Vesicular Transport, the fundamental process by which cells shuttle molecules within membrane-bound vesicles, is critical for cellular function and communication. Disruptions in this intricate system are increasingly recognized as drivers of various diseases, particularly those affecting the nervous system. Among the key regulators of vesicular transport are the evolutionarily conserved TRAPP (Transport Protein Particle) complexes. This article delves into recent research highlighting the crucial role of TRAPPC4, a core subunit of the TRAPP complex, in neurodevelopmental disorders, emphasizing the link between TRAPPC4 deficiency and impaired vesicular transport and autophagy.
Recent exome sequencing efforts have identified a novel homozygous variant in TRAPPC4 in multiple unrelated families with children exhibiting severe neurodevelopmental symptoms. These symptoms included early-onset seizures, developmental delays, microcephaly, sensorineural deafness, spastic quadriparesis, and progressive brain atrophy. The shared genetic anomaly, a rare variant affecting a splice site within TRAPPC4 (c.454+3A>G), strongly implicates TRAPPC4 dysfunction in the pathogenesis of these debilitating conditions. Further genetic analysis ruled out a common founder effect, suggesting independent occurrences of this variant.
In silico predictions and experimental validation confirmed that this TRAPPC4 variant disrupts normal mRNA splicing. This aberrant splicing resulted in reduced levels of the full-length TRAPPC4 transcript and an increase in a truncated transcript lacking exon 3. Consequently, affected individuals showed significantly decreased levels of the TRAPPC4 protein. Intriguingly, the levels of other TRAPP complex subunits remained unaffected, suggesting a specific vulnerability arising from TRAPPC4 deficiency. Further investigations using native gel electrophoresis and size exclusion chromatography revealed that the reduced TRAPPC4 levels led to defects in the assembly or stability of the entire TRAPP complex.
To directly assess the impact on vesicular transport, researchers examined Golgi trafficking, a well-characterized process dependent on TRAPP complexes. Using a fluorescent marker protein (VSVG-GFP-ts045), they observed a significant delay in both the entry into and exit from the Golgi apparatus in fibroblasts derived from affected individuals. Importantly, restoring normal TRAPPC4 levels through lentiviral expression rescued this trafficking defect, directly linking TRAPPC4 deficiency to impaired vesicular transport.
Beyond vesicular transport, the TRAPP complex has also been recently associated with autophagy, a critical cellular process for clearing damaged components and maintaining cellular health. Consistent with this link, fibroblasts from affected individuals displayed basal autophagy defects and delays in autophagic flux. This suggests that the TRAPPC4 variant not only disrupts vesicular trafficking but also impairs autophagy, potentially contributing to the neurodegenerative aspects of the observed disorder. These findings were further validated using a yeast model with a temperature-sensitive mutation in the TRAPPC4 orthologue, trs23, which exhibited similar defects in both autophagy and secretion.
In conclusion, this research provides compelling evidence for the pathogenicity of a specific TRAPPC4 splice site variant and its association with neurodevelopmental disorders. The study underscores the critical role of TRAPPC4 in maintaining functional TRAPP complexes, which are essential for proper vesicular transport and autophagy. These findings expand the growing spectrum of TRAPP-associated neurological disorders and highlight the importance of vesicular transport pathways in brain development and function. Further research into TRAPPC4 and TRAPP complex function may pave the way for potential therapeutic strategies for these devastating conditions.
