Biology subjects

Bhopatkar, A. A.

Publications and source records attributed to Bhopatkar, A. A..

3 recordsLinked to original sources

bioRxiv2021

Cytoplasmic Colocalization of Granulins and TDP-43 Prion-like Domain Involves Electrostatically Driven Complex Coacervation Tuned by the Redox State of Cysteines.

Cytoplasmic inclusions containing aberrant proteolytic fragments of TDP-43 are associated with frontotemporal lobar degeneration (FTLD) and other related pathologies. In FTLD, TDP-43 is translocated into the cytoplasm and proteolytically cleaved to generate a prion-like domain (PrLD) containing C-terminal fragments (C25 and C35) that form toxic inclusions. Under stress, TDP-43 partitions into membraneless organelles called stress granules (SGs) by coacervating with RNA and other proteins. We were interested in understanding if and how cysteine-rich granulins (GRNs 1-7), which are the proteolytic products of a genetic risk factor in FTLD called progranulin, interact with TDP-43. We show that extracellular GRNs internalize and colocalize with PrLD as puncta in the cytoplasm of neuroblastoma cells but show no presence in SGs. In addition, we show GRNs and PrLD coacervate to undergo liquid-liquid phase separation (LLPS) or form gel- or solid-like aggregates. Identification of the sequence determinants within GRNs for the observed phase transitions reveal the negative charges to be the drivers of LLPS modulated by the positive charges and the redox state of cysteines. Furthermore, RNA and GRNs compete and expunge one another from PrLD condensates, providing a basis for GRNs absence in SGs. Together, the results illustrate the potential mechanisms by which extracellular GRNs, formed during chronic inflammatory conditions, could internalize, and modulate cytoplasmic TDP-43 inclusions in proteinopathies.

biophysics↗

bioRxiv2020

Are Granulins Copper Sequestering Proteins?

Granulins (GRN 1-7) are short ([~]6 kDa), cysteine-rich proteins that are generated upon the proteolytic processing of progranulin (PGRN). These modules, along with their precursor, have been implicated in multiple pathophysiological roles, especially in neurodegenerative diseases. Our previous investigations into GRN-3 and GRN-5 reveal them to be fully disordered in the reduced form and implicate redox sensitive attributes to the proteins. Such redox-dependent modulation has become associated with proteins involved in oxidative stress regulation and maintaining metal-homeostasis within cells. To probe whether GRNs play a contributory role in such functions, we tested the metal binding potential of the reduced form of GRNs -3 and -5 under neutral and acidic pH mimicking cytosolic and lysosomal conditions, respectively. We found, at neutral pH, both GRNs selectively bind Cu(II) and no other divalent cations. Binding of Cu(II) also partly triggered the oxidative multimerization of GRNs via uncoordinated cystines at both pH conditions. Furthermore, binding did not induce gain in secondary structure and the protein remained disordered. Overall, the results indicate that GRN-3 and -5 have a surprisingly strong affinity for Cu(II) in the pM range, comparable to known copper sequestering proteins. This data also hints at a potential of GRNs to reduce Cu(II) to Cu(I), a process that has significance in mitigating Cu-induced ROS cytotoxicity in cells. Together, this report uncovers a metal-coordinating capability of GRNs for the first time, which could have profound significance in their structure and function.

biophysics↗

bioRxiv2019

Granulins modulate liquid-liquid phase separation and aggregation of TDP-43 C-terminal domain

Tar DNA binding protein (TDP-43) has emerged as a key player in many neurodegenerative pathologies including frontotemporal lobar degeneration (FTLD) and amyotropic lateral sclerosis (ALS). Important hallmarks of FTLD and ALS are the toxic cytoplasmic inclusions of C-terminal fragments of TDP-43 (TDP-43CTD), which are formed upon proteolytic cleavage of full-length TDP-43 in the nucleus and subsequent transport to the cytoplasm. TDP-43CTD is also known to form stress granules (SGs) by coacervating with RNA in cytoplasm under stress conditions and are believed to be involved in modulating the pathologies. Among other factors affecting these pathologies, the pleiotropic protein called progranulin (PGRN) has gained significant attention lately. The haploinsufficiency of PGRN, caused by autosomal dominant mutations in GRN gene, results in its loss-of-function linked to FTLD and ALS. But precisely how the protein contributes to the pathology remains unknown. Recently, cleavage to GRNs were observed to be a significant part of FTLD and ALS progression with specific GRNs exacerbating TDP-43-induced toxicity in C.elegans. In this report, we show that GRNs -3 and -5 directly interact with TDP-43CTD to modulate latters aggregation or stress granule formation in disparate ways in vitro. These results constitute the first observation of direct interaction between GRNs and TDP-43 and suggest a mechanism by which the loss of PGRN function could lead to FTLD and ALS.

biophysics↗