HNRNPC Protein
Overview
HNRNPC (Heterogeneous Nuclear Ribonucleoprotein C) is a highly abundant RNA-binding protein belonging to the heterogeneous nuclear ribonucleoprotein family. Encoded by the HNRNPC gene located on chromosome 14q11.2, this protein exists in two isoforms: HNRNPC1 and HNRNPC2, generated through alternative splicing of exon 2. HNRNPC is one of the most prevalent nucleoplasmic proteins, constituting approximately 0.1-0.2% of total cellular protein. The protein is ubiquitously expressed across human tissues, with particularly high levels in neurons and brain tissue. Its molecular weight ranges from 33-36 kDa depending on the isoform, and it possesses two RNA recognition motifs (RRMs) that enable direct interaction with RNA substrates.
Function/Biology
HNRNPC functions as a multifunctional RNA-binding protein with primary roles in pre-mRNA processing and RNA metabolism regulation. The protein plays critical roles in alternative splicing regulation by binding to nascent pre-mRNA transcripts and modulating splice site accessibility to the spliceosome machinery. Through its RNA recognition motifs, HNRNPC preferentially binds poly-U and poly-UC sequences, commonly found in pre-mRNA introns and intronic regulatory elements. The protein shuttles between the nucleus and cytoplasm, participating in mRNA export and localization. Additionally, HNRNPC regulates mRNA stability and translation efficiency through interactions with other trans-acting factors and involvement in mRNA secondary structure formation. HNRNPC associates with the spliceosome and co-regulates splicing patterns in conjunction with SR proteins and other hnRNPs, creating a balanced regulatory network for gene expression. The protein also localizes to stress granules and processing bodies during cellular stress responses, suggesting roles in mRNA triage and translational control.
Role in Neurodegeneration
Recent research has implicated HNRNPC dysregulation in multiple neurodegenerative diseases, particularly amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Studies demonstrate that HNRNPC becomes abnormally hyperphosphorylated and sequestered into cytoplasmic inclusions in ALS pathology, co-aggregating with other pathological RNA-binding proteins like TDP-43 and FUS. This mislocalization disrupts normal HNRNPC function and contributes to global dysregulation of alternative splicing patterns characteristic of ALS neurons. In Alzheimer's disease, altered HNRNPC expression correlates with cognitive decline and pathological changes, potentially affecting splicing of genes involved in amyloid-beta and tau metabolism. HNRNPC dysregulation appears to impair the processing of transcripts encoding proteins critical for neuronal survival, axonal transport, and synaptic function.
Molecular Mechanisms
The neurotoxic mechanisms involving HNRNPC dysregulation operate through several interconnected pathways. Aberrant hyperphosphorylation of HNRNPC, mediated by kinases including GSK-3β and CDK5, promotes its cytoplasmic accumulation and aggregation propensity. Once mislocalised, HNRNPC loses access to nuclear pre-mRNA substrates, causing genome-wide splicing dysregulation affecting neuron-specific transcripts. Particularly vulnerable targets include genes encoding components of the nuclear export machinery, translation factors, and proteins maintaining cytoskeletal integrity. HNRNPC aggregation also sequesters other regulatory proteins, amplifying dysfunction. The protein's interaction with TDP-43 and FUS suggests shared pathogenic mechanisms in ALS and FTD, where HNRNPC sequestration into pathological inclusions may represent a secondary consequence of primary RNA-binding protein pathology. Additionally, impaired HNRNPC function disrupts the splicing of survival motor neuron (SMN) and other neuroprotective genes, exacerbating neuronal vulnerability.
Clinical/Research Significance
HNRNPC represents an important therapeutic target for neurodegenerative diseases, particularly ALS. Understanding HNRNPC-mediated splicing dysregulation may identify disease-relevant transcript alterations and reveal new intervention points. Research utilizing cell and animal models demonstrates that restoring HNRNPC localization or function ameliorates pathogenic splicing patterns. The protein serves as both a molecular marker for disease progression and a potential therapeutic target through approaches including antisense oligonucleotides targeting pathological splicing events, molecular chaperones to prevent aggregation, or kinase inhibitors reducing pathological phosphorylation. Biomarker studies examining phosphorylated HNRNPC in cerebrospinal fluid and serum may improve disease diagnosis and treatment monitoring.
Related Proteins: TDP-43, FUS, HNRNPA1, HNRNPA2/B1, HNRNPK, SR proteins
Associated Diseases: Amyotrophic lateral sclerosis, Frontotemporal dementia, Alzheimer's disease
Pathways: Alternative splicing regulation, mRNA export, stress granule formation, RNA quality control