEP290, Meckel syndrome type 1 protein (MKS-1), and coiled-coil and C2 domain-containing protein 2A (CC2D2A)

Biochemical studies have demonstrated that TMEM216 directly complexes with Meckelin (TMEM67), which is encoded by another gene mutated in both JSRD and MKS . The tectonic/B9 complex serves three critical functions:

• Facilitation of cilia formation

• Regulation of ciliary membrane protein localization (such as Arl13b)

• Restriction of plasma membrane proteins from entering the ciliary compartment

How does deletion of Tmem216 affect photoreceptor development and function?

Tmem216 knockout zebrafish exhibit multiple defects in photoreceptor development, organization, and survival. The comprehensive effects include:

These findings demonstrate that while TMEM216 deletion does not prevent initial photoreceptor generation, it results in improper development and eventual degeneration . The combination of mislocalized outer segment proteins, shortened ciliary axoneme, and abnormal outer segment disc morphology suggests that TMEM216 plays essential roles in both the transport of proteins to the outer segment and the structural organization of the outer segment itself.

What molecular mechanisms mediate Tmem216's role in ciliogenesis?

Loss of TMEM216 function, either in patient fibroblasts or following siRNA knockdown, causes defective ciliogenesis and impaired centrosomal docking . These defects are associated with concomitant hyperactivation of RhoA and Dishevelled signaling pathways . This suggests that TMEM216 may regulate ciliary formation in part through modulation of these signaling cascades.

As a component of the transition zone complex, TMEM216 contributes to:

• Establishment of proper ciliary composition through selective protein transport

• Maintenance of the diffusion barrier between ciliary and plasma membrane compartments

• Regulation of ciliary membrane protein localization (particularly for proteins like Arl13b)

The interplay between TMEM216 and other transition zone components appears critical for these functions, though the precise molecular interactions governing these processes require further characterization.

What methodological approaches are available for generating Tmem216 knockout models?

The generation of tmem216 knockout zebrafish using CRISPR/Cas9 genome editing has proven effective for studying Tmem216 function . The methodological workflow includes:

• Guide RNA Design and Synthesis

  • Target sequences specific to the tmem216 gene

  • Include appropriate promoter sequences for transcription

• Microinjection

  • Injection of guide RNAs and Cas9 protein into zebrafish embryos at one-cell stage

• Founder Identification

  • PCR genotyping of F0 zebrafish

  • Selection of animals yielding shorter PCR bands than wildtype

• Line Establishment

  • Crossing F0 zebrafish with wildtype fish to obtain F1 generation

  • PCR and sequencing verification of F1 zebrafish

  • Identification of specific mutations in shorter bands (compared to 577bp wildtype band)

• Colony Maintenance

  • Collection of experimental data from F2 and F3 generations

For phenotypic characterization, a comprehensive approach combining multiple techniques is recommended:

• Immunofluorescence staining for ciliary and photoreceptor markers

• Terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) for cell death assessment

• Electron microscopy for ultrastructural analysis

• Western blotting for protein expression quantification

• In situ hybridization for expression pattern analysis

Alternative approaches include siRNA knockdown in cell culture systems, which has been successfully employed to study TMEM216 function in fibroblasts .

How can researchers quantify structural and functional defects in Tmem216 mutant models?

Based on published methodologies, researchers should implement a multi-modal approach to comprehensively characterize Tmem216 mutant phenotypes:

• Photoreceptor Morphology Assessment

  • Immunofluorescence staining with specific markers:

    • Rod photoreceptor outer segment: 4D2 antibody

    • Cone photoreceptor outer segment: G protein subunit α transducin 2 (GNAT2) antibody

    • Ciliary axoneme: acetylated α-tubulin antibody

• Protein Localization Analysis

  • Evaluate outer segment protein distribution (rhodopsin, GNAT2, red opsin)

  • Quantify mislocalization to inappropriate cellular compartments

• Cell Death Quantification

  • TUNEL staining to identify apoptotic nuclei

  • Calculation of TUNEL-positive cells as percentage of total cells in relevant layers

• Ultrastructural Analysis

  • Electron microscopy to evaluate:

    • Outer segment disc morphology

    • Presence of vesicles/vacuoles within outer segments

    • Ciliary transition zone architecture

• Protein Expression Quantification

  • Western blot analysis using:

    • Outer segment markers (e.g., GNAT2)

    • Normalization to housekeeping proteins (e.g., β-actin)

    • Band intensity quantification using software like ImageJ

These methodological approaches provide complementary data to thoroughly characterize the structural and functional consequences of Tmem216 mutation or deletion.

What are the genotype-phenotype correlations for different TMEM216 mutations?

Research has identified several important genotype-phenotype correlations for TMEM216 mutations:

• Mutation Location and Protein Stability

  • All identified nonsynonymous changes occur in evolutionarily conserved residues

  • These mutations lead to unstable protein when expressed in heterologous cells

  • Both truncating mutations (in middle and end of protein) and specific missense mutations (particularly p.R73 transversions) are associated with disease

• Founder Effects

  • The p.R73L mutation represents a clear founder mutation in the Ashkenazi Jewish population

• Phenotypic Spectrum

  • Joubert Syndrome: Characterized by nephronophthisis (45%) and polydactyly (45%), without retinal dystrophy or congenital hepatic fibrosis

  • Oro-Facio-Digital type VI: Distinguished by polydactyly associated with either tongue tumors or multiple oral frenula

  • Meckel Syndrome: More severe presentation with skeletal dysplasia as a common feature

The specific mechanisms by which different mutations lead to distinct clinical presentations remain incompletely understood, highlighting the need for further research into structure-function relationships of the TMEM216 protein.