U.S. stock futures are mixed after markets fell sharply in the prior session; UnitedHealth Group (UNH) shares are plunging in premarket trading after a report that the Justice Department is investigating the insurer's Medicare billing practices; a Japanese group is reportedly planning to propose to Tesla (TSLA) that it invest in struggling automaker Nissan; Chinese e-commerce firm Alibaba Group (BABA) shares are rising on a report that GameStop (GME) CEO Ryan Cohen has raised his stake in the company; and energy-drink maker Celsius (CELH) stock is soaring after the company reported strong earnings and moved to acquire rival Alani Nu. Here's what investors need to know today.

1. US Stock Futures Mixed After Market Selloff

U.S. stock futures are mixed in premarket trading after markets fell in the prior session on investor concerns over consumer health. Dow Jones Industrial Average futures have turned 0.4% lower on the UnitedHealth report after the index fell 1% yesterday, while S&P 500 futures are little changed after it pulled back from record-high levels. Nasdaq futures are 0.3% higher after the tech-focused index declined 0.5% Thursday. Bitcoin (BTCUSD) is up slightly to trade near $99,000, while yields on the 10-year Treasury note are down slightly. Oil and gold futures are lower.

2. UnitedHealth Stock Dives on Reported DOJ Probe of Medicare Billing

UnitedHealth Group (UNH) stock is sinking 10% in premarket trading after a report that the U.S. Department of Justice is investigating the insurer's Medicare billing practices. The Wall Street Journal reported that the DOJ civil fraud investigation is looking into the insurance giant's practices for recording diagnoses that trigger additional payments to its Medicare Advantage plans. Shares of other insurance providers also are falling on the report, with Humana (HUM) down 5% and Cigna Group (CI) 2% lower.

3. Japanese Group to Pitch Tesla on Investment in Nissan

A high-level Japanese group is reportedly planning to propose to Tesla (TSLA) to invest in ailing automaker Nissan after its merger with domestic competitor Honda collapsed earlier this month. Tesla may want to be a strategic investor in Nissan since the group believes the U.S. electric vehicle maker desires the Japanese firm's American factories, according to the Financial Times. Nissan shares closed 9.5% higher in Tokyo trading following the report. Tesla shares are little changed in premarket trading.

4. Alibaba Stock Gains as GameStop CEO Cohen Reportedly Expands Stake

U.S.-listed shares of Chinese e-commerce firm Alibaba Group (BABA) are jumping 4% in premarket trading after a report that GameStop (GME) CEO Ryan Cohen has raised his stake in the firm. The Wall Street Journal reported that Cohen now personally has about $1 billion in stock in the firm, increasing his holdings from several hundred million dollars. The stock soared 8% yesterday after its December-quarter results topped analysts' estimates.

5. Energy-Drink Maker Celsius Stock Soars on Strong Revenue, Acquisition of Rival

Shares of Celsius (CELH) are surging more than 30% in premarket trading after the energy-drink maker reported quarterly revenue that beat expectations and moved to acquire a rival drink brand. Celsius reported fourth-quarter revenue of $332.2 million, ahead of analysts' estimates compiled by Visible Alpha, while its adjusted earnings per share (EPS) of $0.14 came in just short of projections. The firm also announced that it would acquire rival Alani Nu, a brand that markets to female customers, for $1.8 billion, including $150 million in tax assets.